S T U D I E S IN I N T E R F A C E SCIENCE
New Developments in Construction and Functions of Organic Thin Films
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S T U D I E S IN I N T E R F A C E SCIENCE
New Developments in Construction and Functions of Organic Thin Films
STUDIES
IN I N T E R F A C E
SERIES D. M 6 b i u s
SCIENCE
EDITORS and R. M i l l e r
Vol. I
Dynamics of Adsorption at Liquid Interfaces
Theory, Experiment, Application by S.S. Dukhin, G. Kretzschmar and R. Miller Vol. z An Introduction to Dynamics of Colloids by ].K.G. Dhont Vol. 3 Interfacial Tensiometry by A.I. Rusanov and V.A. Prokhorov Vol. 4
New Developments in Construction and Functions of Organic Thin Films edited by T. Kajiyama and M. Aizawa
New Developments in Construction and Functions of Organic lhin Films Edited by T I S A T O KAJ I Y A M A
Department of Chemical Science and Technology Kyushu University 6-10-1 Hakozaki Higashi-ku, Fukuoka 812 Japan MAS U O AI Z A W A
Department of Bioengineering Tokyo Institute of Technology Nagatsuta, Midori-ku Yokohama 226 Japan
x996 ELSEVIER Amsterdam
- Lausanne
- New York-
Oxford - Shannon
- Tokyo
ELSEVIER SCIENCE B.V. Sara Burgerhartstraat 25 P.O. Box 211, iooo AE Amsterdam, The Netherlands
ISBN: o 444 81956 8
9 I99 6 Elsevier Science B.V. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior written permission of the publisher, Elsevier Science B.V., Copyright & Permissions Department, P.O. Box 52I, I OOO AM Amsterdam, The Netherlands. Special regulations for readers in the U . S . A . - This publication has been registered with the Copyright Clearance Center Inc. (CCC), 222 Rosewood Drive, Danvers, MA, oi923. Information can be obtained from the CCC about conditions under which photocopies of parts of this publication may be made in the U.S.A. All other copyright questions, including photocopying outside of the U.S.A., should be referred to the publisher. No responsibility is assumed by the publisher for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions or ideas contained in the material herein. This book is printed on acid-flee paper. Printed in The Netherlands.
PREFACE This book deals with organic thin films by a variety of techniques. The component molecules are relatively simple ones with self-organizing properties, that is, ordered molecular assembly characteristics. The contents of this book are arranged in the order from the fundamental concepts of molecular assembly of self-organizing molecules to the potential biological applications of protein assemblies, supramolecular species. Though the many promising applications for new electric, magnetic or optical devices, biomimetic membranes and so on have been attractively investigated recently, the fundamental studies on molecular assembly characteristics and functions for monolayers, bilayers and multilayers, LangmuirBlodgett films are indispensable to future technological innovations for molecular electronic devices, biological sensors and so on. A Priority Area Research Program for "New Functionality Materials: Design, Preparation and Control" was organized by Professor Teiji Tsuruta, Tokyo Science University in the fiscal years from 1987 to 1992 under the support of the Ministry of Education, Science and Culture, Japan. The main studies on the contention of this book was enthusiastically carded out by the research group for design of functionality of materials with supramolecular structure, design of functionality of materials composed of oriented molecules and information transmission functions of biofunctionality materials. This book is timely in view of the recent surge of interest and effort in "New Developments on Construction and Functions of Organic Thin Films". Tisato Kajiyama Faculty of Engineering, Kyushu University April, 1996
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~
VII
Contributors Masuo Aizawa Tokyo Institute of Technology, Yokohama, Japan Masanao Era KyushuUniversity, Kasuga-shi, Japan Masamiehi Fujihira Tokyo Institute of Technology, Yokohama, Japan Tisato Kajiyama KyushuUniversity, Fukuoka, Japan Masakazu Makino Universityof Shizuoka, Shizuoka, Japan Toshihiko Nagamura Shizuoka University, Hamamatsu, Japan Hiroo Nakahara Saitama University, Urawa, Japan Yushi Oishi Saga University, Saga, Japan Yoshio Okahata Tokyo Institute of Technology, Yokohama, Japan Kenji Okuyama Tokyo University of Agriculture and Technology, Tokyo, Japan Shogo Saito Kyushu University, Kasuga-shi, Japan Masatsugu Shimomura HokkaidoUniversity, Sapporo, Japan Tohru Takenaka ScienceUniversity of Okayama, Okayama, Japan Tetsuo Tsutsui KyushuUniversity, Kasuga-shi, Japan Junzo Umemura Kyoto University, Kyoto, Japan Kenichi Yoshikawa NagoyaUniversity, Nagoya, Japan
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CONTENTS Preface
..............................................
v
1 Novel Concepts of Aggregation Structure of Fatty Acid Monolayers on the Water Surface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ttsato KAJIYAMA and Yushi OISHI 2 Crystal Engineering of Synthetic Bilayer Membranes . . . . . . . . . . . . . . Kenji OKUYAMA and Masatsugu SHIMOMURA
39
3 Control of Molecular Orientation and Packing in Monolayer Assemblies. 9 71 Hiroo NAKA HARA 4 In situ Characterization of Langmuir-Blodgett Films by using a Quartz Crystal Microbalance as a Substrate . . . . . . . . . . . . . . . . . . . . . . . . . Yoshio OKAHA TA
109
5 Application of Vibrational Spectroscopy to the Study of StructureFunction Relationship in Langrnuir-Blodgett Films . . . . . . . . . . . . . . . Tohru TAKENAKA and Junzo UMEMURA
145
6 Construction of Well Organized Functional Langrnuir-Blodgett Films by Mimicking Structures and Functions of Biological Membranes . . . . . Masamichi F UJIHIRA
181
7 Nonlinear Characteristics of Thin Lipid Films . . . . . . . . . . . . . . . . . . . Masakazu MAKINO and Kenichi YOSHIKA WA
211
8 Molecular Control of Photoresponses of LB Films Containing Redox Chromophores . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Toshihiko NA GAMURA
247
9 Design of the Non-Linear Optical Films by Langmuir-Blodgett Technique . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Masanao ERA, Tetsuo TSUTSUI and Shogo SAITO
287
10 Protein Assemblies for Information Transduction . . . . . . . . . . . . . . . . Masuo AIZA WA
323
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New Developments in Construction and Functions of Organic Thin Films T. Kajiyama and M. Aizawa (Editors) 9 1996 Elsevier Science B.V. All rights reserved.
N o v e l c o n c e p t s of a g g r e g a t i o n structure o f fatty acid m o n o l a y e r s on the w a t e r surface T. Kajiyama and Y. Oishi* Department of Chemical Science and Technology, Kyushu University, 6-10-1 Hakozaki, Higashi-ku, Fukuoka 812, Japan. 1. E V A L U A T I O N O F M E L T I N G AND C R Y S T A L L I N E R E L A X A T I O N T E M P E R A T U R E S O F F A T T Y ACID M O N O L A Y E R S ON T H E W A T E R SURFACE
1.1 Monolayer preparation and modulus measurement of monolayer. Myristic(C14), palmitic(Cl6), stearic(C18) and arachidic(C20)acids(chromatographic reference quality) were used without further purification. Benzene with spectroscopic quality was used as solvent. Benzene solutions of myristic, palmitic, stearic and arachidic acids were prepared with concentrations of 3.7x10 -3, 2.8x10 -3, 2.2x10 -3 and 3x10 -3 mol-1-1, respectively. The subphase water was purified by the Milli-QII| system(Millipore Co.,Ltd.). The subphase temperature, Tsp was varied in a temperature range of 274-321 K by circulating constanttemperature water around an aluminum support of a trough. The accuracy of Tsp was _+1 K, which was evaluated by using a thermocouple positioned ca. 1 mm below the water surface. And also, the room temperature was adjusted to the same temperature as Tsp by using an airconditioner and three infrared radiation lamps. Each monolayer was compressed to a given surface pressure at a barrier speed of 48 mm. min- 1. Pressure-area (~t-A) isotherms were obtained at various Tsps with a microprocessorcontrolled film balance system. The static elasticity, KS of the monolayer on the water surface was evaluated from the ~-A isotherm by using the following equation[ 1-3 ]. Ks = -A(dJt/dA)
(1)
Figure 1 shows the ~t-A(solid line) and the logKs-A(broken line) isotherms for the stearic acid monolayer at Tsp of 293 K. LogKs(max) was defined as the maximum of logKs which corresponded to -the collapsing point. At this point, even though the collapsed monolayer fragments were observed as an appearance of patchy pattern on the base monolayer(the substrate monolayer on the water surface), molecules in the base monolayer were packed most densely and homogeneously. Therefore, it is reasonable to consider that homogeneous compression force was transmitted throughout the monolayer. Then, the temperature dependence of logKs(max) was adopted for determination of the melting temperature, Tm and the crystalline relaxation temperature, Tac of the monolayer on the water surface. The hydrophilic SiO substrate(static water contact angle:0=30 ~ was prepared by vapordeposited SiO onto a Formvar substrate[4], with which an electron microscope grid(200-mesh) was covered. The relatively hydrophobic siliconized substrate(0=90 ~ was also prepared by surface siliconized treatment; a collodion-covered electron microscope grid was dipped into an aqueous solution of silane coupling agent. Bright field electron micrographs and electron diffraction (ED) patterns were taken with a Hitachi H-500 electron microscope, which was operated at an acceleration voltage of 75 kV and *Present address: Department of Applied Chemistry, Saga University, 1 Honjo-machi, Saga 840, Japan.
7060-
Stearic Acid Tsp- 293 K 1.11"-A isotherm 2.LOGKs-A isotherm
E50 ~'40 E 30 20 10 0o
'-,n0
area~
0.1
"7
E 2.0 ~" E 1.0 o
0 _.I
[\
0.2 0.3 0.4 A/nm2.moleeule -1
3.0
o .5
Fig. 1. The ~t-A and log Ks-A isotherms of the stearic acid monolayer at Tsp of 293 K. a beam current of 2.5 laA. Electron microscopic observations were carried out at the same temperature as Tsp at which the monolayer was prepared on the water surface, by using a thermostating apparatus. Pt-carbon was vapor-deposited onto the monolayer samples with a shadowing angle of 25 ~ for the bright field electron microscopic observation. Figure 2 shows the ED patterns of arachidic acid monolayers transferred onto a SiO substrate(a) and a siliconized one(b) by an upward drawing method with a drawing speed of 60 mm-min-1 at Tso=293K and ~t=25 mN-m-], respectively. The hydrophilic part of the monolayer contacts with die substrate surface by the upward drawing method. As shown in Figure 2, the crystalline structure of the monolayer depends on the hydrophilic or hydrophobic characteristics of the substrate, maybe due to the difference of interracial interaction. The monolayer transferred on the hydrophilic substrate such as SiO shows that the crystal system of the monolayer is hexagonal. The diffraction spot of 0.42 nm spacing is assigned to the (10) reflection of two-dimensional hexagonal crystal. This hexagonal crystal system agrees with that of the arachidic acid monolayer on the water surface, which was confirmed by a grazing incidence in-plane X-ray diffraction[5]. Since the hydrophilic group of the monolayer contacts
Fig. 2. The ED patterns of the arachidic acid monolayers transferred onto (a) SiO and (b) siliconized substrates.
with the hydrophilic SiO substrate during transfer of the monolayer, this interfacial condition is in a similar fashion to that of the monolayer on the water surface with respect to the magnitude of interfacial free energy between the hydrophilic(polar) group of monolayer and the hydrophilic substrate surface. Therefore, it is reasonable to consider that the hexagonal crystal system of the monolayer on the water surface is transferred and stably maintained on the hydrophilic substrate. On the other hand, the ED pattern of Figure 2(b) shows that the crystal system of the arachidic acid monolayer transferred on the siliconized hydrophobic substrate is face-centered rectangular. The diffraction spots of 0.42 and 0.38 nm spacings are assigned to the (1 I) and (20) reflections of the two-dimensional rectangular crystal, respectively. The hexagonal crystal system on the water surface could not be transferred and maintained on the siliconized hydrophobic substrate because the surface energy of the substrate was quite different from that of the water surface. In this case, since the interfacial interaction between the hydrophilic(polar) group of the monolayer and the hydrophobic substrate surface becomes weaker with an increase in the surface energy difference, fatty acid molecules form their inherent crystalline structure. Then, it seems reasonable to conclude from Figures 2(a) and 2(b) that the crystalline structure of fatty acid monolayers on the water surface can be transferred onto a hydrophilic substrate by the upward drawing method without any change of crystallographical system. Also, since the surface of the hydrophilic SiO substrate in an amorphous state is quite smooth, the SiO substrate is suitable for the electron microscopic observation of the monolayer without any change of the crystallographical structure of the monolayer on the water surface. Then, in order to investigate the thermal behavior of the monolayer structure on the water surface, the monolayer was transferred onto the hydrophilic SiO substrate by the upward drawing method with the drawing speed of 60 mm- min- 1 at various Tsps and at a certain surface pressure where each monolayer was morphologically homogeneous[6,7]. 1.2. Determination of melting temperature of m o n o l a y e r on the water surface. Figure 3 shows the Tsp dependences of logKs(max) for stearic acid monolayer on the water surface and the ED patterns oI the monolayer transferred onto the hydrophilic substrate at the surface pressure of 20 mN-m-1. The homogeneous monolayer was formed on the water surface at this magnitude of surface pressure. The ED patterns were taken at the same temperature as
Fig. 3. The Tsp dependences of log Ks(max) and ED patterns of the stearic acid monolayers.
Fie. 5. The Tsn dependences of log Ks(max), bright field electron micrographs and ED patterns of~he palmitic~cid monolayers. Tsp at which the monolayer was prepared. The magnitude of logK_Sma:'(x) started to apparently decrease at ca. 298 K and3 i7 K. The ED patterns at 313 K and 319 K were a crystalline Debye ring and an amorphous halo, respectively. Therefore, Figure 3 indicates that a fairly remarkable decrease of logKs(max) at around 317 K corresponds to the melting behavior of stearic acid monolayer on the water surface. Also, Tms of palmitic and myristic acid monolayers on the water surface were similarly estimated to be 301 K and 278 K, respectively, on the basis of the Tsp dependence of logKs(ma.x) and ED patterns as shown in Figures 4 and 5. As mentioined above, Tm of the fatty acid monolayer on the water surface is successfully evaluated from the Tsp dependences of static elasticity and ED pattern.
2
350
A
Z~
1 9
,t,,s
C 0 300 a. m~,,,
O1 C O
o 3
o
o
1 9 Fatty Acid (monolayer) 2 z~ F,~j Acid (bulk)
250
3 o n-Paraffin (Broadhurst eq.)
12 '1'4' 16 ' 1 8 . .20 . . . . .22 24 26 Number of Carbon Fig. 6. Variation of Tm with alkyl chain length for (1) fatty acids in a monolayer state, (2) fatty acids in a three-dimensional crystalline state, and (3) normal paraffins in a three-dimensional crystalline state. 0.32 0.30
Stearic Acid(C~ 8)
~0.28
~0.26 E 0.24 0.22
c
o
T~c(298 K)
T
-
f- lO
Z I.iJ
29zo8
fo
0.20
0.30
A / nm z. molecule-1
0.40
Fig. 14. Surface area dependences of surface pressure and frequency maximum of the CH2 asymmetric band for (a) crystalline stearic acid monolayer and (b) amorphous myristic acid monolayer.
15
region. With an increase in surface pressure, the amorphous domains aggregate concurrently with an apparent increase in the conformational order, such as the conformation change from trans to gauche form. Appearance of the plateau region on the ~-A isotherm may correspond to the conformational variation in the amorphous monolayer during surface compression. Finally, all molecules are considerably well aligned on the water surface at higher surface pressure, in spite of an amorphous state. Thus, the crystalline or amorphous structure of fatty acid monolayers on the water surface is fundamentally determined by the relative magnitude of Tsp to Tm of the monolayers. 3. T H E E F F E C T OF IONIC R E P U L S I O N A M O N G H Y D R O P H I L I C GROUPS ON T H E A G G R E G A T I O N S T R U C T U R E OF M O N O L A Y E R 3.1. Aggregation structure of arachildic acid monolayer in a dissociated state. Arachidic acid monolayers were prepared from a benzene solution on the water subphase of pH5.8(pure water) and 12.6(adjusted by addition of NaOH) at Tsp of 303 K below Tm(-328 K) of the monolayer [31 ]. The ionic dissociation state of hydrophilic group was estimated on the basis of the stretching vibrations of carbonyl and carboxylate groups by Fourier transforminfrared attenuated total reflection, FT-IR ATR measurements. 70 arachidic acid monolayers were transferred on germanium ATR prism, resulting in the formation of the multi-layered film. Transfer on the prism was carried out at surface pressures of 25 or 28 mN-m -l. Infrared absorption measurements revealed that almost carboxylic groups of arachidic acid molecules did not dissociate on the water subphase of pH5.8, whereas all carboxylic groups dissociated as carboxylate ions on the water subphase of pill2.6. Figures 15 (a) and (b) show the zt-A isotherms for the arachidic acid monolayers on the water surface of pH5.8(pure water) and of pill2.6 at Tsp of 303 K, respectively, as well as the
Fig. 15. ~t-A isotherms and ED patterns of arachidic acid monolayers at a Tsp of 303 K on the water subphase of pH 5.8 (a) and pH 12.6 (b).
16 ED patterns of the monolayers at several surface pressures. In the case of a neutral state of arachidic acid(pH5.8), the ~-A isotherm showed a sharp rise of surface pressure with decreasing surface area without any appearance of plateau region. The ED patterns at surface pressures of 0 and 25 mN-m -1 showed a crystalline arc and crystalline spot, respectively, indicating the formation of "the crystalline monolayer"[6,7]. Kjaer et al.[5] also reported from synchrotron X-ray diffraction studies that the arachidic acid monolayer on the pure water surface revealed the crystalline phase of the monolayer at various surface pressures. The change of the ED pattern from the crystalline arc to the crystalline spot suggests that crystalline domains were fused or recrystallized at the monolayer domains interface owing to sintering behavior caused by surface compression, resulting in the formation of larger two-dimensional crystalline domains[2,3]. In the case of a dissociated state of arachidic acid on the water subphase of pill 2.6 at Tsp of 303 K, a plateau region of the ~-A isotherm was observed in a range of 0.3-0.5 nm2-molecule -1. The ED pattern at5 mN-m -z showed an amorphous halo, whereas those at 12 and 28 mN-m -1 exhibited crystalline arc or spot. Therefore, Figure 15(b) indicates that the arachidicacid monolayer is crystallized by compression on the water surface of pill2.6. This type of monolayer has been classified as "the compressing crystallized monolayer"[32,33]. It is clearly concluded from Figure 15 (a) and (b) that amphiphile molecules form the crystalline monolayer and the compressing crystallized monolayer at Tsp below Tm in the cases of a neutral state(maybe the low degree of ionic dissociation) and a highly dissociated state of polar groups, respectively. Figure 16 shows the classification based on the aggregation structure of monolayers with respect to thermal(Tsp, Totc, Tm) and chemical(the degree of ionic dissociation of hydrophilic group) factors. This figure is divided into the four quadrants by the two axes of Tsp and the repulsive force among hydrophilic groups. In the case of amphiphiles with nonionic hydrophilic group(corresponding to the third and fourth quadrants), isolated domains grown fight after spreading a solution on the water surface are gathered to be a morphologically homogeneous monolayer by compression. Then, at Tsp below Tm (the third quadrant), the monolayer is in a crystalline phase which is designated "the crystalline monolayer". The crystalline monolayer is further classified into the two types: crystalline domains are assembled as a large homogeneous crystalline monolayer due to a surface compression-induced sintering at interfacial region among monolayer domains at Tsp below Tcxcand also, crystalline domains are gathered without any si:~ecial orientation among domains above Tac[3 ]. The crystalline relaxation phenomena Ionic repulsion among hydrophilic groups
quadrant l Crystalline Monolayer, Compressing Cry,stallized Monolayer
12nd
"
'i I
A
/',
: ,
A
TK.-KH :
I 3rd quadrant
'i
A
i
' "Tsp
To~ i
,i
A
A
~
]1st quadrant, ]
Amorphous Monolayer
!
J A
j
A
A
, Fusing-oriented .RandomlyAssembled Crystalline Monolayer Amorphous Monolayer 14th quad'rant I
Fig. 16. Classification of the aggregation structure of a monolayer on the water surface.
