Surface Charging and Points of Zero Charge (Surfactant Science)

CRC Press Taylor & Francis Group 6000 Broken Sound Parkway NW, Suite 300 Boca Raton, FL 33487-2742 © 2009 by Taylor and Francis Group, LLC CRC Press is an imprint of Taylor & Francis Group, an Informa business No claim to original U.S. Government works Printed in the United States of America on acid-free paper 10 9 8 7 6 5 4 3 2 1 International Standard Book Number: 978-1-4200-5188-9 (Hardback) This book contains information obtained from authentic and highly regarded sources. Reasonable efforts have been made to publish reliable data and information, but the author and publisher cannot assume responsibility for the validity of all materials or the consequences of their use. The authors and publishers have attempted to trace the copyright holders of all material reproduced in this publication and apologize to copyright holders if permission to publish in this form has not been obtained. If any copyright material has not been acknowledged please write and let us know so we may rectify in any future reprint. Except as permitted under U.S. Copyright Law, no part of this book may be reprinted, reproduced, transmitted, or utilized in any form by any electronic, mechanical, or other means, now known or hereafter invented, including photocopying, microfilming, and recording, or in any information storage or retrieval system, without written permission from the publishers. For permission to photocopy or use material electronically from this work, please access www.copyright. com (http://www.copyright.com/) or contact the Copyright Clearance Center, Inc. (CCC), 222 Rosewood Drive, Danvers, MA 01923, 978-750-8400. CCC is a not-for-profit organization that provides licenses and registration for a variety of users. For organizations that have been granted a photocopy license by the CCC, a separate system of payment has been arranged. Trademark Notice: Product or corporate names may be trademarks or registered trademarks, and are used only for identification and explanation without intent to infringe. Library of Congress Cataloging-in-Publication Data Kosmulski, Marek, 1956Surface charging and points of zero charge / Marek Kosmulski. p. cm. -- (Surfactant science series ; 145) Includes bibliographical references and index. ISBN 978-1-4200-5188-9 (hard back : alk. paper) 1. Points of zero charge. 2. Surface energy. 3. Volumetric analysis. I. Title. II. Series. QD571.K787 2009 541’. 335 --dc22 Visit the Taylor & Francis Web site at http://www.taylorandfrancis.com and the CRC Press Web site at http://www.crcpress.com

2009012179

To my wife

Contents Preface ......................................................................................................... xxiii Acknowledgments ..................................................................................... xxvii Chapter 1

Introduction ................................................................................. 1 1.1 1.2 1.3 1.4 1.5

Nomenclature .................................................................... 8 Scope ............................................................................... 10 Inert Electrolytes ............................................................. 12 The Significance of Parks’ Review ................................. 15 Structure of Adsorbents .................................................. 17 1.5.1 Alumina ........................................................... 17 1.5.2 Iron (Hydr)oxides ............................................. 19 1.5.3 Magnanese Oxides ........................................... 20 1.5.4 Silica ................................................................ 20 1.5.5 Titania .............................................................. 20 1.5.6 Clay Minerals .................................................. 20 1.5.7 Nitrides ............................................................ 21 1.6 Solubility ......................................................................... 21 1.6.1 Simple (Hydr)oxides ........................................ 21 1.6.2 Other Materials ................................................ 23 1.7 Solid Phase Transformation at Room Temperature in Contact with Solution ................................................. 24 1.7.1 Alumina ........................................................... 25 1.7.2 CdO .................................................................. 25 1.7.3 CuO .................................................................. 25 1.7.4 Iron (Hydr)oxides ............................................. 25 1.7.5 Other Systems .................................................. 25 1.8 Solid Phase Transformation on Heating ......................... 26 1.9 Kinetics ........................................................................... 26 1.9.1 Proton Adsorption ............................................ 27 1.9.2 Isotope Exchange ............................................. 28 1.9.3 Dissolution ....................................................... 29 1.10 Solution Chemistry—pH Scale ....................................... 30 1.10.1 Problem 1: Concentration versus Activity ....... 32 1.10.2 Problem 2: Experiments at Constant Ionic Strengths ................................................. 32 xiii

xiv

Contents

1.10.3 Problem 3: Buffered versus Unbuffered System ... 1.10.4 Problem 4: Sodium Effect ................................ 1.10.5 Problem 5: Suspension Effect .......................... 1.10.6 Problem 6: Different pH Scales ....................... 1.10.7 Problem 7: Electrolysis .................................... 1.11 Very Dilute Solutions ...................................................... 1.12 Speciation in Solution ..................................................... Chapter 2

33 33 33 34 34 34 36

Methods ..................................................................................... 39 2.1

2.2 2.3

2.4

2.5

2.6

Experimental Setup in Electrokinetic Measurements ...... 2.1.1 Electrophoresis ................................................ 2.1.2 Electro-Osmosis ............................................... 2.1.3 Streaming Potential ......................................... 2.1.4 Sedimentation Potential ................................... 2.1.5 Electroacoustic Methods .................................. Experimental Conditions in Electrokinetic Measurements ................................................................. CO2 and Silica Problem .................................................. 2.3.1 The CO2 Problem ............................................. 2.3.2 The Silica Problem .......................................... Experimental Results: z Potential ................................... 2.4.1 Shapes of Individual Electrokinetic Curves .............................................................. 2.4.2 Position of IEP ................................................. 2.4.3 Aging and Hysteresis ....................................... 2.4.4 Effect of Ionic Strength on the Numerical Value of the z Potential .................................... 2.4.5 Effect of the Nature of the Salt on the Numerical Values of the z Potential ................ Experimental Conditions: Titration ................................ 2.5.1 The Choice of an Inert Electrolyte and the Range of Ionic Strengths ........................... 2.5.2 Solid-to-Liquid Ratio ....................................... 2.5.3 Other Titration Parameters .............................. Results: Titration ............................................................. 2.6.1 Presence or Absence of CIP ............................ 2.6.2 Reproducibility and Reversibility .................... 2.6.3 Shape of Charging Curves and Typical Values of s0 ...................................................... 2.6.4 Effect of Ionic Strength on Charging Curves .............................................................. 2.6.5 Effect of the Nature of the Salt on Numerical Values of s0 .................................... 2.6.6 Surface Charging of Materials Other than Metal Oxides ...........................................

41 41 46 47 48 48 51 55 55 57 59 60 61 61 62 65 66 71 72 72 74 74 76 77 79 79 80

xv

Contents

2.7 2.8

2.9

Chapter 3

Relation of Results Obtained by Different Methods .......................................................................... Other Methods ................................................................ 2.8.1 Methods Involving Nonaqueous Solvents ....... 2.8.2 Electrical Methods ........................................... 2.8.3 Sum Frequency Generation and SecondHarmonic Generation ...................................... 2.8.4 Methods Equivalent to Titration ...................... 2.8.5 Force between Particles ................................... 2.8.6 Nonstandard Methods ...................................... Adsorption Models .......................................................... 2.9.1 Density of Protonable Surface Groups ............ 2.9.2 Electrostatic Models ........................................ 2.9.3 Surface Acidity ................................................

80 81 81 81 82 82 87 88 89 89 92 96

Compilation of PZCs/IEPs ....................................................... 101 3.1

Simple Oxides ................................................................ 3.1.1 Aluminum (Hydr)oxides ................................. 3.1.2 Beryllium (Hydr)oxides ................................. 3.1.3 Bi2O3 .............................................................. 3.1.4 Ca(OH)2 ......................................................... 3.1.5 Cadmium (Hydr)oxides ................................. 3.1.6 Cerium (Hydr)oxides ..................................... 3.1.7 Cobalt (Hydr)oxides ....................................... 3.1.8 Chromium (Hydr)oxides ................................ 3.1.9 Copper (Hydr)oxides ....................................... 3.1.10 Dy2O3 ............................................................. 3.1.11 Er2O3 .............................................................. 3.1.12 Iron (Hydr)oxides ........................................... 3.1.13 GeO2 .............................................................. 3.1.14 Ga2O3 ............................................................. 3.1.15 HfO2 ............................................................... 3.1.16 HgO ................................................................ 3.1.17 Indium (Hydr)oxides ...................................... 3.1.18 IrO2 ................................................................ 3.1.19 Hydroxides of Lanthanides ............................ 3.1.20 La2O3 .............................................................. 3.1.21 Magnesium (Hydr)oxides .............................. 3.1.22 Manganese (Hydr)oxides ............................... 3.1.23 Niobium (Hydr)oxides ................................... 3.1.24 Neodymium (Hydr)oxides ............................. 3.1.25 Nickel (Hydr)oxides ....................................... 3.1.26 Lead (Hydr)oxides ......................................... 3.1.27 PdO ................................................................ 3.1.28 Praseodymium (Hydr)oxides .........................

101 101 193 194 195 195 197 202 207 216 221 221 221 321 321 322 323 323 326 327 328 329 333 353 356 357 364 367 367

xvi

Contents

3.2

3.1.29 PtO2 ................................................................ 3.1.30 PuO2 ............................................................... 3.1.31 Ruthenium (Hydr)oxides ................................ 3.1.32 Sb2O5 .............................................................. 3.1.33 Sc2O3 .............................................................. 3.1.34 Samarium (Hydr)oxides ................................. 3.1.35 Silica .............................................................. 3.1.36 Tin (Hydr)oxides ............................................. 3.1.37 Tantalum (Hydr)oxides .................................. 3.1.38 Thorium (Hydr)oxides ................................... 3.1.39 Titanium (Hydr)oxides ................................... 3.1.40 Tl2O3 .............................................................. 3.1.41 Uranium (Hydr)oxides ................................... 3.1.42 Vanadium (Hydr)oxides ................................. 3.1.43 Tungsten (Hydr)oxides ................................... 3.1.44 Y2O3 ............................................................... 3.1.45 Yb2O3 .............................................................. 3.1.46 Zinc (Hydr)oxides ........................................... 3.1.47 Zirconium (Hydr)oxides ................................ Aluminosilicates, Phyllosilicates, Clays, and Clay Minerals ................................................................ 3.2.1 Adularia ......................................................... 3.2.2 Amelia Albite from Wards ............................ 3.2.3 (Ca,Fe)2(Ln,Al,Fe)3Si3O12OH, Allanite (orthite) from Kabuland, Norway .................. 3.2.4 Amphiboles .................................................... 3.2.5 Andalusite ...................................................... 3.2.6 Andesine ........................................................ 3.2.7 Anorthite ........................................................ 3.2.8 Anorthoclase .................................................. 3.2.9 Anthophyllite ................................................. 3.2.10 Augite, (Al,Ca,Fe,Mg,Ti)2(Al,Si)2O6 .............. 3.2.11 Beidellite, SBCa-1 .......................................... 3.2.12 Bentonite ........................................................ 3.2.13 Be3Al2Si6O18 Beryl from Hoggar, Algeria ..... 3.2.14 Biotite K(Mg,Fe,Mn)3(OH,F)2 (Al,Fe,Ti)Si3O10 .............................................. 3.2.15 Blazer from Huber Na2O · Al2O3 · 2.8 SiO2 · 7 H2O .............................................. 3.2.16 Bronzite from Kraubath .................................. 3.2.17 Bytownite ........................................................ 3.2.18 Chlorite (Mg,Al,Fe)12(Al,Si)8O20(OH)16 .......... 3.2.19 Cleavelandite .................................................. 3.2.20 Clinochlore ....................................................

368 369 369 373 373 373 374 431 440 443 445 502 503 506 507 508 513 514 524 548 548 548 548 548 549 549 549 549 549 549 550 550 550 550 551 551 551 551 552 552

xvii

Contents

3.2.21 3.2.22 3.2.23 3.2.24 3.2.25 3.2.26 3.2.27 3.2.28 3.2.29 3.2.30 3.2.31 3.2.32 3.2.33 3.2.34 3.2.35 3.2.36 3.2.37 3.2.38 3.2.39 3.2.40 3.2.41 3.2.42 3.2.43 3.2.44 3.2.45 3.2.46 3.2.47 3.2.48 3.2.49 3.2.50 3.2.51 3.2.52 3.2.53 3.2.54 3.2.55

Clinoptilolite, Zeolite, Unit Cell: Na6(AlO2)6(SiO2)30 · 24H2O ........................... Clinozoisite from Kirchham .......................... Cordierite 2MgO · 2Al2O3 · 5SiO2 ................. Ca2(Fe,Al)Al2[O/OH/SiO4/Si2O7] Epidote from Knappenwand .......................... Feldspar .......................................................... Garnets ........................................................... Halloysite-7Å ................................................. Hornblende ..................................................... Illite ................................................................ Kaolinite and Kaolin Si2Al2O5(OH)4 ............. Labradorite ..................................................... Laponite Na0.8Mg5.4Li0.4Si8O20(OH)4 from Laporte .................................................. Mica ............................................................... Microcline ...................................................... Montmorillonite ............................................. Montmorillonite–Alumina Composite .......... Mordenite (Synthetic Zeolite) from Huber NaAlSi5O12 · 3H2O ......................................... Muscovite ....................................................... Na3K(AlSiO4)4 Nephelin from Skudesundskjaer ............................................ Oligoclase ...................................................... Olivine from Dreis ......................................... Orthoclase ...................................................... Palygorskite (Mg,Al)2Si4O10(OH) · 4(H2O) from Tunisia ................................................... Perlite from Cumaovasi, Turkey (or from Izmir) ............................................... Pyrophyllite Al2(OH)2Si4O10 .......................... Rhomboporphyr ............................................. Ripidolite ....................................................... Sanidine ......................................................... Saponite ......................................................... Sapphirine ...................................................... Serpentine ...................................................... Smectite ......................................................... Tremolite ........................................................ Turmaline (drawite) NaMg3Al6B3Si6O27 (OH,F)4 .......................................................... Vermiculite from Clay Minerals Society Repository ......................................................

552 553 553 553 554 554 555 555 555 559 572 572 572 574 575 583 584 584 584 584 585 585 585 585 586 586 586 587 587 587 587 588 588 589 589

xviii

Contents

3.3

3.4

3.2.56 Vesuvian from Solberg .................................. 3.2.57 Zeolites .......................................................... 3.2.58 Zinnwaldite ................................................... Mixed Oxides ................................................................ 3.3.1 Materials Containing Aluminum .................. 3.3.2 Bi–Th Mixed Oxides ...................................... 3.3.3 Materials Containing Ce ................................. 3.3.4 Materials Containing Co ................................ 3.3.5 Materials Containing Cr ................................. 3.3.6 Materials Containing Fe ................................. 3.3.7 In–Sn Mixed Oxides ...................................... 3.3.8 Mixed Oxides Containing Mg ....................... 3.3.9 Material Containing Nb ................................. 3.3.10 Materials Containing Ni ................................ 3.3.11 Materials Containing Pb ................................ 3.3.12 Materials Containing Ru ............................... 3.3.13 Silicates .......................................................... 3.3.14 Materials Containing SnO2 ............................ 3.3.15 Materials Containing TiO2 ............................ 3.3.16 Materials Containing WO3 ............................ 3.3.17 Materials Containing Zn ................................ 3.3.18 Materials Containing Zirconia ...................... Salts ............................................................................... 3.4.1 Aluminates and Haloaluminates ................... 3.4.2 Borides and Borates ....................................... 3.4.3 Carbides, Carbonates, and Salts of Organic Acids ................................................ 3.4.4 Chlorides ........................................................ 3.4.5 Chromates ...................................................... 3.4.6 LiCoO2 ........................................................... 3.4.7 Fluorides ........................................................ 3.4.8 Ba Ferrite from Aldrich ................................. 3.4.9 AgI ................................................................. 3.4.10 Manganates .................................................... 3.4.11 Molybdates ..................................................... 3.4.12 Sr1-xNbO3-d .................................................... 3.4.13 Nitrides .......................................................... 3.4.14 Niobates ......................................................... 3.4.15 Phosphates and Apatites ................................ 3.4.16 Silicates .......................................................... 3.4.17 Sulfides and Sulfates ...................................... 3.4.18 Titanates ......................................................... 3.4.19 Tungstates and Tungstophosphates ................ 3.4.20 BaZrO3 ...........................................................

590 590 591 591 592 610 611 611 612 613 624 625 626 627 628 628 630 647 648 654 655 656 665 665 666 666 696 697 698 698 701 701 701 707 708 709 720 720 739 740 769 774 775

xix

Contents

3.5

3.6

3.7

3.8

3.9

Glasses .......................................................................... 3.5.1 Commercial ................................................... 3.5.2 Other .............................................................. Carbon and Carbon-Rich Materials .............................. 3.6.1 Diamond ........................................................ 3.6.2 Graphite ......................................................... 3.6.3 Fullerene C60 .................................................. 3.6.4 Carbon Black, Activated Carbons, and Related Products ..................................... 3.6.5 Activated Carbon Cloths and Fibers .............. 3.6.6 Composite Material ........................................ Other Inorganic Materials .............................................. 3.7.1 Silicon, >99.6% HQ Silgrain from Elkem Materials .............................................. 3.7.2 Sulfur .............................................................. 3.7.3 Ice .................................................................. 3.7.4 D2O Ice .......................................................... 3.7.5 Gas Bubbles ................................................... 3.7.6 Natural Inorganic Materials .......................... Coatings ......................................................................... 3.8.1 Alumina Coatings ........................................... 3.8.2 Hydrous Chromia on Hematite ...................... 3.8.3 Co Oxide on Stober Silica ............................. 3.8.4 Iron (Hydr)oxide Coatings ............................. 3.8.5 Germania on Silica ........................................ 3.8.6 IrO2 on Stober Silica ...................................... 3.8.7 Mn Compounds on Hematite ......................... 3.8.8 Nickel (Hydr)oxide Coatings ......................... 3.8.9 RuO2 on Silica ................................................ 3.8.10 Silica Coatings ............................................... 3.8.11 Sn(OH)4 on Hematite ..................................... 3.8.12 Titania Coatings ............................................. 3.8.13 Yttria on Hematite ......................................... 3.8.14 Zr (Hydr)oxide Coatings ................................ 3.8.15 Passive Layer on Ti6Al4V Alloy ................... 3.8.16 Passive Films on Stainless Steels .................. 3.8.17 NiCO3 · Ni(OH)2 · H2O on MnCO3 .................. 3.8.18 YOHCO3 Coatings ......................................... 3.8.19 Zr2O2(OH)2CO3 and Zr2 (OH)6 SO4 on Polystyrene ................................................ Well-Defined Low-Molecular-Weight Organic Compounds ................................................................... 3.9.1 Hydrocarbons ................................................. 3.9.2 Bromododecane from Sigma-Aldrich ...........

776 776 779 781 781 781 781 782 807 811 811 811 811 812 812 812 813 814 814 820 820 821 823 824 824 824 825 826 828 829 833 834 835 835 835 835 836 837 837 839

xx

Contents

3.10

3.11

3.12

3.13

3.9.3 Fullerol ........................................................... 3.9.4 Acids .............................................................. 3.9.5 Cholesterol, 99+%, Alfa Aesar ...................... Polymers (Macroscopic Specimens) ............................. 3.10.1 Polyamides ..................................................... 3.10.2 Polycarbonates ............................................... 3.10.3 Polyetheretherketone, Victrex, Lite K, from Lipp-Terler ............................................. 3.10.4 Polyetherimide, Molecular Mass 89 100, from Lipp-Terler ............................................. 3.10.5 Polyethylene ................................................... 3.10.6 Poly(ethylene imine) from Polysciences ................................................... 3.10.7 PMMA ........................................................... 3.10.8 Polypropylene from E-Plas ............................ 3.10.9 Polystyrene ..................................................... 3.10.10 PTFE .............................................................. 3.10.11 Polyurethane .................................................. 3.10.12 Polymers, Fibers ............................................ 3.10.13 Polymers, Powders ......................................... Latexes .......................................................................... 3.11.1 Commercial ................................................... 3.11.2 Synthetic ........................................................ 3.11.3 Origin Unknown ............................................ Natural High-Molecular-Weight Organic Substances ..... 3.12.1 Humic and Fulvic Acid .................................. 3.12.2 Marine Colloidal Organic Matter .................. 3.12.3 Suspended Particulate Matter from River Mersey in NW England ....................... 3.12.4 Cellulose ........................................................ 3.12.5 Dextrin ........................................................... 3.12.6 b-Casein ........................................................ 3.12.7 Lysozyme ....................................................... 3.12.8 Chitosan ......................................................... 3.12.9 Chitosan–Polymethacrylic Acid Composites ..................................................... 3.12.10 Asphaltene ..................................................... Microorganisms ............................................................ 3.13.1 Bacterium Bacillus subtilis ............................ 3.13.2 Bacterium Corynebacterium xerosis ............. 3.13.3 Cell Walls of Bacterium Rhodococcus erythropolis .................................................... 3.13.4 Bacterium Rhodococcus opacus from Fundacao Tropical de Pesquisas e Tecnologia Andre Tosello, Sao Paulo ............

