Adsorption at Interfaces
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Adsorption at Interfaces
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
Adsorption at Interfaces K. L. Mittal, Editor
Papers from a symposium honoring
sponsored by the Division of Colloid and Surface Chemistry at the 167th Meeting of the American Chemical Society Los Angeles, Calif., April 2-5,
ACS
1974.
SYMPOSIUM
SERIES
AMERICAN CHEMICAL SOCIETY WASHINGTON, D. C. 1975
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
8
Library of Congress
Data
Adsorption at interfaces. (ACS symposium series; no. 8) Companion vol. to Colloidal dispersions and micellar behavior. Includes bibliographical references and index. 1. Adsorption—Congresses. I. Mittal, K. L., 1945- ed. II. American Chemical Society. Division of Colloids and Surface Chemistry. III. American Chemical Society. IV. Series: American Chemical Society. ACS symposium series; no. 8. QD547.A37 541'.3453 74-32040 ISBN 8412-0249-4 ACSMC8 8 1-290 ( 1975)
Copyright © 1975 American Chemical Society A l l Rights Reserved
PRINTED IN THE UNITED STATES OF AMERICA
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
ACS Symposium Series Robert F. Gould, Series Editor
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
FOREWORD The A C S S Y M P O S I U M SERIES was founded in 1974
to provide
a medium for publishing symposia quickly in book form. T h e format of the SERIES parallels that of its predecessor, A D V A N C E S IN C H E M I S T R Y SERIES, except that in order to save time the papers are not typeset but are reproduced as they are submitted by the authors in camera-ready form. As a further means of saving time, the papers are not edited or reviewed except by the symposium chairman, who becomes editor of the book.
Papers published in the A C S S Y M P O S I U M
SERIES
are original contributions not published elsewhere in whole or major part and include reports of research as well as reviews since symposia may embrace both types of presentation.
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
PREFACE "Professors R. D . Void and M . J. Void retired in June 1974 after more A
than 30 yrs at the University of Southern California. The annual
meeting of the American Chemical Society in April in near-by Los Angeles provided an ideal opportunity to pay tribute to these two outstanding workers in colloid science by holding a symposium in their honor. When the idea for this symposium was broached to me, I accepted it without hesitation as I such an event would elicit an enthusiastic response both from their former students and fellow colloid scientists.
The response exceeded all expecta-
tions as 57 papers covering a wide variety of interfacial and colloidal phenomena by 98 authors from 14 countries were included in the program, and two papers were considered later on.
Such overwhelming
response certainly testifies to their popularity as well as versatility in this research field. In fact, this symposium turned out to be the second largest one at the meeting. In consideration of the contents of this symposium, it might be more appropriate to name it "International Symposium on Interfacial and Colloid Phenomena honoring Professors R. D . and M . J. Void." This volume of 20 papers documents part of the proceedings of the symposium. T h e other part is contained in a companion volume of 24 papers, entitled "Colloidal Dispersions and Micellar Behavior." The papers in this volume deal with the adsorption of a variety of adsorbates on an array of substrates. Thermodynamics of adsorption, insoluble monolayers, adsorption at low energy solids, adsorption at colloidal particles, adsorption of polymers, and other aspects of adsorption are covered. Adsorption plays an important role in many technological, industrial, natural, and biological processes. Adsorption is truly an interdisciplinary field as is evidenced by the vast amount of literature being published from diverse laboratories.
Adsorption at interfaces ranges from the ad-
sorption of simple molecules ( for example, gases ) on bulk solid substrates to the adsorption of polymeric materials on colloidal particles.
Innumer-
able adsorption applications have produced a proliferation of literature on this topic. W i t h the availability of sophisticated instrumentation, a tremendous progress has been made in understanding the nature of the adsorbed species and the absorbate-adsorbent
interactions, but the sub-
ix In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
ject of adsorption is still pregnant with many challenging problems whose solutions will open new vistas in many basic and applied areas. Colloid chemistry, more generally colloid science, has been aptly described by the Voids in their book "Colloid Chemistry" (Reinhold, 1964)
as "the science of large molecules, small particles, and surfaces."
So obviously, adsorption is an integral part of the study of colloids, and many of their characteristics are attributable to the adsorption at colloidal particles. The Voids have been active in a spectrum of research activities.
He
made his research debut in the study of the solubility of organic compounds in aqueous and nonaqueous media while her first paper was on the mechanism of substitution reactions.
After this Robert worked in
industry where he became acutely aware of the inadequacies of the theories of simple system to study the intricacies of colloidal systems.
Subsequently, Marjorie be-
came very interested in studying colloids. They actively pursued the phase rule studies of association colloids ( soaps, greases, etc. ) in aqueous and nonaqueous media using surface chemical, electron microscopic, x-ray diffraction, and thermal analytical ( D T A ) techniques; adsorption at various interfaces; stability of colloidal dispersions; and calculation of van der Waals forces. More recently, their research interests have included understanding the factors influencing emulsion stability using ultracentrifuge; use of computer in floe formation and calculation of the dimensions of coiling type polymers; dispersions of carbon black; phase behavior of lithium stéarate greases; theories of colloidal stability in nonaqueous media; and the hydration of biopolymers like D N A . Obviously, the Voids' research activities have run the gamut from less glamorous colloidal systems like greases to the more fashionable biopolymers. Their work on the phase behavior and properties of nonaqueous soap systems had a significant impact in the petroleum industry (cf. N L G I Spokesman 18, 168
(1964)).
Their research
investigations
have culminated in 136 scientific and technical publications. They have also written the book mentioned above. This small paperback is an extremely good exposition of the principles and the methods of study of colloidal systems.
Owing to its popularity and utility it has been trans-
lated into Japanese. Apart from their research contributions, the Voids have rendered a great service to colloid science by popularizing it on a global basis, and they were very instrumental, along with Professors Adamson, Mysels, and Simha, in establishing an internationally acclaimed center for surface and colloid chemistry in the Chemistry Department at the University of Southern California.
Robert Void organized the Summer Conferences
χ In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
on Colloid Chemistry (1961-1964) at U . S . C . sponsored by the National Science Foundation. The Voids are very meticulous researchers and highly stimulating teachers.
I have found them very adept at inculcating good research
habits in the minds of neophytes in research. N o student can get away with sloppy record keeping.
Furthermore, they know how to bring out
the best in a student. Robert D . Void received his A . B . and M.S. degrees at the University of Nebraska in 1931 and 1932, respectively, and P h . D . degree from the University of California at Berkeley in 1935. During 1935-1937 he was a research chemist with Proctor and Gamble in Cincinnati. H e was a postdoctoral fellow with Professor J. W . McBain at Stanford University from 1937 to 1941.
Since 1941 he has been engaged in teaching and
research at the Universit a Fulbright Senior Research Fellowship with Professor Overbeek at the University of Utrecht, The Netherlands.
In 1955-1957 he served as Visit-
ing Professor of Physical Chemistry at the Indian Institute of Science, Bangalore, India, where he introduced new fields of research and helped to establish a P h . D . program.
In 1965 he served as a consultant for the
Summer Institute at Jadavpur University, Calcutta, India, which is designed to improve the teaching of chemistry in Indian colleges. H e was a member of the advisory board of the Journal of Colloid Science from its inception in 1946 until 1960. Among the various offices he has held in National Associations include Chairman, Southern California Section, A C S , 1967; Committee on Colloid and Surface Chemistry of the National Academy of Sciences, 1964-1967; Chairman,
California Association
of Chemistry
Teachers,
1961; National Colloid Symposium Committee, 1948-1953; and Chairman, Division of Colloid Chemistry, A C S , 1947-1948.
H e was awarded the
Tolman medal of the Southern California Section of the American Chemical Society for his research contributions and service to the profession in 1970. Marjorie J. Void received her B.S. in 1934 and P h . D . in 1936 from the University of California at Berkeley at the unusually young age of 23. She was University Medallist (Class Valedictorian)
at U . C . Berkeley.
After brief experience as a lecturer at the University of Cincinnati and the University of Southern California, she was a research chemist with the Union O i l Co. ( 1942-1946 ). Since then she has held various faculty appointments in the Department of Chemistry of the University of Southern California, with the title of Adjunct Professor for the last 14 yrs. She was awarded a Guggenheim Fellowship in 1953 which was taken at the University of Utrecht, T h e Netherlands.
From 1967 to 1970 she served
xi
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
as a member of the Advisory Board of the Journal of Colloid and Interface Science. She was awarded the Garvan Medal of the American Chemical Society in 1967. Among many other awards and honors include Los Angeles Times "Woman of the Year," 1966; National Lubricating Grease Institute Authors Award for the "Best Paper" presented at the Annual Meeting (in 1967), 1968; and she is listed among the 100 Outstanding Women of the U.S. by the Women's Home Companion 1969. Unfortunately, Mrs. V o i d was struck with multiple sclerosis in the fall of 1958 and has been confined to a wheel chair most of the time since 1960. In spite of her poor health and this terrible handicap, she has shown admirable stamina to deliver advanced colloid chemistry lectures continuously for two hours. O n November 19, 196 Binds" appeared in Los Angele glimpses of the scientific and social lives of the Voids. A special love for science has been maintained in the Void clan for almost a century. Individually speaking, the Voids are different in many respects. Mrs. Void has put it succinctly, "Robert has a sound almost conservative judgment while I tend to go off half-cocked. W e complement each other in our work and our lives." Although the Voids are retiring from active duty, they have no intention of relinquishing their interest in colloid science—a discipline they have cherished for about 40 yrs—as they plan to write a text book on the subject. M a y we join together on this occasion to wish them a very healthy and enjoyable retirement in San Diego. Acknowledgments: First, I am grateful to the Division of Colloid and Surface Chemistry for sponsoring this event. I am greatly indebted tc the management of the I B M Corp., both at San Jose and at Poughkeepsie for permitting me to organize this symposium and edit the volumes Special thanks are due to my manager, E . L . Joba, for his patience and understanding. The secretarial assistance of Carol Smith is gratefully acknowledged. Special thanks are also due to M . J. Dvorocsik and Elizabeth M . Ragnone for helping to prepare this volume for publication. The able guidance and ready and willing help of Paul Becher and K. J. Myseh is deeply appreciated. T h e reviewers should be thanked for their man) valuable comments on the manuscripts. I am thankful to my wife, Usha for helping with the correspondence, proofreading, and above all foi tolerating, without complaint, the frequent privations of an editors wife It would be remiss on my part if I failed to acknowledge the enthusiasn and cooperation of all the participants, especially the delegates fron overseas countries. Poughkeepsie, N.Y., November 19, 1974 K. L . M I T T A L :
xii
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
Adsorption at Interfaces
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
1 Thermodynamics of Adsorption and Interparticle Forces D. H . E V E R E T T School of Chemistry, University of Bristol, Bristol BS8 1TS, England C. J. R A D K E Chemical Engineering Department, Pennsylvania State University, University Park, Penn. 16802
Introduction An understanding of the p r o p e r t i e s of colloidal systems depends critically upon a knowledge of the factors which determine the forces between small particles in a fluid medium. Many systems of practical importance are d i s p e r s i o n s in aqueous e l e c t r o lyte s o l u t i o n s and it was n a t u r a l that the earlier t h e o r i e s of colloid stability were concerned w i t h the way in which e l e c t r o static forces, arising from ions in the s o l u t i o n and the electrical s t a t e of the s u r f a c e , combine w i t h d i s p e r s i o n forces between the particles to determine the thermodynamically s t a b l e (or metastable) s t a t e of the system. With the development of systems (often in non-aqueous media) stabilised by adsorbed molecules, a t t e n t i o n was d i r e c t e d towards the e f f e c t of adsorbed species on interparticle forces. A number of p o s s i b l e contributions to these e f f e c t s were recognised but in earlier treatments they were i n c l u d e d as separate a d d i t i v e c o n t r i b u t i o n s to the interaction energy. The phenomena a s s o c i a t e d w i t h adhesion between particles in powders have been looked upon as u n r e l a t e d problems and no q u a n t i t a t i v e study has been made of the e f f e c t of adsorbed gases on the i n t e r a c t i o n f o r c e s . The present work has sought to provide a u n i f y i n g thermodynamic approach to all these problems. Much of the earlier work on colloid stability was a l s o based on a thermodynamic approach and some of the equations of the present paper have p r e v i o u s l y been a p p l i e d to specific problems, though they were often d e r i v e d by more intuitive methods which l a c k both the r i g o u r and g e n e r a l i t y of a formal approach. There has, as far as we know, been no previous attempt to b r i n g all the above problems together in a s i n g l e general thermodynamic framework. It i s important to s t r e s s that colloidal phenomena are controlled both by thermodynamic and kinetic f a c t o r s and that s i n c e d i f f e r e n t p o s s i b l e processes ( e . g . , the approach of two particles via Brownian motion, and the establishment of adsorption equilibrium at the particle surfaces) may occur on w i d e l y d i f f e r e n t time s c a l e s , the observed phenomena may correspond, in d i f f e r e n t 1
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
2
ADSORPTION
AT
INTERFACES
circumstances, to t h e i r e v o l u t i o n along d i f f e r e n t paths. We l i m i t c o n s i d e r a t i o n here to the case i n which adsorption e q u i l i brium i s maintained d u r i n g the approach, encounter, and subsequent aggregation or s e p a r a t i o n of two p a r t i c l e s . For s i m p l i c i t y we deal w i t h the case of the i n t e r a c t i o n between p a r a l l e l p l a t e s . Rather than develop the theory i n i t s f u l l g e n e r a l i t y at once, i t i s more convenient to t r e a t f i r s t the case of s o l i d p a r t i c l e s i n a vapour phase: t h i s r e v e a l s the more important p r i n c i p l e s which are shown l a t e r to have q u i t e general a p p l i c a bility. We then consider i n t e r a c t i o n s i n n o n - e l e c t r o l y t e systems and f i n a l l y the e l e c t r o l y t e case f o r both i d e a l l y p o l a r i z a b l e and non-polarisable electrodes. In the l a t t e r case, we discuss two s p e c i f i c examples: extensions to other systems are r e a d i l y devised. Gas Adsorption on P a r a l l e The e f f e c t of gas a d s o r p t i o n on the f o r c e between i n t e r a c t i n g p l a t e s has been discussed i n an e a r l i e r paper(1) and we summarise the a n a l y s i s here. An a d s o r p t i v e gas and two p a r a l l e l p l a t e s are enclosed i n a p i s t o n - c y l i n d e r arrangement as shown i n Figure 1 and the p l a t e s are p o s i t i o n e d at an e q u i l i b r i u m separa t i o n by an e x t e r n a l f o r c e Af ( p o s i t i v e i f the p l a t e s r e p e l one another) which i s p r o p o r t i o n a l to the s u r f a c e area of each p l a t e and which i n an i n f i n i t e s i m a l movement dh c o n t r i b u t e s work -Afdh to the system. The d i f f e r e n t i a l Gibbs f r e e energy of the system, w r i t t e n r e l a t i v e to a reference system having the same temperature bulk pressure and volume as those i n F i g u r e 1 but c o n t a i n i n g no adsorbing p l a t e s , i s shown to be d(G - G*)
=
-(S - S^)dT
σ υ
+
ydn
a
+
2adA
-
Afdh
(1)
v
where η = η - n i s the Gibbs adsorption of the vapour at a chemical p o t e n t i a l of μ and σ i s the d i f f e r e n t i a l surface excess f r e e energy ( i n t e r f a c i a l t e n s i o n ) . I n t e g r a t i o n of Equation (1) at constant Τ, u, σ and h w i t h subsequent d i f f e r e n t i a t i o n and s u b t r a c t i o n from (1) leads to a modified Gibbs adsorption equation 1
-2da
=
fdh
1
+
2rdy
,
(constant T)
(2)
where Γ = n°/2A i s the adsorbate surface excess c o n c e n t r a t i o n per u n i t area of s o l i d p l a t e . I f E q u a t i o n (2) i s r e w r i t t e n i n the l i m i t of zero pressure, i n d i c a t e d by a s u p e r s c r i p t , and sub t r a c t e d from ( 2 ) , we o b t a i n
(3)
where Af i s the excess f o r c e over t h a t between the p l a t e s i n vacuum. Thus a change i n the s o l i d / g a s i n t e r f a c i a l t e n s i o n w i t h
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
1.
EVERETT
AND
Adsorption and
RADKE
Interparticle Forces
3
p l a t e s e p a r a t i o n c o n t r i b u t e s to the t o t a l f o r c e . Another more u s e f u l expression f o r Af emerges when the Maxwell r e l a t i o n between f and Γ from Equation (2) i s i n t e g r a t e d w i t h respect to the f u g a c i t y of the adsorbate p*, p*/ 9Γ Af = f - f° = 2RT d In p* (constant T,h) , (4) 9h T,P where the f u g a c i t y i s defined r e l a t i v e to the u n i t - f u g a c i t y i d e a l gas standard s t a t e . This equation presents the a l t e r n a t i v e but e q u i v a l e n t view that changes i n gas adsorption (at constant gas pressure) w i t h p l a t e s e p a r a t i o n i n f l u e n c e the t o t a l f o r c e . The p o t e n t i a l energy between the p l a t e s per u n i t p l a t e area and r e l a t i v e to zero at i n f i n i t e p l a t e s e p a r a t i o n i s defined as the i n t e g r a l of the f o r c e w i t h respect to h at constant Τ and p. A p p l i c a t i o n of t h i s d e f i n i t i o Equatio (3) give V
V
P ' P
=
f
(f
f
" °
)
d
h
=
2 [ ( σ
"
Q 0 )
h
"
( σ
-
a 0 )
«J
(constant Τ,ρ)
· ,
(5)
where ν i s the p o t e n t i a l energy of p l a t e i n t e r a c t i o n i n the zero gas pressure l i m i t and t h e r e f o r e a r i s e s only from the i n t e r molecular forces between the p l a t e s . This vacuum p o t e n t i a l energy may be w r i t t e n approximately as v^ = -k^/l2i\h?- where A^ i s Hamaker's constant f o r the p a r t i c u l a r s o l i d m a t e r i a l ( 2 ) , thus enabling the t o t a l p o t e n t i a l energy curve to be c a l c u l a t e d from knowledge of A^ and the dependence of the i n t e r f a c i a l t e n s i o n on h. A more convenient r e l a t i o n f o r Vp f o l l o w s by combining Equations (4) and (5) and changing the i n t e g r a t i o n order: ΓΡ* ν - v° = 2RT [Γ(«0 - r ( h ) ] d In p* (constant T,h) . (6) Ρ Ρ J 0
Thus the pressure dependence of - Γ(η)] (or the e f f e c t of h on the adsorption isotherm) determines the e f f e c t of the gas adsorbate on the p o t e n t i a l energy of i n t e r a c t i o n . Two opposing e f f e c t s a r i s e when the p l a t e s approach. First the i n t e r m o l e c u l a r p o t e n t i a l f i e l d s emanating from each p l a t e o v e r l a p , causing an increase i n the gas a d s o r p t i o n , and from Equations (4) and (6), i f t h i s increase occurs at a l l pressures an a t t r a c t i v e component to the t o t a l i n t e r a c t i o n ensues. Second, the d i m i n i s h i n g adsorption space decreases the gas adsorption and i f t h i s decrease occurs at a l l p r e s s u r e s , Equations (4) and (6) i n d i c a t e a r e p u l s i v e component to the t o t a l i n t e r a c t i o n . The p r i n c i p l e that an overlapping of force f i e l d s f u r n i s h e s an a t t r a c t i v e increment to the t o t a l p o t e n t i a l energy whereas a d i m i n i s h i n g space f u r n i s h e s r e p u l s i v e increment i s not s p e c i f i c to gas adsorption but, as w i l l be seen l a t e r , can be expressed i n more general terms. These e f f e c t s may be i l l u s t r a t e d q u a n t i t a t i v e l y by con s i d e r i n g the adsorption of an i d e a l gas i n the low pressure or Henry's law r e g i o n ( 1 ) . In t h i s model the gas i s d i s t r i b u t e d i n
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
4
ADSORPTION
AT
INTERFACES
the i n t e r m o l e c u l a r f o r c e - f i e l d between the p l a t e s , presumed to be a d d i t i v e i n the i n d i v i d u a l p l a t e p o t e n t i a l s , according to a Boltzmann c o n c e n t r a t i o n p r o f i l e ( 3 ) and the p l a t e s e p a r a t i o n dependence of Γ i s determined by i n t e g r a t i n g the excess concentra t i o n p r o f i l e over the a v a i l a b l e a d s o r p t i o n space. A p p l i c a t i o n of Equation (6) then permits e v a l u a t i o n of Vp - v . The r e s u l t s of these c a l c u l a t i o n s , using both (10:4) (A) and (9:3) (B) a d s o r p t i o n p o t e n t i a l s , are reviewed i n F i g u r e 2 (compare F i g u r e 5, reference 1) i n which the t o t a l p o t e n t i a l energy curve v i s shown as a f u n c t i o n of h. To estimate the vacuum d i s p e r s i o n - f o r c e p o t e n t i a l Vp ( s o l i d l i n e C) a Hamaker constant of 1 0 " ^ j chosen, corresponding approximately to the experimental value f o r mica (4,5). The dashed curve represents s c h e m a t i c a l l y the short range r e p u l s i o n when the p l a t e s get very c l o s e together. Curves A and Β i n the 0.8-0.9 nm (8-9 X) range i n d i c a t e the e f f e c t of the a d s o r p t i v e gas; they correspon T o r r ) , to a c o l l i s i o n diamete p l a t e a d s o r p t i o n w e l l depth of 8 kT (approximately 22 k J mol"" at room temperature). Although no s t a b i l i s i n g b a r r i e r i s present i n Figure 2, a deep secondary minimum appears even at modest a d s o r p t i o n e n e r g i e s , suggesting t h a t the presence of an a d s o r p t i v e low pressure gas might cause loose f l o c c u l a t i o n of s o l i d a e r o s o l s . At higher pressure, where m u l t i l a y e r a d s o r p t i o n commences, the simple i d e a l gas model i s i n v a l i d and no q u a n t i t a t i v e theory i s p r e s e n t l y a v a i l a b l e to p r e d i c t the v a r i a t i o n of gas a d s o r p t i o n with plate separation. N e v e r t h e l e s s , q u a l i t a t i v e arguments suggest that a r e p u l s i v e f o r c e c o n t r i b u t i o n might occur at a l l separations i n t h i s pressure r e g i o n ( l ) . Adsorbate molecules i n the l a y e r s f u r t h e s t away from the s o l i d surface are not s t r o n g l y i n f l u e n c e d by the s o l i d p l a t e p o t e n t i a l f i e l d and w i l l i n t e r a c t w i t h the outermost molecules adsorbed on the approaching second p l a t e before the p l a t e p o t e n t i a l f i e l d s overlap s i g n i f i c a n t l y . This r e d u c t i o n of a v a i l a b l e adsorption space w i l l decrease at a l l separations and hence lead to a r e p u l s i v e c o n t r i b u t i o n to the t o t a l p o t e n t i a l energy. The p o s s i b i l i t y t h e r e f o r e a r i s e s that at higher gas pressures a p o t e n t i a l b a r r i e r may e x i s t to prevent adhesion of the s o l i d p l a t e s . At pressures between the Henry and m u l t i l a y e r regions intermediate behaviour should be expected (e.g., see F i g u r e 9 of Réf. 1 ) . p
p
w a s
1
N o n - e l e c t r o l y t e Mixture Adsorption on P a r a l l e l P l a t e s The a n a l y s i s of the preceding s e c t i o n i s r e a d i l y extended to the case of a c-component n o n - e l e c t r o l y t e l i q u i d mixture confined between p a r a l l e l p l a t e s ( 1 ) . We review the e s s e n t i a l features by w r i t i n g Equation (2) f o r a multicomponent system and by s u b s t i t u t ing the i s o t h e r m a l and i s o b a r i c Gibbs-Duhem equation of the bulk l i q u i d l e a d i n g to i=c -2da = fdh + 2 £ Γ. dy. (constant Τ,ρ) , (7) i=2 1 , 1
1
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
EVERETT
AND RADKE
Adsorption and Interparticle Forces
Ρ
T,P,V
-L
. Af
Figure 1. Cylinder of volume V containing an amount η of gas at Τ,ρ; and two flat plates each of area A, separated by distance h, held in equilibrium by a force Af
Figure 2. Calculated potential energy curve for two plates in an adsorptive ideal gas A, for 10:4 potential; Β for 9:3 potential at a pressure of 20 Nm' (0.15 Torr) for single plate well depth of 8 kT, and collision diameter 0.34 nm (3.4 A); C van der Waals attraction (Hamaker constant 10' J); dotted line short range repulsion (schematic)
2
19
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
6
ADSORPTION
AT
INTERFACES
c. where Γ. , - Γ. i s the r e l a t i v e adsorption of species i 1,1 ι l w i t h respect to an a r b i t r a r y component 1. Cross d i f f e r e n t i a t i o n of Equation (7) followed by i n t e g r a t i o n w i t h respect to y. y i e l d s a r e l a t i o n analogous to Equation (4) c
1
1
-
f c-1 —00
where
I
dy
3h J
(constant Τ,ρ,η,μ^.
i
i s the f o r c e i n the c-component mixture and f
χ
)
(8) ^ i s the
force i n the l i q u i d mixture from which component i has been eliminated. By a p p l y i n g Equation (8) (c-1) times to e l i m i n a t e s e q u e n t i a l l y the mixtur force e x p r e s s i o n s , we o b t a i far
i=c
-
2
I
3h
i=2
dy. T, ,u P
(constant T,p,h),(9)
ijtl
where f ^ i s the f o r c e between p l a t e s immersed i n the pure l i q u i d of species 1. Equation (9) again r e v e a l s that the i n t e r a c t i o n force i n a l i q u i d mixture i s i n f l u e n c e d by the d i s t a n c e depend ence of the a d s o r p t i o n isotherm and contains e i t h e r a t t r a c t i v e terms when ^ increases w i t h decrease i n h or r e p u l s i v e terms when ?i9l decreases w i t h decrease i n h. F i n a l l y , the p o t e n t i a l energy o f i n t e r a c t i o n i n a none l e c t r o l y t e s o l u t i o n r e l a t i v e t o the p o t e n t i a l energy i n pure l i q u i d 1 i s , c f . Equation ( 6 ) , i=c
v (c) p
-
v (l) p
2
I
Κι™
T
~ i , i ^ i
i=2
(constant T,p,h) , (10) where [Γ£ ι(°°) i ( h ) ] i s the change i n the r e l a t i v e a d s o r p t i o n of component i when the two p l a t e s approach at constant Τ,ρ and y£. This r e l a t i o n suggests a means f o r c a l c u l a t i n g the e f f e c t of s o l u t i o n composition on the i n t e r a c t i o n o f p a r t i c l e s i n a l i q u i d medium and, t h e r e f o r e , may provide a path to estimate the v a r i a t i o n of the e f f e c t i v e Hamaker constant w i t h composition. I f we n e g l e c t the e f f e c t of pressure on l i q u i d phase p r o p e r t i e s , [ v p ( l ) - Vp] i s given by Equation (6) when the upper l i m i t o f that i n t e g r a l i s the s a t u r a t i o n f u g a c i t y of component 1. Thus a d d i t i o n o f t h i s l i m i t i n g form o f Equation (6) to (10) gives the p o t e n t i a l energy i n a l i q u i d mixture v ( c ) r e l a t i v e to t h a t i n vacuum v . Q u a n t i t a t i v e a p p l i c a t i o n o f these ideas must await f u r t h e r development i n our understanding of dense gas and l i q u i d mixture a d s o r p t i o n . p
p
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
1.
EVERETT
AND
RADKE
Adsorvtion and
Interparticle Forces
7
E l e c t r o l y t e Adsorption on I d e a l l y P o l a r i z e d P l a t e s E a r l y attempts to e x p l a i n the e f f e c t of e l e c t r o l y t e s on the s t a b i l i t y of lyophobic c o l l o i d s culminated i n the coherent p i c t u r e of the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory(6^7)· Extension of the present f o r m u l a t i o n to t h i s problem i s s t r a i g h t forward (8) and provides r i g o r o u s expressions from which, on the b a s i s of a simple model, the DLVO theory i s recovered. Since some of the e a r l i e r work, notably t h a t of 0verbeek(9), was a l s o based on a thermodynamic a n a l y s i s , s e v e r a l of the expressions derived below are c l o s e l y s i m i l a r to p r e v i o u s l y known equations. The present work, however, r e v e a l s some i m p l i c i t assumptions i n e a r l i e r d i s c u s s i o n s , provides a b a s i s f o r f u r t h e r developments, and completes the general account of the forces between c o l l o i d a l p a r t i c l e s immersed e i t h e r i n e l e c t r o l y t e or n o n - e l e c t r o l y t e solutions. Since p o l a r i z e d i f f e r e n t l y , we d i s c u s In the f o l l o w i n g , only an o u t l i n e of theory i s given: a d e t a i l e d account w i l l be p u b l i s h e d s e p a r a t e l y ( 8 ) . By analogy w i t h the s e c t i o n on gas adsorption on p a r a l l e l p l a t e s , two i d e a l l y p o l a r i z a b l e p a r a l l e l p l a t e e l e c t r o d e s are immersed i n an e l e c t r o l y t e s o l u t i o n and enclosed i n a p i s t o n - c y l i n d e r arrangement (Figure 3). The e l e c t r o d e s are h e l d at a s e p a r a t i o n h by the force A f , and are connected to an i n t e r n a l reference e l e c t r o d e , w i t h respect to which the p l a t e s can be h e l d at a p o t e n t i a l E. For convenience we d i s c u s s the p a r t i c u l a r case of a system of platinum e l e c t r o d e s , a d i l u t e aqueous KC1 e l e c t r o l y t e s o l u t i o n , and, s i n c e i t i s r e v e r s i b l e to the e l e c t r o l y t e anion, an i n t e r n a l calomel reference e l e c t r o d e . An e x t e r n a l calomel reference • · Ο e l e c t r o d e c o n t a i n i n g KC1 at a standard c o n c e n t r a t i o n c i s a l s o included. P o t e n t i a l s measured r e l a t i v e to t h i s are denoted by Ε . To describe the system, platinum ions and e l e c t r o n s are chosen a r b i t r a r i l y as the components of the e l e c t r o d e phases, and potassium i o n s , c h l o r i d e ions and water molecules as the com ponents of the aqueous phase. Equation (7) may be w r i t t e n i n terms of these components and combined (a) w i t h the bulk e l e c t r o d e and s o l u t i o n Gibbs-Duhem equations, (b) w i t h the i n t e r f a c i a l e l e c t r o - n e u t r a l i t y c o n d i t i o n , Σζ^Γ^ = 0 and (c) w i t h the d i s s o c i a t i o n e q u i l i b r i u m c o n d i t i o n s f o r KC1 and P t ( 1 0 , l l ) . This then y i e l d s the f o l l o w i n g form of the Gibbs adsorption isotherm: β
-2da
=
fdh
where
^
+
2qdE
+
2Γ + Κ
R
Q
du^
(constant Τ,ρ)
,
(11)
i s the r e l a t i v e adsorption of potassium ions w i t h
Q
respect to water at the p l a t i n u m / s o l u t i o n i n t e r f a c e , q i s the i n t e r f a c i a l charge per u n i t area defined by q
=
F(r
p
t
+
-r _) e
=
F(r
c
l
--r
K
+
)
,
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
(12)
8
ADSORPTION
AT
INTERFACES
and Ε i s the c e l l p o t e n t i a l given by fe
=
y . c l
-
y_ e
-
^
-
J y ^ ^
.
(13)
In Equation (13) F i s the Faraday constant and which depends on the e l e c t r i c a l s t a t e of the phase i n which i r e s i d e s ( 1 2 ) , i s the e l e c t r o c h e m i c a l p o t e n t i a l of charged species i ; f o r n e u t r a l components = P£, the o r d i n a r y chemical p o t e n t i a l . When the two p l a t e s are i n f i n i t e l y separated, Equation (11) reduces to the c l a s s i c a l Lippmann equation f o r a p o l a r i z a b l e i n t e r f a c e ( 1 1 ) . By arguments s i m i l a r to those used p r e v i o u s l y , Equation (11) leads to s e v e r a l a l t e r n a t i v e expressions f o r the i n t e r a c t i o n force. I f Equation (11) i s w r i t t e n at the p o i n t (or p o t e n t i a l ) of zero charge (pzc) and, subtracted from Equation ( 1 1 ) , we o b t a i n 3(σ - σ )η f(E) - f f t ) = pzc Sir ·Ρ· · κοι μ
which i s analogous to Equation (3) except that here f at a c e l l p o t e n t i a l Ε i s r e l a t i v e to that i n a s o l u t i o n of the same com p o s i t i o n but at the p o t e n t i a l of zero charge. Since the e l e c t r o l y t e c o n c e n t r a t i o n i s constant Ε - E = Ε - E . Three other force expressions are a v a i l a b l e from the Maxwell r e l a t i o n s of Equation ( 1 1 ) * . The c r o s s - d i f f e r e n t i a l between f and q upon i n t e g r a t i o n over the celjj, p o t e n t i a l y i e l d s p z c
f(E)
-
ftt > pzc
p z c
Ε pzc (constant T , p , y
KC1
,h)
.
(15)
V a r i a t i o n of s u r f a c e charge d e n s i t y w i t h p l a t e s e p a r a t i o n at con stant c e l l p o t e n t i a l and s o l u t i o n composition w i l l a f f e c t the force. Furthermore, the c r o s s - d i f f e r e n t i a l between f and Γ, on i n t e g r a t i o n w i t h respect to the chemical p o t e n t i a l of potassium c h l o r i d e , gives the force i n a s o l u t i o n having a chemical p o t e n t i a l P K C 1 > r e l a t i v e to that i n a standard reference s o l u t i o n at the same i n t e r n a l c e l l p o t e n t i a l . When the c o n c e n t r a t i o n of potassium c h l o r i d e i s changed at constant E, the p o t e n t i a l of the i n t e r n a l e l e c t r o d e , and hence that of the p l a t e s , changes r e l a t i v e to the e x t e r n a l e l e c t r o d e . An i n c r e a s e i n the c o n c e n t r a t i o n of potassium c h l o r i d e c decreases the p o t e n t i a l of the p l a t e s and by a s u i t a b l e choice of c the p l a t e s can be brought to the p o t e n t i a l o£ zero charge (pzc) which, r e l a t i v e to the e x t e r n a l e l e c t r o d e i s Epzc* We choose t h i s c o n c e n t r a t i o n C p as reference concentra t i o n , whence Z C
* There are e i g h t p o s s i b l e Maxwell equations i n v o l v i n g f from Equation ( 1 1 ) . We c i t e only three here.
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
1.
EVERETT
AND RADKE
9
Adsorption and Interparticle Forces
Γ pzc
I
Κ +
· 2°Ί 3h J Η
T
E
'P' '\ci
(constant T,p,E,h) (16) DZC where Pj^Cl * 4cCl n e g a t i v e l y charged p l a t e s and i s greater f o r p o s i t i v e l y charged p l a t e s . A t h i r d Maxwell r e l a t i o n r e s u l t s i n d i r e c t l y from Equation (11) by using the i d e n t i t y 3E 3h [SE - - 1 s l e s s
t n a n
f
o
r
(15)
followed by s u b s t i t u t i o n i n the d i f f e r e n t i a l form o f Equation and i n t e g r a t i o n w i t h respec f(q)
-
f(0) =
i3E
l
dq
9h
Γ
(constant Τ , ρ , ν ^ , η )
(17)
,
where the lower l i m i t of i n t e g r a t i o n i s again taken as the p z c . These f o r c e expressions may be i n t e g r a t e d according t o Equation (5) to o b t a i n the p o t e n t i a l energy o f i n t e r a c t i o n r e l a t i v e t o that when the p l a t e s are at the pzc. Thus Equation (14) y i e l d s 2[(σ
σ ) pzc h
- (σ
σ ) 1 pZC (18) ι
(constant Τ,ρ,h)
.
Again the v a r i a t i o n o f the i n t e r f a c i a l t e n s i o n d i f f e r e n c e (σ - CTp ) w i t h p l a t e s e p a r a t i o n determines the p o t e n t i a l energy of i n t e r a c t i o n ( c f . Equation (5)). The d i f f e r e n c e (σ^ -σ^) i s c o n v e n t i o n a l l y c a l l e d the f r e e energy' o f the charged i n t e r a c t i n g p l a t e s 07,9). Equation (15) with i n v e r s i o n o f the order o f i n t e g r a t i o n gives ZC
f
[q(«0
" q(h)] dE
~pzc (constant T,p,h)
,
(19)
where i t i s assumed that E p i s independent of h. An equation s i m i l a r to Equation (19) has been d e r i v e d e a r l i e r both by an i n t u i t i v e argument and by a l e s s e x p l i c i t thermodynamic treatment (7). From Equation (16) we o b t a i n f o r n e g a t i v e l y charged p l a t e s Z C
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
10
ADSORPTION
A T INTERFACES
^KCl V
E
' W
E
-V
O
-
2
tV,H 0 o
( h
(
d
> " V,H 0 ^ ^Cl 9
Z C
U? TCCl (constant T,p,h)
.
(20)
The general p r i n c i p l e enunciated i n the e a r l i e r s e c t i o n on gas adsorption on p a r a l l e l p l a t e s again a p p l i e s to the approach o f charged p l a t e s a t constant i n t e r n a l c e l l p o t e n t i a l and constant bulk c o n c e n t r a t i o n of potassium c h l o r i d e , provided a t t e n t i o n i s focussed on the p r e f e r e n t i a l l y adsorbed i o n ( c o u n t e r - i o n ) . Thus f o r n e g a t i v e l y charged p l a t e s Γ + i s positive. As the p l a t e s approach the d i f f u s e p a r t f th doubl l a y e r begi t overla and s i n c e Ε i s kept constant between the p l a t e s ; Γ (°°) (h) plate κ. >"2 experience a r e p u l s i v e i n t e r a c t i o n . Conversely, f o r p o s i t i v e l y charged p l a t e s Γ + i s negative and increases w i t h decrease i n h, w h i l e Γ -
i s p o s i t i v e and decreases as the p l a t e s approach. fti^J
LI
Thus, i f the adsorption of the counter-ion decreases as the p l a t e s approach, adsorption e f f e c t s w i l l make a r e p u l s i v e c o n t r i b u t i o n t o the p o t e n t i a l o f i n t e r a c t i o n . Superimposed upon the p u r e l y e l e c t r o s t a t i c i n t e r a c t i o n s w i l l be the i n t e r m o l e c u l a r i n t e r a c t i o n s of the ions and the s o l v e n t which are u s u a l l y neglected as a second-order e f f e c t . F i n a l l y , i n t e g r a t i o n of Equation (17) gives V
p
(
q
' W
"
ν
ρ
( 0
'\α
}
=
2 |^[(E(h) - E C ) ] dq (constant Τ,ρ,η)
,
(21)
where v ( 0 ) i s again the p o t e n t i a l energy a t the pzc. For ideally p o l a r i z a b l e p l a t e s , because no charge can be t r a n s f e r r e d across the i n t e r f a c e , i t i s p o s s i b l e to b r i n g the p l a t e s together a t con s t a n t charge, and a t the same time to maintain an e q u i l i b r i u m i o n i c distribution. The d i s t a n c e dependence o f the changing c e l l p o t e n t i a l a t constant charge now determines the p o t e n t i a l energy of i n t e r a c t i o n . A q u a n t i t a t i v e i l l u s t r a t i o n of the thermodynamic f o r m u l a t i o n i s provided by the simple model o f p o i n t charges ( K and CI f o r convenience) d i s t r i b u t e d i n a uniform l i q u i d d i e l e c t r i c confined between two conducting p a r a l l e l p l a t e s . Since the p o i n t charges i n t e r a c t only w i t h the f i e l d s emanating from the charged p l a t e s , the e l e c t r o c h e m i c a l p o t e n t i a l o f an i o n i c species may be r i g o r o u s l y separated i n t o a p u r e l y chemical and a p u r e l y e l e c t r o s t a t i c part(12), p
+
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
1.
EVERETT
\
=
μ
AND
+
£
Adsorption and
RADKE
z.F
Interparticle Forces
11
,
(22)
where z. i s the i o n charge and φ i s the l o c a l e l e c t r o s t a t i c potential. This s e p a r a t i o n leads to a Boltzmann c o n c e n t r a t i o n d i s t r i b u t i o n and upon combination w i t h the Poisson equation from c l a s s i c a l e l e c t r o s t a t i c s provides a d i f f e r e n t i a l equation to describe the s p a t i a l v a r i a t i o n of the l o c a l e l e c t r o s t a t i c p o t e n t i a l between the i n t e r a c t i n g p l a t e s ( 1 3 ) . For small e l e c t r o s t a t i c p o t e n t i a l s the Poisson-Boltzmann equation i s r e a d i l y i n t e grated and y i e l d s two expressions f o r the r e l a t i o n between the surface charge d e n s i t y q and the surface e l e c t r o s t a t i c p o t e n t i a l Φ depending on the boundary c o n d i t i o n imposed at the p l a t e surface. F i r s t , i f the surface p o t e n t i a l i s h e l d constant then the reduced surface charge d e n s i t y becomes ο
where c i s the bulk molar c o n c e n t r a t i o n of potassium c h l o r i d e , λ i s the Debye length defined f o r a 1:1 e l e c t r o l y t e i n a l i q u i d solvent of uniform d i e l e c t r i c constant, ξ = Ρφ /RT i s a reduced surface p o t e n t i a l and h = h/λ i s the reduced d i s t a n c e between the charged plates. SeconSly, i f the surface charge d e n s i t y remains constant then the surface p o t e n t i a l must vary according to q
W
r ΊΓ
=
C 0 t h
(
V
(constant q)
.
(24)
F i n a l l y , the reduced surface excess c o n c e n t r a t i o n (Γ ) of potassium ions i s Γ
+
Κ ,Η 0
A,
2
F
=
r
TZ
=
ξ
- ο
t a n h
(25)
I "
The reduced surface excess c o n c e n t r a t i o n of c h l o r i d e ions i s equal and opposite. Equations (22) to (25) supply the r e q u i r e d r e s u l t s from the point-charge model. S u b s t i t u t i o n of Equations (23) and (22), w r i t t e n once f o r c h l o r i d e ions and once f o r e l e c t r o n s , and Equation (12) i n t o Equation (19) y i e l d s -
y^w
- y pzc>w E
r
-
°
2XcRT =
ζ [ΐ 2 ο
[q l ) and ν ( f ^ ^ ) are e q u a l , since the P t e l e c t r o d e s ρ pzc ρ Q both have the same p o t e n t i a l E , r e l a t i v e to the e x t e r n a l standard reference e l e c t r o d e wnen the pzc i s independent o f c. As shown i n Figure 4 the p l a t e s experience a r e p u l s i o n a t a l l distances. I t may be noted that whereas i n the case o f gas a d s o r p t i o n , r e p u l s i o n force i n the case o f e l e c t r o l y t e case, from e l e c t r o s t a t i c f o r c e s : i o n s i z e e f f e c t s may a l s o appear as a second c o n t r i b u t i o n . A p p l i c a t i o n of the point-charge model to Equation (21) pro vides a constant-charge p o t e n t i a l energy of i n t e r a c t i o n which, because of the d i f f e r e n t i n t e g r a t i o n path, i s d i s t i n c t from Equation (26). From Equations (13), (21), (22) and (23) we obtain Z C
p Z
0
i
z c
E
K C 1
p z c
y^w - y^w 2 cRT
^o
( h
r
9
" 5
)
f
= ξ^(-)[ ο^ 0
o
( 0 O )
^
d q
1 J q -i] Λ h
0
(constant T,p,h)
.
(27)
This r e s u l t i s that of the DLVO theory a p p l i e d to c o l l o i d a l p a r t i c l e s which c o l l i d e w i t h a constant surface charge(14). To maintain constant charge when two charged polarïzable p l a t e s approach, the e f f e c t of o v e r l a p p i n g of the d i f f u s e p a r t s of the double l a y e r s which tends to decrease the adsorption of the counter-ions has to be o f f - s e t by an increase i n the p o t e n t i a l . Thus, from Equation (27), a r e p u l s i v e p o t e n t i a l energy i s generated. We note (Figure 4) that the r e p u l s i v e e f f e c t i s more pronounced f o r the constant charge i n t e g r a t i o n path and that a t small separations the constant-charge p o t e n t i a l energy approaches infinity. Since the present l i n e a r i s e d point-charge model equations apply only f o r s m a l l values of the surface p o t e n t i a l , Equation (27) must break down f o r s e p a r a t i o n distances near zero. E l e c t r o l y t e Adsorption on I d e a l l y Non-polarizable P l a t e s To discuss the i n t e r a c t i o n between r e v e r s i b l e ( i . e . , i d e a l l y n o n - p o l a r i z a b l e ) p a r t i c l e s the platinum e l e c t r o d e s i n F i g u r e 3 are replaced by s i l v e r / s i l v e r c h l o r i d e e l e c t r o d e s . Since s i l v e r and
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
EVERETT
A N D RADKE
Adsorption and Interparticle Forces
Ρ
A -
A f
c
H 02
KCI(c) Hg Ci 2
r
1] +0 Ε ~?
KCKc^K 5-
metric aqueous electrolyte (potassium chloride) and two plates charged to a potential Ε relative to an internal ref erence calomel electrode and to a potential F? relative to an external standard calomel electrode
Figure 4. Calculated effect of a dilute point-ion electrolyte on the potential energy for two charged plates from Equations 26 and 27
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
14
ADSORPTION
AT
INTERFACES
c h l o r i d e ions are present i n the aqueous s o l u t i o n to an extent determined by the s o l u b i l i t y product o f s i l v e r c h l o r i d e K^gCl they both serve as p o t e n t i a l determining ions f o r the s i l v e r c h l o r i d e electrodes. F o l l o w i n g the e a r l i e r procedure, the modified Gibbs a d s o r p t i o n equation f o r the r e v e r s i b l e e l e c t r o d e system i s -2da
=
fdh +
2Γ +
άμ^
χ
(constant Τ,ρ)
.
(28)
Comparison o f Equation (28) w i t h Equation (11) d i s c l o s e s the absence of the c e l l p o t e n t i a l as an independent v a r i a b l e . For r e v e r s i b l e e l e c t r o d e s a t constant temperature and pressure the c e l l p o t e n t i a l cannot be v a r i e d s i n c e Ε i s determined by the chemical p o t e n t i a l s of pure phases o n l y : F E
-
%
+
% c i -
Because the i n t e r n a l c e l l p o t e n t i a l i s no longer an independent v a r i a b l e , n o n - p o l a r i z a b l e e l e c t r o d e s cannot approach r e v e r s i b l y under the c o n d i t i o n of constant charge. By r e p e t i t i o n of the thermodynamic development given i n the preceding s e c t i o n , the p o t e n t i a l energy o f i n t e r a c t i o n between n o n - p o l a r i z a b l e e l e c t r o d e s becomes pzc ^kci = 2 [V,H 9 0 ( h > " V,H 9 0 ( T O >J%:ci
y w - v©
y
KCl (constant T,p,h)
,
(30)
which i s analogous to Equation (20) and again i n d i c a t e s the r o l e of e l e c t r o l y t e a d s o r p t i o n i n determining the e l e c t r o s t a t i c p a r t o f the t o t a l p o t e n t i a l energy curve f o r i n t e r a c t i n g r e v e r s i b l e electrodes. A p p l i c a t i o n of the point-charge model to Equation (30) y i e l d s a r e s u l t s i m i l a r to t h a t of the constant p o t e n t i a l DLVO theory. S u b s t i t u t i o n of the e l e c t r o c h e m i c a l p o t e n t i a l e q u a l i t y c o n d i t i o n of the c h l o r i d e (or s i l v e r ) ions i n the aqueous and s o l i d s i l v e r c h l o r i d e phase, Equations (22), ( 2 5 ) , and the r e l a t i o n t h a t 2
dy
R C 1
=
RT d I n
( 1
τ
+
Ζ
1
+
/ 4 K AgCl] — 2 - j r
J
g i v e s , as a f i r s t order approximation o f a s e r i e s expansion,
-κ
jxzxt
ξ
=
0
& -
t a n h
[tJJ I·
1
+
- V J
·
( 3 1 )
We n o t i c e , however, a c o r r e c t i o n term t o the constant p o t e n t i a l DLVO theory i n v o l v i n g the s o l u b i l i t y product of the r e v e r s i b l e electrodes. For substances such as s i l v e r c h l o r i d e ( s o l u b i l i t y product M O " ( m o l d m " ) the c o r r e c t i o n term i s unimportant f o r added K C l concentrations of > 10"^ mol dm" . The c o r r e c t i o n 1 0
3
2
3
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
1.
EVERETT A N D RADKE
15
Adsorption and Interparticle Forces
f a c t o r w i l l not be n e g l i g i b l e f o r more s o l u b l e substances, e.g., MgC0 , K = 2.6 χ 10~-> when c i s l e s s than 5 χ 10"" mol dm"". The present thermodynamic treatment thus provides a means f o r extending the DLVO theory to i n c l u d e i n t e r a c t i o n s between more s o l u b l e dispersed s o l i d s . 2
3
3
s
Conclusions A general thermodynamic treatment i s presented t o d e s c r i b e the e f f e c t of adsorption o f gases and mixtures o f both e l e c t r o l y t e s and n o n - e l e c t r o l y t e s on the i n t e r a c t i o n o f s o l i d p a r t i c l e s . The f o r m u l a t i o n provides a coherent p i c t u r e o f the e f f e c t o f a d s o r p t i o n i n terms o f the opposing tendencies o f the overlap o f the i n t e r p a r t i c l e f o r c e f i e l d s ( i n t e r m o l e c u l a r o r e l e c t r o s t a t i c ) and o f the a v a i l a b i l i t y o f adsorption space. The two i d e a l i s e d cases o f p o l a r i z a b l e an to d e s c r i b e the i n t e r a c t i o medium. A q u a n t i t a t i v e point-charge model a p p l i e d t o both types of i n t e r a c t i n g p a r t i c l e s y i e l d s , under d i f f e r e n t c o n d i t i o n s , the constant p o t e n t i a l and constant charge Derjaguin-Landau-VerweyOverbeek t h e o r i e s . Extension o f the DLVO theory t o the case o f less sparingly soluble solids i s outlined.
Literature Cited 1. Ash, S.G., Everett, D.H. and Radke, C.J., J.Chem.Soc.Faraday Trans.II, (1973), 69, 1256 2. Hamaker, H.C., Physica (l937), 4, 1058 3. Barker, J.A. and Everett, D.H., Trans.Faraday Soc., (1962), 58, 1608 4. Tabor, D. and Winterton, R.H.S., Proc.Roy.Soc. (1969), 312A, 435 5. I s r a e l a c h v i l i , J.N. and Tabor, D., Proc.Roy.Soc., (1972), 331A, 19 6. Derjaguin, B.V. and Landau, L., Acta Physicochem.USSR (1941) 14, 633 7. Verwey, E.J. and Overbeek, J.Th.G., "Theory of the S t a b i l i t y of Lyophobic Colloids", Elsevier, Amsterdam, 1948 8. Everett, D.H. and Radke, C.J., i n preparation 9. Overbeek, J.Th.G., in "Colloid Science", H.R.Kruyt, Ed., Vol. I , Chap. 4,6, Elsevier, Amsterdam, 1952 10. Parsons, R., "Thermodynamics of E l e c t r i f i e d Interfaces", i n 'Source Book of Colloid and Surface Chemistry', H. van Olphen, Ed., to be published 11. Newman, J . , "Electrochemical Systems", Prentice-Hall, Englewood Cliffs, New Jersey, 1973 12. Guggenheim,E.A.,"Thermodynamics",5th ed., North-Holland, Amsterdam, 1967 13. Parsons, R., in "Modern Aspects of Electrochemistry", J.O'M. Bockris,Ed.,Vol. 1, Chap. 3, Academic Press, New York, 1954 14. Usui, S., J. Colloid InterfaceSci.,(1973), 44, 107
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
2 Configurational Behavior of Isolated Chain Molecules Adsorbed from Athermal Solutions M.
LAL
and M . A. T U R P I N
Unilever Research, Port Sunlight, Cheshire L62
4XN,
England
K. A. R I C H A R D S O N Department of Polymer and Fibre Science, University of Manchester, Institute of Science and Technology, Manchester, England D. S P E N C E R Unilever Computing Services Ltd., Bromborough Port, Cheshire, England
Models which have been considered i n the various a n a l y t i c a l treatments of s t a t i s t i c a l thermodynamic and conf i g u r a t i o n a l behaviour of adsorbed polymer layers most often ignore such important c h a r a c t e r i s t i c s of the adsorbate molecules as the excluded-volume e f f e c t and bond r o t a t i o n a l hindrance (1,2,3,4,5). Further, the procedures used for introducing solvent and concentration e f f e c t s are less s a t i s f a c t o r y (6). The present a n a l y t i c a l developments are thus limited to oversimplified models that would have only a little correspondence with r e a l systems. It i s , therefore, desirable to investigate alternative approaches which would be e f f e c t i v e i n the study of more r e a l i s t i c models. The success of Monte Carlo computer simulation method i n investigating the configurational and thermodynamic behaviour of systems involving polymer molecules assuming models of varying complexity i s well established (7,8,9). This approach i s based on the assumption that it i s possible to generate a random sample of molecular configurations which would adequately simulate the canonical ensemble corresponding to the model assumed. Then the mean values of various properties over such a sample would converge to the canonical ensemble averages. The extension of the Monte Carlo method to polymers i n t e r a c t i n g with interfaces has so f a r been c a r r i e d out only to a limited extent (10,11). This i s because of a serious d i f f i c u l t y pertaining to sample convergence that one would encounter i n the case of strongly i n t e r a c t i n g systems, i f the usual techniques are used. The first objective underl y i n g the present work i s to introduce a more sophisticated Monte Carlo scheme i n t h i s area with the aim of overcoming the d i f f i c u l t y just mentioned. The successful application of such a scheme should lead to r e l i a b l e studies on the models which adequately take into account the solvent and concentration effects. This i s the first of a series of papers on our 16
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
2.