17
1. 2CnSNa (Anionic Amphiphile) 0 II
CH3(CH2)n_IOC(~H2 C H3(C H2)n. iOICICH--SO~ Na+ O n=12, Tc=293 K (wet) n=14, Tc=317 K (wet) n=16, Tc=328 K (wet) 2. PEI (Cationic Polymer) H2N (CH2C H 2~)x--(CH2C H2NH)~-y
CaeCneN( Mw=40,000-50,000 Amino Groups: primary:secondary:tert iary=1:2: I N + /N=75% (pH=3.2)
Fig. 17. Chemical structures of anionic amphiphile sodium 1,2-bis((tetradecyloxy)carbonyl) ethane- 1-sulfonate (2C 14SNa) and poly(ethylenimine)(PEI). correspond to a change from elastic to viscoelastic characteristics in a crystalline phase due to a remarkable increase from anharmonic thermal molecular vibration[ 10-13]. At Tsp above Tm(the fourth quadrant), the monolayer is in an amorphous phase which is designated "the amorphous monolayer". In the case of amphiphiles with ionic hydrophilic group(the first and second quadrants), a distinct domain structure is not formed at lower surface pressure owing to an electrostatic repulsion among polar head groups. At Tsp below Tm (the second quadrant), amphiphile molecules form a large homogeneous crystallized monolayer(Tsp_7) showed a large bathochromic shift (absorption maxima; 375 nm and 390 nm) relative to the monomeric azobenzene chromophore. The spectral versatility was ascribed to Davydov splitting due to strong intermolecular interaction in the ordered molecular assemblies. Semi-quantitative calculation based on Kasha's molecular exciton theory [12] predicted that hypsochromic shift and bathochromic shift in the absorption spectrum observed in the aqueous bilayer solutions was attributed to the side-by-side (Fig.2 model a) and the headto-tail (model b) orientation of the azobenzene dipole moments, respectively. In the field of spectroscopy, these types of chromophore arrangement are known as "H-aggregation" and ".laggregation", respectively.
Ci'I~CH2)n.1-0
CHs , CH2)m-N+-" CH2CH20H CH3 Br"
=N
C n A z o C m N + BrScheme 1. Ammonium amphiphile having an azobenzene chromophore.
~,
..
200
_
c
-,;
;~',
,
.
/
300 400 Wavelength (nm)
u
500
Figure 1. UV-visible absorption spectra of azobenzene bilayer membranes in water. curve a: C8AzoCIoN+ Br-, curve b: CnAzoC5N+ Br-, curve c: C6AzoC2N+Br -.
41
b
a )
J-type orientation
H - t y p e orientation
I1
K
excited state
_
_
_
A |
I I i
ground state .... monomer
i
monomer
dimer
dimer
head-to-tail
side-by-side N molecules , ~
i
A / /
,
,
,/
r
Figure 2. Molecular orientation model of bilayer membranes and schematic explanation of Kasha's molecular exciton t h e o ~ (see equation (2)).
2.2. U V - v i s i b l e a b s o r p t i o n s p e c t r u m of s o l v e n t cast films Optically transparent films were prepared on quartz plates by casting of water or ethanol solutions at room temperature. Typical absorption spectra of the cast films are shown in Figure 3. Two strong absorption bands attributed to a-:t* electronic transition are observed in the UVvisible region. The absorption band located around 250 nm is attributed to a transition dipole moment along a short axis of the azobenzene chromophore [13]. The long axis transition at lower photo excitation energy (300-400 nm) was found to be strongly affected by the chemical structure of the amphiphile in cast films as well as in aqueous bilayer solutions. Absorption spectra of the cast films are classified into following six groups (Table 1).
42
~
a
,
i,
3 o.6
o02
.
(ps)
258
255
202
214
238
209
214
173
182
0.207 0.200 0.253 0.193 0.281 0.286 0.283 0.225 0.196 33
31
30
29
33
31
29
27
26
0.739 0.742 0.677 0.639 0.575 0.613 0.596 0.715 0.762 171
162
143
129
279
258
212
142
105
..
93 orientation vertical and the medium one for C8NC8. The slope in the case of the C18NC8 acceptor is similar to that for the long-chain bipyridinium acceptor in the composite LB systems with the same donor layer [42]. In addition, Figure 26 shows the fluorescence decay curves of the D layer for the composite LB films containing the C12NC12 or the C18NC8 as the A layer at a distance of 20 - 30/~, in comparison with that for the D layer alone, which decays approximately with a single exponential way [43]. In the presence of the acceptor layer the donor fluorescence decays more rapidly and the decay curves depend upon the spacers of C n. In a conventional way, these curves have been fitted with three components of the lifetimes (1;n) and amplitudes (An). Table 2 summarizes the values of t n and An, in which 1.0 - 1.7 ns, 170 - 260 ps, and 26 ~ 33 ps were obtained for n = 1,2, and 3 of 1;n, respectively. From the values of amplitudes, it can be seen that the quick component of 1;3 is major for the composite systems, as compared with 1;1 and 12 . Approximately, assuming that the change of the lifetime 1;3 can be ascribed to the electron transfer from the donor to the acceptor layers through the hydrocarbon spacer, the reciprocal of 1;3 indicates the rate of the electron transfer. Figure 27 shows plots of of the values of - log O ] T3 a g a i n s t the disIO.60 I 0 . 0 / ~ / ~ / tance, d, between the ~ ~ j ~~~Cl,NCS | D a n d A l a y e r s , using the C12NC12 and C18NC8 acceptors, in which a linear relation10.55 ship between logarithms of the rate of the electron transfer and the D - A distance was I / -~.... ",,._~,_~ "--I T obtained. As an alter,I 1 native treatment of the 10.50 lifetimes, an average lifetime, e.g. < 1; > = Z 1;i O Ai are indicated in the bottom row of Table 2, and also the values ofC,6 C,a C2o C22 IT 10.4" Ct4 It6 ,ira i20 (i2z log < 1; > are plotted 20 25 30 against the distance, d, D I S T A N C E (A) in Figure 27. The Figure 27. Plots of -log ~ ( - - - - - and - - - ---) and relative values of the
-]
~
]
4 9.8~'
t
C12NC12
~~l
- log< 9> ( and -- - - - ) against the distance between the donor and the acceptor layers. 9
I
slopes for these lines are found to be very similar to the results obtained from
94 the statical quenching of the donor fluorescence for the same composite LB systems, as shown in Figure 25. These results indicate the effects of the orientation of the acceptor molecules upon the barrier height for the electron tunneling in the layered structures. 3.6.2. Orientation control of organometaric compounds. Ferrocene derivatives are interested for electrochemical mediators and used to modify some organic compounds such as proteins and lipids. For amphiphilic ferrocene and biferrocene derivatives [44, 45] from the surface pressure - area isotherms of the rnonolayers on the water surface and the polarized UV-visible and IR spectra of the multilayers on solid plates, cyclo- pentadienyl rings of the ferrocene derivatives with one alkyl chain and 2,10-bissubstituted biferrocence are oriented parallel to the surface, whereas those of the ferrocene derivative with two chains on different rings and l',6'-bis-substituted biferrocence are vertical, as shown in Figure 28. Charge transfer complexes of the amphiphilic ferrocene and biferrocene with iodone or a long-chain TCNQ derivative and also the salts with BF4-form stable condensed monolayers, in the absorption spectra for mono- and multilayers of the complexes or salts, the charge transfer or the ferricenium cation
0
Fe
I
0
( <
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I
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(
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o d :~
( co
\
<
O 2E were obtained to be EL "" """ :'~.: :': .':'. .. 10 m N / m , about 800 ps and 5 ns, which are very >-.. short as compared "- "" "" "...'- ": . " - ' 2 I--.- 0~ NJ 000p, " ::? with those in the solution (200 ~ 400 z uJ ns). The more rapid HzN--?H--COOC18H3z z component of 120 ps is found to be c3 tlJ added for the X-type 1",4 1 film prepared at the lower surface pressure. The different oro arrangements of the z chromophore have o been well reflected _10 in therelaxation pro.t. 2 3 4 5 6 7 8 g J0 c e s s of p h o t o TIME / NANOSECONDS excited states in the Figure 39. Fluorescence decay curves of the excimer layered system. observed at 480 nm for the multilayers of L-PyrAlaPolycondensation C18 prepared at different conditions. reactions in the LB films of L-NaphAla-C18 were followed by monitoring change of the IR spectra, as shown in Figure 40. From diminishing the ester band (1735 cm-1) and appearance of the amide I bands (1690 - 1650 cm-1), it can be found that the polycondensation occurs in the LB films. In the X-type film, however, the conversion was very low at 40 ~ This is probably due to the reason that the intralayer reaction resulting in a helical structure favorable to polyalanine [56] is inhibited. For the Y-type film, in the first step for the oligomer was obtained at 25 ~ and then the poly-amino acid in a random coil conformation was produced through increasing the temperature up to 40 oC, as suggested by the amide I bands at 1690 and 1650 cm-1, respectively. It is supposed that the interlayer reaction propagates by "sewing up" the amino and 14.17
~n
O2.tU -4
! |
.
17 t
,
,
.
.
.
,
_ _
",
b
j
I
I
.,
,
K2)
!
I
I
i
I
.....
I
I
_
I
,
104
J
{I ~c~:o'17!5
'P CH2
2921
'
ester A amideI
~~,'=,~o-~:~-
.-~ ,~.-~ ~
Xtype 2!.,~-,
28b0` '; 18(X) ' WAVENUMBER ( cm -~)
14~3
CH2
2920
')C:O omido I (oligomer)
1690{rondom co~} PC:~er 1735 I 1650
850
V'\.H C/\N. 6 ~ o 0 H 0 H ~\/~-H .~\ /~-H O~ HC o~ Hc,
"-
~.v'\~ oc/ .. I
I I i I oc%/NH 0C%/NH
HC Ho. HC
1
40~
t
_f
Y type !
!
....
I~,
I
,,
I
1400 2800 1800 WAVENUMBER ( cm-1) Figure 40. Change of IR spectra for X- and Y-type LB films of UNaphAla-C18 together with the reaction mechanisms.
3200
105
the ester groups facing each other at the adjacent layers in the Y-type films. A similar spectral change was observed for the LB films of L-PyrAla-C18. The percent conversion of the monomer to the polypeptide can be estimated by the quantity 100 ( A o - At) / Ao, where Ao and At are the integrated intensities of the ester band at the start and time t, respectively. At 40 ~ it tended to saturate at about 30 % for the Y-type films of L-NaphAla-C18, and L-PyrAla-C18, although the conversion for the LB film of long-chain ester of alanine (L-Ala- C18) reached to 90 % [52]. This difference is considered to be due to a larger steric hindrance of the aromatic rings. The equimolecular mixture of L-NaphAla-C18 and L-Ala-C18 forms the stable condensed monolayer on the aqueous sub- phase of pH 8 and can be deposited onto solid plates at 30 mN/m and 10 ~ as Y-type films by the vertical dipping m e t h o d . F i g u r e 41 s h o w s plots of the conversion against the reaction time for the - A l a - C ; . , 40 ')C m i x e d LB f i l m , in comparison with that f o r t h e LB f i l m of L-Ala-C~s : L - N a p A l a - C ; ~ = I : 1 L-NaphAla-C18 a lo n e 45 ~ at 40 ~ A consider40"C ~ O~"s"l:l~~ able increase of the 35 ~ conversions was observed for the mixed LB film, although the freaction rates were 0 smaller than that in the L~ pure L-Aia-Ct8 f i l m L. N a p A l a - C t . , 40 ~ > reported in the prec o vious paper [53]. This 0 result indicates that by mixing the ester of amino acid containing the aromatic ring with L-Ala-C18, the steric h i n d r a n c e can be partly reduced. With the temperature elevation from 35 to 45 ~ o ' s'o ;6o =5o the initial reaction rates Time (hr) were somewhat enFigure 41. Plots of the percent conversion against hanced, while the final the reaction time for LB films of L-NaphAla-C18, conversions were not L-Ala-C18 and their equimolar mixture. significantly changed. Further, the circular dichroisms
S
106
for the mixed LB film were changed during the progress of the polycondensation and the Cotton effect due to the interaction of the 1Bb transition of naphthalene was enhanced. It is expected that the polycondensation in LB films of long-chain esters of amino acids with aromatic rings provide an interesting step to control arrangements of n-electron systems in the layered structures, depending upon conformations of the resultant polypeptides [57].
4. C O N C L U S I O N By applying the monolayer assemblying methods to the amphiphilic functionalized molecules, the layered structures with a well-defined orientation and packing of the functional groups could be obtained, in which various molecular orienations and packings could be demonstrated, depending on the chemical modification such as kinds, number and positions of the substituents. Some physical and chemical processes such as relaxation of the excited states and polycondensations in the films, can be investigated in relation to the molecular arrangements to clarify the biological and artificial functions of the molecular organized systems. These studies are expected to be developed in future aspects for molecular electronics.
REFERENCES 1. N.Kimizuka and T.Kunitake, Colloid Surfaces, 19 (1989) 301. 2. M.Losche and H.Mohwald, Rev. Sci. Instrum., 55 (1984) 1968. 3. D.Honig and D.M6bius, J. Phys. Chem., 95 (1991) 4590. 4. H.Nakahara and K.Fukuda, J. Colloid Interface Sci., 69 (1979) 24; 93 (1983) 530. 5. S.Kim, H.Tanaka, and L.S.Pu, Japanese Patents L.O., 60-128453, 60-130558, 1985. 6. H.Takahashi, Y.Irinatsu, M.Tsujihashi, S.Kozuka, and W.Tagaki, Nippon KagakuKaishL (1987) 293. 7. H.Nishi, T.Kawashima, and K.Kitahara, Nippon KagakuKaishL (1990) 1162. 8. H.Nishi, N.Azuma, and K.Kitahara, J. Heterocycle. Chem., 29 (1992) 475. 9. M.Sisido, S.Egusa, and Y.Imanishi, Macromolecules, 18 (1985) 882. 10. K.Kjaer, J.AIs-Nielsen, C.A.Helm, P.Tippman-krayer, and H.MSwald, J. Phys. Chem., 93 (1989) 3200. 11. C.Knobler, Science, 249 (1990) 870. 12. T.Kajiyama, N.Morotomi, M.Uchida, and Y.Oishi, Chem. Lett., (1989) 1047. 13. T.Kajiyama, Y.Oishi, M.Uchida, N.Morotomi, J.Ishikawa, and Y.Tanimoto, Bull. Chem. Soc. Jpn., 65 (1992) 864.
107
14. Z.-h.Lu and H.Nakahara, Chem. Lett., (1995) 117. 15. Z.-h.Lu and H.Nakahara, Chem. Lett., (1994) 2005. 16. S.Henon, D.Honig, D.Vollhardt, and D.M6bius, J. Phys. Chem., 96 (1992) 8157. 17. K.Kobayashi, M.Takasago, Y.Taru, and K.Takaoka, Thin Solid Films, 247 (1994) 248. 18. W.Liang and H.Nakahara, Chem. Lett., (1995) 973. 19. T.Moriizumi, Thin Solid Films, 160 (1988) 413. 20. T.Katsube, T.Yaji, K.Kobayashi, T.Kawaguchi, and T.Shiro, Appl. Surface Sci., 33/34 (1988) 413. 21. T.Kawaguchi, T.Shiro, and K.Iwata, Thin Solid Films, 191 (1990) 369. 22. I.V.Turko, I.S.Yurkevich, and V.L.Chashchin, Thin Solid Films, 210/211 (1992) 710. 23. H.Nakahara. H.Tanaka, K.Fukuda, M.Matsumoto, and W.Tagaki, Thin Solid Films, in press. 24. M.L.Bender and M.Komiyama, Cyclodextrin Chemistry, Springer-Verlag (1978). 25. Y.Kawabata, M.Matsumoto, T.Nakamura, M.Tanaka, E.Manda, H.Takakashi, S.Tamura, W.Tagaki, N.Nakahara, and K.Fukuda, Thin Solid Films, 159 (1988) 353. 26. A.Yabe, Y.Kawabata, H.Niino. M.Tanaka, A.Ouchi, H.Takahashi, S.Tamura, W.Tagaki, H.Nakahara, and K.Fukuda, Chem. Lett., (1988) 1. 27. A.Yabe, Y.Kawabata, H.Niino, M.Matsumoto, A.Ouchi, H.Takahashi, S.Tamura, W.Tagaki, H.Nakahara, and K.Fukuda, Thin Solid Films, 160 (1988) 33. 28. M.Tanaka, R.Azumi, H.Tachibana, T.Nakamura, Y.Kawabata, M.Matsumoto, T.Miyasaka, W.Tagaki, H.Nakahara, and K.Fukuda, Thin Solid Films, 244 (1994) 832. 29. H.Nakahara, K.Fukuda, I.Yamazaki, M.Matsumoto, and W.Tagaki, unpublished data. 30. R.Steiger, R.Kitzing, and P.Junod, Photographic Sensitivity (ed. Cox,R.J.), Academic Press, London (1973) 221. 31. S,Kim, M.Furuki, L.S.Pu, H.Nakahara, and K.Fukuda, Thin Solid Films, 160 (1988) 303. 32. H.Nakahara and D.M6bius, J. Colloid Interface Sci., 114 (1986) 363. 33. H.Nakahara, K.Fukuda, D.MSbius, and H.Kuhn, J. Phys. Chem., 90 (1986) 6144. 34. R.O.Routfy, C.K.Hsiano, and R.M.Kazmaier, Photohr. Sci. Eng., 27 (1983) 5. 35. K.Y.Law, Chem.Rev., 93 (1993) 449. 36. K.Fukuda and H.Nakahara, Colloids and Surfaces, A102 (1995) 57. 37. F.urbach, Phys. Rev., 92 (1953) 1324. 38. M.Schreiber and Y.Toyozawa, J. Phys. Soc. Jpn., 51 (1982) 1544. 39. H.Nakahara, H.Uchimi, K.Fukuda, N.Tamai, and I.Yamazaki, Mol. Cryst. Liq. Cryst., 183 (1990) 345.
108
40. H.Nakahara and K.Fukuda, J. Colloid Interface Sci., 69 (1979) 24; 93 (1983) 530. 41. H.Nakahara, K.Kitahara, H.Nishi, and K.Fukuda, Chem. Lett., (1992) 711. 42. H.Kuhn, Light-Induced Charge Separation in Biology and Chemistry, (eds. Gerischer, H. & Katz,J.J.), Verlag Chemie, Weinheim (1979) 151. 43. H.Nakahara, Y.Sano, I.Yamazaki, T.Yamazaki, K.Kitahara, H.Nishi, and K.Fukuda, J. Phys. Chem., to be submitted. 44. H.Nakahara, M.Sato, and K.Fukuda, Thin Solid Films, 133 (1985) 1. 45. H.Nakahara, T.Katoh, M.Sato, and K.Fukuda, Thin Solid Films, 160 (1988) 153. 46. K.Ogawa, S.Kinoshita, H.Yonehara, H.Nakahara, and K.Fukuda, J. Chem. Soc., Chem. Commun., (1989) 477. 47. H.Nakahara, K.Fukuda, K.Kitahara, and H.Nishi, Thin Solid Films, 178 (1989) 361. 48. H.Nakahara, K.Z.Sun, K.Fukuda, N.Azuma, H.Nishi, H.Uchida, and T.Katsube, J. Mater. Chem., 5 (1995) 395. 49. H.Fujimoto, U.Nagashima, H.Inokuchi, K.Seki, Y.Cao, H.Nakahara, J.Nakayama, M.Hoshino, and K.Fukuda, J. Chem. Phys., 92 (1990) 4077. 50. H.Nakahara, J.Nakayama, M.Hoshino, and K.Fukuda, Thin Solid Films, 160 (1988) 87. 51. A.ishii, Y.Horikawa, I.Takaki, J.Shibata, J.Nakayama, and M.Hoshino, Tetrahedron Lett., 32 (1991) 4313. 52. H.Nakahara, A.Nagasawa, A.Ishii, J.Nakayama, M.Hoshino, K.Fukuda, K.Kamiya, C.Nakano, U.Nagashima, K.Seki, and H.Inokuchi, Mol. Cryst. Liq. Cryst., 227 (1993) 13. 53. K.Fukuda, Y.Shibasaki, and H.Nakahara, J. Macromolecular Sci., Chem. Ed., A15 (1981) 999; Thin Solid Films, 160 (1988) 43. 54. K.Fukuda, Y.Shibasaki, H.Nakahara, and H.Endo, Thin Solid Films, 179 (1989) 103. 55. H.Nakahara, H.Endo, K.Fukuda, T.Ikeda, and M.Sisido, Mol. Cryst. Liq. Cryst., 218 (1992) 177. 56. Ralph W.G.Wyckoff, Crystal Structure, vol.6, Pt.2, Intersci. Pub., New York, (1971) 383, 511. 57. H.Nakahara, K.Hayashi, Y.Shibasaki, K.Fukuda, T.Ikeda, and M.Sisido, Thin Solid Films, 244 (1994) 1055.