839 839 841 841 841 842 842 842 843 843 843 843 844 844 844 845 845 845 845 848 851 852 852 855 856 856 857 857 857 858 858 858 859 859 859 860

860

xxi

Contents

3.14 3.15

3.16 3.17 3.18 3.19 3.20 3.21

3.22. Chapter 4

3.13.5 Bacterium Shewanella putrefaciens .............. 3.13.6 MS2 Bacteriophages ...................................... Metals ............................................................................ Literature Intentionally Ignored .................................... 3.15.1 PZCs/IEPs Not Reported or Not Found ........ 3.15.2 Secondary Sources ........................................ 3.15.3 The Electrolyte Is Not Inert ........................... 3.15.4 Mechanical Mixtures and Complex and Ill-Defined Materials ..................................... 3.15.5 Nonstandard, Incorrect, or Undefined Method, and Nonstandard Terminology ........ 3.15.6 Wrong Citations ............................................. Temperature Effect ....................................................... Pressure Effect .............................................................. Compilations of PZC of Various Materials .................. Correlations ................................................................... Mixed Water–Organic Solvents .................................... Nonaqueous Solvents .................................................... 3.21.1 Allegedly Pure Solvents ................................. 3.21.2 Effect of Water ............................................... 3.21.3 Effect of Inorganic Electrolytes ..................... 3.21.4 Effect of pH ................................................... Conclusion .....................................................................

860 860 861 861 861 861 862 863 864 866 866 868 869 870 873 874 875 875 876 876 876

Ion Specificity .......................................................................... 879 4.1

Affinity Series ............................................................... 4.1.1 Aluminas ........................................................ 4.1.2 Iron (Hydr)oxides ........................................... 4.1.3 MnO2 .............................................................. 4.1.4 Hydrous Niobia .............................................. 4.1.5 Silica .............................................................. 4.1.6 SnO2 ............................................................... 4.1.7 Thoria ............................................................. 4.1.8 Titania ............................................................ 4.1.9 UO2 ................................................................ 4.1.10 WO3 ................................................................ 4.1.11 Zirconia .......................................................... 4.1.12 Mica ............................................................... 4.1.13 Na-Montmorillonite ....................................... 4.1.14 Red Mud ........................................................ 4.1.15 Alkali Metal-Substituted Manganese Oxides .... 4.1.16 d-MnO2 .......................................................... 4.1.17 Si3N4 ............................................................... 4.1.18 Chrisotile .......................................................

879 880 880 881 881 881 882 883 883 883 883 883 884 884 884 884 884 884 884

xxii

Contents

4.2

4.3

4.1.19 Controlled Pore Glasses ................................. 4.1.20 Diamond ........................................................ Uptake of 1-1 Electrolyte Ions at or Near the PZC ....... 4.2.1 Alumina ......................................................... 4.2.2 Gibbsite .......................................................... 4.2.3 CdO ................................................................ 4.2.4 Co3O4 ............................................................. 4.2.5 Magnetite (containing 2.4% of Silica) ........... 4.2.6 Hematite ......................................................... 4.2.7 Goethite ......................................................... 4.2.8 HfO2 ............................................................... 4.2.9 Niobia ............................................................. 4.2.10 Silica .............................................................. 4.2.11 Hydrous Tin Oxide ........................................ 4.2.12 ThO2 ............................................................... 4.2.13 Titania ............................................................ 4.2.14 Zirconia .......................................................... 4.2.15 Alumina–Silica Mixed Oxides ...................... 4.2.16 Silica–Titania and Alumina–Silica– Titania Mixed Oxides .................................... 4.2.17 Titania–Zirconia Mixed Oxides .................... 4.2.18 d-MnO2 .......................................................... 4.2.19 Porous Glasses ............................................... High Ionic Strength ....................................................... 4.3.1 Ions in Solution .............................................. 4.3.2 Experimental Methods .................................. 4.3.3 Electroacoustic Method .................................

884 884 884 885 886 886 886 886 886 886 886 887 887 887 887 887 888 888 888 888 888 889 889 889 890 891

Appendix ....................................................................................................

893

References ..................................................................................................

911

Index ........................................................................................................... 1057

Preface In 1995, I came to the Forschungszentrum Karlsruhe, Germany as an Alexander von Humboldt fellow. The Forschungszentrum (Research Center) had just been renamed from Kernforschungszentrum (Nuclear Research Center), reflecting the change in its research profile from nuclear technology to more general research in natural sciences. I was one of very few experienced surface chemists among numerous non-surface chemists who started new projects more or less related to surface phenomena. Not surprisingly, several colleagues approached me with questions, one being about the points of zero charge (PZCs) of various materials. In the beginning, I advised my colleagues to use the review by Parks [1]. Indeed, [1] used to be the most complete review on PZCs of oxides, and authors who reported their own measurements usually compared their results with those reported by Parks. The popularity of Parks’ review is reflected by the number of citations. Yet my colleagues were not entirely satisfied. Parks reports data only for a limited number of materials. Moreover, my colleagues were concerned about the significance of expressions such as “titania has a PZC at pH 6.” Should we expect the same PZC for all titanias, no matter what method is used and what batch of material is used? Is the scatter of results reported in the literature due to real differences in properties between particular samples or to a real difference between the isoelectric point on the one hand and the PZC obtained by titration on the other? Both approaches are equally attractive, and the truth is probably somewhere in between. Probably the differences in PZC obtained for different samples of a material having the same chemical formula are due to a combination of real differences in properties and experimental errors (e.g., insufficient purity), and it is very difficult to completely exclude either of these factors or to assess their contributions to the observed effect. Yet the question about the existence/nonexistence of a common PZC for all titanias (or other groups of materials with a common chemical formula) cannot be avoided in a review of PZCs. In my previous review [2], all PZCs of materials with a common chemical formula were grouped and analyzed together. The entries were sorted only by chemical formula. In the present review, a completely different approach is adopted. PZC data on well-defined specimens of materials are sorted by trade name and manufacturer (for commercial materials), location (for natural materials), or specific recipe (for synthetic materials). This approach emphasizes the comparison between particular results obtained for different samples of apparently the same or at least very similar material. The classification of materials according to the above criteria was more difficult than originally expected. Detailed sample information is often missing or xxiii

xxiv

Preface

incomplete in scientific publications. Often, literature references are given instead of specific data. Spelling errors in trade and manufacturers’ names are common. Even complete trade name and manufacturer information is not sufficient for correct classification. The results reported in this book were taken from papers published over a few decades. In the meantime, manufacturers, distributors, and other enterprises have merged, split, or changed their names. The same product might have been offered under various brand names. Thus, the number of classes distinguished in this book is probably much larger than the number of significantly different products. On the other hand, it may very well be that the recipe for a commercial product may have been modified without its trade name being changed. The present author does not possess this knowledge. Calcination and washing are other factors that make comparison of results from different sources obtained for apparently the same material more difficult. Original commercial materials often undergo calcination and/or washing before their surface charging is studied. Calcination removes organic impurities from the surface, but it also removes surface hydroxyl groups, which are responsible for the surface charging. Calcination at high temperatures also induces diffusion of impurities from the bulk solid onto the surface. The conditioning of the sample after calcination may strongly affect its surface charging properties. Numerous studies have been devoted solely to the effect of different calcination–rehydration sequences on surface charging. In fact, a new material is produced when the original sample is calcined at sufficiently high temperature. The goal of washing is to remove impurities, which are usually concentrated on the surface. The effect of washing on the isoelectric point (IEP) of titania was systematically studied in [3]. In fact, washing also modifies the surface by leaching the components of the sample, changing the degree of hydration, and replacing substances originally present in the sample by other substances originating from the washing solution. The idea of a washing procedure that removes only impurities is an example of wishful thinking. A description of washing procedures in the literature is often incomplete or missing, but even with detailed information, it is difficult to assess the nature of the changes induced by washing. In the present book, all results obtained for the same original commercial material—uncalcined and calcined, unwashed and washed in different ways— have been analyzed as one group. Many groups consist of a single sample; that is, only one study reporting PZCs/IEPs for certain commercial materials could be found. A few commercial materials have become very popular, and numerous studies reporting PZCs/IEPs could be found. A similar approach applies to home-synthesized materials. Again, certain recipes have frequently been used to synthesize materials for surface charging studies, and numerous studies reporting PZCs/IEPs of such materials could be found, while other recipes have been used only in single studies. A few studies of surface charging of home-synthesized materials reported original recipes. In other studies, the recipes were taken from the literature. When possible in this book, a recipe is reported for each synthetic material, which makes it possible to synthesize similar material. However, certain obvious details are omitted. For

Preface

xxv

instance, the use of distilled water (rather than tap water) to prepare solutions and to wash precipitates is standard. Also, details regarding the equipment (glassware and filters) and the chemicals (manufacturer and purity) are usually omitted. For several specimens, a literature reference is given instead of a specific recipe. Several recipes are reported only in theses, internal reports, and other difficult-toaccess sources. Usually, the original papers reporting synthetic recipes analyze a broad spectrum of experimental conditions. A literature reference is then not sufficient to identify a specific recipe. Problems with identification of specific recipes are explicitly stated in this book when appropriate. Most studies reporting PZCs/ IEPs of home-synthesized materials refer to a specific recipe. Similar recipes are grouped together here; that is, recipes for home-synthesized materials that belong to one group are not necessarily identical. Results obtained for natural materials from the same geographic location (mine or a specific country, etc.) are grouped together. Of course, specimens collected at the same site do not necessarily have identical properties, in contrast with series of commercial materials sold under the same trade names or series of home-synthesized materials prepared according to the same recipe. Finally, results of surface charging studies of materials of unknown (or unreported) origin are also given. These materials very likely belong to one of the groups of commercial, home-synthesized, or natural materials, but as their origin is unknown, each of these materials is treated as a single-member group. For each type of material (commercial, home-synthesized, natural, or “origin unknown”) the physical properties related to surface charging properties are reported when available. As many sources as possible (not limited to the papers reporting surface charging studies) have been used to obtain these data. The results from different sources are often scattered. In this respect, the present book presents more detailed data than previous compilations of PZCs/IEPs. The style of organization and presentation of the PZC/IEP data here follows the style of my previous book [2]. The present compilation of PZC/IEP is critical and selective; that is, numerous studies reporting PZC/IEP data or cited as sources of such data have deliberately been rejected. Many other studies relevant to the present compilation might have been overlooked or were unavailable. A reader may be interested in a certain study apparently reporting PZC/IEP data on materials of interest even when it is not used in the present book. To this end, the references deliberately omitted in this book are listed, together with a short explanation of the nature of the results presented there. A few papers cited as references allegedly containing PZC/IEP information are also mentioned, even if inspection of the original papers indicates absence of such information. Marek Kosmulski

Acknowledgments The collection of publications cited in the present compilation is based on the anonymous work of numerous librarians. Several scientists replied to my requests and sent me reprints of their publications. I take this opportunity to express my gratitude to them. The technical assistance of Piotr Próchniak and Teresa Chlebik from the Department of Electrochemistry, Lublin University of Technology is gratefully acknowledged as are grants from Lublin University of Technology and from the Alexander von Humboldt Foundation. Parts of this book were written at Åbo Akademi (Finland) and the European Institute of Transuranium Elements (Karlsruhe, Germany). Professors Jarl B. Rosenholm and Thomas Fanghänel are acknowledged for their hospitality. Marek Kosmulski

xxvii

1

Introduction

Introductory descriptions of surface charging and the electrical double layer can be found in numerous handbooks of surface and colloid chemistry (e.g., [4–7]), in other books (e.g., [8–10]), and in review articles (e.g., [11]). The reader of the present book is assumed to be familiar with these phenomena and with basic ideas and methods of analytical chemistry. Figures 1.1 and 1.2 show the idealized picture as presented in the handbooks. The s0(pH) curves obtained at different concentrations c1 < c2 < c3 of an inert 1-1 electrolyte shown in Figure 1.1 have a common intersection point (CIP) at s0 = 0. The absolute value of s0 at constant pH increases as the ionic strength increases on the both sides of the point of zero charge (PZC). s0 at constant ionic strength steadily decreases as pH increases. Many examples of such sets of three or more charging curves are reported in the literature. The number of charging curves shown in Figure 1.1 is limited to three for clarity. Differences between c1, c2, and c3 by an order of magnitude are necessary to obtain a clear difference in the absolute value of s0 and a clear CIP. When the differences between the concentrations are smaller, the charging curves obtained at various ionic strengths are likely to overlap rather than intersect. The z(pH) curves obtained at different concentrations c1 < c2 < c3 of an inert 1-1 electrolyte shown in Figure 1.2 show a common IEP. The absolute value of z at constant pH decreases as the ionic strength increases on both sides of the IEP. z at constant ionic strength steadily decreases as pH increases. Studies reporting electrokinetic data for numerous ionic strengths are rare. Electrokinetic data obtained at one ionic strength are sufficient to determine the IEP. Differences between c1, c2, and c3 by an order of magnitude produce a clear difference in the z potential. When the differences between the concentrations are smaller, the electrokinetic curves obtained at various ionic strengths are likely to overlap, as observed in [12] (anatase in 0.001 and 0.0025 M KCl). On the other hand, an increase in KCl concentration from 0.01 to 0.045 M induced a decrease in electroacoustic signal by a factor of about 3 on both sides of the IEP [438]. The following figures illustrate problems that occur in real systems. Let us consider a quantity XYZ (e.g., s0 or z potential) that reverses its sign at pH0. Here, pH0 represents electroneutral conditions without specifying precisely what quantity is meant and how it was measured, and the PZC and IEP are examples of pH0. The line in Figure 1.3 represents an idealized situation: XYZ depends only on pH, and many high-precision data points are available on both sides of pH0. A line drawn 1

2

s0

Surface Charging and Points of Zero Charge

PZC

0 c1 c2 c3 pH

FIGURE 1.1 Expected course of s0(pH) curves. c1, c2, and c3 are different concentrations of an inert 1-1 electrolyte, with c1 < c2 < c3.

z

through these points gives pH0. Actual systems are more complicated. Figures 1.4 through 1.12 show typical problems that make determination of pH0 difficult. Only one problem is illustrated in each figure, but in fact combinations of two or more problems often occur. Figure 1.4 illustrates the effect of quantities other than pH on XYZ. These quantities, such as temperature and the nature and concentration of the 1-1 electrolyte and of impurities present in the system, are usually controlled, although in fact they cannot be set exactly constant, but vary over a limited range. Varying degrees of attention have been paid to controlling these quantities, and the level of control claimed in a scientific paper (e.g., the temperature limits) is not necessarily realistic. Different measurable quantities, for example s0 from titration and z potential from electrokinetic measurements, represent the state of the charged surface. In principle, each of these quantities can reverse

IEP

0 c3 c2 c1 pH

FIGURE 1.2 Expected course of z(pH) curves. c1, c2, and c3 are different concentrations of an inert 1-1 electrolyte, with c1 < c2 < c3.

3 XYZ

Introduction

pH0

0

pH

FIGURE 1.3 Idealized situation as found in handbooks: pH0 depends only on pH. XYZ represents a quantity that reverses its sign at pH0.

XYZ

sign at different pH values, thus producing different pH0 values (Figure 1.5). In the presence of inert electrolytes, the CIP and IEP of the same sample of pure metal oxide often match, but in a few studies they have been substantially different. It is an open question whether the discrepancies between the CIP and IEP of the same sample of metal oxide reported in the literature are only due to experimental errors and impurities or whether they may also occur in properly conducted experiments with very pure materials. Different specimens represented by the same chemical formula often produce different pH0 values (Figure 1.6). This is an experimental fact, and the reason for these discrepancies is not clear. Substantial discrepancies in the pH0 of particular

pH0 (T2) pH0 (T3)

pH0 (T1)

0 T1 T2 T3

pH

FIGURE 1.4 Real situation: pH0 depends on quantities other than pH. Temperature and the nature and concentration of a 1-1 electrolyte and of impurities present in the system are examples of such quantities.

4 X, Y, or Z (scaled)

Surface Charging and Points of Zero Charge

pH0 (Z) pH0 (Y )

pH0 (X)

0 X Y Z

pH

FIGURE 1.5 Real situation: different quantities that represent the sign of the electric charge at the surface produce different pH0 values. An example is s0 from titration compared with z potential.

XYZ

specimens (>1 pH unit) are due to impurities. In principle, different crystallographic structures and different morphologies of crystals (exposure of different faces) of the same chemical structure can also produce different pH0 values, and, for example, numerous studies of the effect of interatomic distances on the acidity of surface oxygen atoms have been published. The discrepancies in pH0 in a series of clean samples that differ only in structure and/or morphology do not exceed 1 pH unit. Figures 1.3 through 1.6 illustrate an idealized picture, in which the measurements produce exact values. Real XYZ measurements produce ranges (average values with limits of uncertainty represented by error bars) rather than

pH0 (specimen 3) pH0 (specimen 2)

pH0 (specimen 1)

0 Specimen 1 Specimen 2 Specimen 3

pH

FIGURE 1.6 Real situation: different specimens produce different pH0 values, although they represent the same chemical formula. Possible reasons include impurities and the effect of interatomic distances on the acidity of surface oxygen atoms.

5 XYZ

Introduction

0

Possible range of pH0

pH

FIGURE 1.7 Real situation: the XYZ measurement produces a range (represented by an error bar) rather than a single quantity (represented by a point). The sign of XYZ is uncertain over a range of pH, which depends on the error in XYZ.

XYZ

single quantities (represented by points). Figure 1.7 shows that the sign of XYZ is uncertain over a particular pH range, which depends on the error in XYZ. Even with very low error in XYZ (Figure 1.8, error bars not shown), the error in the pH measurements (represented by error bars) makes pH0 uncertain in unbuffered systems. In microelectrophoresis, errors in pH measurements are the main source of uncertainity. Section 1.10 discusses pH measurements in more detail. In real measurements, both pH and XYZ values are uncertain.

0

Possible range of pH0

pH

FIGURE 1.8 Even with very low error in XYZ (error bars not shown), the error in pH measurements (represented by error bars) makes pH0 uncertain in unbuffered systems. In microelectrophoresis, errors in pH measurements are the main source of uncertainty.

6

Slow titration

XYZ

XYZ

Surface Charging and Points of Zero Charge

Fast titration pH

0 pH0 (base titration)

pH0 (acid titration) Base titration Acid titration pH

FIGURE 1.9 Real situation: hysteresis. The loop narrows as the titration rate decreases, but it is often difficult to avoid, even with very slow titration.

In Figures 1.1 through 1.8, a tacit assumption was made that the system is in adsorption equilibrium. Real measurements (Figure 1.9) are often carried out in titration mode. Reversibility of titration (acid titration vs. base titration) is not guaranteed, but this is seldom examined; that is, titration in only one direction is reported in most studies. The system tends to “remember” its state from the past: this phenomenon is called hysteresis. The XYZ obtained in base titration starting at low pH is more positive than XYZ at the same pH obtained in acid titration starting at high pH. Therefore, base titration gives a higher pH0 than acid titration, and the actual pH0 is in between, but not necessarily half way. The hysteresis loop narrows as the titration rate decreases (two small loops in the right upper corner of Figure 1.9), but it is often difficult to avoid, even at very low titration rates. Silicate and carbonate anions are omnipresent: they occur as impurities in metal oxides and in other adsorbents, and in water and in other reagents used to prepare solutions, and they are absorbed from air and leached out from parts of the apparatus. Their sorption leads to an increase in negative charge and to a shift in pH0 to low pH (Figure 1.10). The binding mechanism of silicates and carbonates is complex; for example, metal silicates and carbonates are often more stable (in terms of G 0 of pure solid phases) and less soluble than the corresponding oxides. In typical surface charging experiments, the concentrations of silicates and carbonates are reduced by using an inert gas atmosphere and plastic ware rather than glassware, but such attempts do not guarantee the absolute absence of silicates and carbonates. Often, data points very close to pH0 are not available or are scattered, and pH0 is determined by interpolation. This is a typical situation in electrophoresis, because dispersions are unstable near the IEP. IEPs determined by interpolation are usually based on an arbitrary curve connecting the data points (represented by circles in Figure 1.11). The two curves in Figure 1.11 represent two interpolations. Both interpolations look “reasonable,” but they produce very different pH0 values.

7 XYZ

Introduction

pH0 (CO2/SiO2 removed)

0 pH0 (CO2/SiO2 present) CO2/SiO2 removed CO2/SiO2 present pH

FIGURE 1.10 Real situation: adsorption of silicate and carbonate leads to a shift in pH0.

XYZ

Numerous pH0 values reported in the literature, especially at very low or very high pH, were obtained by extrapolation. Extrapolation is used when only data points on one side of the IEP are available or when the data points in the vicinity of the IEP are scattered. For example, in [14], a PZC of MoO3 at pH -0.5 is claimed, which certainly could not have been obtained by direct measurement. pH0 values determined by extrapolation are usually based on an arbitrary curve connecting the data points (represented by circles in Figure 1.12). The two curves in Figure 1.12 represent two extrapolations. Both extrapolations look “reasonable,” but they produce very different pH0 values. When the pH0 falls beyond the range of data points, it is safer to report a limit (e.g., “pH0 < 2 if any”) rather than a specific value of pH0.

pH0 (interpolation 2)

0 pH0 (interpolation 1) Data points

pH

FIGURE 1.11 Data points in the close neighborhood of pH0 are not available (this is often the case in electrophoresis), and the pH0 is determined by interpolation. Circles represent data points. Curves represent two arbitrary interpolations.

8

Surface Charging and Points of Zero Charge pH0 (extrapolation 2) XYZ

0 pH0 (extrapolation 1)

Data points

pH

FIGURE 1.12 pH0 falls outside the range of data points, and is determined by extrapolation. Circles represent data points. Curves represent two arbitrary extrapolations. Numerous pH0 values reported in the literature, especially in ranges of very low or of very high pH, have been obtained by extrapolation.