LAL E T AL.
Isolated Chain Molecules
s t u d i e s of the c o n f i g u r a t i o n a l s t a t e of c h a i n molecules i n the a d s o r b e d s t a t e . H e r e we d e s c r i b e t h e p r e s e n t a p p r o a c h and p r e s e n t i t s a p p l i c a t i o n t o an i s o l a t e d e x c l u d e d - v o l u m e c h a i n i n t e r a c t i n g with a surface. T h i s m o d e l may be c o n s i d e r e d t o r e p r e s e n t a polymer m o l e c u l e adsorbed from an e x t r e m e l y dilute athermal solution. The
Present
Approach
The e q u i l i b r i u m v a l u e o f a q u a n t i t y , q, q, c a n i d e n t i f i e d w i t h the c a n o n i c a l ensemble a v e r a g e :
be
q exp(-U/KT) d i l fexp(-U/KT) dSL where e x p ( - U / K T ) i s t h v a l u e q of the q u a n t i t y integration e q u a t i o n extend o v e r t h e t o t a l c o n f i g u r a t i o n a l phase space, of the system. L e t us draw a random sample o f c o n f i g u r a t i o n s f r o m t h e p h a s e s p a c e SI. The mean o f t h e q u a n t i t y q, , over such a sample i s g i v e n as exp(-UVKT)
= S
where S i s t h e m a g n i t u d e o f t h e s a m p l e . The Monte C a r l o a p p r o a c h assumes t h a t f o r l a r g e S, w o u l d c o n v e r g e t o q . For systems i n v o l v i n g s t r o n g i n t e r a c t i o n s , sampling procedures b a s e d on p u r e l y random c o l l e c t i o n o f c o n f i g u r a t i o n s w i l l n o t be s a t i s f a c t o r y a s t o a c h i e v i n g c o n v e r g e n c e t h a t w i l l p r o d u c e estimates w i t h i n reasonable l i m i t s of u n c e r t a i n t y . The way t o o b t a i n samples w i t h a c c e p t a b l e c o n v e r g e n c e i n such c a s e s i s t o u t i l i s e a s a m p l i n g scheme w h i c h f a v o u r s t h e h i g h d e n s i t y r e g i o n s o f the p h a s e s p a c e . I n a p r e v i o u s p u b l i c a t i o n we p r e s e n t e d a scheme s u i t a b l e f o r t a c k l i n g c h a i n m o l e c u l a r models i n v o l v i n g i n t e r - s e g m e n t a l i n t e r a c t i o n s ( 1 2 ) . This scheme c a n be m o d i f i e d so a s t o be c o n g e n i a l t o t h e m o d e l under the p r e s e n t c o n s i d e r a t i o n . The p r e s e n t method i n v o l v e s g e n e r a t i n g a homogeneous markov s e q u e n c e o f c o n f i g u r a t i o n s w i t h o n e - s t e p t r a n s i t i o n p r o b a b i l i t i e s ( p . .) g i v e n s u c h t h a t t h e c o n f i g u r a t i o n s a m p l e thus o b t a i n e d assumes Boltzmann d i s t r i b u t i o n i n t h e l i m i t of a l o n g sequence. Hence t h e s i m p l e means o v e r s u c h a s a m p l e c a n be i d e n t i f i e d w i t h t h e c a n o n i c a l e n s e m b l e a v e r a g e s . The d e s i r e d c o n f i g u r a t i o n s e q u e n c e s c a n be g e n e r a t e d f o l l o w i n g t h e commonly u s e d a s y m m e t r i c p r o c e d u r e a c c o r d i n g t o w h i c h t h e t r a n s i t i o n p r o b a b i l i t i e s a r e g i v e n as (13)
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
18
ADSORPTION
1 f o r U. J
x
INTERFACES
V
±
1-P
The s e q u e n c e c a n s t a r t f r o m any a l l o w e d c o n f i g u r a t i o n c h o s e n arbitrarily. L e t the e n e r g y c o r r e s p o n d i n g t o t h i s c o n f i g u r a t i o n be U. Consider another c o n f i g u r a t i o n , with e n e r g y 'U, w h i c h c a n be d e r i v e d f r o m t h e p r e v i o u s one by some random p r o c e s s . I f 1J i s l e s s t h a n o r e q u a l t o U, a c c e p t t h e new c o n f i g u r a t i o n i n t h e s e q u e n c e . E l s e , c h o o s e a random number, x, b e t w e e n 0 and 1 and compare i t w i t h r = [exp(-U/KT)/exp(-U/KT)] . I f r > x, a l s o t h e n t h e new c o n f i g u r a t i o n assume O t h e r w i s e , r e j e c t th r e t u r n t o t h e p r e c e d i n g one, w h i c h means t h a t t h e same c o n f i g u r a t i o n i s counted a g a i n i n the sequence. Repeat the above p r o c e s s t o a c q u i r e f u r t h e r moves. I n t h i s way t h e s e q u e n c e c a n be e x t e n d e d t o any l e n g t h . I n t h e p r e v i o u s scheme t h e mechanism by w h i c h t h e s y s t e m moved f r o m one c o n f i g u r a t i o n t o a n o t h e r i n v o l v e d t h e r o t a t i o n of a s i n g l e bond i n t h e c h a i n . T h i s r e s u l t e d i n the changes i n t h e p o s i t i o n s of a l l the segments s u c c e e d i n g t h e r o t a t i n g bond. T h i s scheme, a l t h o u g h s u c c e s s f u l i n c a s e of f r e e chains, i s u n s u i t a b l e i n the p r e s e n c e of an i n t e r f a c e . F o r t h e p r e s e n t m o d e l we d e v e l o p e d a mechanism b a s e d on t h e c o n s i d e r a t i o n t h a t t r a n s i t i o n f r o m one c o n f i g u r a t i o n t o a n o t h e r o c c u r s as a c o n s e q u e n c e o f c h a n g e s i n t h e r o t a t i o n a l p o s i t i o n s o f o n l y a few c o n s e c u t i v e b o n d s . L e t us t e r m s u c h a s e q u e n c e o f b o n d s a s t h e t r a n s i t i o n r e g i o n (figure 1). The p o s i t i o n o f t h e t r a n s i t i o n r e g i o n i n t h e m o l e c u l e i s assumed t o be s t o c h a s t i c i n n a t u r e . In the p r e s e n t scheme t h e number o f bonds c o n s t i t u t i n g t h e t r a n s i t i o n region is four. T h i s i s t h e minimum s i z e o f t h e r e g i o n w h i c h must be c o n s i d e r e d i n o r d e r t o g e n e r a t e a l l t h e p o s s i b l e c o n f i g u r a t i o n s w h i c h c a n be assumed by a l i n e a r c h a i n m o l e c u l e confined to a tetrahedral l a t t i c e . We f u r t h e r assume t h a t b e y o n d t h e t r a n s i t i o n r e g i o n no c h a n g e s i n t h e p o s i t i o n s o f t h e segments w i l l o c c u r . In o r d e r t h a t the samples a c h i e v e the r e q u i r e d convergence, i t i s n e c e s s a r y t h a t t h e e r g o d i c i t y and s t e a d y s t a t e c o n d i t i o n s a r e met. In simple terms, the e r g o d i c i t y c o n d i t i o n s t i p u l a t e s t h a t s t a r t i n g f r o m a c o n f i g u r a t i o n i t i s p o s s i b l e t o r e a c h any p e r m i s s i b l e c o n f i g u r a t i o n i n a f i n i t e number o f s t e p s , and t h a t t h e sample c o n t a i n s no p e r i o d i c i t i e s . The s t e a d y s t a t e c o n d i t i o n states that
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
2.
LAL E T AL.
Isolated Chain Molecules
ZJ i
u. p.. ι J
=
u. J
a
19
for a l l j
where u . a n d u . a r e t h e a b s o l u t e p r o b a b i l i t i e s o f t h e o c c u r r e n c e o f s t a t e s i and j i n t h e sample. We r e c k o n t h a t t h e s a m p l e s g e n e r a t e d by o u r scheme a r e i n r e a s o n a b l e a c c o r d w i t h t h e above c r i t e r i a . J
The
Model
The g e o m e t r y c o r r e s p o n d i n g t o t h e f o u r v a l e n c e bonds o f t h e c a r b o n atom i m p l i e s t h a t p o l y m e r m o l e c u l e s w i t h -C-C-C"back-bone" can be a d e q u a t e l y r e p r e s e n t e d by t h e t e t r a h e d r a l chains. Our model i s t h u s b a s e d o n a t e t r a h e d r a l l a t t i c e i n which t h e l a t t i c e s i t e o r by s o l v e n t m o l e c u l e s C-C^" bond a n g l e s i n such c h a i n s i s 109.28°, a n d t h e v a l u e s o f t h e r o t a t i o n a l a n g l e s w h i c h t h e bonds c a n assume a r e 0 ( t r a n s ) , 120° ( g a u c h e ) and 240° (gauche"). The l a t t i c e m o d e l assumes t h a t t h e a b o v e v a l u e s o f t h e bond a n g l e s a n d t h e bond r o t a t i o n a l a n g l e s a r e i n v a r i a n t . This approximation seems n o t u n r e a s o n a b l e . I t i s necessary t o s t i p u l a t e that t h e c h a i n s w i l l assume o n l y t h o s e c o n f i g u r a t i o n s w h i c h do n o t c o n t a i n m u l t i p l e occupancies (the excluded-volume c o n d i t i o n ) . The a d s o r b e n t i s r e p r e s e n t e d by a t w o - d i m e n s i o n a l s u r f a c e i n the l a t t i c e with t h e a d s o r p t i o n s i t e s l o c a t e d a t the l a t t i c e points. +
In s o l u t i o n s t h e r e e x i s t t h r e e k i n d s of i n t e r a c t i o n s : ( 1 ) segment/segment i n t e r a c t i o n s , ( 2 ) s o l v e n t / s o l v e n t i n t e r a c t i o n s and (3) segment/solvent interactions. Let ^22' with
^12
d e n c r t e
>
respectively,
t h e segment/segment,
pairs.
The e n e r g y
segment/segment
segment/solvent
change a c c o m p a n y i n g t h e f o r m a t i o n
pair,
Δ € = € 1
the energies associated
s o l v e n t / s o l v e n t and
Δ β^,
11
+ €
of a
w i l l be
22
- 2 €
12
Δ^ serves as t h e s o l v e n t parameter i n t h e model. Good s o l v e n t s o p p o s e t h e f o r m a t i o n of segment/segment p a i r s , w h i c h i m p l i e s t h a t f o r such s o l v e n t s Δ ^ assumes p o s i t i v e v a l u e s . I n b a d s o l v e n t s t h e f o r m a t i o n o f segment/segment p a i r s w i l l be f a v o u r e d , hence Δ ^ w i l l be n e g a t i v e . Δ ^ = 0 corresponds t o athermal s o l u t i o n s . The s e c o n d e n e r g y p a r a m e t e r c h a r a c t e r i s i n g o u r m o d e l i s the segment/surface b i n d i n g energy Δ C » which i s d e f i n e d as
a
2
IS
2S
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
20
ADSORPTION
where and
£ ^ *
i s the ^ ^ s
AT
INTERFACES
energy a s s o c i a t e d with a segment/surface a s s o c i a t e d w i t h a s o l v e n t / s u r f a c e bond.
n a
bond
s
Thus * t h e e n e r g y c h a n g e due t o t h e f o r m a t i o n o f a s e g m e n t / s u r f a c e bond a t t h e e x p e n s e o f a s o l v e n t / s u r f a c e b o n d . The a d s o r p t i o n c a n o n l y t a k e p l a c e i f Δ ^ i s negative. Computation A c o m p u t e r p r o g r a m was d e v e l o p e d f o r g e n e r a t i n g c o n f i g u r a t i o n s e q u e n c e s f o l l o w i n g o u r scheme a s s u m i n g t h e m o d e l d e s c r i b e d i n s e c t i o n 3. The p r o g r a m p r o c e e d e d a s follows : 1.
Read t h e Ν
following
= total
of
data:
segment
M L
= l e n g t h of th = l e n g t h of i n t e r v a l i n the sequence c o r r e s p o n d i n g the output of data i n i n t e r m e d i a t e stages Δ€^Δ€ : the energy parameters.
to
2
2.
G e n e r a t e an
i>y f
x
±
initial
x
configuration
;
z ±
; *N> N> V
N]
[_ l ;
(
n
±
, y
i
, z
Χ
l>
>y * ±
z
±
2'^2^2'
····
)
define
the
p o s i t i o n of
3.
Define
the
adsorbent
4.
D e t e r m i n e t h e v a r i o u s q u a n t i t i e s o f i n t e r e s t ( e . g . number o f s e g m e n t s on t h e s u r f a c e , s q u a r e o f t h e e n d - t o - e n d d i s t a n c e , s q u a r e o f t h e r a d i u s of g y r a t i o n , e t c . ) .
5.
G e n e r a t e a new c o n f i g u r a t i o n by r a n d o m l y s e l e c t i n g f o u r c o n s e c u t i v e bonds i n any p a r t of t h e c h a i n and m o v i n g t h e s e bonds t o an a l t e r n a t i v e c o n f i g u r a t i o n , w h i l s t t h e remainder of the molecule i s u n a l t e r e d .
6.
Check t h e new c o n f i g u r a t i o n f o r e x c l u d e d - v o l u m e . c o n f i g u r a t i o n i n v o l v e s m u l t i p l e o c c u p a n c y of any s i t e , go t o 14.
7.
C a l c u l a t e Δϋ, the d i f f e r e n c e between the p r e v i o u s and t h e new configuration.
8.
I f Δυ
9.
Calculate r =
10.
Generate a
11.
I f r > x,
12.
Go
13.
Calculate
to
i s p o s i t i v e or
a
Z
plane such t h a t
zero,
go
to
y = x+
constant.
energies
I f the lattice
of
the
13.
exp(AU/KT).
random number go
segment i .
to
( x ) b e t w e e n 0 and
1.
13.
14. the
desired
q u a n t i t i e s f o r the
new
configuration.
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
2.
LAL E T AL.
21
Isolated Chain Molecules
14.
Accumulate the
calculated
15.
I f the i s any
16.
Print
17.
I f the t o 5.
18.
C a l c u l a t e the simple over the sample.
19.
P r i n t the
number of integer), the
quantities.
configurations go t o 5.
desired
number o f
g e n e r a t e d Φ AL
(where
A
information. configurations
a v e r a g e s of
computed a v e r a g e s and
g e n e r a t e d <M,
the
various
other relevant
go
quantities
information.
The c a l c u l a t i o n s were c a r r i e d o u t on a CDC 7600 c o m p u t e r . C o n v e r g e n c e was assume a v e r a g e s of the v a r i o u s e t l i m i t s as the sampl sampl r e q u i r e d f o r r e a s o n a b l e c o n v e r g e n c e e x c e e d e d 500,000 c o n figurations. I t took a p p r o x i m a t e l y ten minutes to generate a s a m p l e of s u c h a m a g n i t u d e on t h e a b o v e m a c h i n e . If a s a m p l e d i d n o t show s a t i s f a c t o r y c o n v e r g e n c e , t h e c a l c u l a t i o n was r e s t a r t e d f r o m t h e l a s t c o n f i g u r a t i o n g e n e r a t e d and i t s s i z e g r a d u a l l y i n c r e a s e d t i l l t h e r e q u i r e d c o n v e r g e n c e was attained. Results
and
Discussion
An o u t s t a n d i n g f e a t u r e o f t h e c o n f i g u r a t i o n a l s t a t e o f a polymer m o l e c u l e i n t e r a c t i n g w i t h a s u r f a c e i s t h a t , except f o r v e r y h i g h -Δ t h e segments a r e f o u n d t o be i n the adsorbed s t a t e . This state i s p i c t o r i a l l y represented i n f i g u r e 2, where i t i s shown t h a t t h e c o n f i g u r a t i o n i s c o n s t i t u t e d o f " t r a i n s " ( s e q u e n c e s of c o n s e c u t i v e segments i n the a d s o r b e d s t a t e ) , " l o o p s " (sequences of c o n s e c u t i v e segments i n t h e u n a d s o r b e d s t a t e ) and " t a i l s " ( u n a d s o r b e d a r r a y s o f segments a t t h e e n d s ) . Basic quantities r e l a t i n g t o the above c o n f i g u r a t i o n a l s t a t e a r e the f r a c t i o n of segments i n t h e a d s o r b e d - s t a t e (V) and t h i c k n e s s o f a d s o r b e d l a y e r ( T ) . A d d i t i o n a l p a r a m e t e r s such as t h o s e p e r t a i n i n g t o the " t r a i n " , " l o o p " and " t a i l " s i z e s and t h e h e i g h t d i s t r i b u t i o n o f segments a b o v e t h e s u r f a c e w o u l d p r o v i d e a more d e t a i l e d knowledge of the c o n f i g u r a t i o n a l s t a t e . n
o
t
a
1
1
The p r e s e n t c a l c u l a t i o n s h a v e b e e n d i r e c t e d t o w a r d s t h e e v a l u a t i o n of the v a r i o u s c o n f i g u r a t i o n a l p a r a m e t e r s n o t e d above. I n t h i s s t u d y we h a v e c o n s i d e r e d a m o d e l c o n s t i t u t e d o f a l i n e a r c h a i n m o l e c u l e of 100- segment l e n g t h a d s o r b e d from a s o l v e n t f o r which Δ ^ = 0, w h i c h i m p l i e s t h a t f o r t h e p r e s e n t system the s o l v e n t / s o l u t e i n t e r a c t i o n s a r e i n v a r i a n t w i t h r e s p e c t to the i n t r a m o l e c u l a r c o n f i g u r a t i o n a l changes. S u c h s o l u t i o n s a r e known a s a t h e r m a l s o l u t i o n s .
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
22
ADSORPTION
A T INTERFACES
TRANSITION
Figure 1. A transition region is constituted of four consecutive bonds (i, i + 1, i + 2, iΐ + 3) in any part of the chain
LOOP
TAIL
t TRAIN
•
////////////////////////////////// Figure 2.
The train-loop-tail
model for an adsorbed polymer molecule
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
2.
LAL E T AL.
23
Isolated Chain Molecules
The mean f r a c t i o n s o f segments b o u n d t o t h e s u r f a c e c a l c u l a t e d assuming various values o f segment/surface b i n d i n g e n e r g y a r e p r e s e n t e d i n f i g u r e 3. We o b s e r v e t h a t < V> increases r a p i d l y with the increase i n ( - Δ ^ 2 ^ ^ -Δ € / K T = 0.5, l e s s t h a n 1 0 % o f t h e s e g m e n t s a r e a d s o r b e d , while a t -Δ62/ΚΤ 1·4, a p p r o x i m a t e l y 7 5 % o f t h e segments w o u l d be o n t h e s u r f a c e . The < V> v e r s u s - Δ ^ 2 ^ e x t r a p o l a t e d t o d e t e r m i n e t h e p o i n t b e l o w w h i c h < V> i s z e r o . S u c h a p o i n t i s known a s t h e c r i t i c a l p o i n t , and t h e c o r r e s p o n d ing energy i s the c r i t i c a l energy of a d s o r p t i o n . For the p r e s e n t s y s t e m (-à € /KT^ f o u n d t o b e ~0.45. To o u r k n o w l e d g e no c a l c u l a t i o n s h a v e h e r e t o f o r e b e e n r e p o r t e d o n a model i d e n t i c a l t o t h e o n e u n d e r t h e p r e s e n t consideration. However, M c C r a c k i n c a r r i e d o u t Monte C a r l o c a l c u l a t i o n s on t h e s y s t e m chains constrained t o f w i t h r e g a r d t e n e r g y o f a d s o r p t i o n f o u n d by M c C r a c k i n i s q u a l i t a t i v e l y s i m i l a r t o t h a t g i v e n by f i g u r e 3. T h e r e i s , however, a p p r e c i a b l e q u a n t i t a t i v e d i f f e r e n c e i n t h e two r e s u l t s : t h e c r i t i c a l energy f o r the t e r m i n a l l y anchored chains c o n s i d e r e d by M c C r a c k i n was f o u n d t o b e a p p r o x i m a t e l y 0.25 KT, w h i c h i s c o n s i d e r a b l y lower than t h a t o b t a i n e d f o r t h e p r e s e n t system; a l s o , o u r v a l u e s of < V > tend t o be lower than those which would b e o b t a i n e d by e x t r a p o l a t i n g M c C r a c k i n s r e s u l t s t o t h e corresponding adsorption energies and t h e chain length. This may b e due t o t h e r e a s o n t h a t a " t r a i n " i n t e t r a h e d r a l c h a i n / / κ τ
:
a
2
=
K T
i
D
l
o
t
c
a
n
D
e
s
2
1
can e x i s t o n l y i n a s i n g l e c o n f i g u r a t i o n tttt . Hence i t i s i n t h e s t a t e of zero c o n f i g u r a t i o n a l entropy. This means t h a t o n a d s o r p t i o n a s e q u e n c e o f s e g m e n t s w i l l h a v e t o l o s e whole c o n f i g u r a t i o n a l e n t r o p y a s s o c i a t e d w i t h i t i n the s o l u t i o n phase. The a d s o r p t i o n i s , thus, o n l y p o s s i b l e i f t h e segment/surface i n t e r a c t i o n energy i s so l a r g e t h a t i t would more t h a n c o m p e n s a t e t h e e n t r o p y l o s s . Secondly, s y s t e m s s u c h a s t h o s e s t u d i e d by M c C r a c k i n h a v e a l r e a d y l o s t some e n t r o p y o n a c c o u n t o f t h e a d d i t i o n a l c o n s t r a i n t o f one chain-end permanently anchoring t o the s u r f a c e . Thus t h e segment/surface energy r e q u i r e d f o r a d s o r p t i o n t o take p l a c e would be lower, a s i t has lower e n t r o p y t o overcome. F i g u r e 4 g i v e s t h e mean f r a c t i o n s o f segments i n t h e s u c c e s s i v e l a y e r s a b o v e t h e s u r f a c e computed f o r v a r i o u s v a l u e s o f -Δ We n o t e t h a t i n t h e c a s e o f Δ € = -0.5 KT the d i s t r i b u t i o n i s markedly d i f f e r e n t from those o b t a i n e d for higher segment/surface b i n d i n g energies. For Δ β = -0.5 KT, t h e segment d e n s i t y v a r i e s l i t t l e i n t h e f i r s t f e w l a y e r s adjacent t o t h e s u r f a c e , and then d e c l i n e s s l o w l y as the d i s t a n c e from t h e s u r f a c e i n c r e a s e s . For higher i n t e r a c t i o n e n e r g i e s , o n t h e o t h e r hand, t h e i n i t i a l d e c r e a s e i n t h e d e n s i t y with d i s t a n c e i s very r a p i d , which subsequently slows down a s t h e d e n s i t y a p p r o a c h e s z e r o . The t h i c k n e s s o f a d s o r b e d 2
2
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
24
ADSORPTION
A T INTERFACES
Figure 3. Mean fraction of segments bound to the surface () vs. energy of adsorp tion per segment (—Ae /kT) 2
-*e / 2
kT
—^ Ο·70η
0-60 -fl
0-50 Η
j Figure 4. Mean fraction of segments () vs. height above the surface (h). 1: —Ae /KT = 0.5. 2: -A* /KT = 0.7. 3: -Ae /KT = 0.9. 4: -Ae /KT = 1.2. 2
2
2
0-40-
4* 0·δΟ -ft
2
Ο
2
4
6
6
10
12
14
h
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
16
18
20
LAL E TAL.
25
Isolated Chain Molecules
l a y e r ( * G ) c a n be d e f i n e d a s t h e d i s t a n c e f r o m t h e s u r f a c e a t which the segment-density vanishes, v i z . the d i s t a n c e a t which a d i s t r i b u t i o n c u r v e i n f i g u r e 4 meets t h e a b s c i s s a . The v a l u e s ofthus d e t e r m i n e d a r e p l o t t e d a g a i n s t - Δ £ / K T i n f i g u r e 5. T h i c k n e s s o f a d s o r b e d l a y e r i s f o u n d t o b e a r a p i d l y decreasing f u n c t i o n of the segment/surface i n t e r a c t i o n energy. The m a g n i t u d e o f C O a t -Δ € = 0.5 (~20 u n i t s ) 2
2
KT s u g g e s t s t h a t when t h e i n t e r a c t i o n e n e r g y i s s m a l l , t h e m o l e c u l e i n t h e a d s o r b e d s t a t e w o u l d assume d i m e n s i o n s s i m i l a r t o t h o s e i n s o l u t i o n . B u t a t e n e r g i e s e x c e e d i n g -1.0 KT t h e a d s o r b e d m o l e c u l e w o u l d be c o n s i d e r a b l y f l a t t e n e d , a s i s i n d i c a t e d by t h e s m a l l v a l u e s of4"£7. A n a l y s i s of the s i z e d i s t r i b u t i o n s of " t r a i n s " , "loops" and " t a i l s " r e v e a l s t h a narrow d i s t r i b u t i o n s wherea e x t r e m e l y w i d e s h o w i n g l a r g e f l u c t u a t i o n s . The mean s i z e s o f " t r a i n s " ( < L > ) , " l o o p s " (), a n d " t a i l s " ( < L > ) c a l c u l a t e d at s e v e r a l values of Δ ^ a r e p l o t t e d i n f i g u r e 6. The mean " t r a i n " s i z e v a r i e s between t h r e e and s i x i n t h e e n e r g y r a n g e considered. At Δ € = -0.5 KT, t h e mean " l o o p " s i z e i s a s l a r g e a s about 20 u n i t s b u t f o l l o w s a s h a r p d e c r e a s e a s - Δ β increases. As e x p e c t e d , t h e mean " t a i l " l e n g t h d e c r e a s e s w i t h the i n c r e a s e i n -Δ C . i n t e r e s t i n g f e a t u r e which our a n a l y s i s brings out i s that, except a t very high - Δ € , a large p r o p o r t i o n o f u n a d s o r b e d segments c o n s t i t u t e " t a i l s " , and t h a t t h e " t a i l s " c o n t r i b u t e most s i g n i f i c a n t l y t o t h e t h i c k n e s s o f adsorbed layer. t
e
2
2
T
n
e
2
2
P i c t u r e of an i s o l a t e d adsorbed molecule emerging from t h e p r e s e n t s t u d y i s t h a t a t low s u r f a c e / s e g m e n t i n t e r a c t i o n e n e r g i e s o n l y a s m a l l f r a c t i o n o f t h e segments i s bound t o t h e s u r f a c e , and t h e shape and d i m e n s i o n s o f t h e a d s o r b e d m o l e c u l e a r e c o m p a r a b l e t o t h o s e assumed i n s o l u t i o n . A t i n c r e a s e d v a l u e s o f t h e i n t e r a c t i o n e n e r g y , however, t h e number o f s e g m e n t s on t h e s u r f a c e i s s i g n i f i c a n t , a n d t h e c o n f i g u r a t i o n a l s t a t e assumed i s s u c h t h a t s m a l l " t r a i n s " a n d s m a l l " l o o p s " are favoured while " t a i l " lengths are r e l a t i v e l y l a r g e . Most important c o n t r i b u t i o n t o the thickness of adsorbed l a y e r d e r i v e s from " t a i l s " . Concluding
Remarks
I n t h i s s t u d y we h a v e d e l i b e r a t e l y assumed a n e x t r e m e l y s i m p l e m o d e l , f o r o u r m a i n p u r p o s e h e r e was t o a s c e r t a i n t h e v i a b i l i t y o f t h e M e t r o p o l i s s a m p l i n g method i n t h e c a s e o f c h a i n m o l e c u l a r systems i n t e r a c t i n g with a s u r f a c e . Notwithstanding the l a r g e magnitude o f the samples r e q u i r e d f o r s a t i s f a c t o r y c o n v e r g e n c e , t h e Monte C a r l o a p p r o a c h c o n s i d e r e d may b e r e g a r d e d a s s u c c e s s f u l i n t a c k l i n g t h e p r e s e n t s y s t e m s . We hope t h a t t h i s method p r o v e s e q u a l l y s u c c e s s f u l when
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
ADSORPTION
Δ
ι °·β € /kj 2
, ι·ο
ι ι·2
Figure 5. Mean thickness of adsorbed layer () as a function of the energy °f
I 1-4.
—
0 4
0*6
A T INTERFACES
O ô
Ι Ό
1-2
Figure 6. Mean values of "train' (), "loop" (), and "tail" () lengths correspond ing to various values of the adsorption energy per segment t
e
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
2.
LAL ET AL.
Isolated Chain Molecules
27
m o d i f i c a t i o n s such as the trans/gauche bond conformational energy d i f f e r e n c e and non-athermal solvents a r e introduced i n the model. We s h a l l deal with such models i n f u r t h e r studies. An important extension of t h i s work that we envisage concerns the i n v e s t i g a t i o n of the c o n f i g u r a t i o n a l behaviour of adsorbed polymer molecules as the s u r f a c e coverage v a r i e s . This can be achieved by i n t r o d u c i n g p e r i o d i c boundaries i n the system (14). Literature Cited 1.
F r i s c h , H.L., Simha, R., and E i r i c h , F.R., J . Chem. Phys., (1953), 21, 365.
2.
Hoeve, C.A.J., DiMarzio Phys., (1965), 42
3.
Rubin, R.J., J . Chem. Phys.,
4.
Roe, R.J., Proc. N a t l . Acad. Sci., (1965), 53, 5 0 .
5.
Motomura, K., and Matuura, R., J . Chem. Phys., (1969), 50, 1281.
6.
S i l b e r b e r g , Α., J . Chem. Phys., (1968), 48, 2835.
(1965), 43, 2392.
7. Lal, M., R.I.C. Rev., (1971), 4, 97. 8.
L a l ,Μ.,and Spencer, D., J . Chem. Soc. Faraday Trans. I I , (1973), 69, 1502.
9.
L a l ,M.,and Spencer, D., J . Chem. Soc. Faraday Trans. I I , (1974), 70, 910.
10.
Bluestone, S., and Cronan,
J . Phys. Chem., (1966), 70, 306.
11.
McCrackin, F.L., J . Chem. Phys., (1967), 47, 1980.
12. 13.
L a l , M., Mol. Phys., (1969), 17, 57. Wood, W.W., i n "Physics of Simple L i q u i d s " , pp 117-230, North Holland P u b l i s h i n g Co., Amsterdam, 1968.
14.
C l a r k , A.T., and Lal, Μ., t o be p u b l i s h e d .
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
3 Equilibrium Film Pressure on a Flat, Low-Energy Solid R O B E R T J. G O O D Department of Chemical Engineering, State University of New York at Buffalo, Buffalo, Ν. Y. 14214
Introduction The equilibrium f i l m pressure of an adsorbate, πe, i s defined as πe
=
Y
S -
Y
SV
(1)
where ΥS i s the surface free energy of the s o l i d i n vacuum, and YSV i s the surface free energy of the s o l i d i n contact with the saturated vapor of the ad sorbate. The value of this property for an adsorbate which, as a l i q u i d , forms a non-zero contact angle on the s o l i d , has been a matter of uncertainty for some time (1-9). This fact has detracted from the use fulness of measurements of contact angle, θ, for the estimation of s o l i d surface free energies (2,3). See r e f s . (5) and (6) for an important discussion of the problem as it has existed in recent years. The measurement of πe i s not p a r t i c u l a r l y easy; and up to very recently (8) the only determinations that had been reported for low-energy s o l i d s were made on powders (4,5,6), while reported contact angle measurements were made on e s s e n t i a l l y f l a t surfaces. Fox and Zisman (1) found reason to conclude that πe, i s probably n e g l i g i b l e ; and this assumption i s basic to t h e i r "Yc" method of t r e a t i n g contact angle data. Adamson (3) has pointed out that the existence of a t h i c k adsorbed f i l m , and consequent nonn e g l i g i b l e value of πe, are compatible with the e x i s tence of a non-zero contact angle, provided the ma terial in the adsorbed f i l m has a structure that i s s i g n i f i c a n t l y d i f f e r e n t from the bulk l i q u i d . He hypothesizes a degree of "structure" i n a m u l t i l a y e r f i l m , which decays with distance from the surface. If such t h i c k , structured multilayers e x i s t , the low 28
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
3.
GOOD
Equilibrium
29
Film Pressure
entropy associated with such a structure could account for the existence of a non-zero contact angle. But of course, one cannot argue that the existence of a non zero contact angle proves the presence of a thick, structured f i l m . And indeed, i t i s hard to imagine that a m u l t i l a y e r f i l m of, say, CCl^ on Teflon could have a "structure that would meet the requirements noted i n Ref. 3. Whalen (6) pointed out that the Hill-deBoer equation (11,12,13), with empirical con stants obtained from measurements with hexane and octane on Teflon TFE i n the low-coverage region, pre d i c t s a submonolayer l i m i t i n g adsorption at saturation. The purpose of this paper i s to develop a method of estimating 7r , a p r i o r i , on a molecularly f l a t , homogeneous, low-energy surface f o r adsorbates of l i q u i d s f o r which θ physical i n t e r a c t i o n quantitative expressions are a v a i l a b l e i n well-known physical theory (10). As a f i r s t step i n this a n a l y s i s , an attempt was made to compute, by methods which w i l l be described below, the constants for a BET m u l t i l a y e r adsorption isotherm for CCI/, on a homogeneous, molecularly smooth fluorocaroon surface. I t was found that the BET "c" constant was very small. I f the adsorbent were a powder instead of a f l a t surface, a very low value of c would indicate a BET type I I I isotherm, the f i n a l up-turn of which (near ρ = p ) was due to enhanced adsorbate-solid i n t e r a c t i o n s at points of s o l i d - s o l i d contacts, e.g. as pendular r i n g s , which Wade and Whalen (.5,(5) have discussed. Such s t r u c t u r a l features are, by d e f i n i t i o n , absent from a molecularly smooth surface. These preliminary r e s u l t s led us to turn to a Langmuir model, as being f a r easier to treat than a BET model. It was a n t i c i p a t e d that a s t r i c t l y Langmuirian model might break down when some possible values of molecular parameters were assumed — for example, the model might p r e d i c t high enough coverage that l a t e r a l i n t e r a c t i o n s would be important. For such a regime, a Hill-deBoer isotherm would be more s u i t a b l e . However, i n the present computations, we confined that aspect of the study to the estimation of the conditions under which the Langmuir postulates break down. To a n t i c i p a t e some of our r e s u l t s , a breakdown of the Langmuir assumptions was i n f a c t found, and i n a very i n t e r e s t i n g region of the range of chain-length for n-alkanes, on Teflon. 11
e
Q
ff
11
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
30
ADSORPTION
AT
INTERFACES
Theory Consider the low-coverage sub-monolayer region of an adsorption isotherm on a uniform s o l i d . We w i l l estimate the energy and entropy of adsorption, with the bulk l i q u i d as the reference state. The p a r t i a l molal entropy of the adsorbed molecules, i n a Langmuir adsorbed f i l m on a s o l i d , i s given by (14). S - - Rln [ x / ( l - x ) l a
(2)
a
where x i s the "θ" of the Langmuir adsorption equa t i o n , i . e . the mole f r a c t i o n of surface s i t e s that are occupied. This entropy term i s c o n f i g u r a t i o n a l ; i t arises from the p o s s i b i l i t y of permuting the molecules among the occupied molal entropy of transfer, liqui sorbed f i l m , i s given by, X W AS = - Rln τ — + Rln ^r- + AS. . - S (3) l-x W internal ql ' The l a s t two terms i n Equation 3 can probably be neglected. These correspond, r e s p e c t i v e l y , to the changes i n i n t e r n a l , molecular degrees of freedom, and to the configurational entropy of the l i q u i d , considered as a q u a s i - l a t t i c e with occupied and vacant s i t e s . The l a t t e r term i s small, probably less than 0.5 entropy unit (15). The second term on the r i g h t involves the number of angular configurations accessible to a molecule i n the l i q u i d (W^) and i n the adsorbed state (W ). For a quasi-spherical molecule such as C C I 4 , this term i s zero. For an elongated molecule, such as an n-alkane longer than propane or η-butane, we may estimate this term as follows: The molecule i s treated as a r i g i d c y l i n d e r , of length I and diameter d. In the l i q u i d , i t s axis may be at any angle, r e l a t i v e to a fixed coordinate system; i n e f f e c t , i t has a volume 0.75ΤΓ(Ί/2)3 accessible to i t . In the adsorbed state, the energy of a t t r a c t i o n between the s o l i d and the extended molecule renders i t improbable that the molecule w i l l have i t s axis i n any plane that i s at an appreciable angle to the surface. So i t may be r e garded as having, accessible to i t , a volume 2ir(l/2)^d. This entropy term, then, i s approximately Rln(3d/4^). A further refinement on t h i s term can be made by counting the numbers of bent configurations e x p l i c i t l y . These are tedious to enumerate, but t h e i r e f f e c t on the equilibrium w i l l be to predict even lower coverage than that estimated with t h e i r neglect. a
Q
Q
Ί
Ί
v
a
L
a
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
3.
Equilibrium
GOOD
31
Film Pressure
Thus, we can write as a reasonable approximation, f o r the entropy of transfer per mole between l i q u i d and adsorbed state: N k t l n ^ J - - ln(ff)] (4) a where Ν i s the Avogadro number. For symmetrical ad sorbate molecules, the l a s t term i n the brackets i s omitted. Volume changes are small, i n transfer from l i q u i d to adsorbed state, so we may estimate the enthalpy change as follows: The p a r t i a l molal energy change, i n transfer from the pure l i q u i d to the adsorbed state, i s : AS = -
Δϋ « AU
V
- ÂÏÏ
V
Here, AU i s the molar energy of vaporization, and AU the p a r t i a l molal energy of adsorption from the vapor. For the purpose of a s t r i c t l y Langmuir-type computation, we assume A U to be constant. Estimates of A U have not been b r i l l i a n t l y successful i n the past; but we are not d i r e c t l y interested i n the heat of adsorption from the gas. Rather, we are interested i n the energy of transfer from the l i q u i d . Thus, i f we use a model that i s moderately reasonable to estimate AU , and the same model to estimate AU * , we w i l l have a good probab i l i t y of making a better estimate of the difference between these two terms. It w i l l develop that we use the macrospic heat of vaporization of the pure l i q u i d as our experimental parameter i n p r e d i c t i n g coverage. Molecular considerations w i l l enter only i n regard to estimating the energy of i n t e r a c t i o n between unlike molecules or segments, and to taking molecular shape into account. Thus, we can write, for substance i , taking the l i q u i d as the reference state, a d s
a d s
acis
V
ac
A
U
N
€
z
/
s
2
( 6 )
i " ii iL Here, ζΐχ i s the coordination number for substance i i n the l i q u i d , and €±± i s the energy at the minimum of the pair p o t e n t i a l function, f o r substance i . Let the molecules of the l i q u i d be designated 1, and those of the s o l i d 2. Then the energy change per mole, on transfer of molecules of type 1 from the l i q u i d to the adsorbed state, i s : Δϋ ^ N < *
1 L
€ /2 u
- «
l a
€
1 2
/2)
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
(7)
ADSORPTION
32
AT
INTERFACES
Here €^2 i s the energy at the minimum of the p o t e n t i a l function, f o r 1-2 bimolecular i n t e r a c t i o n , and z ^ i s the number of nearest neighbor surface molecules, or groups, that can i n t e r a c t with a molecule of adsorbate, The l a s t term the brackets i n Equation 7 corresponds to the energy of adsorption from the vapor, estimated according to the same model used f o r Equation 6· Equating Δϋ and TAS, we obtain a
-
NkTUn^-H
ln(|f)] = N ( N Z
1L
Z l L
€ n
z
l a
€
flafl2
11
1 2
)/2
(8)
I f the dominant types of a t t r a c t i v e forces f o r the two species are the same, i t has been shown that 6^2 *- given, approximately, by ( 1 0 ) : s
(9)
'12
The r i g h t side of Equation 8 then becomes: Ν
ζ
η Λ ι
(10)
τ
I f the coordination number of a molecule (or group) of type 1, i n the l i q u i d , z ^ , i s the same as that of a molecule (or group) of type 2 i n i t s l i q u i d state, ζοτ, and i f there i s the same degree of v a l i d i t y f o r trie assumption of pairwise a d d i t i v i t y of energies f o r substances 1 and 2, and the same f r a c t i o n a l contribution due to neighbors outside the f i r s t coordination s h e l l , then, L
€
ll
/ €
22
Ξ
(ID
AUJf/AUj
Combining Equations 6, 10 and 11 with Equation 8, we obtain: log1-x.
2.3RT
^la
1 Z
(12)
1 L VAUj
A simple case, to which we can apply Equation 12, i s C C I 4 on polytetrafluorethylene (Teflon TFE). We estimate the r a t i o of energies, ^22^11 > a
s
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
3.
A U
Equilibrium
GOOD
£C1//
a u
$FO
β
Film Pressure
F
o
r
t
h
i
s
P
33
u r
P°se,
A U
ÏFo
2
i
s
estimated
z
from ^heat of vaporization data for CF4 (16). Using the experimental energy of vaporization f o r CCI4, i t i s found that χ = 3 χ 10"^. The f i l m pressure f o r a d i l u t e monolayer can be computed from s,™ x 7Γ (13) e l-x o
σ
a
where a i s the area per molecule i n a close-packed monolayer. For CCIA on Teflon, assuming σ = 30A , the r e s u l t i s ir = 4 χ 10"^ ergs/cm . Thus we obtain a f i r s t , important conclusion: For one l i q u i d that forms a non-zero contact angle on a low-energy s o l i d (!) > ^e should be n e g l i g i b l e , provided the s o l i d i s homogeneous and molecularl An important syste applied i s the series of homologous n-alkanes on polytetrafluoroethylene. For t h i s purpose, we must modify Equations 8 to 12. We may assume the T e f l o n surface to consist of extended chains. The zigzag structure of the fluorocarbon chain j u s t matches the period of zigzag i n a saturated hydrocarbon chain. We may neglect the h e l i c a l configuration of the f l u o r o carbon, because the p i t c h i s small, about 14 carbons for a turn of the h e l i x . The fact that the l a t e r a l spacing between ( C F 2 ) n chains i s wider than between ( C H o ) chains does not a f f e c t t h i s computation. For an n-alkane, the methyl groups must be treated separately from the methylenes, because the p o l a r i z a b i l i t y of a C H 3 i s considerably greater, and a terminal C H 3 w i l l have more nearest neighbors i n the l i q u i d than w i l l a mid-chain C H 2 . For a long-chain hydrocarbon, most of the neighbors of a C H 3 i n the l i q u i d state are C H 2 groups, so the energy of i n t e r a c t i o n between C H 3 groups i n the l i q u i d may be neglected. Then Z - Q ^ H , Equation 7, may be replaced with the expression Z
n
^ LCH2
and z
l a
( n
€
"
1 2
2 ) z
CH2 9
L
C
H
^
L
C
H
^
,
^
^
, by €
aCH CH ,CF 2
2
+ 2
2z
G
aCH CH ,CF 3
3
With t h i s model, the expression i n Equation 10, becomes
2
for AU,
(15) employed
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
34
ADSORPTION
Δϋ - f
+ 2z
^
^ LCH^ CH^CH^
LCH,
CH2CH2
CH^CH^
1 -
'aClk
"CF CF
'LCH.
•CH C
2
0
2
C
H
'aCH.-
-CF CF
'LCH,
:
2
A T INTERFACES
2
2
2
(16)
CH2 CH„
We can estimate the 6's for methylene and methyl groups i n terms o f energies of vaporization, as was done f o r use i n Equation 1 2 . As before, we approximate the r a t i o s under the square roots by the r a t i o o f energies of vaporizatio the r e s u l t with the /Δυ
'aCR-, log-
1-x
1
(η-2)Δυ;
2.3RT
'LCH,,
«2
2 y
A
U J
,
AU,
CH,
1
CH„
A U
CH
4
i
/Δυ,
'aCH, +
CF.
-
CF.
- log(|f) ( 1 7 )
-
Δϋ,CH.
'LCH-
We have c a r r i e d out computations f o r n-alkanes on Teflon, using A U C H 3 2 5 0 joules/mole and ΔΗ^Ηβ = 8 2 0 0 joules/mole, from vapor pressure data ( 1 6 ) , and 2
Z
/ z
2
/
4
=
z
/ z
1
,
1
T
h
e
r
a
t
i
o
d
/
l w
a
s
aCH LCH " ' aCH LCH = ' estimated assuming the van der Waals diameter of a n-alkane to be 4.8A, and the projection of the C-C distance on the chain axis to be 1.252A. Table I shows the r e s u l t s of this c a l c u l a t i o n . Values f o r pentane and butane are i n parentheses because the assumptions that most of the neighbors of a C H 3 group are C H 2 groups, and that the molecule l i e s f l a t on the surface, break down f o r short-chain alkanes. We note at once, from Table I, that x and are e s s e n t i a l l y zero f o r the higher hydrocarbons. Below hexane, χ increases r a p i d l y ; and the assumption of no l a t e r a l interactions quickly becomes inapplicable. So this computation leads d i r e c t l y to a p r e d i c t i o n of the region where the method of computation should break down. Thus, i t 2
2
3
3
a
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
3.
Equilibrium
GOOD
Film Pressure
35
Table I. n-alkane
2
X
octadecane
1.4 χ
a 10~
hexadecane
4.2 χ
decane
1.1 χ
TT ,ergs/cm e
6
4.4 χ
10~
5
10"
6
1.5 χ
10"
4
10~
4
5.6 χ
10"
3
10"
2
octane
4.0 χ
10"
4
hexane
1.7 χ
10"
3
pentane
(2.9 χ
2.3 χ 0.12
(0.23)
3
10" )
butane i s as expected, that the computed values f o r butane and pentane are much below those observed (4). It i s possible that the estimates of z çHo d aCH3 used above may, f o r c e r t a i n kinds of s i t e s , be too Small. For a r e a l surface, i t i s highly probable that step or ledge s i t e s e x i s t , for which z cHo y ke 3 and z Q^ may be as 5. Such s i t e s could w e l l constitute 5 or 10% of an experimental surface. Examination of models shows that, for elongated molecules, i t i s u n l i k e l y that z cH2 l d be as large as 3 f o r the e n t i r e chain, together with z çH2 s large as 5 for every terminal C H 3 . Indeed the values of these two z's w i l l depend on chain length, and on the l a t e r a l spacing of fluorocarbon chains. I t i s only for convenience that we approximate them by constants, and we w i l l assume average values of 2.5 f o r z çjj2 * 4 for z ç7io. An aiicane molecule i n a s i t e such as j u s t described w i l l not have freedom to take up any angular o r i e n t a t i o n p a r a l l e l to the plane of the surface; indeed there w i l l be just two orientations which have the same energy. So the term, log (3d/4t), i n Equation 17 must be replaced by log (3d2/£ ) Table II shows the r e s u l t computations of this type: The f r a c t i o n x ( tep}°f step s i t e s on Teflon TFE covered by n-alkane molecules, and the contribut i o n to the f i l m pressure due to t h i s coverage, assuming 5% of the surface i s accounted for as step sites. a n
a
z
m a
a
a
c o u
a
a
a
a n c
a
a
2
e
a
s
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
36
ADSORPTION
AT
INTERFACES
Table I I : Estimate of f r a c t i o n a l coverage of step s i t e s on t e f l o n TFE by n-alkane molecules, and c o n t r i b u t i o n of t h i s coverage to equilibrium f i l m pressure, assuming 5% of surface i s accounted f o r as step s i t e s . Table II n-alkane octadecane
χ , a(step) 8.5 χ 10
hexadecane
1.8 χ 10
fc
0.05ττ , ergs/cm e' 1*3 χ ΙΟ"
N
4
3
3.2 χ 10
decane
2.2 χ 10~
2
octane
5.2 χ 10"
2
hexane
1.5 χ
(pentane)
(2.0 χ 10" )
(butane)
2
&
2
5.8 χ 10"
1.5 χ 10 " 1
(0.8)
1
(1.3)
(3.0 χ 10" )
1
The values f o r pentane and butane are given i n paren theses because of the breakdown of an assumption ( i n addition to those noted regarding Table I) that was made i n the s t e p - s i t e model. This i s , that the aver age values of ζ , τ τ and z ~ are less than 3 and 5, respectively. 2 3 We note at once, from Table I I , that the comput ed contributions to ir f o r the higher n-alkanes i s not a p p r e c i a b l e — j u s t as was seen i n Table I. And for the lower alkanes, f o r which the model breaks down, the computed contribution to ir i s s t i l l r e l a t i v e l y small — only 1.9 erg/cm2 for butane, for which i t i s computed that about 30% of the step-sites are occupied. F i n a l l y , we w i l l treat water on Teflon and on polyethylene, using Equation 12 with omission of the term, log(3d/4£). To evaluate ^^_2 * relations : ο Γ
u
a C H
a C H
e
y w
€
d
+
"
+
6
μ
e
u
s
=
e
t
le
" C " )
Κ
1
+
B
«A
+
1
~TET
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
3.
GOOD
Equilibrium
e
2 2
Film Pressure
= ^22
fc
12
Here
37
*12
(
J4l
€
2
0
)
21
22
< >
i s computed by the equation
*
d
22
- -r+M-
< >
where I i s i o n i z a t i o n energy. These r e l a t i o n s are discussed i n d e t a i l i n Ref. (10). The superscripts d, i and μ,, i n Equations 18 - T 2 r e f e r r e s p e c t i v e l y to dispersion, inductio components of intermolecula energy polariz a b i l i t y , I i s i o n i z a t i o n energy, μ i s dipole moment, and Β i s a constant which, f o r molecules c o n s i s t i n g of atoms i n the second and t h i r d rows of the periodic table, i s close to 0.66 (10). For water, the r a t i o computed by Equation 19 i s close to 0.20 (10). Equations 18 and 19, are i m p l i c i t i n the discussion of Fowler and Guggenheim (17,18). Equation 21 can be put i n the form, e
"- "fe-% €
€
- 11
° '
2
(23>
v
< > 24
The reduction i n average coordination number, ùz , f o r water, on going from bulk l i q u i d to adsorbed state, i s probably about 1, i . e . from something near 4, to about 3. Polyethylene i s treated as extended (CH2) chains. The r e s u l t s are given i n Table I I I . It should be emphasized that these computations are f o r water on i d e a l Teflon TFE and polymethylene surfaces, i . e . assuming the s o l i d s to be smooth and homogeneous* n
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
38
ADSORPTION
AT
INTERFACES
Table I I I . Estimates of f r a c t i o n a l coverage and of equilibrium f i l m pressure f o r water on a fluorocarbon s o l i d and on a polymetheylene surface, both assumed to be smooth and homogeneous. Solid
χ a
7Γ , ergs/cm e 7-
Teflon TFE
6 χ 10
2.5 χ 10
Polyethylene Discussion
7 χ 1θ"
6
3 χ 1θ"
5
(a) The Approximations. We must now make a c a r e f u l examination of the important approximations that we have introduced The f i r s t and possibly the most important, approximatio Langmuir equation i t s e l f y computed show that these systems (except for the lower hydrocarbons) should be within the coverage region where the Langmuir equation can s a f e l y be used. We have, of course, neglected l a t e r a l interactions so far; and we now examine the v a l i d i t y of that neglect. For a quasi-spherical molecule such as CCl^, the energy term — i . e . the r i g h t side of Equation 12 — becomes ΔΙΙν
^la
lia
273RT
5
IL
AU;
5
(25)
1L
where z"£ i s the average number of " l a t e r a l " nearest neighbors of an adsorbed molecule. ^ In a perfect 2-dimensional, hexagonal l a t t i c e , z'i would be 6. For CH2 groups i n an n-alkane, z £ i s at most 2 . The entropy of adsorption w i l l be less than the Langmuir entropy (14), so to use the expression ,(25) instead of the corresponding term i n Equation 1 2 w i l l y i e l d the maximum value of x : a
v
a
a
a
l-x-
exp
ΔΙΙ JL RT
'ΔΙΚ
1 - 'la 1L
'la S
5
(26)
1L
For C C I 4 on Teflon TFE, with zî /ziL =0.5, this treatment y i e l d s x < 10" . Thus, the refinement of computing x^ with allowance f o r l a t e r a l i n t e r a c t i o n s does not bring the estimated coverage s e r i o u s l y outside the region of the isotherm where l i n e a r i t y i s a
2
a
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
3.