New Developments in Construction and Functions of Organic Thin Films T. Kajiyama and M. Aizawa (Editors) 9 1996 Elsevier Science B.V. All rights reserved.
Characterization by using a Quartz Substrate
In
situ
Yoshio
of L a n g m u i r - B i o d g e t t Crystal Microbalance
109
Films as a
Okahata
Department of Biomolecular Engineering, Tokyo Institute Technology, Nagatsuda, Midori-ku, Yokohama 226, Japan
of
We review useful usages of a quartz crystal-microbalnce (QCM) as tool of in situ characterization of Langmuir-Blodgett (LB) films" transfer ratio and water incorporation during a transfer process, swelling behavior in water subphase, and detachment at the airwater interface. LB films of cadmium octadecanoate and other amphiphiles were transferred on a quartz-crystal microbalance (QCM, 9 MHz, ATcut) as a substrate with a vertical dipping method. Frequencies of the QCM substrate were followed with time in air, after the QCM was raised from the interface. From the time courses of these frequency changes at each dipping cycle, the transfer amount of dry LB films (W1), the incorporated amount of water (W2), and its evaporation speed (v) could be obtained in nanogram level. The QCM can be used as a monitor of hydration behavior of phospholipid LB films in water. The hydration rate and amount were obtained from time-courses of frequency changes of the QCM deposited with LB f i l m s of various phospholipids. The phosphatidylethanolamino (PE) LB films showed the large hydration (water penetration) only around their phase transition temperatures (To). The LB film of DPPC and DPPG having relatively hydrophilic head groups gradually flaked from the substrate in the fluid state above their To. On the other hand, the LB film of DPPS having relatively water-unacceptable head groups hardly hydrated at any temperatures both below and above T~.
110
The QCM can be used as a monitor of detachment of LB films from the QCM substrate passed through the air-water interface. The removal of LB films mainly occurred when the substrate was lowered slowly into a pure water surface, but not in the raising process. 1. I N T R O D U C T I O N Interest in Langmuir-Blodgett (LB) films is wide spread and formation of ordered thin organic films by transforming lipid monolayers from a water surface is well known [1]. The characterization of LB multilayer films has been studied usually in the dry state by various methods such as FT-IR spectroscopy, X-ray diffraction, ellipsometry, and X-ray photoelectron spectroscopy [2-4]. Recently, the direct observation of the monolayer on the water subphase has been given by fluorescent microscopy [5-7], X-ray diffraction, and electron microscopy techniques [8,9]. However, the in situ evaluation of LB films during a transfer process, stability in water or at the air-water interface has not been fully explored. In this review, we describe our recent results of in situ characterization of LB films during vertical transfer process, swelling or hydration behavior in water subphase, and detachment at the airwater interface during pass through the interface by using a quartzcrystal microbalance (QCM). QCMs are known to provide very sensitive mass measuring devices because their resonance frequency changes upon the deposition of a given mass on the electrode [10]. QCMs are widely employed as a sensor device such as gas sensing [11], trace ion detection [12], detection of odor compounds and other bioactive compounds [13], immunoassay [14], DNA hybridization [15], enzyme reaction [16], surface analysis [17], gelation monitoring [18], liquid chromatographic detection [19], and electrochemical analysis [20-22]. However, only a few preliminary uses of the QCM plate to analyze the transfer processes of LB films in situ [23]. 2. EVALUATION OF LB FILMS DURING A T R A N S F E R PROCESS: T R A N S F E R RATIO AND WATER I N C O R P O R A T I O N Although characterization of LB films after deposited on a substrate or monolayers at the air-water interface have been studied in detail. However, characterization of LB films during a transfer
111 process such as water incorporation between layers and drying time has not been fully given in the conventional methods. When a monolayer moves from a water subphase to a solid substrate, behavior of water might be the key to control the film quality. It has been reported that subtle change of the thin water layer under the phospholipid monolayer during the transfer process caused the crystallization [24]. The polarized micrograph of the 22-tricosanoic acid LB film showed anisotropy due to water flow [25]. Water content must affect the electrical and the optical properties of LB films. For a practical problem, drying time of LB films in air during transfer processes has been determined with our own experience and feeling. Quartz-crystal
microbalance
(QCM) r
Osc,,,a,,ng L I counter F--quenc,.H ecomputer ona, circuit ] - --
1
LB films LB film-forming a m p h i p h i l e s (CH3(CH 2)n.2.CO O " )Cd2+
n = 16, 18, 20, 22
CH3(CH2)lsCOOH CH3(CH2)lsCOOCzHs CH3(CHz)16CONH2 CH3(CH2)lT-OH
Figure 1 E x p e r i m e n t a l set-up for vertical dipping processes of monolayers on a quartz-crystal microbalance (QCM) substrate.
In this chapter, we characterize in situ vertical transfer process of LB films of cadmium octadecanoate and other amphiphiles by using a QCM as a dipping substrate. Experimental set-up is shown in Figure 1. A transfer amount of dry LB films ( W ~ / ng), an incorporated amount of water during a lifting process (W2/ ng), and
112 an evaporation speed of the water under drying in air ( v / ng s-l) are obtained from time courses of the frequency change of the QCM at each dipping cycle. Effect of transfer conditions such as surface pressure, number of layers, and dipping speed, and chemical structures of amphiphiles on these values (W1, W2, and v ) a r e studied.
2.1
Experimental
Measurements of pressure-area (n-A) isotherms and transfers of monolayers on a substrate were carried out by using a computercontrolled film balance system (San-Esu Keisoku, Co., Fukuoka, FSD20). Maximum surface area on the trough was 475 X 150 mm2. The trough surface and the moving barrier were coated with Teflon, and the subphase was temperature-controlled with a thermostat (20 + 0.5 ~ The concentration of lipid solutions was 1 mg/ml and the spreading amount of lipid solutions was 50 - 150 g l. After solvent evaporation, the monolayer was compressed at the speed of 0.60 cm2 s-l. Measurements of n-A isotherms and transfers of monolayer on a QCM substrate were performed automatically with the usual manner [26,27]. AT-cut, 9 MHz quartz-crystal oscillators were purchased from Kyushu Dentsu, Co., Tokyo, in which Ag electrodes (0.238 cm2) had been deposited on each side of a quartz-plate (0.640 cm2). A homemade oscillator circuit was designed to drive the quartz at its resonant frequency both in air and water phases. The quartz crystal plates were usually treated with 1,1,1,3,3,3-hexamethyldisilazane to obtain a h y d r o p h o b i c surface unless otherwise stated [28]. Frequencies of the QCM was followed continuously by a universal frequency counter (Iwatsu, Co., Tokyo, SC 7201 model) attached to a microcomputer system (NEC, PC 8801 model). The following equation has been obtained for the AT-cut shear mode QCM [10]:
zlF = -2F2 Am A~/pqUq
(1)
where A F is the measured frequency shift (Hz), 17o the parent frequency of the QCM (9 X 106 Hz), Am the mass change on the electrode (g), A the electrode area (0.238 cm2), pq the density of quartz (2.65 g cm-3), and ~Zq the shear modulus of quartz (2.95 X 1011
113 dyne cm-2). Thus, the frequency decreases linearly with increasing the mass on the electrode area of the QCM. Calibration of the QCM used in our experiment by a polymer-casting or LB film-depositing method gave the following equation [13,26,27]. Am
=
(2)
- (1.27 + 0.01) x 10-9 AF
It is close to the theoretical equation calculated 1.30 X 10-9 AF). The stability of the QCM examined. The standard deviation of frequencies ng) and no frequency-drift was confirmed by with 95% confidence.
from eq. (1) (Am = frequency was also was ca. 0.5 Hz (0.6 a statistical method
A 0 N "r -
-
cll
200
ID
c t~ e-o
-400
slope,
9
v i/
-600
111111
r
c o" u.
-800 -I000
o '2'o'2o-6'o
1
8o
.
,;o
.
1
1
Time / min Figure 2 Frequency changes of the QCM substrate in air during 4 cycles of vertical dipping processes of cadmium octadecanoate LB films (surface pressure" 20 mN m-l, 20 ~ dipping speed" 100 mm min-~).
Process of LB Films on a QCM Typical time courses of frequency changes of the QCM substrate in air during 4 cycles of vertical dipping processes are shown in Figure 2. The QCM was lowered into the subphase at point A and raised in air at point B with a dipping speed of 100 mm min-~ through the cadmium octadecanoate monolayer (20 mN m-~, 20 ~ The frequency of the QCM in air gradually increased with time and 2.2
Transfer
114 reached a constant value in 15 min at point C. From the decrease of frequencies of 183 + 3 Hz from points A to C, the increased mass with each cycle was calculated to be W1 = 232 + 3 ng according to eq. (2). This value was consistent with the theoretical mass of four dry monolayers (two layers on each side) of cadmium octadecanoate (225 ng) on the Ag electrode of the QCM, which was calculated from the average area per molecule in the monolayer (0.237 nm2 from a rt-A isotherm) and the area of Ag electrode (0.238 cm2). Thus, the frequency decease is affected only with the mass on the electrode area of the quartz plate. The gradual frequency increase from points B to C is explained by the mass decrease due to the evaporation of water deposited b e t w e e n layers from the subphase. The amount of incorporated water (W2 / ng) and its evaporation speed (v / ng min-1) were calculated from the frequency change and the initial slope of the time course between points B and C, respectively. The cadmium octadecanoate LB films were observed to incorporate W2 = 209 + 5 ng of water with 4 layers of LB films (WI = 232 + 3 ng) at the first d i p p i n g cycle: c a d m i u m o c t a d e c a n o a t e were transferred on a substrate with the water of almost the same mass of LB films. Figure 2 also indicates that we should wait ca. 15 min to get the dry LB films during transfer processes in this conditions. When the next deposition was carried out before the complete evaporation of water, a transfer ratio of LB films was gradually decreased from 0.9 to 0.7 with increasing dipping cycles. 2.3
T r a n s f e r R a t i o of LB F i l m s The transfer process was continued at least 10 times at different surface pressures (20, 10 and 5 mN m-~). The total transferred weight (ZW~) of dry cadmium octadecanoate LB films was plotted against the number of transfer cycle as closed circles in Figure 3. The amount of transferred films was also estimated from the conventional method calculated from a moving area of a barrier in kept the surface pressure constant, and plotted as closed triangles in Figure 3. Straight lines indicate the theoretical mass of Y-type two layers on each side of the QCM. At the high surface pressure of 20 mN m-~, the transferred w e i g h t obtained from both the frequency changes of the QCM s u b s t r a t e and the barrier m o v e m e n t was almost equal to the
115 theoretical line, and the obtained transfer ratio was 1.01 + 0.02. McCaffrey et. al. had reported the d e p o s i t i o n of c a d m i u m octadecanoate LB films on a QCM plate and obtained the similar linear correlation between the frequency change and the number of transfer cycle [23]. However, the mass associated with each layer was 20% larger than the theoretical mass of the dry LB films, probably because they deposited LB films with the water lifted still incorporated in them. In this study, the weight calculated from the QCM method was also in accord with the value from the conventional barrier movement, which means all the disappeared monolayers from the air-water interface were transferred on the substrate under this condition.
:L
(a)
20 m N
m "t
(b)
10 m N
rn "1
(c) 5
mN
m-I
2.0 o&
.c I.S
o&
ID
O&
"~ 1.0 ~D (I)
"~ 0.5 c-
~-
~ 2 ~ o 8~oo2
~. ~ 8 ~ b o
2 ~, 6 ~ ~b
Number of Transfer Cycle
Figure 3 The total transferred weight ( E W e ) of cadmium octadecanoate LB films calculated from the QCM method ( 0 ) and the barrier movement ( & during repeated depositions at the surface pressure of (a) 20 mN m-l, (b) 10 mN m-l, and (c) 5 mN m-~. Solid lines represent the theoretical value calculated from the n-A isotherms (dipping speed: 100 mm min-~, 20 ~
At the low surface pressures of 5 and 10 mN m-i, the observed values showed the deviation from the theoretical lines and the transfer ratios were c a . 0.9 - 0.7. The transferred mass obtained from the barrier movement was smaller than that from the QCM method under these conditions. This means that the barrier motion on the surface cannot compensate the disappeared area of the
116 transferred monolayer enough at the low surface pressure. Thus, the QCM method is useful to estimate in situ the real transfer weight on the substrate in comparison with the conventional method even at the low surface pressure. 2.4
Effect of Surface of the S u b s t r a t e and N u m b e r of Layers The amount of incorporated water (W2) and its evaporation speed (v) were obtained as a function of dipping cycles in the transfer of cadmium octadecanoate LB films and the results are shown in Figure 4. Both relatively hydrophilic and hydrophobic surfaces of the QCM were employed as a substrate, in which the former was a bare Ag electrode (contact angle for water: 50 +_ 5 o) and the later was prepared by treating with 1,1,1,3,3,3hexamethyldisilasane (contact angle for water: 110 + 5 o)[28]. The transfer ratio of LB films ( W : ) w a s 0.98 + 0.05 for each dipping cycle on both hydrophilic and hydrophobic substrates. The monolayer could be transferred even on the hydrophilic bare electrode for the first downward process, because the Ag surface is not so hydrophilic (contact angle: 50 + 5 ~
600 c-
60 . m
400 200
E
40
20
00246810 Number of Layers
Number of Layers
Figure 4 Effect of number of layers on the amount of incorporated water (W2) and the evaporation speed of the water (v) in the transfer of cadmium octadecanoate LB films. The hydrophilic (O) and hydrophobic ( 0 ) s u b s t r a t e s were used in this e x p e r i m e n t (surface pressure" 20 mN m-~, dipping speed" 100 mm min-~, 20 ~
117 The amount of incorporated water (W2) and its evaporation speed (v) decreased as the number of layers increased in both hydrophilic and hydrophobic surfaces of the QCM. W2 and v values were particularly large at the first cycle in the case of the hydrophilic surface. It has been shown that several layers on a substrate are disordered by the influence of the substrate surface and this effect disappears as the number of layers increases [29,30]. The first few layers seem to incorporate the large amount of water in the defects of LB films and the water easily evaporates through the disordered monolayer. The hydrophobic alkyl chains contact with the substrate surface at the first down stroke. Such a contact has a disadvantage in energy in the case of the substrate having a hydrophilic surface, then the first monolayer on the hydrophilic surface was particularly disordered and shows the large W2 and v values. The dependency of the evaporation speed (v) on the number of layers seemed to be larger than that of the amount of incorporated water (W2), which means v values are more sensitive parameters reflecting the film disorder than the W2 values. When the larger number of layers was deposited, the longer drying time was required due to the slower evaporation speed of water.
4O
400
,,.,,,. !
._~ 30 E
30O ~
g2o
200 ~",,
> 10
100
06
fo
Sudace PressureImN m"
Figure 5 Effect of surface pressures on the !ncorporated water (W2) and the evaporation ~n the transfer of cadmium octadecanoate (dipping speed" 100 mm min-~, 20 ~ at the cycle).
amount of speed ( v ) LB films 5th transfer
118 E f f e c t of Surface Pressure on Water I n c o r p o r a t i o n The W2 and v values at the 5th dipping cycle were obtained at various surface pressures, and the results are shown in Figure 5. Both W2 and v values increased with decreasing the surface pressure. At the low surface pressures below 20 rnN m-~, the LB films having many defects were transferred with a low transfer ratio below 1.0, and the large amount of water was incorporated in these defects and its evaporation speed was fast through the disordered film. 2.5
2.6
Effect of Dipping Speed W 2 and v values at the 5th transfer cycle of c a d m i u m o c t a d e c a n o a t e at 20 mN m-~ as a function of dipping speed of the QCM substrate are summarized in Table 1. The transfer ratio of LB films (W j) was 0.95 + 0.05 at the dipping speeds of 40 - 100 mm min-~. W h e n the dipping speed was decreased, the evaporation speed (v) was decreased: the well-oriented LB films could be obtained at the low dipping speed. This is consistent with the report by Pitt, et. al. that the lower transfer speed was favorable to obtain the higher quality LB films in the first 10 layers [31]. Table 1 Effect of dipping speeds octadecanoate LB filmsa dipping speed/mm min-1 100 80 60 40
on
the
transfer
of cadmium
W2/ng Wswen/ng W2-Wswell/ng v/ng min-X 209 186 212 262
40 50 66 95
169 136 146 167
15.8 14.2 10.0 7.7
a Surface pressure: 20 mN m -1, 20 ~ at the fifth dipping cycle. The incorporated amount of water (W2) seems to increase with decreasing the dipping speed. Since the W2 value may include both the really i n c o r p o r a t e d water and the swelling with water when the substrate exists in the water subphase, the effect of dipping speed on W2 values should be divided in two factors. We have already reported
119 that cadmium octadecanoate LB films swelled largely in the subphase, compared with other LB films [26]. The swelling amount (W,,,,u)was separately obtained from the frequency decreases (mass increase) when the LB film-deposited QCM was soaked for the time in the subphase calculated from each dipping speed. The (W,w,l~- W z) value reflects the true amount of water pulled into the outer layer during the lifting-up process and was almost independent of dipping speeds of 40 - 100 mm min-~ (see Table 1). The W2 value seems to increase with decreasing the dipping speed due to the swelling amount. In the case of the transfer of octadecanol monolayers, the different results were obtained as shown in Table 2. The (W,,,,l~-W 2) value decreased largely with decreasing the dipping speed for the octadecanol LB films. The (W,~,t~-W 2) value for octadecanol LB films was more than two times larger than those for cadmium octadecanoate (369 ng and 169 ng at the dipping speed of 100 mm min-~, respectively), which indicates the OH head groups interact with water strongly compared with the CO0head groups (see the latter section). Therefore, the effect of dipping speed on the incorporated amount of water depends on the hydrophilic head groups of LB films. Table 2 Effect of dipping speeds octadecanoate LB filmsa dipping speed/ram min -1 100 80 60 40 20 5
W2/ng 393 267 184 111 81 62
on
the
transfer
Wswen/ng 24 25 30 36 44 56
of
cadmium
W2-W~,en/ng 369 242 154 75 37 6
a Surface pressure: 20 mN m -1, 20 ~ at the fifth transfer cycle.
2.7
Effect
of
Lipid
Structures
The W2 and v values were obtained for various single-chain a mp h ip h iles with the same chain length (C18) and the different
120 hydrophilic head groups, and the results are summarized in Table 3. All LB films could be transferred with the transfer ratio of 1.0 _+ 0.1 in these conditions. The octadecanol LB film showed particularly large W2 and v values, which means that much amount of water was incorporated into the octadecanol LB film and the water was easily evaporated. The OH groups of octadecanol seem to form the hydrogen bond network in the subphase. The LB films transferred with much amount of water might have defects and disorders in LB films, which make the evaporation speed large. The LB films except octadecanol showed the tendency that the W2 depended on their hydrophilicity of head groups and the v value was independent of the head groups.
Table 3 Effect of hydrophilic head single-chain amphiphilesa
groups
on
the
transfer
amphiphiles
W2/ng
v/ng min -1
CH3(CH2)I7--OH [CH3(CH2)16-COO-] Cd2+ CH3(CH2)I6--COOH CH3(CH2)16--CON'I-I2 CH3(CH2)16-COOC2H5
389 155 52 33 42
19.7 4.27 3.74 4.24 2.93
of
a Surface pressure: 30 mN m -1, 20 ~ at the fifth dipping cycle.
The W2 and v values for LB films prepared from various aliphatic acid cadmium salts having different alkyl-chain lengths (C16- C22) were also obtained and the results are shown in Figure 6. All LB films could be transferred with the transfer ratio of 1.0 _+ 0.1 in these conditions. The W2 value was constant and independent of the chain length, but the v value decreased with decreasing the chain length. This indicates that the chain length of lipids mainly affects the evaporation speed of water in LB interlayer and has no effect on the amount of the incorporated water. Thus, the incorporated water seems to exist near the hydrophilic head groups, so that the W2 value depended on their hydrophilicity, but not on the alkyl chain length. On the contrary, the evaporation speed v
121
depended on the alkyl chain length, but not on the hydrophilic head groups, since the rate-limiting process of evaporation of water is to pass through the hydrophobic alkyl-chain part.
,ol
9-, , m
E
,A
-
=
f-
5
&
r
--
400 200
Q
A
> ,
~
16
9
18 2 0
!