1.1 NOMENCLATURE The terms “point of zero charge” and “isoelectric point” and the corresponding abbreviations PZC and IEP are used in the present book according to IUPAC recommendations [15,16]. The PZC is defined as the conditions at which the surface charge density equals zero; for metal oxides and related materials, it is determined by potentiometric titration or by related methods as the point at which the apparent surface charge density determined in the presence of an inert electrolyte is independent of ionic strength. Zero net surface charge density does not imply the absence of any charges, but rather the presence of equal amounts of positive and negative charge. The IEP is defined as the conditions at which the electrokinetic charge density and thus the electrokinetic (z) potential equals zero; it is determined by electrokinetic methods (see [17] for measurement and interpretation of electrokinetic phenomena). Different versions of these abbreviations—lower- and upper-case, with or without periods—are used in the literature. The same abbreviations also appear in the form of subscripts, for example, pHIEP. This notation emphasizes that there are species other than protons that may produce a reversal in sign of the z potential, and the concentration of such a species (e.g., a polymer [18,19]) that is required to reverse the sign of the z potential can also be termed the IEP. The present book is devoted to pH-dependent surface charging, and there is no need to emphasize repeatedly that the IEP is a pH value. However, in other publications, the abbreviation “IEP” may refer to species other than protons, and certain situations require a clear indication of which species induced sign reversal. For example, the primary surface charging of silver halide colloids is governed by silver and halide ions in solution, and their IEP is expressed in terms of pAg or pX. One of these

Introduction

9

quantities is sufficient, since their sum is equal to pKs, where Ks is the solubility product of the silver halide. In most studies of pH-dependent surface charging reported in the present book, the pH was adjusted by addition of an acid or base that has an anion or cation in common with the inert electrolyte. In an electrokinetic study of apatite [20], the pH was adjusted with KOH (standard procedure), and in another series of measurements, the pH was adjusted with K2HPO4. The z potentials were substantially different in two series of measurements, but the IEP was consistent. In apatite and other materials that undergo selective leaching out of components, the concentrations of the leaching products in solution affect the surface charge. They are not all independent variables, because they are interrelated by solubility product and by equilibria in solution. Not surprisingly, the PZC/IEP of apatite and other materials that undergo selective leaching out of components, obtained from studies in which pH was the sole adjusted and/or controlled variable, are less consistent than the PZC/IEP of materials that show negligible solubility. In numerous IUPAC publications (e.g., [21]), pI is used as an abbreviation for isoelectric point. Although the above recommendations refer chiefly to electrophoresis of proteins, the nature of electrokinetic phenomena in proteins and in colloids is basically the same, and there is no need for two different abbreviations for isoelectric point. Not surprisingly, several authors in the colloid chemistry literature have also used the abbreviation pI. The present author prefers IEP as a more common and less confusing abbreviation (pI may suggest minus the logarithm of iodide concentration—and indeed it is used with such a meaning in this book). An interesting semantic problem is faced in [22], which reports isoelectric points of protein molecules in solution as well as those of larger particles formed by these molecules. Another abbreviation for isoelectric point, namely pH(I), is recommended in [21]. Several authors ignore IUPAC recommendations and use their own terms and abbreviations. The term “zero point of charge” (abbreviated as ZPC), which has the same meaning as PZC defined above, has been used in a popular textbook [5] and in several other publications (e.g., [23]). The term “point of zero zeta potential” (PZZP) has been used for the IEP. In the present book, “ZPC” and other atypical terminology (e.g., “pHz”) have been translated into the recommended terminology when necessary and possible. Already in 1968, Somasundaran [24] complained about use of the terms IEP and PZC outside their normal meaning. IEP and PZC are two different physical quantities, and they must be distinguished even when they happen to be numerically equal. Numerous examples of confusion between IEP and PZC can be found in the literature. For example, in [25], the IEP is termed the PZC. In such cases, the proper terminology has been used in the present book and the terminology used in the original papers has been ignored. The present study is focused on materials with variable (pH-dependent) surface charge, and the methods and definitions are adjusted to this type of materials. Clay minerals and other materials with a dominant role of permanent charge need a different approach. Clay minerals do not show a clear CIP of charging curves

10

Surface Charging and Points of Zero Charge

obtained at various ionic strengths; thus, the above definition that identifies the PZC with the CIP is not applicable. For example, [26] defines five different zero points for soils, and recommends detailed methods for their determination. Reference [26] also gives a list of PZCs (determined by different methods) for common materials. In [27], Sposito discussed PZCs of materials with permanent charge. In [28] and [29], he challenged the application of well-established methods (designed for materials having variable surface charge). Sposito argued that particles with local positive and negative charge may show substantial electrophoretic mobility when their net electrokinetic charge is zero.

1.2 SCOPE The present book reviews PZC/IEP data reported for well-defined, homogeneous materials without surface coating; that is, ill-defined materials (e.g., most natural soils), physical mixtures consisting of grains of various materials (as in [30]), and surface-engineered materials are deliberately omitted. For example, commercial pigments (pigments used as obtained) often consist of core materials with organic and inorganic coatings. Such coatings constitute a small fraction of the mass of the pigment, but severely affect their surface charging properties. In several studies (e.g., [31,32]), the presence of such coatings is explicitly stated. In a few other studies, the composition of the core material is reported, but the presence of the coating is not mentioned. The present author does not possess knowledge about the presence of surface coatings in commercially available materials unless this is explicitly reported in the cited papers. It may very well be that several PZC/IEP values reported for allegedly pure core materials were in fact obtained for surface-engineered materials. The PZCs/IEPs presented in this book are organized primarily according to the chemical formula of the adsorbent. The materials considered here have been organized into the following classes: 1. Simple, sparingly soluble (hydr)oxides. Within this class, compounds are sorted alphabetically by chemical symbol of the electropositive element (usually metal), then by degree of oxidation (lower degree of oxidation first), and then by degree of hydration (lower degree of hydration first). 2. Aluminosilicates and clay minerals. Within this class, compounds are sorted alphabetically by their names. There are numerous, often multilevel, classifications of clay minerals, and different names are often assigned to the same material or to very similar materials. Such materials are listed under the names used in the original publications and interconnected by cross-references. 3. Mixed oxides, that is, materials composed of two or more sparingly soluble (hydr)oxides. The solubility of the components is a key factor distinguishing between mixed oxides and salts (see Class 4 below). For example, MgSiO3 is considered as mixed oxide, since both MgO and

Introduction

4.

5. 6. 7. 8. 9.

10. 11. 12.

13.

11

SiO2 are sparingly soluble, whereas CaSiO3 is considered as a salt, since CaO is soluble. Mixed oxides are organized alphabetically by chemical symbol of the electropositive element in the main component, and then by chemical symbols of the electropositive elements in the other components. The reader looking for a given mixed oxide is advised to check under all components, since information appears only once in the book and no cross-references are provided. The class of mixed oxides comprises salt-type stoichiometric compounds on the one hand and series of nonstoichiometric compounds with broad ranges of compositions on the other. IEPs of mixed oxides are often very different from the weighted average of the IEPs of their components [33]. Salts. These are sorted alphabetically according to the chemical symbols of the anion-forming elements. Salts that can be considered as composed of two sparingly soluble oxides are considered as mixed oxides (see Class 3 above). Glasses. Carbon. Data are given for natural diamond, graphite, and fullerene, and then for commercial and home-made activated carbons. Other well-defined inorganic materials. These include sulfur, ice, and air bubbles. Natural inorganic materials. Coatings. Composite materials with a thick external layer and a core, where the latter practically does not contact the solution, are organized based on the nature of the coating (external layer) according to the principles explained under Classes 1–4 above. Well-defined low-molecular-weight organic compounds and their mixtures. Synthetic polymers (macroscopic specimens). These are sorted alphabetically by chemical names. Latexes. Commercial products are sorted by manufacturer’s name and trade name as the primary identifiers (irrespective of the chemical nature of the monomers). These are followed by home-made latexes. Natural high-molecular-weight organic compounds. These include humic substances and natural organic matter, asphaltene, and cellulose.

Different specimens with the same chemical formula are arranged into three subclasses: A. Commercially available materials. These are sorted alphabetically by manufacturer’s/retailer’s name and/or trade name. When manufacturer’s/ retailer’s name and trade name were reported in the original publication, the materials are sorted primarily by manufacturer/retailer name and then by trade name. Otherwise, the trade name is used as a sole identifier. Cross-references are provided between categories that are likely to represent the same material.

12

Surface Charging and Points of Zero Charge

B. Home-synthesized materials. In a few instances, the recipes for certain chemical compound are organized into a few smaller subclasses according to the method, precursor, etc. C. Natural materials. These are organized alphabetically by country of origin. Most scientific papers report sufficient information to assign the material of interest to one of subclasses A–C and then to a smaller subclasses. Several specimens could not be classified because of insufficient information in the original papers. Although the usefulness of the information about PZCs/IEPs of such materials is limited, they are also reported in the present compilation. Such materials are referred to as “origin unknown.”

1.3 INERT ELECTROLYTES The idea of an inert (indifferent) electrolyte was coined in the context of electrocapillary studies using the Hg electrode. Grahame [34] found a series of electrolytes that produced the same PZC (determined as the electrocapillary maximum) irrespective of the nature or concentration of the electrolyte. Such behavior suggests that the ions of these electrolytes interact with the surface only by a Coulombic force. In contrast, many other electrolytes induced a shift in the PZC, with the magnitude and direction of this shift depending on the nature or concentration of the electrolyte. A shift in the PZC suggests that ions of these electrolytes can be positively adsorbed in spite of electrostatic repulsion; that is, they interact with the surface by a noncoulombic force. This phenomenon is termed specific adsorption. Thus, an inert electrolyte does not show specific adsorption of either ion. The above terminology (“inert” vs. “specific”) was adopted for studies of the surface charging of colloids. Different experimental methods are used and different quantities are measurable for colloids than for the Hg electrode, but the model of an electrical double layer is analogous. Studies of pH-dependent surface charging of colloids are usually carried out in the presence of an inert electrolyte and an acid or base (used to adjust the pH) with an anion or cation in common with the inert electrolyte. Products of dissolution of the solid are also present in solution at low concentration (we are only interested in sparingly soluble solids), but are ignored in most studies. Sometimes, the concentration of dissolution products is measured, and very occasionally the concentration of dissolution products (which are water-soluble salts) is controlled by addition of these salts to the dispersion. The effect of addition of Al(iii) salt on the z potential of alumina was studied in [35]. At the IEP, the solubility of Al species is low; thus, the IEP was not very different from that in a 1-1 electrolyte. The solubility problem is discussed in more detail in Section 1.6. Parks [1] found that any combination of Na or K on the one hand and of Cl, NO3, or ClO4 on the other constitutes an inert electrolyte with respect to metal oxides, and this has been generally accepted since then. Interestingly, some

Introduction

13

electrolytes that are inert with respect to Hg show specific adsorption of either ion by metal oxides, and vice versa. Halide anions are usually inert with respect to metal oxides, but are potential-determining ions for silver halides. Thus, the term “inert electrolyte” is relative. In surface charging studies of nonconductive materials, a shift in the IEP induced by addition of a salt may be used as a criterion for the presence or absence of specific adsorption. Increasing the concentration of an inert electrolyte at constant pH induces an asymptotic decrease in the absolute value of the z potential without sign reversal. In contrast, increasing the concentration of specifically adsorbing counterions at constant pH leads to sign reversal. Several experimental studies of the effect of ionic strength on the z potential at constant pH have confi rmed this rule. The z potential as a function of ionic strength was also studied in [36], but the pH was not reported. Electrolytes that show inert behavior at concentrations up to about 0.1 M may induce a sign reversal of the z potential at concentrations of about 1 M in aqueous solution. In mixed and nonaqueous solvents, 1-1 electrolytes that are inert in water show specific adsorption of cations, which induces shifts in the IEP to high pH [37]. In principle, specific adsorption of anions induces an increase in the negative electrokinetic charge and a shift in the IEP to low pH, and specific adsorption of cations induces an increase in the positive electrokinetic charge and a shift in the IEP to high pH. However, sorption of heavy metal cations often induces surface precipitation, and then the IEP of the new surface is similar to that of the surface coating, that is, of the (hydr)oxide of the heavy metal cation. In such systems, the direction of the shift in the IEP depends upon the relative position of the IEP of the original surface on the one hand and that of the surface coating on the other. For example, in the presence of U(vi) (cationic species dominate in the pH range of interest), the IEP of hematite shifts to low pH [38]. This is because the IEP of U(vi) oxide is lower than that of hematite. Group 1 metal ions other than Na + and K+ are often used as constituents of inert electrolytes. The applicability of Li+ as a constituent of inert electrolytes is limited by the low solubility of its salts (e.g., the carbonate). Bromides and iodides show indifferent behavior toward metal oxides and related compounds, but adsorption of fluorides is usually specific. Reference [16] discusses F- as an inert ion, and specific adsorption of F- on alumina is considered as an exception. Reference [39] describes the adsorption of nitrate, perchlorate, and chloride as nonspecific on quartz, titania, and alumina, but as specific on zirconia and thoria. With SnO2 and Fe2O3, adsorption of chloride was found to be specific, and adsorption of nitrate and perchlorate to be nonspecific. Further examples of such exceptions are discussed in Chapter 2. Reference [40] shows that sodium and potassium trichloroacetate, trifluoroacetate, and trifluoromethanesulfonate also act as inert electrolytes. Ammonium and tetraalkylammonium salts are possible candidates for inert electrolytes for metal oxides, but not for silica. Namely, 10-3 M solutions of TMA, TEA, and TPA salts induce shifts in the IEP of silica to high pH, and 10-2 M solutions induce shifts to even higher pH [41]. The IEP of Si3N4 shifts to high pH in the presence of (C2H5)4NCl with respect to NaCl [42,43].

14

Surface Charging and Points of Zero Charge

The usual approach to inert electrolytes assumes that a broad pH range is covered, and the inert character of both ions of the electrolyte is essential. In studies that cover a narrow pH range far from the PZC, the character of the counterion (the ion and the surface have charges of opposite sign) is essential, and the inert/ specific character of co-ions, which are practically absent in the interfacial region, is less important. In the presence of CaCl2 in the strongly acidic range and in the presence of Na2SO4 in the strongly basic range, Fe(OH)3 behaves as in the presence of NaCl (the molarity of a 1-1 salt must be twice as high as the molarity of a 2-1 salt to produce the same concentration of monovalent ions). The CIP of goethite in Na2SO4 was only marginally different from those found in NaCl or NaNO3 [44]. The nature of the co-ion can be ignored over a limited pH range, and electrolytes with inert counterions act as inert electrolytes. Electrokinetic studies in which only NaOH and H2SO4 were used to adjust the pH, and no inert electrolyte was added, belong to a similar category. Although H2SO4 may induce a shift in the IEP, a pristine IEP may still have been obtained in such studies. That is, the results in neutral and basic pH in such studies are obtained without H2SO4 addition; thus the specific/nonspecific character of anion adsorption can be ignored. The presence or absence of a CIP of charging curves is not a criterion for an inert electrolyte. Figure 1b in [45] shows charging curves of alumina-coated titania at three KCl concentrations, and the figure caption claims that “these curves do not intersect at a common point, suggesting that Cl is specifically adsorbing on the oxide.” Such an interpretation is not acceptable. Charging curves of metal oxides at different concentrations of a heavy metal nitrate show a CIP [46], in spite of specific adsorption of heavy metal cations. That CIP falls at different pH values for different salts, and it does not correspond to the point of zero proton charge. On the other hand, coincidence of the CIP and IEP supports the hypothesis that an electrolyte is inert. Different research groups use different terminology in describing surface charging behavior in the presence of specific adsorption. Specifically adsorbed ions contribute to the surface charge. Some authors use the term “surface charge” as a synonym for proton charge, whereas others consider surface charge as the sum of proton charge and the charge due to adsorption of species other than the proton. Numerous electrokinetic studies (see, e.g., [47]) have been carried out in the presence of pH buffers. These results are not used in the present compilation, because the components of pH buffers usually show specific adsorption. Mixed evidence is found in the literature regarding specific/nonspecific character of adsorption of short-chain carboxylic acids. Reference [48] suggests an absence of a shift in the IEP of alumina (0.5 g/L) in the presence of >0.001 M organic acids. The effect of specific adsorption on the electrokinetic curves became visible at pH < IEP. No shift in the IEP of titania in the presence of CH3COONa or C2H5COONa was observed [49], but sodium salts of higher carboxylic acids induced a shift in the IEP to low pH. The PZC/IEP under pristine conditions is (by definition) independent of the nature and concentration of the electrolyte, and these details are often omitted in

15

Introduction

scientific publications. Otherwise, in the presence of specifically adsorbing ions, pH0 refers to a specific nature and concentration of electrolyte, which should be clearly indicated, for example, in the caption or key of a figure presenting surface charging behavior. Such information is generally provided, with some exceptions; for example, in [50], the IEP was most likely obtained in the presence of a dispersant (phosphate), although this was not indicated on the figure or in its caption, and the reported IEP could be easily confused with the pristine IEP. In a few studies (e.g., [51]), potentiometric titrations were carried out in the presence of NH4NO3. The disadvantage of this electrolyte and of other salts involving weak acid or weak base is substantial buffer capacity. The electrolytebackground-corrected uptake of protons is obtained as a difference of two large and almost equal numbers; thus, the value and even the sign of the difference is uncertain. This problem is less significant in electrokinetic methods, except that larger amounts of acid/base have to be used to adjust the pH than with salts of a strong acid and a strong base.

1.4

THE SIGNIFICANCE OF PARKS’ REVIEW

Parks’ review [1] introduced several ideas in the field of surface charging of metal hydr(oxides) that seem obvious now but at the time were revolutionary. Examples include the collection of PZC/IEP data from different sources, inert electrolytes (Section 1.3), and the possible correlation between PZC and wellestablished physical quantities such as the bond valence and the degree of oxidation or hydration. Not surprisingly [1] has been a source and an inspiration for many followers, and with over 2000 citations it is one of the most successful papers in the field of colloid chemistry ever published. Figure 1.13 presents the history of citations of [1]. Even now, the knowledge of many scientists about pH-dependent surface charging of metal oxides is chiefly based upon that

120 100

Citations

80 60 40 20 0 1965 1970 1975 1980 1985 1990 1995 2000 2005

FIGURE 1.13 Citations of Parks’ review [1].

16

Surface Charging and Points of Zero Charge

classical publication. For example, as recently as in 2006, one of the plenary lectures in a specialized colloidal conference was based on Parks’ ideas originally published in [1]. Also, the present compilation is in some senses a continuation of Parks’ work. Certainly, the results presented in a review can be only as good as the results in the publications upon which that review is based. The experimental techniques upon which the determination of PZC/IEP is based have improved considerably over the last four decades. Therefore, the experimental results reported in recent publications are (on average) more credible than those that were available in the literature in the mid-1960s. Surprisingly, much from [1] remains valid over 40 years after publication, but a few results and hypotheses have turned out to be incorrect. Both correct and incorrect results from Parks’ review have been repeated in recent papers. A few examples of such uncritical citations will be presented below. In a few instances, the PZC/IEP value, experimental conditions, or methods reported in [1] differ from those in the original paper cited as the source of these results. For example, the results cited in [1] from [52] are substantially different from those in the latter paper. Numerous papers have cited the incorrect value, following [1], rather than the value from the original paper [52]. Further similar examples are presented in Chapter 3. I would like to emphasize, however, that the rate of erroneous citations in [1] is not particularly high, compared with that in other publications. A few scientific papers cited in [1] report results that do not represent PZCs/ IEPs by today’s standards, but these results are quoted as PZC/IEP in [1]. For example, the solubility of W(vi) in HCl was studied in [53] by titration of Na2WO4 with HCl, and the authors found the pH of the solubility minimum (at a molarity of HCl of about 0.5, observed in a certain kinetic regime) and termed it the “isoelectric point of tungstic acid solubility.” The corresponding pH value (0.43) was cited in [1] as the “isoelectric point of hydrous WO3 obtained by electrophoresis,” and it was then cited following [1] in numerous papers. More examples like this are presented in Chapter 3. Although these results do not represent actual PZCs/ IEPs, some of them “made careers” as PZCs/IEPs widely cited in the scientific literature, and they are discussed as such in the present review. Even theories were built upon these results. The relationship between PZC and valency (PZC < 0.5 for M2O5 oxides, etc.) often quoted following Parks [1] is limited. Recent experiments with Nb2O5 and Ta2O5 indicate that the PZCs of these oxides are substantially higher than 0.5 (cf. Chapter 3). Similarly, the relationships between PZC and hydration (less hydrated compounds have lower PZC) and between PZC and degree of oxidation (oxides at a higher degree of oxidation have a lower PZC) often quoted following Parks [1] are also limited. Comparison between hematite and hydrous iron oxide on the one hand and between magnetite and hematite on the other (cf. Chapter 3) does not confirm these rules. The above criticism does not refer to Parks’ review, but rather to uncritical quotation of hypotheses (which might seem reasonable 40 years ago) without survey of more recent literature.