GOOD
Equilibrium
Film Pressure
39
expected. Hence, the use of the Langmuir isotherm as an acceptable approximation, f o r molecules such as C C I 4 , appears to be j u s t i f i e d . (For step s i t e s , the s i t e s themselves w i l l be i s o l a t e d from each other, and so there i s no l a t e r a l i n t e r a c t i o n of adsorbate molecules even when most of the step s i t e s are covered). For l i q u i d s having much lower b o i l i n g points than CCl^, however, i t i s to be expected that l a t e r a l interactions and two-dimensional condensation w i l l be important, and hence that larger values of ττ w i l l be found. A second approximation i s the assumption of chemical uniformity of the s o l i d surface. It has been established (19) that hydrophilic s i t e s e x i s t on at l e a s t some, ancT possibly a l l samples of Teflon TFE Such s i t e s are, no doubt the majority of s i t e s containing groups are l i k e l y to be present at the surface; and on other low-energy surfaces, i t i s to be expected that high-energy heterogeneities should commonly be present i n a f i n i t e concentration. Such s i t e s would i n t e r a c t with water molecules, and could contribute to a large value of ττ as computed from water adsorption, yet not make an excess c o n t r i b u t i o n to hydrocarbon adsorption. Heterogeneity w i l l also be present for geometric reasons, such as microscopic roughness or microporosity beyond the l e v e l of the step-sites considered above. So some adsorbed molecules w i l l have more nearest neighbors, i n terms of groups such as C F 2 , C F 3 etc. on Teflon, than w i l l others. But i n general, i t i s very u n l i k e l y that z-^ w i l l be larger than 5 or 6. So the r a t i o z^ /z-n w i l l be, at most, about 0.5; and the large values or this r a t i o w i l l pertain to only a very small f r a c t i o n of surface s i t e s , on a surface which i s smooth enough for contact angle measurements to be made. The r a t i o , ^ 2 2 ^ 1 1 ' ^ l° " §y s o l i d , 2, i n contact with any l i q u i d , 1, w i l l seldom be very large; for a hydrocarbon on a fluorocarbon, i n the computations given above, i t turned out to be at most 1.25. There w i l l be no more than a small f r a c t i o n of the s i t e s for which the energy term i n Equation 12 or 17 i s , for geometric reasons, very much larger than that computed above. And f o r elongated molecules on such s i t e s , i t has already been noted that the angular entropie term (the l a s t term i n Equations 12 and 17) w i l l be much more negative, because to occupy such a high-energy s i t e , an extended molecule w i l l have at most two configurations access i b l e to i t . Hence, the coverage of an e s s e n t i a l l y Β
Θ
a
a
o r
a
w
e n e r
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
40
ADSORPTION
AT
INTERFACES
f l a t surface w i l l not be s e r i o u s l y l a r g e r than that c a l c u l a t e d here. Of course, surfaces can be prepared which are very much more heterogeneous, f o r geometric reasons, than j u s t indicated, e.g. by abrasion. But as pointed out by Neumann and Good (20), such surfaces are not suitable for contact angle measurements ; and the r e p r o d u c i b i l i t y of data obtained would be very poor, and the hysteresis extremely large. Also, the heterogeneity of a surface that has been modified, by p a r t i a l oxidation or by g r a f t i n g short hydrophilic chains onto i t , w i l l be much more serious than that of an unmodified surface. The f i l m pressure on such surfaces i s a very d i f f e r e n t problem from that on an e s s e n t i a l l y homogeneous low-energy surface and i t w i l l not be discusse The term Rln(W /WL) rigorou expressio the change i n entropy associated with angular conf i g u r a t i o n s . The approximation of evaluating i t as Rln(3d/4£) has already been discussed. The error thus introduced r e s u l t s i n p r e d i c t i o n of too high an adsorption; so the conclusion as to the n e g l i g i b l e value of ir f o r higher hydrocarbons i s not weakened by i t . Regarding the expressions containing the €'s, e.g. Equation 10, i t may be seen at once that € ^ i s by f a r the most important parameter, because the r a t i o , la/ 1L generally small, e.g. 3/12, or r a r e l y larger than perhaps, 5/11. Since i s evaluated from AUÏ, i t i s c l e a r that the dominant energetic component of the computation i s the heat of vapori z a t i o n of the l i q u i d . There i s considerably less energy than A U "recovered , on adsorption, because the coordination number i s so much lower, f o r the adsorbed molecule with the groups i n the adsorbent surface. The estimation of €^2 from >/€^^Z2 3 i ° 9, i s i f anything, on the high side, even f o r systems where the cohesion of the bulk l i q u i d and of the s o l i d are both dominated by the London force. A more exact expression, replacing Equation 9, i s a
e
Z
Z
i s
V
11
9
€
1 2 = * - ^ Λ 2 "
E
where Φ can be computed a p r i o r i , from molecular properties (10), e.g., by Equation 22 f o r nonpolar molecules. The fact that we express the €'s i n terms of A U s (e.g. Equations 11 to 17) i s , i n a p r a c t i c a l V f
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
3.
GOOD
Equilibrium
Film Pressure
41
sense, a strong point of this theory. We have already noted the assumptions m a d e — s e e the paragraph preceding Equation 11. The main factor leading to inequality of z^j and (see above) i s , differences i n decree of expansion or the l i q u i d s , considered as f r a c t i o n of q u a s i - l a t t i c e s i t e s that are vacant, at the temperatures where the heats of vaporization are measured. Since the b o i l i n g point i s , to a f a i r approximation, a "corresponding temperature within the meaning of the theory of corresponding states, we can conclude the equality of coordination numbers i s a good approximation. Even i f these errors do not cancel t o t a l l y , they should do so p a r t i a l l y . And since this r a t i o appears under a square root sign Equation 12 and then i s m u l t i p l i e d by a facto the s e n s i t i v i t y of y i s small. Equations 16 and 17 introduce approximations needed to treat highly asymmetric molecules. The asymmetry cannot be handled i n terms of any e x i s t i n g , macroscopic treatments which use heats of vaporization d i r e c t l y . These assumptions are, however, a p r i o r i , reasonable, and so can be counted on as leading to a v a l i d p r e d i c t i o n of the trends. Equations 18 to 22 are based on Good and Elbing (10) and (as already noted) on the e a r l i e r treatments of Good and G i r i f a l c o (2.,Z>18) and Fowler and Guggen heim (17). A notation basecPon that of Fowkes (21) has been employed; but the quantities are not obtain able from empirical treatment of surface tension data, as i s the case with Fowkes Y '. Indeed, the dispersion component of the t o t a l surface energy can not i n general be evaluated from contact angle data, because of the lack of thermodynamic uniqueness of the U function, i n a binary system. (It i s only when there i s no i n t e r f a c i a l excess mass of e i t h e r . component that U i s unique. In general, U ( '£ U ( ), where the superscript designates the Gibbs d i v i d i n g surface: (1) r e f e r s to the surface located such that Γ\ = 0, and (2), to the Γο = 0 surface.) Thus, i s a function which has physical meaning only i n terms of the components of intermolecular a t t r a c t i o n constants as computed from molecular properties, e.g. by Equation 18 and 19. The c o e f f i c i e n t , 0.2, i n Equation 24, arises from t h i s treatment of the energy components f o r i n t e r a c t i o n of water and a nonpolar s o l i d . I t s use i n the theory for water depends f o r accuracy, not so much on the v a l i d i t y of Equation 18 or 19, as on the assumptions 11
1
lf
df
s
s
S
L
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
s
2
42
ADSORPTION
AT
INTERFACES
about coordination number and second-nearestneighbors, which j u s t i f y the use of the r a t i o of^ energies of vaporization i n Equation 24. We estimate the uncertainty here, as no worse than 50% i n ^i\l^22 which means an uncertainty that i s less than 25% a f t e r the square root i s extracted. The q u a l i t a t i v e conclusion that x and rr are n e g l i g i b l e , f o r H2O on Teflon and polyethylene, are not s e r i o u s l y changed by allowance for such uncertainty, because A U i s so large, f o r water, and as a consequence, the computed values of x and ΊΤ are so small. In summary, the approximations of t h i s theory are probably not serious; and indeed, the majority of the approximations contribute errors i n the d i r e c t i o n of too high an estimate of x and ττ · It would take very much greater c o r r e c t i o l i k e l y to appear, i the q u a l i t a t i v e conclusion that x and ττ are generally small for l i q u i d s that b o i l above room temperature. It must be emphasized, however, that these compu tations are not intended as quantitative predictions of x arri 7r for these systems. The approximations made above, including those about structure of the s o l i d surface, are s u f f i c i e n t l y serious that quanti tative agreement cannot be expected. But for the p r e d i c t i o n of trends, as with the n-alkanes, and for the conclusion that ττ i s small for high b o i l i n g l i q u i d s , the approximations should not detract from the v a l i d i t y of this theory. (b) Comparison with Experiments: i r for Organic Compounds on Teflon. Graham (4) has found ir to be much smaller, f o r η-octane on powdered Teflon TFE, than for alkanes having surface tension below Y , e.g. butane. ir has been reported to be large f o r low-boiling gases such as N on Teflon. Graham (4) noted a d i f f i c u l t y a r i s i n g from bulk s o l u b i l i t y of an alkane i n the substrate, such that the quantity sorbed could not be uniquely assigned as between adsorption and absorption. (This trouble was not encountered by Whalen and Wade (5,6.)). Graham concluded that his value of ir for octane on Teflon, about 1.7 erg/cm^, was an upper l i m i t ; he could not estimate the r e a l value. Wade and Whalen (5,j5) also used a powder form of Teflon, and also oBtained a small value for i r : 3.3 f o r hexane and 2,9 for octane. They e x p l i c i t l y corrected for pendular r i n g condensation, and i n so doing, they obtained estimates of ir i n t h e i r systems that were very probably more v a l i d than Graham's were f o r h i s . Our computations are lower than these experimental r e s u l t s y
a
e
V
a
θ
a
a
e
e
e
c
e
2
e
e
e
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
3.
Equilibrium
GOOD
Film Pressure
43
e.g. ττ =0.1 f o r hexane i n the absence of step s i t e s . The r e s u l t , 7 r =0.5 for Teflon with step s i t e s accounting for 5% of the area, shows the d i r e c t i o n of change i n the computed r e s u l t s when a more complex surface i s postulated. The preliminary computations made f o r CCl^ indicate that the e x p l i c i t employment of a two-dimensional van der Waals equation (11,12) would be worthwhile with the lower hydrocarbons. We w i l l investigate such two-dimensional condensation i n another communication. We note, however, that our equations have successfully predicted the observed trend of ir with chain length. As has already been noted, water adsorption measurements at Lehigh (19) have shown that the surface of Teflon powder i s heterogeneous and about 0.757 of the surface of the sampl which adsorbed wate t i o n of water, reported i n Table I I I , i s f o r the 99% of the surface exclusive of the hydrophilic s i t e s . The Lehigh measurements (19) could not q u a n t i t a t i v e l y d i s t i n g u i s h the increment of water adsorption due to t h i s f r a c t i o n of the surface. The water isotherm was described as resembling a type II isotherm, with surface area less than 1% of the nitrogen area, superimposed on a type I I I isotherm. We have not attempted to analyse the contribution to the adsorption of hydrocarbons or other nonpolar molecules, due to the hydrophilic s i t e s on Teflon. Wade (6) has discussed the evidence f o r interactions of high-energy s i t e s on Teflon with hydrocarbon molecules. Whalen (22) has recently measured the adsorption of water on Teflon powder, and estimated a value of 1.9 ergs/cm for ?r . He estimates the hydrophilic s i t e s on this s o l i d to account f o r 3%> of the surface area (6,23). Adsorption measurements on Teflon powders are of l i m i t e d d i r e c t pertinence to the contact angle question because there i s every reason to suspect that the surface of a powder i s quite s i g n i f i c a n t l y d i f f e r e n t from that of a smooth slab that i s suitable for contact angle measurement. This point i s hard to v e r i f y d i r e c t l y , however, because while electron microscopic examination of a grossly smooth s o l i d could reveal pores, i t would not reveal chemical heterogeneity, p a r t i c u l a r l y that i n the form of i s o l a t e d , high-energy s i t e s ; and the p r e c i s i o n measure ment of contact angle on a powder i s not easy. e
e
2
e
(c) Comparison With Adamson's Model, and Results For Water On Polyethylene.
His
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
44
ADSORPTION
AT
INTERFACES
(1) Theory. Our model i s , i n p r i n c i p l e , not incompatible with that of Adamson and Ling (3), i n that t h e i r isotherm graph, i n i t s lower pressure region, resembles a BET type I I isotherm. I t i s w e l l known that, by lowering the value of the " c " constant, a type I I I isotherm i s obtained which, i n i t s lowcoverage region, i s not experimentally d i s t i n g u i s h a b l e from the Henry's law region of a Langmuir isotherm. Figure 1 shows a schematic graph of the isotherm we propose. Since the coverage at saturation i s f a r below a monolayer, there i s no need to make any hypotheses about the structure of the absorbed mater i a l i n the region above monolayer coverage. (2) Experiment Adamson i n 1973 reported (8,9.) a value of 42 ergs/c These r e s u l t s appea I I I . They are also i n apparent c o n f l i c t with the w e l l known conclusion (24) that the Y polyethylene i s i n the neighborhood dF"31 ergs/cm . The use of Equation 28, [Y (cos0+l)- 7Γ 2 ] •Li e Y c
2
T
S
4Λ
\
(28)
Y (cose+i)
2
L
4Φ
2
with Φ about 0.9 to 0.95 (1,10) leads to an estimate of about 36 ergs/cm f o r Yg. I t would be rather anomalous f o r polyethylene i n water vapor to have a negative surface free energy Ygy, which would be obtained by subtracting 42 from"36 as Equation (1) would seem to prescribe. A r e c o n c i l i a t i o n can be achieved by hypothesizing that water on the polyethylene sample that was used (8.,9.) behaved i n a s i m i l a r fashion to water on Teflon TFE, (19) noted above. The water molecules might adsorb i n clusters on i s o l a t e d hydrophilic s i t e s , y i e l d i n g a f i l m with the observed average thickness and the reported ττ^. With respect to water adsorption, the e f f e c t i v e Y g of this s o l i d would then be, not the value derived from contact angles of nonpolar l i q u i d s , but a larger value: 2
Y
S
=
Y
+
S ^e(water) -36+42 , = 78 ergs/cm
(29)
2
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
3.
GOOD
Equilibrium
Film Pressure
45
Figure 1. Schematic: A possible isotherm for adsorption of a liquid such as carbon tetrachloride or hexadecane on a molecularly smooth, homogeneous solid such as Teflon TFE. Extrapohtions shown on log-log plot are speculation, and no physical reality is intended, particularly beyond the region where the curve starts to swing to the left.
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
46
ADSORPTION
AT
INTERFACES
1
I f the water c l u s t e r s on Adamson s p o l y e t h y l e n e were i s o l a t e d from each o t h e r , t h e i r p r e s e n c e would not reduce t h e water c o n t a c t a n g l e t o z e r o . And i n the h y d r o p h o b i c r e g i o n s between c l u s t e r s , t h e amount o f water adsorbed p e r u n i t a r e a , and ττ , would be n e g l i g i b l e — a s computed above, f o r T a b l e I I I . β
Conclusions The e q u i l i b r i u m f i l m p r e s s u r e , 7 r , s h o u l d be n e g l i g i b l e on a smooth, homogeneous s u r f a c e o f a lowenergy s o l i d such as T e f l o n , f o r most l i q u i d s t h a t form non-zero c o n t a c t a n g l e s , and p a r t i c u l a r l y f o r those w i t h h i g h h e a t s o f v a p o r i z a t i o n . ir s h o u l d be large f o r l o w - b o i l i n g substances such as N 2 ethane e t c . , on t h e s e s o l i d s d i c t i o n s are i n accor e
e
Acknowledgement T h i s work was s u p p o r t e d by t h e N a t i o n a l S c i e n c e F o u n d a t i o n under Grant GK10602. Literature
Cited
1.
Fox, H.W. and Zisman, W.A., (1950) 5, 514.
2.
Good, R.J. and (1960) 64, 561.
3.
Adamson, A.W., and L i n g , I . , in " C o n t a c t A n g l e , Wettability and A d h e s i o n " , p. 57, Advan. Chem. S e r No. 43, American C h e m i c a l S o c i e t y , Washington, D.C., 1 9 6 4 .
4.
Graham, D.P., J. Phys. Chem., (1965), 69, 4387.
5.
Wade, W.H., and Whalen, J.W., J . Phys. Chem. (1968), 72, 2898.
6.
Whalen, J.W., J . Colloid (1968), 28, 443.
7.
Good, R.J., in " C o n t a c t A n g l e , W e t t a b i l i t y and Adhesion", Advan. Chem. S e r . No. 43, American C h e m i c a l S o c i e t y , p. 74, Washington, D.C., 196 4 .
8.
Adamson, A.W., 4 7 t h N a t i o n a l Ottawa, Canada, June, 1973.
Girifalco,
J.
Colloid
Sci.,
L.A., J. Phys. Chem.,
and
Interface
Colloid
Sci.,
Symposium,
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
3.
9. 10.
GOOD
EquilibriumFilmPressure
47
Adamson, A.W., 167th National Meeting of the American Chemical Society, Los Angeles, April, 1974. Good, R . J . and Elbing, Ε . , Ind. Eng. Chem. (1970), 62, (3), 54.
11.
de Boer, J . H ., "The Dynamical Character of Adsorp tion," Oxford University Press, London, 1953.
12.
Ross, S., and O l i v i e r , J . P . , "0n Physical Adsorp tion," Interscience Publishers, New York, 1964.
13.
Hill, T . L . , "Introduction to S t a t i s t i c a l Thermo dynamics," Addison-Wesley Publishing Co., Reading, Mass., 1960.
14.
Everett, D.H., Proc. Chem. Soc., Feb., 1958, p. 57.
15.
Glasstone, S., L a i d l e r , K.J., and Eyring, Η., "Theory of Rate Processes," McGraw-Hill, N . Y . , 1941.
16.
C.R.C. Handbook, 51st ed., Chemical Rubber Publishing Co., Cleveland, Ohio, 1970.
17.
Fowler, R . H . , and Guggenheim, E . A . , " S t a t i s t i c a l Thermodynamics," Cambridge University Press, London, 1952.
18.
Berghausen, P . E . , Good, R.J., Kraus, G . , Podolsky, Β., and S o l l e r , W., "Fundamental Studies of the Adhesion of Ice to S o l i d s , " WADC Technical Report 55-44, 1955.
19.
Chessick, J.J., Healey, F . H . , and Zettlemoyer, A . C . , J . Phys. Chem. (1956), 60, 1345.
20.
Neumann, A.W. and Good, R.J., J . Colloid and Inter face Sci., (1972), 38, 341.
21.
Fowkes, F . M . , J. Phys. Chem. (1957), 61, 904.
22.
Whalen, J.W., Vacuum Microbalance Techniques, A.W. Czanderna, E d . , v. 8, p. 121, Plenum Press, 1971.
23.
Whalen, J.W., (personal communication,
24.
Zisman, W.A., i n "Contact Angle, Wettability and Adhesion," Advan. Chem. Ser., No. 43, pp. 1-51, American Chemical Society, Washington, D.C., 1964.
1974).
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
4 Binding of Solute and Solvent at the Interface and the Gibbs Surface Excess D. K. C H A T T O R A J and S. P. M O U L I K Department of Food Technology and Biochemical Engineering and Department of Chemistry, Jadavpur University, Calcutta-32, India
Introduction With the help of an imaginary mathematical plane placed at the interfacial region, a liquid-gas system, according to the Gibbs concept, may be divided into two phases. From the thermodynamic analysis of such a system, Gibbs also derived his well -known equation relating the surface excess of solute with the surface tension and bulk activity of the solute in solution. The physical concepts associated with the Gibbs surface excess have been examined more critically by Guggenheim and Adam (1) and also by Defay and Prigogine (2). Guggenheim (3) has also given an alternative derivation of the Gibbs adsorption equation assuming certain arbitrary but finite values for the physical thickness of the interfacial phase. In some cases, the interfacial thickness may be estimated from the experimental data (4). Goodrich (5) has recently analyzed the Gibbs adsorption equation with the help of an algebric method in which no mention is made of the mathematical dividing surfaces. Surface excesses of solute and solvent in a two-component system are, however, relative to each other and their values may be positive or negative. An attempt will be made in this paper to obtain relations between the surface excesses and the absolute amounts of solvent and solute bound to the interfacial layer. It is well-known that the surface tension of water increases with addition of various inorganic electrolytes to its bulk. In the dilute region of electrolyte concentrations, this increase is explained in terms of image forces (6^Z.>jL) * ^ extensive application of the Gibbs adsorption equation for the highly concentrated electrolyte solutions did not attract many workers because interpretation of the negative surface excess of electrolyte in terms of its interfacial adsorption is difficult. An attempt will also be made here to estimate the composition of the interfacial phase of an electrolyte solution from the observed negative value of the solute surface excess. T
48 In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
ie
4.
cHATTORAj A N D MOULiK
Binding of Solute and Solvent
Binding o f Two Components at the L i q u i d I n t e r f a c e Let us imagine t h a t a c e r t a i n amount of a l i q u i d component 1 i s mixed w i t h a d e f i n i t e amount of another l i q u i d or s o l i d component 2 as a r e s u l t o f which a b i n a r y s o l u t i o n i n the shape of a column (Figure 1) i s formed. With the help o f a plane set a r b i t r a r i l y at p o s i t i o n P j p j , t h i s column may be d i v i d e d i n t o two d i s t i n c t regions A and B. The exact nature and the charac t e r i s t i c f e a t u r e of t h i s d i v i d i n g plane w i l l be discussed l a t e r on i n d e t a i l . Region B, composed s o l e l y of the bulk l i q u i d phase, contains ηχ and T12 moles of components 1 and 2 respec t i v e l y . The components i n the bulk phase are s a i d t o remain i n the f r e e s t a t e of mixing. In region A, nj and τΐ2 moles of two components are present as a whole. Out o f these, ηχ and η 2 moles e x i s t i n g i n the f r e e s t a t e of mixing may form the bulk phase. Remaining Δ n with moles of componen 2 because of the existence of the i n t e r f a c i a l energy at the upper extreme top s i d e o f t h i s r e g i o n . They are termed to remain i n the bound s t a t e o f mixing and considered to form one square centimeter of an i n t e r f a c e . ?
Let Γ ι and Γ 2 be the r e l a t i v e excesses of components 1 and 2 r e s p e c t i v e l y i n r e g i o n A which may be d e f i n e d by the r e l a t i o n s (2) ,
1 *2
= n{ - n^
Τ
= n2 - ni jh_
2
...
χ
Γ^
...
(1) (2)
*i where and X2 are bulk mole f r a c t i o n s of the r e s p e c t i v e com ponents i n r e g i o n B. The concentrations o f s o l u t e and solvent i n the gas phase are neglected i n w r i t i n g Equations (1) and (2). Γ j and Τ2 w i l l be zero when the composition n^Av? of r e g i o n A becomes equal to the bulk composition X1/X2 of region B. Combining r e l a t i o n s (1) and (2), Equation (3) w i l l be obtained. f
Χ
χ
Τ2
+ X
Γ 2
1
= 0
...
(3)
For given values o f X^ and X2, the values o f ^ \ and Γ 2 are thus r e l a t e d t o each other according to Equation (3) and o b v i o u s l y the two excesses a r e o p p o s i t e i n s i g n . F u r t h e r , a t d e f i n t e values o f X^ and X2, n^ and n2 i n Equations (1) and (2) are not f i x e d but are s i g n i f i c a n t l y dependent on the a r b i t r a r y p o s i t i o n o f the d i v i d i n g plane. The absolute composition of the surface cannot t h e r e f o r e be expressed i n terms o f Γι, f
Γ2,
ni
f
a n c
* 2· η
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
50
ADSORPTION
AT
INTERFACES
Let us now assume f u r t h e r t h a t the d i v i d i n g plane at p-^p^ behaves as a semi-permeable membrane which allows o n l y the com ponents i n t h e i r f r e e s t a t e o f mixing t o pass from r e g i o n A to r e g i o n Β and v i c e v e r s a . However, the components i n t h e i r bound s t a t e o f mixing i n r e g i o n A are not allowed t o permeate through t h i s membrane. The chemical p o t e n t i a l M 2 °f component 2 i n r e g i o n A, remaining i n the f r e e s t a t e may be given by the r e lation, If
=
β 2
μ
2
+
R
T
l
n
f
2
n
2
Π n
=
M°
If +
l
n
2
+
(nj
-
&n{)+in
2
-
Δη ) 2
(4)
a r e
t n e
Here f and μ. r a t i o n a l a c t i v i t y c o e f f i c i e n t and standard chemical p o t e n t i a l o f component 2 i n the f r e e phase r e s p e c t i v e l y . The chemical p o t e n t i a l o f the same component i n r e g i o n Β i s s i m i l a r l y given by Equation ( 5 ) . 2
2
μ
=
2
μ
+
2
RT l n f X 2
...
2
(5)
From^the p r i n c i p l e o f membrane e q u i l i b r i u m , JLt i s equal t o U>2 t h a t combining Equations (4) and ( 5 ) , one w i l l o b t a i n 2
s o
n
2
nj
-
Δη
-
Δη*
X
2
...
2
(6)
X
1
Rearranging t h i s , n
2
-
n{
x
2
=
Δη
2
-
X
2
An[
...
(7)
Equation (7) i s s i m i l a r i n form to t h a t used by B u l l and Breese (9) f o r the c a l c u l a t i o n s o f the extents, o f s o l u t e and.solvent b i n d i n g on the p r o t e i n boundary. I n s e r t i n g Equation (7) i n Equation ( 2 ) ,
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
4.
CHATTORAJ
AND
Γ
=
2
Binding, of Solute and Solvent
MOULiK
Δη
2
-
An
x
x
2
51
...
(8)
I t may s i m i l a r l y be shovm t h a t
Γι
=
AnJ
-
Δη
*1_
2
...
(9)
X2 For given values o f and X , Δ η | and Δη? (and hence 1^ and Γ ) are not dependent on the p o s i t i o n o f the d i v i d i n g membrane. Absolute composition o f the bound phase a t the i n t e r face may be obtained i experimental data. Equation (2) may a l s o be w r i t t e n i n the form, 2
2
Γ
2
=
(Δη^ -
Δη[
X
2 x
}
l
+ (n' 2
X
2 j
..
(10)
X
From the comparison r e l a t i o n s (8) and (10), one w i l l a t once f i n d , X
η'' 2
η" 1
2
...
(10a)
X. 1
This means t h a t the mole r a t i o n o f the components i n the f r e e phases o f regions A and Β are i d e n t i c a l . I n the r i g h t s i d e o f Equation ( 2 ) , two equal and opposite v a r i a b l e s η and η Xo/Xj become i m p l i c i t l y i n c l u d e d owing t o the a r b i t r a r y placement o f the d i v i d i n g plane i n c l u d i n g a p a r t o f the bulk region. The numerical values o f these v a r i a b l e s then depend upon the p o s i t i o n o f t h i s d i v i d i n g plane. The terms i n r e l a t i o n s (8) and (9) are independent o f the p o s i t i o n o f the d i v i d i n g membrane. The Gibbs Adsorption Equation Let us now imagine t h a t the d i v i d i n g plane i s r a i s e d from p o s i t i o n p-jP-[ t o P P s l o w l y and r e v e r s i b l y , so t h a t a l l the f r e e components from r e g i o n A permeate t o region B. Region Β i s now occupying the t o t a l bulk o f the l i q u i d system whereas r e g i o n A, s o l e l y composed o f Δ η ^ and Δ η ^ moles o f the bound components, may be regarded t o form the surface phase o f one square centimeter i n t e r f a c i a l area. The s p e c i a l mixing o f the components a t the i n t e r f a c e i s o b v i o u s l y due t o the e x i s t e n c e 2
2
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
52
ADSORPTION
AT
INTERFACES
o f the i n t e r f a c i a l energy. In the absence of the imaginary membrane a l s o , the bound s t a t e should remain as a p h y s i c a l r e a l i t y because the i n t e r f a c i a l f r e e energy does not disappear under t h i s c o n d i t i o n . The surface bound phase may thus be imagined t o be separated e n t i r e l y from the l i q u i d i n b u l k , by an imaginary p h a s e - d i v i d i n g plane placed at the p o s i t i o n P P 2 Applying the Gibbs-Duhem r e l a t i o n s f o r the surface (bound) and bulk phases separated w i t h such a plane set a p p r o p r i a t e l y , we can o b t a i n the f a m i l i a r expressions f o r the Gibbs adsorption equation which at constant temperature and pressure w i l l read (3), ?
2
Γ
2
=
( Δη
=
f
-
2
x
Δη|
2
)
2
RT
d f X 2
2
and (
Δ
Δη.
Π ι
X
=
- f x x
RT
2
dy
x
d
... X £ 1
(12)
1
Here y stands f o r the s u r f a c e t e n s i o n o f the l i q u i d mixture corresponding t o a given bulk mole f r a c t i o n X o f the s o l u t e , f l and f , the r a t i o n a l a c t i v i t y c o e f f i c i e n t s of the s o l v e n t and s o l u t e r e s p e c t i v e l y , R the gas constant and Τ the absolute temperature. In the conventional forms of the Gibbs equation, Γι and Γ are r e l a t e d to n j and n according to Equations (1) and (2) r e s p e c t i v e l y which on t h e i r t u r n depend s i g n i f i c a n t l y on the p o s i t i o n of an a r b i t r a r y d i v i d i n g plane P i P i . Δηj and Δη are, however, independent of the p o s i t i o n o f the d i v i d i n g plane. 2
2
2
2
1
2
Evaluations of
^2
The surface t e n s i o n , y , o f water u s u a l l y increases w i t h increase i n c o n c e n t r a t i o n o f an i n o r g a n i c e l e c t r o l y t e provided the s o l u t e content i n the aqueous s o l u t i o n i s high. For the present a n a l y s i s , the data on the surface t e n s i o n o f water f o r d i f f e r e n t high concentrations of an e l e c t r o l y t e have been c o l l e cted form the l i t e r a t u r e . These e l e c t r o l y t e s are the f o l l o w ing: L i C l (10) 25°C; NaCl (11) 25°C; KC1 (12) 25°C; NaBr (10)
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
4.
CHATTORAJ
AND
53
Binding of Solute and Solvent
MOULiK
25°C; NaN0 (10) 20°C; K S 0 (12) 25°C; M g C l (10) 20°C; MgS0 (13) 25°C and A l (S04)3 (10) 25°C The surface t e n s i o n data of n o n - i o n i c sucrose (10) s o l u t i o n s have a l s o been considered here s i n c e t h i s s o l u t e i s observed t o increase the boundary t e n s i o n o f water. The p r a c t i c a l a c t i v i t y c o e f f i c i e n t s (on the m o l a l i t y s c a l e ) f o r v a r i o u s concentrations o f these e l e c t r o l y t e s and sucrose can be obtained from the l i t e r a t u r e (14). The values o f t h e corresponding r a t i o n a l a c t i v i t y c o e f f i c i e n t s ( f ) of these s o l u t i o n s may e a s i l y be c a l c u l a t e d u s i n g a p p r o p r i a t e conversion f a c t o r s (14). Here water and s o l u t e stand f o r com ponents 1 and 2 r e s p e c t i v e l y . For each s o l u t e , y has been p l o t t e d against f X and t h e slopes d y /d (f2X2) o f curve f o r v a r i o u s v a l u e s o f f2X2 have been c a l c u l a t e d u s i n g the chordarea method o f slope a n a l y s i s (15). The negative surface excesses - Γ 2 , o f the e l e c t r o l y t e s f o r v a r i o u s values o f X have been c a l c u l a t e d w i t h t h e hel and 5, - Γ 2 f o r v a r i o u against the mole r a t i o X / X i . The open and c l o s e d signs i n the f i g u r e s represent data obtained from experiments and from i n t e r p o l a t i o n o f 7-f2X2 curves, r e s p e c t i v e l y . 3
2
4
2
4
2
2
t
2
n
e
2
2
Solute-Solvent Binding C h a r a c t e r i s t i c s I t i s o f i n t e r e s t t o note t h a t i n a l l these p l o t s i n Figures 2, 3, 4 and 5 increases l i n e a r l y w i t h i n c r e a s i n g mole r a t i o , X 2 / X 1 , provided the s o l u t e content i n the s o l u t i o n i s not too high. The slope and i n t e r c e p t o f such l i n e a r p l o t s may e v i d e n t l y represent the r e s p e c t i v e values o f Δ η ι and Δ 2 according t o Equations (8) o r (11). The l e a s t square values o f Δηχ and Δ η f o r v a r i o u s systems are presented i n Table I along w i t h t h e i r r e s p e c t i v e s t a n d a r d d e v i a t i o n s . From t h e values o f Δ η ^ i n Table I , i t may be observed t h a t one square centimeter o f a s o l u t i o n i n t e r f a c e may bind s i g n i f i cant amount o f water whose magnitude may be o f t h e order 1 0 " ° t o 10" moles. Δη{ f o r a l k a l i c h l o r i d e s f o l l o w the order: L i C l < NaCl < KCl. The water b i n d i n g c a p a c i t i e s o f the i n t e r face i n the presence o f u n i - u n i v a l e n t sodium s a l t s (NaCl, NaBr and NaN03) do d i f f e r from each other widely. The magnitudes of Δ η ^ f o r u n i - u n i v a l e n t and u n i - b i v a l e n t s a l t s ( K S 0 , M g C l ) are a l s o comparable t o each other. However, Δ η ^ f o r p o l y v a l e n t e l e c t r o l y t e s , A l ( S 0 ) 3 and MgS0 , are s i g n i f i c a n t l y high. The agreement o f t h e order o f Δ η ! f o r v a r i o u s e l e c t r o l y t e s w i t h that t o be expected from the t y o t r o p i c s e r i e s i s o n l y p a r t i a l . From Figures 2, 3, 4 and 5, i t may a l s o be noted t h a t the p l o t o f - Γ against Χ /Χχ d e v i a t e s from l i n e a r i t y f o r L i C l , NaBr (not shown i n f i g u r e ) , NaNU3, K S 0 (not shown), and A l (S0 )3 when t h e mole r a t i o n r e l a t i v e l y high. This may i n d i c a t e t h a t t h e surface bound components Δη^ and Δ η are not constant but become v a r i a b l e f u n c t i o n s o f X A i a t r e l a t i v e l y higher s o l u t e c o n c e n t r a t i o n s . When X A l f o r NaNU3, K S 0 , K C l , η
2
f
n
o
t
2
2
2
4
4
2
4
2
2
a
r
4
2
e
4
2
2
2
2
4
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
5.57
KC1 (cone)
(b)
3.74
K2SO4 (cone)
155
MgSÛ4 (cone)
00
52.7
MgS0 ( d i l )
8 (a)
4
MgCl
7
(b)
6.66
4.71
K2SO4 ( d i l )
6 (a)
2
8.36
NaNU3 (cone)
00
3.28
NaN0 ( d i l )
5 (a)
3.06
NaBr
4
3
5.05
KC1 ( d i l )
3 (a)
3.70
NaCl
2
2.95
+
+
+
+
+
+
+
+
+
+
+
+
9
0.83
0.90
0.29
0.04
0.96
0.70
0.07
0.05
0.18
0.01
0.07
0.31
2
A n j χ 10 moles/cm
LiCl
Electrolytes
1
Serial No.
-0.05
0.10
0.51
-0.01
-0.08
57.7
-0.39
0.01
-1.51
-1.31
-0.15
0.34
+
+
+
+
+
+
+
+
+
+
+
+
0.61
0.35
0.23
0.03
0.12
9.24
0.08
0.08
0.79
0.39
0.12
1.04
2
1 1
A n j x 10 moles/cm s
Δη
93.0
31.6
3.99
2.24
2.82
3.45
1.97
1.83
3.33
3.02
2.21
1.77
L
Table I . Least Square Values o f Δ η { and
0.069
^ J L Δ ηj
2
s
3.90
m
S
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
s
2.60 9.00
2.62 +_ 1.70 18.8 + 1.86
15.1 +_ 0.89
Sucrose
19.2
L
4.34 +_ 1.73
1 1
Sucrose ( d i l )
(cone)
(cone)
*not l e a s t square v a l u e s .
(b)
10(a)
0
2
χ 10
moles/cm
2
10.5
32.0
2
Δη
4.66 + 4.23
3
Al2(S04)
(b)
*(dil)
9
mo l e s / c m
Δ n{ χ 1 0
17.6 +_ 2.92
3
4
A1 (S0 )
2
Electrolytes
9(a)
No.
Serial
Table I . (Contd.)
1
2
s
0.33 0.69
0.0124
0.14
m
0.0060
0.0026
Δη 1
Δ n
ο
a
Si"
IT
56
ADSORPTION
P
2
AT
INTERFACES
y///////7777}\ P2
p; θ
Figure 1. Hypothetical binary solution in the shape of a column
5 10
10 20
15 50
20 40
( X /X|> X I 0 2
25 SO
90 60
I Π
3
Figure 2. Surface excess of solute as a function of mole ratio. 1: KCl (Scale I-I). 2: KCl (Scale I-II). 3: MgSO (Scale II-II). 4: MgSO (Scale I-I). h
/f
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
CHATTORAJ
AND
Binding of Solute and Solvent
MOULiK
CM
(X /X,
Figure 3.
) Χ ΙΟ
Surface excess of solute as a function of mole ratio. 1-4: LiCl, NaBr, NaCl, MgCl . 2
I
π
m
-3 -60 L
Figure 4. Surface excess of solute as a function of mole ratio. 1: NaN0 (Scale II-II). 2: NaNO, (Scale I-I). 3: K SO (Scale III-III). 4:K,SO,,(ScaleUl-l). 3
2
h
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
58
ADSORPTION
AT
Figure 5. Surface excess of solute as a function of mole ratio. 1: Sucrose (Scale I-I). 2: Sucrose (Scale II-I). 3 and 4: Al (SO., ), (Scale I-I). 2
t
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
INTERFACES
4.
CHATTORAJ
AND
MOULiK
Binding of Solute and Solvent
59
MgSU4, A l (S04)3 and sucrose are very h i g h , - Γ v s X / X i p l o t s become again l i n e a r having d i f f e r e n t slopes and i n t e r cepts. The l e a s t square values o f Δ I L and Δ η , c a l c u l a t e d again from Equation (11) a t these regions o f h i g h s o l u t e con t e n t , are a l s o i n c l u d e d i n Table I . Compared t o Δ η | , values o f Δ η f o r d i f f e r e n t e l e c t r o l y t e s are indeed q u i t e s m a l l . Δ η o f NaNC3 (concentrated) i s very h i g h and the s u r f a c e m o l a l i t y (m ) i n t h i s case i s as h i g h as 4 molal compared t o i t s bulk m o l a l i t y v a r y i n g between 5 t o 10. For A l (SU4)3 and sucrose, Δ η i s s i g n i f i c a n t and m l i e s between 0.10 t o 0.70. For a l l other cases, Δ η and m are l e s s than 1 0 " and 1 0 ~ r e s p e c t i v e l y . The observed experimental e r r o r f o r the e v a l u a t i o n o f Δ η are q u i t e high so t h a t the signs and magnitudes o f Δ η f o r these e l e c t r o l y t e s have no p h y s i c a l meaning. For these e l e c t r o l y t e s one may e a s i l y neg l e c t Δ η i n Equatio i n the e v a l u a t i o n o f Δ η ^ . £ni i s o f the order 10 and i t s signs are p o s i t i v e both i n the nigh and low ranges o f mole r a t i o . Δ η | f o r low and high KCl concentrations are found t o be 5.0 χ 10"" and 5.6x10 moles per u n i t area r e s p e c t i v e l y . Since negative values f o r Δη do not bear any p h y s i c a l meaning, probably Δ η i n Equation (11) f o r KCl i s a l s o n e g l i g i b l e but Δ η | instead of remaining constant s l o w l y i n c r e a s e s w i t h gradual increase o f X / X j . The i n i t i a l slope r e p r e s e n t i n g Δ n-^ o f KCl i n a short range o f e l e c t r o l y t e c o n c e n t r a t i o n w i l l not be, however, t o o f a r o f f from the observed v a l u e 5.0 χ 10"^. In g e n e r a l , we conclude t h a t o n l y a small amount o f e l e c t r o l y t e i s a s s o c i a t e d w i t h the surface-bound water even when a l a r g e amount o f the same e l e c t r o l y t e remains d i s s o l v e d i n the bulk water phase. I n t e r f a c i a l water f o r these cases i s h i g h l y s u r f a c e - a c t i v e and i t has a strong d i s l i k e f o r i n o r g a n i c e l e c t r o l y t e s and sucrose. The r e l a t i v e p o s i t i v e surface excess o f water i n the con v e n t i o n a l i n t e r p r e t a t i o n o f the Gibbs equation may be i d e n t i f i e d w i t h the amount o f water adsorbed on one square centimeter o f the l i q u i d i n t e r f a c e when the adsorbed s o l u t e i s a r b i t r a r i l y taken as zero. In many o f the cases considered here, Δ η i s i n f a c t n e g l i g i b l y small so t h a t according t o Equations ( 3 ) , (8) and ( 9 ) , Γι i s v e r y c l o s e t o Δ η | . R e s u l t s i n Table I f o r NaN03 and a few e l e c t r o l y t e s a l s o i n d i c a t e t h a t Δ η ^ may be s i g n i f i c a n t l y g r e a t e r thann i f Δ η i s not n e g l i g i b l y s m a l l . F u r t h e r , the r e l a t i v e excess r which i s negative i s observed t o increase c o n s i d e r a b l y w i t h increase o f Χ?/Χχ. Under these c o n d i t i o n s , e l e c t r o l y t e a c t u a l l y bound, Δ η , i s e i t h e r n e g l i g i b l y small o r s i g n i f i c a n t l y p o s i t i v e and at c e r t a i n range o f e l e c t r o l y t e c o n c e n t r a t i o n , i t becomes independent o f the mole r a t i o X /X^. Assuming the e f f e c t i v e c r o s s - s e c t i o n a l area o f a water molecule t o be 1 nm a t the i n t e r f a c e , the moles ( Δη·^) o f water r e q u i r e d t o form one square centimeter (6) o f an i n t e r 2
2
2
2
2
2
s
2
2
s
2
11
s
2
2
2
2
9
2
2
2
2
2
2
2
2
2
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
60
ADSORPTION
AT
INTERFACES
9
f a c i a l monolayer i s 1.7 χ 10" . Neglecting the c o n t r i b u t i o n o f Δ η , the number ( L ) o f water l a y e r s i n the bound i n t e r f a c i a l region may be estimated from the r a t i o Δη·[/ Δ η ^ . L f o r v a r i o u s e l e c t r o l y t e s and sucrose are a l s o included i n Table I . The r e s u l t s i n d i c a t e that the surface bound water i s m u l t i molecular f o r a l l s o l u t e s . For u n i - u n i v a l e n t and u n i - b i v a l e n t e l e c t r o l y t e s , as w e l l as f o r non-ionic sucrose, the number o f such surface bound l a y e r s may l i e between the values 2 t o 6. Values o f L , f o r A l ^ 0 4 ) 3 and sucrose (concentrated) are moderately high whereas those f o r MgS0 i n both high and low concentration ranges are u n u s u a l l y l a r g e . Higher values o f L f o r p o l y v a l e n t e l e c t r o l y t e s may be r e a l . I t may a l t e r n a t i v e l y be p o s s i b l e that these p o l y v a l e n t e l e c t r o l y t e s undergo consider able h y d r o l y s i s o r i o n - a s s o c i a t i o n a t high s o l u t e concentrations. Under these c o n d i t i o n s , s e v e r a l types o f s o l u t e components w i l l e x i s t i n the system s needed f o r the c a l c u l a t i o o f the Gibbs adsorption equation (16). 2
s
s
s
2
4
s
Conclusions I t i s evident from a l l these d i s c u s s i o n s t h a t a l i q u i d column i n Figure 1 may be d i v i d e d by a phase demarcating plane 2 2 every part o f the bulk phase below t h i s plane has the mole r a t i o composition Χ /Χι· Above t h i s plane, t h e mole r a t i o o f any part o f the surface-bound l i q u i c i phase mav deviate from Χ /Χχ because o f the b i n d i n g o f Δ η ^ and Δ η moles o f the components. S i m i l a r r e a l concept f o r the surface phase has a l s o been discussed by others (_2, 3 ) . With the help of the Gibbs Equations (11) and (12), the a c t u a l composition o f the surface region i n terms o f Δ η * and Δ η may be evaluated from the surface t e n s i o n - c o n c e n t r a t i o n data. This e v a l u a t i o n i s only p o s s i b l e when the composition o f the surface bound l i q u i d phase becomes independent o f the bulk mole r a t i o so that - Γ becomes a l i n e a r f u n c t i o n o f Χ /Χι· The l i q u i d column i n Figure 1 may f u r t h e r be d i v i d e d by p l a c i n g a semipermeable membrane a t an a r b i t r a r y p o s i t i o n P^Pj so that region A now contains the bulk and the surface-bound phases r e s p e c t i v e l y below and above the phase d i v i d i n g plane P P . The r e l a t i v e surface excesss Γ y thus be shown t o be equal t o e i t h e r o f the expressions o c c u r r i n g on the l e f t or r i g h t s i d e o f Equation (7). The magnitudes o f the terms n j and n^ i n t h i s equation depend s i g n i f i c a n t l y on the a r b i t r a r y p o s i t i o n o f P-^Pjwhereas those o f Δ η | and Δ η are i n v a r i a n t t o ΡχΡχ. In the conventional d e r i v a t i o n o f the Gibbs adsorption equation, sometimes the p h a s e - d i v i d i n g plane i s a r b i t r a r i l y s h i f t e d from i t s f i x e d p o s i t i o n P P t o a v a r i a b l e p o s i t i o n 1 1* r e g i o n o f A i s then a r b i t r a r i l y imagined t o be the surface phase composed o f n j and n moles o f the components. P
P
s u c n
t n a t
2
2
2
2
2
2
m
2
a
2
2
2
2
P
P
T
n
e
2
w n o l e
2
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
4.
CHATTORAJ
AND
MOULiK
Binding of Solute and Solvent
61
The i n d i v i d u a l values o f n] and n^ w i l l not bear any p h y s i c a l s i g n i f a n c e . However, the r e l a t i v e surface excess Γ2 defined i n terms o f n^ and n^ according t o Equation (2) may be c o r r e c t l y c a l c u l a t e d from the 7-X2 data s i n c e the r e l a t i v e surface ex cess does not depend on the p o s i t i o n o f Ρ ι Ρ χ . The i n v a r i a n t character o f the r e l a t i v e excess may be c l e a r from the c l o s e s c r u t i n y o f Equations ( 7 ) , (10) and (10a). U n l i k e t h e long-chain i o n s , the c a t i o n s and anions o f the i n o r g a n i c e l e c t r o l y t e s are not s p e c i a l l y adsorbed and o r i e n t e d at the i n t e r f a c e o f water. Both surface and bulk phases d i v i d e d by the plane P2P2 should be i n d i v i d u a l l y e l e c t r o n e u t r a l (2) i n terms o f the net charge o f c a t i o n s and anions. With the help o f the Gibbs Equation (11), one can o n l y c a l c u l a t e the macroscopic composition o f the surface phase i n terms o f the water and e l e c t r o l y t e components bound t o each other The m i c r o s c o p i c f e a t u r e of the i o n i c double l a y e i n the i n t e r f a c i a l r e g i o analysis. From the values o f L i n Table 1, i t may be concluded that the i n t e r f a c e s o f d i f f e r e n t e l e c t r o l y t e s o l u t i o n s u s u a l l y poss ess p h y s i c a l t h i c k n e s s o f the order 1 t o 10 nm. This order i s i n agreement w i t h t h a t suggested by Guggenheim (17) f o r the t h i c k n e s s o f the p h y s i c a l l y defined surface phase. In c o n t r a s t to the uniform composition o f the bulk r e g i o n , the s o l u t e and solvent bound s u r f a c e r e g i o n should be p h y s i c a l l y inhomogeneous (17, 18). Assuming t h i s p h y s i c a l p i c t u r e o f the surface and bulk phases, Guggenheim (17, _18) has deduced the Gibbs a d s o r p t i o n equation which i s s i m i l a r i n form t o Equations (11) and (12). However, the i n d i v i d u a l amounts o f the s o l u t e and s o l v e n t com ponents bound a t the i n t e r f a c e i n h i s treatment remain a r b i t r a r y and u n c e r t a i n s i n c e the bulk phase i s separated from surface phase by a r b i t r a r y placement o f a d i v i d i n g plane. From the d i s cussions presented above, i t i s noted t h a t Δ η } and Δ η ^ a r e independent o f the imaginary placement o f the d i v i d i n g plane. T h e i r values are absolute and under c e r t a i n c o n d i t i o n s may be evaluated w i t h i n the l i m i t s o f experimental e r r o r . In a separate paper i t w i l l be shown t h a t t h e present approach can a l s o be use f u l t o c a l c u l a t e the i n t e r f a c i a l b i n d i n g o f components r e s u l t i n g from the mixing o f an organic l i q u i d w i t h water. s
Acknowledgement The authors a r e g r a t e f u l t o Dr. Κ. K. Kundu f o r many h e l p f u l discussions. Literature Cited 1.
Guggenheim, E.A. and Adam, N.K., Proc. Roy. Soc. London, Ser. A, (1933) 139, 231.
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
62
2.
3. 4. 5. 6.
7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18.
ADSORPTION AT INTERFACES
Defay, R., P r i g o g i n e , I . , Belleman, A. and E v e r e t t , D.H., "Surface Tension and A d s o r p t i o n " , John Wiley and Sons, Inc., New York, 1966. Guggenheim, E.A., Trans. Faraday Soc., (1940), 36, 397. C h a t t o r a j , D.K. and C h a t t e r j e e , A.K., J . C o l l o i d I n t e r f a c e Sci., (1966) 21, 159. Goodrich, F.C., Trans. Faraday Soc., (1968) 64, 3403. Randles, J.E.B., i n "Advances in E l e c t r o c h e m i s t r y and E l e c t r o c h e m i c a l E n g i n e e r i n g " , P. Delahay, Ed., V o l . 3, p.1, I n t e r s c i e n c e , New York 1963. Onsager, L. and Samaras, N.N.T., J. Chem. Phys., (1934) 2, 528. Buff, F.P. and Stilbinger, F.H., Jr., Chem. Phys., (1956) 25, 312. B u l l , H.B. and Breese Arch Biophys Biochem. (1970) 137, 299. "Hand Book o f Chemistry and P h y s i c s " , 47t ed., F25, The Chemical Rubber Company, C l e v e l a n d , Ohio, 1966-67. Jones, G. and Ray, W.A., J . Amer. Chem. Soc., (1941) 63, 3262. Jones, G. and Ray, W.A., J . Amer. Chem. Soc., (1937) 59, 187. Jones, G. and Ray, W.A., J . Amer. Chem. Soc., (1942) 64 2744. Robinson, R.A. and Stokes, R.H., " E l e c t r o l y t e S o l u t i o n s " , 2nd ed. ( r e v i s e d ) , B u t t e r w o r t s , London, 1959. C h a t t e r j e e , A.K. and C h a t t o r a j , D.K., J . C o l l o i d I n t e r f a c e Sci., (1968) 26, 140. C h a t t o r a j , D.K. and P a l , R.P., Indian J. Chem., (1970) 10, 417. Guggenheim, E.A., "Thermodynamics", North H o l l a n d , Amsterdam, 1961. Adam, N.K., "The P h y s i c s and Chemistry o f S u r f a c e s " , Dover P u b l i c a t i o n s , New York, 1968.