22
Acyl Chain Length i l
i
1
|
,
|
-
-
16 18 20 22
Acyl Chain Length n
Figure 6 Effect of alkyl-chain length of aliphatic acid cadmium salts monolayers, [CH3(CH2),_2COO-]2 Cd2+, on the amount of incorporated water (W2) and the evaporation speed of the water (v). The broken line is calculated from eq. (3). (surface pressure" 30 mN m-l, dipping speed: 100 mm min-l, 20 ~ at the 5th dipping cycle).
The evaporation speed is expressed in the following equation according to Fick's low, where the water evaporation is supposed to occurs only from the outer layer, but not from the side part of LB films
[32]-
v
-
60
=
D
x
A
W-Wo
x
~
d
(3)
where v / 6 0 (g s-l) is the evaporation speed, D the diffusion coefficient of water in LB film, A the cross-sectional area of diffusion (the electrode area), w o the vapor pressure of water outside, w the pressure of water vapor equilibrating with balk water in the LB film, and d the thickness of the monolayer. The W o and w values can be calculated with 60% in
122 relative humidity (outside) and 100% (inside, saturated) at 25 ~ The dependency of the film thickness d on acyl chain length n is expressed in the following equation on the assumption that the chain forms a t r a n s zig-zag conformation with the 2.54 A C-C spacing. d (cm) = 1.27 • 10 -8 (n - 2)
(4)
The curve fitting for the observed value was done with the least square method, and the obtained curve is shown as a broken line in Figure 6. This model seems to reproduce the experimental value qualitatively. From this curve fitting, the apparent diffusion coefficient D = 9.7 x 10-12 cm2 s-~ was obtained. Since the diffusion coefficient of free water is 2.6 X 10-1 cm2 s-~ at 25 ~ LB films of the fatty acid cadmium salts are calculated to have the 1/(2.7 X 1011) free space for water diffusion to whole area. The experimental value had the sharper slope than that calculated from the model, which means the diffusion coefficient D decreased as chain length increased. The change in quality depending on the chain length might also affect the water evaporation. According to eq. (3), the evaporation speed depends on the humidity in air, and is independent of the amount of water remaining in LB films. Thus, the water amount in LB film should decrease by zero-order with the amount of water. This is why the frequencies increase due to the water evaporation was linear in Figure 2 (points from B to C). 2.8
Conclusions LB films of various lipids were transferred on a QCM as a substrate under various conditions. The mass of the transferred film (W l, the transfer ratio), the amount of incorporated water (W2), and the evaporation speed (v) were evaluated from the frequency changes of the QCM during transfer processes in situ. We could estimate the deposition state and structures of LB films from these values. When the LB films deposited at the lower surface pressure and at the higher dipping speed on the more hydrophilic surface, the smaller transfer ratio and the larger amount of incorporated water, and the larger evaporation speed of the water were observed, which indicates the deposition of the disordered LB films. On the contrary, when the wellpacked LB films are obtained, the good transfer ratio (W~), the small W2
123 and v values are observed. The W2 and v values also reflects the hydrophilic head groups and alkyl-chain length of amphiphiles, respectively. A QCM system will become a useful sensor system to evaluate LB films during a transfer process in situ. 3. H Y D R A T I O N
B E H A V I O R OF LB FILMS IN W A T E R S U B P H A S E
Hydration of phospholipid head groups is essential properties not only for stabilizing bilayer structures in an aqueous environment, but also for fusion or endocytosis of biological membranes including protein transfers [33-35]. Hydration or swelling behavior has only been studied by indirect methods such as X-ray diffraction [36],differential scanning calorimetry (DSC) [37], and 2H-NMR [38,39]. m
lPersonal [computer,] 9
,
_
{Frequency[ counter ~
,o
mometer
J I_
i
Quartz-Crystal Microbalance
! Oscillating I ,,, |circuit 1111
,_
r--
(QCM)
O
DLPE (n = 12) DMPE (n = 14) DPPE (n = 16) CH3-(CH2) f-4-.-COO ~----] CH3.(CHz)fZ.COO~ DPPC
O',
^ . . +it';N3
O
CH3
C H3-(C H2)/-4,'-COO----."] CH3.(CH2)t.rCOO-- I HO
rol
O" /~. / ~
'--o-P-o y OH
DPPG
0
OH
C H 3-(C H2)f-a.--COO -"'-! C,3.(CH2),T.COO--. ~ DPPS
O" ~,,,~NH3. I.
o-~-o- T0
COO"
Figure 7 Apparatuses for frequency measurements of the LB film-deposited QCM in water and structures of lipid molecules
124 In this chapter, we determined directly the hydration behavior and stability (flaking) of LB films of various naturally-occurring lipids such as dilauroyl-, dimyristoyl-, dipalmitoylphosphatidylethanolamine (DLPE, DMPE, and DPPE, respectively), dipalmitoylphosphatidylcholine (DPPC), dipalmitoylphosphatidylglycerol (DPPG), dipalmitoylphosphatidylserine (DPPS) and cholesterol (see Figure 7). The frequency of the QCM deposited with phospholipid LB films is expected to decrease (mass increase), when hydration occurred at head groups of LB films on the QCM in water. The initial hydration rate and the hydration amount were obtained at various temperatures below and above Tr changing acyl-chain length and hydrophilic head groups in phospholipids. Twodimensional morphology of phospholipid monolayers during hydration was also observed by fluorescent microscopy.
er
N
500-
--5(30
D
0
1000 <E
14.
"q -1000
C 0
-1500 Initial Rate
2000 ~
Vo Time / hour
Figure 8 Typical time-courses of frequency changes when the QCM deposited with 10 layers of DMPE (T~ = 49 ~ LB films on each side is immersed in water (a) at 25 ~ (b) at 70 ~ and (c) at 50 ~
3.1
Hydration
of
Phosphatidylethanolamine
LB
Films
Ten layers of dimyristoylphosphatidylethanolamine (DMPE) LB films were deposited on the QCM. The QCM was immersed into temperaturecontrolled water phase, and the frequency was followed with time. Typical time-courses of frequency changes arc shown in Figure 8. The phase transition temperature from solid to liquid crystalline states of
125 DMPE membranes was determined to be Tc = 49 ~ from differential scanning calorimetry. The frequency hardly changed at 25 ~ (below To) and at 70 ~ (above T~). On the contrary, the frequency largely decreased at 50 ~ (near To), which indicates that the mass increase due to hydration around head groups of phospholipid LB films. After reaching the equilibrium of the frequency decrease, the QCM was picked up to air phase and dried. The frequency was gradually reverted to the original mass of the dry LB films before immersing in water due to the evaporation of water in air. Thus, the frequency decrease indicates simply the mass increase due to hydration on LB films. The resonance frequency has been reported to be also affected with the change of viscoelasticity of the membrane on the QCM, especially when the membrane is thick and fluid [26,40,41]. The resonance resistance reflecting viscoelasticity of the membrane was measured by an impedance analyzer to be constant (800 +_ 50 f~) during the large resonance frequency change (AF - -1400 _+ 10 Hz) at 50 ~ (near T~). We [26] and Muramatsu e t . a l . [40] have already mentioned that the viscoelasticity change does not affect the frequency change when the membrane is very thin such as 10 layers of LB films even in the fluid state, because the energy loss in the thin film is very small. Therefore, the frequency decrease of 10 layers of DPPE LB films around their Tc shown in curve (c) in Figure 8 is attributed mainly to the mass increase due to the hydration (swelling) of lipid membranes on the QCM. Twenty layers (1130 _ 5 ng) of DMPE LB films (each 10 layers on both sides of QCM electrodes) were observed to be hydrated with 1 7 8 0 _ 20 ng of water near T~ at 50 ~ 61 mol of H20 per 1 tool of DPPE. It has been reported that DPPE has 7-8 mol of non-frozen water molecules per lipid even in the dry state [44]. The initial hydration rate V o and the equilibrium hydration amount W** were obtained as parameters reflecting the hydration behavior of LB films (see Figure 8). Temperature dependencies of the hydration behavior (voand W~,) of 10 layers of DMPE (To = 49 ~ LB films are shown in Figure 9. Large W.~ and v o values were observed only around the phase transition temperature (T~) of DMPE membranes. Thus, DMPE LB films were hydrated only near the To, but not in the solid state below the T~ and in the fluid state above the T~. This indicates that the
126
coexistence of two phases (solid and fluid domains) near the T~ caused the large hydration rate and amount in the LB films.
2500
600 "7.
~2000
S00 E e-
400 ~
1500
300 :~
looo
200 ~
500 0
0
A - -
20
30
.. w
w
40
50
60
Temperature
I
70
~
Figure 9 Effect of temperatures on the hydration amount W** ( 9 and the initial hydration rate Vo ( ~ of 10 layers of DMPE LB films
r
'r /,00 E
4OOO
,,.,...
300 Q.=
2000
:~ 200 tO
looo "~
"~
A
0
o
_
T
100
=
zo
" 30
~
~
2-0
30
Number of Layers Figure 10 Effect of the number of layers of DMPE LB films on each side of the QCM on the swelling behavior (W.~ and V o ). [3 at 20 ~ (below T~), 0 at 50 ~ (near T~), z~ at 65 ~ (above Tr
127
Figure 10 shows effects of the membrane thickness of DMPE LB films on the h y d r a t i o n behavior at three different temperatures. The hydration amount (W**) increased linearly with increasing the number of layers of LB films only around T~, but not temperatures below and above To. This indicates that water molecules deeply penetrate into LB layers around T~. The hydration rate ( v o) was very large and hardly depended on the membrane thickness around T~. This means that water can penetrate from the top surface of the membrane, but not from the side part of LB films.
o~ c
near Tr
2500
700 600 '7c:
o
500 E
0
e-
400 ~
g soo
300 | q,.
~ 1000
200 N 100 --~ 0
9
C12
DLPE
C14
C16
0
D M P E DPPE
Acyl Chain Length Figure 11 Effect of acyl chain length of phospholipid (PE) LB films (10 layers on each side) on the swelling behavior (W.~ and v o ) around each T~ (DLPE: Tc = 30 ~ DMPE: T~ = 49 ~ and DPPE: T~ = 63 ~
Figure 11 shows effect of acyl chain length of phospholipid on the hydration behavior around the each Tc of 10 layers of LB films of DLPE (C12, Tc = 30 ~ DMPE (CI4, Tc = 49 ~ and DPPE (Cl6, Tc = 63 ~ The h y d r a t i o n amount W~. was constant and independent of acyl chain length, which i n d i c a t e s that the hydration water exists around
128 hydrophilic head groups, but not in hydrophobic acyl chain region. On the other hand, the hydration rate Vo decreased sharply as the chain length increased. This means that water molecules penetrate through the defects coexisting solid and fluid states in the acyl chain regions near the T~ and the hydration rates decreased largely in the long chain lipid membranes, and water molecules exist in the hydrophilic regions between layers.
500
at 50 *C -500
250
-250
at 25 *C 0 9 0
. . . -!. . I
! 2
! 3
I , 4
,-! 5
Time / hour Figure 12 Frequency changes of the QCM deposited with 10 layers of DPPC LB films, when the QCM is immersed in water at 25 ~ (below T~ ) a n d at 50 ~ (around the T~ of DPPC).
3.2
Hydration of Other phospholipids and Cholesterol Ten layers of DPPC (T~ = 42 ~ LB films were deposited on a QCM plate by a horizontal lifting method on one side and immersed in water. Typical time-courses of frequency changes of the DPPC-deposited QCM are shown in Figure 12. At 25 ~ in the solid state below the T~, DPPC LB films were stable and hardly swelled in water. However, the frequency gradually increased (mass decreased) at 50 ~ (above the T c) and reached equilibrium at AF = 450 + 50 Hz (-Am = 575 + 50 ng), which is equivalent to the loss of 10 layers mass of the dry LB films. Frequency
129 measurements after drying in air indicated that most of LB films flaked from the QCM plate into water. The similar flaking behavior in water at temperatures above the Tc was also observed in the LB film of DPPG (T~ = 42 ~ Thus, LB films of DPPC and DPPG having relatively hydrophilic head groups such as phosphocholine and phosphoglycerol are easily hydrated and then flaked in the fluid state above their Tc in water.
2500
600
2000
500 E
,") e-
L,0O ~
1500
30o
0
E ~ 1000 "o
:~ (3)
200 ~:
500
~
I(X)
"~
@
0 -~- 20
" 30
/~0
.
.
50
Temperature
.
.
/
60
.
70
~
Figure 13 Effect of temperature on the hydration amount W** (C)) and the initial hydration rate V o ( ~ of 10 layers of DPPS LB films on each side of the QCM
The hydration behavior of DPPS (To = 55 ~ LB films is shown in Figure 13. The DPPS LB film having phosphoserine head groups little hydrated ( A m = 600 + 10 ng, 20 mol of water per lipid) even near the phase transition temperature. Hydration ability has been reported from adsorption experiments of water vapor to lipid powder to be in the order of PC > PE > PS lipids [43]. It has been determined from calorimetry that the amount of non-frozen water around lipid molecules is 10 mol, 7-8 mol, and 0 mol for 1 mol of PC, PE, and PS lipids, respectively [44]. This tendency is consistent with our results that PC molecules are easily hydrated and flaked from the substrate, and PE and
130
PS lipids are hydrated with 61 and 20 mol of water per lipid around their T~, respectively.
250O
600
t,,-
2000
500
~= 1500,
z.O0
0
~,
'=::
,'-
R=,.
E I:::
3OO r----
soo
200
lOO ~
20
9
A "F
30
60
Temperature
/
7b
~
Figure 14 Effect of temperature on the hydration amount W.o ( O ) and the initial hydration rate V o ( ~ of 10 layers of Cholesterol LB films on each side of the QCM
The hydration behavior of LB films of cholesterol is shown in Figure Interestingly both the hydration amount W** and the hydration rate V o decreased with increasing temperatures. Since cholesterol molecules are very hydrophobic and are thought to be hardly hydrated, the incorporated water might exist in the structure defects near the hydrophilic OH groups of LB films and these defects could disappear at high temperature by annealing effect. After aging of cholesterol LB films at 70 ~ for 1 h in water, cholesterol membranes are hardly hydrated at all temperatures (W,, = 50 + 10 ng, 3-4 mol of H20 per cholesterol). 14.
3.3
Observation of LB films by Fluorescence Microscopy In order to confirm the large hydration behavior of phospholipid LB films only around their T~, two-dimensional m o r p h o l o g y of DMPE monolayers was observed by a fluorescence microscope. The two-
131 dimensional morphology of transferred monolayers was observed by using a fluorescent microscope (Olympus, Co., Tokyo, model BSH-RFK). The fluorescence image was detected with the high sensitive SIT camera (Hamamatsu Photonics, Co., Tokyo, model C2741) and the image processor (Hamamatsu Photonics, Co., model DVS-1000) [42].
Figure 15 Fluorescent images of the DMPE monolayer on a slide glass before and after aging at different temperatures.
A DMPE monolayer containing 2 mol% of octadecylrhodamine as a fluorescent dye was transferred onto a slide glass and aged in water at
132 different temperatures for 10 minutes. After drying, the fluorescent image was observed and photographs are shown in Figure 15. A dark region represents the crystalline phase, because the fluorescent dye is refused from the crystalline domain and exists in the disordered domain. The 1 0 - 20 ~tm size of crystalline domains were observed in the samples without aging and aged at 20 ~ (below Tc~. The sample after aging at 60 ~ (above T~) showed large crystalline (dark) domains due to an annealing effect. On the other hand, when the sample aged at 50 ~ (around T~ very small crystalline domains were observed, which indicates the microscopic coexistence of crystalline and liquid crystalline phases near To. The total area of the disordered region (white region) after aging near T~ was larger than those at any temperatures above and below To. These observations are consistent with the large hydration behavior of DMPE LB films only around T~, which may occur in the defects between two domains. We observed also the morphological pictures of cholesterol LB films. The small disordered (white) area was frequently observed in the prepared cholesterol LB films and they were largely decreased after aging at 70 ~ for 1 h (photographs are not shown). This is consistent with the hydration behavior that the hydration of cholesterol LB films was largely decreased with increasing temperatures (see Figure 14). 3.4
Conclusion From the frequency measurements of the LB-film-deposited QCM plate in water, the behavior of phospholipid LB films can be classified into three types: (i) phospholipids having relatively hydrophilic head groups such as DPPC and DPPG are hydrated and then easily flaked from the substrate in the fluid liquid crystalline state a b o v e To; (ii)DPPE and DPPS having the less hydrophilic head groups are hydrated only n e a r their T~; (iii) cholesterol LB films show relatively large hydration behavior even at low temperatures due to the water penetration into the structure defects in the membrane. This is the first example to demonstrate directly and quantitatively the hydration behavior of phospholipid membranes in water. The combination of the QCM and the LB method is a useful tool for characterization of lipid membranes in water.
133 4. D E T A C H M E N T OF LB FILMS F R O M A QCM SUBSTRATE AT THI AIR-WATER INTERFACE In this chapter, we focused on the stability of LB films at the airwater interface: the detachment of LB films from a substrate was followed from the frequency increase (mass decrease) of the QCM that was passed through the air-water interface (see Figure 16). It is necessary to know the stability (detachment) of LB films at the airwater interface for the application of LB films in aqueous systems. We have quantitatively obtained the detached amount of LB films at the air-water depending on dipping (lowering or rising) speed of a substrate, conditions of water surface (surface tension or temperature), chemical structures of amphiphiles (chain length or hydrophilic groups), and structures of LB films (monomeric or polymeric). LB film-forming amphiphiles (CH 3(CH 2),.2.CO O " )Cd2+
n = 16, 18,20,22
OI i
CH3(CH 2)n-l--O-C-~ H--NH3+
CI
CH 3(CH =).. ~ --O ~-(CH2)~ O 2Cn-glu-NH3 +
n = 16, 18
o CH3 CH 3(CH 2)--1 --O --C --~H --NHCO -CH2 -~1, -CH 3 "O3S CH 3(CH 2).- 1 --O-C-1CH2)2
CH3
0
2Cn-glu-N "3C 1 / PSS "
n = 16, 18
OEt O I CH 3(CH 2)18~ N C II ~ / C O N H , ~ ~ / S i - O E t | CH 3(CH 2)18 /
OEt 2 C 18-Si
Figure 16 Measuring system for detachment of LB films from a QCM substrate at the air-water interface, and chemical structures of LB film-forming amphiphiles.
LB films
134
1Ol.
.Jill
N
e=
-r ',2,,
200
150
150
u}
ca 100
14. m ..I q~
100
0
r'--
~ , 50 0
5O
c:
~.
m
E .=.
0
0
0 c 0
E = n ~0 < costp >
(9)
where n is the volume concentration of molecules, ~t0 the molecular dipole moment in the ground state, h a unit vector of the surface normal, and tp the angle between I.to and h. Therefore, p -
dP _/-to ~T + n d~0 ~ + n ~ d dT
(10)
The first term represents the temperature dependence of the film density, being directly related to the thermal expansion coefficient of the film, and also indirectly related to that of the substrate. Generally, the thermal expansion coefficient of the organic substance is much larger than that of the inorganic substance like glass. In the case of the alternate LB film of fatty acid/alkylamme or fatty acid/alkylanilme, the pyroelectric coefficient p decreased as the thermal coefficient of the substrate increased [ 18]. The second term in Eq. (10) originates from the change in dipole moment itself with temperature, and the third term from that in the orientation angle with temperature. These microscopic problems can be well investigated by vibrational spectroscopy, as demonstrated in the following sections.
5. R E L A T I O N S H I P B E T W E E N M O L E C U L A R O R I E N T A T I O N AND PYROE L E C T R I C I T Y IN ALTERNATING LB FILMS
5.1. Temperature dependence It has been known that alternating LB films consisting of two different amphiphiles have noncentrosymmetric structures, and therefore, are expected to provide piezoelectric, pyroelectric and nonlinear optical properties. Therefore, we prepared alternating LB films consisting of 5-(p-dodecyloxyphenyl)pyrazine-2-carboxylic acid (DOPC)
C,2H25 0
C0 0 H
synthesized by Takehara et al. [ 19] and deuterated stearic acid (St-d35). Hereafter, we will symbolize DOPC by P and deuterated stearic acid by S. The alternating LB film
159 was prepared by n-time depositions of the unit PS bilayer on the first monolayer of S (Figure 14). We designate this structure by S(PS)n. Then, we measured pyroelectricity and temperature dependence of the molecular orientation in these films using the above-mentioned method [3]. The same studies were also performed for the alternating LB films of their barium salts designated by S(PS)n-Ba. For pyroelectric measurements, we used two AI electrodes on both sides of the alternating LB film as shown in Figure 14. The electric current generated on linearly heating the LB film was measured by a picoammeter in the temperature range from -30 ~ to 60 ~ The pyroelectric coefficient p is calculated from the observed pyroelectric current/by
(dT) -1
(11)
P= /'dt"
Here, A was 170 nm 2 and dT/dt was 2.2 K/min. Figure 15 shows the current-temperature (I-T) curves for the alternating S(PS) 9 and S(PS)9-Ba films [7]. For reference, the I-T curve for the homogeneous (centrosymmetric) SPIs-Ba film which consists of 18-monolayers of DOPC-Ba on the first monolayer of St-d35-Ba is also shown in Figure 15. For both alternating LB films, the negative current increases on heating above 0 ~ After passing through the minimum points around 40 ~ the curves rise rapidly on further heating. For homogeneous SP18-Ba film, on the other hand, almost no current is obtained up to 40 ~ and then the positive
jAL II / ./!11/
., / /
/'1/
//13
I
I
I
I
t
t
I
I
I
I
I
I
i
I
i
I
I
i
I
I
,
i
!