17

Introduction

1.5 STRUCTURE OF ADSORBENTS The acidity of surface oxygen atoms in the adsorbents of interest depends on the spatial distributions of atoms. Various representations have been used to illustrate these distributions. A perspective view of a few dozens of MO4 tetrahedra or MO6 octahedra (M = metal) that share corners, edges, or faces is the most common representation (Figures 1.14 and 1.15). A few models show just the polyhedra, and other models indicate possible locations of surface groups or possible mechanisms of binding of various species to the surface. In ball-and-stick models, particular atoms are represented as small balls in different colors (or shades), and the neighboring balls are connected by sticks of different lengths (Figure 1.16). The balls represent the positions of the centers of atoms, but not their size. The ball-and-stick and polyhedral representations may be combined. Wire-frame models show only bonds (sticks), and the atoms are not explicitly shown. In ball models, particular atoms or ions (metal, oxygen, and OH-) are represented as balls in different colors (or shades), but, in contrast with ball-and-stick models, the bonds are not explicitly shown, the balls are relatively large and touch each other, and the sizes of the balls usually represent the sizes of the corresponding atoms. A perspective view of a few dozens of balls shows positions of atoms in a particular crystallographic face. Different types of software are available to create these models. A few literature references reporting such models in graphical form are collected below. The crystallographic data upon which models are based are collected in the Appendix.

1.5.1

ALUMINA

Octahedral and ball models of gibbsite are presented in [54]. Octahedral models of the 0001 and 1-102 faces of a-alumina shown in Figure 3 of [55] and (a)

(b)

(010)

(001)

FIGURE 1.14 Structure of manganite: (a) the 010 plane is in the plane of the paper; (b) the 001 plane is in the plane of the paper. The 010 plane is indicated by the dashed lines. (Reprinted from Ramstedt, M. et al., Langmuir, 20, 8224, 2004. Copyright 2004 American Chemical Society. With permission.)

18

Surface Charging and Points of Zero Charge

FIGURE 1.15 An origami by Michał Kosmulski representing the octahedral model.

Figure 1 of [56] indicate the location of possible adsorption sites for metal cations. The octahedral models shown in Figures 3.10 and 3.15 of [57] illustrate the formation of gibbsite, bayerite, and boehmite from solution monomers. Balland-stick models of the 100, 010, and 001 surfaces of gibbsite are shown in Figure 9 of [58]. An original model of different planes of a-alumina was used

d¢

d

a

c

c¢ a

b

(110)

c¢ d¢

b

a

d c

(100)

FIGURE 1.16 Structure of rutile: white circles, O2-; black circles, Ti4+; large gray circles, O of adsorbed water; small gray circles, H of adsorbed water. Lower-case letters denote various surface species. (Reprinted from Imanishi, A. et al., J. Am. Chem. Soc., 129, 11569, 2007. Copyright 2007 American Chemical Society. With permission.)

19

Introduction Surface OH– Subsurface OH– 1st layer Al3+ 1st layer O2– 2nd layer Al3+

FIGURE 1.17 View of the 0001 plane of sapphire. The projection of multiple unit cells is indicated by the dashed lines. (Reprinted from Kershner, R.J., Bullard, J.W., and Cima, M.J., Langmuir, 20, 4101, 2004. Copyright 2004 American Chemical Society. With permission.)

in Figures 1 and 10 of [59] (Figure 1.17). Different layers of atoms in alumina were indicated by different colors. An octahedral model of diaspore is shown in Figure 7d of [60].

1.5.2

IRON (HYDR)OXIDES

Ball-and-stick models of the 001 planes of goethite (Figure 1), akanegeite (Figure 2), and lepidocrocite (Figure 3), the 110 plane of solvated goethite (Figure 5, also showing a wire-frame model), and the 1120 plane of hematite (Figure 4), are shown in [61]. Ball-and-stick models of the 001 plane of goethite (Figure 3) and the 111 plane of magnetite (Figure 2) are shown in [62]. Octahedral models of different iron (hydr)oxides are shown in [63]. Octahedral models of goethite and hematite, and of the phase transformation from goethite to hematite, are shown in Figures 1 and 11 of [64]. An octahedral model of hematite is shown in Figure 3.11 of [57]. A ball-and-stick model of the 012 surface of hematite in the XZ and YZ planes is shown in Figure 1 of [65]. Figure 5 of [66] shows a ball-and-stick model of the 100 face of goethite in plan and in section. An octahedral model shown in Figure 3.12 of [57] illustrates the formation of goethite from solution monomers. Color was used in an octahedral model of goethite shown in Figure 4 of [67]. Octahedral models of the 001 and 110 faces of goethite generated by the computer program ATOMS and shown in Figure 1 of [68] indicate a possible binding mechanism of phthalate. Octahedral models of the 100 and 110 faces of goethite shown in Figure 5 of [69] and of the 001 and 100 faces of lepidocrocite shown in Figure 8 indicate a possible binding mechanism of metal cations. An octahedral model of the ab plane of goethite shown in Figure 11 of [70] indicates a possible binding mechanism of metal cations. An octahedral model of goethite shown in Figure 1 of [71] indicates possible binding mechanisms of U(vi). Figure 1 of [72] shows typical morphologies of FeOOH crystals (goethite and lepidocrocite), and Figure 2 shows an octahedral model of FeOOH projected onto the 001 plane. Octahedral models of the 110 plane of goethite shown in Figure 12 of [73] and Figure 1 of [74], and of the 110 and 001 planes shown in Figure 5 of [75] and Figure 4 of [76], indicate the locations of different types of surface oxo and

20

Surface Charging and Points of Zero Charge

hydroxo groups. Ball-and-stick models of the 101 and 001 faces of goethite shown in Figure 6 of [77] indicate the locations of different types of surface oxo and hydroxo groups. Ball models of dry and hydrated 100 surfaces of goethite are shown in Figure 1 of [78].

1.5.3

MAGNANESE OXIDES

Octahedral models of various Mn oxides are shown in Figure 4 of [79] and Figure 1 of [80]. Octahedral models of manganite are shown in Figure 4 of [81] and Figure 1 of [82]. Octahedral models of the 001 and 010 planes of manganite are shown in Figure 2 of [83] and Figure 9 of [84]. An octahedral model of hausmannite is shown in Figure 1 of [82]. An octahedral model of birnessite shown in Figure 1 of [85] indicates a possible binding mechanism of metal cations.

1.5.4

SILICA

A tetrahedral model of quartz is shown in Figure 1 of [86] (cited from [87]). A ball-and-stick model of b-cristobalite is shown in Figure 1 in [88].

1.5.5

TITANIA

An octahedral model of the 101 plane of anatase shown in Figure 4 of [89] indicates the locations of different types of surface oxo and hydroxo groups and a possible binding mechanism of metal cations. A ball-and-stick model of the 110 surface of rutile is shown in Figure 3 of [90]. Figures 2 and 3 of [91] and Figure 10 of [92] show the 110 and 100 planes of rutile. A modified ball-and-stick model is used in [91], in which atoms are represented by their chemical symbols rather than by balls. Octahedral models of the 110 plane of rutile shown in Figure 1 of [93] and Figure 2 of [94] indicate the locations of different types of surface oxo and hydroxo groups. Octahedral models of rutile and brookite and a ball-and-stick model of anatase are shown in Figure 1 of [95]. Ball models of the 110, 100, and 001 faces of rutile are shown in Figure 9 of [96]. Octahedral models shown in Figure 3.20 of [57] illustrate the formation of anatase and rutile from solution monomers. Figures 3.16 and 3.18 of [57] show analogous processes for other metal (hydr)oxides.

1.5.6

CLAY MINERALS

A polyhedral model of hectorite is shown in Figure 3 of [97]. Figure 1 of [98] shows ball-and-stick model of kaolinite. A combined polyhedral/ball-and-stick model of kaolinite is shown in Figure 1 of [99]. A ball-and-stick model of kaolinite is shown in Figure 1 of [100]. A ball-and-stick model of the 010 plane of montmorillonite is shown in Figure 1 of [101]. Ball-and-stick models of kaolinite, pyrophyllite, and illite are shown in Figure 7 of [60].

Introduction

1.5.7

21

NITRIDES

A ball-and-stick model of b-Si3N4 is shown in Figure 1 of [88].

1.6 SOLUBILITY The present review is devoted to pH-dependent surface charging of relatively insoluble materials. Materials of solubility higher than about 0.001 M or 0.1 g/dm3 are outside the scope of this book, although PZCs/IEPs of relatively soluble materials can be found in the literature. The above solubility limit refers to nearly neutral pH, and at extreme pH values the solubility of so-called insoluble materials often increases by many orders of magnitude with respect to that at neutral pH. The solubility of so-called insoluble materials is often ignored in surface charging studies, but it must be realized that a certain fraction of the adsorbent undergoes dissolution in the form of various species. In some experiments, this solubility is in fact immaterial, but in a few other experiments, solubility matters. Solubility may be responsible for irreproducibility of experiments and for scatter in the PZCs/IEPs reported in the literature. Solubility depends on temperature, pH, and ionic strength. Solubilities of thermodynamically stable forms are lower than those of less stable forms, and solubilities of small crystals are higher than those of large crystals. Moreover, dissolution is a slow process, and the concentration of dissolved species in solution in many experiments is well below saturation. Thus, thermodynamic (equilibrium) data on solubility are of limited relevance to surface charging experiments with short equilibration times. Chemical dissolution of metal oxides and related materials is reviewed in [102]. Most studies were devoted to dissolution in the presence of organic ligands, which form stable complexes with metal ions in solution and/or enhance the dissolution of iron and manganese oxides by reduction of Fe(iii) and Mn(iv) [103]. Few studies have been carried out solely in the presence of inert electrolytes. Specific information on solubility of particular specimens can be found in the original literature. Usually, the experimentally determined concentration of a given element in a given kinetic regime is plotted against pH. Plotting dissolved amount as a function of time is another common mode of presentation. Ney [104] expressed the solubilities of various materials in terms of conductivities of saturated solutions. A kinetic study was also carried out.

1.6.1

SIMPLE (HYDR)OXIDES

1.6.1.1 Alumina Reference [105] reports solubilities of two aluminas as functions of pH. The solubility of alumina was also studied in [106] (Figure 3), [107] (Figure 1), [108] (Figure 3), [109], [110] (Figure 1), [111] (Figure 7), and [112] (Figure 4). The solubility of alumina in the basic range is substantially depressed in the presence of

22

Surface Charging and Points of Zero Charge

silicate [113]. Dissolution of boehmite was studied in [114] (Figure 8), and measured and calculated solubilities of gibbsite are compared in [115]. 1.6.1.2 Indium (Hydr)oxides The solubility of indium (hydr)oxides was studied in [116]. 1.6.1.3 Iron Oxides The solubility of magnetite was studied in [117] and that of iron(iii) hydroxide in [118]. Figure 2 of [119] shows the solubilities of different iron (hydr)oxides: amorphous >> goethite >> hematite. The lowest, pH-independent solubility is at pH 6–10, and ranges from 10-8 M (amorphous) to 10-12 M (hematite). 1.6.1.4 Manganese (Hydr)oxides Dissolution of manganite was studied in [84], and the solubilities of three samples of MnO2 are reported in Figure 9 of [120]. d-MnO2 dissolves at pH < 2 [121]. 1.6.1.5 NiO The solubility of NiO was studied in [122]. 1.6.1.6 PbO The concentration of Pb(ii) in solutions of different initial pH in contact with PbO at 40°C as a function of time (0–24 h) was studied in [123]. Only numerical values are reported and units are not specified. 1.6.1.7 Silica Silica is more soluble than most other materials studied in this book. Not surprisingly, more studies have been devoted to the solubility of silica than to solubilities of less soluble materials. An overview of the older literature was presented in [124]. The solubility of silica was studied in [1787]. Solubilities of various silicas in the range from 10-3.75 M (quartz) to 10-2.5 M is reported in [126]. The solubility increased with specific surface area. Silicate concentration was measured at different pH values and NaCl concentrations in [127]. A solubility of silica of 10-2.43 M at pH 2 was found in [128]. The kinetics and temperature dependence of the solubility of silica were studied in [129] and the kinetics and ionic strength dependence in [130], in both cases at pH 2–10. The kinetics, pH dependence, and effect of alkali pretreatment were studied in [131]. Solubilities of 11 ppm for quartz and 116 ppm for amorphous silica are reported in [86]. The same study reports 10–80 ppm of silica in natural waters. 1.6.1.8 Titania The low solubility of titania is documented in [132]. Figure 9 of [133] reports the solubility of titania.

Introduction

23

1.6.1.9 ZnO The solubility of ZnO in water and in 0.001 M KCl was studied in [134]. The solubility of original (commercial) and washed ZnO after 3-day equilibration was studied in [135]. Solubilities of Zn oxides and carbonates were studied in [136]. ZnO is soluble in dilute acids and bases, and remains sparingly soluble over a relatively narrow pH range.

1.6.2

OTHER MATERIALS

Solubility of materials other than simple oxides is a complex phenomenon, and usually leads to selective leaching of different elements and to changes in the chemical character of the surface. The concentration of more than one element in solution has to be followed. 1.6.2.1 Clay Minerals Reference [137] reports the absence of significant dissolution of illite. In contrast, [138] reports considerable release of Si and Al from illite. Dissolution of illite, that is, Al, Si, Ca, and Mg concentrations at pH 3–9, was studied in [139]. Dissolution of kaolinite at acidic pH was studied in [140]. Reference [141] reports the release of Si, Fe, Al, and Mg from montmorillonite as a function of pH. Concentrations of Si and Al in solution during titration of smectite are reported in [142]. Solubility of laponite is discussed in [143] and references therein. 1.6.2.2 Aluminum Silicate The Al concentration in solutions of synthetic aluminum silicate was studied in [144]. 1.6.2.3 Carbonates The solubility of FeCO3 and MnCO3 at 5 × 104 Pa CO2 was studied in [145]. 1.6.2.4 Apatite Calcium and phosphate concentrations were measured as a function of pH in [20]. The pH was adjusted by the addition of KOH, KF, Ca(NO3)2, or K2HPO4. Dissolution is inhibited in the presence of high-molecular–weight organic compounds [146]. 1.6.2.5 Niobate Reference [147] reports nonstoichiometric leaching of components from PbMg1/3Nb2/3O3 (Mg > Pb >> Nb in neutral and basic pH). The IEP depends on the solid-to-liquid ratio. 1.6.2.6 Titanate Acid treatment of BaTiO3 (Ba-containing supernatant removed) gives a product with an IEP similar to that of TiO2 [148]. Selective leaching of Ba from BaTiO3

24

Surface Charging and Points of Zero Charge

was also studied in [149]. The IEP depends on the solid-to-liquid ratio. Probably, similar effects are observed for other salts.

1.7

SOLID PHASE TRANSFORMATION AT ROOM TEMPERATURE IN CONTACT WITH SOLUTION

Numerous surface charging experiments have been carried out with materials that are not thermodynamically stable in contact with aqueous solution. Theoretically, the unstable phase can be transformed into a stable phase, but the rate of transformation is difficult to predict or control. In numerous experiments, the degree of transformation is negligible, and it may take years to see any change. On the other hand, a substantial difference in standard Gibbs energies between stable and unstable forms may induce an appreciable degree of phase transformation in the course of adsorption experiments, especially when equilibration times are long. It can also happen that, for kinetic reasons, one unstable form is converted to another unstable form rather than to a stable form. Thus, the initial state of the adsorbent (specific surface area, chemical formula, and crystallographic form), which is usually reported in scientific papers, is not necessarily relevant to the state at which the adsorption measurement was carried out. The thermodynamic data relevant to assessment of stability of different chemical compounds and of different crystallographic forms of the same compound are compiled in the Appendix. The Gibbs energy is not directly related to the rate of transformation; that is, the existence of a driving force does not imply that the transformation actually occurs. The phase transformation is usually ignored in surface charging experiments, and this may be responsible for irreproducibility of experiments and for scatter in the PZCs/IEPs of certain materials reported in the literature. A few specific examples of studies of phase transformation under experimental conditions relevant to studies of surface charging are discussed below. Hydration or dehydration and transformation between different crystallographic forms of the same compound have attracted more attention than redox reactions. Redox conditions are seldom controlled in surface charging experiments, with the exception of studies of surface charging of metal sulfides and nitrides and a few other redoxsensitive compounds. Thermodynamic stability of metal (hydr)oxides against formation of carbonates or basic carbonates is another relevant, but often ignored, aspect. Elementary thermodynamic calculations indicate that, for many sparingly soluble metal oxides, the equilibrium oxide + CO2  carbonate is shifted to the right at a partial pressure of CO2 that is found in the atmosphere; that is, oxides may convert into carbonates, and preformed carbonates will not spontaneously decompose. Surface charging experiments are often carried out in an atmosphere of inert gas in order to minimize the amount of CO2, but the effectiveness of measures aimed at exclusion of CO2 is limited. The experimental problems related to the presence/exclusion of CO2 are discussed in more detail in Chapter 2.

Introduction

1.7.1

25

ALUMINA

Spontaneous conversion of g-alumina into bayerite in contact with solution is reported in [110]. A diffuse reflectance Fourier transform infrared (DR-FTIR) study demonstrated transformation of g-alumina into bayerite-like phase on the surface [150]. Surface regions in a- and g-alumina convert to hydrated alumina similar to gibbsite or bayerite after exposure to water, according to [136]. Incorvati [151] observed transformation of a-alumina into bayerite or gibbsite at room temperature within 1 day.

1.7.2

CdO

CdO is transformed into hydroxide in NaClO4 medium and into basic chloride in NaCl medium [152].

1.7.3

CUO

Reference [153] reports conversion of Cu(OH)2 into CuO (a change in color is visible after 18 hours).

1.7.4

IRON (HYDR)OXIDES

Transformation of goethite into hematite by grinding at room temperature was detected by X-ray diffraction (XRD) [154]. Amorphous iron hydroxide was found to crystallize to goethite, according to [118]. Changes in the specific surface area and in the chemistry of the surface were observed. Storage of hydrous ferric oxide (HFO) for 16 weeks as a dispersion under nitrogen induced no substantial change in its crystallinity [155]. The fraction of oxalate-extractable Fe in aged HFO varied over the same period (in [155], Figure 11 and the text report opposite trends). The effect of aging at different pH values in the presence of different anions on the conversion of ferrihydrite into goethite or hematite at 20°C was studied in [156]. The conversion was slow at pH 7 (half-conversion time > 1 year), but it was faster at high pH. At pH 11, the half-conversion times ranged from 37 days (in the presence of nitrate) to 53 days (in the presence of sulfate). Goethite was the predominant product of conversion at pH 11, while at pH 8 and 9, hematite was the main product. Partial conversion of ferrihydrite into goethite in dispersion after over 4 weeks’ storage at room temperature was reported in [157], but no trace of goethite was detected after 10 days. Magnetite was partially converted into maghemite and akageneite on storage in aqueous medium for 3 years [158].

1.7.5

OTHER SYSTEMS

The above examples refer to simple systems containing only metal oxides and aqueous solution of salts of alkali metal cations. Other metal cations originally present in solution can be built into new phases formed upon sorption, as observed in studies of interaction of heavy metal cations with quartz [86] and with kaolinite [159].

26

Surface Charging and Points of Zero Charge

1.8 SOLID PHASE TRANSFORMATION ON HEATING Calcination of powders in the presence of different gases may induce solid phase transformation, which in turn affects the PZC/IEP. Hydrogen-treated and untreated zirconia were studied in [160], but no substantial shift in CIP was detected. Two titanias were heated in O2 or in H2 at 530 or 600°C, but no substantial change in IEP or CIP was observed in one sample [161]. Dehydration of titania (rutile) as a function of temperature was studied in [162]. The s0 of silica was depressed by a factor of 10 by heating at 800°C for 3 hours. Further heating (up to 36 hours) did not affect s0. Rehydration of heated powders for 3–56 days brought about a gradual increase in s0 [163]. A few examples of different phase transformations induced in the same initial material by calcination at various temperatures are presented in Chapter 3.

1.9 KINETICS In principle, kinetics is beyond the scope of the present book, which presents and discusses results obtained under pseudo-equilibrium conditions. However, some information about the kinetics of processes relevant to surface protonation is necessary to properly design pseudo-equilibrium experiments and to understand the significance of their results. Reference [164] presents an overview of the kinetics of adsorption. As the present book is focused on pH-dependent surface charging, the following types of kinetic experiments are directly relevant. 1. Solid is added to a solution. 2. Acid, base, or inert salt is added to a pre-equilibrated dispersion. 3. D2O or T2O is added to a pre-equilibrated dispersion. The time dependence of the following quantities has been studied: 1. 2. 3. 4. 5.

pH The concentration of products of dissolution of the solid The concentration of ions of inert electrolyte Distribution of nuclides Electrokinetic potential

Obviously, the time dependence of pH is of primary interest, and the other quantities have attracted less attention. The course of kinetic curves of adsorption/ isotope exchange at solid/liquid interfaces is qualitatively similar to that referring to kinetics in solution; that is, the changes are rapid at the beginning of the process, becoming slower in course of the experiment. This phenomenon is important in the planning of pseudo-equilibrium experiments. For example, in a system that has attained a certain degree of equilibration within 1 hour, equilibration for the next 5 minutes is unlikely to bring about substantial change.