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
5 Mechanism o f Sulfonate A d s o r p t i o n at the Silver Iodide-Solution Interface K. OSSEO-ASARE, D. W. F U E R S T E N A U , and
R. H . O T T E W I L L *
Department of Materials Science and Engineering, University of California, Berkeley, Calif. 94720
Introduction The e a r l y work on silver i o d i d e d i s p e r s i o n s (1) showed that the s o l s prepared in the presence of an excess of i o d i d e ions were more s t a b l e than those prepared w i t h silver ions present in ex cess. This was found to be due to the f a c t that the p o i n t of zero charge d i d not c o i n c i d e w i t h the equivalence p o i n t , i.e. in the presence of potential determining ions only, the equivalence p o i n t was found to be at pAg 8 whereas the net charge on the s u r f a c e became zero at pAg 5.5. Historically silver i o d i d e has been clas sified as a hydrophobic sol, a definition l a r g e l y based on the f a c t that silver i o d i d e s o l s are not r e v e r s i b l e , i.e. coagulated s o l s cannot be redispersed by diluting w i t h water (see Frens and Overbeek (2)). R e c e n t l y , s t u d i e s of the a d s o r p t i o n of water vapor on silver i o d i d e powders have shown t h a t the s u r f a c e is a l s o hydrophobic in terms of the u s u a l definition of wettability. Zettlemoyer et al. (3) found that the s u r f a c e area r a t i o s water/ argon and water/nitrogen were much l e s s than u n i t y and concluded that approximately three out of five s u r f a c e s i t e s were hydro phobic; they a t t r i b u t e d the h y d r o p h i l i c - h y d r o p h o b i c balance to oxide i m p u r i t i e s . The r e s u l t s of P r a v d i c and M i r n i k (4) showed that at pI 5 (pAg 11) and pH 5 hexylamine can reverse the charge of a silver iodide particle. This behavior i s in marked c o n t r a s t to the e f f e c t of alkylamine on q u a r t z , a h y d r o p h i l i c s u b s t r a t e , where as far as a d s o r p t i o n is concerned the octylammonium i o n appears to behave in the same way as an ammonium i o n ( 5 ) . Long chain alkyl sulfates, even at low c o n c e n t r a t i o n s , a l s o a f f e c t the z e t a p o t e n t i a l of the silver i o d i d e s u r f a c e ( 6 ) . Moreover, according to B i j s t e r b o s c h and Lyklema (7) even short c h a i n a l c o h o l s , e.g. η-butyl, adsorb w i t h t h e i r hydrophobic p a r t s towards the silver i o d i d e . Recently measurements of the contact angle of silver i o d i d e surfaces in water have shown that values of ca. 23° can be obtained (8,9). These f a c t o r s all suggest t h a t there i s strong hydrophobic i n t e r a c t i o n between hydrocarbon chains and 63
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
64
ADSORPTION
AT
INTERFACES
s i l v e r i o d i d e surfaces which can play an important r o l e i n s i l v e r i o d i d e - s u r f a c e a c t i v e agent s o l u t i o n i n t e r a c t i o n s . In a d d i t i o n , recent s t u d i e s u s i n g e l e c t r o n microscopy have shown that a l k y l p y r i d i n i u m compounds appear t o undergo a chem i c a l r e a c t i o n w i t h s i l v e r i o d i d e surfaces at s u r f a c t a n t concen t r a t i o n s approaching the c r i t i c a l m i c e l l e c o n c e n t r a t i o n (10). I t appears t h e r e f o r e that the a d s o r p t i o n of surface a c t i v e agents at a s i l v e r i o d i d e s u r f a c e may i n v o l v e a complex i n t e r p l a y of e l e c t r i c a l , hydrophobic ( d i s p e r s i o n f o r c e s ) and chemical i n t e r actions . The purpose of the present paper i s t o e l u c i d a t e a d s o r p t i o n mechanisms i n the a l k y l s u l f o n a t e - s i l v e r i o d i d e system through e l e c t r o p h o r e t i c m o b i l i t y measurements. The s u l f o n a t e s used i n t h i s study ranged from p e n t y l to t e t r a d e c y l . Further i n f o r m a t i o n on the nature of the A g i s u r f a c e and s u l f o n a t e a d s o r p t i o n was obtained through study systems. Experimental
- M a t e r i a l s and Methods
M a t e r i a l s . T r i p l y d i s t i l l e d water used f o r a l l s o l u t i o n s was prepared by f i r s t passing tap water through a Barnstead Laboratory s t i l l followed by a two-stage Haraeus quartz s t i l l . The f i n a l d i s t i l l a t e was kept under p u r i f i e d n i t r o g e n u n t i l used. S i l v e r n i t r a t e and potassium i o d i d e of A.R. grade were used to prepare the s i l v e r i o d i d e s o l . Iodine was obtained as resublimed c r y s t a l s from M a l l i n c k r o d t and s i l v e r w i r e of 99.5 to 99.8% p u r i t y was obtained from Sargent-Welch. The sodium s a l t s of C m , 12> 10> 8 s u l f o n i c a c i d s were s u p p l i e d by A l d r i c h Chemical Company w h i l e the C 5 was from Κ & Κ L a b o r a t o r i e s . c
c
a n d
c
P r e p a r a t i o n of S i l v e r Iodide S o l . A stock s o l was prepared by adding 50 ml of 1 0 " Μ ΚΙ s o l u t i o n to an equal volume of 1 0 ~ M AgNÛ3 s o l u t i o n w i t h s t i r r i n g . A f t e r aging f o r 12 - 18 hours, t h i s was d i l u t e d to a s o l c o n c e n t r a t i o n of 10~** M Agi f o r e l e c t r o phoresis measurements. The i o n i c s t r e n g t h was c o n t r o l l e d w i t h 10~ M KNO3 and a p p r o p r i a t e volumes of AgNU3 and s u r f a c t a n t s o l u t i o n s were added to g i v e t h e r e q u i r e d pAg and s u r f a c t a n t concen tration. z
2
3
E l e c t r o p h o r e s i s . The e l e c t r o p h o r e t i c measurements were con ducted w i t h the R i d d i c k Zetameter (11), a product o f Zetameter I n c . , New York. Room temperature was maintained a t 20 ± 2°C f o r most of these experiments. I n g e n e r a l , twenty m o b i l i t y readings were taken f o r each system and averaged; the p o l a r i t y of t h e a p p l i e d v o l t a g e was reversed f o r a l t e r n a t e measurements. U s u a l l y 100 V was used; however, f o r slower p a r t i c l e s i t was o f t e n neces sary to go to v o l t a g e s as high as 300 V to observe any a p p r e c i able motion. Care was taken not to keep the s o l too long before use s i n c e on standing f o r extended periods of time, the s o l
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
5.
OSSEO-ASARE
ET
Sulfonate Adsorption
AL.
65
p a r t i c l e s have been found to decrease i n m o b i l i t y - probably a r e s u l t of the desorption which accompanies c o a g u l a t i o n ( 6 ) . The average diameter of the s o l p a r t i c l e s , 2a, was found by e l e c t r o n microscopy (12) to be 20 nm. The i o n i c s t r e n g t h was c o n t r o l l e d u s i n g ΙΟ" M KNO3; t h i s gave a Debye-Huckel r e c i p r o c a l l e n g t h 1/K of 9.5 nm. Based on the f a c t that under these c o n d i t i o n s the product Ka has a v a l u e of 1.05, the r e s u l t s of Wiersema et a l . (13) can be used to convert the measured e l e c t r o p h o r e t i c m o b i l i t i e s to z e t a p o t e n t i a l s . Below a m o b i l i t y , u , of 2 urn/ sec per volt/cm, as a good f i t to the c a l c u l a t i o n s of Wiersema et a l . , the m o b i l i t y can be converted i n t o zeta p o t e n t i a l by 3
ζ = 20u
(1)
Q
e
where ζ i s the z e t a p o t e n t i a l i n m i l l i v o l t s b i l i t y v a l u e s , however zeta p o t e n t i a l becomes Wetting
At the higher
mo
Behavior.
P r e p a r a t i o n of t h i n A g i f i l m s . F o l l o w i n g the method of B i l l e t t and O t t e w i l l ( 8 ) , g l a s s microscope s l i d e s , cut i n t o pieces approximately 1 cm χ 2.5 cm χ 0.1 cm, were cleaned w i t h aqua r e g i a and stored under t r i p l y d i s t i l l e d water. To p l a t e the g l a s s w i t h s i l v e r , a watch g l a s s w i t h the s l i d e s i n i t , was p o s i t i o n e d 3 cm v e r t i c a l l y below a s i l v e r source (approximately 6 cm s i l v e r w i r e ) held i n a tungsten basket. The vacuum u n i t was pumped down to about 666.6 yPa through a l i q u i d n i t r o g e n t r a p . The s i l v e r m i r r o r s were t r a n s f e r r e d q u i c k l y from the vacuum evap o r a t o r and immersed i n a 0.0025 Ν s o l u t i o n of i o d i n e i n 0.01 Μ ΚΙ s o l u t i o n f o r 20 - 30 seconds. The f i l m s were then aged i n 10" * Μ ΚΙ s o l u t i o n f o r 1.5 hours and kept under d i s t i l l e d water u n t i l used. I t was found that l e a v i n g a t h i n l a y e r of s i l v e r between the g l a s s p l a t e and the Agi f i l m improved the adhesion of the f i l m to the s u b s t r a t e . This procedure was t h e r e f o r e followed i n the p r e p a r a t i o n of the f i l m s . 1
Measurement of contact angle. The c a p t i v e bubble technique (8) was used to determine the contact angles. For making the measurement, the s o l u t i o n to be s t u d i e d was f i r s t added to a c e l l made of o p t i c a l g l a s s and then the s l i d e w i t h the A g i f i l m was placed i n t o the d e s i r e d s o l u t i o n f o r about 15 minutes before t a k i n g measurements. A g l a s s c y l i n d r i c a l tube of 3 mm i n t e r n a l diameter was placed above the f i l m s u r f a c e and the a i r pressure i n the bubble was regulated by means of a screw arrangement at the upper end of the tube. A telescope supplied w i t h an o c u l a r p r o t r a c t o r was used to observe the bubble p r o f i l e . The bubble was allowed to touch the f i l m surface by g e n t l y i n c r e a s i n g the pressure, then a f t e r s i t t i n g on the f i l m f o r about 15 seconds, the pressure was increased by a s m a l l amount. This
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
ADSORPTION
A T INTERFACES
Figure 1. The effect of pAg on the electro phoretic mobility of silver iodide in the pres ence of sodium pentyl sulfonate at millimolar ionic strength with potassium nitrate
3
4
5
6
7
8
ρ Ag
Figure 2. The effect of pAg on the electro phoretic mobility of silver iodide in the pres ence of sodium octyl sulfonate at millimolar ionic strength with potassium nitrate
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
5.
ossEOASARE E T A L .
Sulfonate Adsorption
67
s i t u a t i o n y i e l d s the receding angle θ^. The pressure was then r e l e a s e d u n t i l the contact boundary on the p l a t e j u s t moved. The r e s u l t i n g angle i s the advancing angle, θ^. S u r f a c t a n t s o l u t i o n s of a given c o n c e n t r a t i o n were prepared and the contact angles were measured as a f u n c t i o n of pAg f o r C I Q and C ^ . In a l l cases, i n c l u d i n g the experiments i n the absence of s u r f a c t a n t , the i o n i c s t r e n g t h was c o n t r o l l e d w i t h ΙΟ" M KNO3. The contact angles reported here are the average of s i x or more values ob tained by p l a c i n g the bubbles on d i f f e r e n t samples or on d i f f e r ent p o s i t i o n s on the same sample. 3
Results E l e c t r o p h o r e s i s . Figures 1 to 5 present the e l e c t r o p h o r e t i c m o b i l i t y of Agi as a f u n c t i o f pA f o v a r i o u a l k y l s u l f o n a t concentrations and d i f f e r e n s u r f a c t a n t , the p o i n t o charg Ag at pAg 5.6, i n agreement w i t h previous values reported i n the l i t e r a t u r e ( 1) . I n a l l cases, w i t h i n c r e a s i n g concentrations of s u r f a c t a n t at any pAg values below the pzc, the p o s i t i v e m o b i l i t y of the s o l p a r t i c l e s decreased, passed through z e r o , and then became i n c r e a s i n g l y negative. The most i n t e r e s t i n g f e a t u r e s of these r e s u l t s are (1) The short c h a i n s u l f o n a t e s are able to reverse the z e t a p o t e n t i a l on A g i although much higher concentrations are r e q u i r e d than f o r longer chain s u l f o n a t e s . (2) A l l the curves appear to c o i n c i d e i n the neighborhood of pAg 7 independent of chain l e n g t h or s u r f a c t a n t c o n c e n t r a t i o n . (3) The zeta p o t e n t i a l goes through a maximum at higher concentrations of s u r f a c t a n t , an e f f e c t which increases w i t h chain l e n g t h . Wetting Behavior. The magnitude of the contact angles ob t a i n e d both i n the absence of s u r f a c t a n t and i n the presence of v a r i o u s concentrations of and C\q have been p l o t t e d as a f u n c t i o n of pAg i n Figures 6 to 8. In the absence of s u r f a c t a n t , both the receding and advanc ing contact angles go through a maximum at about pAg 5.4. The curves shown i n F i g u r e 6 are drawn through the weighted average of the experimentally determined contact angle values f o r each pAg. The spread i n the θ values f o r a given pAg i s l a r g e l y the r e s u l t of the inherent d i f f i c u l t y of o b t a i n i n g p e r f e c t l y r e p r o d u c i b l e s u r f a c e s . These r e s u l t s are i n good agreement w i t h those reported by B i l l e t t and O t t e w i l l (14) who a l s o observed a maximum at about pAg 5.4, which i s c l o s e to the pzc. Figures 7 and 8 show that when the b u l k c o n c e n t r a t i o n of the s u r f a c t a n t i s low, the w e t t i n g behavior i s very s i m i l a r to that of the s u r f a c t a n t - f r e e system except at higher p o s i t i v e s u r f a c e charge (low pAg) where a s i g n i f i c a n t r i s e i n contact angle i s observed. In other words, i n F i g u r e 8, the receding contact
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
ADSORPTION
3
4
5
6
7
A T INTERFACES
8
Ρ Ag
Figure 3. The effect of pAg on the electrophoretic mobility of silver iodide in the pres ence of sodium decyl sulfonate at millimolar ionic strength with potassium nitrate
3
4
5
6
7
PAg
Figure 4. The effect of pAg on the electrophoretic mobility of silver iodide in the pres ence of sodium dodecyl sulfonate at millimolar ionic strength with potassium nitrate
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
5.
OSSEO-ASARE
ET AL.
Sulfonate Adsorption
Figure 5. The effect of pAg on the electrophoretic mobility of silver iodide in the presence of sodium tetradecyl sulfonate at millimolar ionic strength with potassium nitrate
Figure 6. The contact angle on silver iodide in the absence of surfactant as a function of pAg at millimolar ionic strength with potassium nitrate
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
69
ADSORPTION
120
AT
Γ
—ι C S0 No 0
3
( Ι 0 " Μ KNO3) 3
100 h
0 Δ
Ι«Ι0" Μ 4
ο 801
60|
ο
ο < 40|
2 Ο
5
pAg
6
Figure 7. The contact angle on silver iodide in the presence of sodium decyl sulfonate as a func tion of pAg at millimolar ionic strength with potassium nitrate
120
100 Ul UJ
ο ω 80(-
ζ
where a£g and a°^- are the a c t i v i t i e s of Ag+ and I r e s p e c t i v e l y at the pzc. Table I summarizes the r e s u l t s obtained f o r AG p u s i n g Equation (10) f o r pAg 3, 4, and 5, r e s p e c t i v e l y . There i s a strong dependence on c h a i n l e n g t h , the values o f AG p i n c r e a s i n g from 4.7 RT f o r C to 10.9 RT f o r C ^ . The e f f e c t o f pAg does not seem to be s i g n i f i c a n t f o r the s h o r t c h a i n s u l f o n a t e s , e.g. +
S
S
5
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
74
ADSORPTION
Table I.
C a l c u l a t i o n of A G
g p
AT
INTERFACES
w i t h Equation (10)
f o r t h e A d s o r p t i o n of A l k y l Sulfonates on S i l v e r Iodide (C m
=
2
15yF/cm ,
r
=
0.29 nm)
3 pAg
A l k y l Sulfonate
cm
Q
3 3
c 0
C
1 Q
1.3x10
C
1 2
3.2xl0~
C
17
C
C
32.5x10 .5xl0" -4
5 8
C
12 14
C C
5
Q
C C C
4.7 RT
1.5xl0"
5.0 RT
7.5xl0"
5
8.0 RT
2.0xl0"
5
9.4 RT
5.6xl0"
6
10.6 RT
8.0xl0"
4
4.6 RT
1.5xl0"
4
6.2 RT
D
8.1 RT 5
3.4xl0""
1 4
4.7 RT RT 5.0
3
l.lxlO"
1 2
gp
3
2.3x10
1 Q
-AG _
8.0 RT
7.0xl0" 2.2xl0~
10
C
5
c x ! 0 mol
C
C
4
-3
8.9 RT
6
10.0 RT
C5 has a constant v a l u e of 4.7 RT f o r the three pAg values con sidered. C maintains the same v a l u e of A G = 8.0 RT f o r pAg 3 to 5. The values f o r the longer c h a i n s u l f o n a t e s , however, ap pear to show some pAg dependency. Thus a t pAg 3 and 4, A G f o r C i s 9.4 RT, but t h i s decreases to 8.9 RT a t pAg 5. This i s i n reasonable agreement w i t h the r e s u l t s of O t t e w i l l and Watanabe (6) who found A G = 8.8 RT a t pAg 3 f o r C12 s u l f o n a t e . S i m i l a r l y f o r C , AG p decreases from pAg 3 to 5 i n the order 10.9 RT, 10.6 RT, and 10.0 RT. 1 0
g p
s p
1 2
s p
li+
s
E v a l u a t i o n of A G at the p o i n t of coincidence of m o b i l i t y pAg curves. The second c o n d i t i o n f o r e v a l u a t i n g the s p e c i f i c ad s o r p t i o n f r e e energy, i . e . u t i l i z i n g the pAg a t which a l l the m o b i l i t y - pAg curves c o i n c i d e , provides a d i f f e r e n t means of a n a l y z i n g a d s o r p t i o n phenomena. At t h i s p o i n t , i t seems t h a t the A g i s u r f a c e charge i s so h i g h l y negative that the consequent a d g
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
5.
OSSEO-ASARE E T
AL.
Sulfonate Adsorption
Table I I . C a l c u l a t i o n of AG Surface A c t i v e c Agent C
C
C
5 8 10
C
12
C
14
χ 10
3
0
75
f o r Hydrophobic I n t e r a c t i o n (pAg 5)
-AG from -AG Equation (10) S
- 3.1
Chainlength
R T
sp
8.0xl0~
4
4.6 RT
1.5 RT
2.5
1.5xl0~
4
6.2 RT
3.1 RT
4
2.3xl0~
5
1.1x10
c
D
3.4x10"°
8.1 RT
5.0 RT
5
8.9 RT
5.8 RT
6
10.0 RT
6.9 RT
7
l a r g e AG ^ almost completely opposes s p e c i f i c a d s o r p t i o n This means that a t t h i s p o i n t should j u s t be counterbalance to the a d s o r p t i o n f r e e energy. Thus, under t h i s c o n d i t i o n , e
AG
sp
«
(12)
-AG , - - zFiK el 6
Since a l l the curves come together by pAg 7 , independent of c h a i n length and c o n c e n t r a t i o n of s u r f a c t a n t , ψβ * - 7 8 mV when spe c i f i c a d s o r p t i o n ceases. This y i e l d s a v a l u e of 3 . 1 RT f o r the s p e c i f i c a d s o r p t i o n f r e e energy. I n a d d i t i o n , because t h i s ad s o r p t i o n phenomenon does not e x h i b i t any c h a i n l e n g t h dependency, i t cannot be a t t r i b u t e d to the c h a i n - c h a i n and c h a i n - s o l i d i n t e r a c t i o n s . I t i s t h e r e f o r e suggested that t h i s behavior i s caused by the s p e c i f i c i t y of the p o l a r head of the s u r f a c t a n t . I n par t i c u l a r , the i n t e r a c t i o n of the s u l f o n a t e head w i t h Ag+ i n the l a t t i c e gives r i s e to a chemisorption bond and t h i s l a t t e r c a l c u l a t i o n estimates i t s magnitude, i . e . , A G = - 3 . 1 RT. Beekley and Taylor ( 2 0 ) have reported a marked s p e c i f i c i n f l u e n c e of c e r t a i n anions, i n c l u d i n g C10H7O3", i n the a d s o r p t i o n of Ag+ on A g i . The recent work of O t t e w i l l and co-workers ( 1 0 ) a l s o i m p l i e s t h a t there might be chemisorption i n the a d s o r p t i o n of dodecyl p y r i d inium bromide on A g i . Therefore, t a k i n g t h e AG value calcu l a t e d w i t h Equation ( 1 2 ) as A G - - 3 . 1 RT, t h e hydrophobic c o n t r i b u t i o n to the t o t a l s p e c i f i c a d s o r p t i o n f r e e energy can be c a l c u l a t e d by s u b t r a c t i n g A G = - 3 . 1 RT from the A G values given by Equation ( 1 0 ) . These have been t a b u l a t e d i n Table I I . There i s a s t r i k i n g s i m i l a r i t y between the values of (-AG p - 3 . 1 RT) and the numbers obtained by d i v i d i n g the r e s p e c t i v e c h a i n lengths by 2 , e s p e c i a l l y f o r the longer c h a i n surfactants. The f r e e energy decrease accompanying the complete removal of a hydrocarbon c h a i n from water i s about 1 RT per mole of CH2 groups ( 1 7 ) . Thus, on t h i s b a s i s , i f a l l the chains are removed from water, the decrease i n f r e e energy w i l l be 5 RT, 8 RT, 1 0 RT, 1 2 RT, and 1 4 RT r e s p e c t i v e l y f o r C5, Ce, C I Q , C , and C . c n e m
c h e m
c h
sp
s
1 2
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
1 4
76
ADSORPTION
AT
INTERFACES
I f on the other hand, the adsorbed s u r f a c t a n t c o n s i s t s of h o r i z o n t a l l y o r i e n t e d c h a i n s , then the accompanying decrease i n f r e e energy might be 1/2 RT per mole of CH groups ( i n s t e a d of 1 RT) s i n c e even though h a l f of the c h a i n surface i s next to the s o l i d s u r f a c e , the other h a l f i s s t i l l exposed to the water. On t h i s b a s i s t h e r e f o r e , the f r e e energy a s s o c i a t e d w i t h complete h o r i z o n t a l o r i e n t a t i o n of the chains w i l l be 2.5 RT, 4 RT, 5 RT, 2
and
7 RT
respectively
for C , 5
C I Q , 0χ2>
Ce,
a n c l
c
l*f
0 n
compar
ing these values w i t h the corresponding (-AG - 3.1 RT) values (Table I I ) v i z . 1.5 RT, 3.1 RT, 5.0 RT, 5.8 RT, 6.9 RT respec t i v e l y f o r C5, CQ CIQ, Ci2> and C^, i t seems reasonable to con clude t h a t the adsorbed s u l f o n a t e ions o r i e n t t h e i r chains p a r a l l e l to the s o l i d s u r f a c e i n d i c a t i n g the presence of c h a i n - s o l i d hydrophobic i n t e r a c t i o n s ( A G * = A G + 3.1 RT). The f a c t sp
9
C H 2
s p
t h a t i n comparison w i t v a l u e s of AG^^ which ar i s the number of carbon atoms i n the chain) values shows t h a t C5, and Ce are l e s s surface a c t i v e than t h e i r longer c h a i n counter parts . At higher s u l f o n a t e concentrations and e s p e c i a l l y w i t h the Cm s u l f o n a t e (Figure 5 ) , the slope of the m o b i l i t y - pAg curves i s reversed as the pAg i s reduced. T h i s r e v e r s a l probably i n d i c a t e s the onset of hemimicelle f o r m a t i o n , which i s the c o n d i t i o n where the a d s o r p t i o n f r e e energy becomes more negative because of the a s s o c i a t i o n of the hydrocarbon chains of the adsorbed s u l f o n a t e i o n s . Contact angle behavior. In view of the f a c t that water seems to be only weakly bonded to the A g i surface (3,21,22), there w i l l be a strong tendency f o r incoming s u l f o n a t e ions to d i s l o d g e water molecules at the s u r f a c e . The r e s u l t w i l l be an i n t e r f a c e c o n s i s t i n g of CH groups w i t h a consequent i n c r e a s e i n the hydrophobic nature of the s o l i d - s o l u t i o n i n t e r f a c e . T h i s means t h a t i n the presence of adsorbed s u l f o n a t e s the contact angle at the s o l i d - l i q u i d - a i r i n t e r f a c e should i n c r e a s e w i t h the number of CH2 groups at the s o l i d s u r f a c e , i . e . w i t h both c h a i n l e n g t h and amount of adsorbed i o n s . F i g u r e s 7 and 8 c l e a r l y support the above a n a l y s i s . A comparison of the contact angles f o r C I Q and shows t h a t at pAg 3 a receding angle of about 50 degrees i s achieved i n 5 χ ΙΟ" M C I Q s u l f o n a t e but only 3 χ 10~ M i s necessary to a t t a i n the same contact angle w i t h Cii+ s u l f o n a t e . Again, f o r each s u r f a c t a n t c o n c e n t r a t i o n , the angles i n c r e a s e w i t h decreasing pAg, showing t h a t the amount of s u r f a c t a n t adsorbed increases w i t h higher p o s i t i v e surface charge. When the pAg i s low, the s o l i d surface has a h i g h p o s i t i v e charge and t h i s leads to an i n c r e a s e i n e l e c t r o s t a t i c a t t r a c t i o n of the a n i o n i c s u r f a c t a n t s to the A g i s u r f a c e . This means that more s u r f a c t a n t i o n s w i l l be drawn towards the s u r f a c e and consequently the c o n t r i b u t i o n s of c h a i n - s o l i d i n t e r a c t i o n s to 2
4
5
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
5.
OSSEO-ASARE E T A L .
Sulfonate Adsorption
77
the f r e e energy of a d s o r p t i o n w i l l be expected t o i n c r e a s e w i t h increased surface charge, and the longer the chain l e n g t h , the more t h i s i n c r e a s e w i l l be apparent, both i n e l e c t r o k i n e t i c be h a v i o r and i n contact angle behavior. Summary The e f f e c t of n - a l k y l s u l f o n a t e s on the e l e c t r o p h o r e t i c and w e t t i n g behavior of s i l v e r i o d i d e has been s t u d i e d . The r e s u l t s show that i n the r e g i o n of negative charge, a l l the s u r f a c t a n t s cease to be surface a c t i v e a t the same pAg, i n the neighborhood of pAg 7. This f a c t has been used to p o s t u l a t e a chemical con t r i b u t i o n of the p o l a r head to the t o t a l s p e c i f i c f r e e energy of a d s o r p t i o n . I t i s demonstrated by means of the Stern-Grahame theory of s p e c i f i c a d s o r p t i o th doubl l a y e tha th s u l fonates adsorb a t lowe o r i e n t e d to the A g i s u r f a c e energy chang c i a t e d w i t h t h i s s p e c i f i c c h a i n - s o l i d i n t e r a c t i o n i s found to be 0.5 RT per mole of CH2 group. At higher a d s o r p t i o n d e n s i t i e s , and e s p e c i a l l y f o r longer chain l e n g t h s , a s s o c i a t i o n c o n t r i b u t e s to the a d s o r p t i o n process. Acknowledgements The support of t h i s research by the N a t i o n a l Science Founda t i o n i s g r a t e f u l l y acknowledged. Literature Cited
1. Kruyt, H. R., Ed., "Colloid Science", Vol. I, Elsevier, Amsterdam, 1952. 2. Frens, G., Overbeek, J . Th. G., J . Colloid Interface Sci., (1971), 36, 286 3. Zettlemoyer, A. C., Tcheurekdjian, Ν., Chessick, J . J., Nature (London), (1961), 192, 653 4. Pravdic, V., Mirnik, Μ., Croat. Chem. Acta, (1960), 32, p. 1. 5. Fuerstenau, D. W., J . Phys. Chem., (1956), 60, 981 6. Ottewill, R. H., Watanabe, Α., Kolloid-Z., (1960), 170, 132 7. Bijsterbosch, B. H., Lyklema, J., J. Colloid Sci., (1965), 20, 665 8. Billett, D. F . , Ottewill, R. H., in "Wetting", S. C. I. Mono graph No. 25, p. 253, Society of Chemical Industry, London, 1967. 9. Billett, D. F . , Hough, D. B., Lovell, V., Ottewill, R. H., (1973), unpublished data. 10. Billett, D. F . , Ottewill, R. H., Thompson, D. W., in "Parti cle Growth in Suspensions", S. C. I. Monograph No. 38, p. 195, Academic Press, London, 1973. 11. Zeta-Meter Manual, Zeta-Meter Inc., New York, N.Y.
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
78
12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22.
ADSORPTION AT INTERFACES
Horne, R. W., Ottewill, R. H . , J. Photo. S c i . , (1958), 6, 39 Wiersema, P., Loeb, Α . , Overbeek, J . Th. G . , J. Colloid Interface S c i . , (1966), 22, 78 Billett, D. F., Ottewill, R.H., (1971), 161st A. C. S. Na tional Meeting, Los Angeles, Ca. Stern, O., Z. Elektrochem., (1924), 30, 508 Grahame, D. C . , Chem. Rev. (1947), 41, 441 Fuerstenau, D. W., Journal of the International Union of Pure and Applied Chemistry, (1970), 24, 135 Osseo-Asare, Κ., Fuerstenau, D. W., Croat. Chem. Acta, (1973), 45, 149 Mackor, E., L., Recl. Trav. Chim., (1951), 70, 763 Beekley, J . S., Taylor, H. S., J . Phys. Chem., (1925), 29, 942 Tcheurekdjian, N. Phys. Chem., (1964), 68, 773 Hall, P. G . , Tomkins, F. C . , Trans. Farad. Soc., (1962), 58, 1734
*Visiting Professor, Department of Materials Science and Engineering, University of California, Berkeley; Permanent address: School of Chemistry, University of Bristol, Bristol BS8 1TS, England
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
6 Adsorption of Dyes on Water-Swollen and Non-Swelling Solid Substrates S. R. SIVARAJA I Y E R and A. S. C H A N E K A R Bombay University, Department of Chemical Technology, Bombay, India G. S R I N I V A S A N Hindustan Lever Research Centre, Bombay, India
Introduction Extensive investigations carried out by Sivaraja Iyer and coworkers (1,2,3) on adsorption of anionic dyes on cellulose surfaces revealed that the extent of dye uptake was strongly influenced by the concentration and nature of theelectrolyte ionic species added to the dyebath. An important effect observed (3) was a marked influence of the nature of the electrolyte cation on dye uptake in the presence of equivalent concentrations (0.1M) of different alkali metal chlorides, the order of increasing dye adsorption being Li < Na < K < Rb < Cs. An examination of the usual factors in the cellulose - aqueous dye solution system which are influenced by the addition of electrolytes such as the Donnan potential (4,5,6,7), Donnan distribution of ions (8,9,10), increase in chemical potential of the dye anion (11,12) and changes in solubility of the dye (3) shows that these factors for the series of the cations studied cannot explain the marked difference in dye adsorption in the presence of Li and Na on one hand and K Rb and Cs on the other. This additional influence of the nature of the cations however, finds a reasonable explanation in terms of their varying abilities to disrupt the structure of ordered water molecules surrounding the cellulose surface and the adsorbing dye anions (3). In the present work, these studies have been extended to cover a wider range of concentrations of KCl and NaCl and also the influence of different halide anions on the extent of dye adsorption. It is well known that cellulose is a hydrophilic material having a negative surface charge due to ionization of intrinsic carboxyl groups and when immersed in water,swells up considerably, exposing a large internal surface, also hydrophilic in nature. The influence of this opening up of its structure on dye and electrolyte sorption, water uptake etc. has been extensively reviewed (12,13,14). The question arises whether the above mentioned effect of electrolyte cations on the adsorption of dyes is confined to a swelling hydrophilic textile fiber such as vis+
+
+
+
+
79 In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
80
ADSORPTION
A T INTERFACES
c o s e , o r i s a phenomenon a s s o c i a t e d w i t h h y d r o p h i l i c s u r f a c e s i n general. Hence s t u d i e s o n t h e a d s o r p t i o n o f t h e a n i o n i c d y e C h l o r a z o l Sky B l u e F F on o t h e r h y d r o p h i l i c s u b s t r a t e s s u c h as r i g i d amorphous t i t a n i u m d i o x i d e a n d amorphous s i l i c a h a v i n g t h i c k e n i n g and t h i x o t r o p i c c h a r a c t e r have been u n d e r t a k e n . I t was a l s o c o n s i d e r e d i m p o r t a n t t o compare t h e dye a d s o r p t i o n c h a r a c t e r i s t i c s o f a completely hydrophobic surface l i k e that o f G r a p h o n and a s u r f a c e e x h i b i t i n g d u a l c h a r a c t e r i s t i c s s u c h a s activated charcoal. The s p e c t r u m o f s u r f a c e s i n v e s t i g a t e d i n t h i s work was f o u n d t o e x h i b i t i n t e r e s t i n g c h a r a c t e r i s t i c s and r e s p o n s e s t o t h e p r e sence o f e l e c t r o l y t e s i n t h e aqueous d y e b a t h . The r e s u l t s o f these comparative i n v e s t i g a t i o n s a r e discussed i n t h i s paper. Experimental Preparation, Purificatio a n i o n i c d i r e c t d y e C h l o r a z o l S k y B l u e F F ( C . I . D i r e c t B l u e 1, s o d i u m s a l t ) u s e d i n t h e s e s t u d i e s , was o b t a i n e d i n a h i g h l y p u r e f o r m ( p u r i t y > 9 9 · 2 # ) f r o m a c o m m e r i c a l sample u s i n g t h e s t a n d a r d s a l t i n g o u t method o f R o b i n s o n a n d M i l l s (15) f o r p u r i f i c a t i o n . T h i s dye w i l l be r e f e r r e d t o i n t h e subsequent p o r t i o n s o f t h i s p a p e r a s CSBFF. The v i s c o s e r a y o n sample u s e d i n t h e p r e s e n t i n v e s t i g a t i o n s was a G w a l i o r R a y o n s a m p l e h a v i n g a d e n i e r o f 1.5 a n d was p u r i f i e d u s i n g s t a n d a r d m e t h o d s (l6). The s p e c i f i c s u r f a c e a r e a o f t h i s v i s c o s e sample a s d e t e r m i n e d f r o m a B.E.T. n i t r o g e n a d s o r p t i o n i s o t h e r m f o r a w a t e r s w o l l e n u n c o l l a p s e d sample (2) a s w e l l a s b y n e g a t i v e s o r p t i o n o f c h l o r i d e i o n s i n a v i s c o s e - a q u e o u s KC1 s o l u t i o n s y s t e m (17) was f o u n d t o b e 205 m /g. The amorphous t i t a n i u m d i o x i d e (Ti02T~sample o b t a i n e d t h r o u g h t h e c o u r t e s y o f P r o f e s s o r R.D. V o i d , U n i v e r s i t y o f S o u t h e r n C a l i f o r n i a , was a N a t i o n a l L e a d sample h a v i n g c o d e No. MP 1391-1 and a s p e c i f i c s u r f a c e a r e a o f 100 m^/g. The G r a p h o n was a p u r e sample o b t a i n e d t h r o u g h t h e c o u r t e s y o f P r o f e s s o r A.C. Z e t t l e m o y e r , L e h i g h U n i v e r s i t y B e t h l e h e m P a , a n d h a s a c o r r e c t e d s p e c i f i c s u r f a c e a r e a o f 120 m2/g (l8). G r a p h o n p r o v i d e s a homogeneous a n d h y d r o p h o b i c n o n p o r o u s s u r f a c e a n d i s t h e r e f o r e w i d e l y u s e d i n a d s o r p t i o n s t u d i e s . The s i l i c a (S1O2) sample u n d e r t h e t r a d e name A e r o s i l 200 was o b t a i n e d a s a g i f t f r o m M/S. D e g u s s a , F r a n k f u r t , West Germany and h a s a s p e c i f i c s u r f a c e a r e a o f a b o u t 200 m2/g. The a c t i v a t e d c h a r c o a l was m i c r o p o r o u s E . M e r c k CNo. 2184) s a m p l e a n d was f u r t h e r p u r i f i e d b y t r e a t i n g i t w i t h d i l u t e h y d r o c h l o r i c a c i d a t 70 C f o r 1 h r . t o remove a c i d s o l u b l e i m p u r i t i e s f o l l o w e d b y r e p e a t e d washings with c o n d u c t i v i t y water. T h i s sample was t h e n d r i e d a t 100°C. The s p e c i f i c s u r f a c e a r e a o f t h e sample was 800 m /g. The a c i d i t y o f s u r f a c e g r o u p s was a b o u t 0.8 e q u i v . p e r k g . 2
2
The s u r f a c e a r e a s o f a l l t h e s e a d s o r b e n t s were o b t a i n e d B.E.T. n i t r o g e n a d s o r p t i o n isotherms.
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
from
6.
IYER
E T AL.
Adsorption of Dyes
81
Measurement o f Dye A d s o r p t i o n . Known w e i g h t s o f t h e s o l i d s (0.1 t o 0.35 g d e p e n d i n g on t h e s p e c i f i c s u r f a c e a r e a o f s a m p l e ) were p l a c e d i n C o r n i n g g l a s s "bulbs w h i c h were d e g a s s e d u n d e r a vacuum o f 10""°mm o f Hg f o r a f e w h o u r s a t 100°C a n d t h e n s e a l e d . T h e s e s e a l e d b u l b s c o n t a i n i n g t h e s o l i d s were b r o k e n u n d e r known volumes o f dye s o l u t i o n s i n s t a n d a r d ground j o i n t s t o p p e r e d Corning glass c o n i c a l f l a s k s . The f l a s k s a n d t h e i r c o n t e n t s were e q u i l i b r a t e d i n a thermosated water b a t h having an accuracy o f + 0.1°C. A s u i t a b l e a r r a n g e m e n t was made f o r a g i t a t i n g t h e f l a s k s during the p e r i o d r e q u i r e d f o r e q u i l i b m m . The e q u i l i b r i u m t i m e s o f a d s o r p t i o n f o r t h e d i f f e r e n t s y s t e m s w e r e f o u n d t o be a b o u t 150 n r . f o r v i s c o s e , 100 n r . f o r T1O2 a n d S i 0 2 , 60 h r s . f o r G r a p h o n a n d a b o u t 120 n r . f o r a c t i v a t e d c h a r c o a l . Stock s o l u t i o n s o f d y e a n d e l e c t r o l y t e were p r e p a r e d i n d o u b l e d i s t i l l e d c o n d u c t i v i t y w a t e r a n d s t o r e d i n t h o r o u g h l y c l e a n e d , steamed a n d d r i e d Corning c o n i c a l f l a s k s the o r i g i n a l and e q u i l i b r i u b a t h were made s p e c t r o p h o t o m e t r i c a l l y w i t h a H i l g e r S p e k k e r i n strument u s i n g B e e r ' s law. The c o l o r i m e t r i c measurements were c a r r i e d o u t a t a w a v e l e n g t h o f 625 nm w h i c h i s t h e w a v e l e n g t h o f maxiumum a b s o r p t i o n . Results
and D i s c u s s i o n
A d s o r p t i o n o f CSBFF on V i s c o s e . The a d s o r p t i o n isotherms d e t e r m i n e d a t 50°C i n t h e p r e s e n c e o f i n c r e a s i n g c o n c e n t r a t i o n s of NaCl and K C l are i l l u s t r a t e d i n F i g u r e 1. Increase i n t h e e x t e n t o f dye u p t a k e w i t h i n c r e a s i n g e l e c t r o l y t e c o n c e n t r a t i o n c l e a r l y n o t i c e d i n t h i s f i g u r e i s an expected and p r e v i o u s l y r e ported behaviour (2.,3_,12.) · More i n t e r e s t i n g i s t h e o b s e r v a t i o n t h a t a t any given e q u i v a l e n t e l e c t r o l y t e c o n c e n t r a t i o n i n t h e r a n g e 0.045-0.2M t h e p r e s e n c e o f KT i o n s i n d u c e s much g r e a t e r d y e a d s o r p t i o n on v i s c o s e t h a n N a i o n s . T h e s e r e s u l t s amply c o n f i r m and e x t e n d t h e r e s u l t s on d y e u p t a k e r e p o r t e d p r e v i o u s l y (3.) a t one e q u i v a l e n t c o n c e n t r a t i o n , n a m e l y , 0.1M. Whereas c a t i o n s a r e seen t o s t r o n g l y i n f l u e n c e t h e dye uptake by v i s c o s e , a n i o n s a r e r e p o r t e d t o h a v e v e r y l i t t l e i n f l u e n c e (12). E q u i l i b r i u m adsorpt i o n o f CSBFF on v i s c o s e a t 50°C c a r r i e d o u t i n t h e p r e s e n c e o f 0.1M c o n c e n t r a t i o n o f K C l , K B r a n d K I a r e a l l f o u n d t o f a l l o n one i s o t h e r m , a s shown i n F i g u r e 2. These d a t a i n d i c a t e c l e a r l y t h a t a n i o n s h a v e no i n f l u e n c e . 1-
+
A number o f p o i n t s o f c l a r i f i c a t i o n a r e now r e q u i r e d t o e x plain the specific d i f f e r e n t i a l effect of cations. The s p e c i f i c a d s o r p t i o n o f c a t i o n s on t h e n e g a t i v e l y c h a r g e d c e l l o s e s u r f a c e i s u n l i k e l y t o be a c o n t r o l l i n g f e a t u r e f o r a d s o r p t i o n , s i n c e t h e c a t i o n s o f t h e a l k a l i m e t a l h a l i d e s a r e known t o b e o n l y d i f f u s e l y a d s o r b e d i n the s w o l l e n aqueous f i b e r phase. Evidence f o r d i f f u s e adsorption o f c a t i o n s i n t h e e l e c t r i c a l double l a y e r present i n c e l l u l o s e - a q u e o u s dye + e l e c t r o l y t e s o l u t i o n i n t e r f a c e s h a s b e e n o b t a i n e d f r o m t h e e x t e n s i v e i n v e s t i g a t i o n s on Donnan p o t e n t i a l s
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
Figure 1. Adsorption isotherms for CSBFF on viscose at different concentrations of NaCl and KCl,at50°C
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
6.
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E T AL.
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and Donnan d i s t r i b u t i o n o f e l e c t r o l y t e i o n s i n s u c h s y s t e m s u s i n g d i f f e r e n t a l k a l i h a l i d e s (^,5.,6,7,8,9,10), s t u d i e s o n n e g a t i v e a d s o r p t i o n o f c h l o r i d e i o n s on c e l l u l o s e f i b r e s u r f a c e s ( 1 7 ) , e l e c t r o k i n e t i c p o t e n t i a l s a t c e l l u l o s e - a q u e o u s KC1 and N a C l s o l t i o n s y s t e m s (l£) a n d a c o r r e l a t i o n b e t w e e n t h e o r y and e x p e r i ment f o r t h e a d s o r p t i o n o f d i r e c t d y e s on c e l l u l o s e ( 1 , 2 , 3 , 1 1 , 2 0 , 21,22). Hence s p e c i f i c a d s o r p t i o n o f t h e c a t i o n s t o d i f f e r e n t e x t e n t s c a n n o t e x p l a i n t h e d i f f e r e n c e s i n dye a n i o n u p t a k e . The o b s e r v e d d i f f e r e n c e s i n t h e p r e s e n c e o f N a C l a n d KC1 c a n n o t a l s o b e a t t r i b u t e d t o d i f f e r e n c e s i n s o l u b i l i t y o f t h i s dye since i t has b e e n shown (3_) t h a t t h e r e a r e n o s i g n i f i c a n t c h a n g e s i n s o l u b i l i t y o f the dye a t a g i v e n e l e c t r o l y t e c o n c e n t r a t i o n . However, a s a t i s f a c t o r y e x p l a n a t i o n f o r t h e i n f l u e n c e o f t h e t y p e o f e l e c t r o l y t e c a t i o n o n dye a d s o r p t i o n b y c e l l u l o s e a t equivalent concentration f NaCl d KC1 b obtained fro c o n s i d e r a t i o n o f the i n t e r a c t i o n species i n v o l v e d i n the i t s e l f i n i t i a l l y a d s o r b s w a t e r f o r m i n g a l a y e r o f s t r o n g l y bound water molecules. E x i s t e n c e o f s t r u c t u r e d water i n t h e immediate v i c i n i t y o f the c e l l u l o s e s u r f a c e due t o i n t e r a c t i o n s between t h e p o l a r g r o u p s o f t h e s o l i d ( v i s c o s e ) and w a t e r i s e v i d e n t f r o m t h e work o f Ramiah and G o r i n g (23) a n d D r o s t - H a n s e n (2*0. Adsorption o f any other s p e c i e s w i l l be hindered by the presence o f t h i s v i c i n a l water. I n the s o l u t i o n phase, d i s s o l v e d i o n s c a r r y around e a c h o f them a number o f r i g i d l y bound w a t e r m o l e c u l e s . These hydrated ions by v i r t u e o f t h e i r strong e l e c t r o s t a t i c f i e l d tend t o d i s r u p t the s t r u c t u r e o f water and i n c r e a s e the o r i e n t a t i o n a l d i s o r d e r o f water surrounding t h e i r hydrated envelope. F r a n k and E v a n s (25) c a l l e d t h i s e f f e c t t h e w a t e r s t r u c t u r e b r e a k i n g e f f e c t . T h i s e f f e c t i n c r e a s e s with i n c r e a s i n g i o n i c r a d i u s i n the case o f c a t i o n s (26), (27). When s u c h h y d r a t e d i o n s a r e d i f f u s e l y a d sorbed i n the e l e c t r i c a l double l a y e r adjacent t o c e l l u l o s e , they can b r e a k the w a t e r - c e l l u l o s e s u r f a c e bonds o r i n o t h e r words, c a n d i s r u p t the adsorbed l a y e r o f water. I n a s i m i l a r manner t h e s e c a t i o n s can a l s o b r e a k the o r d e r e d s t r u c t u r e o f w a t e r ("icebergs") around n o n - p o l a r p a r t s o f the dye anions (28,29,30,31,32,33). Such d i s t u r b a n c e s o f the a d s o r b e d water m o l e c u l e s on the c e l l u l o s e s u r f a c e a n d t h e b r e a k d o w n o f " i c e b e r g s " a r o u n d t h e dye m o l e c u l e w i l l increase with increasing i o n i c strength. I n e f f e c t , the dye anions w i l l approach the c e l l u l o s e s u r f a c e very c l o s e l y thereby e n a b l i n g a n i n c r e a s e i n t h e i n t e r m o l e c u l a r f o r c e s b e t w e e n them. The g r e a t e r w a t e r s t r u c t u r e b r e a k i n g e f f e c t o f K a s compared t o Na t h u s l e a d s t o t h e e n h a n c e d dye u p t a k e i n t h e p r e s e n c e o f K as shown i n F i g u r e 1. +
+
+
T h e r m o d y n a m i c s o f A d s o r p t i o n o f CSBFF o n V i s c o s e . I s o t h e r m s , a f t e r c o r r e c t i n g f o r a Donnan e q u i l i b r u m a c c o r d i n g t o methods r e f e r r e d t o e a r l i e r (2,3.), were f o u n d t o g i v e good l i n e a r r e c i p r o c a l Langmuir p l o t s . From t h e s l o p e s a n d i n t e r c e p t s o f t h e s e p l o t s , s a t u r a t i o n v a l u e s a s w e l l a s s t a n d a r d t h e r m o d y n a m i c param-
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
ADSORPTION
Δ—Δ
Κ Br
0-1 M
• - •
ΚΙ
0-1M
A T INTERFACES
1*
U
6
8
10
Dfj- [ M O L E S / LITER ) Χ 10
4
Figure 2. Adsorption isotherms for CSBFF on viscose in the presence of 0.1M concentrations of KCl, KBr and Kl at 50°C y
100
200
300
400
500
600
700
8 00
9 00 1000
1100
1200
LIMITING C O - A R E A (/? /mol ecu le) 2
Figure 3. Variation of —ΔΗ° with limiting coarea of the adsorbed dye molecule on viscose
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
6.
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m e t e r s were c a l c u l a t e d i n t h e u s u a l manner. F i g u r e 3 w h i c h shows t h e v a r i a t i o n o f e n t h a l p y c h a n g e s - Δ H° w i t h l i m i t i n g c o a r e a p e r a d s o r b e d d y e m o l e c u l e was c o n s t r u c t e d u s i n g t h e p r e s e n t a s w e l l a s p r e v i o u s d a t a (3). These r e s u l t s i n d i c a t e t h a t ~ Δ Η ° indepen d e n t o f t h e l i m i t i n g c o a r e a o f t h e a d s o r b e d dye m o l e c u l e u n t i l a b o u t 6nm^. A g r a d u a l c h a n g e i s now s e e n t o o c c u r i n t h e r a n g e o f c o a r e a s b e t w e e n 6-3.5 n m i n d i c a t i n g i n c r e a s i n g l a t e r a l d y e - d y e interactions. A t a b o u t a c o a r e a o f 3.66 n m f o r t h e a d s o r b e d dye molecule, i t s o r i e n t a t i o n w i l l correspond t o a molecule l y i n g f l a t on t h e s u r f a c e w i t h t h e p l a n e o f t h e b e n z e n e r i n g s p a r a l l e l t o t h e surface. T h i s l i m i t i n g c o a r e a f o r f l a t o r i e n t a t i o n was c a l c u l a t e d from a C o u r t a u l d ' s atomic model o f t h e dye. The f u r t h e r s h a r p r i s e i n - Δ Η ° shown i n F i g u r e 3 i n t h e r e g i o n f r o m 3.5-2.0 n m i n d i c a t e s i n c r e a s e d s t r o n g e r l a t e r a l dye-dye i n t e r a c t i o n s . The c h a n g e s i n l i m i t i n g c o a r e a o f t h e d y e m o l e c u l e f r o m 3.5-2.0 n m can be a t t r i b u t e d t o a s o r b e d d y e t o one i n w h i c perpendicular t o the surface. This l a t t e r o r i e n t a t i o n corresponds t o a p r o j e c t e d c o a r e a o f 2 nm a s d e t e r m i n e d from a C o u r t a l d s atomic model o f t h e d y e . i
s
2
2
2
2
2
!
A d s o r p t i o n o f CSBFF o n T i O p . The a d s o r p t i o n i s o t h e r m s o f CSBFF o n T1O2 i n t h e p r e s e n c e o f N a C l a n d K C l a t 35°C a n d a t 3 5 ° and i+5°C i n t h e p r e s e n c e o f K C l a r e g i v e n i n F i g u r e s h a n d 5 respectively. I t i s i n t e r e s t i n g t o note that the a d s o r p t i o n be h a v i o u r e x h i b i t e d b y c e l l u l o s e i s r e f l e c t e d s t r o n g l y b y T1O2 a l s o . I n t h e a b s e n c e o f a n y e l e c t r o l y t e , T1O2 d o e s n o t a d s o r b CSBFF j u s t as i n t h e c a s e o f v i s c o s e . B y i n c l u d i n g some N a C l i n t h e d y e b a t h slight adsorption takes place. However, t h e amount o f d y e u p t a k e e v e n a t t h e h i g h c o n c e n t r a t i o n o f 0.2M N a C l i s v e r y s m a l l u n l i k e i n t h e c a s e o f v i s c o s e , where a d s o r p t i o n i s c o n s i d e r a b l e e v e n a t 0.0^5M N a C l . I n c o n t r a s t t o t h i s s l i g h t a d s o r p t i o n when N a C l i s p r e s e n t , c o n s i d e r a b l e dye a d s o r p t i o n t a k e s p l a c e even i n t h e p r e s e n c e o f 0.1M K C l . J u s t a s i n t h e c a s e o f v i s c o s e , e l e c t r o l y t e a n i o n s C I " , B r " o r I " were f o u n d t o h a v e no e f f e c t o n d y e a d s o r p tion. The d y e a d s o r p t i o n i s o t h e r m a t 0.2M K C l i s f o u n d t o o b e y t h e Langmuir t y p e e q u a t i o n and t h e a d s o r p t i o n p r o c e s s i s m a r k e d l y e x o t h e r m i c a s c a n b e n o t i c e d f r o m t h e i s o t h e r m s a t 3 5 ° a n d U5°C shown i n F i g u r e 5· From t h e r e c i p r o c a l L a n g m u i r p l o t f o r t h e i s o t h e r m a t 35°C i n t h e p r e s e n c e o f 0.2M K C l t h e c o a r e a o f t h e a d s o r b e d d y e m o l e c u l e was c a l c u l a t e d t o b e a b o u t 3.5nm . This c o r r e s p o n d s t o a f l a t o r i e n t a t i o n o f adsorbed dye m o l e c u l e s on t h e Ti0 surface indicating saturation adsorption. T h e same s a t u r a t i o n on t h e v i s c o s e f i b e r s u r f a c e i s reached a t a K C l concen t r a t i o n o f 0.1M. T h e s e r e s u l t s show t h a t T1O2 w h i c h i s a h y d r o p h i l i c r i g i d s u r f a c e , e x h i b i t s a s i n t h e c a s e o f v i s c o s e , t h e same c h a r a c t e r i s t i c behaviour towards dye a d s o r p t i o n i n the presence o f electrolytes. Hence t h e m o d e l s u g g e s t e d f o r e x p l a i n i n g t h e d y e a d s o r p t i o n behaviour on v i s c o s e i n t h e presence o f d i f f e r e n t 2
2
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
ADSORPTION
AT
Figure 5. Adsorption isotherms for CSBFF on Ti0 in the presence of 0.1M and 0.2M KCl at 35 and45°C 2
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
INTERFACES
6.