,
|
i
I
|
,
I
i
i
I
!
I
i i i
p
Picoammeter
' ! !
S
'
P } (PS)~
,.
--I l l
/ i i Z I
I / i l l
iI'.I
tl
GLassslide Figure 14. Alternating LB films S(PS). and the electric circuit for pyroelectric measurements [7].
160 current increases. Therefore, the negative current observed only for the alternating films can be regarded as pyroelectric currents which may be due to changes in spontaneous polarization in the alternating films. Maximum pyroelectric coefficient, pmax, calculated by Eq. (5) is 1.8 ~tCm2K -~ at 43 ~ for the S(PS)9-Ba film and 1.2 lxCm2K1 at 37 ~ for the S(PS) 9 film. These values are comparable to previously reported data for some alternating LB films by other investigators. The positive currents observed above 50 ~ for all LB films examined will be discussed later in connection with the molecular orientation. Figure 16 shows thermal stability of the pyroelectricity for the S(PS)9S4-Ba film [7]. After the current was measured on the heating process to 40 ~ (the curve a), the sample was cooled to -30 ~ and then the current was measured again m the second heating process to 60 ~ This I-T curve (the curve b) is almost identical with the first one. However, the third and fourth I-T curves (the curves c and d) which were obtained after the repeated heating of th sample to 60 ~ show smaller negative currents than the previous ones. These results indicate that the heating of the sample
20
< r
*E
10
SPIe-Bo
0
-
|
/
I._
L_ =
S(PS)9
-
1~=:l.2pCm'tK ''
(j
-lO
-
-40
!. -20
S(PS)9-Ba pNx=l.g/JCm-=E' ,.! I .... 0 20
Temperature
~ I
I
40
/
I ! 60
,
80
~
Figure 15. Current-temperature curve of alternating S(PS) 9 and S(PS)9-Ba films and homogeneous SPls-Ba film [7].
161 above 60 ~ induces the depolarization of the alternating LB films and consequently reduces the pyroelectricity. Figures 17 and 18 represent FT-IR transmission and RA spectra, respectively, of the alternating S(PS)9-Ba films at various temperatures from 0 ~ to 120 ~ [7]. Two intense bands at 2919 and 2852 cm -~ are the antisymmetric and symmetric CH 2 stretching bands of DOPC, and two bands at 2192 and 2088 cm ~ are the antisymmetric and symmetric CD 2 stretching bands of St-d35, respectively. Apparently, all these bands decreases their intensities with the increase in temperature in Figure 17. At the same time, intensity differences of the respective bands are evident between the transmission and RA spectra. From these data, we calculated temperature dependence of the orientation angle T of the hydro-carbon chain axes of the constituent molecules in the alternating S(PS) 9 and S(PS)9-Ba films using Eqs. (2) and (3). The results are shown in Figure 19 [7]. Apparently, the Y values of the respective constituents in the S(PS)9-Ba film are much smaller than those of the corresponding molecules in the S(PS) 9 film. This reveals that the barium salt molecules are more highly oriented as compared with the
20
~176
I-
~176
I:
.~
,,~
I: I:
10
~176
d ,,e-,
fi
0
oo
(13 i.. L_ :3
~
~:
"~.~..'~.....
~176176 ~
(.)
~~
-... -10
-40
I
,,1/
C
-~
a
~
~ i
t
b 1
-20
1
0
I
20
Temperature
,
I
I
40
I
60
,L
.
80
~
Figure 16. Current-temperature curve of alternating S(PS)4-Ba film [7]; (a) first curve measured on heating to 40 ~ (b) second curve measured on heating to 60 ~ after the first measurement and cooled to -30 ~ (c) third curve measured on heating to 60 ~ after the second measurement and cooled to -30 ~ (d) fourth curve measured on heating after the third measurement and cooled to -30 ~
162
ILj~// V,(CH2) \
V,(Cl-.~)
v,(cl::m)
~ V,(CD2)
~2~176~
. ~
80 *
uJ ro z
V,(CO0")
c
T
J
~
60 *C
> k.~. This assumption is consistent with the observed stability of the bilayers formed at the zero surface pressure point. The kinetics of [D] can be then e x p r e s s e d as NDv d[D_....~]dt= k1[S]2 f(x) + A----~ (2-3) and the kinetics of ND can be obtained finally from
dND =k~ NS2 dt -~-- fix)
(2-4)
where Ns is the number of PhDA2-8 molecules at the air/water interface. Now we consider the relationship between the effective concentration(F,ff) and the surface pressure(x) at the air/water interface. Ideally, the surface pressure is directly proportional to the concentration of surfactants. However, as the actual n-A isotherms show several specific effects, such as limiting area and points of inflexion, we shall assume the following relationships:
Fen = [S] + a[D],
f
(2-5)
Feff Feff,L ~ ,
/1; or (Feff - Feff,L) m
where F is the surface concentration of lipid molecules and it is inversely proportional to A, and a is the correction factor representing the contribution of [D] (related to [S]) to the effective concentration, respectively. The effective concentration at the limiting area is expressed as Feff.L. Judging from the actual x-A isotherms, the relationship between n and Feff can be expressed in terms of a function of higher order. To simplify the analysis without losing generality, we put m equal to 2, as shown in Figure 8B. To illustrate the inhomogeneities of the surface pressure we adopt a rectangular cell model, as is schematically shown in Figure 9.
224
Cell Number 1
Compression
Figure 9. Schematic representation of the model adapted for the numerical simulation.
This model assumes that the air/water interface f r o m the blade to the Wilhelmy plate can be divided into a number of equal small cells. We apply a simple a r g u m e n t that the rate of mass t r a n s f e r by d i f f u s i o n is p r o p o r t i o n a l to the d i f f e r e n c e in concentration between the n e i g h b o r i n g cells, while the c o n c e n t r a t i o n and the surface p r e s s u r e within each cell are a s s u m e d homogeneous. To give a full interpretation of e x p e r i m e n t a l results o b t a i n e d in this work, we have to note that the i n h o m o g e n e i t y of the s u r f a c e p r e s s u r e at the air/water interface is one of the most important factors g i v i n g rise to the characteristic f e a t u r e s in the n - A curves. In the right-hand part of F i g u r e 10 are s h o w n simulation results obtained by using the above kinetic equations and the r e c t a n g u l a r cell model which divides the air/water interface into one h u n d r e d cells. In this simulation, the relative magnitudes of the rate of relaxation p r o c e s s e s and the rate of c o m p r e s s i o n were set up as f o l l o w s . : F i g u r e 10A: the rate of c o m p r e s s i o n > the rate of p r o c e s s (I) > the rate of p r o c e s s (II), F i g u r e 10F: the rate of process (I) > the rate of c o m p r e s s i o n > the rate of p r o c e s s (II), F i g u r e 10I: the rate of p r o c e s s (I) > the rate of p r o c e s s (II) >> the rate of compression. F i g u r e 10A shows the oscillation of the surface p r e s s u r e . H o w e v e r , if we include the damping factors and increase the n u m b e r of ceils in the simulation, the oscillation would be s m o o t h e d to a flat plateau such as it is r e p r e s e n t e d by the dotted line. It is evident that the above simulation results can r e p r o d u c e essentially all characteristic features such as "flat plateau", "zero s u r f a c e p r e s s u r e " , and " o v e r s h o o t hump" o b s e r v e d in the actual n - A curves. T h e s e properties are characteristic examples of nonlinearity in the n o n e q u i l i b r i u m state of a thin
225
film [32]. In the future, it will be necessary to obtain direct s p e c t r o s c o p i c i n f o r m a t i o n on the stable aggregates ( p o s s i b l y bilayers) o f P h D A 2 - 8 fatty acids. U t i l i z a t i o n of s p e c t r o s c o p i c methods, such as e p i f l u o r e s c e n c e m i c r o s c o p y [25, 3 3 - 3 6 ] , may give us direct information on the unique nature in the transition of the thin film.
ii
ill
E (a) inml~l~lmi~lml r~
n 4o
-.
10
20
30
10
I 20
i30
s so
I 60
I 50
I 60
-'-1
i
!
z.
| =
i-
i
ii
........
I 40
ii
I
I
I
I
iii
I,m
l..
8
qa
(i) {o}
I
I
1o
20
I 3O
I, 40
o,)
i ,, I
50 60 Surface Area [ ~.2/molecule]
I
I
i
Surface Area
Figure 10. Comparison between the experimental n-A curve(left) and the corresponding curve obtained by computer simulation(right). The left-hand parts of Figures 10(a), (f) and (i) are the same as in Figures 6. Parameters used in the simulation are; a = 0.60, k] = 0.75, (A) v = 1, (F) v = 0.5, (I) v = 0.01.
226 3. N O N L I N E A R VISCOELASTICITY AIR/WATER INTERFACE
OF
THIN
FILM
AT
In the preceding section we have shown several interesting phenomena, such as the overshoot hump and the zero surface pressure for a thin film with PhDA2-8, observed during the compression process. In order to evaluate nonlinear viscoelastic property in a quantitative manner, we have developed a method to observe the dynamic n-A characteristics with repeated cycles of c o m p r e s s i o n and expansion. Before we shall describe the m e t h o d o l o g y and results of the observation of the dynamic ~-A characteristics, we w o u l d like to discuss first the thermodynamic aspects of a lipid monolayer with special emphasis on the effects of the Coulombic interaction between the charged head groups of the lipid molecules. Inclusion of the Coulombic interaction enables to obtain a new insight into the origin of the nonlinear viscoelastic properties of lipid monolayer at the air/water interface.
3. l . I m p o r t a n c e e f f e c t s of C o u l o m b i c I n t e r a c t i o n s on the f o r m of the i s o t h e r m of the p h o s p h o l i p i d thin f i l m The ensemble of lipid molecules situated at the air/water interface can be regarded as composed of interacting "particles" in t w o - d i m e n s i o n a l space. Different types of physico-chemical interactions exist among the "particles", such as hydrophobic interaction, hydrogen bonding, van der Waals interaction, etc. Therefore we can expect that the essential feature of the thermodynamic state of the ensemble of the t w o - d i m e n s i o n a l "particles" can be interpreted similarly as in the model of t w o - d i m e n s i o n a l non-ideal gas. In other words, the isotherm of the lipid film at the interface, or the equilibrium pressure(rt)-concentration(l") characteristics, may be expressed by means of the virial expansion in a two-dimensional space. In the following we would like to discuss the actual isotherm of p h o s p h o l i p i d film, by comparing the standard virial expansion with a different type of expansion implying the specific aspect of Coulombic interactions. If we consider the thermodynamic behavior of t w o - d i m e n s i o n a l "particles", in the neighborhood of the phase transition region, the n-A isotherm of the film will behave as it is depicted by the line I in Figure 11. H o w e v e r , it is well known that in actual measurement, the surface pressure does not generally show the flat plateau, as in line II in Figure 11. Such a general experimental trend may be, at least to some degree, attributed to the nonequilibrium effect as is discussed t h r o u g h o u t this chapter. Actually, Mingins et al. has carried out a careful measurement of the equilibrium n-A characteristics for phospholipid molecules at an oil/water interface and observed nearly the ideal trace similar to a t w o - d i m e n s i o n a l van der Waals gas as it is slow by the line I(see Figure 11) [37]. Recently we have p r o p o s e d a theoretical model to interpret the re-A, or n-I', relationship.
227
fluid/solid coexistence
llI g I
A Figure 11. Schematic representation of the n-A relationship of a lipid monolayer
When one takes into account the effects of interaction between the polar head groups using similar degree of the approximation as in the Debye-Hiickel theory, the following relationship results [38]: n = R T F (1 - ~ F 1/2 + ctF),
(3-1)
where 0t is the second virial coefficient in the usual two-dimensional van der Waals equation, and ~ is the parameter proportional to the Debye-Htickel length around the charged group. Let us compare the theoretical isotherm of Equation 3-1 with the n-F relationship represented as the standard expansion with the second ot and third 13 virial coefficients as in rc = R T F
(1 + czF + [~1"2).
(3-2)
Both Equations 3-1 and 3-2 contain two "adjustable" parameters. Thus, the c o m p a r i s o n of these equations with the results of actual measurements will allow us to the determine the validity of the unique expansion in Equation 3-1. Equation 3-1 can be re-arranged into
228
f(r)
=
/r
RT1-'2
1
- --
= a-
~ r -~/2
F
.
(3-3)
Similarly, Equation 3-2 gives rc f(r')
=
1 - --
RTI ~
= a
+ I~r"
F
.
(3-4)
Using Equations 3-3 and 3-4, we can plot f(F) v s . F 1/2 or F based on the experimental data on the rc-F characteristics (see Figure 12) [39]. Figure 13 indicates that Equation 3-3 is more realistic than Equation 3-4. The correlation coefficient, R, for Equation 3-3 is nearly unity, suggesting that the assumption of the "Coulombic" interaction between the hydrophilic headgroups is essential in the interpretation of the isotherm, especially for the low F(or the large A) region. Equation 3-1 implies that the Ir-F relationship becomes "N"-shaped when becomes sufficiently large. Indeed, Mingins et al. have reported that at the oil/water interface the ~-A isotherms of the phospholipids with Cls, C20 and Czz alkyl chains show clear first-order transitions [37].
4o E z E
ii
5 30 2o IO
0.5
~.0
t5
z.o
z.5
A (nm2/moLecute)
Figure 12. n-A isotherm for 1,2-dimyristoyl lecithin at an n-heptane-aqueous NaC1 solution interface. The numerals on the curves are the temperatures given in Celsius.
229
oo
(A)
4
O3)
"7, 3.5 t
~ ~"* x.-'~ o. " .. >~" / ~" X."" A ~'tx
3.
AX-~
1
"o.
~""la.laA %
x~x,."..., "l.
2.
2.5
!
0.3 0.5 o.7 0.9 ~.1 ].3 ~.5 F x 1018
, ,
0.7
t
i
i
0.9
i
i
i
I
i
1.1
,
i , , ,
1.3 1
1"'~
|
1.5
1/,,~
,
.7
x 10.9
D Equation (3-3) E Equation (3-4)
(C) 1 0.98 ~0.96 0.94 0.92 0.9
5
10
15
20
Temperature Figure 13. (A) The plot of f(I') v s . F based on the data of Figure 12. 03) The plot of f(l") v s . F -1/2 for the following temperatures: O , 5 ~ ~ , 10 ~ X, 15 ~ 0 , 20 ~ (C) The comparison of the correlation coefficients, R, between the two different analysis.
3.2.Dynamic
surface
pressure
as a m e a s u r e
of the nonlinearity
In the above subsection it was demonstrated that the inclusion of electrostatic interactions into the pressure-area-temperature equation of state provides a better fit to the observed equilibrium behavior than the model with t w o - d i m e n s i o n a l neutral gas. Considering this fact, we would like to devote our attention now to the character of the lipid film under the dynamical, nonequilibrium conditions. In the following we shall describe the dynamical behavior of the p h o s p h o l i p i d ( 1 , 2 - d i p a l m i t o y l - 3 - s n - p h o s p h a t i d y l e t h a n o l a m i n e s ; DPPE) thin films in the course of the compression and expansion cycles at air/water interface. The dynamic surface pressure was measured with a trough equipped with a couple of moving Teflon blades and an electronic balance, CHAN/Ventron, Cerritos, CA, U . S . A . , similar to the apparatus describe by Mendenhall and Mendenhall [40], and by B i e n k o w s k i and Skolnick [41]. The trough coated
230 with Teflon was 116 mm long, 61 mm wide, and 14 mm deep; it was rinsed carefully with methanol and distilled water before each experimental run. The surface pressure was monitored as the tension of the piano steel wire connected to the platinum plate(9.8 x 20 x 0.03 mm), previously polished with 250-mesh emery, dipped one half into the aqueous subphase. DPPE was gently spread onto the aqueous phase(ca. 100 ml), containing various chemical species. The surface area was then changed successively between 1300 and 4600 mm z, by moving the Teflon blades with a cycle of 60 see. When the n - A curve began to trace a single closed-line after several cycles, the curve of the dynamic surface pressure was recorded. All measurements were performed at 2 0 + 1~ Figure 14 shows the dynamic surface pressure for the thin f i l m ' o f DPPE after several compression and expansion cycles. Figure 15 s h o w s the characteristic response of the dynamic ~-A curve to the chemical stimuli. It is apparent that the dynamic n-A curves traced closed-lines exhibiting hysteresis and the hysteresis loops varied with the addition of chemical c o m p o u n d s . The characteristic response may be explained taking into the consideration the specific effects of added chemicals on the dynamic behavior of the DPPE thin film. Nicotine is rather hydrophobic and is expected to change the manner of packing of alkyl-chains in the aggregates of the DPPE molecules. Sodium chloride changes the Debye length of the charge effect of the DPPE head group and, thus, changes the aggregation of the DPPE molecules.
(B)
(A)
8
r~
!k
! JLLA_J I
120
!
,
,
,
,
180
240
300
360
420
Time
,,.
|
,
,
,
,
,
-
.
_
480 20 30 40 50 60 70 80901 00
see Surface Area
Figure 14. (A) Time trace of surface pressure with periodic change of the surface area for the DPPE thin film at an air/water interface. (B) Dynamic n-A curve obtained after two cycles.
231
(A)
(B)
l, .
.
.
.
|
(C)
(D)
i|1
.|
r~
Surface Area Figure 15. Characteristic changes in the dynamic n-A curve caused by the addition of various kinds of chemical compounds to aqueous solution containing DPPE. (A) 0.05 mM nicotine, (B) 0.5 mM sodium chloride, (C) 0.1 M sucrose, (D) 0.5 mM citric acid.
S u c r o s e changes the dynamic structure of water molecules, which, in turn, affects the manner of aggregation of the DPPE. Citric acid changes the degree of dissociation of the head group of the DPPE molecules. It becomes, therefore, apparent that each chemical species affects the viscoelastic behavior of the lipid thin film in a characteristic manner. It is noted that, as is shown in Figure 15, the chemicals with different t a s t e - r e s p o n s e s show markedly different effects on the dynamic behavior of the p h o s p h o l i p i d film. Detail d i s c u s s i o n on the chemical r e s p o n s e in relation to the mechanism of taste sensation has already been given in a series of studies from our research group [ 3 , 4 2 , 4 3 ] .
3 . 3 . A n a l y s i s o f the d y n a m i c s u r f a c e p r e s s u r e After the a cco mp lis h men t of the above mentioned experiment on the nonlinear viscoelasticity of the DPPE thin film, we have tried to construct a new instrument for the m e a s u r e m e n t of the dynamic surface tension. We have noticed that, the blades used to change the surface area in the commercial instrument, did not show genuine triangle or sinusoidal trajectory but rather mathematically undefined. With our newly designed instrument, the time change of the surface area can be controlled according to a chosen function with the aid of a micro-computer. Experimental procedure for the preparation of the thin film was similar to that described in the preceding subsection.
232
(E)
tlIi
9
(c)
03)
Figure 16. Experimental arrangement used for the measurement of the dynamic behavior(n-A curve) of the DOPC thin film at an air/water interface. (A) trough, (B) blades, (C) platinum plate(Wilhelmy plate), (D) water circulating outlet, (E) electric balance, (F) blades controller, (G) data processor.