Introduction

27

There are two classical ways to design a kinetic experiment: 1. The volume of samples taken to control the concentrations of the reagents is negligibly small compared with the volume of the system, or the concentrations are controlled without sample withdrawal (ion-selective electrodes). 2. The reaction is started in many identical reactors at the same time. Only one sample is taken from each reactor (at different times). Both solutions assure that the course of the reaction is not affected by sample withdrawal, and the reaction proceeds toward the same equilibrium state. Results obtained in experimental systems designed in this way often fit theoretical equations corresponding to certain transport models. Certainly, the fact that the results match a model calculation does not imply that the model is physically correct. Several kinetic studies reported in the literature disobey the above rules; that is, the volume of the samples withdrawn during the kinetic experiment is comparable to the volume of the system. Results of such kinetic experiments may be still interesting, but they are unlikely to fit any theoretical equation that assumes that the system tends to the same equilibrium state during the entire experiment. Each sample withdrawal changes the proportions of components in the system, and thus the equilibrium state also changes. Protonation/deprotonation reactions are among the fastest reactions in solution, and it is believed that surface protonation/deprotonation reactions are also fast. Therefore, the experimentally observed kinetics in surface protonation experiments is transport-controlled. Different models of kinetics of ion exchange with intraparticle rate control are discussed in [165]. Kinetic models based on a series of consecutive and/or branched reactions and experimental setups for kinetic measurements are reviewed in [166]. The experimentally observed pH after addition of a reagent (e.g., in potentiometric titration) does not reach a constant value, but changes at a variable rate even over very long times. Some arbitrary assumption is necessary to establish the “equilibrium” value. A few examples of kinetic experiments of surface protonation are briefly presented in this section.

1.9.1

PROTON ADSORPTION

1.9.1.1 Alumina The kinetics of proton adsorption by alumina was studied in [167,168]. The z potential was studied as a function of exposure time (1–14 days) in [169]. 1.9.1.2 Cr2O3 The z potential was studied as a function of exposure time (1–14 days) in [169].

28

Surface Charging and Points of Zero Charge

1.9.1.3 Iron (Hydr)oxides The kinetics of equilibration at a hematite single-crystal aqueous interface was studied in [170]. Relaxation times for proton adsorption–desorption on hematite and magnetite were studied as functions of pH in [171] using a pressure jump technique. The kinetics of proton desorption from natural hematite was studied in [172]. Rate constants were calculated for different salts at different concentrations. The time dependence of the pH during titration of ferrihydrite is shown in [173]. Figure 1 of [174] shows the time of equilibration after addition of base to a goethite dispersion as a function of pH. The kinetics of proton adsorption by goethite was also studied in [168]. The final pH of dispersions of hematite and corundum (with different amounts of acid or base added) after 2 hours’, 1 day’s, and 4 days’ equilibration is reported in [175]. The curves obtained for different equilibration times differ significantly over the pH range 5–7. The z potential of natural hematite as a function of aging time was studied in [176]. The variation of pH with time on addition of base to a hematite dispersion is reported in [119]. 1.9.1.4 Manganese Oxides The kinetics of OH and alkali metal ion uptake by l-MnO2 was studied in [177]. 1.9.1.5 Silica The kinetics of proton adsorption by silica was studied in [168]. After addition of quartz to a solution [178], the fast stage (the first 4 minutes) was followed by a slower, linear decrease of pH with time. 1.9.1.6 Titania The kinetics of proton adsorption/desorption on anatase was studied using a pressure jump technique in [179], and rate constants were calculated. Reference [161] presents the kinetics of proton adsorption for TiO2. References [180–182] report the changes in pH in a fresh titania dispersion at natural pH in water and in 0.005 M NaCl. A constant value was established in about 10 hours. The z potential was studied as a function of exposure time (1–14 days) in [169]. The variation of pH with time on addition of base to titania is reported in [183]. 1.9.1.7 Apatite The kinetics of proton uptake at was studied under different conditions. Proton uptake was accompanied by calcium release [184].

1.9.2

ISOTOPE EXCHANGE

The kinetics of tritium exchange between water and d-MnO2 was studied in [185]. The kinetics of tritium exchange between water and five different samples of silica was studied in [186].

Introduction

1.9.3

29

DISSOLUTION

1.9.3.1 Alumina The concentration of dissolved species as a function of exposure time (1–14 days) was studied in [169]. The rate of dissolution of alumina is reported in [187]. The rate of dissolution of alumina as a function of pH is reported in [188]. The rate of dissolution of corundum is reported in [189]. The kinetics of dissolution of alumina was studied in [110], and that of gibbsite in [190]. 1.9.3.2 BeO The rate of dissolution of BeO was studied in [187]. 1.9.3.3 Cr2O3 The concentration of dissolved species was studied as a function of exposure time (1–14 days) in [169]. 1.9.3.4 Cu(OH)2 Kinetics of dissolution was studied in [153]. 1 ppm of Cu was found at pH 5, and maximum concentration in solution was reached after 2 h. 1.9.3.5 Iron (Hydr)oxides The kinetics of dissolution of goethite in 0.5–2 M NaCl and NaNO3 was studied in [191]. 1.9.3.6 Silica The rate of dissolution of quartz, also in the presence of Al(iii), was studied in [192]. The rate of dissolution of quartz at 25°C and higher temperatures as a function of pH is reported in [188]. The dissolution rate of silica at pH 10 increases as the Na concentration in solution (as chloride or sulfate) increases [193]. The dissolution kinetics of silica as the function of pH and ionic strength was studied in [194]. The kinetics of dissolution of silica at low pH was studied in [195]. The rate of dissolution of bamboo phytoliths, which are composed chiefly of silica, is reported in [196]. 1.9.3.7 Titania The concentration of dissolved species was studied as a function of exposure time (1–14 days) in [169]. The kinetics of dissolution of titania was also studied in [197]. 1.9.3.8 Clay Minerals The kinetics of dissolution of kaolinite was studied in [189]. The kinetics of dissolution of aluminosilicates was studied as a function of pH in [188]. 1.9.3.9 Silicates Reference [198] reports dissolution rates of silicate minerals. The kinetics of forsterite (MgSiO4) dissolution was studied in [199].

30

Surface Charging and Points of Zero Charge

1.9.3.10 Carbonates The rates of dissolution of various carbonates were studied in [200]. The kinetics of magnesite dissolution was studied in [201] and that of dolomite dissolution in [202,203].

1.10 SOLUTION CHEMISTRY—PH SCALE Probably many readers of the present book do not expect that something as selfevident as pH measurements might cause a problem. Yet, apparent easiness and obviousness is a pitfall. Different aspects of the pH scale and of pH measurements have been discussed in numerous handbooks of chemistry. Reference [204] is a special monograph devoted solely to pH measurements. A few aspects of the pH of solutions, not directly related to surface charging or adsorption, are discussed in this section. The approach to proton adsorption from aqueous solution must be different from the approach to adsorption of other solutes, because water molecules can provide or absorb a practically unlimited number of protons (higher by several orders of magnitude than the concentration of any other species in solution and the concentration of surface sites) to balance the changes induced by adsorption. Thus, adsorption isotherms based on the concept of a distribution of a limited amount of adsorbate molecules between solution and surface are not applicable. Most authors accept this obvious fact, but a few others have used the same formalism for proton adsorption as is used for other solutes. For example, in [205], the surface charging of alumina is discussed in terms of adsorption isotherms (amount adsorbed vs. equilibrium concentration). Positive adsorption of protons is equivalent to negative adsorption of OH-, and vice versa. In adsorption experiments, uptake of protons and release of OH- cannot be distinguished. Only the net result of uptake/release of H+ and OH- can be obtained, and independent curves of H+ and OH- adsorption reported in the literature [206,207] must be based on measurements of other quantities. The normal pH scale is from 0 to 14 (the upper end of the pH scale corresponds to the logarithm of the reciprocal autoprotolysis constant of water, pKw at 25°C), but extremely acidic or extremely basic solutions induce numerous difficulties. Most commercially available pH electrodes give reliable results only in the pH range from about 2 to about 12. Very acidic and very basic solutions are caustic, and they dissolve many materials of interest that are insoluble at neutral pH. Very acidic and very basic solutions have intrinsically high ionic strengths; thus, concepts and methods that require low ionic strength are not applicable, and there are limited possibilities to study the effect of ionic strength on different quantities. For example, an ionic strength of 0.01 M is often sufficient to induce rapid coagulation of colloidal dispersions. Therefore, most studies of pH-dependent surface charging at solid/liquid interfaces are limited to the pH range 2–12. An experiment to confirm a hypothetical PZC/IEP at pH < 2 or >12 would be very difficult. Certainly, the limits 2 and 12 are not sharp, and the difficulties mentioned above gradually increase as the acid or base concentration increases. The width of the

Introduction

31

pH scale practically available for surface charging studies is then about 10 pH units; that is, 1 pH unit constitutes about 10% of the available pH scale. A difference of 1 pH unit is then significant, and a difference in excess of 2 pH units (20% of the available scale) is huge. A measurement, calculation, or estimation that produces an error of 2 pH units must not be considered as successful. An error of ±2 pH units covers up to 40% of the available pH scale, that is, such result is not much better than lack of any pH information. A few commercially available pH meters display a pH value with 3 decimal digits, and most commercially available meters display 2 decimal digits. The 3rd decimal digit displayed by a pH meter is not stable, even in a well-buffered system. The 2nd decimal digit displayed by the meter may be stable under certain circumstances and over limited periods of time, but it must not be considered as significant in many pH measurements, especially in an unbuffered system. In measurements related to pH-dependent surface charging, the 2nd decimal digit is usually uncertain. Thus, the PZCs/IEPs reported in the literature are usually rounded to the next 0.1 pH unit. Rare examples of PZCs/IEPs reported with 2 decimal digits are due to carelessness (insignificant digits are not rounded off), rather than to extraordinary accuracy. The uncertainty in the measured pH values is often ignored in scientific papers. For instance, in graphs presenting the electrokinetic potential as a function of pH, vertical error bars are often plotted, reflecting the uncertainty in the measured electrokinetic potential (Figure 1.7). It often happens that, even with a broad margin of error in the electrokinetic potential, it is still not possible to draw a smooth electrokinetic curve through all data points, and the outstanding points usually occur at nearly neutral pH. This problem can be fixed when the uncertainty in the pH values is taken into account (Figure 1.18). The procedure of pH measurement is not limited to insertion of a combination electrode into a solution (dispersion) and waiting until a constant value is displayed. Some pH value will be displayed, even when the rules given in every user manual are disobeyed. Typical errors are inadequate calibration of the pH electrode, use of outdated pH buffers, old electrodes (2 years is a typical lifetime), insufficient flow in the salt bridge between reference electrode and solution (or incorrect level and/or composition of the solution in the bridge), and insufficient electrical contact between solution and electrode. These examples of carelessness are commonplace in scientific laboratories, and typically induce errors in the range of a few tenths of a pH unit. It is not obvious to all scientists that the pH reported in z(pH) or s0(pH) plots (from which the IEP or PZC is determined) is the equilibrium pH of the dispersion used for the measurements. The following description was found in [208]. The authors equilibrated their particles in a solution 1 of pH 1, 1.9, 3, 5, 7, 8, 11, or 13. The particles were then separated from solution 1 and redispersed in pure water. The new dispersion (particles in solution 2) was used to measure the electrophoretic mobility. Obviously, the pH of the solution 2 formed by equilibration of pretreated particles with water was different from the pH of solution 1, and most scientists would have plotted the z potential against the pH of solution 2 to

32

z

Surface Charging and Points of Zero Charge

Data points

0

pH

FIGURE 1.18 Electrokinetic curve with one outstanding point. In order to explain the difference between the outstanding point and the smooth curve connecting all other points solely in terms of the error in the z potential, broad margin of error has to be allowed. The same discrepancy may be explained by allowing relatively narrow margin of error in the pH.

obtain the IEP. But, in [208], the z potential was plotted against the pH of solution 1. Not surprisingly, unusual values of the apparent IEP were obtained. The following problems related to pH measurement and interpretation are directly related to the problems discussed in this book.

1.10.1

PROBLEM 1: CONCENTRATION VERSUS ACTIVITY

The pH is minus the logarithm of the activity of protons rather than of their concentration. For ionic strengths less than 0.001 M, the difference in numerical value between the activity and the concentration is immaterial, but with concentrations of a 1-1 electrolyte greater than 0.01 M, the difference is in excess of 0.1. This is especially important when the recorded pH values are used to calculate the chemical speciation (solution of the chemical equilibrium problem), for which the measured activity has to be properly converted into concentration in the mass balance calculation.

1.10.2

PROBLEM 2: EXPERIMENTS AT CONSTANT IONIC STRENGTHS

When the ionic strength is high enough, and the required pH is sufficiently close to 7, the acid or base added to adjust the pH does not contribute to the total ionic strength. In contrast, with low ionic strengths, the pH adjustment often affects the overall ionic strength. For example, [209] reports measurements in a 0.00001 M 1-1 electrolyte at pH 3–12; [210] reports measurements in a 0.00001 M electrolyte solution at pH 2; according to [211], the IEP of silica in 0.0001 M NaCl (pH 1–2) is supposed to be different from that in HCl only; and [212] reports measurements

Introduction

33

in a 0.00001 M 1-1 electrolyte at pH 3–9, with the negative z potential supposed to be substantially greater than in a 0.0001 M 1-1 electrolyte. In those studies, the contribution of 1-1 salts to the total ionic strength at pH 3 was insignificant, and the total ionic strength (due to the presence of acid or base) was much higher than the salt concentration. It is impossible to carry out an experiment at pH 3 and ionic strength (total electrolyte concentration) less than 0.001 M, and the apparent effects of salt concentration were in fact due to some other factor that was not sufficiently controlled.

1.10.3

PROBLEM 3: BUFFERED VERSUS UNBUFFERED SYSTEM

Attempts at pH measurement in an unbuffered system at nearly neutral pH (e.g., in high-purity water) result in a very unstable display on a pH meter, and the amplitude of the oscillations may be of the order of 1 pH unit. This is an intrinsic property of electrometric pH measurements, and nothing can be done about it. Very dilute dispersions in 1-1 electrolytes used in microelectrophoresis and solutions in contact with monoliths (in capillary electro-osmosis) are typical examples of unbuffered systems. Results of pH measurements in such systems are sometimes quoted in the literature with two or even three decimal digits, while in fact even the first decimal digit is uncertain. IEPs based on such measurements are only rough estimates. The problem of unstable pH reading may be fi xed by using pH buffers. However, the components of these buffers are usually surface-active, and induce shifts in the IEP. Moreover, pH adjustment in a buffered solution requires a substantial amount of acid or base, thus leading to the effects discussed above in Section 1.10.2. In concentrated dispersions (e.g., in potentiometric titrations and electroacoustic measurements), the solid surface acts as a buffer, and the problem disappears.

1.10.4

PROBLEM 4: SODIUM EFFECT

In spite of progress in the technology of glasses for pH electrodes, their sensitivity to ions other than protons at high pH is still a problem. Even the highest-quality pH electrodes are sensitive to sodium and lithium salts at pH > 12. The difference between the display of the pH meter and the proton activity increases as the ionic strength and pH increase. This problem can be reduced (but not completely avoided) when low-sodium-effect electrodes (specially designed for high-pH measurements) are used.

1.10.5

PROBLEM 5: SUSPENSION EFFECT

When two separate electrodes (glass and reference) are used in concentrated dispersions, the display of the pH meter depends on the location of the electrodes in the dispersion. Moving the electrodes up and down can induce a difference in the apparent pH in excess of 0.2 pH units. The dispersion effect in pH measurements is discussed in [213].

34

1.10.6

Surface Charging and Points of Zero Charge

PROBLEM 6: DIFFERENT PH SCALES

The standard pH scale is designed for 25°C, atmospheric pressure, ionic strength less than 1 M, and solvent water. The upper end of the pH scale equals pKw, which depends on temperature, pressure, and ionic strength. The center of the pH scale, which corresponds to an ideally neutral solution, equals 0.5pKw. Measurements at high or low temperatures, high pressures (>107 Pa), or very high ionic strengths, or in mixed or nonaqueous solvents, require calibration by means of special pH buffers. In other words, a pH electrode calibrated under standard conditions must not be used to measure pH under nonstandard conditions. A pH meter calibrated under standard conditions will certainly display some value under nonstandard conditions, but that value is not the actual pH. Standard pH buffers for common mixed solvents are commercially available. There is no simple relationship between the pH scales for standard and nonstandard conditions. pH measurements at high ionic strengths are discussed in detail in [214]. Autoprotolysis constants of water are reported in [215] for numerous nonaqueous and mixed solvents, in [216] for numerous mixed solvents, and in [217] for aqueous dioxane. Gibbs energies of proton transfer from water to mixed solvents are reported in [218,219].

1.10.7

PROBLEM 7: ELECTROLYSIS

The pH measured just before injection of dispersion into an electrophoretic cell is not necessarily relevant to the pH in the cell during the measurement. The products of electrolysis in electrophoretic cell make the pH more basic. The effect of products of electrolysis on the pH in the cell is more significant with a short distance between the electrodes, high voltage, long measurement time, and low solid-to-liquid ratio. This problem is discussed in more detail in [220].

1.11

VERY DILUTE SOLUTIONS

This book focuses on surface charging of solids dispersed in solutions of inert electrolytes, in the absence of surface-active species. In real experiments, surface-active species are not completely absent, but occur at very low concentrations. Such impurities are often referred to in order to explain unexpected results. The state of matter in very dilute solutions is seldom considered in the scientific literature, and it has been the subject of numerous misinterpretations. The principles of solution chemistry known from elementary handbooks apply in a concentration range of about 10-6 –1 M. The procedures and methods that work very well at concentrations greater than 10-6 M must not be extrapolated to lower concentrations. This is because distilled water used in the laboratory is not pure water, but contains dissolved components of air, materials leached from the container (e.g., silica), and other components. The contribution of the solutes to the physical and chemical properties of water is negligible when their concentrations are low enough, and the presence of impurities in distilled water can be

Introduction

35

ignored in typical solution chemistry experiments. It is tacitly assumed that impurities are absent and that the solute is homogeneously distributed in the entire volume of solution, but this is an idealized model rather than physically realistic. The range of very high dilution requires a different approach. Typical distilled (deionized) water contains a few solutes at a level greater than 10-5 M (a few ppm), a few dozens of solutes at a level greater than 10-6 M (a few hundred ppb), and hundreds of solutes at a level greater than 10-8 M (a few ppb). The concentrations of impurities vary from one sample of distilled/deionized water to another, and it is difficult to select a representative sample. Very likely, the distribution of impurities is nearly Zipfian. Zipf’s law was originally formulated to describe the frequency of occurrence of words in English language, but it also applies to many natural phenomena. It was found that the product of the rank in a list (ordered by descending frequency) and the frequency is nearly constant. In other words, the frequency is inversely proportional to the rank. Exact inverse proportionality is seldom observed, but the type of frequency distribution in an assembly of different species in which a few most frequent species constitute a large part of the assembly and numerous less frequent species constitute a small part is more common than the type of distribution in which all species are equally represented. The validity of Zipf’s law has been examined for distribution of concentrations of various compounds [221], but not of impurities in distilled water. It would be a difficult task to find a complete distribution of the concentrations of impurities, which are often beyond the range of available analytical methods, and their distribution in solution is not necessarily homogeneous. Namely, adsorption on walls of a vessel or on dust particles, which has a negligible effect on the distribution of a solute in a concentrated solution, may substantially affect the distribution of solutes in a dilute state. This effect is well known for radioactive nuclides (radiocolloids), for which it can be easily followed, but it occurs for nonradioactive substances as well. Relatively concentrated solutions can be obtained by dilution of more concentrated solutions by a certain factor. It is tacitly assumed that the water used for dilution does not contain the solute of interest. Such an assumption is acceptable when relatively concentrated solutions (>10-4 M) are diluted. Even when water used for dilution contains, say 10-10 M of the solute of interest as an impurity, the contribution of that impurity to the total amount of the solute of interest in diluted solution is still negligible. The assumption that the solute of interest is absent in water used for dilution becomes risky when a diluted solution (> 1, where k is the reciprocal Debye length and a is the particle radius and for low z potential. The relationship between ue and z has been extensively studied, and many more or less complex equations

42

Surface Charging and Points of Zero Charge

have been proposed for moderate or low ka and for high z potential. An analytical relationship for the most general case (any value of ka and z potential) is not available. Electrophoresis is especially useful for fine particles that form stable dispersions. Large particles will settle down before the measurement can be carried out. Unsuccessful electrokinetic measurements with two samples of relatively coarse (1 and 8 μm diameter) hematite particles are reported in [244]. Materials in the form of large particles or macroscopic specimens can be ground to a finer size, but the new surface exposed by grinding may have different properties from the original (external) surface. Sedimentation is slower for particles with specific densities close to that of the solution. To be visible, the material of interest must have a refractive index sufficiently different from that of the dispersing medium. Electrophoresis is especially useful when a limited amount of material is available, because a very small amount of the material of interest (a fraction of a milligram for sufficiently fine particles with a high refractive index) is sufficient to perform a measurement in modern devices. Electrophoresis is suitable for dilute dispersions, but it cannot be used directly for dense dispersions or in colored liquids that are not transparent to light. The electrophoretic mobility of particles in a concentrated dispersion is determined by dilution of a portion of the dispersion with a supernatant obtained from a larger portion. Dilution of a dispersion with water or with some solution other than the supernatant in order to produce a transparent dispersion is not recommended, because the z potential of particles in equilibrium with the new solution (of different composition) may be substantially different from the z potential of the same particles in the original solution. Electrophoresis is not recommended for measurements at high ionic strengths. A typical limit is 0.1 M, but a few modern devices can be used up to about 1 M. Electrophoresis can be used to independently determine the z potentials of two or more kinds of particles present in the same dispersion. Many devices produce results in the form of mobility histograms (shown, e.g., in [245]), in which peaks corresponding to various types of particles can be distinguished (Figure 2.1). Basically, an electrophoretic measurement can be carried out with a dispersion of conductive particles, although the popular equations used to calculate the z potential from the mobility are valid for insulating particles only. Electrophoresis of metal particles was studied in [246]. Reference [247] reports experimental problems that precluded an electrophoretic study of sulfur at pH < 4. The nature of those problems was not specified. Electrophoresis does not require calibration against a standard dispersion, but standard dispersions (usually latexes) are offered for control of the performance of instruments. A goethite–phosphate system with a controlled solid-to-liquid ratio, pH, ionic strength, and phosphate concentration was recommended as a standard dispersion in [248]. Similar z potentials for that standard dispersion were obtained in five laboratories, with three types of instruments. Electrophoresis is always accompanied by Brownian motion; that is, the mobility of a single particle measured at given time point is not necessarily representative of that particle or of the entire population. Brownian motion is responsible for