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e l e c t r o l y t e s c a n be e x t e n d e d t o T1O2 also. The d i f f e r e n c e s i n t h e m a g n i t u d e o f t h e c a t i o n e f f e c t s on t h e a d s o r p t i o n o f dye on. T1O2 as compared t o v i s c o s e c a n now be e x p l a i n e d a s f o l l o w s : T1O2 has a s t r o n g a f f i n i t y f o r w a t e r . From t h e h e a t o f immersion s t u d i e s c a r r i e d out by Z e t t l e m o y e r et a l . ( 3 M the i n t e r a c t i o n o f v a t e r w i t h T1O2 was f o u n d t o h a v e an e n e r g y o f t h e o r d e r o f a b o u t 8-9 kcal/mole. T h i s i s much more t h a n t h e e n e r g y o f i n t e r a c t i o n o f water w i t h c e l l u l o s i c m a t e r i a l which f a l l s w i t h i n the r a n g e 1+.9-6 k c a l / m o l e ( l U ,35.). B e c a u s e o f t h e s t r o n g e r T1O2 - H2O bonding, stronger f o r c e s are r e q u i r e d t o d i s r u p t the s t r u c t u r e a r o u n d T1O2 a s compared t o v i s c o s e . In view of the o b s e r v a t i o n t h a t t h e a d s o r p t i o n o f t h e dye b y T1O2 i s o n l y o f s m a l l magnitude i n t h e p r e s e n c e o f e v e n 0.2M NaCl i t i s surmised t h a t the water structure breaking e f f e c t of Na i o n s i s somewhat d e f i c i e n t t o e n c o u r a g e dye a d s o r p t i o n and t h e i r e f f e c t i s r e f l e c t e on T1O2. Evidence i n suppor argumen s t u d i e s b y B e r u b e and D e B r u y n (36) on t h e c a p a c i t y o f t h e e l e c t r i c c a l d o u b l e l a y e r i n a T1O2 - aqueous e l e c t r o l y t e s o l u t i o n system, u s i n g KC1 and N a C l a s t h e e l e c t r o l y t e s . These a u t h o r s have r e a c h e d t h e same c o n c l u s i o n s , n a m e l y , N a w i l l have l i t t l e i n f l u e n c e on w a t e r s t r u c t u r e s u r r o u n d i n g t h e T1O2 s u r f a c e whereas K+ i o n s d i s r u p t i t . +
+
A d s o r p t i o n o f CSBFF on S i O p . Dye a d s o r p t i o n e x p e r i m e n t s were c a r r i e d o u t a t e l e c t r o l y t e c o n c e n t r a t i o n s i n t h e r a n g e 0 t o 3.5 M N a C l and KC1 r e s p e c t i v e l y . T h e s e e x p e r i m e n t s i n d i c a t e t h a t no a d s o r p t i o n o f t h e dye t a k e p l a c e a t any e l e c t r o l y t e c o n c e n t r a t i o n . T h i s i s a s i g n i f i c a n t o b s e r v a t i o n d i f f e r e n t from the observations made i n t h e c a s e o f v i s c o s e o r T1O2. S i l i c a i s known t o b i n d w a t e r more s t r o n g l y w i t h an e n e r g y o f t h e o r d e r o f 9 t o 20 kcal/ m o l e a s c a l c u l a t e d f r o m h e a t o f i m m e r s i o n s t u d i e s (3^_3T_). T h e s e b i n d i n g e n e r g y v a l u e s a r e o f much g r e a t e r m a g n i t u d e t h a n f o r t h e b i n d i n g o f w a t e r w i t h c e l l u l o s e and T1O2. From a NMR study of w a t e r on s i l i c a , P i c k e t t and R o g e r s (38) observed very high surface coverage. The p o l a r i z a t i o n and o r i e n t a t i o n o f t h e f i r s t few l a y e r s o f w a t e r i n t u r n c a u s e s f u r t h e r l a y e r s t o be b u i l t up w i t h a l a t t i c e l i k e order. Above 50 l a y e r s t h e o r d e r e d s t r u c t u r e b r e a k s down l e a d i n g t o a more m o b i l e g e l - l i k e s t r u c t u r e . In view o f t h i s v e r y s t r o n g i n t e r a c t i o n o f water w i t h s i l i c a s u b s t r a t e s and t h e p r e s e n c e o f a t h i c k e n v e l o p e o f s t r u c t u r e d w a t e r l a y e r s s u r r o u n d i n g s i l i c a , t h e i n a b i l i t y o f t h e a n i o n i c dye t o a d s o r b i s e a s i l y understandable. E v i d e n t l y , the e l e c t r o l y t e c a t i o n cannot i n f l u e n c e t h i s s t r o n g l y bound water envelope s u r r o u n d i n g silica e v e n a t t h e h i g h e s t c o n c e n t r a t i o n s t u d i e d and h e n c e c a n n o t p l a y i t s u s u a l r o l e i n f a v o u r i n g dye a d s o r p t i o n . 5
A d s o r p t i o n o f CSBFF on G r a p h o n . I t c a n be n o t i c e d f r o m F i g u r e 6 t h a t t h e s a t u r a t i o n v a l u e o f dye u p t a k e i s t h e same b o t h i n t h e a b s e n c e and p r e s e n c e o f e l e c t r o l y t e . Furthermore,
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
the
88
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+
type of e l e c t r o l y t e c a t i o n K+ or N a has no i n f l u e n c e on the s a t u r a t i o n v a l u e or on the extent o f e q u i l i b r i u m dye a d s o r p t i o n at any stage of the isotherm, since the isotherms i n the case of KCl and NaCl o v e r l a p each o t h e r . The i n f l u e n c e o f e l e c t r o l y t e i s n o t i c e d o n l y i n the i n i t i a l stages of the a d s o r p t i o n isotherm and i s understandable from the point of view of r e d u c t i o n , by the e l e c t r o l y t e c a t i o n , of the e l e c t r o s t a t i c r e p u l s i o n due t o adsorbed dye anions f o r other dye anions approaching the adsorbent s u r f a c e . As i n the systems d i s c u s s e d e a r l i e r , anions d i d not i n f l u e n c e dye adsorption. Figure 7 shows a d s o r p t i o n isotherms of CSBFF on Graphon i n the temperature range 30° t o 50°C and 0.13 M NaCl. Here i t can be observed t h a t the s a t u r a t i o n v a l u e i s u n a f f e c t e d by temperature, but d u r i n g the e a r l i e r stages of the dye a d s o r p t i o n isotherms t h a t t h e r e i s a s l i g h t increase i n e q u i l i b r i u m a d s o r p t i o n w i t h tempera t u r e i . e . the a d s o r p t i o are o f the Langmuir typ l i t e r a t u r e f o r a n i o n i c dye a d s o r p t i o n on Graphon (39_ h0). The r e c i p r o c a l Langmuir p l o t s f o r these isotherms are shown i n F i g u r e 8 and the c a l c u l a t e d s a t u r a t i o n v a l u e i n the temperature range 30° - 50°C again corresponds t o a closepacked monolayer w i t h a l i m i t ing coarea of 3.67 nm per adsorbed dye molecule. This coarea as d i s c u s s e d e a r l i e r , i n d i c a t e s an o r i e n t a t i o n of the adsorbed dye molecule i n a f l a t c o n f i g u r a t i o n w i t h the benzene r i n g s p a r a l l e l to the Graphon s u r f a c e . 9
2
Thermodynamics of A d s o r p t i o n of CSBFF on Graphon. From these r e c i p r o c a l Langmuir p l o t s the standard thermodynamic parameters shown i n Table I have been c a l c u l a t e d i n the u s u a l way. In t h i s Table, ^ H ° i s seen t o be a s m a l l p o s i t i v e v a l u e and the entropy of a d s o r p t i o n i s a l a r g e p o s i t i v e q u a n t i t y . S i m i l a r p o s i t i v e entropy values have been observed by Schneider et a l . (kl) f o r a d s o r p t i o n of a l i p h a t i c a c i d s and a l c o h o l s on p o l y s t y r e n e . These r e s u l t s i n d i c a t e t h a t strong hydrophobic i n t e r a c t i o n s are i n v o l v e d i n the a d s o r p t i o n of the a n i o n i c dye CSBFF on graphon. An impor t a n t f e a t u r e o f hydrophobic bond formation d i s c u s s e d by Nemethy and Scheraga (k2) Kauzmann (k3) and Schneider et a l . (kl) is t h a t the enthalpy of formation of the hydrophobic bond i s p o s i t i v e at low temperatures and becomes more negative p a s s i n g through zero as the temperature i n c r e a s e s . This behaviour i s l o g i c a l since the enthalpy o f hydrophobic bond formation i s a net e f f e c t of two f a c t o r s namely, the enthalpy of " i c e b e r g " d e s t r u c t i o n and the enthalpy o f the bonding between the i n t e r a c t i n g species. The f i r s t f a c t o r which i s p o s i t i v e , predominates at low temperatures and decreases w i t h i n c r e a s i n g temperature. The second f a c t o r i s negative and remains more or l e s s u n a f f e c t e d i n the narrow temp erature ranges u s u a l l y s t u d i e d . Experimental evidence f o r these enthalpy changes w i t h i n c r e a s i n g temperature has been provided by Schneider et a l . (kl), who observed a decrease i n a d s o r p t i o n at higher temperatures in- the case o f p o l y s t y r e n e - hydrocarbon 9
9
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
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ET AL.
Adsorption of Dyes
ol 0
1 2
U Ο
σ
6
8
10
12
14
(MOLES/LITER]X 10*
Figure 6. Adsorption isotherms for CSBFF on graphon in the absence and presence of electrolyte at 30°C
6
0-0
30 C
Δ - Δ 4.0°C • - •
U 0
ι 2
ι
ι
50°C
ι
ι
L
6 8 10 Ο [MOLES/LI TER] Χ 10 σ
ι 12
k %
4
Figure 7. Adsorption isotherms for CSBFF on graphon at different temperatures in the presence of 0.13M NaCl
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
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h y d r o p h o b i c i n t e r a c t i o n s . Some p r e l i m i n a r y o b s e r v a t i o n s o n t h e a d s o r p t i o n o f d y e on G r a p h o n , c a r r i e d o u t a t t e m p e r a t u r e s h i g h e r t h a n 50°C, i n d i c a t e a s i m i l a r d e c r e a s e i n a d s o r p t i o n . These r e s u l t s give f u r t h e r evidence f o r hydrophobic i n t e r a c t i o n s i n t h e Graphon-dye system. T a b l e I Thermodynamic P a r a m e t e r s f o r t h e A d s o r p t i o n o f C h l o r a z o l Sky B l u e F F o n G r a p h o n i n P r e s e n c e o f 0.13M N a C l . #
Temperature (°C)
Free Energy o f Adsorption
Heat o f Adsorption
Entropy o f Adsorption
ΔΗ°
As°
(kcal/mole
30 1+0 50
-6.26 -6.62 -6.98
+U.65
+36
A d s o r p t i o n o f CSBFF on A c t i v a t e d C h a r c o a l . Adsorption o f CSBFF on a c t i v a t e d c h a r c o a l p r e s e n t s a v e r y i n t e r e s t i n g s t u d y . F i g u r e 9 i l l u s t r a t e s a d s o r p t i o n i s o t h e r m s a t 30°C o n a c t i v a t e d c h a r c o a l i n t h e p r e s e n c e and absence o f e l e c t r o l y t e s i n t h e d y e bath. Two i n t e r e s t i n g f e a t u r e s c a n b e n o t i c e d f r o m t h i s f i g u r e . I n t h e a b s e n c e o f e l e c t r o l y t e s , a c t i v a t e d c h a r c o a l shows c o n s i d e r a b l e a d s o r p t i o n o f CSBFF j u s t a s i n t h e c a s e o f G r a p h o n a n d i n c o n t r a s t t o t h e behaviour observed f o r h y d r o p h i l i c surfaces l i k e v i s c o s e a n d T1O2. T h e p r e s e n c e o f e l e c t r o l y t e h o w e v e r , e n h a n c e s t h e t e n d e n c y f o r a c t i v a t e d c h a r c o a l t o a d s o r b t h e dye t o a considerable extent. This behaviour i s s i m i l a r t o that observed i n t h e case o f h y d r o p h i l i c s u b s t r a t e s b u t i n c o n t r a s t t o t h a t o f Graphon. Thus a c t i v a t e d c h a r c o a l e x h i b i t s a d u a l b e h a v i o u r with r e s p e c t t o dye a d s o r p t i o n . I n t h e absence o f e l e c t r o l y t e s , t h e L a n g m u i r t y p e a d s o r p t i o n t e n d s t o a l i m i t i n g v a l u e o f 3.5 x 10"^ moles/kg. M a k i n g t h e a s s u m p t i o n t h a t t h e a d s o r b e d dye m o l e c u l e o c c u p i e s a l i m i t i n g c o a r e a o f 3.67 nm p e r m o l e c u l e a s i n t h e case o f Graphon, dye a d s o r p t i o n w i l l c o r r e s p o n d t o a coverage o f 80 m /g. T h i s a r e a i s o n l y 10$ o f t h e t o t a l s u r f a c e a n d p e r h a p s represents t h e t o t a l l y hydrophobic surface o f c h a r c o a l which i s a v a i l a b l e f o r adsorption o f l a r g e molecules. I t i salso reason able t o c o n s i d e r t h a t t h i s i n t e r a c t i o n i s o f t h e hydrophobic type as i n t h e c a s e o f G r a p h o n s i n c e a f e w e x p e r i m e n t s ( n o t r e p o r t e d h e r e ) , i n d i c a t e a s l i g h t endothermic a d s o r p t i o n p r o c e s s . The dye u p t a k e was f o u n d t o i n c r e a s e e n o r m o u s l y o n a d d i n g e l e c t r o l y t e s t o t h e d y e b a t h a s shown i n F i g u r e 9· The p r e s e n c e o f n e g a t i v e l y 2
2
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
91
Adsorption of Dyes
Figure 8. Reciprocal Langmuir plots. l/ϋφ vs. 1/D for the adsorption of CSBFF on graphon at different tempera tures. NaCl 0.13M. and D refer to the amount of dye adsorbed and the a
a
0-005
0-0
[LITERS/MOLE ]
Figure 9. Adsorption isotherms for CSBFF on activated charcoal in the presence and absence of electrolyte at 30°C
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
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ADSORPTION
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charged a c i d i c s i t e s on a c t i v a t e d carbons i n c l u d i n g c h a r c o a l i s v e i l known (39,*^»]+5.). The e l e c t r o l y t e c a t i o n s screen the e l e c t r o s t a t i c r e p u l s i o n between the approaching dye anion and the n e g a t i v e l y charged a c i d i c s i t e s and hence enable a g r e a t e r ad s o r p t i o n o f dye anions. The d i f f e r e n t i a l e f f e c t s o f the e l e c t r o l y t e c a t i o n s K** and N a are a l s o c l e a r l y n o t i c e d , the adsor p t i o n i n the presence o f KT*" being always g r e a t e r than t h a t i n the presence o f N a . The observed increase i n dye a d s o r p t i o n i n the presence o f KT" as compared t o N a can be explained as being due t o a greater d i s r u p t i o n o f t h e water s t r u c t u r e on t h e h y d r o p h i l i c p a r t s o f the c h a r c o a l surface by K i o n s and p a r a l l e l s the behav i o u r f o r dye a d s o r p t i o n on h y d r o p h i l i c v i s c o s e and T1O2 s u r f a c e s . The h y d r o p h i l i c c h a r a c t e r o f c h a r c o a l surfaces a r i s e s from the o r d e r i n g o f water molecules i n the v i c i n i t y o f the a c i d i c surface groups (U6 hj_,1*8). Fo t h c h a r c o a l sampl d i th t study, the a c i d i c group +
+
1
+
+
9
The present i n v e s t i g a t i o n s u s i n g the dye a d s o r p t i o n t e c h nique, b r i n g t o l i g h t not o n l y the v a r i a t i o n s i n the i n t e r f a c i a l "behaviour of d i f f e r e n t h y d r o p h i l i c s u b s t r a t e s , but a l s o the d i f ferences between t h e a d s o r p t i o n f e a t u r e s o f h y d r o p h i l i c and hydro phobic s u r f a c e s . The b i n d i n g o f water w i t h the surface and i t s f u r t h e r s t r u c t u r i n g i n the v i c i n i t y o f the surface are charac t e r i s t i c features of hydrophilic substrates. Disruption of t h i s s t r u c t u r e of v i c i n a l water t o d i f f e r e n t extents by d i f f e r e n t e l e c t r o l y t e c a t i o n s seems t o p l a y an important r o l e i n r e g u l a t i n g dye adsorption on h y d r o p h i l i c s u r f a c e s . The hydrophobic surface i s p r a c t i c a l l y u n a f f e c t e d by the presence o f e l e c t r o l y t e s i n t h e aqueous medium, dye a d s o r p t i o n being promoted by hydrophobic i n t e r a c t i o n s t h a t i n v o l v e entropy d r i v e n d i s r u p t i o n o f " i c e b e r g " water. Literature Cited 1.
S i v a r a j a I y e r , S.R., S r i n i v a s a n , G., Baddi, N.T., and R a v i k r i s h n a n , M.R., Text. Res. J., (1964), 34, 807.
2.
S i v a r a j a I y e r , S.R., and Baddi, Ν.Τ.,in Proc. Symp. "Con tributions t o Chemistry o f S y n t h e t i c s Dyes and Mechanism o f Dyeing", p. 36, Univ. Dept. Chem. Tech., Bombay, I n d i a (1967)
3.
S i v a r a j a I y e r , S.R., S r i n i v a s a n , G. and Baddi, N.T., Text. Res. J., (1968), 38, 693.
4.
Neale,S.M.,Trans. Faraday Soc.,
(1947), 43, 325.
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
6.
IYER
ET AL.
Adsorption of Dyes
93
5·
N e a l e , S.M., a n d
6.
N e a l e , S.M., a n d S a n a , P.K., J. S o c . D y e r s
7.
Sivaraja
Standring,
P.T.,
P r o c . Roy. S o c . L o n d o n ,
S e r . A, (1952), 213, 530. (1957),
Colour.,
73, 381. I y e r , S.R., a n d J a y a r a m , R., J. S o c . D y e r s
Colour.,
(1970), 86, 398. 8.
U s h e r , F . L . , a n d Wahbi, A.K., J. S o c . D y e r s 58, 221.
9.
N e a l e , S.M., a n d
10.
Sivaraja
11.
H a n s o n , J., N e a l e F a r a d a y S o c . , (1935),
Farrar,
I y e r , S.R.
J.,
J.
d Kalbag
Colloid
Colour.,
Sci.,
(1942),
(1952), 7, 186.
V.N., U n p u b l i s h e d
results.
31, 1738.
12.
Vickerstaff, T., "The Physical and B o y d , L o n d o n , 1954.
13.
S i v a r a j a I y e r , S.R. in "The C h e m i s t r y of Synthetic D y e s , " K. V e n k a t a r a m a n , E d . , Vol. VII, Chap. I V , A c a d e m i c P r e s s , New Y o r k , in press.
14.
Stamm, A . J . , "Wood a n d Cellulose S c i e n c e " , p . 248, hold P r e s s C o . , New Y o r k , (1964).
15.
R o b i n s o n , C., a n d
Mills,
C h e m i s t r y of D y e i n g " ,
Oliver
The R e i n
H.A.T., P r o c . Roy. S o c . L o n d o n , S e r .
A, (1931), 131, 596. 16.
17.
W h i s t l e r , R.L., E d . , "Methods in C a r b o h y d r a t e Vol. 3, p . 3 A c a d e m i c P r e s s , New Y o r k , 1963. Nemade,
B.I.,
Sivaraja
Chemistry",
I y e r , S.R. a n d J a y a r a m , R., T e x t . R e s .
J., (1970), 40, 1050.
18.
Z e t t l e m o y e r , A.C., in " H y d r o p h o b i c S u r f a c e s " , F.M. F o w k e s , E d . , p . 1, A c a d e m i c P r e s s , New Y o r k , 1969.
19.
S i v a r a j a I y e r , S.R., a n d J a y a r a m ,
R., J. S o c . D y e r s C o l o u r . ,
(1971), 87, 338. 20. 21.
C r a n k , J., J. S o c . D y e r s P e t e r s , R.H. a n d
Colour,
Vickerstaff,
(1947), 63, 293.
T., P r o c . Roy. S o c . London S e r .
A, (1948), 192, 292.
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
ADSORPTION
22. 23.
S t a n d i n g ,H.A.,a n d W a r w i c k e r , 40, T175. Ramiah, M.B. a n d
J.O.,
J.
Text.
A T INTERFACES
(1949),
Inst.,
D.A.I., J. Poly. Sci. Part C, 2,
Goring,
(1965), 27. (1969),
24.
D r o s t - H a n s e n , W., I n d . E n g . Chem.,
25.
F r a n k , H.S., a n d E v a n s , M.W., J. Chem. P h y s . ,
61(11),10 (1945), 13,
507. 26.
F r a n k , H.S., a n d Wen, W.Y.,
27.
W i c k e , Ε., Angew. Chem
28.
Sivaraja
29.
Nemethy, G., Angew. Chem.
30.
M u k e r j e e , P., a n d G h o s h ,
F a r a d a y Soc., (1957),
Disc.
24, 133.
Iyer,
Internat.
(1966), 5, 106.
Edit.,
S.R.
(1970), 242, 1196. Internat.
Edit.,
(1967), 6, 195. (1963), 67,
A.K., J. P h y s . Chem.,
193. 31.
R o h a t g i , K.K., a n d Singhal, G.S., J. P h y s .
Chem.,
70, 1695. 32.
K a t a y a m a , A.
Sivaraja
T.,
Konishi,
Κ . , a n d Kuroki, Ν.,
P o l y m , (1965), 206, l62.
Kolloid-Z.Z. 33.
Takagishi,
(1966),
I y e r , S.R., a n d S i n g h , G.S., J. S o c . D y e r s
Colour,
(1973), 89, 128. 34.
Z e t t l e m o y e r , A.C., a n d C h e s s i c k , J.J., Advan.
Catalysis,
(1959), 11, 263. 35.
B a d d i , N.T.,
36.
B e r u b e , Y.G., a n d D e B r u y n ,
37.
M a c k r i d e s , A.C., a n d Hackerman, Ν . ,J.P h y s .
Cell.
Chem. T e c h . ,
(1969), 3, 56l.
P.L., J. Colloid a n d
Interface
Sci., (1968), 28, 92. Chem.,
(1959),
63, 594. 38.
Pickett,
J.H., a n d R o g e r s , L.B.,
Anal.
Chem.,
1892.
39.
Graham, D., J. P h y s .
40.
N a n d i , S.P. a n d W a l k e r , Jr. P . L . ,
Chem.,
(1967), 39,
(1955), 59, 896. Fuel,
(1971), 50, 345.
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
6.
41.
IYER
Adsorption of Dyes
ET AL.
95
S c h n e i d e r , H., K r e s h e c k , G.C., a n d S c h e r a g a , H.A., J. P h y s .
Chem. (1965), 69, 1310.
42.
Nemethy, G., a n d S c h e r a g a , H.A., J. P h y s .
43.
Kauzmann, W., Adv.
44.
C o u g h l i n , R.W., E z r a , F . S . , a n d T a n , R.N., in " H y d r o p h o b i c Surfaces", F.M. Fowkes, E d . , p . 44. A c a d e m i c P r e s s , New York, 1969.
45.
Boehm, H.P., Chem.
46.
47.
Chem.,
(1962),
in 14,
1.
66, 1773.
Diehl,
Internat.
Protein
Ε.,
Edit.,
W a l k e r , Jr. P . L . F.M. Fowkes, E d . , p
(1959),
Chem.,
Heck., W., a n d S a p p o c k ,
R., Angew.
(1964), 3, 669. 107,
1969.
,
D u b i n i n , M.M. in " C h e m i s t r y a n d
Physics
W a l k e r , Jr., E d . , Vol. 2,
M a r c e l - Dekker,
p . 51,
o f C a r b o n " , P.L. New Y o r k
1965. 48.
M a t t s o n , J . S . , a n d Mark Jr., H.B., f a c e C h e m i s t r y and Adsorption from New Y o r k , 1971.
"Activated Solution",
Carbon - Sur Marcel-Dekker,
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
7 Adsorption of Polystyrene on Graphon from Toluene VICTOR K. D U N N * and ROBERT D. V O L D Department of Chemistry, University of Southern California, Los Angeles, Calif. 90007
Introduction Recently many investigators have been concerned with the stability of non-aqueous suspensions, and a number of alternative hypotheses have been proposed to account for the observed facts (1,2,3). All involve consideration of the overlap region of the adsorbed stabilizer surrounding the suspended particle, and predict an increase in stability with increasing molecular weight of an adsorbed macromolecule. In connection with a project to determine quantitatively the effect of the molecular weight of adsorbed polystyrene on the stability of Graphon suspensions in toluene, it therefore became necessary to obtain data on the extent of adsorption and the thickness of the adsorbed layer under the same identical conditions under which subsequent determinations of the rate of flocculation of the suspension could be carried out. Such data are also useful for comparison with the predictions of computer simulations of adsorption of macromolecules (4). Determination of adsorption on Graphon in suspension is greatly complicated by the fact that the material is present as aggregates rather than primary particles. Hence the amount of polystyrene adsorbed will depend on the degree of dispersion of the Graphon, which in turn is a function of the level of mechanical agitation, and also dependent on whether the agitation takes place in the presence or absence of the polystyrene. Since it was desired to compare stabilities at saturation adsorption of a number of polystyrene samples of different molecular weights, it was first necessary to determine adsorption isotherms for each of the polystyrenes so as to insure use of an initial concentration which would result in saturation adsorption. Since flocculation measurements were to be carried out in Graphon suspensions it was also necessary to study the rate of adsorption and *Present address: Xerox Corporation, Webster Research Center, W130, Rochester, New York 14644. 96 In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
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determine the time r e q u i r e d to reach a t l e a s t a p p r o x i m a t e l y the equi1ibrlum value. The r e s u l t s o b t a i n e d y i e l d i n t e r e s t i n g i n f o r m a t i o n c o n c e r n ing t h e p r o b a b l e c o n f o r m a t i o n o f t h e a d s o r b e d m o l e c u l e s and t h e dependence o f t h e t h i c k n e s s o f t h e a d s o r b e d l a y e r on the m o l e c u l a r weight of the p o l y s t y r e n e . M a t e r i a l s and
Methods
Materials. G r a p h i t i z e d c a r b o n b l a c k (Spheron 6 ) , p r e p a r e d by h e a t i n g i n t h e a b s e n c e o f a i r t o 2700°C, o r h i g h e r , was o b t a i n e d f r o m t h e Cabot C o r p o r a t i o n . Samples were e x t r a c t e d c o n t i n u o u s l y f o r 48 h o u r s i n a S o x h l e t e x t r a c t o r w i t h c y c l o h e x a n e and t o l u e n e successively. A f t e r t h i s p u r i f i c a t i o n n e i t h e r w a t e r nor t o l u e n e e q u i l i b r a t e d w i t h the e x t r a c t e d Grapho showed absorptio f r o m 340 t o 800 nm i n d i c a t i n ing i n the v i s i b l e r e g i o spectru s u r f a c e a r e a o f t h e e x t r a c t e d Graphon was f o u n d t o be 95.4 m /g by low t e m p e r a t u r e a d s o r p t i o n o f n i t r o g e n by t h e B.E.T. method, u s i n g 16.2Â as t h e a r e a p e r n i t r o g e n m o l e c u l e . T o l u e n e ( A n a l y t i c a l R e a g e n t , M a l l i n c k r o d t Co.) was distilled t h r o u g h a 1-1/2 f o o t column ( A l l i n n c o n d e n s e r ) and c o l l e c t e d between 110.0 t o 110.6°C. S i n c e i t had been shown (5_) t h a t non aqueous d i s p e r s i o n s o f powdered q u a r t z were d e s t a b i l i z e d by t r a c e s o f w a t e r , i t was n e c e s s a r y t o work under s t r i c t l y anhy d r o u s c o n d i t i o n s . A c c o r d i n g l y , a b o u t t h r e e f e e t o f sodium w i r e o f 2 mm d i a m e t e r was p r e s s e d i n t o a g a l l o n o f f r e s h l y d i s t i l l e d t o l u e n e and l e f t f o r 24 h o u r s , and t h e t o l u e n e t h e r e a f t e r s t o r e d o v e r sodium w i r e . G l a s s w a r e used i n t h e e x p e r i m e n t s was d r i e d a t 160°C and used i m m e d i a t e l y a f t e r d r y i n g . P r e c a u t i o n s to main t a i n a b s o l u t e d r y n e s s were n e c e s s a r y , not o n l y b e c a u s e o f t h e d e s t a b i 1 i z a t i o n p o s s i b l e b e c a u s e o f c a p i l l a r i t y e f f e c t s between w a t e r f i l m s on the p a r t i c l e s , but a l s o b e c a u s e any s u c h f i l m c o u l d make p o s s i b l e i o n i z a t i o n o f s u r f a c e i m p u r i t i e s i n t h e G r a p h o n , s u c h as c a r b o x y l g r o u p s , and r e s u l t i n e l e c t r o s t a t i c c h a r g e s on t h e Graphon p a r t i c l e s . P o l y s t y r e n e was o b t a i n e d f r o m t h e P r e s s u r e C h e m i c a l Company. F i v e s a m p l e s o f n o m i n a l m o l e c u l a r w e i g h t s * 20,400, 110,000, 200,000, 498,000 and 1,800,000 were u s e d . For t h e f i r s t t h r e e s a m p l e s t h e r a t i o o f w e i g h t a v e r a g e t o number a v e r a g e m o l e c u l a r w e i g h t i s g i v e n as l e s s t h a n 1.06, and l e s s t h a n 1.20 f o r the l a s t two s a m p l e s . S e v e r a l o f the p r e l i m i n a r y e x p e r i m e n t s were c a r r i e d o u t w i t h Dow R e s i n P-65, a p o l y s t y r e n e o f b r o a d m o l e c u l a r w e i g h t d i s t r i b u t i o n and n o m i n a l m o l e c u l a r w e i g h t 2.3x10^ as d e t e r m i n e d f r o m v i s c o s i t y measurements. Samples were used d i r e c t l y f r o m t h e b o t t l e s i n c e t h e r e was no d e t e c t a b l e change i n w e i g h t ( w i t h i n 0.00002 g) a f t e r d r y i n g two h o u r s a t 112°C and 10"3 torr. "Weight a v e r a g e m o l e c u l a r w e i g h t s o f t h e s e s a m p l e s a r e to be 20,400; 111,000; 200,000; 507,000; and 1 . 9 x 1 0 .
reported
6
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
98
ADSORPTION
A T INTERFACES
Determination of A d s o r p t i o n Isotherms. The amount o f a d s o r p t i o n was d e t e r m i n e d f r o m t h e change i n c o n c e n t r a t i o n o f p o l y s t y r e n e in t h e Graphon s u s p e n s i o n s i n t o l u e n e p r e p a r e d by u l t r a s o n i c a g i t a t i o n a f t e r e q u i l i b r a t i o n i n the presence o f v a r y i n g concen t r a t i o n s o f p o l y s t y r e n e . S i n c e t h e amount a d s o r b e d i s dependent on t h e d e g r e e o f d i s p e r s i o n o f t h e G r a p h o n , t h e t i m e a l l o w e d f o r a t t a i n m e n t o f e q u i l i b r i u m , and t h e amount o f a g i t a t i o n d u r i n g t h i s p e r i o d , e x t e n s i v e p r e l i m i n a r y e x p e r i m e n t s were n e c e s s a r y t o e s t a b l i s h a s a t i s f a c t o r y p r o c e d u r e w h i c h c o u l d be used i n s u b s e q u e n t e x p e r i m e n t s on t h e s t a b i l i t y o f t h e s u s p e n s i o n s . The p r o c e d u r e a d o p t e d f o r the f i n a l d e t e r m i n a t i o n s was a s follows. The Graphon sample was o u t g a s s e d f o r 2h hours a t 160 C, at 10"^ t o r r . A 0.5000 g sample was then w e i g h e d i n t o a 125 ml E r l e n m e y e r f l a s k , f o l l o w e d by 25.0 ml o f t o l u e n e . Each f l a s k was i n d i v i d u a l l y i r r a d i a t e d f o 20 m i n u t e i 100 w a t t 15 KH Delta Sonic u l t r a s o n i intensity. A 25.0 ml a l i q u o t i o n o f p o l y s t y r e n e i n t o l u e n e was added t o e a c h f l a s k i m m e d i a t e l y f o l l o w i n g i r r a d i a t i o n , and t h e f l a s k s t h e n r o t a t e d t h r e e m i n u t e s a t 56 r.p.m. a t an a n g l e o f 30° t o t h e v e r t i c a l by a Borg Equipment D i v i s i o n model 1007-4N r o t o r . They were t h e n l e f t un d i s t u r b e d i n a c o n s t a n t t e m p e r a t u r e room {2k° ±1°C) f o r 14 d a y s . The s u p e r n a t a n t l i q u i d was then d e c a n t e d , and a b o u t 30 ml c e n t r i f u g e d f o r t e n m i n u t e s a t 4800 r.p.m. i n an I n t e r n a t i o n a l C l i n i c a l C e n t r i f u g e , model CL, t o remove r e s i d u a l Graphon R a r t i c l e s . The c e n t r l f u g e d l i q u i d was c l e a r and c o l o r l e s s and had z e r o a b s o r bance a t 710 nm u s i n g t o l u e n e as a r e f e r e n c e l i q u i d . The c o n c e n t r a t i o n o f p o l y s t y r e n e a f t e r a d s o r p t i o n was d e t e r m i n e d by p l a c i n g 25.0 ml o f t h e c e n t r i f u g e d l i q u i d r e c o v e r e d from t h e d i s p e r s i o n i n w e i g h e d " b o a t s " made f r o m aluminum f o i l , and d r y i n g to c o n s t a n t w e i g h t (±0.05 mg) a t 112°C. The " b o a t s " , made by f o l d i n g t h e f o i l , were about 1" h i g h and 3" by 3" s q u a r e a t t h e base. About f o u r t o f i v e hours were r e q u i r e d t o r e a c h c o n s t a n t weight. D u r i n g t h e e q u i l i b r a t i o n the f l a s k s were c l o s e d w i t h two l a y e r s o f S a r a n wrap and a l a y e r o f aluminum f o i l h e l d i n p l a c e by a r u b b e r band a r o u n d t h e n e c k . S i n c e s i m i l a r l y c l o s e d f l a s k s containing i n i t i a l l y 50 ml o f t o l u e n e l o s t an a v e r a g e o f o n l y 0.2523 g w e i g h t In f o u r t e e n d a y s , i t i s a p p a r e n t t h a t any change in c o n c e n t r a t i o n o f t h e p o l y s t y r e n e due t o e v a p o r a t i o n o f t o l u e n e i s n e g l i g i b l e under t h e s e c o n d i t i o n s . The r e s u l t s o f the p r e l i m i n a r y e x p e r i m e n t s c a r r i e d o u t t o e s t a b l i s h t h i s f i n a l procedure are of value i n themselves f o r t h e i n s i g h t t h e y a f f o r d i n t o t h e e f f e c t on a d s o r p t i o n o f macrom o l e c u l e s on d i s p e r s e d p a r t i c l e s o f t h e t i m e a l l o w e d f o r e q u i l i b r a t i o n , o r d e r of a d d i t i o n of r e a g e n t s , degree of a g i t a t i o n of t h e s u s p e n s i o n s , and d e g r e e o f d i s p e r s i o n o f t h e G r a p h o n . Figure 1 shows t h e e f f e c t o f t h e d e g r e e o f d i s p e r s i o n o f Graphon a t 23 ±1°C In a s y s t e m p r e p a r e d by i r r a d i a t i o n o f 0.5000 g o f Graphon In 25.0 ml o f t o l u e n e f o r v a r y i n g l e n g t h s o f t i m e , f o l l o w e d by
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
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addition of 25.0 ml of a 4.00 mg/ml toluene solution of Dow Resin P-65. The suspension was swirled gently for three minutes and l e f t to equilibrate undisturbed for seven days. Each point shown in Figure 1 is the average from three separate experiments. Up to 15 minutes the amount adsorbed increases s i g n i f i c a n t l y with increasing time of i r r a d i a t i o n , as might be expected from the presumed decrease in average aggregate size. Further i r r a d i ation beyond 15 minutes had l i t t l e effect, the amount adsorbed remaining f a i r l y constant at 45.3 to 46.5 mg polystyrene per gram of carbon. It can therefore be assumed that after 15 min utes no additional subdivision of aggregates occurs with further irradiation, analogous to e a r l i e r results (6) showing attainment of a reversible equilibirum particle size distribution of carbon and f e r r i c oxide in aqueous suspensions dependent on the inten sity of agitation. Accordingly i t concluded that 20 minute period of irradiatio to insure reaching the g adsorption The f u l l c i r c l e represents the amount of adsorption found when the solutions were swirled three minutes daily during the equilibration period rather than being left undisturbed. The increase in adsorption shows that even under these mild condi tions some additional deflocculation occurred as a result of the agitation in the presence of the polystyrene, as was also found by other workers (j) who reported increases in adsorption of polymer with agitation. It is significant that even with no ultrasonic irradiation of the suspension 39.4 mg of polystyrene were adsorbed per gram of Graphon. This is about 85 % of the limiting adsorption reached after 20 minutes of irradiation, and demonstrates that poly styrene can reach most of surface area of the Graphon in suspen sion even though most of the particles are present in aggregates. The precision of the measurements improved with longer peri ods of irradiation up to 20 minutes as shown in Table I . This Table gives the relative error (defined in terms of the average deviation from the average value) in the weight of polystyrene adsorbed per gram of Graphon at each time of irradiation for a suspension containing i n i t i a l l y 2 mg of polystyrene (Dow Resin P-65) per ml of toluene. After irradiation the samples were left seven days without agitation before determination of the amount adsorbed, except for one set which were swirled for three minutes daily. Apparently i t is more d i f f i c u l t to reproduce a given degree of dispersion at short periods of irradiation than at longer. The reproducibility was much poorer for samples which were agitated after addition of the polystyrene solution to the Graphon suspension, presumably because the polystyrene could coat and s t a b i l i z e any fresh surface resulting from the agitation, thus increasing the surface area of Graphon available for adsorp tion .
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
ADSORPTION
10
20
A T INTERFACES
30
TIME OF IRRADIATION (minutes )
Figure 1. Adsorption of polystyrene (2 mg/ml, M.W. = 2.3 X 10 ) onto Graphon from toluene as a function of time of ultrasonic irradiation. Time of equilibrium, 7 days. Q, agitation; Φ, 3 minutes of agitation daily. s
1X0
A
0
I I
I 2
I 3
I I I I I I 4 5 6 7 8 9 EQUILIBRATION ,(days)
I 10
1 II
1 12
1 13
1 14
Figure 2. Adsorption of polystyrene (1 mg/ml) onto Graphon from toluene, O, M.W. = 1,800,000; •, M.W. = 498,000; A, M.W. = 250,000, broad distribution
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
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T a b l e I . P r e c i s i o n as a F u n c t i o n o f I r r a d i a t i o n Time I r r a d i a t i o n Time ( m i n u t e s ) 0 10 15 20 30 Average % d e v i a t i o n 1.6 4.5 3-0 1.4 2.2 *These s a m p l e s were s w i r l e d 3 m i n u t e s d a i l y a f t e r
20* 8.0 irradiation
E x p e r i m e n t s such as t h e s e l e d t o t h e a d o p t i o n o f a s t a n d a r d p r o c e d u r e i n v o l v i n g i r r a d i a t i o n o f t h e s u s p e n s i o n f o r 20 m i n u t e s , and no f u r t h e r a g i t a t i o n a f t e r m i x i n g t h e p o l y s t y r e n e s o l u t i o n w i t h t h e Graphon s u s p e n s i o n . I t s h o u l d be n o t e d t h a t i r r a d i a t i o n must be c a r r i e d o u t p r i o r t o a d d i t i o n o f p o l y s t y r e n e . When G r a phon s u s p e n s i o n s c o n t a i n i n g p o l y s t y r e n e were i r r a d i a t e d f o r h a l f a m i n u t e i t was f o u n d t h a t t h o s e c o n t a i n i n g p o l y s t y r e n e o f h i g h e r m o l e c u l a r w e i g h t had a much h i g h e r a b s o r b a n c e , i n d i c a t i v e o f a s m a l l e r average aggregate s i z e . C o n s e q u e n t l y no d i r e c t d e t e r m i n a t i o n o f t h e dependenc lar weight o f the polystyren so p r e p a r e d b e c a u s e o f t h e d i f f e r e n c e s i n t h e i r i n i t i a l s t a t e . U s i n g t h e s t a n d a r d method o f p r e p a r a t i o n o f s u s p e n s i o n s and d e t e r m i n a t i o n o f amount a d s o r b e d a l r e a d y d e s c r i b e d , s e v e r a l e x p e r i m e n t s were p e r f o r m e d t o d e t e r m i n e t h e amount o f a d s o r p t i o n as a f u n c t i o n o f t i m e o f e q u i l i b r a t i o n and o f t h e m o l e c u l a r weight of the polystyrene. These d a t a showed how much t i m e had to be a l l o w e d t o r e a c h t h e s a t u r a t i o n v a l u e o f t h e a d s o r p t i o n . On t h e b a s i s o f a l l t h e s e p r e l i m i n a r y e x p e r i m e n t s i t was t h e n p o s s i b l e t o s p e c i f y f o r t h e p o l y s t y r e n e samples o f d i f f e r e n t m o l e c u l a r w e i g h t s an i n i t i a l c o n c e n t r a t i o n s u c h t h a t t h e e q u i l i b r i u m c o n c e n t r a t i o n r e m a i n i n g i n t h e s o l u t i o n w o u l d be s u f f i c i e n t to m a i n t a i n m o n o l a y e r c o v e r a g e o f t h e Graphon. R e s u l t s and D i s c u s s i o n Data on t h e r a t e o f a d s o r p t i o n f o r p o l y s t y r e n e o f h i g h , low and i n t e r m e d i a t e m o l e c u l a r w e i g h t a r e shown i n F i g u r e 2. Few mea s u r e m e n t s were made a t s h o r t p e r i o d s o f e q u i l i b r a t i o n s i n c e t h e p r i m a r y p u r p o s e o f t h e s e e x p e r i m e n t s was t o e s t a b l i s h t h e l e n g t h of time necessary t o reach apparent a d s o r p t i o n e q u i l i b r i u m . Where s t u d i e d , a d s o r p t i o n was r e l a t i v e l y r a p i d d u r i n g t h e f i r s t two d a y s , a m o u n t i n g t o a b o u t 70% o f t h e q u a n t i t y a d s o r b e d a f t e r f o u r t e e n d a y s . The l i m i t i n g v a l u e o f t h e amount a d s o r b e d a p p e a r s to have been r e a c h e d a f t e r t e n days e q u i l i b r a t i o n w i t h a l l t h r e e polystyrenes. The a d s o r p t i o n i s o t h e r m s o f p o l y s t y r e n e s o f d i f f e r i n g m o l e c u l a r w e i g h t s as d e t e r m i n e d by o u r s t a n d a r d p r o c e d u r e a r e shown i n F i g u r e 3. The amount a d s o r b e d a t f i r s t i n c r e a s e s r a p i d l y w i t h c o n c e n t r a t i o n , and t h e n l e v e l s t o a p l a t e a u a t an e q u i l i b r i u m c o n c e n t r a t i o n o f p o l y s t y r e n e o f 1 mg/ml. The shape o f t h e s e i s o t h e r m s i s s u g g e s t i v e o f m o n o l a y e r a d s o r p t i o n , and r e s e m b l e s t h a t f o u n d by S c h i c k and H a r v e y ( 8 ) . They s t u d i e d t h e a d s o r p t i o n o f a p o l y s t y r e n e o f M 292,000 o n t o Graphon f r o m s e v e r a l s o l v e n t s ,
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
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A T INTERFACES
Figure 3. Adsorption of polystyrene from toluene onto Graphon. ·, M.W. = 110,000; A, M.W. = 200,000; O, M . W . = 498,000; • , M.W. = 1,800,000.
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
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i n c l u d i n g t o l u e n e , and a l s o f o u n d t h a t a l i m i t i n g a d s o r p t i o n c h a r a c t e r i s t i c o f m o n o l a y e r a d s o r p t i o n was r e a c h e d . The p r e s e n t work shows t h a t t h e maximum amount o f p o l y s t y r e n e a d s o r b e d on Graphon i n c r e a s e s f r o m 39 t o 69 mg/g as t h e m o l e c u l a r w e i g h t i n c r e a s e s f r o m 110,000 t o 1,800,000. In F i g u r e k t h e s a t u r a t i o n a d s o r p t i o n i s p l o t t e d a g a i n s t t h e square root o f the molecular weight o f the adsorbed p o l y s t y r e n e . The amount a d s o r b e d v a r i e s l i n e a r l y w i t h t h e s q u a r e r o o t o f t h e molecular weight according t o the equation, (1) where W Is mg o f p o l y s t y r e n e a d s o r b e d p e r gram o f Graphon and M i s the molecular weight of the p o l y s t y r e n e . The l i n e e x t r a p o l a t e s t o a n o n - z e r o i n t e r c e p t a n o t uncommon r e s u l t i n t h e a d s o r p t i o n o f polymers The f a c t t h a t t h E q u a t i o n (1) l e a d s t o some i n t e r e s t i n g c o n c l u s i o n s c o n c e r n i n g the c o n f o r m a t i o n o f t h e a d s o r b e d p o l y s t y r e n e . As e x t r e m e l i m i t i n g c a s e s t h e p o l y m e r m o l e c u l e c o u l d be h e l d c o m p l e t e l y f l a t on t h e s u r f a c e , o r i t c o u l d be p r e s e n t as e s s e n t i a l l y r i g i d rods in c l o s e packing o r i e n t e d p e r p e n d i c u l a r l y t o the s u r f a c e . If the m o l e c u l e s a r e f l a t t h e w e i g h t a d s o r b e d a t s a t u r a t i o n s h o u l d be v e r y n e a r l y i n d e p e n d e n t o f t h e m o l e c u l a r w e i g h t , whereas i n a p e r p e n d i c u l a r o r i e n t a t i o n I t w o u l d be d i r e c t l y p r o p o r t i o n a l t o the molecular weight. S i n c e t h e d a t a g i v e n i n F i g u r e k show t h a t n e i t h e r o f t h e s e p r e d i c t i o n s i s c o r r e c t , i t c a n be c o n c l u d e d t h a t t h e c o n f o r m a t i o n o f t h e a d s o r b e d m o l e c u l e must be somewhere In between t h e two e x t r e m e s i t u a t i o n s . Another p o s s i b i l i t y would be f o r t h e m o l e c u l e t o be a n c h o r e d t e r m i n a l l y b u t t o have a r a n dom c o i l c o n f i g u r a t i o n i n s o l u t i o n . T h i s would r e q u i r e t h a t t h e amount a d s o r b e d be d i r e c t l y p r o p o r t i o n a l t o t h e s q u a r e r o o t o f t h e m o l e c u l a r w e i g h t , w h i c h i s c o n t r a d i c t e d by t h e n o n - z e r o i n t e r c e p t i n d i c a t e d by E q u a t i o n ( 1 ) . A model c l o s e l y r e s e m b l i n g t h o s e I l l u s t r a t e d by Eîrich e t a l (10), which i s i n accord w i t h the e m p i r i c a l e q u a t i o n r e p r e s e n t i n g t h e d a t a , i s shown i n F i g u r e 5. The a d s o r b e d l a y e r may be d i v i d e d i n t o two r e g i o n s . In r e g i o n (1) t h e monomer segments a r e a n c h o r e d f l a t d i r e c t l y on t h e s u r f a c e , t h e w e i g h t o f segments a d s o r b e d i n t h i s f a s h i o n b e i n g I n d i c a t e d as W . In r e g i o n (2) t h e segments a r e p r e s e n t as l o o p s o r t a i l s i n t h e s o l u t i o n n o t i n a c t u a l c o n t a c t w i t h t h e s u r f a c e , t h e w e i g h t so a d s o r b e d b e i n g i n d i c a t e d as W. Accordingly, i f W i s the t o t a l weight o f poly mer a d s o r b e d , i t f o l l o w s t h a t t
Q
t
= w + w (2) t
8
15
25
8
Number of P o i n t s
6
13
19
6
Number of P o i n t s i n ~ 6*
Average Temperatures of Thermal Anomalies O c c u r r i n g i n Regions I I - V .
II
Region
Table V I I I .