F i g u r e 16 s h o w s the experimental a r r a n g e m e n t for the m e a s u r e m e n t of the s u r f a c e p r e s s u r e . The trough (200 mm long, 50 mm wide and 10 mm deep) was coated with Teflon. The s u b p h a s e temperature was controlled within + 0.1 ~ by means of a jacket connected to a thermostated water circulator, and the e n v i r o n m e n t a l air temperature was kept at 18 ~ The surface tension was m e a s u r e d with a Wilhelmy plate of p l a t i n u m ( 2 4 . 5 x 10.0 x 0.15 mm). The s u r f a c e p r e s s u r e monitored by an electronic balance was s u c c e s s i v e l y stored in a m i c r o - c o m p u t e r , and then Fourier t r a n s f o r m e d to a frequency domain. The s u r f a c e area was changed s u c c e s s i v e l y in a sinusoidal manner, between 37.5 /~2/molecule and 62.5 /~2/molecule. We have chosen an u n s a t u r a t e d phospholipid(1,2-dioleoyl-3-sn-phosphatidyl-choline; DOPC) as a curious s a m p l e to m e a s u r e the dynamic surface tension with this novel instrument, as the u n s a t u r a t e d lipids play an important role in b i o m e m b r a n e s and, m o r e o v e r , such a "fluid" lipid was expected to exhibit marked dynamic, nonlinear characteristics. The spreading solution was 0 . 1 3 3 mM c h l o r o f o r m solution of DOPC. The c h l o r o f o r m was purified with three consecutive distillations. The experimental traces of the t i m e - d e p e n d e n t change of the surface p r e s s u r e and the dynamic n-time and n-A curves are shown in Figure 17.
233
03)
(A)
50
0
100
1 0 2 250 Time(sec)
300+
354045~0~5 Surface Area
(~k2/mole, culc)
Figure 17. (A) Time trace of surface pressure with periodic change of the surface area for the DOPC thin •rn at an air/water interface. (B) Dynamic n-A curve. After two or three cycles, the curve begins to trace a single closed loop.
(A) ~ !
~)
; - ......... ~_ . . . .
Ii
0
V
~,.
I4
coo 2 ~ o 3 ~ o 4 ~
,,
Frequency
354045 505560
,
,
,
Surface Area
(~/mol~l~)
Figure 18. (A) Fourier transformation spectra of the time trace of surface pressure for the steady loop (see Figure 17). Top: real part (elastic component), bottom: imaginary part (viscous component). (B) Inverted Fourier Spectra for the real and imaginary parts.
234
After several cycles of the compression and expansion, the dynamic n-A curve becomes a single closed loop, somewhat distorted from a genuine ellipsoid. In order to analyze the forms of the hysteresis loop under stationary conditions, we have measured the time trace of the dynamic surface pressure after five cycles of the compression and expansion, and then Fouriertransformed it to the frequency domain. The Fourier-transformation was adapted to evaluate the nonlinear viscoelasticity in a quantitative manner. The detailed theoretical consideration for the use of the Fourier transformation to evaluate the nonlinearity, are contained in the published articles [8,43]. F i g u r e 18A shows the Fourier spectra thus obtained. The real and imaginary parts correspond to the elastic and viscous components of the DOPC thin film, respectively. We can see that the spectrum is composed not only from the fundamental (tOo) but also from the higher (2o)0, 3~0, 4c00,..) harmonic components. Such a trend indicates that the DOPC thin film exhibits rather large nonlinearity in the viscoelastic characteristics. In other words, higher harmonics serve as a useful quantitative measure of the nonlinear characteristics of the thin film. Using the inverse Fourier transformation, the real and imaginary parts can be represented as the elasticity and viscosity of the film in a separate manner as is shown in Figure 18B.
3 . 4 . Theoretical interpretation of hysteresis loops Next, we discuss the physico-chemical meaning of the hysteresis or the imaginary components in the dynamic ~-A curve. Generally, DOPC molecules form a soluble monolayer at an air/water interface. When the monolayer is c o m p r e s s e d rapidly, the DOPC molecules are dissolved into the subphase and form micellar aggregates. If the aggregates are dissolved continuously into the subphase, in other words, if the total amount of the lipid present at the interface decreases with each cycle of the repeated compression and expansion, the dynamic n-A curve can not exhibit a single closed loop. In the actual experiment, we have observed a single closed line for the n-A curve (see Figure 17B), indicating that under our experimental conditions, DOPC molecules dissolve into the subphase in the course of the compression and then return into the interface. The proposed scheme for the cooperative adsorption/desorption of the DOPC molecules at the air/water interface is given in Figure 19. On the basis of the above discussions, we would like to propose the f o l l o w i n g kinetic model. Let [S] and [Dint] be the concentration of DOPC molecules in the "regular" and "aggregated" states at the air/water interface, respectively. The concentration of the aggregated state in the vicinity of the air/water interface(sublayer) is expressed as [Dsub].
235
kl ~ -" k_1
S+S
Din t
k2 S + Din t
"k.2
D sub
Based of this mechanism, the time course of c o n c e n t r a t i o n s is given by the f o l l o w i n g set of differential equations,
diS]
-
_ kl[S] 2 + k.l[Din t] - k2[S][Dint] + k_2[Dsub],
dt
(4-1)
d[DinO 9 = dt d[Dsub] dt
,,
=
kl[S] 2 - k-l[Din0 - k2[S][Dint] + k-2[Dsub],
(4-2)
k2[S][Dint] - k.2[Dsub], (4-3)
w h e r e k I is the aggregation rate constant, k.l the d i s s o c i a t i o n rate constant, k. 2 the a d s o r p t i o n rate constant, respectively, and the d e s o r p t i o n rate coefficient k 2 is represented by a nonlinear function of the surface p r e s s u r e , k2
=
F(rr).
(4-4)
Air-Water Interface
Sublayer
, , . . ~ . , .
.
.
.
.
. . , . . . . .
.
.
.
.
.
.
.
.
.
.
.
.
.
,
.
.
.
.
.
.
.
.
i
.
.
.
.
.
.
o
.
I
-
-
VA Subphase Figure 19. Schematic representation of the cooperative adsorption and desorption of DOPC molecules between an air/water interface and a sublayer.
236
(A)
r.~
I
I
(B) m|l
I,
I
I
'
35 40 45 50 55 60
.
.
.
.
Surface Area
Surface Area
(.~2/molecule)
Figure 20. Comparison between the experimentally observed dynamic surface behavior and the results obtained by computer simulation. The left-hand parts of Figure 20(A) is the same as in Figure 1703).
This function has been introduced to account for the first order like transition in the process of the compression of the film. The function F(rc) may be thus represented as an "S"-shape function (Figure 8) [30,31]. In analogy with the section 2, the time dependent changes of concentrations, [S], [Dint] and [Dsnb] are calculated from the above equations and the rectangular cell model based on division of the air/water interface into twenty cells. In the present work, we take the approximation that the dynamic surface pressure is directly proportional to [S] and [Dint] [44,45]. The simulation results are shown in Figure 20. We can observe that the simulation can reproduce the characteristic feature, the hysteresis loop, o b s e r v e d in the dynamic rc-A curves.
4.SPONTANEOUS AND INTERFACE SYSTEM
OSCILLATION T E N S I O N IN
OF E L E C T R I C A L POTENTIAL AN OIL-WATER-SURFACTANT
Recently we have found that sustained electrical oscillations are generated in a liquid membrane consisting of water/oil phase, where the aqueous phase contains surfactant and alcohol [3,10]. The difference between the concentrations in the aqueous and the oil phases forms the driving force of electrical oscillations. In the last section, we would like to describe this type of rhythmic oscillation from the point of view of the nonlinearity in the kinetics of transfer of lipid molecules through the oil/water interface. It is
237 closely related to the cooperative character of lipid molecules at the oil/water interface.
4.1.Oscillatory DPPE
pattern in liquid membrane with s o d i u m oleate and
Experiments for the measurement of oscillations were performed using a cell in the form of a concentric hollow cylinder shown schematically in Figure 21 [9]. Nitrobenzene solution (13 ml) of 0.5 mM tetraethylammonium bromide (TEAB) was placed at the base of the cell. TEAB was used to decrease the electrical resistance of the bulk organic phase. An aqueous solution (8 ml) containing both surfactant, sodium oleate or DPPE, and alcohol, butanol, was carefully introduced into the inner cylinder and, simultaneously, 0.5 M NaC1 aqueous solution (14 ml) was introduced into the outer cylinder. All measurements were carried out at 20 + 1 ~ The voltage across the aqueous/organic/aqueous phase, or the thin lipid film, was measured with a high-impedance voltmeter connected by two salt bridges to two Ag/AgC1 electrodes. Interracial tension was monitored by the Wilhelmy method [46] using a hydrophilic glass plate (15 x 15 x 1 ram).
(A)
I
(B)
~_~
i
", I
,
'
I
i~
i
30mm ---50mm
'
2mm
I .k.--~_4.: I_J ....
t Figure 21. (A) Side view of the experimental apparatus for the simultaneous measurements of electrical potential and interfacial tension. (a) High impedance voltmeter, (b) electronic balance, (c) salt bridge. 03) Bird-eye view of the apparatus.
238
(A) o
rO
:
t
20min
(B)
I
20rain
(C)
E
I
20rain
-t
Figure 22. Variations of the electrical potential and the interfacial tension. The aqueous phases in the inner cylinder are; (A) 0.1 mM sodium oleate plus 5 vol% butanol. 03) 0.07 mM DPPE plus 5 vol% butanol, and (C) 5 vol% butanol without surfactant.
239 Figure 22 shows the results of the simultaneous m e a s u r e m e n t of electrical potential and interfacial tension for the liquid membrane with an aqueous phase of (A) 0.1 mM sodium oleate plus 5 vol% butanol, (B) 0.07 mM DPPE plus 5 vol% butanol and (C) 5vo1% butanol. Rhythmic changes of the electrical potential (Figure 22A and 22B) continued for 1-2 h. The amplitude, frequency and wave form of the oscillations were found to be essentially the same for each experiment performed under fixed conditions. On the other hand, in the absence of the surfactant irregular pulses were o b s e r v e d (Figure 22C). Such an irregular change in the electrical potential in Figure 22C is attributable to the so-called Marangoni effect [46], i.e. spontaneous interfacial turbulence accompanied by mass transfer across the interface. Let us note that the electrical potential and the interfacial tension change in a synchronized manner as it is shown in Figures 22A and 22B, whereas no apparent change in the interfacial tension can be observed in Figure 22C. When experiments were performed with DPPE and without butanol, only a few pulses were observed and sustained oscillations were not generated. Thus, the main driving force for the oscillations appears to be the diffusion of alcohol from the bulk aqueous phase to the organic phase through the interface, and the surfactant molecules induce a simultaneous or cooperative change over the entier interface. As the interfacial tension is directly related to the concentration of surfactant molecules at the interface [46], the rhythmic changes in the interfacial tension suggest that the concentration of surfactant, sodium oleate in Figure 22A and DPPE in Figure 22B, at the oil/water interface changes repeatedly between high and low values.
State A
State B ,|
,,
WaterPhase FA
f
~
Oil Phase
~
~a~hol
i-' Oil Phase
~
Figure 23. Schematic representation of the repetitive formation and reconstruction of the lipid thin film.
240
Table 1. Repeated change of the interfacial state.
State m
-
'
'
~
B
AE
low
high
Force change on the glass plate
z~tF
low
high
Surface tension
A7
high
low
Concentration of the surfactant
AI-"
low
high
Electrical potential ,,
.
.
.
.
The increase in the concentration of the surfactant molecules at the interface corresponds to the growth of the monolayer, which increases the electrical potential of the electronic bilayer. The synchronized change is shown schematically in Figure 23 together with Table 1.
4.2.
T h e o r e t i c a l e x p l a n a t i o n of the o s c i l l a t i o n s
Based on the above results and discussion, the mechanism for the rhythmic oscillations at the oil/water interface with surfactant and alcohol molecules may be explained in the following way [3,47,48] with reference to Table 1. As the first step, surfactant and alcohol molecules diffuse from the bulk aqueous phase to the interface. The surfactant and alcohol molecules near the interface tend to form a monolayer. When the concentration of the surfactant together with the alcohol reaches an upper critical value, the surfactant molecules are abruptly transferred to the organic phase with the formation of inverted micelles or inverted microemulsions. This step should be associated with the transfer of alcohol from the interface to the organic phase. Thus, when the concentration of the surfactant at the interface decreases below the lower critical value, accumulation of the surfactant begins and the cycle is repeated. Rhythmic changes in the electrical potential and the interface tension are thus generated.
241
Aqueous phase
Interface ! !
Organic phase
! !
I i Negative Feed back
Xb "
9 :'%11
i ~.~Zi'
;- Bulk Organic Phase
~,y.d/~ ,
Y t --D
t
,
r
1
],
Scheme 1. The process of transfer of surfactant and alcohol from the aqueous phase to the organic phase through the interface.
The foregoing interpretation of the mechanism, although necessarily somewhat speculative, provides a useful kinetic model [10]. Let X, Y, and Z be the concentrations of the key chemicals; X i, the concentration of surfactant at and/or near the interface; Yi, the concentration of alcohol near the interface; Zi the concentration of the aggregate or complex of surfactant and alcohol at and/or near the interface. Scheme 1 can be considered as a possible explanation for the mechanism of oscillation in a liquid membrane. Xb and Yb are the concentration of the surfactant and alcohol, respectively, in the bulk aqueous phase. Scheme 1 is composed of the following steps (i)-(iv).
Dx Xb ~
Xi
(i)
Vi
(ii)
Dy Yb ~
Xi + Yi
Zi
~
k4
~
k3
Zi
bulk organic phase
(iii)
(iv)
242 Steps (i) and (ii) correspond to the migration of surfactant and alcohol from the bulk aqueous phase toward the oil/water interface. Step (iii) is the formation of aggregates of surfactant and alcohol at the interface, and is related to the construction of the monolayer associated with surfactant and alcohol. Step (iv) indicates the migration of the aggregates of surfactant and alcohol from the interface into the bulk organic phase, forming inverted micelles or microemulsions in the organic phase. It may be expected that the rates of transfer of surfactant and alcohol, Dx and De, are affected by the negative feedback of Zi. In other words, the diffusion rate, Dx, of surfactant from the aqueous phase to the interface may decrease with the net increase in the concentration of surfactant at the interface, Xi plus Zi. A similar situation may hold for the diffusion rate, Dr, of alcohol from the aqueous phase to the interface. Hence, the system kinetics may be considered under the following assumptions: (a) the concentration of surfactant and alcohol in the bulk aqueous phase, Xb and Yb, remain constant; (b) the rates of diffusion of surfactant alcohol from the bulk aqueous phase to the interface are expressed as Dx(Xb - Xi) and Dv(Yb - Yi), respectively; (c) the negative feedback of Zi on the diffusion of X and Y are given Yb- kl Zi and - k2Zi, respectively; (d) the rate of step (iv) is expressed as a function, F(Xi, Yi),with the rate constant k3; and (e) the rate of step (iv) is expressed as a function, G(Zi), with the rate constant k4. Under these assumptions, the kinetics of the migration of surfactant and alcohol are described by the differential equations:
dX i dt
-
Dx (Xb
-
X
-
D y (Yb
-
Yi)
dYi dt dZi dt
i)
-
k l Zi,
-
k2 Zi,
- k 3 F(Xi,Yi) - k 4 G(Zi),
(4-1)
(4-2)
(4-3)
F(Xi, Yi) may be given by (Xi + Yi), a simple form for the synergetic effect of Xi and Yi. which has the physical meaning that the monolayer is formed together with surfactant and alcohol. Self-oscillatory states can be obtained if G(Zi) has "N"-shape nonlinearity.
243
(A) o (Do_ I
i
Time
~
I
-~.o Zi
1.0
(B)
o (-9 o I
I
I
-1.0 Zi 1.0
"Time
(c)
9 T -
Time
,
-~.0 Zi
1.0
Figure 24. Computer simulation of the oscillation with variation of the nonlinear function G(Zi). Parameters; (A) Dx=0.1, Dy=0.025, Xb=0.4, Yb=2.56, k~--0.2, k2--0.05, k3=2.5, k4=2.5; (B) Dx--0.2, Dy=0.05, Xb=0.4, Yb=2.5804, k~=0.4, k2=0.1, k3=5.0, k4=5.0; (C) Dx--0.1, Dy=0.025, Xb=0.4, Yb=2.8, kl=0.2, k2=0.05, k3=2.5, k4=2.5. The shapes of G(Zi) are shown on the right side of Figure 24.
244 As has been discussed in the preceding sections, it is expected that the surfactant monolayer exhibits "N"-shape nonlinearity in its dynamic x - F characteristics. Thus, we would like to discuss the kinetics, assuming that G(Zi) is a cubic function. Figure 24 shows numerical results for the set of Equations 4-1, 4-2, 4-3. Comparison with the experiments, Figure 24A and 24B indicates that the oscillation patterns have been well reproduced. Thus it becomes evident that repetitive formation and destruction of the monolayer are the key steps for the rhythmic pulsing. In the right of Figure 24, the shape of the "N"-type nonlinearity, G(Zi) versus Zi. is shown. When G(Zi) is a smooth function as in Figure 24C, growing oscillations with regular bursting pulses are generated. On the other hand, the period between the bursting pulses becomes irregular when the derivative of G(Zi) is discontinuous as in Figure 23A and 23B. The steepness on the right-hand side of the "N"-shape function is the important factor to induce the small pulses between the great bursting pulses. In conclusion, we have shown that lipid film generally exhibits marked nonlinear characteristics. Nonlinear characteristics become particularly significant under the dynamic and/or nonequilibrium conditions. Further experimental and theoretical studies of nonlinear characteristics of lipid films are desirable.
ACKNOWLEDGEMENTS The authors express their sincere thanks prof. M. Marek (Prague Institute of Chemical Technology) to the helpful comments on the present articles. The authors thank Prof. H. Kawakami (Tokushima University), Dr. T. Ishii (Tsurumi University), and Dr. S. Nakata (Nara University of Education) for their helpful suggestions and to Dr. Y. Yoshioka (Kanegafuchi Chemical Industry Co., Ltd.) for the advice on the experimental technique of rc-A measurement of PhDA2-8.
REFERENCES 1. R. J. Field and M. Burger(eds.), Oscillations and Traveling Wave in Chemical Systems, John Wiley and Sons, New York, 1985. 2. G. Nicolis and I. Prigogine, Self-organization in Nonequilibrium Systems, John Wiley and Sons, New York, 1977. 3. K. Yoshikawa and Y. Matsubara, J. Am. Chem. Soc., 105 (1983) 5767. 4. K. Yoshikawa and Y. Matsubara, J. Am. Chem. Soc., 106 ( 1 9 8 4 ) 4 4 2 3 . 5. K. Yoshikawa, T. Omochi and Y. Matsubara, Biophys. Chem., 23 (1986) 211.
245 6. K. Yoshikawa, T. Fujimoto, T. Shimooka, H. Terada, N. Kumazawa and T. Ishii, Biophys. Chem., 29 (1988) 293. 7. K. Yoshikawa, S. Nakata, T. Omochi and G. Colaccico, Langmuir, 2 (1986) 715. 8. K. Yoshikawa, M. Shoji, S. Nakata, S. Maeda and H. Kawakami, Langmuir, 4 (1988) 759. 9. K. Yoshikawa and M. Makino, Chem. Phys. Let., 160 (1989) 623. 10. K. Yoshikawa, Excitable Liquid Membranes, in T. Araki and H. T s u k u b e ( e d s . ) , CRC press, 1990. 11. S . J . Singer and G. L. Nicolson, Science, 175 (1972) 720. 12. G . W e i s s m a n and R. Claiborne(eds.), Cell Membranes: Biochemistry, Cell Biology & Pathology, HP Publishing Co., Inc., New York, 1975. 13. I. Langmuir, Trans. Far. Soc., 15 (1920) 62. 14. K . B . Blodgett, J. Am. Chem. Soc., 57 (1935) 1007. 15. H . - J . Cantow(ed.), Polydiacetylenes, Advances in Polymer Science, New York, Vol. 63, 1984. 16. G. Wegner, Recent Progress in the Chemistry and Physics of Poly(diacetylenes), in W.E.Hatfield(ed.), Molecular Metals, Plenum, New York, 1979. 17. M. Brenton, J. Macromol. Sci. Chem., 21 (1981) 61. 18. A. Laschewsky, H. Ringsdorf, G. Schmidt and J. Schneider, J. Am. Chem. Soc., 109 (1987) 788. 19. B. Ostermayer, O. Albrecht and W. Vogt, Chem. Phys. Lipids, 41 (1986) 265. 20. B. Hupfe and H. Ringsdorf, Chem. Phys. Lipids, 33 (1983) 263. 21. D . J . Scoberg, D. N. Furlong, C. J. Drummond, F. Grieser, J. Davy and R. H. Prager, Colloids and Surfaces, 58 (1991) 409. 22. Y. Yoshioka, N. Nakahara and K. Fukuda, Thin Solid Films, 133 (1985) 11. 23. H. Matsuo, D. K. Rice, D. M. Balthasar and D. A. Cadenhead, Chem. Phys. Lipids, 30 (1982) 367. 24. B. Tieke and G. Lieser, J. Colloid Interface Sci., 88 (1982) 471. 25. S . W . Hui and H. Yu, Langmuir, 8 (1992) 2724. 26. D. Day and H. Ringsdorf, J. Polym. Sci.: Polym. Let. Ed., 16 (1978) 205. 27. T. Kato, Y. Hirobe and M. Kato, Langmuir, 7 (1991) 2208. 28. Y. Kawabata T. Sekiguchi, M. Tanaka, T. Nakamura, H. Komizu, K. Honda and E. Manda, J. Am. Chem. Soc., 107 ( 1 9 8 5 ) 5 2 7 0 . 29. T. Kurata, A. Tsumura, H. Fuchigami and H. Koezuka, J. Phys. Chem., 95 (1991) 8831. 30. N . L . Gershheld, Annu. Rev. Phys. Chem., 27 (1976) 349. 31. H . W . Horn and N. L. Gershbeld, Biophys. J., 18 (1977) 301. 32. M. Makino, M. Kamiya, T. Ishii and K. Yoshikawa, Langmuir 10 (1994) 1287. 33. A. Miller and H. M6hwald, J. Chem. Phys., 86 (1987) 4258.