43

Intensity

Methods

Mobility

FIGURE 2.1 Mobility histogram. The narrow peak represents large particles of one material and the broad peak represents fine particles of another material simultaneously present in the same dispersion.

the shapes of mobility histograms (see [249] for specific examples), and it can be used to estimate particle size (smaller particles produce broader peaks; Figure 2.1). The errors due to Brownian motion can be eliminated by taking an average mobility of many particles. A few illustrations of home-made electrophoresis cells can be found in the older literature [225,250,251]. Several designs have gained popularity, and they are named after their inventors: Abramson (flat cell), Briggs, Briggs–Mattson, Hamilton–Stevens, Mattson [252,253], Riddick, and van Gils. A moving-boundary cell is shown and described in [7,47]. Nowadays, the use of commercial instruments prevails, and studies using home-made machines are rare. The tables in Chapter 3 (with the literature search having been completed in 2007) report results obtained by the following commercial instruments based on electrophoresis, in which the mobility is calculated from the velocity of colloidal particles: • • • • •

From Bel Japan: Zetasizer 4 From Brookhaven (BI, BIC): 90 Plus, ZetaPlus, and Zeta PALS From Coulter: Delsa models 440 and 440 SX From Jiangsu: DXD-11 From Malvern: models 3000 HSA (HAS) and 5000; AZ-6004; Nano ZP; Nano ZS; Zetamaster S, 5002, and PCS; Zetasizer (Z-sizer) 2, 2c (IIc), IIe, 3 (III, illustrated in [254]), 4 (IV, MK IV), 2000, 3000, 3000 HS, 5000, ZEN 2010, ZEN 3600, and ZET 5004 • From Microtec (Japan): Zeecom and ZC-1500 • MRK (Japan): velocity of single particles in a flat cell

44

Surface Charging and Points of Zero Charge

• Nicomp 380/ZLS and Particle Sizing Systems • From Otsuka: Leza 600, Photal ELS (ELS) 800, 3800 (Doppler shift, flat cell), and ELS-8S (Doppler shift) • From Pen Kem: models 102, 500, 3000, and 3000 S (S 3000); Laser Zee Meter models 500 and 501 • From Perkin-Elmer: model 39 (moving-boundary electrophoresis) • From Rank Brothers (Rank): Mark I and Mark II (MK II, illustrated and described in [255]). • From Repap (Sweden): velocity of single particles in a flat cell • From Sephy (CAD): Zetaphoremeter models II (or Sephy 2100, Z 2300, and Z 3000: velocity of single particles in a rectangular cell), III (rectangular cell, velocity of single particles processed by digital image analysis software), IV (Z 4000: velocity of single particles processed by digital image analysis software), 4000, and 4.20 • From Shanghai Zhongshun: Powereach JS94 H and JS94G+ • From Sugiura (Japan): 2VD • From Arthur H. Thomas & Co. (flat cell, reported in pre-World War II papers). • From Zeiss (Karl Zeiss): Cytophoremeter (rectangular cell) • From Zeta-Meter (ZM, Z-Meter, Riddick): Zeta-Meter (there is a photograph and description of a model without a number in [104]) models 2.0, 3.0, 3.0+ (illustrated in [256–258]), 77, and 80 Technical data on current versions of these instruments can be found on the manufacturers’ Web pages. Most of the above instruments are no longer available on the market, but many pieces of older equipment are still running in different laboratories. Various spellings of the above trade and company names occur in the literature. The number of different trade names listed above is probably higher than the number of substantially different designs of instruments. The above information about the names and principle of operation of instruments in based on the scientific literature rather than on manufacturers’ data. In older commercial instruments, the velocity of individual particles was observed in a microscope and measured by means of a stopwatch. In more advanced models the particles were displayed on a screen and various facilities were added to eliminate the subjectivity factor. Measurement of Doppler shift of the wavelength of light scattered by moving particles made it possible to follow many particles in a short time. Home-made electrophoresis machines have been designed for special applications that are beyond the operational range of commercial instruments. A hightemperature electrophoresis apparatuses is described in [259–262]. Illustrations of home-made apparatuses for measurements of the electrophoretic mobility of gas bubbles can be found in [263,264]. A few methods of determination of the z potential based on electrophoresis use quantities other than particle velocity. The z potential can be calculated from the mass of particles transported in an electric field. This idea was commercialized by Micromeritics in an instrument called EMTA 1202 and by Numinco in an

45

Methods

instrument called MIC 1201. Home-made machines based on electrophoretic mass transport have also been used [265–267]. Illustrations of a cell are presented in [265–268]. The IEPs obtained by means of these machines are classified as “Electrophoresis” in the “Instrument” columns in the tables in Chapter 3. Electrophoresis is also accompanied by electro-osmosis, that is, the movement of liquid in an electrophoretic cell caused by the electric field. Analytical equations for electro-osmotic flow are known for the most popular cell geometries, namely, cylindrical and with a rectangular cross-section. The general equation for a rectangular cell is complicated, but it simplifies for a flat cell (high aspect ratio). Different ideas have been proposed to eliminate electro-osmosis or to properly correct for it. The measured velocity of particles with respect to fixed elements of the apparatus is the sum of the electrophoretic velocity of the particles and the electro-osmotic velocity of the liquid. Fortunately, for simple, highly symmetrical cell geometries, a stationary layer can be found; that is, at a certain distance from the cell walls, the electro-osmotic velocity is zero and the measured velocity of particles is not affected by the movement of the liquid. In the idealized case shown in Figure 2.2, there are two stationary levels, which are situated symmetrically with respect to the cell axis. Vertical lines represent the cell walls, and a uniform electric field is applied along the cell axis. The measured velocity of the particles (circles) depends on their position in the cell and is equal to the sum of the electrophoretic velocity of the particles with respect to the liquid (which is independent of position in the cell) and the electro-osmotic velocity of the liquid with respect to the cell, which depends on its position in the cell. At the stationary levels, the electro-osmotic velocity is zero and the measured velocity of the particles is equal to their electrophoretic velocity. Away from these levels, the measured velocity of particles is different from their electrophoretic velocity (indicated by a

Apparent velocity

Actual velocity of particles

Stationary levels Cell walls

Position in the cell

FIGURE 2.2 Electro-osmotic velocity of liquid (dashed parabola) and apparent velocity of particles (circles) in a flat cell. Zero velocity is represented by the horizontal solid line.

46

Surface Charging and Points of Zero Charge

two-headed arrow). In practice, the actual electro-osmotic profile often shows deviations from the theoretically calculated shape. There are also technical limitations related to the focusing of the optical system at the desired position in the dispersion. Therefore, electro-osmosis causes errors in electrophoretic measurements, especially in the early devices, which used elongated cells. These problems can be controlled in the “parabola method,” that is, measurement of the entire profile of apparent mobility across the symmetry axis of the cell rather than the mobility at a fixed distance from the cell wall (Figure 2.2). The shape of such a mobility profile nearly corresponds to a second-degree polynomial in a rectangular cell. The parabola method is tedious and not very popular, and the number of data points in one profile (seven points in Figure 2.2) is a question of subjective choice. Examples of such studies can be found in [269–272]. In several modern devices, the distance between the electrodes is comparable to the cross-section of the cell, and special cell designs reduce the problems with electro-osmosis. The parabola method makes it possible to measure the z potential of cell walls. Usually, the cell is made of quartz, and the parabola method thus offers the possibility of determining the IEP of one material that has already been extensively studied. The z potentials of macroscopic specimens of other materials can also be determined from the mobility profile [273–275] by replacement of the original cell wall of a commercial electrophoretic cell by a flat specimen of the material of interest. For example, in [276], the IEP of a basal plane of mica found from the mobility profile was different from the IEP of a mica dispersion. Only a few types of electrophoretic devices (most of which are no longer available on the market) can be used to determine z potentials by means of electro-osmosis.

2.1.2

ELECTRO-OSMOSIS

When the solid phase is fixed (e.g., as a capillary, membrane, or porous plug), an electric field induces a flow of liquid termed electro-osmosis. The character of the flow depends on the construction of the apparatus. For example, in an electrophoretic cell, the liquid flows in one direction near the walls and in the opposite direction in the center of the cell, and the net flow across the cell cross-section is zero (Figure 2.2). Electro-osmosis can also be demonstrated as a difference in pressure (height of a water column) generated as a result of an electric field applied to a capillary, membrane, or porous plug. Electro-osmosis has been chiefly discussed as a phenomenon accompanying electrophoresis (Section 2.1.1), but it also occurs separately and can be used to determine the z potential and the IEP. Electro-osmosis is recommended for macroscopic specimens, for fibers, and for large particles, which do not form stable dispersions. Opaque solutions do not pose a problem. A difference in the refractive index between solution and particles is not required. A relatively large amount of material is necessary to carry out a measurement. Electro-osmosis is not recommended for measurements at high ionic strengths (>0.1 M). Electro-osmosis in a mixture of different materials produces an averaged result, without the possibility of separating the components.

Methods

47

In principle, electro-osmosis is designed for nonconductive particles. At high field strengths and with conductive particles, the velocity may be higher by one or two orders of magnitude than that predicted by the Smoluchowski equation [277]. This phenomenon is termed electro-osmosis of the second kind. The other limitation of electro-osmosis is that the cell of the apparatus (the housing used to fix the porous plug) also induces an electro-osmotic flow. The measured quantity is a combination of the flow induced by the material of interest and that induced by the cell. Under certain conditions, the contribution of the latter can be negligible, but it can be significant in some systems. The same recommendations and limitations apply to measurements based on streaming potential (see Section 2.1.3, where a specific example of error induced by the streaming potential of the material of the cell is discussed). Equipment for measurements of z potentials based on electro-osmosis is not commercially available. A few descriptions of home-made electro-osmosis devices can be found in the literature [278–283].

2.1.3

STREAMING POTENTIAL

When the solid phase is fixed (e.g., as a capillary, membrane, or porous plug), a forced flow of liquid induces an electric field. The potential difference is sensed by two identical electrodes. The streaming potential or streaming current can be used to determine the z potential. The streaming potential and electro- osmosis can be observed in similar experimental setups, except that the natures of the force and the flux are reversed. Thus, the recommendations and limitations discussed in Section 2.1.2 also apply to measurements based on the streaming potential. For example, the instrument cell induces a streaming potential, which may contribute substantially to the result of the measurement. A linear dependence between the z potential obtained by electrophoresis and the streaming current measured by a commercial apparatus was observed in [284], but the line did not cross the origin; that is, a streaming current of zero was not equivalent to the IEP obtained by electrophoresis. This discrepancy was caused by the cell in the streaming current apparatus (used to fi x the porous plug), which had an IEP much lower than the IEP of the material of interest, and which contributed to the measured streaming current. In principle, the streaming potential method applies to nonconductive materials, but [285] reports z potentials of Ti and Ti6Al4V alloy (or, rather, of their corrosion products) obtained using the EKA apparatus (Paar). The tables in Chapter 3 report results obtained using the following commercial instruments based on streaming potential: • • • •

EKA from Brookhaven (BI, BIC, illustrated in [286]) Zetacad from CAD Inst. (France) ECA 2000 from Chemtrac Systems, USA EKA and SurPASS from Paar (Anton Paar, an illustration of the measurement cell can be found in [287])

48

Surface Charging and Points of Zero Charge

A few illustrations and descriptions of home-made streaming potential devices can be found in the literature [235,288–297]. An illustration and a description of a radial flow apparatus designed to measure streaming potential can be found in [298]. A streaming current detector based on a completely different principle than the above instruments is presented in [299]. The dispersion is in a narrow space between a vertical cylindrical vessel and a coaxial piston, which moves back and forth along the axis. The potential between two gold electrodes on the wall of the cylinder at different heights is measured, and its zero value is identified with the IEP. The apparatus’ own response corresponds to the electrokinetic behavior of the piston and cell materials. In the presence of a colloid, the piston and the cell are assumed to be covered with colloidal particles. The above design has been utilized in some commercial instruments: • PCD 02 and 03 from Mutek (illustrated in [286]) (the name of this instrument, which is derived from “particle charge detector,” is somewhat misleading) • Milton Roy Generation 1 from Pryde Instruments Pty. Ltd A rotating disk in an electrolyte solution induces a difference of electric potential between an electrode placed near the center of the disk and another electrode placed far from the disk [300,301]. The measured voltage is proportional to the 3/2 power of the rotation rate and to the z potential of the surface of the disk.

2.1.4

SEDIMENTATION POTENTIAL

The electric field induced by sedimentation of colloidal particles under gravity is termed the sedimentation potential. The potential difference is sensed by two identical electrodes placed at different heights. No commercial or home-made apparatus for measurements of z potential and IEP based on sedimentation potential have been reported in the recent literature. For the following commercial instruments referred to in the literature, the principle of operation (probably one of classical electrokinetic phenomena) has not been reported: ZP-10 B from Shimadzu, Japan; Zeta Reader Mark 21, Mitamura Riken; and Sugiura 2 VD.

2.1.5

ELECTROACOUSTIC METHODS

In classical electrokinetic phenomena, the forces and fluxes are independent of time. Electroacoustic effects are analogs of electrophoresis and sedimentation potential in which the forces and fluxes are variable in time. Alternating forces induce alternating fluxes of the same frequency, with a time delay. The phenomenological coefficients between the force and coupled flux can be used to calculate the z potential. The phase shift is a source of additional information about the system. The electric sonic amplitude (ESA) is the amplitude of the ultrasonic field

49

Methods

induced by an alternating electric field. The alternating electric field induces vibrations of colloidal particles and of ions of the supporting electrolyte. The contribution of the latter to the overall signal is negligible at low ionic strengths and high solid loads, but becomes significant at high ionic strengths and low solid loads. The ESA signal of a 1-1 electrolyte is proportional to [302] m- - rV- - (m+ - rV+ )r 1+ r

(2.2)

where r is the specific density of the solution; m and V are, respectively, the molar masses and volumes of the anion and the cation (subscripts + and -); and r is the anion-to-cation mobility ratio. The ESA signals of a few salts (e.g., LiNO3) are low owing to the mutual compensation of the anion and cation signals, while other salts (e.g., NaI and CsCl) produce strong ESA signals. A solution containing two salts each of which produces a strong ESA signal may have a negligible ESA signal owing to the compensation for the signals of the two salts. O’Brien derived a theory for ESA-based instruments. The signal of the colloidal particles is given by Ê Dr ˆ ESA = A(w ) f Á · m ÒZ Ë r ˜¯ D

(2.3)

where A(w) is an empirical frequency-dependent instrument constant, f is the solid volume fraction, r is the specific density of the solution, Dr is the difference in specific density between particles and the solution, Z is another empirical instrument constant, and · mD Ò is the particle-averaged dynamic mobility, defined as mD =

2 ez Ê wa 2 ˆ [1 + f (l, w¢ )] G 3h ÁË v ˜¯

(2.4)

where

G(a ) =

Ê1 ˆ 1 + (1 + i ) Á a ˜ Ë2 ¯ Ê1 ˆ 1 + (1 + i ) Á a ˜ Ë2 ¯

f (l, w¢ ) =

1/2

1/2

(2.5)

Ê1 ˆ + i Á a ˜ (3 + 2 Dr /r ) Ë9 ¯

1 + iw¢ - (2l + iw¢er / e ) 2(1 + iw¢ ) + 2l + iw¢er / e

(2.6)

50

Surface Charging and Points of Zero Charge

w¢ =

we K•

(2.7)

l=

Ks K• a

(2.8)

In Equations 2.4 through 2.8, K is the conductivity, a is the particle radius, and n (= h/r) is the kinematic viscosity. The subscripts relate to the following: S, liquid near the surface; •, bulk solution; and r, solid particles. The colloid vibration current (CVI) is the amplitude of the alternating current induced by an ultrasonic field. The potential difference is sensed by two identical electrodes. The theory of instruments based on CVI is discussed in detail in [303]. Recommendations and limitations are similar for CVI and ESA. Both techniques require calibration against standard dispersions or solutions, and then the quality of the results depends on the quality of the standard. The electroacoustic method can be used for a broad range of particle sizes, as well as for unstable dispersions. Sedimentation is prevented by a stirrer or peristaltic pump. The specific density of the particles must be substantially different from that of the solution. The refractive index of the particles is of limited significance. The electroacoustic method requires a substantial amount of material to produce reliable results. A low solid-to-liquid ratio implies a weak signal of the particles (Equation 2.3). Thus, the method is not suitable for dilute dispersions, but it can be used directly for dense dispersions. The electroacoustic method can be used at high ionic strengths. Such measurements require correction for the electrolyte signal. Special high-ionic-strength calibration is required in certain types of commercial equipment [304]. Commercial instruments have an electrolyte-correction procedure built into their software. The electroacoustic method produces an overall signal when two or more kinds of particles are present in the same dispersion, and the contributions of the components cannot be distinguished. The particles must not be electrically conductive. An introduction to electroacoustic methods and a discussion of their advantages and disadvantages can be found in the review [305]. The tables in Chapter 3 report results obtained by means of the following commercial instruments based on the electroacoustic method: • From Colloidal Dynamics: Acustosizer, Acustosizer II, and Zeta Probe • From Dispersion Technology: DT models 300 and 1200 (illustrated in [306]) • From Matec: ESA models 8000, 8050, and 9800; MBS models 8000 and 8050 • From Pen Kem: Acustophoretic Titrator 7000

Methods

51

2.2 EXPERIMENTAL CONDITIONS IN ELECTROKINETIC MEASUREMENTS A typical preparation of dispersions for electrophoretic or electroacoustic measurements consists of the following steps: 1. The original powder (e.g., from a commercial source) is subjected to washing, drying, and/or calcination. 2. A dispersion is prepared by mixing the powder with a solution of 1:1 salt (or with water). 3. The dispersion is aged for a certain time and/or ultrasonicated. 4. The solid-to-liquid ratio, pH, and ionic strength of a portion of the dispersion are adjusted by dilution, and a salt, acid, or base added. 5. The new dispersion is aged for a certain time and/or ultrasonicated. 6. A portion of the new dispersion (or of the supernatant obtained by settling under gravity or by centrifugation) is injected into the cell of the instrument. The pH is measured just before or during the electrokinetic measurement. Some of the above steps can be omitted, for example, in instruments equipped with an automatic titrator and pH probe. Steps 1 through 6 can be modified by using different types of labware (glass or plastic), protection against atmospheric CO2 (using freshly boiled water or bubbling of solutions and dispersions with nitrogen or another inert gas), thermostatting (many commercial instruments are equipped with a thermostat, but some are not), different types of stirrers or shakers set to different speeds, different concentrations of acid or base, different times of aging and modes of ultrasonication (time and intensity), different modes of filling and washing the cell of the instrument and different delay times between the injection and the measurement, and finally different instrument settings (adjustable parameters depend on the type of instrument). The above examples indicate how many experimental protocols can be designed. In many publications, the preparation of samples for electrokinetic measurements and the measurement itself are described in detail. In a few other papers, the description is limited to the type of instrument. The z potential is unequivocally defined by the composition of the solution in contact with solid particles. Therefore, conditions such as the solid-to-liquid ratio, aging, and ultrasonication should not affect the IEP. The use of optimum conditions for electrokinetic measurements may improve the reproducibility of results, but the IEP under different experimental conditions will be the same. In fact, in only a few studies was the experimental protocol indeed optimized, and in many other studies the experimental protocol was copied from other publications or arbitrarily selected. When the z potential and IEP depend on the experimental conditions, information on these experimental conditions is essential. For example, the acidic branch of the electrokinetic curves (using the Acoustosizer) reported in [307] was rather

52

Surface Charging and Points of Zero Charge

insensitive to the solid load, and on the basic branch the z potential increased with the solid load. The IEP of commercial aluminum hydroxycarbonate reported in [308] depended on dilution. The usefulness of an IEP that applies only to strictly defined experimental conditions is very limited. However, this is not obvious if the parameters that define the experimental conditions and that are easy to control are alone responsible for the observed discrepancies. Other parameters that are beyond control might have caused the discrepancies. In modern devices, certain parameters (e.g., time of measurement) are adjusted by the instrument software, and the experimentalist has limited control over them. The electrophoretic mobility depends on the z potential and on the shapes and sizes of the particles (see the more detailed discussion in Section 2.4.4). Particles that have the same z potentials but different shapes and sizes have different mobilities. In polydispersed colloids with irregular particle shapes, the z potential can only be estimated from the electrophoretic mobility, and the result depends on the approach to the definition of the size of an irregularly shaped particles. The size of a particle with irregular shape may be defined as: • The diameter of a sphere with same volume as the particle • The diameter of a sphere with same surface area as the particle • The diameter of a sphere with same surface-to-volume ratio as the particle • The diameter of a sphere with same drag resistance (or free-falling speed) as the particle • The diameter of a circle with same projected area as the particle (in selected or random orientation) • The width of the minimum square aperture through which the particle has passed In polydispersed assemblies of particles, different types of average size can be calculated, for example, as follows: • • • •