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anomalies are mainly changes i n s l o p e , displacements of base l i n e , or ( l e s s f r e q u e n t l y ) s m a l l endothermic peaks ( i n heating curves). The changes i n slope and the changes i n b a s e l i n e w i l l be discussed f i r s t . As a l l the r e s u l t s were obtained w i t h a c a l o r i m e t e r c e l l , the changes r e f l e c t v a r i a t i o n s i n s p e c i f i c heat and/or heat c o n d u c t i v i t y . I n e i t h e r case, such changes are most l i k e l y a s s o c i a t e d w i t h changes i n the s t a t e of water at the s i l i c a / w a t e r i n t e r f a c e . Most l i k e l y , the changes are due p r i m a r i l y to v a r i a t i o n s i n heat c a p a c i t y . However, whether the changes are i n the heat c a p a c i t y or due to changes i n thermal c o n d u c t i v i t y , these changes must r e f l e c t s t r u c t u r a l phenomena. Unusual changes i n thermal c o n d u c t i v i t y of b u l k water have been reported by Frontasev (27). However, those changes are l i k e l y spurious e f f e c t s due to i n t e r f a c i a l phenomena, as discussed by one of us (9, 2J3, 29) r a t h e r than m a n i f e s t a t i o n s of a b u l k pro p e r t y . Metzik and Aidanov thermal c o n d u c t i v i t i e b r i e f survey a r t i c l e by M e t z i k , et a l . , (31). F i n a l l y , note t h a t any change i n s t r u c t u r e i s l i k e l y to r e s u l t i n changes i n both the s p e c i f i c heat and the thermal c o n d u c t i v i t y . DTA has f r e q u e n t l y been used f o r the study of anhydrous l i p i d s , aqueous l i p i d systems and various b i o l o g i c a l l y important membrane systems (32). Extremely complicated thermograms have been reported f o r human e r y t h r o c y t e membranes (33) and an i n t e r e s t i n g c l i n i c a l a p p l i c a t i o n of DTA has been reported by Heyd i n g e r , et a l . (34). Only r a r e l y has d i f f e r e n t i a l thermal a n a l y s i s been used s p e c i f i c a l l y to study the p r o p e r t i e s of i n t e r f a c i a l water (see, however, B u l g i n and Vinson) (35). We observe i n passing that thermal anomalies are found at or near the same temperature regions i n i n t e r f a c i a l p r o p e r t i e s of water adjacent to a l a r g e v a r i e t y of m a t e r i a l s , i n c l u d i n g i o n i c , p o l a r and non-polar s o l i d s . This s u r p r i s i n g observation has been r e f e r r e d to as the " p a r a d o x i a l e f f e c t " (12, 36). The r o l e of adsorbed water on v a r i o u s h y d r o p h i l i c surfaces has been s t u d i e d by Bernett and Zisman (37), who conclude t h a t "the c r i t i c a l s u r f a c e t e n s i o n , as w e l l as the s u r f a c e energy of c l e a n , high-energy h y d r o p h i l i c s o l i d s ( l i k e b o r o s i l i c a t e g l a s s , fused quartz and c r y s t a l l i n e ctf-alumina) a f t e r exposure to a humid atmosphere i s dependent upon the s u r f a c e concentration of water adsorbed on that s u r f a c e , but that i t i s independent of the chemical nature of the underlying s o l i d " . They f u r t h e r conclude that t h i s observation may l i k e l y be extended to any other h i g h energy h y d r o p h i l i c s u r f a c e (metals and metal o x i d e ) . An i n t e r e s t i n g review of the p r o p e r t i e s of water near surfaces has been reported by Hartkopf and Karger (38), who have s t u d i e d aqueous i n t e r f a c i a l p r o p e r t i e s by gas chromatography. This study - w h i l e not n e c e s s a r i l y g e n e r a l l y c o n s i s t e n t w i t h the f i n d i n g s by Drost-Hansen [compare p a r t i c u l a r l y w i t h Ref. (3)] - i s i g n i f i c a n t , as i t provides an experimental approach to the im p o r t a n t , but f r e q u e n t l y neglected, area of study of l a y e r s of s
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water w i t h thicknesses ranging from 3 nm to about 200 nm. Other techniques f o r the study of l a y e r thicknesses i n t h i s r e g i o n have been reported by Tschapek and N a t a l e (39); see a l s o the a r t i c l e by Adamson regarding s t u d i e s of f i l m thicknesses w e l l exceeding monolayers (40). The s m a l l endothermic peaks observed i n some thermograms during h e a t i n g present a problem i n i n t e r p r e t a t i o n . Such peaks were f r e q u e n t l y , but not i n v a r i a b l y , observed. I t i s impossible to determine i f these peaks are " r e a l " or a r t i f a c t s . Assuming the peaks are indeed r e a l , and i f a l l the water i n the sample undergoes a f i r s t - o r d e r phase t r a n s i t i o n , the apparent heat of t r a n s i t i o n i s between 10 to 100 c a l / g mole water. While extremely s m a l l , such values are not e n t i r e l y unreasonable. R e c a l l that the l a t t i c e f r e e energies between Ice I and Ice I I i s only 19 c a l / g mole and that th hea f transitio adjacen high-pressure polymorph most, by 550 c a l / g mole and f r e q u e n t l y by only 200-300 c a l / g mole (6, 12) . One of us has already suggested that i n view of the ease w i t h which water molecules may be packed i n v a s t l y d i f f e r e n t s t r u c t u r e s , at r e l a t i v e l y minor e n e r g e t i c expenses, i t i s r e a sonable to assume that many d i f f e r e n t types of v i c i n a l water s t r u c t u r e may a l s o e x i s t - each w i t h i t s own thermal s t a b i l i t y domain. On the other hand, the f a c t that the endothermic peaks are not i n v a r i a b l y observed, as w e l l as the general behavior of the (thermal) p r o p e r t i e s of v i c i n a l water, suggest that the changes observed at the " c r i t i c a l temperature r e g i o n s " may r e f l e c t h i g h e r - o r d e r phase t r a n s i t i o n s (and t h u s , not be expected to be a s s o c i a t e d w i t h a heat of t r a n s i t i o n ) . A l a r g e change i n s p e c i f i c heat may p o s s i b l y g i v e r i s e to the "apparent" heat of t r a n s i t i o n . Furthermore, although i n p r i n c i p l e s t r a i g h t f o r w a r d , there appears to be some u n c e r t a i n t y as to the d i s t i n c t i o n be tween f i r s t and second-order phase t r a n s i t i o n s . See, f o r i n stance, the comments by Lieb (41) and by Mayer (42). While thermal anomalies are reported f r e q u e n t l y i n the l i t erature - where s u f f i c i e n t l y c l o s e l y - s p a c e d measurements have been made on i n t e r f a c i a l systems - n o t a b l e exceptions have been encountered. See, f o r i n s t a n c e , the u l t r a s o n i c study reported by Younger, et_ a l . (43). I t i s of i n t e r e s t to note that i n the p r e sent experiments, anomalous thermograms have been obtained under very w i d e l y d i f f e r i n g c o n d i t i o n s . Thus, comparisons of samples of porous glass exposed merely to d i f f e r e n t r e l a t i v e h u m i d i t i e s (say, 52% v s . 95%) have produced unmistakeable changes i n s l o p e . Q u a l i t a t i v e l y s i m i l a r r e s u l t s are a l s o obtained i n s t u d i e s where l a r g e amounts of water have been present ( f o r i n s t a n c e , 5 micro l i t e r s per 5 mg of s o l i d ) . This suggests that the phenomenon i s indeed an i n t e r f a c i a l phenomenon ( 9 ) , although the anomalies are c e r t a i n l y not p r i m a r i l y c h a r a c t e r i s t i c of a mono-molecular l a y e r or pauci-molecular l a y e r ; compare the d i s c u s s i o n of the depth of the v i c i n a l water as estimated on the b a s i s of the measurements by P e s c h e l and A d l f i n g e r (19). The study of the nature of the
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
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thermal anomalies is continuing in our laboratory by Differential Scanning Calorimetry (using sealed "cups" to eliminate the spur ious evaporation effects). Summary The data obtained from a DTA calorimeter study of water in samples of porous glass beads ("Bioglas", average pore diameters 20, 100, 200 or 250 nm) suggest that the water in these systems undergoes relatively sharp transitions over certain narrow tem perature intervals (notably near 30 to 33°C, 44 to 46°C and 59 to 52°C) in good agreement with earlier observations by DrostHansen C3, 8, 9, 10, Π ) , Peschel and Adlfinger (JU3, 19) and others (23, 24). Because of experimental difficulties (notably evaporation effects), i tainty i f the observed first-order phase transitions or higher-order phase transitions, as proposed earlier by one of us (9, 12). However, the data do confirm the existence of anomalous temperature dependencies in the thermal properties of interfacial water. Acknow1edgement One of us (W. Drost-Hansen) wishes to thank Dr. S. A. Bach for introducing him to the use of DTA (calorimetric) measure ments for the study of thermal properties of aqueous systems. The authors also wish to thank Professor Curtis R. Hare for helpful discussions; Mr. Lawrence Korson for his assistance in solving various experimental problems, and Ms. Lynda Weller for preparing the manuscript. Literature Cited 1.
Henniker, J . C.
Rev. Mod. Phys., (1949), 21(2), 322.
2.
Low, P. F.
3.
Drost-Hansen, W.
4.
Deryagin, Β. V., Ed. "Research i n Surface Forces", Vol. 3, Consultants Bureau, New York, 1971.
5.
Franks, F e l i x , Ed. "Water, A Comprehensive Treatise", Vols. 1,2,3,4,5, Plenum Press, New York, 1972, 1973, 1974.
6.
Eisenberg, D. and Kauzmann. "The Structure and Properties of Water", Oxford University Press, New York, 1969.
7.
Home, R. Α., Ed. "Water and Aqueous Solutions: Structure, Thermodynamics and Transport Processes", Wiley-Interscience, New York, 1972.
Advan. i n Agronomy, (1961), 13, 269. Ind. Eng. Chem., (1969), 61(11), 10.
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
154
ADSORPTION AT INTERFACES
8.
Drost-Hansen, W. Ind. Eng. Chem., (1965), 57(4). (Also published in "Chemistry and Physics of Interfaces". Sympo sium Proceedings, Sidney Ross, Ed., Am. Chem. S o c ., Washing ton, D. C., (1965).
9.
Drost-Hansen, W. Chem. Phys. Lett., (1969), 2, 647.
10. Drost-Hansen, W. J . Geophys. Res., (1972), 77(27), 5132. 11. Drost-Hansen, W. Soc. Exp. Biology, Symposium Proceedings, (1972), XXVI, 61. 12. Drost-Hansen, W. in "Chemistry of the Cell Interface", B, H. D. Brown, Ed., Chapter 6, pp. 1-184, Academic Press, New York, 1969. 13. Drost-Hansen, W. Naturwissen., (1956), 43, 512. 14. Temperley, Η. Ν. V . , "Changes of State", Cleaver-Hume Press Ltd., London, 1956. 15. Ubbelohde, A. R., "Melting and Crystal Structure", Clarendon Press, Oxford, 1965. 16. Stanley, H. E. "Introduction to Phase Transitions and Critical Phenomena", Oxford University Press, 1971. 17. Peschel, G. Zeit. Phys. Chemie, Neue Folge, (1968), 59, 27. 18. Peschel, G. and Adlfinger, Κ. H. Naturwissen., (1971), 56, 558. 19. Peschel, G. and Adlfinger. Zeit. F. Naturforsch., (1971), 26a, 707. 20. Kerr, J . E. D., "Relaxation Studies on Vicinal Water", un published Ph.D. Dissertation, University of Miami, Coral Gables, Florida, 1970. 21. Thurn, Η., Materialprűf., (1963), 5(3), 114. 22. Kratky, O., Leopold, H. and Stabinger, H. Zeit. Angew. Physik., (1969), 27, 273. 23. Forslind, E. Svensk Naturv., (1966), 2, 9. 24. Wiggins, P. Μ., "Thermal Anomalies in Ion Distribution". Submitted for publication, 1974; see also Biophys. J., (1973), 13, 385.
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
9.
LING AND DROST-HANSEN
Water in Porous Glass
155
25.
Wiggins, P. M. In press, J. Theoret. Biol., (1974).
26.
Cooper, A. R., Cain, J. H., Barrall, Ε. M., II and Johnson, J. F., Separ. Sci., (1970), 5, 787.
27.
Frontas'ev, V. P., Dokl. Akad. Nauk SSSR, (1956), III(5), 1014.
28.
Drost-Hansen, W. in equilibrium Concepts in Natural Water Systems", Adv. Chem. Ser., No. 67, pp. 70-120, American Chemical Society, Washington, D. C., 1967.
29.
Drost-Hansen, W. Proceedings First International Symposium on Water Desalination, Vol. I, pp. 382-406, U. S. Govern ment Printing Office
30.
Metzik, M. S. and Aidanova, 0. S. in "Research in Surface Forces", Vol. 2, Β. V. Deryagin, Ed., pp. 169-180, Consul tants Bureau, New York, 1966. See also ibid., Vol. 3, pp. 34-35, 1971.
31.
Metzik, M. S., Perevertaev, V. D., Liopo, V. Α., Timoschtchenko, G. T. and Kiselev, A. B. J . Colloid Interface Sci., (1973), 43, 662.
32.
Oldfield, E. and Chapman, D. Fed. Am. Soc. Exp. Biol. Letters, (1972), 23(3), 285.
33.
Jackson, W. Μ., Kostyla, J., Nordin, J. H. and Brandts, J. F. Biochem., (1973), 12(19), 3662.
34.
Heydinger, D. Κ., Hammer, E. J., Pgeil, R. W. and Taylor, P. H. J. Lab. Clin. Med., (1971), 77(3), 451.
35.
Bulgin, J . J . and Vinson, L. J. Biochimica Biophysica Acta, (1967), 136, 551.
36.
Drost-Hansen, W. Ann. Ν. Y. Acad. Sci., (1973), 204, 100.
37.
Bernett, K. and Zisman, W. A. J . Colloid Interface Sci., (1969), 29(3), 413.
38.
Hartkopf, A. and Karger, B. L. Accounts Chem. Res., (1973), 6, 209.
39.
Tschapek, M., Natale, I. and Santamaria, R. Kolloid-Z., Z. Polym., (1965), 211, 143.
40.
Adamson, A. J . Colloid Interface Sci., (1973), 44, 273.
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
156
ADSORPTION AT INTERFACES
41.
Lieb, E. in "Phase Transitions", Solvay Institute, Proceedings XIV, Conference on Chemistry at University of Brussels, pp. 14-15, Interscience Publishers, London, 1971.
42.
Mayer, J. E. in "Phase Transitions", Solvay Institute, Proceedings XIV, Conference on Chemistry at University of Brussels, p. 15, Interscience Publishers, London, 1971.
43.
Younger, P. R., Zimmerman, G. O., Chase, C. E. and DrostHansen, W. J. Chem. Phys., (1973), 58(7), 2675.
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
10 Monolayer Studies V . Styrene-Acrylic Acid Films on Aqueous Substrates ERWIN SHEPPARD and NOUBAR TCHEUREKDJIAN Chemical Research Department, S. C. Johnson and Son, Inc., Racine, Wis. 54303
Introduction Low molecular weight styrene-acrylic acid copolymers of varying degrees of polymerization were spread as monolayers on water and various salt solutions at neutral and alkaline pH's. The monolayer properties of high molecular weight styreneacrylic acid systems were studied by Müller (1), who could not attain complete spreading and consequently monolayers were not formed. Monomolecular film behavior of other high molecular weight copolymers have been studied successfully. For example, Fowkes et al. (2) have reported on C8-C18 α-olefins-vinyl acetate systems, Labbauf and Zack (3) have reported on methyl methacrylate-styrene systems, and Hironaka et al. (4) have discussed films of methyl acrylate and n-butyl acrylate copoly mers. Glazer (5) has reported on the spreading properties of ethoxylin resins with molecular weights from 400-2500 prepared by the reaction of epichlorohydrin and 4,4-dihydroxydiphenylpropane. The adhesive properties of these resins were corre lated with their interfacial spreading properties. Experimental Table I lists the styrene-acrylic acid, S/AA, copolymers studied, the number average molecular weights, Mn, and the acid numbers or neutralization values defined as milli-equivalents of base needed to neutralize one gram of the copolymer. The theoretical neutralization value was 4.5 meq. The starting monomer mole ratio for the polymerization was 0.41 for acrylic acid and 0.59 for styrene. The initiator used was benzoyl peroxide. The molecular weights were determined by vapor phase osmometry using ethanol as the solvent. Pressure-area isotherms at 22°C were determined by the Wilhelmy platinum hanging plate method with automatic recording of both the film pressure and the area (6). Minor modifications were incorporated in the design of Mauer's (7) analytical 157 In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
158
ADSORPTION A T INTERFACES
Table i . Sample 1 2 3 4 5 β 7
Description of Styrene/Acrylic Acid Copolymers Initiator Level (mole %) 0.5 1 2 3 5 6 7
Meq. of base/g of copolymer 4.89 4.32 4.18 4.12 3.96 3.96 3.80
M
n
2540 2400 2250 1750 1500 1280
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
10.
SHEPPARD
AND
TCHEUREKDjiAN
Styrene-Acrylic Acid Films
159
balance for recording changes in weight keeping the sensing plate nearly stationary. International Rectifier Type CS120V6 photocells were used in the Wheatstone bridge c i r c u i t , miniatur ized equivalent tubes in the amplifier c i r c u i t , and a 75 V D.C. power supply. The film trough and barriers used to clean the substrate surface and compress the monolayers were FEP Teflon-coated tool aluminum. The trough was rinsed with deionized water before each determination. Deionized d i s t i l l e d water and materials of highest purity available were used throughout. The mono layers were spread from 10% ethanol and 90% benzene (w/w) solu tions and were formed on deionized d i s t i l l e d water, on sub strates adjusted to pH=9, on dilute solutions containing various cations and on their hydroxides. About 15 minutes were allowed for the spread films to equilibrate before compression was started at a constant rat Gaines (8) has summarized most of the pertinent experi mental techniques and precautions to be used in measuring mono layer properties. Results and Discussion Terminology. Monolayers of styrene/acrylic acid low molec ular weight copolymers were studied i n order to determine the interfacial properties among the several samples of different molecular weights and acid values and to determine the effect of pH and of various cations on their spreading behavior at air/aqueous solution interfaces. The specific area, Ao> at zero film pressure was obtained by extrapolating the linear portion of pressure-area isotherm to the area axis as shown in Figure IB. In this study atypical 7Γ-Α curves as represented i n Figure 1A were obtained for the S/AA films under certain conditions. The pressures at which the isotherms deviated from linearity in the direction of higher compressibilities, - A " (dA/d7r)-r, were designated as 7 T , collapse pressure for the expanded region, and TTçç, collapse pressure for the condensed region. The former represented the i n i t i a l collapse point and the maximum film pressure in going from the higher areas to the transition region while the latter represented the f i n a l collapse pressure and the highest pressure attained prior to f i n a l collapse of the spread f i l m . The designations for the various regions of the isotherm were made purely for convenience i n this discussion and to describe the high and low area regions, and the transition area regions (areas between AQE and AQC)· Here AQE represents the extrapolated limiting value of the high area region and AQC represents the extrapolated limiting value of the low area region. 1
CE
Spreading of S/AA Copolymers on Water. Hydrocarbon homopolymers such as polyethylene and polystyrene do not spread
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
160
ADSORPTION A T INTERFACES
completely on water and water soluble polymers such as polyacrylic acid give unstable films on neutral substrates (8). Thus a proper balance of water soluble and water insoluble groups i s required in order to obtain stable, completely spread monolayers of copolymers. In Figure 2 are shown three typical pressurearea isotherms for the S/AA copolymers spread as monolayers at the air-water interface. Each of the seven copolymers exhibited the three distinct monolayer regions which are shown schematical ly i n Figure 1A. They have a highly compressible intermediate transition region as characterized by the A A Q = A O E - A O C value which i s the magnitude of the transition region. Monolayer film areas, collapse pressures, and A A Q ' S for the copolymers spread on water are recorded in Table II. A plot of A A Q VS. copolymer acid number i s given in Figure 3. When the linear region of the A A Q vs. copolymer acid number curve was extrapolated t o A A g ^ O was found to be about 4.5. In other words, a copolymer of this series with an acid number of at least 4.5 i s required i n order to enhance spreading without intermediate collapse pressure and transition regions. Copolymers with acid numbers less than 4.5 would have their characteristic p a r t i a l l y collapsed transition regions which divide the high and low film area regions of the isotherms. It i s fortuitous that the polymerization process u t i l i z e d a starting monomer composition with a theoretical neu tralization value of 4.5 meq. which i s numerically identical to the extrapolated limiting value of 4.5 meq. for the spread films when ΔΑο=0. In spite of the hydrophilic nature of the S/AA copolymers, collapse of the monolayers started at r e l a t i v e l y high film areas. The region over which collapse occurred i s controlled by the copolymer composition as stated above and shown in Figure 3. Therefore, the interfacial film properties of the copolymers may be altered at w i l l to give any desired surface intermolecular cohesion and film-substrate adhesional properties. The transition region, A A Q , extended over larger monolayer areas when the copolymer acid number was decreased. The lower molecular weight S/AA copolymers were relatively more hydrophobic when compared to the higher molecular weight samples as indicated in Table I. As the surface film pressure i s increased the nonfilm forming, hydrophobic portions of the copolymers are assumed to be squeezed out of the film carrying along some of their film forming neighbors. Depending upon the structure of the copoly mers, i t appears that more of a polymer chain could be squeezed out of the surface film with increasing polystyrene concentra tion i n the copolymer. At high pressures, the composition and structure of the copolymers appeared to dominate the AQC values while at lower pressures the strongly hydrophilic nature of the acrylic acid groups appeared to control the spreading character i s t i c s of the films without regard to the polystyrene content as indicated by the almost constant AQE values except for Sample 1 which appears to be a special case. These data are shown in
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
SHEPPARD
AND
TCHEUREKDjiAN
Styrene-Acrylic Acid Films
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
162
ADSORPTION AT INTERFACES
i9 g
W O
H
CM ^ -
^
^
ΙΛ
Ο
Ο
Ο
Ο
Ο
' Q
35) that in stearic acid monolayers each calcium ion coordinates with four carboxyl ions to produce a copolymeric soap l a t t i c e which i s shown i n two dimensions i n Figure 6(a). The copolymeric structure composed of ionized carboxyl groups and calcium ions permits a maximum charge interaction i n stearic acid monolayers, and hence, gives a very low (negative) value of average potential per molecule (Figures 4 and 5). However, the presence of about 10 mole % or more of stearyl alcohol i n stearic acid monolayer causes considerable disruption of the copolymeric structure, and hence, converts the copolymeric structure into a calcium distearate form. Any further increase i n the fraction of mole % of stearyl alcohol simply dilutes the calcium distearate units i n the mixed monolayer. This interpertation i s supported by the results shown i n Figure 4. 2
2
2
2
2
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
11.
177
Interaction of Calcium Ions
SHAH
/TRIS + NaCl
Figure 5. The maximum surface potential of stearic acid-stearyl alcohol mixed monohyers on various subsolutions at pH 8.8 in the presence (A) and absence (Q) of CaCl . For subsolutions of 0.05M tris + 0.02M NaCl at pH 8.8, the kinks in the surface potentials occur at molar ratios 9:1,3:1, and 1:3. 2
I ACID (0)
.75 .50 25 0 MOLE FRACTION (I ALCOHOL)
Figure 6. Schematic of stearic acidr-stearyl alcohol —I 1 1 1 1—^—monohyers on subsolutions of 0.05M tris buffer + )>
e
()
d h
5
00
e
Upon i n t e g r a t i o n of Equation then f o l l o w s that γ = 2σ
α
+ AF
+ IIh
e
(4) at constant Τ and y£
( i > 2) i t
(6)
e
The Gibbs a d s o r p t i o n equation f o r the b u l k s u r f a c e reads ά σ
α
=
_ s a s
g
.
d T
Γ
α
ά μ
^
( 7 )
which s u b s t r a c t e d from Equation d(AF
(4) gives
+ IIh ) = - 2A sdT + hdll - 2$ s
e
s
s
Ar dy
2
e
f
s
i
(8)
i
a
where A s = s - s and ΔΙ\ = Γ? - Γ? are excess q u a n t i t i e s of the f i l m surface over that of the bulk s u r f a c e . In our case i = l i s the s o l v e n t , i=2 i s sodium dodecyl sulphate and i=3 i s NaCl. Surface Excess of S a l t and Detergent. I t w i l l be apparent from Equation (8) that ΔΓ = A fl DDS ^ salt obtained from 9 A F / 8 y 2 and 3 A F / 8 y , r e s p e c t i v e l y . From the graphs i n Figure 1 and known mean a c t i v i t y c o e f f i c i e n t s ΔΓ3 can be c a l c u l a t e d . (The term Ilh i s négligeable i n our case and a s l i g h t c o r r e c t i o n f o r the change of \iz w i t h y was taken i n t o account.) The r e s u l t s are given i n Table I . r
2
a n d
Δ 1 ? 3
=
r
c a n
b e
a
3
3
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
12.
DE
FEijTER
AND
VRij
187
Ή ewton-Black Soap Films
Table I . C a l c u l a t e d Values o f the Adsorption o f E l e c t r o l y t e i n NB-soap-films drawn from an Aqueous S o l u t i o n c o n t a i n i n g 0.05% Na-dodecyl Sulphate and NaCl r e s p e c t i v e l y Na2S0if.
ΔΓ V
3
χ 10
1 1
2
( e q u i v a l e n t cm" )
Τ °C 20.5
22.0
23.5
25.0
26.5
29.5
1.39
1.35
1.34
1.33
1.29
1.22
1.72
1.71
salt NaCl NazSOi»
Δ Γ was found t o be s m a l surface Γ = 42 χ 1 0 " u n i t area at a f i l m surface minus t h a t a t the b u l k s u r f a c e . I t w i l l be c l e a r from Table I t h a t Δ Γ i s p o s i t i v e and o n l y s l i g h t l y dependent on temperature. F u r t h e r , Δ Γ 3 i s f o r NaaSOi* l a r g e r than f o r NaCl. We f i r s t i n v e s t i g a t e d what the d i f f u s e e l e c t r i c a l double l a y e r p r e d i c t s about Δ Γ . At s i n g l e e l e c t r i c a l double l a y e r s counterions are a t t r a c t e d and co-ions are e x p e l l e d from the s u r f a c e . The o v e r a l l e f f e c t i s an e x p u l s i o n o f the ( n e u t r a l ) e l e c t r o l y t e ( s o - c a l l e d negative a d s o r p t i o n ) . For h i g h s u r f a c e potentials Γ ^ -2C3K" , where κ" i s the d i f f u s e double l a y e r t h i c k n e s s . I n a f i l m the e l e c t r i c a l double l a y e r s overlap and Γ3^ w i l l be s m a l l e r i n magnitude, i . e . Δ Γ i s p o s i t i v e i n accordance w i t h the experimental r e s u l t s i n Table I . The p r e c i s e c a l c u l a t i o n o f Δ Γ f o r d i f f u s e , o v e r l a p p i n g double l a y e r s i s complicated and i n v o l v e s e l l i p t i c a l i n t e g r a l s (1_, 2). We made an attempt t o i n t e r p r e t the numbers i n Table I (at 23.5°C) i n the f o l l o w i n g way. I n our system the d i f f u s e p a r t of the double l a y e r s i s probably s m a l l . Moreover i n the f i l m we expect a n e a r l y t o t a l overlap o f these d i f f u s e double l a y e r s , so t h a t T ^ 0 and Δ Γ * 0 - (-Γ ) - Γ . Thus t h i s would mean t h a t we can ( i n an i n d i r e c t way) o b t a i n i n f o r m a t i o n about the d i f f u s e double l a y e r at the s i n g l e (bulk) s u r f a c e . The d i s t a n c e between the Outer Helmholz (OH)-planes i n the f i l m , 2 d , i s d i f f i c u l t t o estimate. From the e q u i v a l e n t water t h i c k n e s s , r ^ , of the f i l m and an assessed d e n s i t y and r e f r a c t i v e index of the hydrocarbon chains we obtained a t h i c k n e s s h = 1.6 nm f o r the aqueous core o f the f i l m . S u b s t r a c t i n g from £h the diameter o f the hydrated SOi* -group (say, 0.48 nm) and the r a d i u s o f the c o - i o n (Cl' ) (say, 0.24 nm) gives f o r d a value i n the order of one o r a few tenths o f nm. For our f u r t h e r c a l c u l a t i o n s we took two c h o i c e s , i . e . d = 0.1 nm and d = 0.4 nm. From the Δ Γ value of Table I , the p o t e n t i a l , ψ °°, at the OH-plane o f the bulk surface was c a l c u l a t e d , e i t h e r by assuming that the p o t e n t i a l , 2
α
2
3
3
α
1
1
3
3
3
f
3
α
3
3
α
3
2
2
3
0
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
188
ADSORPTION AT INTERFACES
ψο, at the OH-planes i n the f i l m was equal to ψο* or equal to The r e s u l t s are given i n Table I I . Table I I . C a l c u l a t e d i ^ - v a l u e s of the Bulk S u r f a c e , f o r the System NaDS + NaCl at 23.5°C. The Last Column gives the ζ-potentials of the NaDS-micelles.
C (NaCl)
d = 0.1
3
(mol
3
dm"
ψ =-οο
nm
Ψο=Ψο°°
0
d = 0.4 ψ =-οο 0
nm
ψο=Ψο°°
ζ
0.2
-41mV
-40mV
-75mV
-70mV
-73mV
0.3
-34
-32
-70
-64
-70
0.4
-30
0.5
-28
-25
-71
-62
-66
I t w i l l be apparent from t h i s t a b l e t h a t the choice of ψο i s not c r i t i c a l which indeed means that there i s n e a r l y t o t a l o v e r l a p p i n g of double l a y e r s . The obtained values f o r ψο°° seem reasonable i n magnitude but are s t i l l very s e n s i t i v e to the choice of d. The values f o r d = 0.4 nm conform r a t h e r w e l l w i t h ζ-potentials c a l c u l a t e d from m i c e l l e m o b i l i t i e s (9). S i m i l a r c a l c u l a t i o n s were performed f o r a ( l - 2 ) e l e c t r o l y t e (NaaSOiJ. These p r e d i c t a r a t i o of 1.42 between the A r - v a l u e s of the two systems; the experimental value f o r t h i s r a t i o from Table I i s found to be 1.28. 3
s
Excess F i l m Entropy. From Equation (8) i t f o l l o w s t h a t A s can be d e r i v e d from the v a r i a t i o n of A F , and thus of Θ, w i t h temperature. In p r a c t i c e , however, A s i s not a very convenient q u a n t i t y , as i t i s q u i t e d i f f i c u l t to keep the y^'s constant when Τ i s v a r i e d . Instead of A s we s h a l l t h e r e f o r e use the i n t e r a c t i o n entropy, AS , d e f i n e d by e
s
s
e
AS
e
= -dAF /dT
(9)
e
From Figure 1, A F vs T-plots can be obtained which are almost s t r a i g h t l i n e s w i t h slopes t h a t are independent of C ^ ^. From t h i s i t f o l l o w s t h a t AS * - 0.066 HH 0.003 mJm" Κ" , i n a l l cases. For the CB-films the absolute value of AS i s at l e a s t a f a c t o r of 50 s m a l l e r . From A F and AS one can f i n d the i n t e r a c t i o n energy, AU i n the f i l m e
N
2
e
AU
e
C
1
e
=AF + TAS e
e
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
e
(10)
12.
DE
FEijTER
AND
vRij
Newton-Black Soap Films
189
Because A F v a r i e s from -0.01 t o -1 mJm~ , the value o f Δ υ i s approximately -20 mJm~ . Thus the formation o f an N B - f i l m from a C B - f i l m i s e n e r g e t i c a l l y a h i g h l y f a v o r a b l e but e n t r o p i c a l l y a h i g h l y unfavorable process, w i t h the q u a n t i t i e s Δ υ and ~TAS tending to compensate each other. The l a r g e (negative) value of A S i m p l i e s t h a t the NB-films have a r a t h e r organized s t r u c t u r e , d i f f e r e n t from t h a t i n CB-films. This was a l s o found by Jones and Mysels (10) and by den Engelsen and Frens (11). I t i s u n l i k e l y that the l a r g e (negative) value o f Δ ϋ can be due t o the van der Waals f o r c e s . We surmise that i t i s caused by s p e c i f i c e l e c t r o s t a t i c i n t e r a c t i o n s between ions of d i f f e r e n t charge signs juxtaposed w i t h respect t o each other. No model i s however a v a i l a b l e t o d e s c r i b e t h i s i n q u a n t i t a t i v e terms a t present. 2
β
e
2
β
e
e
β
Summary Contact angles were measured w i t h the l i g h t d i f f r a c t i o n technique o f P r i n c e n and F r a n k e l . The dependence o f contact angles on e l e c t r o l y t e c o n c e n t r a t i o n was i n t e r p r e t e d i n terms o f adsorption d i f f e r e n c e s o f e l e c t r o l y t e between f i l m and bulk s u r f a c e s . The contact angles were very temperature dependent which d i r e c t s to a k i n d o f "phase t r a n s i t i o n " between the Newton Black and the common b l a c k f i l m s as found p r e v i o u s l y . The l a r g e e n t r o p i e s and energies o f i n t e r a c t i o n imply t h a t the Newton Black f i l m i s an organized s t r u c t u r e d w i t h s p e c i f i c i n t e r a c t i o n s other than d i f f u s e double l a y e r and van der Waals forces. Literature Cited 1. de Feijter, J.Α., "Contact Angles in Soap Films", Ph.D. thesis, University of Utrecht, 1973. 2. J. Colloid Interface Sci., to be published. Princen, H.M., "Shape of Fluid Drops at Fluid-Liquid Interfaces and Permeability of Soap Films to Gases", Ph.D. thesis, University of Utrecht, 1965. 4. Princen, H.M., J. Colloid Sci., (1965) 20, 156. 5. Derjaguin, B.V., Acta Physico Chim., (1940) 12, 181. 6. Huisman, F. and Mysels, K.J., J. Phys. Chem., (1969) 73, 489. 7. Princen, H.M. and Frankel, S., J. Colloid Interface Sci., (1971) 35, 386. 8. Everett, D.H., Pure Appl. Chem., (1972) 31 (4), 614.
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
190
ADSORPTION
AT
INTERFACES
9. Stigter, D. and Mysels, K.J,, J. Phys. Chem., (1955) 59, 45. 10· Jones, M.N., Mysels, K.J. and Scholten, P.C., Trans. Faraday Soc., (1966) 62, 1336. 11. den Engelsen, D. and Frens, G., J. Chem. Soc. Faraday Trans. I, (1974) 70, 193.
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
13 Stratification in Free Liquid Films J. W. KEUSKAMP and J. LYKLEMA Laboratory for Physical and Colloid Chemistry of the Agricultural University, De Dreijen 6, Wageningen, The Netherlands
Introduction Stratification is layer-lik In concentrated surfactant solutions it is a familiar phenomenon. The smectic liquid crystalline phase is a characteristic example. It is less known that under the proper conditions stratification occurs also in free surfactant films. Long ago this has been established by Johonnott (1) and Perrin (2) for aqueous films in air, drawn from concentrated Na-oleate solutions. Later, Bruil and Lyklema (3) observed the same for free aqueous films from concentrated solutions of Na- or Li-dodecylsulphate (NaDS or LiDS respectively). They inferred the occurrence of stratification from some peculiarities in the drainage behavior. The normal drainage behavior, as observed from the intensity of reflected monochromatic light, is a continuous thinning till the first or common black film is formed, in some cases followed by a step-wise formation of the second black or Newton film. The thickness of the common black is sensitive to the electrolyte concentration. It can vary between 10 and 80 nm and is mainly determined by van der Waals compression and double layer repulsion. On the other hand, Newton films are essentially bilayers. Their thicknesses are not very sensitive to the electrolyte concentration but they form only if special electrolytes are present in sufficient amounts. For example, Na -ions promote but Li -ions inhibit the formation of Newton films of Na-dodecylsulphate films. The nature of the transition between the common and Newton film is still not clear, but there are many indications that specific structural effects, induced by ions, play a role (4). See also (5, 6) for such inference from contact angle measurements. Under conditions conducive to stratification, the film initially drains gradually. However, once a thickness of the or+
+
For paper 1 in this series, see ref. (3.). Detailed information obtainable on request from first author.
191 In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
192
ADSORPTION A T INTERFACES
der of a few tens of nm has been reached (the precise value de pending on the composition of the film) the further thinning pro ceeds step-wise. Several steps after each other can be observed, denoted as the . . . . t h i r d , second, f i r s t order, the highest order pertaining to the thickest f i l m . Bruil and Lyklema (3>) observed that the thickness difference between consecutive steps was larger than, but nevertheless comparable with the thickness of the Newton films. This suggests that s t r a t i f i e d films are built up of repeating units of pseudo-Newton films, v i z . aqueous slabs flanked by a surfactant layer. Hence, the study of s t r a t i f i c a t i o n can be informative with respect to the factors determining the Newton film formation. The study to be described i s an extension of ( 3 , ) . The main emphasis i s on the effects of the nature and concentration of added electrolytes. Materials and Methods Surfactants. NaDS, LiDS and NH^DS have been ing Dreger et a l . (J). These chemicals contained dodecanol (DOH) as judged from shallow minima in curves of 2 . U , 3 . 3 and 2 . U mN m" respectively. v e r i f i e d that DOH does not affect s t r a t i f i c a t i o n trations up to 3% of the surfactant. The CMC's, 1
prepared follow small amounts of the γ - l o g c However, i t was even in concen as determined
conductometrically are 8 . 3 * 1 0 ~ 3 , 8 . 8 x 1 0 " ^ and Τ · 1 1 0 ~ 3 mol dm~3 χ
respectively. The f i r s t value agrees well with general experience ( 8 J . CMC's of the other two compounds are not available i n l i t e r ature as far as we are aware. Comparison between these values suggests that NHj^ binds stronger to the DS~-anion, whereas L i binds weaker than the Na -ion. +
+
+
Electrolytes, obtained from Merck were of analytical grade and used without further p u r i f i c a t i o n . Thickness Measurements have been made on v e r t i c a l films with an area of a few cm2 using a light reflection method essentially identical to that used by Lyklema et a l . ( £ ) . The measuring site in the film was 5 mm above the meniscus. The wavelength of the monochromatic l i g h t was 5^6 nm and the temperature was kept at 2 5 . 0 0 ± 0.01°C. In order to derive the real thickness d from the equivalent water thickness the procedure by Frankel and Mysels ( H)) was adopted to correct for the differing optical properties of the surfactant surface layers. For a three-layer f i l m , consisting of an aqueous core, flanked by two dodecylsulphate layers of thicknesses 1 . 0 6 nm and index of refraction 1.U3 the correction (d - d^) i s - 0 . U nm. In order to arrive at this figure, the index of refraction of the aqueous core ( 1 . 3 6 ) was assumed to be identical to that of a 2 M solution of NaHSO^, NH^HSO^ or LiHSO^. For a film with ρ of such units repeating, we applied a correction of - 0 . U ρ nm. Here, ρ i s the order of the stratification.
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
13.
KEUSKAMP
AND
LYKLEMA
Free Liquid Films
193
I n order t o prevent e v a p o r a t i o n , the vapor pressure i n the box has been c a r e f u l l y adjusted and always s u f f i c i e n t time was allowed f o r a c c l i m a t i s a t i o n . The r e p o r t e d t h i c k n e s s e s are a l l the average o f s e v e r a l measurements and are independent o f d e t a i l s i n the a c t u a l drainage p a t t e r n . R e s u l t s and D i s c u s s i o n Drainage Behavior. P l a t e s 1 and 2 are photographs of s t r a t i f y i n g NaDS-films i n an advanced s t a t e o f drainage. The b r i g h t s i l v e r r e g i o n i s the λ A maximum, w i t h d^. above 100 nm. Above t h i s s i l v e r band s e v e r a l r e g i o n s o f d i f f e r i n g t h i c k n e s s can be observed, d i s t i n g u i s h a b l e as d i f f e r e n t shades o f grey. T h e i r t h i c k n e s s can be p r o p e r l y measured i f t h e i r area i s l a r g e enough, which i s o n l y the case f o r the lower ( i . e . b l a c k e r ) o r d e r s . Up t o f i v e orders have been thu presence o f more o f them I t i s important t o r e a l i z e t h a t the p** order develops o f t e n spontaneously i n the middle o f the (p+1)*" order. T h i s shows t h a t the drainage i s not a " g l i d i n g o f f " of one r e p e a t i n g u n i t from the remainder (or from between the remainder) but due t o a spon taneous d i s p r o p o r t i o n a t i o n . The f l u i d e x p e l l e d by t h i s process flows downward. I n some cases i t i s v i s i b l e as a b r i g h t e r r i m around a dark spot, i n other cases as b r i g h t spots. From t h i s o b s e r v a t i o n we conclude t h a t upon drainage the f i l m passes through a number o f metastable s t a t e s , the Gibbs energy as a f u n c t i o n of t h i c k n e s s being o f the shape of F i g u r e 1. The hump between the consecutive minima, e.g. the t r a n s i t i o n a c t i v a t i o n Gibbs energy i n c r e a s e s w i t h decreasing o r d e r , as judged from the i n c r e a s i n g s t a b i l i t y o f the lower o r d e r s . The t r e n d i s (see below) t h a t l e s s orders can be observed and t h a t the t h i c k n e s s e s o f the minima are g r e a t e r i f the s u r f a c t a n t c o n c e n t r a t i o n i s lower. The dashed l i n e i n F i g u r e 1 g i v e s the probable shape o f G(d) f o r a lower s u r f a c t a n t c o n c e n t r a t i o n . S t r a t i f i c a t i o n occurs at s u r f a c t a n t c o n c e n t r a t i o n s lower than those at which i n bulk not yet elongated f l a t or c y l i n d r i c a l m i c e l l e s develop (JJ_). I n view of t h i s , we conclude from the f a c t t h a t laminar s t r u c t u r e s do develop as soon as a c e r t a i n c r i t i c a l t h i c k n e s s i s reached upon drainage, t h a t t h i s s t r a t i f i c a t i o n i s induced by the s u r f a c t a n t l a y e r s on the s u r f a c e s , and more so, the c l o s e r these surface l a y e r s are ( i . e . the t h i n n e r the f i l m ) . Consequently, f o r a thermodynamic d e s c r i p t i o n of the s t r a t i f i c a t i o n , the mutual i n t e r a c t i o n between the l a y e r s must be taken i n t o account, t h i s i n c o n t r a d i s t i n c t i o n t o the m i c e l l i z a t i o n i n b u l k which can be adequately t r e a t e d as a monomer-micelle e q u i l i b r i u m without accounting f o r the presence of other m i c e l l e s . 1
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
Plate 2. Stratified film of a NaDS solution in a different stage of drainage than the film of Phte 1
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
13.
KEUSKAMP
AND
Free Liquid
LYKLEMA
Films
195
E f f e c t o f Nature and Concentration o f S u r f a c t a n t . F i g u r e 2 c o l l e c t s data f o r f i l m s o f LiDS, NaDS and NH^DS i n the absence o f e l e c t r o l y t e . The general trends agree w i t h those o f Ref. (3.)» a l though t h e r e are some minor d i s c r e p a n c i e s i n t h a t i n our case the t h i c k n e s s f o r NaDS i s a few percent lower and t h a t now s m a l l but systematic d i f f e r e n c e s between the s u r f a c t a n t s w i t h d i f f e r e n t c a t i o n are observed: d (NH^DS) > d (LiDS) > d (NaDS). A l s o common ( n o n - s t r a t i f i e d ) f i l m s are s l i g h t l y t h i c k e r w i t h L i than w i t h N a (22). T h i s can be explained by the l e s s e r s p e c i f i c ad s o r p t i o n o f L i on the s u r f a c t a n t surface w i t h the ensuing stronger e l e c t r o s t a t i c r e p u l s i o n . T h i s e x p l a n a t i o n agrees w i t h the observed CMC-order o f LiDS and NaDS, but not f o r NH^DS (see above). The i n f l u e n c e o f the s u r f a c t a n t c o n c e n t r a t i o n ( c p ) i s two f o l d . I n the f i r s t p l a c observable.This i s probabl cooperative s t r u c t u r e formation. The second e f f e c t , the decrease of d w i t h i n c r e a s i n g c p i s more d i f f i c u l t t o e x p l a i n . B r u i l and Lyklema (3.) found a dependence on c ' ^ f NaDS-films. We found the same dependence f o r NH^DS. The v a r i o u s orders are roughly e q u i d i s t a n t . The d i f f e r e n c e s d - d j = Ad decrease w i t h i n c r e a s i n g c p . At the highest s o a p s t u d i e d ( 0 . 3 0 M f o r LiDS, 0 . 6 0 f o r NaDS and 0 . 5 0 f o r NH^DS) they are 6 . 6 , 5.5 and 6 . 3 nm r e s p e c t i v e l y . The mutual d i f f e r e n c e s are not s i g n i f i c a n t . These increments decrease f u r t h e r w i t h c p and upon e x t r a p o l a t i o n c _ « a t t a i n l i m i t i n g values of 5 . 2 ± 0 . 6 nm. T h i s tends t o the t h i c k n e s s o f a Newton f i l m . Under these c o n d i t i o n s the s t r a t i f i e d f i l m i s apparently b u i l t as r e p e a t i n g Newton-like l a y e r s . +
+
+
s o a
g o a
1
o r
s o a
c
s o a
g o a
E f f e c t o f E l e c t r o l y t e s . The i n f l u e n c e o f added e l e c t r o l y t e s ( c o n c e n t r a t i o n c ) can be summarized i n t o two p o i n t s : ( i ) For a given o r d e r , d decreases somewhat w i t h c . For example, i n a f i l m drawn from a 0.2U M LiDS s o l u t i o n d-j decreases from 8 . 8 t o 7 . 8 nm and dg from 15-9 t o 1 5 · 5 nm i f L i C l i s added up t o 0.12 M. I n a f i l m , drawn from 0.2k M NH^DS, d^ decreases from 9 . 3 t o 8 . 0 nm i f NH^Cl i s added up t o 0 . 3 M. The decrease i s roughly l i n e a r . The measurements are not accurate enough t o say whether the steepness o f t h i s f a l l - o f f i s d i f f e r e n t f o r the d i f f e r e n t o r d e r s . A p o s s i b l e e x p l a n a t i o n i s sought i n terms o f e l e c t r o s t a t i c s h i e l d i n g . Note t h a t the c o n c e n t r a t i o n s are s o h i g h t h a t the Debye-Huckel l i m i t i n g law, which would p r e d i c t a c | de pendency, i s no longer v a l i d . ( i i ) E l e c t r o l y t e s i n h i b i t s t r a t i f i c a t i o n , but the i n h i b i t i n g e f f e c t i s s t r o n g l y dependent on thé nature of the c a t i o n . Con s i d e r i n g the occurrence o f the second order above the f i r s t , i t appears t h a t o n l y 0 . 0 0 8 M o f added NaCl s u f f i c e s t o subdue i t s occurrence i n NaDS-films, whereas as much as 0.1k M L i C l i s need ed t o suppress the occurrence o f more than one l a y e r i n LiDSs t a b i l i z e d f i l m s . For the NH^ -case t h i s f i g u r e i s 0.16 M. g
s
n
+
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
196
ADSORPTION A T INTERFACES
Figure 1. Probable thickness dependency of the Gibbs . . . etc. represent 2nd . . . order. high surfactant concen tration, low surfactant concentration. d/nm 40
5th order
•
NaDS
*
Li DS
ο
NH DS 4
30
20h
^- —
- A
4th
order
3rd
order
2nd order
10 Γ·
0
0.2
0.A*
1st
0.6 Surfactant
order
cone/M
Figure 2. Metastable equilibrium thicknesses of stratified films, stabilized by LiDS, NaDS, or NH^DS. No electrolyte added, Τ = 25°C.
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
13.
KEUSKAMP AND LYKLEMA
Free Liquid Films
197
There i s a certain parallelism with the influence of elec trolytes on common (not-stratified) black films: salts decrease d, and L i - and NH^ -films are s l i g h t l y thicker than Na -films, but i t depends c r i t i c a l l y on the nature of the cation whether or not a jumpwise t r a n s i t i o n to the Newton film occurs. The d i f f e r ence i s that i n t h i s case Na promotes Newton film formation, whereas Na i n h i b i t s s t r a t i f i c a t i o n . The nature of t h i s contra diction i s not c l e a r , but phenomenologically i t can be i n t e r preted as a blunting of the activation energy humps i n Figure 1, and more strongly so i n Na than i n L i , so that for common films with L i s t i l l one high barrier remains, preventing the Newton f i l m formation. The behavior of NH^ compares better with that of L i than that of N a . The dramatic differences between L i and Na point out to a very specific ionic effect I t i s not known to us whether these occur also i n concentrate these phenomena deserv +
+
+
+
+
+
+
+
+
+
+
S t r a t i f i c a t i o n of free l i q u i d films i s the occurrence i n i t of l a y e r - l i k e structures. I t can be observed i f free f i l m s , drawn from concentrated surfactant solutions, are drained. In such films after some time several regions can then be seen, d i s t i n guishable i n reflected l i g h t as different shades of gray. These regions are denoted as orders, the lowest order being the t h i n nest f i l m . The thicknesses of the various orders have been o p t i c a l l y determined for films drawn from NaDS, LiDS or NH^DS, both i n the absence and presence of added electrolytes. The orders differ between each other by an increment ΔςΙ that i s roughly constant at given surfactant concentration. For extremely high surfactant concentrations Ad (as well as the thickness of the f i r s t order film) approach that of a Newton (also known as a Perrin or second black) f i l m . Electrolytes i n h i b i t the s t r a t i f i c a t i o n , but Na -ions do so far more effectively than L i - or NH^ -ions. +
+
+
Ac knowledgement The authors acknowledge the s k i l l f u l technical assistance of Mr. R . A . J . Wegh.
Literature Cited 1. Johonnott, E.S., Phil. Mag. (6) (1906) 11, 746 2. Perrin, J., Ann. Phys. (1918) 10, 160 3. Bruil, H.G. and Lyklema, J., Nature Phys. Sci. (1971) 233, 19 4. Jones, M.N., Mysels, K.J. and Scholten, P.C., Trans. Faraday Soc. (1966), 62, 1336 5. Ingram, B.T., J . Chem. Soc. Faraday Trans. I (1972) 68, 2230
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
198
ADSORPTION AT INTERFACES
6. De Feijter, J.A., Ph.D. Thesis, State Univ. Utrecht (1973) 7. Dreger, E.E., Keim, G.I., Miles, G.D., Shedlovsky, L. and Ross, J., Ind. Eng. Chem. (1944) 36, 610 8. Mukerjee, P. and Mysels, K.J. "Critical Micelle Concentrations of Aqueous Surfactant Systems," NSRDS-NBS 36, p.51 Superintendent of Documents, Washington, D.C., 1971 9· Lyklema, J., Scholten, P.C. and Mysels, K.J., J . Phys. Chem. (1965) 69, 116 10. Frankel, S.P. and Mysels, K.J., J . Appl. Phys. (1966) 37, 3725 11. Reiss-Husson, F. and Luzzati, V., J . Phys. Chem. (1964) 68, 3504 12. Lyklema, J., Recl. Trav. Chim. Pays-Bas (1962) 81, 890
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
14 Surface Chemical Properties o f H i g h l y Fluorinated Polymers MARIANNE K. BERNETT and W. A. ZISMAN Laboratory for Chemical Physics, Naval Research Laboratory, Washington, D.C. 20375
Introduction Highly fluorinated linear polymers are characterized by a low free surface energy and concomitant low wettability, as evidenced by the large contact angles of drops of organic and aqueous liquids. A comprehensive set of workable principles had been built up by Zisman and co-workers relating the chemical and spatial constitution in the outermost surface of a polymer with its surface energy (1, 2). During the last few years Wall, Brown, and Lowry of the National Bureau of Standards synthesized several new highly fluorinated ethylene polymers and copolymers (3-8) and established (4) that according to Wunderlichs "bead" theory and "rule of constant heat increment" (9, 10), the ethylenic polymers with fluorinated side groups generally contribute two carbon atoms to the backbone chain. This 2-carbon moiety plus the sub stituent constitute the smallest unit whose oscillations affect surface lattice equilibrium in a polymeric material, which repre sents the lowest free energy configuration. This paper discusses the critical surface tensions of several of these radiation-induced polymers and copolymers, and compares them to those of polymers with related structures and surface constitutions. '
Critical Surface Tensions of Wetting Table I lists the experimental fluoropolymers and the data on their structural formulae, intrinsic viscosities [η] in hexafluorobenzene, and glass transition temperatures Tg (3-8), along with the code numbers used for this report. By casting a polymer from solution in hexafluorobenzene as a film on a clean glass slide, a smooth surface is obtained which is characterizable for wetting properties by contact angle (θ) measurements with freshly percolated liquids. Table II shows the average values of the advancing contact angles (± 1°) obtained from such liquids of two homologous series, the n-alkanes and the 199 In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
3
2
3
n
2
61
4 2
6
i n hexafluorobenzene;
2
6
5
5
n
+ [-CF^CF,,-]^
b e i n acetone; i n benzene
n
n
[-CF -CF(C F )-]
2
XIV
1 1
[-CF -CF(C H )-]
5
XIII
2
[-CH -CH(C F )-] 2 6 ρ η
a
3
[-CF -CF(n-C F )-]
3
XII
XI
n
[-CHF-CH(CF )-]
X
3
n
2
[-CH -CF(CF )-]
2
IX
2
3
+ [-CF^CF^]^
16.5 16.5 18.8 17.5
29 21 49 87
1.63 1.30
e
0.27
1.0
4.0
α 25000
0.23
25
2
2
194
25.4
202
17.8
22.5
14.1 105
235
16.2
45
[-CH -CH(CF CF CF )-]
n
15.5
58
VIII
2
3
3
52
2
2
2
[-CH -CH(CF CF CF )-]
2
2
VII
2
2
0.74
VI
V 79
Μ
16.3
41
0.33
18.0
9
2.10
21.1
19
21.5
5.00
[-CH -CH(CF CF CF )-]
+ [-CFg-CFg-]^
2
2
0.63
[-CH -CH(CF CF CF )-]
2
2
[-CF -CF -]
+ C-CF -CF -]
[-CH -CH(CF CF )-]
2
+
c
7 , dyn/cm 27
a
1.10
[η], d l / g
n
IV
39
[-CH -CH(CF )-]
III
3
5Ô
2
[-CH -CH(CF )-]
n
II
3
[-CH -CH(CF )-]
2
Polymer and Composition. mol%
I
Code
Table I . P h y s i c a l P r o p e r t i e s of E t n y l e n i c Fluoropolymers
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
n-ALkanes Hexadecane Tetradecane Trideoane Dodecane Undecane Decane Nonane Octane .methylsiloxanes DC200 3.0 c S t DC200 2.0 c S t DC200 1.5 c S t 12 (DM3) 10 (OMS) 9 (DMS) 8 (DMS) 7 (DMS) 6 (DMS) 5 (DMS)
Liquids
19.4 18.9 18.1 19.7 19.5 19.3 19.1 19.0 18.6 18.2
27.8 26.7 26.0 25.4 24.6 23-9 23.1 21.8
(dyn/cm)
56
42 35 29
45
36 29
50 47
63
III
51
II
50
I
41 40 39 38 37 33 32
50 46 42
55
61 58
IV
54 51
44 40 36
43 40 36
59
64
VI
55 51
61
67
V
Code
48 44
37 33 29
37 33 28
53
59
48 45
55
61
VII VIII
19 16 14 10 7 6 5
39 36 30
45
52 49
IX
30 29 28 26 24 22 18
44 41 36
49
56 53
Χ
51 50 49 48 47 45 43
55 53 49
60
66 64
XI
< 5
22 13 9
31
41 37
XII
Table I I . Advancing Contact Angles (Deg) of L i q u i d s on Fluoropolymers (25°C)
< 5
< 5
14 10
XIII
40 38 36 34 33 29 27
32 28 25
46
52 50
XIV
202
ADSORPTION A T INTERFACES
d i m e t h y l s i l o x a n e s , where the l a t t e r were e i t h e r the Dow Corning 200 s e r i e s o r the w e l l - c h a r a c t e r i z e d s e r i e s c o n t a i n i n g from 6 t o 12 d i m e t h y l s i l o x a n e (DMS) -units. (11, 12). When the cos θ of each member of such a homologous s e r i e s of l i q u i d s on a smooth, c l e a n , s o l i d , low-energy surface i s p l o t t e d a g a i n s t the s u r f a c e t e n s i o n (ïjJ) f o r each of those l i q u i d s , a s t r a i g h t l i n e r e s u l t s ; the i n t e r c e p t a t cos θ = 1 (θ = 0°) i s r e f e r r e d t o as the c r i t i c a l s u r f a c e t e n s i o n of w e t t i n g (7 ) f o r t h a t p a r t i c u l a r s u r f a c e (13). Figure 1 shows such a grapn f o r polymer IV, [-CHp-CHiCpF^)-] , and i s r e p r e s e n t a t i v e o f the graphs f o r t h e other polymers w i t h the e x c e p t i o n of the p e r f l u o r o p h e n y l s u b s t i t u t e d polymers X I I and XIV. Here the s t r a i g h t l i n e f o r the n-alkanes d i s p l a y s a marked d i s c o n t i n u i t y i n the r e g i o n of 7 = 24-25 dyn/cm, r e s u l t i n g i n two values of 7 , d i f f e r i n g by 1 dyn/cm. Experimental phenomena suggest t h a t tne s m a l l e r mole c u l e s a r e capable of s l i p p i n adlineated structures o and b u l k i e r molecules are r e t a i n e d on the s u r f a c e , thus b e i n g more r e l i a b l e r e p r e s e n t a t i v e s f o r the t r u e value of 7 . Values of 7 thus obtained f o r each polymer f i l m are given i n Table I . Wettability f o r low-energy s u r f a c e s , as d e f i n e d by 7 , i s determined e s s e n t i a l l y by the nature and packing of the exposed surface atoms of t h e s o l i d and i s otherwise independent o f the nature and arrangement of the u n d e r l y i n g atoms and molecules ( l , 2). The arrangement o f the s u r f a c e atoms, of course, must represent the lowest f r e e energy c o n f i g u r a t i o n f o r a given s e t of r e s t r a i n i n g c o n d i t i o n s such as the nature, s i z e of the u n d e r l y i n g atoms, l e n g t h of the c h a i n , e t c . I n p a r t i c u l a r , f o r polymeric m a t e r i a l s i t depends on d e f i n i t i o n of the s m a l l e s t u n i t , which f o r ethylenic polymers c o n s i s t s o f 2 carbons i n the backbone (Sb IQ) p l u s the s u b s t i t u e n t . Table I I I l i s t s 7 values of f l u o r i n e - c o n t a i n i n g e t h y l e n i c homopolymers, wherS the formulae shown are the r e p e a t i n g u n i t s i n the polymer s t r u c t u r e , as they would appear i f a l l c o n s t i t u e n t s were present i n the s u r f a c e . The order o f s t r u c t u r e s i s arranged t o show p r o g r e s s i v e s u b s t i t u t i o n s of e i t h e r a hydrogen o r a f l u o r i n e atom i n the backbone c h a i n by e i t h e r a f l u o r i n e atom or a p e r f l u o r o group. S e v e r a l observations can be made from i n s p e c t i o n of values of 7 : (a) When the ethylene c h a i n i s f u l l y hydrogenated, replacement of one hydrogen by a - C F group lowers 7 a p p r o x i mately 10 dyn/cm. (Compare XV and i f XVI and IX, and XVI and X.) (b) When one carbon atom i n the 2-carbon moiety of the ethylene c h a i n i s f u l l y f l u o r i n a t e d , replacement of a hydrogen by a -CF^ group on the other carbon atom lowers 7 o n l y about 5 dyn/cm. (Compare XVII and XIX.) (c) Nearly equSl y values a r e observed on s e v e r a l p a i r s of monomers of u n l i k e molecular c o n s t i t u t i o n s , such as I and X V I I , I X and X V I I I , and X and XIX. I n s p e c t i o n o f S t u a r t - B r i e g l e b molecular models shows t h a t , i n c e r t a i n s t e r i c arrangements, the 7 -determining packing of f l u o r i n e atoms a t the s u r f a c e o f these paîrs could be v e r y s i m i l a r , s i n c e the pendant ?