246 34.
H . M . McConnell, D. Keller and H. Gaub, J. Phys. Chem., 90 (1986) 1717. 35. N. Kimizuka and T. Kunitake, J. Am. Chem. Soc., 111 (1989) 3758. 36. M. Shimomura, K. Fujii, T. Shimamura, M. Oguchi, M. Shinohara, Y. Nagata, M. Matsubara and K. Koshiishi, Thin Solid Films, 210/211 (1992) 98. 37. J. Mingins, J. A. G. Taylor, B. A. Pethica, C. N. Jackson and B. Y. T. Yue, J. Chem. Soc., Faraday Trans. I, 78 (1982) 323. 38. K. Yoshikawa, S. Maeda and H. Kawakami, Ferroelectrics, 86 (1988) 281. 39. K. Yoshikawa, M. Makino, $. Nakata and T. Ishii, Thin Solid Films, 180 (1989) 117. 40. R . M . Mendenhall and A. L. Mendenhall Jr., Rev. Sci. Instrum., 34 (1963) 1350. 41. R. Bienkowski and M. Skolnik, J. Coll. Interface Sci., 39 (1972) 323. 42. K. Yoshikawa, M. Shoji, and T. Ishii, Biochem. Biophy. Res. Commun., 160 (1988) 699. 43. M. Makino, K. Yoshikawa, and T. Ishii, Nippon Kagaku Kaishi 10 (1990) 1143. 44. S. Nakata, K. Yoshikawa, M. Shoji, H. Kawakami and T.Ishii, Biophys. Chem., 34 (1989) 201. 45. M. Makino, M. Kamiya, N. Nakajo and K.Yoshikawa, Progress in Anesthetic Mechanism, in press. 46. A . W . Adamson, Physical Chemistry of Surface, 5th ed., John Wiley & Sons Inc., New York, 1990. 47. M. G. Velarde(ed.), Physicochemical Hydrodynamics Interfacial Phenomena, Plenum Press, New York, 1988. 48. I. Langmuir, J. Am. Chem. Soc., 39 (1917) 1848.
New Developments in Construction and Functions of Organic Thin Films T. Kajiyama and M. Aizawa (Editors) 9 1996 Elsevier Science B.V. All rights reserved.
M o l e c u l a r control of p h o t o r e s p o n s e s of LB films containing
247
redox
chromophores Toshihiko Nagamura Crystalline Films Laboratory, Research Institute of Electronics, Shizuoka University, 3-5-1 Johoku, Hamamatsu 432, Japan Langmuir-Blodgett (LB) films containing redox chromophores such as 4,4'bipyridinium, carbazole, aUoxazine, porphyrin, and Ru(II)-bipyridine complexes showed various photoresponses including photoinduced electrochromism, photocurrents, amplified fluorescence quenching, or photochemical modulation of second harmonic generation. The molecular control of these photoresponses in LB films was successfully achieved by controlling the molecular arrangements, molecular orientation, and/or the extent of aggregation of redox chromophores. The relationship between the structure of organized redox chromophores and the photochemical or photophysical properties will be discussed. 1. INTRODUCTION Organized molecular assemblies containing redox chromophores show specific and useful photoresponses which cannot be achieved in randomly dispersed systems. Ideal examples of such highly functional molecular assemblies can be found in nature as photosynthesis and vision. Recently the very precise and elegant molecular arrangements of the reaction center of photosynthetic bacteria was revealed by the X-ray crystallography [1]. The first step, the photoinduced electron transfer from photoreaction center chlorophyll dimer (a special pair) to pheophytin (a chlorophyll monomer without magnesium ion) separated by about 1.7 nm occurs in about 3 ps [2]. Then electrons are transported between precisely aligned redox chromophores in bilayer membrane with a rate about two orders of magnitude faster at each step than the corresponding reverse reactions. As can be found in this elegant model molecular design of functional compounds and molecular control of their arrangements will be essential to construct highly efficient photoresponsive systems. The Langmuir-Blodgett deposition is one of the best methods to prepare highly organized molecular systems, in which various molecular parameters such as distance, orientation, extent of chromophore interaction, or redox potential can be controlled in each monolayer. We have been studying
248 photophysical and photochemical properties of LB films in order to construct molecular electronic and photonic devices in the near future. In this review article recent results on structural properties, spectroscopic characterization and various photoresponses of organized redox chromophores in LB films will be presented mainly based on our studies. 2. SPECIFIC ORIENTATION AND INTERACTIONS OF CHROMOPHORES IN LB FR,MS The molecular orientation and interactions of redox chromophores are very important in controlling photoresponses at the molecular level. Absorption and fluorescence spectra will give important information on them. We have studied, photoresponses, specific interactions, in-plane and out-of-plane orientation of various chomophores in LB films composed of amphiphiles shown in Figure I [3-12]. 2.1. Specific absorption due to the interactions of chromophores in LB films Chromophores in LB films show specific absorption spectra reflecting their
DDA
RCN'~R2
~ c ~ O
CUA
~N C (.H2hoCOOH
BCZ~NC (.H~~ )oCO HO
PV2+
HV2+ AV2§ TFPB-
C19H39C~OH PA
AMP
DHA
R~=((CH~kO)..R2--(CHz), Re~=C~.. R.=C~.,c,N(c,eH~h, R2=C2Hs o
,,~'~
C15H31C00H
RuC18B
C16H33 Br
~16"33 ~~"~,0CI6 H33
0 II
~ , , , CNICH2)I?CH3
-~.'~K.~
H .2c~o2 ICIN(Ch]I?CH3 O
2C18NB c~{C~)r~,,:~,-cHs. e; CH3(CH~I7/"-, C.3 H
k
0
N
"-2~
n= 5 x 10~
Figure I Structuresand abbreviationsof compounds employed.
249 interactions due to dose packing. Absorption spectrum of four monolayers of 1,3-didodecylaUoxazine (DDA) deposited at 20 ~ and 20 m N o m -z by a vertical dipping method on a quartz plate coated with three monolayers of cadmium arachidate is shown in Figure 2 together with that in n-hexane solution (0.01 0.15 -
i
|'
i
1 ....
10.5
l
0.4
lL_
0.10
0.3~
o r .
6 was reported in phthalocyanine LB films [26]. Minari et al. [31] made a model calculation on the in-plane orientation of merocyanines and reported that it was caused by their flow during a transfer process from air/water interface to the substrate surface. Bird et al. [32] obtained a sort of epitaxial LB films by depositing 3,3'-dioctadecyl-9ethylthiacarbocyanine on the cleavage ac-face of gypsum. The amphipathic chromophores stand with long edge against the gypsum face, and with chromophore short axes nearly vertical. The polarized absorption spectra with polarization parallel and perpendicular to a-axis showed almost perfect orientation of long axes along a-axis with dichroic ratio higher than 20. Similar amphipathic thiacarbocyanine without 9-ethyl group showed little dichroism [32]. The out-of-plane orientation of chromophores can be more easily controlled in LB films as compared with the in-plane orientation. Many chromophores are known to show anisotropic orientation in the surface normal direction. The molecular structure of chromophores and their position in amphiphile molecules, the surface pressure, the subphase conditions are among those affect their out-of-plane orientation. The out-of-plane orientation has been studied by dichroic ratio at 45 ~ incidence, absorbance ratio at normal and 45~ incidence, and incident angle dependence of p-polarized absorption [3,4,27,33-41]. The evaluation of the out-of-plane orientation in LB films is given below using amphipathic porphyrin (AMP) as an example [5,10,12]. Mixtures of AMP and arachidic acid (AA) showed two solid condensed phases below and above about 30 mNom -1 as in Figure 4. The extrapolated limiting area of both solid condensed phases decreased with decreasing molar fraction of AMP. The molecular areas (Alow and Ahi~) occupied by AMP were estimated from the molecular area of cadmium arachidate (0.20 n m 2) and the molar fraction of AMP; Alow - 1.96 + 0.15 rtm2 and Ahi~ = 0.92 + 0.12 nm 2. These values correspond to the molecular area of a porphyrin ring of AMP (about 2 nm2), estimated from a Corey-Pauling-Koltun (CPK) molecular model, and to one-half of its value. Ruaudel-Teixier et al. [42] reported a value of 2.2 nm 2 for the molecular area of tetraarylporphyrin derivatives. Thus at surface pressures lower than 30 mNom -1, molecules of AMP are dispersed in mixed monolayers with their rings almost parallel to the water surface. The ~-A isotherms in Figure
252 4 indicated that the structural changes of surface monolayers should occur at about 30 m N , m q either by orientational change of a porphyrin ring from fiat to vertical or by the squeezing out of AMP or AA from mixed monolayers at the a i r / w a t e r interface. 60
'.
,
.
,
'
"',
,
"'
"r~: E " 40L,.
3
~-
:
b
a
cL 2 0 -
O u
t,.
U~ 0
I
0.2
I
0.4 Area
0.6
.
t
0.8
1.0
/ n m l - m o l e c u l e -!
Figure 4 Surface pressure-molecular area (~-A) isotherms for mixtures of AMP and AA at 18~ pH6.3. The molar ratios (AMP:AA) are as follows: (a) 1-.5, (b) 1:10, (c) 1:15, (d) 1".27.2. Z
M
J Figure 5 The incident ~ and polarization angle a dependence of polarized absorption. The y and z axes indicate the dipping direction and the surface normal. The transition dipole M and its xy projection makes # and 0 from the z and y axes, respectively. The angular dependence of the polarized absorption spectra of LB films containing AMP deposited at lower (20 mN*m -1) and higher (43 or 50 m N . m -1) surface pressures was studied to determine the molecular orientation of p o r p h y r i n s as schematically shown in Figure 5. No polarization angle (a) dependence was observed at normal incidence. This indicates that the projections of the transition dipole moments of the porphyrins are statistically
253 distributed in the film plane. If chromophores are homogeneously dispersed in the LB films, more chromophores are excited by increasing the incident angle as far as the width of illuminated LB films is smaller than that of a monitor light. The correction for this cannot be made directly by the incident angle 13 to the substrate because of the refractive index. The absorbance of s-polarized light with electric vector perpendicular to the incident plane should not depend on the incident angle if the same numbers of chromophores are excited. The absorbance of s-polarized light for LB films of a 1:5 mixture of AMP and AA increased with the incident angle in practice [5,10]. Since porphyrins were found to be homogeneously distributed in the film as mentioned above, this dependence was attributed to the increase of optical path with the incident angle. Then, the correction factor cos~' was estimated at each incident angle so as to make the spolarized absorbance constant [5,10]. The estimated values of cosl3' were smaller than those of cosl3 and corresponded with the result reported by Yoneyama et al [25] for phthalocyanine LB films. The dependence of the p-polarized absorption of AMP at 435 nm on the incident angle is shown in Figure 6 ( a ) a n d 6(b)for LB films from a 1:5 mixture deposited at 20 and 50 m N - m 4, respectively. In these figures the observed absorbance and that corrected by cosl3~ are shown by open and filled circles, respectively. Similar angular dependencies were observed for LB films from a 1:27.2 mixture deposited at 20 and 43 mNom 4. The maximum was observed at normal incidence in all LB films. A larger absorbance was observed in LB films deposited at higher surface pressures compared with those deposited at lower surface pressures in both mixtures. The full lines in these figures were calculated by the least-squares method from the corrected data using the Eq. (2) to describe the dependence of the absorbance on the incident angle, A(I3) = constant{ + cos2~ o(1 - 3)} (2)
.30 k) c-
cO .(3 K..
0 o1
0.05. Only gradual quenching was observed for a system with fc= 0.02. Similar results were observed for mixed LB films irradiated in argon [51]. The time dependencies are not expressed by a first-order nor a second-order kinetics, which will be explained below. No effects of the excitation wavelength on the fluorescence quenching behavior were observed.
I"'I0.5
t. 0
,
! 45
I o0
t ime/rain Figure 17 Time dependences of normalized fluorescence intensity at 355 nm for two mixed LBfilms of CUA and PA irradiated at 290 nm and 1.0 mW/cm2 in air at 15 ~ The fc values are 0.02, 0.05, 0.15, 0.25, 030 and 0.40, respectively, from the top. No changes in the absorption spectra of LB films were observed in argon during irradiation for up to 225 h. The decay of fluorescence during irradiation in argon was also found to depend on the temperatures. It became slower at lower temperatures and only 16% decayed at 80~ after 90 min irradiation, which indicated some contribution of thermal process to the fluorescence decay. From these results the fluorescence quenching in argon is most probably due to the efficient energy transfer to the nonradiative sites formed by some changes of aggregation structure of carbazolyl chromophores in LB films. Similar fluorescence quenching without changes of absorption spectra was reported in vacuum deposited films of pyrenecarboxylic acid upon irradiation by a Xe lamp or an excimer laser [59]. It was attributed to the structural changes of aggregates of pyrenyl chromophores to the non-fluorescent aggregate [60]. Meanwhile irradiation of LB films in air caused gradual spectral changes suggesting the photooxidation of carbazolyl chromophores [61]. The absorbance at 290 nm, however, decreased only 4% during irradiation for 90 min in air, where about 80% of fluorescence was quenched as shown in Figure 17. All these results indicated that the fluorescence was quenched with much higher extent than the changes of absorbance at the excitation wavelength. Such amplified fluorescence quenching in mixed LB films of CUA and PA was most probably
264 caused by the very efficient and molar fraction dependent energy transfer to a trace amount of nonradiative sites formed by photooxidation or changes of aggregation structure of carbazolyl chromophores during irradiation. The efficient energy transfer among carbazolyl chromophores occurred by the singlet exciton migration as mentioned above [50]. The mean displacement rh for randomly hopping exdtons corresponding to the number of jumps is 1.9 nm at fc= 0.05 and 20.0 nm at fr 0.27 [50]. Then the fluorescence from CUA molecules located in a circle with a radius of rh will be quenched if one CUA molecule within that circle would become nonradiative by photooxidation or changes of aggregation structure. The dependencies of fluorescence quenching on the irradiation time shown in Figure 17 and on the molar fraction can be explained by the mechanism that carbazolyl chromophores are distributed or aggregated inhomogeneously in mixed LB films and the extent of their aggregation varied with the molar fraction of CUA. The time dependencies of fluorescence quenching in LB films shown in Figure 17 can be understood as the result of dispersive electronic excitation transport, i.e. a set of single exciton hopping processes with different rates depending on the spatial distribution. As a small number of the energy trap sites are formed during irradiation, the rate of exciton transport slows because the density of carbazolyl chromophores available to accept exciton is reduced. The time dependence of such dispersive processes is known to be expressed by I(t) = I0-exp(-kt a) (0< cz oooooooooooo oooooo -
oooroooooo ooo oooooooooooo ooooooooo!ooooooo!,ooooooooo!ouooooo!
Figure 22 Schematic representation of LB films containing AMP deposited at (a) 20 and (b) 50 mN~ at 20 m N . m q. The larger absorbance and polarized absorption spectroscopy of mixed porphyrin LB films prepared at higher surface pressures than 30 m N . m q strongly suggested that porphyrins were squeezed-out and loosely stacked in LB films as mentioned in section 2.2. Then comparing the intermolecular tunneling rate (~hish-1) from a porphyrin pair p to a porphyrin pair q in LB films containing a loosely stacked pair as schematically shown i ! !
I! I o8~ 9
Ipairp~
"p~~
r,a~ f"
~
Figure 23 Schematicrepresentation of the intermolecular tunneling interaction between a pair p to a pair q in LB films containing loosely stacked porphyrins (ellipses).
270 in Figure 23 with that (Zlow-1) for monomolecularly dispersed porphyrins in LB films prepared at lower surface pressure, we obtain ZlowI'rhi~ = exp(l~((dx2+dy2) 1/ 2 _ (dx2+dy2+4_4dycosO+4dxsinO)l / 2)) + ~+Jay--j ~1t2 - (dx~dy2+4+4dycos0.4dxsin0)l/2)) + 2 exp(kb(( d x~
(8)
where rx=bdx and ry=bdy. From the spectroscopic results mentioned above, we can set 0= 0~ [10]. The calculated values of Tlow/Xhir,h for kb= 1.74, based on the damping constant for through-space tunneling (k= 0.58 A-l) and the typical distance (2b) between tetraphenyl-type porphyrin rings (about 6/k), varied depending on dxand dy values [87]. The minimum value of dy is 2 (ry= 6.0~) for porphyrin in adjacent (face-to-face arrangement) hydrophilic regions as schematically shown in Figure 22. The maximum value of dy is 14.3 for porphyrin pairs in hydrophilic regions separated by two arachidate monolayers as schematically shown in Figure 22. The former is expected to predominantly contribute to the observed photocurrents , since the probability of perpendicular intermolecular tunneling decreases exponentially with the distance. The observed value of Zper20/X~erS0 at higher bias voltages is estimated to be 5.5 from Figure 21 and eq. (7) as mentioned above. Although the distance between porphyrins in the parallel direction will be distributed over a wide range, we can estimate the average value of dx to be 5.4 and 6.9 for 1:5 and 1:10 mixtures, respectively, from the observed limiting area at both solid condensed phases and the molar ratio. The calculated values of Zlow/Zhighwith dy = 2 and dx = 57 corresponded well with the observed value of Zper20/X~er50 at higher bias voltages. This result indicates that the average distribution of porphyrins in LB films contributes to the photoconduction at higher bias voltages where intermolecular tunneling of photocarriers occurs rather easily because of a decreased tunnel barrier, as suggested by the Poole - Frenkel theory [27]. Meanwhile, the observed value of X?er20/ZperS0 at lower bias voltages corresponds with the calculated values of Xlow/Xhigh with dy = 2 and dx = 1 - 2. This result strongly suggests that at lower bias voltages only porphyrins located much closer than the average distribution value can contribute to the photoconduction, probably due to a larger intermolecular tunneling time because of a much less decreased barrier height. The present model of intermolecular tunneling is thus supported for photocurrents in LB films deposited from two solid condensed phases with different molar fractions of amphipathic porphyrins which were loosely stacked and monomolecularly dispersed depending on the surface pressure. Thus the supermonomolecular structure of porphyrin pairs in LB films are concluded to greatly facilitate intermolecular photoconduction. 5. PHOTOINDUCED ELECTROCHROMISM IN LB FILMS CONTAINING IONPAIR CHARGE-TRANSFER COMPLEXES Various photochromic systems employing polymeric thin films or LB
271 films have recently attracted much interest in view of their promising applicability to high-speed and high-density photon-mode optical memory. The photochromism reported so far involves changes of chemical bonds such as heterolytic cleavage of a pyran ring in spiropyrans or cis-trans isomerization in azobenzenes. Very recently we have reported novel photochromism (photoinduced electrochromism) in organic solutions [89,90], microcrystals [91,92], LB films [7,9,93,94], and polymer films [95-98] which was due only to the photoinduced electron transfer reaction via the excited state of specific ion-pair charge-transfer (IPCT)complexes [99,100] of 4,4'-bipyridinium salts with tetrakis[3,5-bis(trifluoromethyl)phenyl]borate [101] (abbreviated to TFPB-). The photochemical colouring and the thermal fading due to the reverse electron transfer were highly reversible in deaerated atmosphere in all systems [7,9,89-98]. The lifetime of coloured (blue) state was found to depend markedly on the microenvironments and temperatures. From steady and laser photolysis results it has been strongly suggested that 4,4'-bipyridinium radical cations escaped from the geminate reaction immediately after the photoinduced electron transfer in less than 20 ps [102] upon IPCT excitation became metastable owing to the bulk and chemical stability of TFPB-, to the restriction of molecular motion by the microenvironment, and also probably to the very high exothermicity of the reverse reaction in the Marcus inverted region [103]. Higkly sensitive detection of photoinduced electrochromism in ultra-thin LB and polymer films has also been achieved by the optical waveguide method [104,105]. Such photoinduced electrochromism may be thus applied to ultrafast photon-mode optical memory and to redox sensors. In this section photoinduced electrochromism and molecular control of orientation of photogenerated radicals in LB films will be discussed. 5.1 Steady photolysis and control of molecular orientation of radicals in LB films Tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (TFPB-) salts of N,N'dihexadecyl-4,4'-bipyridinium (HV2+) and N-ethyl-N'-(2-ethylamide)-N",N"dihexadecyl-4,4'-bipyridinium (AV2+) were prepared from corresponding bromide salts and Na§ -. The structures of amphipathic 4,4'-bipyridinium ions are shown in Figure 1 together with that of TFPB-. Monolayer properties of several mixtures of AA with TFPB- salts of HV 2+ or AV2+ were studied on an aqueous subphase containing 0.25 mM CdC12 and 0.05 mM NaHCO3 pH 6.3 at 18~ LB films were deposited at 20 m N . m -1 and 18~ on a quartz plate for UV/vis or on a poly(ethyleneterephthalate) film for ESR measurements from 1:1 and 4:1 mixtures of AA and HV 2§ or AV2+. The deposition ratio was almost unity during 30 deposition cycles for all mixed monolayers. For steady photolysis these samples were irradiated in degassed condition by a Hamamatsu 150 W XeHg lamp equipped with a Toshiba L-39 cut off filter (~ex> 365 nm) and a 10 cm water filter to excite their IPCT absorption band alone. The incident angle dependencies of both s- and p-polarized absorption for 4,4'-bipyridinium radical cations were measured in degassed condition together with the polarization angle dependence at normal incidence.