An average of linear dimensions Based on the square root of an average of surface areas Based on the cube root of an average of volumes Based on the ratio of the sum of surface areas to the sum of linear dimensions • Based on the ratio of the sum of volumes to the sum of surface areas • Based on the square root of the ratio of the sum of volumes to the sum of linear dimensions The above averages may be taken over actual dimensions or over dimensions of equivalent spheres of a certain type (see above). Different methods of calculation of equivalent sizes of irregularly shaped particles and of an average in an assembly of polydispersed particles can produce very different average particle sizes in the same assembly of particles. The problem of equivalent size of irregularly shaped

Methods

53

particles and different types of averages in assemblies of polydispersed particles is discussed in more detail in handbooks of colloid chemistry. Numerous electrokinetic studies have been carried out with monodispersed colloids, that is, assemblies of particles that have the same shape and size (with some scatter). Preparations and properties of monodispersed colloids are reviewed in [309,310]. The electrokinetics of monodispersed colloids are reviewed in [311]. In assemblies of identical spherical particles, the z potential can be calculated exactly from the electrokinetic mobility. The advantage of spherical shape and monodispersity, which make exact calculation of the z potential possible, is attained at the expense of purity; that is, monodispersed particles are usually not very pure. z potentials at one concentration of a specific inert electrolyte are sufficient to determine the IEP. Yet several publications report z potentials obtained at various concentrations and/or in the presence of various inert electrolytes. These studies demonstrated the independence of the IEP of the nature and concentration of the inert electrolyte. For example, [312–320] report z potentials obtained at three different ionic strengths, [321–324] report z potentials obtained at four different ionic strengths, and [325,326] report z potentials obtained at five different ionic strengths. The IEP of rutile in the presence of three alkali chlorides is reported in [327], and [328] reports a similar study of hematite (three sodium salts). Reference [329] reports z potentials obtained at two ionic strengths in five electrolytes, and [330] reports z potentials in the presence of two salts, each at two ionic strengths. Reference [331] reports z potentials obtained by means of electrophoresis with two instruments, at five and two ionic strengths, respectively. Numerous publications report specific solid-to-liquid ratios used in electrokinetic experiments. These ratios are of limited significance when the dispersion is unstable, and the dispersion in the instrument cell has a different composition from that originally prepared. The range of solid-to-liquid ratios used in electrophoresis is illustrated in Tables 2.1 and 2.2. The volume fraction can be converted into mass fraction, and vice versa, when the specific densities of the components are known. The specific densities of most powders of interest are in the range of 2000–6000 kg/m3, and their mass fractions are about two to six times higher than their volume fractions, thus both quantities are of the same order of magnitude. The range of solid-to-liquid ratios shown in Tables 2.1 and 2.2 is very broad, covering over four orders of magnitude. The range 0.00005–0.001 is very popular, and studies with very high and very low solid-to-liquid ratios are rare. The range of aging conditions used in electrophoretic measurements is illustrated in Table 2.3. The options for the preparation of dispersions for electroacoustic measurements are similar to those discussed above for electrophoresis, except that the experiments are usually carried out in titration mode and long equilibration before each measurement is not practiced, although long equilibration and/or sonication before the titration are commonplace. Various experimental protocols in electroacoustic measurements were compared in [423], for example, using a stirrer versus a pump and simultaneous measurements of the z potential and size versus using a pre-assumed size.

54

Surface Charging and Points of Zero Charge

TABLE 2.1 Mass Fractions of Solid Particles Used in Electrophoretic Measurements Mass Fraction 0.00000025 0.000001 0.00001 0.000014–0.00014 0.000015 0.00002 0.00003 0.00003–0.00005 0.00005 0.00005–0.002 0.000075 0.0001 0.0001–0.0003 0.0001–0.001 0.0001–0.01 0.00014 0.00015 0.0002 0.0003 0.0004 0.0005 0.0008 0.001 0.00125 0.0013 0.00167 0.0025 0.003 0.005 0.01 0.015 0.2

Reference [332] [333] [334] (latex), [335] [314] (results obtained with different solid mass fractions are similar) [336] [249] [337] [338] [334] (hematite), [339–347] [348] (results obtained with different solid mass fractions differ substantially) [349] [350–370], [371] (allowed to settle down, and only supernatant taken for measurements), [372,373] [374,375] [104] [376,377] [378] [379] [271,380–383] [123,382], [181] (allowed to settle down, and only supernatant taken for measurements) [384] [43,60,385–387] [388] (allowed to settle down, and only supernatant taken for measurements) [259,318,389–392] [393] [394] (centrifuged, and only supernatant taken for measurements) [395] [396] [258,397] (allowed to settle down, and only supernatant taken for measurements) [398], [399] (only supernatant taken for measurements) [400] (only supernatant taken for measurements), [401] [402] (allowed to settle down, and only supernatant taken for measurements) [403] (mass transport electrophoresis)

55

Methods

TABLE 2.2 Volume Fractions of Solid Particles Used in Electrophoretic Measurements Volume Fraction 0.00001 0.0001 0.0005 0.001

Reference [404] [147,405–411] [412] [413]

The range of solid-to-liquid ratios used in electroacoustics is illustrated in Tables 2.4 and 2.5. Generally, the solid-to-liquid ratios in electroacoustics are much higher than in electrophoresis.

2.3

CO2 AND SILICA PROBLEM

Silica and CO2 are omnipresent impurities that affect surface charging and may induce a shift in the PZC/IEP in certain systems. The discrepancies in the PZC/ IEP of the same material reported in various sources may be due to limited control over the silica, CO2, and other surface-active impurities.

2.3.1

THE CO2 PROBLEM

CO2 is absorbed from the atmosphere by solutions and dispersions, and induces a shift of their pH to lower values and an increase in their buffer capacity, especially at neutral and weakly basic pH. This pH shift is especially important in the interpretation of the results of potentiometric titration (Section 2.5) but is less significant in electrokinetic measurements. Moreover, dissolved carbonates show a certain degree of surface activity, which is important in all kinds of surface charging studies. Typical CO2 concentrations are 102.42 Pa in the atmosphere and 10-3 M (total inorganic carbon) in river or marine water [445]. In many studies, freshly boiled water and bubbling of solutions and dispersions with inert gases were used as protection against atmospheric CO2. For example, rigorous exclusion of silica and CO2 in [446] resulted in a relatively high PZC for goethite. In many other studies, less attention was paid to the exclusion of silica and CO2. For example, in [447], the IEP was determined in the presence of 0.001 M NaHCO3. That result was quoted in [448] as a pristine IEP. It is likely that IEPs obtained in the presence of carbonates are cited as pristine IEPs in many other secondary sources. A few systematic studies of the effect of CO2 on PZC/IEP have been carried out. Reference [449] reports an insignificant effect of CO2 on the charging of goethite. In contrast, [450] reports a shift in the IEPs of aluminum and iron oxides to low pH by about 1 pH unit in dispersions titrated with a Na2CO3 solution with respect to dispersion titrated with NaOH solution.

56

Surface Charging and Points of Zero Charge

TABLE 2.3 Aging and Ultrasonication before Electrophoretic Measurements Aging

Ultrasonication Yes 30 s 10 min 15 min >30 min

0–21 d 0–33 d 15 min 20 min–overnight 25 min 30 min

10 min, before aging

2h Overnight

15 h 15–18 h 1d

Before aging

1d 1d 1d 1d

15 min, before aging 3–5 min, before aging 15 min, before aging 5 min, after aging

>1 d 2d 3 and 15 d 10 d

15 min, before aging 3 min, before aging

Reference [414] [373] [411] [407] [384] (otherwise the results were irreproducible) [415] [416] [60,393,417] (at 25°C under nitrogen) [376,377] [402] (only supernatant taken for measurements) [181] (only supernatant taken for measurements) [418] [371] (after aging, dispersion was diluted, ionic strength and pH were adjusted, dispersion was aged for an additional 15 min with stirring, allowed to settle for 5 min, and supernatant was taken for measurements) [397] (after aging, pH of dispersion was adjusted and dispersion was aged for another 20 min and supernatant taken for measurements) [352] (argon) [336] [258] (dispersion was allowed to settle for 5 min, and supernatant was taken for measurements), [346] (in darkness), [398] (25°C, in polyethylene bottle), [419] (only supernatant taken for measurements) [123,347,396] [410] [271] (20°C, in polyethylene flasks) [420] [388] (only supernatant taken for measurements) [421] (dispersion aged after pH adjustment) [422] [349] (under nitrogen) [394] (centrifuged, and supernatant taken for measurements)

57

Methods

TABLE 2.4 Mass Fractions of Solid Particles Used in Electroacoustic Measurements Mass Fraction

Reference

0.025 0.03–0.05 0.05 0.05–0.07 0.05–0.45 0.1 0.13 0.2

[424,425] [376] [426] [377] [427] [428,429] [430] [431,432]

There is also mixed evidence about the effect of CO2 on the CIP of charging curves. Reference [451] reports an insignificant role of CO2 in the titration of alumina. In contrast, a substantial effect was found at a carbonate-to-aluminum ratio greater than 0.1 [452]. Shifts in the CIP in opposite directions in the presence of CO2 have been reported. Reference [453] reports a shift in the CIP of titania to high pH by 0.4 pH unit in the presence of 0.001 M NaHCO3. On the other hand, the CIP of goethite was shifted to low pH by about 1 pH unit in a system equilibrated with the atmosphere with respect to a CO2-free system [66,454].

2.3.2

THE SILICA PROBLEM

Numerous examples of underestimated IEPs in the literature are due to the adsorption of silicate on the materials of interest. The solubility of silica in water was

TABLE 2.5 Volume Fractions of Solid Particles Used in Electroacoustic Measurements Volume Fraction

Reference

0.001 0.003–0.025 0.005 0.01 0.01 or 0.05 0.02 0.03 0.05 0.1

[433] [350] [357,434] [435–437] [438,439] [440,441] [442] [443] [444]

58

Surface Charging and Points of Zero Charge

discussed in Section 1.6. The silica problem was not generally recognized until the 1970s. Cleaning methods described in early publications (e.g., Soxhlet extraction) might have induced silica contamination as a result of long contact of the material of interest with solutions saturated with products of dissolution of glass at high temperature. Silica leached out from glassware at room temperature induces shifts in the IEPs of certain materials to low pH. Long contacts of dilute dispersions of metal oxides with neutral and basic solutions in glass containers usually resulted in low IEPs. A few systematic studies of this effect have been carried out. The effect of storage of alumina and latex [455], and titania [455,456] dispersions in glass containers on their IEPs has been studied. Aging of alumina dispersions in Schott glass containers induced a shift in the IEP to low pH, while aging in a plastic container did not [457]. The effect of storage in glass on the IEPs of alumina and chromia were studied in [458] and [331], respectively. The effect of storage in glass on the PZC of iron oxide was studied in [459]. Silica present in natural iron (hydr)oxides produces low IEPs of natural materials compared with silica-free synthetic materials. Leaching of silica from natural ferrihydrite induced a shift in the CIP to a higher pH [460]. A correlation between the CIP and the silica-to-iron ratio was found in a series of natural and synthetic silica-containing ferrihydrites [460]. Uptake of silica from a 110ppm solution by alumina, iron(III) oxide, and clay minerals was studied in [461]. At pH 7–10, iron(III) oxide adsorbed silica completely. The uptake by alumina peaked at pH 9, but residual silica was also present in solution. Uptake of silica by clay minerals was observed in the basic range; in the acidic range, the silica concentration in dispersions was higher than the initial concentration, owing to leaching. Nowadays, measures against silica contamination are often undertaken, and this has become a standard procedure, too obvious to be specified in scientific papers. Plastic labware was used in the synthesis of iron (hydr)oxides in [462]. Contact with sources of silica was avoided in a cleaning procedure of alumina flat plates in [463]. Attempts to avoid silica are not always successful. An abnormally low IEP was found in a study of the force between two single-crystal sapphire platelets [464]. The authors explained the positive surface potential at pHs as low as 6.7 by the sorption of silica or other species from the solution. Silica is not the only surface-active anionic impurity that may be responsible for the abnormally low IEPs reported in the literature. The other impurities are less abundant (see Section 1.11), but they may be more surface-active than silica, and in certain systems the shift in the IEP to a low pH may be chiefly due to anionic impurities other than silica. The silica problem is a specific case of a general problem of surface-active anionic impurities, with silica being the most wellknown example of such an impurity. The specific nature of surface-active anionic impurities other than silica is not known. Thus, it is difficult to control and avoid them, and measures undertaken against silica contamination are not necessarily efficient against other surface-active anionic impurities. The discrepancy between the IEP of alumina monoliths (pH ⬇ 5) and that of alumina powders (pH ⬇ 9) has divided the scientific community into two groups. One group considers the low IEP of monoliths to be a well-established experimental

Methods

59

fact, resulting solely from alumina–inert electrolyte solution interactions. Another group challenges the validity of these low IEP measurements. Incorrect procedures or insufficient purity of the reagents have been invoked as the reason for the low IEPs of monoliths. For example, a shift in the IEP to low pH may be due to specific adsorption of anions. A low solid-to-liquid ratio makes the monoliths more sensitive to impurities as compared with the same material in the form of a fine powder. The ability of certain low-molecular-mass surface-active solutes to induce a substantial shift in the IEP can be assessed by comparing the number of surface-active molecules with the number of surface sites in the system of interest. The number of surface-active molecules must be at least comparable to the number of surface sites to induce a substantial effect. Otherwise, the contribution of surface-active molecules to surface charging is negligible. In potentiometric titrations and electroacoustics, a typical solid-to-liquid ratio of 100 g/dm3 (Table 2.4) produces 10,000 m2/dm3 for a 100 m2/g powder. Assuming 6 surface sites/nm2, which is a typical value used in model calculations (see Section 2.9.1), 10,000 m2/dm3 (titration and electroacoustics) corresponds to 0.1 mol/dm3 of surface sites. The concentration of surface sites exceeds the concentrations of impurities by several orders of magnitude; thus, the contribution of impurities to the surface charge is negligible. In microelectrophoresis, a typical solid-to-liquid ratio of 0.1 g/dm3 (Table 2.1) produces 10 m2/dm3, and 10-4 mol/dm3 of surface sites with a 100 m2/g powder. The concentration of surface sites is comparable to typical concentrations of silica found in laboratory water, and it exceeds the concentrations of other surface-active impurities. Under such circumstances, a shift in the IEP can be caused by silica, and the concentrations of other impurities are too low to induce a substantial shift. With a monolith, the solid-to-liquid ratio is in the region of 0.1 m2/dm3 (1 cm2 of sample in contact with 1 cm3 of solution), that is, 10-6 mol/dm3 of surface sites for the same material. The concentration of surface sites is much lower than the typical concentrations of silica found in laboratory water, and it may be comparable to the concentrations of other surface-active impurities. Under such circumstances, a shift in the IEP is almost certain. Even very strict measures against silica can hardly assure a silica concentration below 10-6 mol/dm3, and a few other impurities can also contribute to the shift, because their concentrations are comparable to the concentrations of surface sites.

2.4 EXPERIMENTAL RESULTS: z POTENTIAL The IEPs are compiled in Chapter 3. Other aspects of electrokinetic curves are discussed in this section. Figure 1.2 shows a typical set of three z(pH) curves obtained at different concentrations of an inert electrolyte. Similar curves (sets of curves) have been obtained in numerous real systems. With typical data sets, it is easy to determine the IEP. A few examples of less typical courses of electrokinetic curves reported in the literature will be discussed below. Specific values of the z potential are not indicated in Figure 1.2. Sufficiently far from the IEP, the z potentials assumed values in the range of about ±100 mV in the presence of a 0.001 M (or more dilute) 1-1 electrolyte, about ±60 mV in the

60

Surface Charging and Points of Zero Charge

presence of a 0.01 M electrolyte, and about ±40 mV in the presence of a 0.1 M electrolyte in many studies (see [2] for numerous examples). In several studies, the absolute values of the z potential at the respective ionic strengths were substantially lower, even very far from the IEP. The Smoluchowski equation, which is built into the software of most commercial zetameters, substantially underestimates the z potentials of fine particles at low ionic strengths (low ka, where k is the reciprocal Debye length and a is the particle radius), especially in the range of high z potentials. Underestimated values of the z potential may be due to erroneous calculations of the z potential from experimental data. A few unusually high values are reported in [465]: +120 mV for Fe(OH)2 and -180 mV for ZrO2 (at pH 10). A z potential in excess of -250 mV in an aqueous medium (from streaming current, using a home-made apparatus) is reported in [466]. Electrophoretic mobilites are usually below ±8 × 10-8 m2 V-1 s-1. A mobility in excess of -30 × 10-8 m2 V-1 s-1 in an aqueous medium is reported in [212]. Calculation of the z potential from experimental data is discussed in more detail in Section 2.4.4.

2.4.1

SHAPES OF INDIVIDUAL ELECTROKINETIC CURVES

The idealized electrokinetic curves shown in Figure 1.2 are smooth. Real curves are less smooth, and local minima and maxima are found. These represent a scattering of results rather than actual minima or maxima. The z potentials of silica in 0.01 M KCl (from electrophoretic mobility) plotted in the form of error bars showed standard deviations of 5–10 mV [249]. The z potentials of SiC plotted in the form of error bars showed standard deviations of about 2 mV [272]. Such a range of scattering is typical for electrophoretic measurements. Reference [397] reports a standard deviation below 0.5 mV in a series of 10 electrophoretic measurements. The reported electrokinetic curves are far from smooth, and this suggests that the standard deviation was underestimated in [397]. The rest of discussion in this section relates to the shapes of smoothed electrokinetic curves, disregarding the local minima and maxima caused by the scattering of results. The idealized electrokinetic curves shown in Figure 1.2 are symmetrical with respect to the IEP, and most experimental electrokinetic curves reported in the literature are nearly symmetrical. An asymmetrical z(pH) curve with a high z potential on the basic side is reported in [467]. An asymmetrical z(pH) curve with a high z potential on the acidic side is reported in [468]. The idealized curves shown in Figure 1.2 steadily decrease with pH and have no more than one IEP, but numerous electrokinetic curves reported in the literature show different shapes. The electrophoretic mobility of alumina in 0.1 M NaNO3 was pH-independent at pH 4–8 [469]. The apparent z potential of MoO3 increased with increasing pH [470]. Also, in one of the hematite samples studied in [471], a positive z potential is reported at high pH and a negative z potential at low pH. An increasing electrokinetic curve (e.g., curve “a” in Figure 8 of [472]) may even show z = 0 at a certain pH, but only sign reversal from positive to negative as the pH increases is considered as an IEP

Methods

61

in the present book. Electrokinetic curves with maxima and minima and multiple IEPs are commonplace in materials that undergo selective leaching (Section 1.6). A nearly symmetrical eletrokinetic curve for one sample of synthetic yttrium–iron garnet and atypical shapes (broad pH ranges with very low positive z below the IEP) for three other samples are reported in [473]. Many examples of materials that show positive z potentials at very low and very high pHs and negative z potentials at moderate pH (two IEPs) are reported in [104]. ZrY0.8O3.2 [474] and PbSO4 [475] showed three points of charge reversal. Numerous examples of atypical electrokinetic behavior, with negative z potential at low pH and positive z potential at high pH or with multiple points of sign reversal, were observed for sulfides [391]. Several publications report electrokinetic curves with maxima or minima of the z potential (beyond the scatter of data points) but without multiple IEPs. A minimum (maximum in absolute value) in the basic range is reported in all systems but one studied in [225], in [476], and in [266,477] (silica, with various KCl concentrations). A maximum in the z potential of FeOOH at pH ª 4.5 was found in [478]. These maxima or minima may be caused by an increase in ionic strength when the dispersion is adjusted to very high or very low pH.

2.4.2

POSITION OF IEP

Many experimental studies confirmed an insignificant effect of the nature or concentration of an inert electrolyte (Section 1.3) on the IEP. For example, the IEP of rutile at pH 6 was observed in the presence of three alkali chlorides [327], and the IEP of hematite at pH 7.5 was observed in the presence of three different sodium salts [328]. Only a few exceptions are reported in the literature. Reference [479] reports a shift in the IEP of titania induced by 0.001 M NaCl with respect to a salt-free system. Measurements of single planes of sapphire using a commercial streaming potential analyzer [59] resulted in irregular changes in the IEP (shifts in excess of 1 pH unit) with KCl concentration (0.001–0.1 M). Reference [480] reports a shift in the IEP of thoria by 2 pH units on increasing the NaCl concentration from 0.0001 to 0.001 M, and a shift by an additional 4 pH units on increasing the concentration from 0.001 to 0.01 M. The IEPs of iron (hydr)oxides found in [332] in 0.01 M KNO3 and NaNO3 matched, but those of manganese oxides showed substantial discrepancies. IEPs of a few specimens of MnO2 were found to increase with increasing ionic strength (>0.001 M NaNO3) in [481], but the IEPs of other specimens in the same study were independent of ionic strength (up to 1 M). The IEP of cobalt spinel obtained from streaming potential measurements [482] depended on NaCl concentration. The IEP of kaolinite shifited to low pH as the NaCl concentration increased, according to [483].