Q
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
14.
BERNETT
AND
Highly Fluorinated Polymers
zisMAN
203
Table I I I . E f f e c t on C r i t i c a l Surface Tensions of Wetting by Replacement of Hydrogen w i t h F l u o r i n e and/or Pendant Group Code
7 (dyVcm)
Polymer
31
XV
[CH -CH -]
I
[CH -CH(CF )-]
IV
[-CH -CH(CF -CF )-]
V
[-CH -CH ( C F ^ C F ^ C F ^ ) -]
15.5
XVI
[-CH -CFH-]
28
IX
[-CH
X
[-CH(CF )-CFH-]
XVII
[-CF -CFH-]
n
22
XVIII
C-CF -CF -]
n
18.5
XIX
[-CF -CF(CF )-]
XI
[-CF -GF(nC F
XX
[-CH -CH(C H )-]
XII
[-CH -CH(C F )-]
XIII
[-CF -CF(C H )-]
n
25.4
XIV
C-CF -CF(C F )-]
n
17.8
2
2
2
n
3
2
21.5
n
2
3
2
n
b
2
3
2
2
2
2
3
2
2
2
2
2
Reference 14;
16.3
n
n
2
a
5
6
6
6
6
i:L
17.5
n
17
n
)-]
5
5
5
5
n
n
n
b
C
d
14.1 33-35
e
22.5
Reference 15; °Reference 13; Reference 16;
References 17, 18
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
Macromolecules
Figure 2. Various configurations of polymer [—CHz—CHtCeFs)—']». (a) (top left) syndiotactic, exposure of flat side; (b) (top right) syndiotactic, exposure of edge; (c) (bottom left) isotactic (12).
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
14.
BERNETT
AND
Highly Fluorinated
zisMAN
205
Polymers
-CF^ groups may be so l o c a t e d as t o p a r t i a l l y obscure the hydro gen atoms . The t o t a l e f f e c t achieves a balance between the hydrogen and the -CF- c o n t r i b u t i o n s which approximates the s u r face c o n s t i t u t i o n of the l i n e a r unbranched c o n f i g u r a t i o n . (d) Polymers IX and X demonstrate a c o n f i r m a t i o n of the c o n c l u s i o n of Pittman and co-workers ( 1 £ ) , t h a t i n a p a r t i c u l a r f l u o r i n a t e d polymer y i s not n e c e s s a r i l y dependent on the t o t a l f l u o r i n e content: deSpite the i d e n t i c a l f l u o r i n e content, the molecular s t r u c t u r e s d i f f e r s u f f i c i e n t l y t o r e s t r i c t f r e e r o t a t i o n of the -CF~ groups i n polymer X w i t h the n e t r e s u l t of c l o s e r surface packing of the f l u o r i n e atoms and thus a lower y . (e) As a n t i c i p a t e d , the lowest y , 14-1 dyn/cm, i s obtained f o r polymer X I which i s not only fulîy f l u o r i n a t e d but a l s o d i s p l a y s the l o n g e s t pendant p e r f l u o r i n a t e d group. The i n c r e a s e i n l e n g t h from 1 carbon t o 5 carbons decreases 7 about 3 dyn/cm (XIX and XI) because of b e t t e r a d l i n e a t i o longer c h a i n , ( f ) Fo hydrogen w i t h f l u o r i n e i n e i t h e r the phenyl group, X I I , or the backbone c h a i n , X I I I , lowers 7 about 10 dyn/cm from the 33-35 dyn/cm of p o l y s t y r e n e . V a r i a t i o n s o r spread of y values f o r a given polymer can be now explained by the v a r i o u s o r i e n t a t i o n s of the phenyl group i n the s u r f a c e , such as exposure of the f l a t s i d e o r the edge, or the t a c t i c i t y of the arrangement (Figure 2 a, b, c ) . T o t a l f l u o r i n a t i o n , as i n polymer XIV of course r e s u l t s i n even lower 7 , s i n c e only f l u o r i n e atoms a r e exposed i n the s u r f a c e . An i n t e r e s t i n g p a r a l l e l can be observed and i s demonstrated i n F i g u r e 3: When the hydrogen atoms i n the e t h y l e n i c backbone are r e p l a c e d by f l u o r i n e atoms, r e g a r d l e s s of whether the pendant group i s the a l k y l -CF~ or the aromatic -CvF , 7 i s lowered by about 4 · 5 dyn/cm. When, on the other hand, ?he S l k y l -CFo group i s r e p l a c e d by the aromatic -C,F group, whether on a f u l l y hydrogenated or f u l l y fluorinaïed backbone, 7 i s r a i s e d by approximately 1 dyn/cm. Q
c
5
6
0
S t e r i c Configurations The problem o f determining the arrangement of the atoms and s u b s t i t u e n t s of the f i r s t l a y e r of the s o l i d s u r f a c e of a polymer or copolymer needs t o be s o l v e d before we can r e l a t e the observed value of 7 t o the most probable surface composition of the polymeric s o l i d . F o r simple u n s u b s t i t u t e d polymers such as [-CF -CF -] o r [-CH ~CH -] the surface conformation can be r a t i o n a l i z e d by S t u a r t - B r i e g l e b molecular models arranged i n p o s s i b l e conformations on a f l a t t a b l e . However, where s i d e chains a r e introduced i n t o the model, s t e r i c hindrances are a l s o i n t r o d u c e d . I t i s obvious from Figure 4, where only some of the p o s s i b l e arrangements of the atoms and s u b s t i t u e n t s i n the f i r s t l a y e r of the s o l i d [-CH -CH(C F^)-] polymer surface a r e shown, t h a t i n a three-dimensional a r r a y a m u l t i t u d e of a l t e r n a t e 2
2
2
2
2
n
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
ADSORPTION A T INTERFACES
206
- CH, - CH(CF ) -
- CF - CF(CF,) -
}
2
21-5
l+l.O
4.5 17.0
+0.8 I
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
14.
BERNETT AND
zisMAN
Highly Fluorinated Polymers
207
conformations are p o s s i b l e , l i m i t e d o n l y by s t e r i c considerations. Lacking s p e c i f i c i n f o r m a t i o n on the t a c t i c i t y of the r a d i a t i o n induced polymer, we assumed them t o be a t a c t i c , i . e . , randomly o r i e n t e d . Any p r e d i c t i o n of surface p a c k i n g i s thus p r o b l e m a t i c . In a d d i t i o n t o the s p a t i a l arrangements, polymers such as X a l s o e x h i b i t c i s and trans isomerism w i t h respect t o the o r i e n t a t i o n of the -F and -CF^ s u b s t i t u e n t s on the backbone, f u r t h e r compli c a t i n g and expanding the number of p o s s i b l e c o n f i g u r a t i o n s . Electrostatic
Dipoles
A t the juncture of the -CH CH< backbone and the - ( C F j F s i d e group there exists an uncompensated e l e c t r o s t a t i c d i p o l e . I f one assumes t h a t the s i d e group i s d i r e c t e d away from the polymer s o l i d surface i n t closer t o the interfac have shown t h a t as χ becomes s m a l l e r , θ a l s o becomes s m a l l e r . Thus, when χ changes from 1 t o 3, 7 i s lowered from 21.5 t o 15.5 dyn/cm (Figure 5), w i t h the l a r g e r Secrease of 5.2 dyn/cm when 1 < χ < 2 and the s m a l l e r decrease of 0.8 dyn/cm when 2 < χ < 3· I n t h e i r study of adsorbed monolayers of p r o g r e s s i v e l y f l u o r i n ated f a t t y a c i d s of the general formula F ( C F ) ( C H j ^ C O O H and 2
p
F
C F
C H
C 0 Q H
&
Γ
Γ
Ϊ
η
z
i
s
m
a
n
^ 2^ 2^10 ' ^ · ^ (20)/sSowed TOat the uncompensated d i p o l e has a l a r g e e f f e c t on w e t t i n g when χ < 7, but becomes l e s s s i g n i f i c a n t when χ ^ 7. Measurements of e l e c t r i c a l and mechanical p r o p e r t i e s of i n s o l u b l e monolayers on water by Bernett and Zisman (21), supported t h e i r view a l s o . S h a f r i n and Zisman a l s o n o t i c e d an abrupt r e v e r s a l of the e f f e c t of homology a t 2 < χ < 3 where the d i f f e r e n c e i n y was only 0.8 dyn/cm because of random t i l t i n g , of the fluoroSarbon group. A s i m i l a r abrupt d i s c o n t i n u i t y a t 2 < χ < 3 f o r the e t h y l e n i c polymers can be explained by r e s t r i c t i o n of r o t a t i o n of the s u b s t i t u e n t and the subsequent s h i e l d i n g of the e l e c t r o s t a t i c d i p o l e . The l a t t e r i s accentuated by the f a c t t h a t the e t h y l e n i c hydrocarbon, t o which the p e r f l u o r o a l k y l s i d e groups are connected, imposes r e s t r i c t i o n s on the p a c k i n g of these s u b s t i t u e n t groups, n e c e s s i t a t i n g p r o g r e s s i v e l y l a r g e r i n t r a molecular r o t a t i o n s and bending w i t h i n the c h a i n . This r e s u l t s i n exposure of the -CF - atomic grouping i n an outermost surface of randomly o r i e n t e d perfluoroethyl o r -propyl groups. I t would be i n t e r e s t i n g t o study polymers w i t h p r o g r e s s i v e l y longer p e r f l u o r o a l k y l s i d e groups t o a s c e r t a i n whether r e g u l a r decreases i n y can be observed or whether a l i m i t i n g value has been approacheS. When the ethylene backbone i s f u l l y f l u o r i n a t e d , no l a r g e uncompensated d i p o l e s are present, and a gradual and u n i n t e r r u p t e d decrease i n y i s observed w i t h i n c r e a s e i n χ (Figure 5 ) . The shape o f tne two curves i n Figure 5 seems t o p o i n t t o an eventual asymptotic approach t o the y -vs-x curve obtained from adsorbed monolayers o f f u l l y f l u o r i n a t e d c a r b o x y l i c ?
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
ADSORPTION AT
208
INTERFACES
a c i d s , as shown i n Figure 6 (20). Since t o date no curve of anyother f a m i l y or s e r i e s of r e l a t e d compounds (22), i n c l u d i n g the two present ones, has crossed or gone beyond the curve of the p e r f l u o r o a c i d s , i t i s suggested t h a t the l a t t e r represents an envelope of l i m i t i n g v a l u e s . Copolymers
I n t h e i r study on copolymers, Hu and Zisman (11) p l o t t e d y f o r each polymer a g a i n s t the mole % of [-CF -CF -] i n the copolymer (Figure 7 ) . The three graphs show the comparative case of p r e d i c t i n g the e f f e c t on y of c o p o l y m e r i z a t i o n of [ C F - C F - ] w i t h [-CH -CH -] (upper curve) (22) and the greater d i f f i c u l t y (middle graph predictin y c o p o l y m e r i z a t i o n w i t h [-CH c
2
2
C
2
2
n
2
2
n
F
e f f e c t w i t h [ - C H - C H ( C o ) - ] ("bottom c u r v e ) . I t should be evident t h a t we are d e a l i n g w i t h s e v e r a l e f f e c t s of adding f l u o r i n a t e d carbon atoms: (a) those due t o London d i s p e r s i o n f o r c e changes (upper graph) and (b) those combining e l e c t r o s t a t i c e f f e c t s and s t e r i c hindrance e f f e c t s (lower two graphs). I t i s obvious from the multitude of p o s s i b l e s t e r i c c o n f i g u r a t i o n s and d i p o l e c o n t r i b u t i o n s t h a t we face the d i f f i c u l t problem of r a t i o n a l i z i n g how t o compute the c o r r e c t molecular conformation t o a r r i v e a t a minimum s u r f a c e energy f o r s u b s t i t u t e d ethylene f luoropolymers and copolymers. 2
7
n
Summary
Wetting p r o p e r t i e s of new, w e l l - c h a r a c t e r i z e d , h i g h l y f l u o r i n a t e d l i n e a r e t h y l e n i c polymers and copolymers w i t h t e t r a f l u o r o e t h y l e n e were i n v e s t i g a t e d . F l u o r i n a t i o n i n the p o l y ethylene backbone was v a r i e d by degree of f l u o r i n e atom s u b s t i t u t i o n ; n - a l k y l s i d e chains of i n c r e a s i n g number were f u l l y f l u o r i n a t e d , whereas phenyl s i d e groups were e i t h e r f u l l y or nonf l u o r i n a t e d . C r i t i c a l s u r f a c e tensions of w e t t i n g obtained on t h i n c a s t f i l m s of these fluoropolymers were compared t o those of polymers w i t h r e l a t e d s t r u c t u r e s and surface c o n s t i t u t i o n s . Because of the presence of the b u l k y f l u o r i n e atoms and aromatic s i d e groups, some of these molecules are extremely s t e r i c a l l y blocked, which makes the p r e d i c t i o n of an e q u i l i b r i u m surface conformation very d i f f i c u l t . The r e s u l t s are discussed i n terms of s o l i d s u r f a c e c o n s t i t u t i o n , s t e r i c hindrance, and e l e c t r o s t a t i c dipole contribution. Literature Cited
1.
Zisman, W. Α., in "Contact Angle Wettability and Adhesion," Adv. Chem. Ser., No. 43, p. 1, Am. Chem. Soc., Wash., D.C., 1964.
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
14.
BERNETT
0
J
AND ZISMAN
I 2
L
4
Highly Fluorinated Polymers
J 6
1
ι 8
209
Ο
F(CF ) C00H
Δ
F ( C F ) ( C H ) C 0 0 H (FROM SOLUTION)]
•
F ( C F ) ( C H 2 ) C 0 0 H (FROM MELT)
•
H(CH ) C00H
ι
2
2
x
2
2
X
2
I 10
(FROM SOLUTION)
X
| 6
|6
(FROM SOLUTION)
| 7
I
l 12
L
X=NUMBER OF FLUORINATED CARBON ATOMS PER
14
I
I 16
I
18
MOLECULE Journal of Physical Chemistry
Figure 6.
Effect of fluorination of the adsorbed acid monolayer on y ( 20 )
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
c
ADSORPTION A T INTERFACES
Macromolecules
Figure 7. Effect of copolymerization with [—CF -CF —] on y { 11) ?r
2
n
c
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
14.
Highly Fluorinated Polymers
BERNETT AND ZISMAN
211
2. Zisman, W. Α., J. Paint Tech., (1972), 44, 42. 3. Brown, D.W., Wall, L. Α., J . Polym. Sci. A-1, (1968), 6, 1367. 4. Brown, D. W., Wall, L. Α., J . Polym. Sci. A-2, (1969), 7, 601. 5. Brown, D. W., Lowry, R. E . , Wall, L. Α., J . Polym. Sci. A-1, (1970), 8, 2441. 6. Brown, D. W., Lowry, R. E . , Wall, L. Α., J. Polym. Sci. A-1, (1970), 8, 3483. 7. Brown, D. W., Lowry, R. E . , Wall, L. Α., J . Polym. Sci. A-1, (1971), 9, 1993. 8. Brown, D. W., Lowry, R. E . , Wall, L. Α., J. Polym. Sci. (Polym. Chem. Ed.) (1973), 11, 1973. 9. Wunderlich, B., J. Phys. Chem., (1960), 64, 1052. 10. Wunderlich, B., Bodily, D. M., Kaplan, H. M., J . Appl. Phys., (1964), 35, 95. 11. Hu, W. Κ. H., Zisman 12. Bernett, M. K., Macromolecules (In Press) (Sep 1974), 7. 13. Fox, H. W., Zisman, W. Α., J. Colloid Sci., (1950), 5, 514. 14. Fox, H. W., Zisman, W. Α., J. Colloid Sci., (1952), 7, 428. 15. Ellison, A. H., Zisman, W. Α., J . Phys. Chem., (1954), 58, 260. 16. Bernett, M. K., Zisman, W. Α., J . Phys. Chem., (1961), 65, 2266. 17. Ellison, A. H., Zisman, W. Α., J. Phys. Chem., (1954), 58, 503. 18. Fox, R. F., Jarvis, N. L . , Zisman, W. Α., in "Contact Angle Wettability and Adhesion," Adv. Chem. Ser., No. 43, p. 317, Am. Chem. Soc., Wash., D.C., 1964. 19. Pittman, A. G., Sharp, D. L . , Ludwig, Β. Α., J. Polym. Sci. A-1, (1968), 6, 1729. 20. Shafrin, E. G., Zisman, W. Α., J . Phys. Chem., (1962), 66, 740. 21. Bernett, M. K., Zisman, W. Α., J . Phys. Chem., (1963), 67, 1534. 222. Pittman, A. G. "Fluoropolymers," p. 419, Wiley Interscience Press, N.Y., 1972. 23. Fox, H. W., Zisman, W. Α., J . Colloid Sci., (1952), 7, 109. ;
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
15 Effect o f Surface O x y g e n Complexes o n Surface Behavior o f Carbons BALWANT RAI PURI Department of Chemistry, Panjab University, Chandigarh, 160014, India
Introduction It is now well know that microcrystalline carbons contain appreciable amounts of combined oxygen which gives rise to stable carbon-oxygen surface complexes (1). The work reported from our laboratories (2,3) and from elsewhere (4,5) indicates that there are definite surface groups or complexes which evolve carbon dioxide and, similarly, there are distinct surface entities which evolve carbon monoxide on evacuation at increasing temperatures. The effect of combined oxygen on various surface properties, such as wettability (6), heats of immersion in various liquids (7-9), adsorbability of water and other polar vapours (10-12), selectivity in adsorption from binary mixtures (13,14) has been reported. However, the effect of the individual oxygen complexes such as acidic CO2-complex (15), nonacidic CO2-complex (16) and CO-complex (3) on some of the surface properties mentioned above has not received adequate attention. An attempt has been made in the present paper to spell out the effect of each complex on (i) selective adsorption from mixtures of methanol and benzene, and (ii) adsorption of (a) benzene vapour, (b) dry ammonia, and (c) phenol from dilute aqueous solutions. Experimental Mogul (a colour black), Spheron-6 (a channel black) and a charcoal prepared by the carbonisation of recrystallised cane sugar (17) were used as such as well as after (i) outgassing at different temperature upto 1000°C, and (ii) treatment with aqueous hydrogen peroxide or potassium persulphate (l6) so as to get materials associated with different amounts of surface oxygen complexes. The amount of combined oxygen and the form of its disposition was obtained by evacuating 0.5 g portion in a resistance tube furnace, raising the temperature to 1200°C and analysing the gases evolved in the usual manner (17). 212 In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
15.
puRi
Surface Oxygen Complexes
213
The base a d s o r p t i o n c a p a c i t y , estimated by t i t r a t i n g against aqueous barium hadroxide (17), f i x e d the amount o f a c i d i c CO2complex. This when subtracted from the amount o f carbon d i o x i d e evolved on evacuation gave the amount o f n o n - a c i d i c C0 -complex (l6) which a r i s e s through the f i x a t i o n o f oxygen at unsaturated s i t e s (17). The amount o f CO-complex was obtained from the amount o f carbon monoxide evolved on evacuation. The n i t r o g e n surface areas and the t h r e e main types o f surface oxygen com plexes of the v a r i o u s samples are recorded i n Table I . S e l e c t i v e a d s o r p t i o n from methanol-benezene s o l u t i o n s was s t u d i e d by mixing 0.25 g carbon w i t h a known weight (5-6 g) o f the s o l u t i o n i n a s m a l l g l a s s tube drawn out a t one end. The l a t t e r was s e a l e d , a f t e r c o o l i n g i n a f r e e z i n g m i x t u r e , t o min imise evaporation. Several such tubes c o n t a i n i n g 0.25 g carbon mixed w i t h s o l u t i o n s o f d i f f e r e n t c o n c e n t r a t i o n placed i a shaker (12 rev/min) h e l 0.05°C f o r ho hours. Th chang compositio liqui was determined i n t e r f e r o m e t r i c a l l y . Adsorption isotherms o f benzene as w e l l as d r y ammonia were determined at 35 + 0.05°C by u s i n g McBain's t o r s i o n balance technique. A d s o r p t i o n isotherms of phenol from aqueous s o l u t i o n s were determined by mixing 0.5 g p o r t i o n s w i t h a known weight ( ~ 5 g) o f s o l u t i o n s of v a r i o u s c o n c e n t r a t i o n s , shaking the suspensions i n a r e v o l v i n g wheel h e l d i n a thermostat a t 35 + 0.05°C f o r 2k hours. The change i n c o n c e n t r a t i o n o f the s o l u t i o n was determined i n t e r f e r o m e t r i c a l l y . The experiments were conducted i n the low c o n c e n t r a t i o n range upto 20 m m o l e s / l i t r e . 2
R e s u l t s and D i s c u s s i o n The composite a d s o r p t i o n isotherms, reproduced i n F i g u r e 1 from a recent r e p o r t from the author's l a b o r a t o r y (l8), show how the preference o f Mogul f o r a d s o r p t i o n from methanol-benzene mix t u r e s of v a r i o u s c o n c e n t r a t i o n s i s a l t e r e d i n the presence o f v a r i o u s surface oxygen complexes. I t i s seen t h a t 1000°-outgassed Mogul, which i s e s s e n t i a l l y f r e e o f combined oxygen, shows strong preference f o r benzene at a l l c o n c e n t r a t i o n s , g i v i n g a t y p i c a l l y U-shaped isotherm (marked A ) . The 7 0 0 ° - outgassed Mogul, which i s f r e e o f COg-complex but r e t a i n s over 2.5 per cent oxygen capable o f e v o l v i n g carbon monoxide (C0-complex), shows, s u r p r i s i n g l y enough, even g r e a t e r preference f o r benzene, the l e s s p o l a r component of the m i x t u r e , a l l along the c o n c e n t r a t i o n range. This i s c o n t r a r y t o the view g e n e r a l l y h e l d (l3 lh) t h a t combined oxygen imparts g r e a t e r preference f o r the more p o l a r component o f the mixture. I t appears t h a t the presence o f q u i nonic groups which form a p a r t of the C0-complex (l£) promotes preference f o r benzene due t o p o s s i b i l i t y of i n t e r a c t i o n o f e l e c t r o n s of benzene r i n g w i t h the p a r t i a l p o s i t i v e charge on the carbonyl carbon atom (20). The l*00°-outgassed and the o r i g i n a l samples of Mogul, which c o n t a i n i n c r e a s i n g amounts o f CO2 com9
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
ADSORPTION A T INTERFACES
214
Table I . Surface Area and Surface Oxygen Complexes o f the Carbons used i n t h e Present Work.
D e s c r i p t i o n o f the sample
Surface area m /g 2
Acidic CO2complex moles/ 100 g
NonC0acidic complex CO2mmoles/ complex 100 g mmoles/ 100g
Mogul Original 30 30 Outgassed a t 1*00°C 3lk Outgassed at 600°C 325 Outgassed at T00°C 318 Outgassed at 850°C Outgassed a t 1000°C 326 Original, treated 310 w i t h aq.H 02 Original, treated with K S 08 312 Outgassed at 1000°, t r e a t e d w i t h aq. H 02328 2
2
2
2
269 198 163 52
1 nil nil nil nil
nil nil nil nil
nil
1*5
nil
281*
189
nil
315
nil
120
20
15.5
nil nil nil nil
122.5 108 nil
nil
135
Spheron-6 Original Outgassed at 600°C Outgassed at 850°C Outgassed a t 1000°C Original, treated w i t h aq.H202 Outgassed a t 1000°, t r e a t e d w i t h aq.H202
110 112 109
116 11^
2.5 nil nil
61
51
106
nil
10
1*
1*12 51* 502
355 51 nil
nil nil nil
1*91
1*1*3
*53
nil
56Ο
488
nil
165
20
Sugar Charcoal Original Outgassed a t 600°C Outgassed a t 1000°C Original, treated w i t h aq.H202 Outgassed a t 1000° t r e a t e d w i t h aq.H 02 2
531 nil
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
15.
puRi
Surface Oxygen Complexes
215
p l e x , though n e a r l y equal amounts o f CO-complex ( c f . Table I ) , show, on the other hand, p r e f e r e n t i a l a d s o r p t i o n o f methanol over a p p r e c i a b l e range o f c o n c e n t r a t i o n ( c f . isotherms marked C and D). This remarkable change i n the preference may be a s c r i b e d t o the presence o f a c i d i c C02-complex. This view r e c e i v e s support from the f a c t t h a t when the amount of the a c i d i c complex i s r a i s e d from 6 8 . 5 mmoles t o 1*5 mmoles on treatment w i t h aqueous hydrogen peroxide and 189 mmoles on treatment w i t h potassium per s u l p h a t e , the preference o f the s u r f a c e i s s h i f t e d i n c r e a s i n g l y i n favour of methanol ( c f . isotherms Ε and F ) . The comparison o f the isotherms Β and F shows c l e a r l y how the presence o f a l a r g e amount o f a c i d i c C02-complex has brought about almost complete r e v e r s a l of the preference o f the carbon s u r f a c e . I t i s a l s o i n t e r e s t i n g t o note t h a t the 1000°-outgassed Mogul y i e l d s almost i d e n t i c a l isother a f t e f i x a t i o f about * per cent o f oxygen, a aqueous hydrogen p e r o x i d e , y oxyge (cf. isotherms marked G and A ) . The presence of n o n - a c i d i c com p l e x , e v i d e n t l y , produces l i t t l e o r no e f f e c t on the preference o f the s u r f a c e . Thus the view h e l d by p r e v i o u s workers ( 1 3 » 1 * ) t h a t the e n t i r e combined oxygen imparts p o l a r i t y t o the surface and p r o motes preference f o r the more p o l a r component o f a b i n a r y mixture needs r e v i s i o n i n the l i g h t o f the observations d i s c u s s e d above. A d s o r p t i o n o f Benzene Vapour. The above c o n c l u s i o n s were checked by s t u d i n g a d s o r p t i o n of benzene vapour d i r e c t l y on a number o f carbon b l a c k s (21). The s o r p t i o n - d e s o r p t i o n isotherms (35±0.05°C) on some of the samples o f Mogul ( F i g u r e 2 ) almost superimpose showing almost complete r e v e r s i b i l i t y o f the process. The amount o f s o r p t i o n a t each r e l a t i v e vapour pressure i s seen to i n c r e a s e a p p r e c i a b l y as Mogul i s outgassed at i n c r e a s i n g tem p e r a t u r e s . The maximum e f f e c t i s produced when the b l a c k i s out gassed at 600°C. I t appears t h a t w i t h the e l i m i n a t i o n o f the p o l a r C02-complex and the emergence o f CO-complex as the o n l y predominant s u r f a c e oxygen complex, the s o r p t i o n o f benzene i n creases a p p r e c i a b l y on account of reasons advanced i n the p r e v i o u s paragraph. The outgassing o f Mogul a t 850°C lowers the amount o f CO-complex c o n s i d e r a b l y and t h e r e i s s i g n i f i c a n t f a l l i n the s o r p t i o n v a l u e at a l l r e l a t i v e vapour pressures. With the complete e l i m i n a t i o n o f the complex at 1000°C, t h e r e i s a f u r t h e r f a l l i n the s o r p t i o n of benzene. A d s o r p t i o n of Dry Ammonia. A d s o r p t i o n o f dry ammonia on m i c r o c r y s t a l l i n e carbons at d i f f e r e n t temperatures has been i n v e s t i g a t e d by a number o f workers amongst which mention may be made of the work o f Anderson and Emmett ( 2 2 ) , V o s k r e s e n s k i i (23.), Holmes and Beebe ( 2 * ) , Studebaker ( 2 5 ) and Dupupet e t a l ( 2 6 . There i s a g e n e r a l agreement t h a t a d s o r p t i o n i s enhanced appre c i a b l y i n the presence o f combined oxygen. However, the exact
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
216
ADSORPTION A T INTERFACES
1-2
0·8
OA
Ε
OO
•fraction
Methanol->-
8
c° - 0 - 4
-1-2
Figure 1. Composite adsorption isotherms of methanolbenzene mixtures on Mogul before and after various treat ments. A = Mogul outgassed at 1000° C, Β = Mogul out gassed at 700°C, C = Mogul outgassed at 400°C, D = Mogul original, Ε = Mogul original, treated with aq. H 0 , F = Mogul original treated with acidified K S 0 , G = Mogul outgassed at 1000°C, treated with aq. H 0 . 2
2
2
8
2
~ΟΛ
0·2 0·3 0 4 0 5 0 6 0·7 Ο θ 0·9 Relative vapour pressure—*:
2
PÔ"
Figure 2. Adsorption isotherms of benzene on Mogul. The solid points denote desorption data. Ο = original, Δ = outgassed at 600°C, • = outgassed at 850°C, · = oxygen-free.
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
2
15.
puRi
Surface Oxygen Complexes
217
r o l e o f the v a r i o u s surface oxygen complexes, which c o n s t i t u t e t h i s oxygen, has not "been e l u c i d a t e d . Adsorption isotherms o f ammonia on sugar c h a r c o a l , Mogul and Spheron-6 "before and a f t e r v a r i o u s treatments are shown i n Figures 3» h and 5, r e s p e c t i v e l y . I t i s seen t h a t , i n each case, there i s a considerable f a l l i n the s o r p t i o n o f ammonia a t each pressure on outgassing and t h a t the e f f e c t i s r e l a t i v e l y more when a carbon i s outgassed a t 600°C, causing e l i m i n a t i o n o f most of C02-complex ( c f . Table I ) , than t h a t when i t i s outgassed i n 600-1000° temperature range, causing e l i m i n a t i o n o f CO-complex. This shows t h a t the s o r p t i o n o f ammonia i s i n f l u e n c e d more by the former than by the l a t t e r s u r f a c e oxygen complex. The treatment of o r i g i n a l samples w i t h aqueous hydrogen peroxide which r e s u l t s i n an a p p r e c i a b l e r i s e i n the amount o f the a c i d i c complex w i t h out a f f e c t i n g much the valu f C0-comple ( c f Tabl I ) i t o cause increase i n th sure, as can be seen o compariso of the F i g u r e s . The treatment o f 1000° - outgassed carbons w i t h aqueous hydrogen peroxide r e s u l t s i n a p p r e c i a b l e f i x a t i o n o f oxygen which, however, gives r i s e mostly t o non-acidic C02-comp l e x ( c f . Table I ) . The isotherm on t h i s sample i s seen t o be almost i d e n t i c a l w i t h t h a t on the corresponding oxygen-free sam p l e . These observations show c l e a r l y t h a t the e n t i r e combined oxygen does not have a uniform e f f e c t on the s o r p t i o n o f dry ammonia by carbon. The oxygen present as a c i d i c C02-complex i n f l u e n c e s the s o r p t i o n o f ammonia t o a r e l a t i v e l y l a r g e r extent than t h a t present as C0-complex w h i l e the oxygen-present as nona c i d i c C02-complex has h a r d l y any e f f e c t a t a l l . Adsorption isotherms on sugar c h a r c o a l , Mogul and Spheron-6, a l l p r e v i o u s l y outgassed a t 1000° and t h e r e f o r e e s s e n t i a l l y f r e e of oxygen, are p l o t t e d i n F i g u r e 6. I t i s seen t h a t the v a r i o u s p o i n t s f i t around a s i n g l e curve showing t h a t the extent o f sorpt i o n / m o f carbon surface when f r e e o f oxygen i s about the same i n every case i r r e s p e c t i v e o f d i f f e r e n c e s i n p o r o s i t y o f these m a t e r i a l s . I t appears t h a t ammonia, being a s m a l l molecule w i t h t h i c k n e s s equal t o 2.36 A, becomes e a s i l y a c c e s s i b l e t o the inner surface o f carbons as w e l l . 2
Adsorption o f Phenol from Aqueous S o l u t i o n s . Adsorption o f phenol from aqueous s o l u t i o n , b e i n g o f i n t e r e s t from the standpoint o f water treatment,was s t u d i e d u s i n g Mogul,Spheron-6,Graphon(ahighl y g r a p h i t i s e d carbon b l a c k ) and sugar c h a r c o a l . The isotherms (35° C) p l o t t e d on the b a s i s o f amounts adsorbed ( μ moles)per m o f surface i n the v a r i o u s carbons i n the o r i g i n a l s t a t e are pre sented i n Figure 7. I t i s seen t h a t the extent o f a d s o r p t i o n a t a given c o n c e n t r a t i o n i s maximum i n the case o f Graphon which i s f r e e o f oxygen and decreases i n the order Graphon>Spheron-6 > Mogul > Sugar c h a r c o a l . This i s a l s o the order o f decreasing oxy gen content o f these m a t e r i a l s . The r o l e o f chemisorbed oxygen i n adversely a f f e c t i n g the amount o f a d s o r p t i o n i s , t h e r e f o r e , q u i t e evident. 2
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
ADSORPTION A T INTERFACES
Figure 3. Adsorption isotherms of ammonia on various samples of sugar charcoal. Ο = original, Δ = 600°-out gassed, • = 1000°-outgassed, · = original, treated with aq. H 0 , A = 1000°-outgassed, treated with aq. H 0 . 2
2
2
2
Figure 4. Adsorption isotherms of ammonia on various samples of Mogul. Ο = original, Δ = 600°-outgassed, • = 1000°-outgassed, · = original, treated with aq. H 0 , A = 1000°-outgassed, treated with aq. H 0 . 2
2
2
2
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
15. puni
Surface Oxygen Complexes
219
Figure 5. Adsorption isotherms of ammonia on vanous samples of Spheron-6. Q = original, Δ = 600°outgassed, • = 1000°-outgassed, · = original, treated with aq. H O , A = 1000°-outgassed, treated with aq. H O . 2
z
2
g
Pressure ^ T o r r ) — • ·
Figure 6. Adsorption isotherms of ammonia on oxy gen-free (1000°-outgassed) samples of sugar charcoal, Mogul and Spheron-6. Q = 1000°-outgassed sugar charcoal, Δ = 1000°-outgassed Mogul, • = 1000°outgassed Spheron-6.
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
220
ADSORPTION A T INTERFACES
I n order t o study the e f f e c t of CO2- and CO-complexes, sep a r a t e l y , the isotherms were a l s o determined on 6 0 0 ° - and 1 0 0 0 ° outgassed samples. The r e s u l t s are p l o t t e d i n F i g u r e s 8, 9 and 10 f o r Spheron-6, Mogul and Sugar Charcoal. I t i s h i g h l y s i g n i f i c a n t t o note t h a t when the a c i d i c CO2- complex i s e l i m i n a t e d s u b s t a n t i a l l y on outgassing a t 600° and CO-complex becomes the predominant surface complex, the extent of a d s o r p t i o n a t each c o n c e n t r a t i o n i n c r e a s e s a p p r e c i a b l y i n the case o f each carbon and even surpasses the corresponding values f o r the oxygen-free ( i . e . , 1000°-outgassed) samples. This shows t h a t the adverse e f f e c t o f combined oxygen, as r e p o r t e d by previous workers (27, 28), can not be t r u e f o r the whole o f the oxygen. A p a r t o f the oxygen disposed o f as carbon monoxide, i n f a c t , enhances adsorb a b i l i t y o f phenol t o an a p p r e c i a b l e extent. The p o s i t i v e e f f e c t of a p a r t o f the combine i enhancin sorptio phenol i s being r e p o r t e d s u l t s c l e a r l y i n d i c a t e t h a t when the a c i d i c C02~complex i s pre dominant, the surface p r e f e r s the s t r o n g l y p o l a r molecule o f water (the s o l v e n t ) , which a d v e r s e l y a f f e c t s the a d s o r b a b i l i t y o f phenol. However, w i t h the e l i m i n a t i o n o f t h i s complex and w i t h the emergence o f CO-complex, as the predominant surface complex, the preference o f the surface f o r phenol r i s e s a p p r e c i a b l y . This appears t o be due t o i n t e r a c t i o n o f OH groups o f phenol w i t h p h e n o l i c and quinonic oxygens a s s o c i a t e d w i t h CO-complex. Summary The e f f e c t o f carbon-oxygen surface complexes on s e l e c t i v e a d s o r p t i o n from methanol-benzene mixtures as w e l l as adsorb a b i l i t y of benzene vapour, dry ammonia and phenol from d i l u t e aqueous s o l u t i o n s by a few samples o f m a c r o c r y s t a l l i n e carbons has been i n v e s t i g a t e d . The view o f previous workers t h a t the combined oxygen a f f e c t s these p r o p e r t i e s more or l e s s u n i f o r m l y has not been s u b s t a n t i a t e d . Thus, w h i l e the presence o f a c i d i c C02-complex enhances preference of the surface f o r methanol, the more p o l a r component o f methanol-benzene s o l u t i o n s , t h a t o f COcomplex enhances preference f o r benzene, the l e s s p o l a r component of these s o l u t i o n s . The presence o f non a c i d i c C02-complex has h a r d l y any e f f e c t . Again, w h i l e the a c i d i c C02-complex supresses the s o r p t i o n o f pure benzene from vapour phase, t h a t of CO-com p l e x enhances i t a p p r e c i a b l y and t h a t o f non a c i d i c complex has h a r d l y any e f f e c t at a l l . Adsorption o f d r y ammonia, which f o r oxygen-free carbons i s l a r g e l y a f u n c t i o n o f surface a r e a , i s enhanced c o n s i d e r a b l y by a c i d i c C02-complex, t o a much smaller extent by CO-complex and not a l a l l by non a c i d i c C02-complex. Adsorption o f phenol from d i l u t e aqueous s o l u t i o n s i s i n f l u e n c e d a d v e r s e l y by a c i d i c C02-complex, f a v o u r a b l y by CO-complex but not by non a c i d i c complex. S u i t a b l e explanations have been o f f e r e d f o r the apparent anamolies.
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
15. puRi
Surface Oxygen Complexes
Equilibrium concentration^ moles/litre J —>-
Figure 7. Adsorption isotherms of phenol on Graphon, sugar charcoal, Mogul, and Spheron-6. X = Graphon, • = sugar charcoal, Δ = Mogul, • = Spheron-6.
Equilibrium concentration (m moles/litre) —
Figure 8. Adsorption isotherms of phenol on Spheron-6 before and after outgassing at 600 and 1000° C. · = original Spheron-6, Δ = 600°-outgassed Spheron-6, • = 1000°-outgassed Spheron-6.
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
221
ADSORPTION A T INTERFACES
Figure 9. Adsorption isotherms of phenol on Mogul before and after outgassing at 600 and 1000°C. · = original Mogul, Δ = 600°-outgassed Mogul, • = 1000°-outgassed Mogul.
τ q2>q3 are the three roots of the cubic term in the denominator then ««ι-i - t o Y
q
2
=
2
= !
c
o
s
t
+
i
4 COS (| + |2L)+ 1
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
(
1
9
)
(20)
238
ADSORPTION AT INTERFACES
ν ! where
+
(21)
^t
(22)
c o s
cos ψ = 1 -
2
and the solution of Equation (18) as given by Grohner and Hofreiter (10) for q^iq^l is q i F ( M ) - (q! - q )E(k,) 3
'a(qi - q3)'
0
+
(qi - q3)(tan Φ)
/l - k
s i n ψ"
2
2
(23)
where F and Ε are e l l i p t i defined by: de
F(k,4>) and
ο Λ - k Φ
, A - k
E(M)
2
2
(24)
sin e' 2
sin
2
, θ de
(25)
where k
=
2
and
q2 - ^3
Π - qi [1 - q
Φ = Arcsin
(26)
qi - q3 0 < ψ < ϊ
(27)
2
The volume of the drop is obtained by taking Equation (8) in the form . 1 = Y[l - f Y ] (28) [1 + (dY/dX) ]* 2
2
4
If one differentiates this equation with respect to Y and inte grates with respect to X one has
_d ο dY
1
Y dX 2
[l+(dY/dX) ]2 2
The l e f t hand side of Equation (29) can be integrated i f the substitution dY = tan τ is made. This yields dX
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
(29)
17. CAYIAS ET AL.
Low Interfacial Tension
239
cos τ dT = 1
(30)
and V "P
2π
3
Y dX = 2
ψ fr
y
(31)
where r is the radius of a sphere of the same volume as the drop. Then combining Equations (29),(30), and (31) yields
r(r)
ο
£
( x
o-i>
(«)
r\
v
This can be reduced to
3 =
J
Combining Equations (6) and (33) yields Cr
Ύ Ϊ3
3
1
(34)
which will be an important equation in computing γ. With the aid of a high speed computor (CDC 6600), one can construct a table of values for r/p , C r , x^/r, yg/r, and x / y with α as the independent variable by the r o l l owing scheme: 3
Q
0
1.
Assign value to a.
2.
Calculate Y for 0 < Y
by Equation (11) or Equation (19)
0
< \
0
3.
Calculate X Q by Equation (23).
4.
Calculate —
by Equation (33).
o
p
5.
Calculate Cr
6.
Calculate x / r by X t (r/p ).
7.
by Equation (34)
3
q
Q
0
Calculate y /r by Y * ( r / p ) Calculate xjy„ by X * Υ ο ο 0 0 n
8.
0
A
n
Λ
This produces Princen s table except the authors have expanded and extended i t to x / y = 4.0 (a = .592589328), and because of the ease of extracting f u l l accuracy from a computer, 1
0
0
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
240
ADSORPTION AT INTERFACES
the values are recorded to ten significant figures, this accuracy however is not necessary.* The authors method of using the table to obtain a value of γ is to take the ratio of the length to width (2x /2y = ο/^ο) P determine the parameter C by using the table; then calculate γ from Equation (7). A flow diagram of the procedure is as follows: 1
0
χ
o
f
t
h
e
d r o
a
n
0
d
When the ratio of the length to width is greater than 4.0, the authors assumed that the shape can be adequately described by assuming a cylindrical tube with hemispherical ends. This is Vonneguts method and equation (15) can be used to determine γ with less than 0.05% error. The error in calculating the volume is less than 0.1%, using this geometry. Experimental The apparatus used (Figure 2) is a refined version of that employed by Princen et a l . A hysteresis synchronous motor was used. Its speed was controlled by varying the frequency from a frequency generator. The rotational s t a b i l i t y was 1 part in 10 as determined by a period averaging counter stable to 1 part in 10 . The range of speeds used was from 1,200 RPM to 24,000 RPM. A Gaertner traveling microscope with a f i l a r eyepiece was used to measure the length and width of the drop and to calibrate the glass tubes for their magnification of the drop diameter. This effect was determined to be a constant for the experiment of y measured/y true=1.332 for a l l aqueous phases studied. The tube housing and assembley (Figure 3) was designed to accept a precision ground .245"0.D. pyrex glass tube (C) rounded on one end and sealed against a rubber septum (G) on the other. A glass cell enclosed the apparatus to permit temperature studies. Thermostating of the system to + 0.5C° was obtained. The procedure for loading the cell is to f i l l the glass tube and metal cap (H) completely with the more dense phase. Holding the tube upside down (capillary pressure retains the more dense phase in the tube), the less dense phase is then injected with a microliter syringe, the tube is then placed in the cap and the 5
8
*
A copy of the program is available from the authors on request
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
Low Interfacial Tension
CAYiAS E T A L .
TUBE
WALL
MOTOR
POWER
CONTROL
AMPLIFIER
BOX
H
y
SCOPE
ROTOR FREQUENCY
TEMPERATURE
COUNTER
CONTROLLER
SIGNAL
ASS'Y
STROBE
GENERATOR
SUPPLY
SPINNING-DROP
Figure 2.
APPARATUS
Schematic of spinning-drop apparatus
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
242
ADSORPTION A T INTERFACES
Figure 3. A = less dense phase (drop), Β = more dense phase, C = glass tube, D = shaft O-rings, Ε = shaft, F = cap O-ring, G = silicone rubber septum, H = cap, I = ball bearings, J = outer support housing, Κ = O-ring for loading bearings, L = load adjusting cap, M = Bakélite plate, Ν = aluminum box tubing, Ο = non-slip positive drive belt, Ρ = pully, R = motor shaft adaptor, and S = threads for heater wire windings
PLOT OF G vs H FOR BENZENE vs. WATER
0
1
2
3 H= Δ < 1 ω / 4 2
Figure 4.
xlO"
AT 27 C e
4 4
Plot for obtaining a using the alternate Princen et al. method
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
17.
CAYIAS ET A L .
Low Interfacial Tension
243
whole assembly placed in the shaft and secured by screwing the cap onto the shaft. A small hypodermic needle is inserted into the cell through the septum to release any pressure buildup caused by screwing the cap on. This method, though d i f f i c u l t at f i r s t , is easily mastered after a few attempts. In a test of the equipment using the benzene/water system with a literature value (11) for the interfacial tension of 35.0 dynes/cm at 20°C the authors obtained at 27°C 34.50 dynes/cm using the principal Princen et a l . method of length and volume measure ment and 34.65 dynes/cm using the alternate Princen et a l . method of frequency differences (Figure 4). Figure 5 is a photograph of the drop. The slightly lower values could possibly be due to im purities in the system or the higher temperature or both. As a final test of the system, the butanol/water system was measured and a tension of 1.80+.0 pares very well with th at 20°C To check the applicability of the system for measuring low interfacial tensions a commercial surfactant, Petronate TRS 10-80 obtained from Witco Chemical Company, was used. A stock solution was made up with the following components in water: 0.2 wt. % Petronate and 1.0 wt. % NaCl. This solution was then compared against several hydrocarbons at 27°C The results of this exper iment are in Figure 6. A picture of one of these drops is included in Figure 7. The octane/surfactant system verified the capability of the apparatus to measure low tensions. The reproducibility of the measurements was 2% but a confidence l i m i t of 10% is probably more r e a l i s t i c for tensions in this range. Using the 10% confidence limit, tensions of 10" dyne/cm could be measured with the Gaertner f i l a r eyepiece (accurate to 5 Χ 10" cm) with some d i f f i c u l t y . This is not a lower limit of the method, only of the optical equipment used. The limit could possibly be extended by choosing a different measuring system. The main d i f f i c u l t y encountered was to deliver small volumes of 10" cc or less. The discharge as a single drop of such small volumes from a syringe was found to be very d i f f i c u l t . The drop has a tendency to stay attached to the needle and quick removal of the needle from the more dense phase caused the drop to detach i t s e l f from the needle partially. This partial detachment made accurate volume measurements almost impossible. A further complication of the problem was that as the needle was retracted from the liquid, some small drops of the less dense phase would be l e f t behind and after starting the rotation these small drops could coalesce with the large drop changing its volume. Another d i f f i c u l t y with the measured volume method is i f the drop breaks up after injection then the experiment would have to be term inated. The last d i f f i c u l t y with measuring the volume of the drop was volume change due to solubilization. For low interfacial 6
5
3
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
ADSORPTION A T INTERFACES
SURFACE
TENSION OF HYDROCARBONS
vs. 0.2% PETRONATE / 1.0% Να CI AT 27 °C AFTER
24 HOURS OF STIRRING
NUMBER OF CARBON ATOMS Figure 6.
Results of hydrocarbon studies against the surfactant
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
Figure 7.