272 The ~-A isotherms are shown in Figure 24 for three mixtures of AV 2+ and AA. The ~-A isotherms exhibited several transitions. A similar ~-A isotherm was observed for a mixture of HV 2+ ( 18.8 ~ ) and AA [94,95]. The apparent limiting area observed at each transition for mixtures of A V 2+ and AA corresponded well with the calculated values based on the molecular area of TFPB- (1.4 nm 2) and 4,4'-bipyridinium ion (0.82 nm 2) for a stepwise squeezingout of 4,4'-bipyridinium ion and TFPB-, which does not dissolve in water, as schematically shown in the inset of Figure 24. From an X-ray analysis on a single crystal of N,N'-dimethyl-4,4'-bipyridinium tetraphenylborate (TPB-), Moody et al. [106] reported that the 4,4'-bipyridinium ion was sandwiched between two TPBions. Ion-pair charge-transfer (IPCT) complexes of TFPB- salts were expected to have similar configuration from several spectroscopic data [89,92,107]. Such a structure of IPCT complexes corresponds well to that schematically shown in the inset (A) of Figure 24 based on the limiting area. AV 2+ and AA systems showed larger molecular areas than HV 2+ and AA systems in all corresponding mixtures. This result may reflect the different orientation of 4,4'-bipyridinium ions as mentioned below. ~
m
=40 Q.
20
b.
l.n
~
9
0".2
"
0.4 0.6 0.8 Molecular area l n m 2
1.0
Figure 24 The a-A isotherms for mixtures of AA with AV2+ by a molar fraction of (a) 0.10, (b) 0.188, and (c) 0.50 at pH 6.3 and 18~ The inset shows the schematic representation of surface monolayers during compression processes (C)-> (B) -> (A). The circle and rectangle in the inset represent TFPB- and 4,4'-bipyridinium group of AV2+, respectively. Upon irradiation of an IPCT band in degassed condition (Xe• 365 nm), the colour of both LB films changed from pale yellow to blue. The UV/vis absorption spectrum after irradiation is shown in Figure 25, which is characteristic of 4,4'-bipyridinium radical cation monomer[108]. Coloured species photogenerated in mixed LB films of AV2+/AA or HV2+/AA systems decayed almost exponentially in the dark in vacuo with a lifetime of about 4 h at 20 ~ [93,94]. The lifetime of 4,4'-bipyridinium radical cations in LB films was almost
273
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Figure 25 Absorptionspectrum of mixed LBfilms (60x2) of HV2+ and AA (1:4) after excitation (>365 nm) in degassed condition at 20~ for 10 min using non-irradiated LBfilms ~ a reference. the same as that in microcrystalline films [91], which indicated the microenvironment around photogenerated radical cations in LB films is similar to that in microcrystals. Such photochemical colouring and thermal fading was repeated reversibly. A broad single line ESR spectmnn was observed upon irradiation of both LB films. In the HV2+/AA system it showed little anisotropy upon rotation around the dipping direction. In the AV2+/AA system, the spectral width (Amsl= 1.42 mT) for the magnetic field parallel to the film plane was larger by about 6% than that (1.33 roT) of the magnetic field perpendicular [9]. Polarized absorption spectra of photogenerated 4,4'-bipyridinium radical cations were measured in vacuo for LB films of HV2+/AA and AV2+/AA systems as a function of polarization angle and incident angle. The thermal decay of radicals during measurements of polarized absorption spectra was corrected by their lifetime [7,9,93,94]. The different optical path length in the incident angle dependence measurements was also corrected from an apparent incident angle dependence of s-polarized absorption in a similar way mentioned above for amphipathic porphyrin [5,10]. No polarization angle dependencies were observed at normal incidence in both LB films. The p-polarized absorption of 4,4'-bipyridinium radical cations at 400 nm, which corresponds to the shortaxis transition, are shown in Figure 26 for (a) HV2+(18.9 %)/AA and (b) A V2+(18.9 %)/AA. Figure 26 shows a minimum absorbance in HV2+/AA and a maximum in AV2+/AA at normal incidence. The solid lines in Figure 26 are calculated by the least square method taking the angle (~) distribution of the transition dipole moments with respect to the surface normal into account. The best fit curves gave following value of ~; 45 ~ < ~ < 46 ~ for HV2+ / A A and 89 ~ < ~ < 90 ~ for A V2+/AA systems, respectively. Similar incident angle dependencies were observed at 614 nm which is due to a long-axis transition of 4,4'-bipyridinium radical cations. From these results and the simulation of angular dependencies,
274
it was shown that both the long and short axes of 4,4'-bipyridinium radical cations lay almost fiat in LB films of AV2+/AA and inclined by about 46 ~ to the substrate surface in LB films of HV2+/AA as schematically shown in Figure 27.
-'
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',
. . . . . . .
,
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,
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,
~' .20 u r
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365 nm) as shown schematically in Figure 28. A linearly polarized He-Ne laser (632.8 nm) was used as a monitor light. A 150-fold sensitivity of the OWG method as compared with the conventional method was demonstrated from the colour change measurement in about 180 nm thick film of pV2+CFFPB-)2 containing 4,4'bipyridinium groups as part of the main chain as shown in Figure 1. The absorbances calculated from the OWG signal, using that before irradiation as a reference, are plotted in Figure 29 against the irradiation time for pV2+Cl~FPB-)2 thin films of various thickness; (a) 10.0, (b) 40.4, (c) 64.9, (d) 95.5, and (e)179.6nm. It is clearly seen that the number of photogenerated 4,4'bipyridinium radical cations increased linearly with irradiation time. The rate of absorbance changes was proportional to the film thickness in the range studied. |o
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1000
2000
300U
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Time / s
Figure 28 Schematic representation of the OWG system for detecting photoinduced electrochromism of ultrathin films (S) in the evacuation chamber shown in an inset.
Figure 29 Changes in the OWG absorbance of pV2+(TFPB-)2 thin films of various thickness during IPC~ excitation: (a) 10.0, (b) 40.4, (c) 64.9, (d) 95.5, and (e) 179.6 nm.
276 These results strongly suggested that pV2+(TFPB-)2 thin films of various thickness are homogeneous and that 4,4'-bipyridinium groups are distributed randomly throughout the polymer films. Photoinduced colour change in a single-monolayer LB film was successfully detected as shown in Figure 30(a) for the HV 2+ /AA system [105]. Comparison of this result with Figure 25 also demonstrated more than 120fold sensitivity of the OWG method. Changes of OWG absorbance are also shown for Y-type LB films deposited on glass slides covered with three monolayers of cadmium arachidate; (b) 2, (c) 4, and (d) 6 monolayers. The absorbance changes increased with the number of monolayers deposited. In contrast with the almost linear increase in absorbance of polymer fiJms as shown in Figure 29, the absorbances in LB films tended to saturate at longer irradiation times. The "saturated" absorbances increased almost proportional with the number of monolayers. Similar results were obtained for LB films of AV2+/AA systems. In LB films the 4,4'-bipyridinium ions are not distributed randomly but are confined to and aligned in a layer of a few tmgstr6ms thick periodically distributed in the direction of the surface normal. The long spacing of LB films of HV2+/AA was evaluated as 55 7~ by small angle X-ray scattering [93,95]. Such structural properties and much smaller thickness of LB films most probably contributed to the "saturation" tendency shown in Figure 30. It should be possible in principle to determine the orientation of chromophores in a single monolayer on an OWG by the absorption of transverse electric (TE, s-polarized) and transverse magnetic (TM, p-polarized) modes laser. Swalen et al. [109] reported that much stronger absorption was observed for a thin evaporated film of 4-dimethylamino-4'-nitrostilbene with the TM mode and for seven monolayers of cyanine dyes with the TE mode. These results corresponded .4
e o n~ J:3 o ul J3
.3 .2
1200
2400
3600
Time / s Figure 30 Changes in the OWG absorbance of LB films of HV2+/AA (1:4) with various numbers of monolayers during IPCT excitation in degassed condition: (a) 1, (b) 2, (c) 4, and (d) 6 monolayers.
277 with the predicted molecular orientation of the two dyes, perpendicular and parallel to the substrate surface, respectively [109]. 4,4'-Bipyridinium radical cations photogenerated in polymer thin films showed the same absorbance for both TE and TM modes. This result corresponded with the random orientation of radical cations, which is consistent with the result from the time dependence of photogeneration mentioned above that 4,4'-bipyridinium groups are randomly distributed throughout the polymer films. In LB films of HV2+/AA with 1-6 monolayers the OWG signals after photoexcitation displayed anisotropic absorption for TE and TM modes. Both the subsituents of the 4,4'-bipyridinium ions and the nature of the substrate surface were found to affect the anisotropic absorption in LB films by the OWG method. It is thus strongly suggested that photogenerated 4,4'-bipyridinium radical cations show specific orientation even in a single-monolayer LB films controlled at the molecular level as found in 120 monolayers LB films by a conventional method [7,9]. 6. PHOTOCHEMICAL MODULATION OF SECOND HARMONIC GENERATION IN LB FILMS CONTAINING METAL COMPLEXES Organic compounds with highly polarizable ~-electron systems have recently attracted much interest in view of their much larger optical nonlinearity compared with inorganic materials which are in practical use. The LB technique is suited to prepare a system without inversion symmetry which is another essential prerequisite to achieve second order nonlinear responses. Organic molecules with donor and acceptor groups at each end of molecular struct~es showing an intramolecular charge-transfer character have been extensively studied in search of large second order nonlinearity. Metal complexes showing a metal-to-ligand charge-transfer are also expected to have large hyperpolarizability, but only a few reports have been made on second order nonlinearity from them [110,111]. We have reported that ruthenium(II)-bipyridine complexes are capable of second harmonic generation (SHG) in alternate LB films and also reported that the SHG can be optically modulated in such LB films by an additional UV laser [112-116]. This is a sort of optical switching based on the changes of second order optical nonlinearity at the ground and excited states. The development of optical phenomena and materials to control light by light is very important for making ultrafast optical switches and phototransistors. Photobistability using third-order optical nonlinearity is one of the most promising phenomena for them, and many studies have been made. The basic concept of photobistability is to combine the photoinduced third-order refractive index change with fight feedback in the cavity. Transmitted light intensity from the cavity is controlled by input light intensity and an ultrafast optical switching is expected by this method if materials with sufficiently high this nonlinear optical coefficient can be developed. Our approach combining second order optical nonlinearity, the efficiency of which is much higher than the third-order one, with photoexcitation will give a new means for controlling light by light. The n-A isotherms for several mixtures of N,N'-dioctadecyl-4,4'-
278 dicarboxamide-2,2'-bipyridine)-bis(2,2'-bipyridine)-ruthenium(II) perchlorate (RuC18B) and dioctadecyldimethylammonium bromide (2C18NB) as shown in Figure I were observed at 14- 25 ~ on an aqueous subphase containing 0.25 mM dextranesulfate polyanion (Dex). A monolayer of a 1:4 mixture of RuC18B and 2C18NB was deposited alternately with that of 2C18NB alone at 25 ~ and 20 m N . m q with a deposition ratio of unity. Thirty-six alternate Y-type LB films were deposited on each side of a glass slide treated with a silane coupling agent. The second harmonic signal from LB films irradiated with an Nd:YAG laser at 1064 nm (100 mJ cm -2, 10 ns) was detected with a photomultiplier through an aqueous CuSO4 solution, two IR-cut filters and monochromator. In some cases LB films were excited by the third harmonic (355 nm, 0.1 - 10 mJ cm -2) of an Nd:YAG laser 10 ns before irradiation of a 1064 nm pulse. In the subpicosecond time-resolved measurement of optical modulation of SHG, 590 nm laser pulse having a temporal width of 2 ps was used as SHG probe pulse from LB films. UV laser pulses at 378 nm, 1 ~tJ, were used for the pump pulses that were generated by sum-frequency mixing of 590 nm and Nd:YLF 1053 nm laser pulses passing through the rhodium dihydrogen phosphate (RDP) crystal. The SH intensity from the LB film was detected while changing the delay time between the pump and probe pulses. No signal at 532 nm upon irradiation with a 1064 nm pulse was observed for usual Y-type LB films, whereas a strong signal with the same temporal width as an incident laser pulse was detected from alternate LB films. From these results, the latter was attributed to SHG from RuC18B. The intensity of second harmonic light (SHL) increased with increasing incident angle. A periodic fringe pattern was observed for LB films deposited on both sides of the substrate, while the intensity increased monotonously in LB films deposited on one side [112]. The absorbance at 480 nm of LB films containing RuC18B, which is attributed to the metal-to-ligand charge-transfer (MLCF) transition to the 2,2'-bipyridine moiety with two amide groups [117], increased with the incident angle for ppolarized light [112]. The observed SHG is thus most probably due to the MLCr transition to the 2,2'-bipyridine containing two amide groups [117]. The intensity of SHL observed from alternate LB films decreased upon excitation at 355 nm before the irradiation of a 1064 nm pulse. The SHG was varied reversibly many times as shown in Figure 31 without ( 0 ) and with (O) 355 nm laser excitation [113,114]. The extent of the decrease in SHG increased with the intensity of the UV laser, about 50% decrease at 10 mJ cm -2. The decrease of SHG upon UV excitation may be caused by various reasons such as the changes in the orientation of chromophores, the phase matching condition, and the refractive indices, by the thermal lens effect, or by the possible absorption of 532 or 1064 nm light at the excited state. Detailed investigation were made on them. The transient absorption of RuC18B upon excitation at 355 nm was resolved into absorption bands of the bipyridine anion radical and Ru 3+, the depletion of the ground state MLCT band, and luminescence above 600 nm [113]. These results clearly indicated the formation of a charge-separated excited state of metal complex [118,119]. Only very weak absorption was observed at 532 nm upon UV laser excitation, which was too weak to explain the observed changes in SHL. No
279
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O.5 355 nm on
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Times of excitation Figure 31 The intensity of SHL at 532 nm from alternate LB films of RuC18B and 2C18NB (O) with and (0) without excitationby a 355 nm pulse as a function of the number of irradiation of a 1064 nm pulse. absorption was detected in LB films at 1064 nm upon excitation at 355 nm. The possibility of the change in refractive indices for both 1064 and 532 nm light can also be excluded from these results. The change of orientation of chromophores and the thermal lens effect were excluded from experiments done at exactly the same conditions except the wavelength of an exciting laser (460 nm, 10 ns, 10 mJ cm -2) using alternate LB films containing 1-methyl-4-(4'-(N-octadecyl-Nmethylamino)styryl)pyridinium iodide (C18STZ). The intensity of SHL did not change in C18STZ LB films upon excitation with a 460 nm laser, while the excitation with a 355 nm laser at the same power density caused about 50% decrease in the intensity of SHL in RuC18B LB films. It was most probably due to the fact that the lifetime of the excited state is much longer in RuC18B (about 600 ns) than that of C18STZ (< 1 ns). The thickness of RuC18B LB films was confirmed to be much smaller than the coherent length, which led to the pseudophase matching. From these experimental results, we have excluded the abovementioned possibilities for the cause of the observed decrease in SHG. The observed photochemical modulation of SHG is most probably attributed to the changes of molecular hyperpolarizability upon formation of excited states, which is confirmed below by time-resolved measurements. From comparison with the SHG of hemicyanine and the UV laser power dependence we roughly estimated the molecular hyperpolarizability (~) at the ground and the excited state to be 70 x 10-30 esu and 36 x 10-30 esu, respectively [114]. The dynamics of the SHL intensity after subpicosecond UV laser excitation of RuC18B LB films is shown in Figure 321115,116]. The SHL intensity decreased to 70 % of its initial value upon excitation and returned to almost the initial value within several hundred picoseconds as shown by a bold line. The fluorescence decay of RuC18B LB films measured by the single photon-counting
280
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. . . .
.
.
.
.
.
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:
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(A)
i
i
.
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-800
:
:
:
-400
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400
800
Delay time I ps
Figure 32 Temporal profile of relative SHL intensity from alternate LB films of RuC18B and 2C18NB with subpicosecond laser pulses at 378 nm: bold curve (A) experimental data, fine curve 03) calculated SHL intensity based on the excited-state lifetime of RuC18B in LB films. method was best fitted with three components of 178 ps, 3.9 ns and 63 ns. This result suggested that at least three kinds of molecular aggregates for RuC18B exist in the LB films due to the high concentration of chromophores. The fine line in Figure 32 shows the predicted time dependence of relative SH intensity resulting from the excited-state decay described by the following equations. The deviation of these equations is based on a kinetic analysis similar to that in ref. 120: ISH(eX)/IsH(0) = (1 + CF(t)) 2 where C = (r- 1)(1 - exp(-oD~))
and
F(t)= IF(t) / I~0)
(9) (10)
IsH(ex) and ISH(0) indicate the SH intensity with and without UV pump laser pulses, r = / is the ratio of orientationaUy averaged molecular hyperpolarizabilities of RuC18B in the excited and ground states, o is the absorption cross section, and Dwr is the total photon dose from the UV laser pulse delivered within a volume of the probe pulse. F(t) is the temporal profile of the excited-state concentration defined by Eq. (10), where IF(t) is the fluorescence intensity at time t after the UV pump pulse. These equations are applicable in the case where there are no decay processes faster than the laser pulse width. The calculated curve using experimentally measured F(t) agrees well with the experimental time dependence of the SH intensity. The best fit was obtained for C=-0.16. This result suggested that the change of SH intensity with UV laser excitation was caused by the change of molecular hyperpolarizability between the ground and the excited states. The negative value of C indicated that is smaller than or they are opposite sign. The former corresponds with the rough estimate of ~ by ns laser experiments as mentioned above. The
281 intensity of subpicosecond UV laser for modulating SHG is much higher than that for the fluorescence measurement, which may cause ultrafast processes such as $1-$1 annihilation within the pulse width of UV laser. The ultrafast dynamics will be necessary to accurately determine the C-value. The temporal profile of SHL intensity was found to change with time resolution of the system (about 2 ps), which dearly demonstrated the ultrafast optical modulation of SHG. In principle optical amplification of SHG will be also possible using the photoexcited state as predicted by calculations [121]. 7. CONCLUSIONS We have shown that redox chromophores organized in LB films with respect to their orientation, alignment, or electronic interactions make very useful and specific photoresponses such as amplified fluorescence quenching, photocurrents controlled at the molecular level photoinduced anisotropic electrochromism, and photochemically modulated second harmonic generation. These results may contribute to facilitate the design and construction of novel photonic devices in the near future.
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