Octane surfactant system at 6,000 RPM
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
246
ADSORPTION AT INTERFACES
tensions, where the drop was composed of several components, solubilization could result in indeterminate drop composition. For example the volume of the drop in the octane/surfactant system when the two phases were not pre-equilibrated, initially increased and then slowly decreased from the initial volume making the measured volume method useless and the frequency difference method inaccurate since the volume was not constant. For these reasons it was necessary to abandon the Princen et al. methods and use the authors' method described previously of measuring both the length and diameter of the drop and from this calculating.the tension. Both Vonnegut and Princen et al. observed a rippled surface for an air buble in water. Princen et al. suggested it was probably due to the vibration from the motor drive, however the authors in examining thi t found distortion f th surface but had vibration possible explanation of the lack of these ripples is the small size of the authors' system in comparison to that of Princen et al. Conclusions The spinning drop method for measuring low surface and interfacial tensions using the methods described in this paper is one of simplicity and ease of operation as compared to other methods. The method does not involve a third (solid) phase in contact with the interface of the drop as Ryden and Albertsson (9) pointed out. The Princen et al. method works well in systems where accurate determination of the volume is possible but for systems the authors examined where accurate determination of the volume is a difficult task, the method of measuring both the length and width of the drop is more accurate. Acknow!edgement The authors wish to express their appreciation to the Robert A. Welch Foundation and the National Science Foundation for sponsoring this research and their special thanks and apprécia* tion to H.M. Princen for valuable discussions and furnishing the authors with some of his original data. The authors also wish to express their gratitude to James Gardner for the drawings which appear in the text.
Literature Cited 1.
Plateau, J.A.F., "Statique Experimentale et Theorique des Liquides, etc.", Gauthier-Villars, Paris, 1873.
2.
Beer, Α., Annalen der Physik und Chemie von Poggendorff, (1855) 96, 210.
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
17. CAYIAS ET AL.
Low Interfacial Tension
247
3.
Raleigh, Lord, Phil. Mag., (1914) 28, 161.
4.
Vonnegut, Β., Rev. Sci. Instrum. (1942) 13, 6.
5.
Silberberg, Α., Ph.D. Thesis, Basel University, Switzerland, 1952.
6.
Rosenthal, D.K., J. Fluid. Mech., (1962) 12, 358.
7.
Princen, H.M., Zia, I.Y.Z., and Mason, S.G., J. Colloid Interface Sci., (1967) 23, 99.
8.
Patterson, H.T., Hu, K.H., and Grindstaff, T.H., J. Polymer Sci., (1971) 34, 31.
9.
Ryden, Jan and Albertsson Sci. (1971) 37, 219.
10.
Grobner, W. and Hofreiter, N., "Integraltafel," 3rd ed., Vol. I, p. 78, Springer Verlag, Vienna, 1961.
11.
Adamson, Arthur W., "Physical Chemistry of Surfaces," 2nd ed., pp. 44-45, Wiley, New York, 1967.
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
18 Adhesion of Ice Frozen from Dilute Electrolyte Solutions H. H. G. JELLINEK Department of Chemistry, Clarkson College of Technology, Potsdam, Ν. Y. 13676
Introduction The adhesion of ice has long been studied but a satisfactory solution, from the practical standpoint of decreasing adhesion or increasing abhesion, has not yet been found. Complete abhesion can be achieved un der laboratory conditions but in practice any surface becomes rapidly contaminated after a few abhesions have taken place and the adhesion of ice continually i n creases with further abhesions. Experimental Data by Smith-Johannsen and Discussion Smith-Johannsen (1) studied the effect of impuri ties in water on the adhesion of ice by freezing such solutions to various substrates. He ascertained that small i n i t i a l concentrations (1 X 10 mole/liter(M) or less) of electrolytes (salts) decrease ice adhesion considerably. The adhesive strength was measured by the force per square centimeter needed to shear the ice off the substrate. The experiments were carried out at -10°C. The freezing point lowering of water by elec trolytes of such concentrations is only about 0.005°C. The ice was frozen rapidly enough so that the electro lyte remained homogeneously distributed throughout the frozen system, and did not have sufficient time to dif fuse away from the substrate/solution interface. Air bubbles appeared during the freezing process. Table I gives relevant data (S = salinity of i n i t i a l solution; p = salinity of grain boundary solution). The effectiveness of decreasing adhesion, or i n creasing abhesion, varies considerably with the partic ular electrolyte in solution. Thus 1 Χ 1 0 Μ Th(NO ) decreases adhesion by 97% on a wax-treated aluminum -3
-3
3 4
248 In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
3
h
2
2
2
3
3
3
3
s
2.91 3.08 1.95 4.00 1.93 2.99 1.17 1.40 1.15 0.88 1.54 1.20 1.27 2.17
10 2,45 2.20 0.85 0.65 0.76 2.78 0.39 1.38 1.30 0.11 6,_7,8)that the process of smelling involves adsorption of the odor producing material on to the surface of the mucous membrane of the nerve c e l l , producing a biochemical or biophysical mechanism to f a c i l i t a t e exchange of ions of sodium and potassium (3.,£,8) to f i r e a nerve impulse. If this is the case then there should be a relation between the threshold value of concentration of the vapor for giving a detectable odor and the adsorbability on mucous membrane. . Human nose is not a reliable instrument for measuring the threshold value, therefore different physical models have been employed by different workers. (£,]O,JU,12,13) The surface excess according to Gibbs equation was c a l culated for each alcohol. The surface excess is highest for ethyl alcohol and decreases up to carbon atom 6. Then again the surface excess increases. Adsorption or surface excess parallels published (3)values for olfactory threshold and the adsorption decreases with increasing molecular cross section up to carbon atom 6 for n-alcohols. The concentration of each alcohol solution in water required to reduce the interfacial tension of oil-water interface by 1 dyne/cm was determined. This concentration also decreased from alcohol with two carbon atoms to six carbon atoms. Thus these concentrations can be used to calculate olfactory threshold values. If the oil-water interface is accepted as a simulated olfactory epithelium, the odorant molecules adsorbed at the interface at low concentration can be a measure of the olfactory threshold. In order to produce the sensation of odor a certain minimum reduction in the interfacial tension has to occur. For
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
266
ADSORPTION AT INTERFACES
purposes of comparing different odorants 1 dyne/cm reduction is used in this investigation. This is equivalent to a decrease in the surface free energy of one erg per square cm. The concentration of certain flavoring agents required to reduce the interfacial tension by about 2 dynes/cm at the o i l / water interface was determined. With the exception of vanillin which was soluble in water, the others were soluble in Squibb mineral o i l . Figure 2. No conclusions could be drawn since olfactory threshold values of flavoring agents are not available. It was found that the extent of adsorption of the n-alcohols at the oil/water interface is related linearly to the published (3) threshold concentrations for the same alcohols. Figure 3. Concentration of η-alcohol in the aqueous phase required to produce a lowering in interfacial tension of 1 dyne/cm also bears a linear relationship to the olfactory threshold values Figure 4. These correlation satisfactory model fo y Summary of Olfaction Mechanism It is generally agreed that a odor can be detected i f the odor carrying substance makes physical contact with the interior part of the nose, namely, the membrane of the nerve cell. The human nose is very sensitive in detecting odorants in low concentrations and at low vapor pressures and the human nose is very selective. Simulated systems lack this kind of sensitivity and selectivity. Interaction between the stimulant and the receptor namely the trigeminal nerve endings takes place as a result of which stimulus is formed. The molecules of the stimulant or the products formed are removed from the inter action zone and the stimulus is transmitted to the olfactory region of the brain and translated into the sensation of odor. The possible mechanism is the odorant molecules interact with the nerve cells in the olfactory epithelium and the molecules are adsorbed on the lipid-water interface of the cell membrane. The odorant molecules penetrate the lipid-cell membrane which results in an exchange between potassium ions in the cell and the sodium ions outside the cell. This process triggers an impulse in the olfactory nerve. The oil/water interface can be likened to the lipid/water interface.
Literature Cited 1.
Bikerman, J.J. "Physical Surfaces" pp.118-121. Academic press, New York, 1970 2. Salzberg, H.W.,Morrow, J . I . , and Cohen, S.R. "Laboratory Course in Physical Chemistry" p. 103-104. Academic Press, New York, 1966. 3. Davies, J.T., Taylor, F.H. 2nd International Symposium on Surface Activity (1957) 4 Butterworth, London, pp.329-340.
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
19.
Mechanism of Olfaction
RATHNAMMA
-0.3
.3
-2.3 log c
-3.3
c » m o l e s / I iter
267
-4.3 aqueous
-5.3
-6.3
7.3
phase
Figure 1. Interfacial tension vs. concentration for a variety of ahohoh. Ο = ethanol, Δ = η-propyl alcohol • = n-butanol, ft = amyl alcohol, φ = n-hexyl alcohol, A = lauryl alcohol, • = n-hexadecanol.
-2
log c
-3 c = moles/liter
-4 -5 aqueous phase
-6
Figure 2. Interfacial tension vs. concentration for flavoring agents. Q = vanillin, φ = zinnamic alcohol, Δ = eugenol, • = isoeugenol, • = safrol, ft = phenyl propyl alcohol.
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
268
ADSORPTION A T INTERFACES
Figure 3. adsorption vs. threshold concentration
ιο μ ,5
10'1-6
10,-5
Cone,
10"
of η - a l c o h o l
r 3 10"
in aqueous phase
10"
10"
moles/liter
Figure 4. Concentration of alcohol required to produce interfacial tension reduction of 1 dyne/cm vs. olfactory threshold
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
19. RATHNAMMA
4.
5. 6. 7. 8.
9. 10. 11. 12. 13. 14
Mechanism of Olfaction
269
Rosano, H . L . , and Friedman, H . H . , in "Flavor Chemistry," Adv. Chem. Ser. No. 56, pp. 53-63, American Chemical Society, Washington, D . C . , 1966. Beets, M . G . J . , Mol., Pharmacol. (1964) 11, 1. Davies, J.T., J. Theor. B i o l . (1965) 8, 1. Renshaw, E . H . , Aust. J . Dairy Tech. (1964) 19(3), 128. Davies, J.T., "Recent Developments in the Penetra tion and Puncturing Theory of Odour," Reprinted from the Ciba Foundation Symposium of Taste and Smell in Vertebrates, pp. 265-291, J an A Churchill, London, 1970. Dravnieks, Α . Ann N.Y Acad S c i (1964) 116,357 Amoore, J.E., (October 1962) , Dravnieks, Α . , Ann. N.Y. Acad. S c i . (1964)116,429. Hartman, J . D . , Proc. Am. Soc. Hort. S c i . (1954) 64, 335. Wilkens, W.F., Hartman, J . D . , J. Food S c i . (1964) 29(3), 372. Harkins, W.D., "The Physical Chemistry of Surface Films," pp. 209-210, Reinhold Publishing Corpora tion, New York, N.Y., 1952.
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
20 Role of Double Interactions and Spreading Pressure in Particulate Soil Removal HERMANN LANGE Henkel and Cie., GmbH, Düsseldorf, Federal Republic Germany
Introduction In the theory of particulate s o i l removal the i n fluence of electric charges and double layers at the surface of s o i l particles and substrates has been discussed for many years (1-3). In analogy to the Derjaguin-Landau-Verwey-Overbeek theory one takes into account the van der Waals-London energy of attraction P and the repulsive energy P due to electrical double layer. By combining both types of interaction energy one obtains diagrams of the type shown in Figure 1. The curves have been calculated for two d i f ferent values of the potential of the diffuse part of the double layer using the tables given by Verwey and Overbeek. (4). The curve of the electrical repulsion ΡR as well as the maximum of the resultant curve is shifted upwards when the potential rises. The poten tial may be increased, for example, by adsorption of surface active ions. This model has sometimes led to the conclusion that not only the deposition of s o i l particles but al -so the removal of adhering particles w i l l be inhibited if the potential of the diffuse layers and, conse quently, the maximum of the resultant curve is high. It w i l l be shown in this paper that this conclusion is not justified because the choice of the common zero point of the scale of potential energy is misleading. Furthermore, the primary step of the removal of adhering particles, namely the beginning of the pene tration of an extremely thin layer of liquid between particle and substrate allowing for the formation of an adsorption layer w i l l be discussed in the second part. A
R
270 In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
20. LANGE
Particulate Soil Removal
0
271
2
4
6
DISTANCE x,nm Figure 1. Potential energies of attraction Ρ and of repulsion P as function of the distance and the resultant Ρ of them, ζ = potential in units of ψ · e/(kT). φ = potential of the diffuse layer. The curves have been calculated for a Hamaker con stant A = 2 · 10~ erg and an electrolyte concentration of 10~ mol/l (univalent ions). A
R
α
α
13
2
PLATE MODEL
SPHERE MODEL
Figure 2. Two models for a particle adhering at a substrate
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
ADSORPTION A T INTERFACES
Figure 4.
Free energy of the double layer per unit area as a function of the distance. Flate model.
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
20.
LANGE
Particulate Soil Removal
273
I n f l u e n c e o f the E l e c t r i c a l Double L a y e r on t h e Removal of S o i l P a r t i c l e s Two models o f a d h e r i n g s o i l p a r t i c l e s w i l l be t a k e n i n t o account as s c h e m a t i c a l l y shown i n F i g u r e 2. With t h e " p l a t e m o d e l " the c o n s i d e r a t i o n s a r e s i m p l e r t h a n w i t h t h e 'tephere model". However, t h e sphere model g i v e s a b e t t e r d e s c r i p t i o n o f most r e a l s o i l p a r t i c l e s . P l a t e M o d e l . The s t a r t p o i n t and the f i n a l s t a t e o f t h e removal o f a p l a t e - l i k e p a r t i c l e i n an e l e c t r o l y t e s o l u t i o n i s shown i n F i g u r e 3. In the b e g i n n i n g t h e r e w i l l be an e l e c t r i c a l d o u b l e l a y e r o n l y a t the o u t e r s u r f a c e o f the system b u t n o t i n t h e c o n t a c t z o n e . A f t e r removal t o a d i s t a n c e l a r g e enough t h a t no i n t e r a c t i o n s between p a r t i c l e and s u b s t r a t e a r e p r e s e n t new d o u b l e l a y e r t a c t a r e a . The f o r m a t i o n o f the d i f f u s e l a y e r s r e s u l t s i n a d e c r e a s e o f the f r e e energy o f the s y s t e m . A t s m a l l e r d i s t a n c e s an i n t e r a c t i o n o f t h e d o u b l e l a y e r s on b o t h s i d e s o c c u r s . C o n s e q u e n t l y , the f r e e energy i s a f u n c t i o n o f the d i s t a n c e as shown i n F i g u r e 4. The d e c r e a s e o f f r e e energy approaches a s y m p t o t i c a l l y t o a l i m i t i n g v a l u e F c o r r e s p o n d i n g t o t h e s t a t e o f no i n t e r a c t i o n between the d o u b l e l a y e r s . The amount o f t h i s v a l u e i n c r e a s e s w i t h r i s i n g p o t e n t i a l of the d i f fuse l a y e r . In o r d e r t o b r i n g a p a r t i c l e from a l a r g e d i s t a n c e i n c o n t a c t w i t h the s u b s t r a t e s u r f a c e one has t o do a work o f Ρ = 2 FQO . The f a c t o r 2 i s caused by the f a c t t h a t the d o u b l e l a y e r i s p r e s e n t a t p a r t i c l e and s u b s t r a t e s u r f a c e . Hence, the v a l u e o f F ^ i s the c o r r e c t r e f e r e n c e l e v e l i f one c o n s i d e r s the a p p r o a c h i n g o f a p a r t i c l e from a l o n g d i s t a n c e t o the s u b s t r a t e s u r f a c e up t o t h e c o n t a c t . C o n s e q u e n t l y , the common z e r o l e v e l o f t h e p o t e n t i a l energy i n t h e f i r s t diagram means two d i f f e r e n t v a l u e s f o r F ^ f o r the lower (z = 2) and f o r the h i g h e r (z = 4) p o t e n t i a l . However, when c o n s i d e r i n g t h e removal o f a d h e r i n g p a r t i c l e s a z e r o l e v e l chosen i n t h i s way i s no l o n g e r a c o r r e c t common r e f e r e n c e l e v e l f o r d i f f e r e n t p o t e n t i a l s , because i n the s t a t e o f a d h e r i n g no d i f f u s e l a y e r i s p r e s e n t between the a d h e r e n t s . C o n s e q u e n t l y , the c o r r e c t r e f e r e n c e l e v e l f o r the removal o f a d h e r i n g p a r t i c l e s i s d e f i n e d by the absence o f any i n t e r a c t i o n energy between p a r t i c l e and s u b s t r a t e as w e l l as o f any f r e e energy o f d o u b l e l a y e r . S o , f o r t h e p o t e n t i a l energy one o b t a i n s c u r v e s as shown i n F i g u r e 5. P* i s the f r e e energy o f a d h e s i o n caused by van der w â a l s w
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
274
ADSORPTION A T INTERFACES
DISTANCE x,nm
Figure 5. Potential energy of attraction P and free energy of double layer 2 F ; and 2 F ) for the potentials ζ = 2 and ζ = 4. The resultant curves are shown by the solid lines. Plate model. A
(2
(i
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
20.
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London f o r c e s . The r e s u l t a n t s o f the f r e e e n e r g i e s o f a t t r a c t i o n and o f the d i f f u s e l a y e r s have been c o n structed . From t h i s diagram the i m p o r t a n t c o n c l u s i o n can be drawn t h a t w i t h i n c r e a s i n g p o t e n t i a l t h e energy b a r r i e r s n o t o n l y f o r the d e p o s i t i o n b u t a l s o f o r t h e r e moval o f p a r t i c l e s w i l l be lowered ( 5 ) . Sphere M o d e l . The c o n s i d e r a t i o n s w i t h the sphere model are somewhat more c o m p l i c a t e d t h a n w i t h the p l a t e m o d e l . I n i t i a l l y , t h e r e w i l l be an i n t e r a c t i o n between the d i f f u s e l a y e r s o f p a r t i c l e and s u b s t r a t e i n a c e r t a i n r e g i o n around the p o i n t o f c o n t a c t as i s t o be seen from F i g u r e 6. Hence, the p o t e n t i a l energy o f the system i n t h e s t a t e o f a d h e s i o n i s no l o n g e r i n d e p e n dent o f the p o t e n t i a d o u b l e l a y e r . The b o r d e r o f t h e r e g i o n o f d o u b l e l a y e r i n t e r a c t i o n i s a r b i t r a r y but i t must be chosen t o be l a r g e enough t o i n c l u d e a l l s i g n i f i c a n t i n t e r a c t i o n . F i g u r e 7 shows the f r e e energy o f t h e d i f f u s e l a y e r i n the r e g i o n o f i n t e r a c t i o n . I n d e e d , the f r e e energy F i n the r e g i o n o f i n t e r a c t i o n f o r the s t a t e o f a d h e s i o n i s i n d e f i n i t e and depends on the c h o i c e o f the b o r d e r o f t h i s r e g i o n . However, t h e d i f f e r e n c e o f the f r e e energies P i n the s t a t e o f a d h e r i n g and a t long distance i s d e r l n i t e . A l t h o u g h the term F i s i n d e f i n i t e an a r b i t r a r y a l t e r a t i o n of i t causes o n l y a p a r a l l e l s h i f t i n g along the o r d i n a t e a x i s . T h e r e f o r e , i t i s u s e f u l l t o chose a r b i t r a r i l y a v a l u e o f z e r o f o r F . In t h i s c a s e , a l l curves f o r d i f f e r e n t p o t e n t i a l s and, consequently, f o r d i f f e r e n t v a l u e s o f F have the same o r i g i n on t h e o r d i n a t e a x i s . A t l o n g a i s t a n c e s , they approach asymptot i c a l l y the l e v e l o f - P below the a b s c i s s a a x i s . By c o m b i n i n g w i t h ihie c u r v e f o r t h e van der W a a l s London a t t r a c t i o n one o b t a i n s t h e diagram shown i n F i g u r e 8. The r e s u l t i s , i n p r i n c i p l e , t h e same as t h a t one o b t a i n e d w i t h the p l a t e m o d e l , namely, the h e i g h t o f the energy b a r r i e r t o be surmounted a t the removal of p a r t i c l e s decreases with r i s i n g p o t e n t i a l . In a l l t h e s e c o n s i d e r a t i o n s , however, i t has been assumed t h a t the p o t e n t i a l s a t p a r t i c l e and s u b s t r a t e are e q u a l . As l o n g as t h e d i f f e r e n c e between b o t h p o t e n t i a l s i s n o t t o o l a r g e the c o n s i d e r a t i o n s w i l l remain v a l i d i n a q u a l i t a t i v e s e n s e . I f a n i o n i c s u r f a c t a n t s a r e adsorbed a t the s u r f a c e s o f p a r t i c l e t h e y do not o n l y i n c r e a s e the negative p o t e n t i a l but they w i l l a l s o tend to e q u a l i z e i t to a c e r t a i n degree. R
R
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
276
ADSORPTION A T INTERFACES
Figure 8. Potential energy of attraction P and the difference F-F as function of the distance, and the resultants of them. Sphere model. A
0
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
20.
LANGE
Particuhte Soil Removal
277
Some p r e d i c t i o n s f o l l o w i n g from the assumption that e l e c t r i c a l double l a y e r f o r c e s are r e a l l y e f f e c t i v e i n the washing p r o c e s s have been v e r i f i e d by washing t e s t s w i t h homologous sodium a l k y l s u l f a t e s and v a r i a b l e c o n c e n t r a t i o n o f added e l e c t r o l y t e (5-6.) . The P r i m a r y Step o f the Removal o f A d h e r i n g
Particles
The f o r e g o i n g c o n s i d e r a t i o n s account o n l y f o r the t o t a l h e i g h t o f the energy b a r r i e r t o be surmounted a t the removal o f a d h e r i n g p a r t i c l e s . However, s p e c i a l a t t e n t i o n must be p a i d t o the b e g i n n i n g o f the p e n e t r a t i o n o f an e x t r e m e l y t h i n l a y e r o f l i q u i d between p a r t i c l e and s u b s t r a t e a l l o w i n g f o r the f o r m a t i o n o f an adsorption l a y e r . The s p r e a d i n g p r e s s u r e o f the adsorbed l a y e r s o f surfactants exerts a and s u b s t r a t e . T h i s f o r c e f a v o u r s the p e n e t r a t i o n o f l i q u i d i n t o t h e zone o f c o n t a c t . F i g u r e 9 shows a model o f the system under c o n s i d e r a t i o n f o r a s p h e r i c a l p a r ticle. The d i s j o i n i n g f o r c e may be c a l c u l a t e d i f t h e v a l u e s of the s p r e a d i n g p r e s s u r e s o f the adsorbed l a y e r s a t the s u r f a c e o f p a r t i c l e and s u b s t r a t e are known. From F i g u r e 9 the f o l l o w i n g e q u a t i o n can be d e r i v e d by simple geometrical c o n s i d e r a t i o n s : f
d
= 2irr
(f
p
+ Tg)
fa i s the d i s j o i n i n g f o r c e , Jf™ ^ ITc ^ spread i n g p r e s s u r e s o f the adsorbea l a y e r s a t p a r t i c l e and s u b s t r a t e s u r f a c e r e s p e c t i v e l y , r i s the p a r t i c l e r a dius · The q u e s t i o n a r i s e s wether t h i s d i s j o i n i n g f o r c e i s s u f f i c i e n t f o r p l a y i n g an e s s e n t i a l p a r t i n the r e moval o f a d h e r i n g p a r t i c l e s . F o r n u m e r i c a l c a l c u l a t i o n s , v a l u e s o f the s p r e a d i n g p r e s s u r e s TT and ]j" are n e c e s s a r y f o r the e v a l u a t i o n o f the d i s j o i n i n g f o r c e . F u r t h e r m o r e , e x p e r i m e n t a l d a t a on the f o r c e o f a d h e s i o n a r e r e q u i r e d f o r c o m p a r i s o n . I t i s somewhat d i f f i c u l t t o f i n d o u t s u i t a b l e d a t a i n the l i t e r a t u r e . A c c o r d i n g t o r e s u l t s o b t a i n e d by V i s s e r (7) t h e average f o r c e s o f a d h e s i o n f of carbon b l a c k p a r t i c l e s o f 0.1 yum r a d i u s on a c e l l u l o s e f i l m immersed i n water amount t o a p p r o x i m a t e l y 2 3 . 1 0 " dyn a t pH 3.3 and 2 . 1 0 " dyn a t pH 10. The s p r e a d i n g p r e s s u r e o f the adsorbed l a y e r o f s u r f a c t a n t s on h y d r o p h o b i c p a r t i c l e s i n e q u i l i b r i u m w i t h the s u r f a c t a n t c o n c e n t r a t i o n c may be e v a l u a t e d from a d s o r p t i o n i s o t h e r m s by the f o r m u l a (8) a
n
a
r
e
e
P
g
6
6
1
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
s
ADSORPTION A T INTERFACES
278
Ο where i s the a d s o r p t i o n as a f u n c t i o n o f the c o n c e n t r a t i o n c . For i o n i c s u r f a c t a n t s , t h i s formula i s o n l y v a l i d when the c o u n t e r i o n c o n c e n t r a t i o n i s k e p t constant. The a d s o r p t i o n i s o t h e r m shown i n F i g u r e 10 has been o b t a i n e d f o r sodium d o d e c y l s u l f a t e w i t h added NaCl f o r c o n s t a n t sodium i o n c o n c e n t r a t i o n a t g r a p h i t i z e d carbon b l a c k (Graphon, Cabot C o r p . B o s t o n , Mass.X The shape o f t h e i s o t h e r m i s v e r y s i m i l a r t o t h a t o b t a i n e d by D a y , Greenwood and P a r f i t t (9) f o r the same system w i t h o u t N a C l p / c v s . c a value of = 29 dyn/cm a t c ' = 8.10"" mol/1 ( i . e . a t the c r i t i c a l m i c e l l e c o n c e n t r a t i o n ) has been o b t a i n e d . A l t e r n a t i v e l y , the s p r e a d i n g p r e s s u r e o f the a d s o r b e d s u r f a c t a n t l a y e r may be o b t a i n e d from the d i f f e r e n c e between the w e t t i n g t e n s i o n s f o r pure water and f o r the s u r f a c t a n t s o l u t i o n . A number o f measure ments o f w e t t i n g t e n s i o n s a t p a r a f f i n wax w i t h aqueous s o l u t i o n s o f sodium a l k y l s u l f a t e s (10) has been p e r formed by the G u a s t a l l a method (11) . V a l u e s o f about 20 dyn/cm have been o b t a i n e d a t c o n c e n t r a t i o n s just below the c r i t i c a l m i c e l l e c o n c e n t r a t i o n . I n f o r m a t i o n s on the s p r e a d i n g p r e s s u r e o f the a d s o r b e d l a y e r on t e x t i l e m a t e r i a l s are h a r d l y a v a i l a b l e . Some measurements o f t h e w e t t i n g t e n s i o n on d i f f e r e n t f o i l s o f t e x t i l e - l i k e m a t e r i a l s have shown t h a t t h e v a l u e s are g e n e r a l l y r e l a t i v e l y low (10). By i n s e r t i n g a r a t h e r low v a l u e f o r the sum o f b o t h s p r e a d i n g p r e s s u r e s , namely 20 dyn/cm i n t o t h e e q u a t i o n one o b t a i n s f o r the r a d i u s 0.1 urn f
s
« 125
· 10~
6
dyne
T h i s v a l u e i s much h i g h e r than the range o f the e x p e r i m e n t a l v a l u e s o f the f o r c e o f a d h e s i o n o b t a i n e d by V i s s e r . Hence, one may c o n c l u d e from t h e s e e s t i m a t i o n s , t h a t the d i s j o i n i n g f o r c e caused by the s p r e a d i n g p r e s s u r e o f the adsorbed l a y e r s w i l l be s u f f i c i e n t t o i n i t i a t e the f i r s t s t e p o f the removal o f a d h e r i n g p a r ticles · Summary In the t h e o r y o f the washing p r o c e s s , sometimes the i n f l u e n c e o f e l e c t r i c c h a r g e s and t h e i r i n c r e a s e by
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
20. LANGE
Particulate Soil Removal
279
Figure 9. Adsorption layers at particle Ρ and sub strate S, causing a disjoining force.
Γ
1
Ο
2
1
4
1
6
8
r———Γ10
CONCENTRATION, 10" mol/l 3
Figure 10. Adsorption isotherm of sodium dodecyl sulfate (SDS) at Graphon with added sodium chloride. Sum of con centrations of SDS and sodium chloride constant and equal to 8.10- mol/l Temperature 23±1°C. 3
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
280
ADSORPTION AT INTERFACES
adsorption of ionic surfactants at the surfaces of solid s o i l particles and substrate have been discussed. Whereas the inhibition of the deposition of particles by electric charges may be easily explained, a satisfactory interpretation of the influence of the charge on the elementary step of washing, i . e . the removal of adhering particles, is lacking. This d i f f i culty can be eliminated by choosing the correct reference states for the potential energies both for the process of deposition and for that of removal. This problem is discussed for the models of plate-like as well as spherical particles. It follows that in both cases an increase of the surface charge contributes not only to the removal of particles. For the primary step of the removal f adherin particles th d i s j o i ning force caused b adsorbed layer of surfactants s also o importance. An estimation shows that this force may be sufficient to provoke the primary step of removal. Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.
Durham, K . , J. Appl. Chem. (1956) 6, 153 Lange, H . , Kolloid-Z. (1957) 154, 103 Lange, H . , Kolloid-Z. (1958) 156, 108 Verwey, E.J.W., Overbeek, J . T h . G . , "Theory of the Stability of Lyophobic Colloids", p. 82 and 141, Elsevier Publishing Company, New York, 1948 Lange, H . , in "Solvent Properties of Surfactant Solutions", K. Shinoda, E d . , pp. 144-150, Marcel Dekker, New York, 1967 Lange, H . , Fette, Seifen, Anstrichm. (1963) 65, 231 Visser, J., J. Colloid Interface Sci. (1970)34,26 Fu, Y., Hansen, R . S . , Bartell, F.E., J. Phys. & Colloid Chem. (1949) 53, 1141 Day, R . E . , Greenwood, F . G . , P a r f i t t , G.D., in Proc. 4th Intern. Congr. Surface Activity,Vol.2, p. 1005, Gordon and Breach, London, 1967. Lange, H . , (1956), unpublished data Guastalla, J., Guastalla, L., C.R. Acad. Sci. (1948) 226, 2054
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
INDEX A Abhesion 248 Acid number, copolymer 160 Acidic carbon dioxide complex 212 Acidic sites, negative charged 92 Acrylic acid copolymer (S-AA), styrene157,158 Activated carbons 92 Activated charcoal 90, 91 Activation energy humps 197 Activation time 226 Adamson's model 43 Additivity rule 17 Adhering particles in electrolyte solution 272, 274 particles, removal of 277 soil particles 273 Adhesion of ice 248 Adhesion to substrate, particle 271 Adhesional properties, film-substrate .... 160 Adhesive strength 253, 256, 258 Adsorbed isolated chain molecules 16 layers 107 thickness of the 25, 26, 105 polymer molecule 22 polystyrene 104, 105 Adsorption of benzene vapor 215 calculation 265 at and below the c.m.c 117 of co-ions, negative 108 critical energy of 23 density 71 of dry ammonia 215 dye 79, 81, 83 of electrolyte 7, 12,187 energy per segment 24, 26 equation, Gibbs 48, 51 equation, modified Gibbs 2 equilibrium 2, 98 free energy of 72 of HDPB and TDPB to Bio-Glas .112,114 of ionic surfactants to porous glass ... 107 of ammonia 218, 219 isotherm BET multilayer 29 above the c.m.c 119 for CSBFF on activated charcoal 91 Graphon 86, 89 titanium dioxide 86 Viscose 82,84 determination of 98 Gibbs 4,7,184 of hexadecyl pyridinium chloride .... 124 of liquid on solid 45
Adsorption isotherm (continued) of methanol-benzene of phenol of polystyrenes of differing molecular weights of SDS layers causing a disjoining force of liquid on solid of macromolecules on dispersed particles maxima measurements on Teflon powders
216 221 101 279 279 45 98 107, 122 43
of phenol from aqueous solutions 217 physical 72 of polystyrene 96 segment-surface energy for 23 space, diminishing 3 of STDS 112,115 thermodynamics of 1 on titanium dioxide 85 values 263 on Viscose 83 Air-solution interfaces 164 Alcohol concentration 264 Alcohols, straight chain 261 n-Alkanes on Teflon T F E 33, 36 Ammonia, dry 215 Ammonia, adsorption of 218, 219 Angle, contact (see Contact angle) Anionic surfactants 118 Anomalies, thermal 133 Approximations, evaluation of 38 Aqueous dye solution system 79 interfacial layers 133 solutions 165,217 substrates 157 Argon surface areas, BET 227 Arrhenius plot of viscosity 130 Atactic 207 Athermal solutions 16,21 Attraction, potential energy 271, 275, 276 Β Bead theory 199 Behavior, wetting 65, 66 Benzene 215, 216, 244 BET argon surface areas 227 BET multilayer adsorption isotherm 29 Binary solution, hypothetical 56 Binding of ionic surfactants 123 at the liquid interface 49 solute-solvent 48, 53
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
ADSORPTION A T
284 Bio-Glas adsorption of HDPB and TDPB to adsorption of SDS to adsorption of STDS to Black film, common Blade method, wettable Bond conformational energy difference, trans-gauche consecutive formation, hydrophobic rotational hindrance Bound layers, surface Boundaries, grain Breaking effect, water structure Bubble technique, captive Bulk solute species Bulk surface
109 112,114 115 112,115 191 262 27 22 88 16 60, 105 253 83 65 108 188
C Cadmium iodide crystal structure 225 Calcium ions : 170 Calorimeter cell 151 Calorimetric study 131 Capillary, hairpin 134, 136 Captive bubble technique 65 Carbon activated 92 black graphitized 97 dioxide complex, acidic and nonacidic 212 microcrystalline 212 surface behavior of 212 tetrachloride 32 Cationic surfactants 117,121 Cations 81,83,169 Cavity effect, intermolecular 171 Cell, calorimeter 151 Cellulose-aqueous dye solution system ... 79 Cellulose, dye adsorption by 83 Chain alcohol 261 ethylene 202 homologues, shorter117 ionic surfactants, homologous long- .... 117 isolated excluded-volume 17 molecules in the adsorbed state , 17 molecules,, configurational behavior of 16 -solid hydrophobic interactions 76 terminating group 163 Charcoal, activated 90, 91 Charge densities 71 model, point10 negative surface 79 potential energy, constant 12 Chemisorption 72 Chlorazol Sky Blue F F (CSBFF) 80 Cis and trans isomerism 207 Clathrates of water, polyhedral 131 C.m.c. (critical micellization concentration) 107, 117, 119 Coarea, limiting 85 Coefficients, virial 123 Cohesion properties, surfaces intermolecular 160
INTERFACES
Co-ion exclusion 118 Co-ions, negative adsorption 108 Collapse pressures 164,167,179 Colloid stability 1,7 Common black film 184, 191 Complex, nonacidic and acidic carbon dioxide 212 Complex surface oxygen 212 Component excesses 52 Computer simulation method, Monte Carlo 16 Condensation nuclei 176 Conductivities, thermal151 Configurational behavior of isolated chain molecules .... 16 sequences 20 state of chain molecules 17 state of a polymer 21 Configurations homogeneous Markov sequence of 17 Conformation of adsorbed polystyrene ... 104 Conformational energy difference, trans-gauche bond 27 Constant-charge potential energy 12 Constant heat increment 199 Contact angle behavior 76 of liquids on fluoropolymers 201 measurement of 28, 65 in Newton-Black soap films 183 on silver iodide 69, 70 Contact points, substrate 163 Copolymer acid number 160 Copolymeric structure 178 Copolymers 208 interfacial film properties of 160 monolayers of 157 radiation-induced 199 (S-AA) styrene-acrylic acid 158 on water, spreading S-AA 159,162,166 Coverage, fractional 36, 38 Critical energy of adsorption 23 micellization concentration (c.m.c.) .... 107 surface tensions of wetting 199, 203 temperature regions 152 Crystal structure, cadmium iodide 225 Crystalline phase, smectic liquid 191 CSBFF (Chlorazol Sky Blue FF) 80 adsorption of on activated charcoal 90, 91 on Graphon 86, 87, 88, 90 on silicon dioxide 87 on titanium dioxide 85, 86 on Viscose 81-84 D Decomposition, thermal Dehydration, of nickel hydroxide Dendrites at the solution-ice interface .... Densities, low negative charge Density, adsorption Derjaguin-Landau-Verwey-Overbeek (DLVO) theory
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
225 225 254 71 71 7
285
INDEX
Detergent, surface excess of 186 Differential effect of cations 81,92 Differential method, double 137 Diffuse double layers 118,187 Dilute electrolyte solutions 248 Dilute point-ion electrolyte 13 Diminishing adsorption space 3 Dipole, electrostatic 207 Dipole interaction, ion170 Disjoining pressure 130, 132,279 Dispersed particles 98 Dispersions, silver iodide 63 D L V O theory (Derjaguin-LandauVerwey-Overbeek) 7 Double differential method 137 Double layer diffuse 187 electrical 187,273 free energy of 272, 276 interactions in particulate soil removal 270 noninteracting 12 Drainage behavior 19 Drop, spinning (see Spinning drop) Dry ammonia, adsorption of 215 D T A study of water in porous glass 129 Dye adsorption 79, 81, 83 Dye solution system, cellulose-aqueous 79
Ε Electric balance 131 Electrical double layer 107, 119, 187, 273 Electrolyte adsorption of 7, 12, 187 cations 83, 92 concentrations 52 dilute point-ion 13 negative surface excess of 48, 53 solutions 248, 272, 274 Electrolytes 7, 192, 195 Electron micrographs 227 Electrophoresis 64, 66 Electrophoretic mobility measurements 64, 66, 68, 69 Electrostatic dipoles 207 Energy of adsorption, critical 23, 72, 73 of attraction, potential 271, 275, 276 barriers 275 change from specific surface interactions 72, 74, 75 change of transfer, partial molal 31 curve, calculated potential 5 difference, trans-gauche bond conformational 27 of double layer, free 272, 276 film free 183 film interaction 188 humps, activation 197 interaction free 185 of interaction, potential 6 potential constant-charge 12 of repulsion, potential 271 per segment, adsorption 24, 26 Enthalpy changes with limiting coarea 85 Enthalpy of hydrophobic bond formation 88 Entropy, excess film 188
Entropy of transfer, partial molal 30 Envelope of structured water layers 87 Epitheliom, simulated olfactory 265 Equilibrium adsorption 2, 98 film pressure 28, 38 solutions, p H of 124 thickness of stratified films 196 Equivalent water thickness 184 Ergodicity 18 Ethylene chain 202 Ethylenic fluoropolymers 200 Evaporation effects 142 Excesses, component 52 Excluded-volume chain, isolated 170 Exclusion co-ion 118 of micelles 107, 122 of polymers 125 volume 120, 123
F Fields, intermolecular potential 3 Film areas 160,168 common black 191 entropy 188 folding 165 free energy 183 free liquid 191 Gibbs adsorption isotherm 184 interaction energy 188 Newton 191 Newton-Black soap 183 pressure ' 28, 38 -area isotherms 161 properties, interfacial 160 silver iodide 65 stratified 196 styrene-acrylic acid 157 -substrate adhesional properties 160 tension 186 First-order phase transitions 131 Flavoring agents 264 Fluorinated polymers, highly 199 Fluorine, replacement of hydrogen with 203 Fluoropolymers, of 200, 201 Forces, adsorption and interparticle 1 Forces, repulsive 4 Fractional coverage 36, 38 Free energy of adsorption of 72 change 72,74 of double layer 272, 276 film 183 interaction 185 solid surface 28 Free liquid films 191
G Gas adsorption on parallel plates 2 Generation of configuration sequences .... 20 Gibbs adsorption equation 2, 48, 51 Gibbs adsorption isotherm 4, 7, 184
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
286
ADSORPTION
Gibbs surface excess 48 Glass, porous (see Porous glass) Grain boundaries 253 Graphitized carbon black 97 Graphon 80 adsorption of CSBFF on 87, 88, 90 adsorption isotherm of phenol on 221 adsorption isotherm of SDS 279 adsorption of polystyrene on 96 H Hairpin capillary 134, 136 HDPB (Hexadecyl pyridinium bromide) 111,112,114 Heat increment, rule of constant 199 Hemimicelle formation 76 Hexagonal layers 227 Hexagonal packing 178 Heterogeneities, high-energy 39 Hexadecyl pyridinium bromide (HDPB Hexadecyl pyridinium chloride 12 High-energy heterogeneities 39 Higher-order phase transitions 131 Hindrance, bond rotational 16 Homogeneous Markov sequence of configurations 17 Homologous n-alkanes on Teflon T F E ... 33 Homologous long-chain ionic surfactants 117 Homologues, shorter-chain 117 Hydrocarbon studies against surfactant 244 Hydrogen, replacement of 203 Hydrophilic material 79 polymers 123 sites 39,43 surfaces 151 Hydrophobic bond formation, enthalpy 88 interactions, chain-solid 76 sol 63 Hysteresis loop 163 I Ice frozen from dilute electrolyte solution 248 Ice interface 250,254 Icebergs 83 Ideally non-polarizable plates 12 Ideally polarized plates 7 Initial solutions 253 Instrumentation 139 Interacting solid particles 122 Interaction chain-solid hydrophobic 76 energy, film 188 free energies 185 specific surface 72, 74, 75 particle 6 potential energy 6 of polymer molecule with a surface .... 21 of water with silica substrates 87 Interface air-solution 164 liquid 49 polarizable 8 silver iodide-solution 63
AT
INTERFACES
Interface (continued) solute and solvent at the solution-ice structure of water near substrate-ice thermal conductivities of water near Interfacial film properties of copolymers layers, aqueous liquid solution layer, shearing tension measurements 234, between mineral oil and water reduction 234, thickness Intermolecular cavity effect cohesion properties, surface potential fields Interparticle forces
....
48 254 129 250 151
160 133 259 234,264 261, 262 261 264, 266 48,255 171 160 3 1
-dipole interaction 170 electrolyte, dilute point 13 surfactant 71 Ionic micelles 108 Ionic surfactants 107, 117, 123 Irradiation, ultrasonic 99 Isolated chain molecules 16 Isolated excluded-volume chain 17 Isomerism, cis and trans 207 Isotactic 204,206 Isotherm adsorption (see Adsorption isotherm) film pressure-area 161 water 43
Κ K\ effects of
87, 92
L Lamellae, liquid 183 Langmuir equation 38 model 29 plots 83,91 Lateral interactions 38, 118 Lattice, tetrahedral 19 Layer adsorbed 25, 26, 105, 107 adsorption 279 aqueous interfacial 133 diffuse double 187 double (see Double layer) electrical double 107,119,273 hexagonal 227 interracial liquid solution 259 interfacial solution 255 thin liquid solution 250 Stern 71 structured water 87 surface bound 60 Limiting areas 164, 167, 168 Lippman equation 8
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
287
INDEX
Liquid crystalline phase, smectic 191 films, free 191 on fluoropolymers 201 interface, binding at the 49 lamellae 183 on solid, adsorption of 45 solution layer 250,259 Long-chain ionic surfactants, homologous 117 Low energy solid, flat 28 energy surfaces 202 interfacial tension 234 Lyophobic colloid stability 7 M Macromolecules, adsorption of 98 Markov sequence 17 Maxima, adsorption 107,122 Maxwell relation 9 Metastable equilibrium thicknes stratified films 196 Methanol-benzene, adsorption of 216 Metropolis sampling method 25 Micelles 108,122 Micellization concentration (c.m.c), critical 107 Microcrystalline carbons 212 Micrographs, electron 227 Mineral oil 261 Mixed monolayers average area of 172,174 interaction of calcium ions with 170 maximum surface potential 177 Mobility measurements, electrophoretic 64 Mobility-pAg curves, of 74 Molecular models, Stuart-Briegleb 205 Molecular weights, of polystyrenes of differing 101 Mongul adsorption isotherm of ammonia on..218, 219 adsorption isotherm of methanolbenzene on 216 adsorption isotherms of phenol on 221 Monolayer film areas 160 Monolayers on aqueous solutions 165 near their collapse pressures 179 of copolymers 157 mixed (see Mixed monolayers) on subsolutions 177 Monomer activity 119 Monte Carlo computer simulation method 16 Multilayer adsorption 4, 29 Ν Na , effects of Negative adsorption of co-ions charge densities surface charge surface excess of electrolyte Negatively charged acidic sites Newton-Black soap films Newton films +
87, 92 108 71 79 48, 53 92 183 191
Nickel hydroxide exfoliation of physical and x-ray properties of surface properties of thermal decomposition of weight loss of Nickel oxide nickel hydroxide dehydration to platelets physical and x-ray properties of surface areas of Nonacidic carbon dioxide complex Nonaqueous suspensions Non-athermal solvents Non-electrolyte mixture adsorption Noninteracting double layers Non-ionic surfactants Non-polarizable plates, ideally Non-swelling solid substrates Nuclei, condensation
Octane surfactant system Olfaction, mechanism of Olfactory epithelium, simulated Olfactory threshold Organic compounds on Teflon Oxygen complexes, surface
229 228 225, 228 225 226 225 232 228 228 212 96 27 4 120 123 12 79 176
245 261 265 265, 268 42 212
Ρ pAg curves, mobility74 pAe on electrophoretic mobility, effect of 66, 68, 69 Packing, hexagonal 178 Paradoxial effect 151 Parallel plates 2,4 Partial molal energy change of transfer ... 31 Partial molal entropy of transfer 30 Particle adhering 271, 272, 274, 277 dispersed 98 interaction 6 Particulate soil removal 270 Partition coefficient 135 Pendant group 203 pH 113,124,165 Phenol, adsorption of .217,221 Phase transitions 131 Physical absorption 72 Plane section of rotating drop 241 Plate model 273 Platelets, nickel oxide 232 Plates ideally non-polarizable 12 ideally polarized 7 parallel 2, 4 quartz 130 Point-charge model 10 Point-ion electrolyte 13 Polar head, surfactant 75 Polarizable interface 8 Polarized plates 7 Polydispersity 231 Polyethylene 43 Polyhedral clathrates 131
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
288
ADSORPTION
Polymer configuration 21, 204, 206 exclusion of 125 highly fluorinated 199 hydrophilic 123 molecule, adsorbed 22 radiation-induced 199 wettability of 204 Polymethylene surfaces 37 Polymorphs of water 131 Polystyrene 97 adsorbed 104,105 of differing molecular weights 101 on Graphon from toluene, adsorption of 96 Polytetrafluorethylene (Teflon T F E ) .... 32 Positive surface excess of water, relative 59 Potential energy of attraction 271, 275, 276 constant-charge 12 curve, calculated 5 of interaction 6 of repulsion 271 fields, intermolecular 3 of mixed monolayers 172, 175, 177 surface 171 zeta 65 Powders, Teflon 43 Pressure collapse 164,167 disjoining 130, 132 film 28, 38,161 spreading 270 Q Quartz plates Quinonic groups
130 213
R Radiation-induced polymers and copolymers Rayon, viscose Receptor sites Reciprocal Langmuir plots Replacement of hydrogen Repulsion, potential energy of Repulsive forces Rotating drop, plane section of Rotational hindrance, bond
199 80 261 91 203 271 4 241 16
S S-AA (styrene-acrylic acid copolymers) 158,159,162,166 Salinities 253 Salt concentration 258 solutions, aqueous 165 surface excess of 186 Salts, effect of 163 Sampling, method Metropolis 25 SDS (Sodium dodecyl sulfate) ...111, 115, 279 Segment, adsorption energy per 24, 26 Segment-surfactant energy for adsorption 23 Segments, mean fraction of 24 Selectivity coefficient 136 Sequences, configuration 17, 20
AT
Short-range interactions Shorter-chain homologues Silica substrates, water Silica surfaces, water adjacent to Silicon dioxide Silver iodide contact angle on dispersions electrophoretic mobility of films -solution interface Simulation, Monte Carlo computer Sites acidic hydrophobic receptor step Smetric liquid crystalline phase Soap films, Newton-Black Sodium chloride structure
INTERFACES
107 117 87 131 87 69, 70 63 66, 68, 69 65 63 16 92 43 261 36 191 183 225
Soil particles, adhering 273 Soil removal, particulate 270 Sol, hydrophobic 63 Solid adsorption of liquid on 45 flat low-energy 28 hydrophobic interactions, chain76 particles, interacting 122 substrates 79 surface free energies of 28 Solutes from adsorbed layers 107 solvent binding 48, 53 species, bulk 108 surface excess of 56, 57, 58 Solution aqueous 165, 217 athermal 21 composition on particle interaction, effect of 6 concentrated surfactant 191 electrolyte 248, 272, 274 equilibrium 124 hypothetical binary 56 -ice interface 254 initial 253 interactions 19 interface, air164 interface, silver iodide63 layer 250, 255, 259 silver iodide 64 system, cellulose-aqueous dye 79 Solvent binding, solute48, 53 Solvents, non-athermal 27 Specificity of the surfactant polar head .... 75 Sphere model 275 Spheron-6 219,221,222 Spinning drop apparatus 241, 242 Spreading pressure 270 Stability, colloid 1,7 STDS (sodium tetradecyl sulfate) 111,112,115 Steady state conditions 18 Stearic acid 170 Stearyl alcohol 170
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
INDEX
289
Step sites 36 Steric configurations 205 Stern-Grahame theory 71 Stern layer 71 Straight chain alcohols 261 Stratification in free liquid films 191 Stratified films 196 Strength, adhesive 253,256,258 Structure breaking effect, water 83 Structured water layers 87 Stuart-Briegleb molecular models 205 Styrene-acrylic acid copolymers (S-AA) 158 Styrene-acrylic acid films 157 Subsolutions 177 Substrate adhesional properties, film160 aqueous 157 contact points 163 -ice interface 250 particle adhesion to 271 silica 8 water-swollen and non-swellin Sugar charcoal, adsorption isotherm of ammonia and phenol on 218, 219, 221, 222 Sulfonate adsorption 63 Surface adsorbed polystyrene not bound to ... 105 surface areas 117,227,228 behavior of carbons 212 bound layers 60 bulk 188 charge, negative 79 chemical properties 199 energy, segment23 excess of electrolyte, negative 48, 53 Gibbs 48 of salt and detergent 186 of solute 56, 57, 58 of water 59 free energies, solid 28 ideal Teflon T F E and polymethylene 37 interactions 72, 74, 75 intermolecular cohesion properties 160 low-energy 202 oxygen complexes 212 potential 171,177 properties of nickel hydroxide 225 tension 234 measurement of 262 of water 52 of wetting 199, 203 Surfactant 192 anionic 118 cationic 117, 121 effect of 195 hydrocarbon studies against 244 ionic 107,117,123 ions 71 non-ionic 123 polar head 75 solutions 191 system, octane 245 Suspension effect 116 Suspensions, non-aqueous 96 Syndiotactic 204, 206
Τ TDPB (tetradecyl pyridinium bromide) 111,114 Teflon hydrophilic sites on 43 organic compounds on 42 powders 43 T F E (polytetrafluorethylene) 32 homologous n-alkanes on 33 step sites on 36 surfaces, ideal 37 water on 36 Temperature regions, critical 152 Tension film 186 interfacial (see Interfacial tension) surface (see Surface tension) 234 wetting 278 Terminating group, chain 163 Thermal anomalies 133 conductivities 151 decomposition 225 Thermodynamics of adsorption and interparticle forces 1 Thermodynamics of CSBFF adsorption 83 Thermogram of water in porous glass beads 138 Thickness of adsorbed layer 25, 105 equivalent water 184 interfacial 48 measurements 192 of nickel oxide platelets 232 Thin liquid solution layer 250 Threshold concentration 268 olfactory 265 values, olfactory 268 Time, activation 226 Titanium dioxide 80, 85, 86 Toluene 96,97 Train-loop-tail model 22 Trans-gauche bond conformational energy difference 27 Trans isomerism, cis and 207 Transfer, partial molal energy change of 31 Transfer, partial molal entropy of 30 Transition region 22,160,168 Trivalent cations 169 Tween 80 124 U Ultrasonic irradiation
99
V Vapor, benzene 215 Virial coefficients 123 Viscose, CSBFF 81-84 Viscose rayon 80 Viscosity of water between quartz plates 130
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
290
ADSORPTION
w Water adjacent to silica surfaces 131 near interfaces 129, 151 interfacial tension between mineral oil and 261 isotherm 43 layers 87 on polyethylene 43 polyhedral clathrates of 131 polymorphs of 131 in porous glass 129 beads 138 relative positive surface excess of 59 with silica substrates, interaction of .... 87 spreading S-AA copolymers on 159,162,166 structure 83, 87 surface tension of 52 -swollen solid substrates 7 system, benzene24
AT
Water (continued) on Teflon T F E thickness, equivalent viscosity of Weight loss of nickel hydroxide Wettability for low-energy surfaces Wettability of polymer Wettable blade method Wetting behavior critical surface tensions of tension
INTERFACES
36 184 130 226 202 204 262 65, 66 199, 203 278
X X-ray properties of nickel hydroxide and nickel oxide Ζ
In Adsorption at Interfaces; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
228