2.4.3

AGING AND HYSTERESIS

Numerous systematic studies of the effect of aging on electrophoretic mobility have been carried out. No substantial difference between samples aged for 1–14 days was observed in [484]. Also, the difference in electrophoretic mobility

62

Surface Charging and Points of Zero Charge

between dispersions of titania aged for 3 hours and 1 day was insignificant [485]. A drift in the z potential in time, and even sign reversal, were observed in [104]. The effect of aging at different pHs and ionic strengths on the z potential of quartz was studied in [486]. A substantial effect of aging (in glass and Teflon, at different pHs) on the electrophoretic mobility of mica is reported in [487]. Aging affected the electrophoretic mobility of silica–alumina composite material [488]. Aging of natural apatite in water induced sign reversal in 1 hour [489]. The effect of aging in these systems may be due to selective leaching out of components. The substantial difference in IEPs between fresh and aged dispersions of SiC [272] and Si3N4 [490] was probably due to oxidation. Electroacoustic measurements are usually carried out in titration mode, and several papers report results of both acid and base titration. Substantial hysteresis (Figure 1.9) was found in [242,427,491–493]. Other studies in similar systems report negligible hysteresis [444,449,494,495]. Four cycles of titration in ESA measurements reported in [496] produced different values of the z potential in the acidic range but a common IEP. Similar IEPs were observed in titrations of alumina from pH 2 to 12 and back, but in titrations from pH 1 to 12, the IEP was shifted to high pH [497]. Generally, more pronounced hysteresis is expected in titrations over a wider pH range.

2.4.4

EFFECT OF IONIC STRENGTH ON THE NUMERICAL VALUE z POTENTIAL

OF THE

The effect of ionic strength on the numerical value of the z potential is usually presented in the form of graphs similar to Figure 1.2. A few studies present plots of z potential as a function of ionic strength at constant pH in addition to (e.g., [249]) or instead of (e.g., [235]) graphs similar to Figure 1.2. High ionic strength induces a depression in the z potential of all kinds of surfaces. Many publications report a decrease in the z potential at constant pH as the ionic strength increases, according to expectations (Figure 1.2), but surprisingly many exceptions to this rule can be found in the literature. The papers reporting unusual ionic strength effects seldom discuss or attempt to explain this atypical behavior. A few types of abnormal effects of ionic strength on the numerical value of the z potential can be distinguished. An increase in the absolute value of the z potential at constant pH with increasing ionic strength (which is a trend opposite of expectations) is reported in [498]. The z potential of polystyrene latex increased in absolute value with increasing ionic strength [499], but silica studied in the same paper behaved normally. The mobility of goethite was found to increase with increasing ionic strength in [468]. The absolute value of z potentials reported in [343,500] decreased with increasing ionic strength in the acidic range but increased in the basic range. In contrast, the absolute value of z potentials reported in [501] increased with increasing ionic strength in the acidic range but decreased in the basic range. The mobility of kaolinite was found to increase in absolute value with NaCl concentration at pH < 5, but the usual behavior was observed at pH > 5 [483]. z potentials (from

Methods

63

the Smoluchowski equation) in the absence of an electrolyte reported in [257] are lower in absolute value than in 0.001 M NaCl in the range of very negative z potentials. In the range of less negative z potentials, the ionic strength effect complied with expectations. z potentials (from the Smoluchowski equation) in the absence of an electrolyte reported in [502] are lower in absolute value than in 0.005 M NaCl in the basic range. In the acidic range, the ionic strength effect roughly complied with expectations. z potentials (from the Smoluchowski equation) in 0.0002 M NaCl reported in [503] were lower in absolute value than in 0.002 M NaCl in the basic range. In the neutral and acidic range, the z potentials in 0.0002 and 0.002 M NaCl were equal, and higher electrolyte concentrations depressed the z potentials, as expected. In several studies, insignificant effects of ionic strength on electrophoretic mobility [66,382,504] and on the z potential [83,144,323,325,393,413,505–507] were reported. The effect of ionic strength on the absolute value of the z potential was insignificant in one sample of zirconia studied in [508], but two other samples behaved normally. Another common type of abnormal behavior is that the electrophoretic mobility or the z potential is independent of ionic strength over a certain range of pH or ionic strengths, with normal behavior being observed outside that range. For example, the electrokinetic curves reported in [319] are insensitive to ionic strength (0.01–0.3 M) in the acidic range. In the basic range, the electrokinetic curves obtained in 0.01 and 0.03 M electrolytes match, and only in the 0.3 M electrolyte was the z potential substantially lower in absolute value, according to expectations. Increasing the ionic strength from 0.01 to 0.1 M had a rather insignificant effect on the positive z potentials of alumina and zirconia determined in [509] by the electroacoustic method. The absolute value of the negative z potential was clearly depressed as the ionic strength increased, according to expectations. The z potential of hematite reported in [510] was rather insensitive to the ionic strength on the basic branch, but the acidic branch showed normal behavior. The difference in the mobility of silica in 0.001 and 0.01 M NaCl reported in [511] was insignificant, but 0.1 M NaCl significantly depressed the absolute value of mobility. An insignificant effect of KCl concentration on the z potential of anatase over the pH range 2–10 was reported in [512]. The effect of KNO3 was insignificant only in the basic range, and normal behavior was observed in the acidic range. Irregular effects of ionic strength, that is, minima and maxima in the z potential at constant pH as a function of ionic strength, have been reported in [513], [514] (there are many exceptions to the expected behavior of the system), [515] (the highest absolute value of the z potential is reported for the highest ionic strength in the basic range), [59] (the z potential of single planes of sapphire measured by a commercial streaming potential analyzer), and [516] (for a titania pigment, surface-modified with silica, alumina, and organic groups). The unusual effects of ionic strength on the apparent z potential may, to some extent, be due to improper calculation of the z potential from directly measured quantities. The Smoluchowski equation applies for large particles, high ionic

64

Surface Charging and Points of Zero Charge

strengths (large ka), and low z potentials. It underestimates z potentials of fine particles at low ionic strengths. Many numerical values of the z potential of fine particles at low ionic strengths reported in the literature were calculated by means of the Smoluchowski equation; thus, they are underestimated. Recalculation of these results using other methods that are more suitable for fine particles than the Smoluchowski equation would produce substantially higher z potentials at low ionic strengths, and only slightly higher z potentials at high ionic strengths. An example of such a correction for spherical particles a = 100 nm is presented in Figure 2.3. The apparent z potentials calculated using the Smoluchowski equation are equal for three different KCl concentrations (the thick solid line in Figure 2.3). The z potentials calculated from the same experimental results using the O’Brien– White theory [517] depend on the KCl concentration. With 0.1 M KCl, the difference between the z potentials from the Smoluchowski equation and those from exact theory is insignificant, but with 0.01 M KCl, the Smoluchowski equation substantially underestimates the z potential, and with 0.001 M KCl, the difference between the Smoluchowski equation and exact theory is even more significant. The results calculated using the O’Brien–White theory follow the expectations; namely, the absolute value of the z potential at constant pH decreases with increasing ionic strength. With 0.001 M KCl, the highest and lowest electrophoretic mobilities exceed the theoretical maximum/minimum, which corresponds to ±4.73 × 10-8 m2 V-1s-1 at ±122 mV. The calculations presented in Figure 2.3 apply to spherical particles of a specific size. With larger particles, the Smoluchowski equation is valid up to higher electrolyte concentrations, and a substantial difference between the Smoluchowski equation and exact theory is observed at higher ionic strengths. 100 80 0.001 M KCI 0.01 M KCI 0.1 M KCI

60

z (mV)

40 20 0 –20 –40 –60 –80 –100 pH

FIGURE 2.3 z potentials of spherical particles a = 100 nm at various KCl concentrations. The thick solid line represents apparent z potentials calculated by means of the Smoluchowski equation, which produced identical results for three ionic strengths. Thin lines with points represent z potentials calculated from the same experimental results by means of O’Brien–White theory. With 0.001 M KCl, the highest and lowest electrophoretic mobilities exceeded the theoretical maximum/minimum, which corresponds to ±4.73 ¥ 10-8 m2 V-1 s-1 at ±122 mV.

65

Methods

Exact theory does not produce an analytical solution, and the mobility at a given z potential is obtained as a result of successive approximations. Numerous equations give a good approximation to the exact theory over a broader range of ka than the Smoluchowski equation, for instance, the following Dukhin– Semenikhin equation: ÏÔ ˘ Ê1 ˆ È Ê1 ˆ Ê 1 ˆ Ô¸ 6 Ì y(1 + 3m) sinh 2 Á y˜ + Í2 sinh Á y˜ - 3my ˙ ln cosh Á y˜ ˝ Ë4 ¯ Î Ë2 ¯ Ë 4 ¯ ˛Ô 3heue 3 y Ô ˚ = - Ó 2 ekT 2 Ê1 ˆ Ê1 ˆ ka + 8(1 + 3m)sinh 2 Á y˜ - 24 m ln cosh Á y˜ Ë4 ¯ Ë4 ¯ (2.9) where ue is the electrophoretic mobility, y = ez/kT is the dimensionless z potential, and m ª 0.15 for aqueous solutions. Equation 2.9 is valid for a 1-1 electrolyte and ka >> 1.

2.4.5 EFFECT OF THE NATURE OF THE SALT ON THE NUMERICAL VALUES OF THE z POTENTIAL

z

The IEP is rather insensitive to the nature of an inert electrolyte, and the numerical values of the z potential far from the IEP are insensitive to the nature of the co-ion, but they do depend on the nature of the counterion. Thus, the acidic branch of an electrokinetic curve depends on the nature of the anion, and the basic branch depends on the nature of the cation. Therefore, in a series of inert electrolytes with a common anion (Figure 2.4), the acidic branch is common for all salts, and the differences are observed in the basic range; and in a series of inert electrolytes with a common cation (Figure 2.5), the basic branch is common for all

IEP

0

Cation 1 Cation 2 Cation 3 pH

FIGURE 2.4 z potentials in the presence of equal concentrations of three inert electrolytes with a common anion.

66

z

Surface Charging and Points of Zero Charge

Anion 1 Anion 2 Anion 3 IEP

0

pH

FIGURE 2.5 z potentials in the presence of equal concentrations of three inert electrolytes with a common cation.

salts, and the differences are observed in the acidic range. The specific effects of anions and cations of inert electrolytes on surface charging will be discussed in more detail in Chapter 4. Numerous examples of the usual behavior can be found in the literature, but there are also surprisingly many examples of abnormal behavior. Those papers reporting unusual effects of the nature of the electrolyte seldom discuss or attempt to explain that atypical behavior. The electrokinetic curves of alumina in the presence of NaCl and NaClO4 reported in [518] differ over the entire pH range (only a difference in the acidic range is expected). The acidic branch of the z potential (pH) curves (KBr vs. KNO3) was independent of the anion, while the basic branches were slightly different [519]. The anion effect (0.01M KCl, KBr, and KI) reported in [322] was negligible in the acidic range, but KI produced more negative z potentials than other salts in the basic range. A significant cation effect on both sides of the IEP is reported in [520]. A significant cation effect (with z potentials in KCl higher than those in NaCl) in the acidic range is reported in [521]. Unusual effects of the nature of 1-1 electrolytes on the electrokinetic behavior of a titania pigment (surface-modified with silica, alumina, and organic groups) are reported in [516].

2.5

EXPERIMENTAL CONDITIONS: TITRATION

Surface charging curves similar to those presented in Figure 1.1 are obtained in two steps. A dispersion containing a known amount of solid and a known amount of inert electrolyte is stirred (usually in a thermostated vessel, under an atmosphere of inert gas), and its pH is recorded. Small volumes of acid or base solution are added in pre-assumed time intervals or once the pH has reached a stable value (according to a certain criterion) after the previous addition. The titration is continued until a pre-assumed pH value is reached. Potentiometric titrations of dispersions produce plots of pH (the stable value or just before an addition of the

67

Methods

Electrolyte (experimental) Electrolyte (theoretical) Dispersion

pH

DV

Apparent PZC

DV

Volume of acid

FIGURE 2.6 Determination of DV (Equation 2.10) from potentiometric titration data (correction for inert electrolyte titration).

next portion of acid or base) as a function of volume (number of moles) of acid or base added (circles in Figure 2.6). A titration curve of dispersion alone is not sufficient to determine the PZC, and further information is required. Namely, analogous titrations are carried out for the electrolyte in the absence of colloidal particles (triangles in Figure 2.6). The titration curve of dispersion or electrolyte may be composed of two segments obtained by acid and by base titration starting at the natural pH. Alternatively, once base titration is complete, the same dispersion can be titrated with acid back to the natural pH and further in a more acidic direction; or acid titration can be carried out first, and, once it is complete, the same dispersion can be titrated with base back to the natural pH and further in a more basic direction. In another modification, the initial base titration is replaced by the addition of one large portion of base, and the dispersion is then titrated with acid back to the natural pH and further in a more acidic direction; or one large portion of acid is added, and the dispersion is then titrated with base back to the natural pH and further in a more basic direction. In all cases, proper bookkeeping of all added portions of acid or base is necessary. A blank titration curve can be easily calculated, and, at least in the nearly neutral pH range, the experimental curves do not differ much from theoretical predictions. A blank titration of a supernatant obtained at a natural pH rather than of pure electrolyte was recommended in [522]. The raw surface charge density is obtained by subtraction of the volume (number of moles) of acid or base (1 mole of base is equivalent to minus 1 mole of acid: double-headed arrows in Figure 2.6) that was used to bring about the same pH in the dispersion on the one hand and in the electrolyte solution on the other; that is, s0 =

Fc DV mA

(2.10)

68

Surface Charging and Points of Zero Charge

where c is the concentration of acid and base used in the titration, DV is the difference in volume of acid used to bring about the same pH in the dispersion on the one hand and in the electrolyte solution on the other, F is the Faraday constant, m is the mass of solid particles, and A is their specific surface area. Equation 2.10 assumes that the molarities of acid and base used in titration are equal and constant. In a more general case of variable molarity, s0 =

F Dn mA

(2.11)

where Dn is the difference in number of moles of acid (1 mole of base is counted as minus 1 mole of acid) used to bring about the same pH in the dispersion on the one hand and in the electrolyte solution on the other. The apparent s0 (in C/m2) is obtained from Equation 2.10 or 2.11. The surface charge density can also be expressed as charge per unit mass (in C/g) [523,524], especially when A is not available. The two representations (charge per unit mass or per unit surface area) produce different numerical values of charge density but the same PZC. In [525] and other papers from the same research group, the surface charge density is expressed as Z, the number of protons reacted per surface site. Z = 0 in their terminology is not necessarily the PZC [75]. Such results can be obtained when the number of surface sites per unit mass or per unit surface area is known. The procedure illustrated in Figure 2.6 (titration at one ionic strength) produces an apparent PZC (the apparent s0 from Equation 2.11 is equal to 0), which, under certain circumstances, can be equal to the actual PZC. When a charging curve is available at just one ionic strength, the apparent s0 from Equation 2.11 and the apparent PZC are the final results, which have been reported in many publications. Such results are indicated as “pH” in the “Method” columns in the tables in Chapter 3. The apparent PZC is nearly equal to the natural pH of the dispersion (with no acid or base added) and is equal to the actual PZC when the powder does not contain acid or base. This is theoretically possible, but it does not occur very often in reality. Even very pure materials contain certain amounts of acid or base, which are very difficult to remove or control, and which contribute to the apparent s0 from Equation 2.11. Therefore, determination of the PZC based solely on titration at one ionic strength is not particularly reliable. For example, the natural pH of dispersions in a series of iron hydroxides precipitated at different pHs (6–9) and from different iron precursors (chloride, nitrate, and sulfate) was similar to the pH of precipitation [526]. The natural pH of dispersions in a series of treated carbons was acidic for acid-treated and basic for base-treated carbon [527]. These examples indicate that the equilibrium pH of a dispersion is influenced by the liquid occluded in the powder. The natural pH values of dispersions reported in the literature were usually obtained at one ionic strength. Only in a few studies are the natural pH values of dispersions at different ionic strengths reported. For example, the natural pH of chromia dispersions was independent of KCl concentration, but decreased when the KNO3 concentration increased [528].

69

s0

Methods

CIP = common PZC

New “0” Original “0” Apparent PZC (c3)

c1 c2

Apparent PZC (c2) Apparent PZC (c1)

c3

pH

FIGURE 2.7 Correction for acid or base associated with solid particles (Equation 2.12).

The amount of acid or base associated with the powder (irrespective of the character of the interaction) can be estimated when a set of titration curves at various ionic strengths is available. The raw surface charging curves obtained at various ionic strengths have a CIP, which often corresponds to an apparent s0 π 0 (Figure 2.7), and the apparent PZC depends on the ionic strength. Such curves are corrected by the addition of a constant number to the raw values s0 from Equation 2.10; that is, the corrected s0 is calculated from the following equation: s0 =

Fc DV + sx mA

(2.12)

where sx is adjusted such that the CIP corresponds to s0 = 0, and the PZC is independent of ionic strength (Figure 2.7). The actual PZC coincides with the CIP (crossover point) of the charging curves obtained at various ionic strengths; such results are reported as “cip” in the “Method” columns in the tables in Chapter 3. The actual PZC (CIP) is usually different from the apparent PZC determined from titration at one ionic strength. The term “crossover point” has been used as a synonym for CIP, but it also has another meaning. For instance, in [529], the term “crossover point” denotes the equilibrium pH of a dispersion in 0.001 M KCl (“Method” = “pH”). sx reflects the difference between the original horizontal axis (the dot-dashed line in Figure 2.7) and the new horizontal axis (the solid line), and it can be calculated when titration curves at different ionic strengths are available. In [530], a CIP was not observed directly but was estimated by extrapolation. In principle, two ionic strengths are sufficient to obtain an intersection point and to calculate the correction. In [531], the intersection of two charging curves was recommended as a method to determine the PZC. However, the fact that the charging curves obtained at two different ionic strengths intersect at a certain point does not imply that all charging curves obtained at different ionic strengths will intersect at the

70

Surface Charging and Points of Zero Charge

s0

same point. The reproducibility and reversibility of charging curves are not perfect (see Figures 1.4 and 1.6 through 1.10), and this may result in a set of charging curves without a CIP. Each pair of charging curves in Figure 2.8 produces a different intersection point. The assessment of whether three charging curves have one (Figure 1.1) or more (Figure 2.8) intersection points is subjective. More precisely, the three intersection points shown in Figure 2.8 fall in a certain pH range. A narrow range (0.1 pH unit) is interpreted as the absence of a CIP (three intersection points). The absence of a CIP can be due to experimental error or to the intrinsic properties of the system of interest. The CIP is obtained by taking into account titrations at three or more ionic strengths. Apparent PZCs obtained from the intersection of two charging curves (only two different ionic strengths) are indicated as “Intersection” in the “Method” columns of the tables in Chapter 3 to distinguish them from PZCs obtained from the intersection of three or more charging curves. The fact that the charging curves obtained at three different ionic strengths intersect at a certain point implies that most likely the charging curves obtained at other ionic strengths will also intersect at the same point. The correction in Equation 2.12 reflects the presence of a certain amount of acid or base associated with solid particles, which is very difficult to remove (e.g., chemisorbed or occluded in narrow pores) and which contributes to the acid–base balance. The steps illustrated in Figures 2.6 and 2.7, that is, correction for acid or base used for the titration of an electrolyte and for an acid or base introduced with solid particles, are standard procedures, which were described in early publications and appear in numerous handbooks. They are seldom specified in scientific publications; only corrected titration curves are usually presented. In a few studies, the second correction (sx in Equation 2.12) was ignored. For example, in [532], a CIP was obtained, but uncorrected PZCs for particular ionic strengths

c1 c2 c3 pH

FIGURE 2.8 A set of charging curves without CIP.

Methods

71

were reported, and uncorrected charging curves were modeled. A linear scale is usually used to plot corrected and uncorrected s0, and nonstandard presentations are rare. The logarithm of the absolute value of an uncorrected s0 was plotted as a function of pH in [533]. It should be emphasized that the correction illustrated in Figure 2.7 applies for surface charging curves obtained by titration, but not for electrokinetic curves. A series of matching (or not matching) IEPs (z = 0) at different ionic strengths is obtained from a series of individual electrokinetic curves. In a few papers (e.g., [512]), the intersection of electrokinetic curves obtained at various ionic strengths is used to determine the IEP. Such an approach was used in [534,535], and the intersection of electrokinetic curves occurred at a high negative z potential. Thus, the numerical values of “PZC” reported in [534,535] are erroneous. The correct IEPs are reported in the tables in Chapter 3, and the original authors’ incorrect values are ignored. The same symbol is used for corrected (Equation 2.10) and uncorrected (Equation 2.12) s0, and the details of data handling are seldom reported in the experimental parts of scientific papers. A few publications report PZCs obtained by titration without clear explanations of whether they were obtained as CIPs or if titration was performed at only one ionic strength. Such results are indicated as “Titration” in the “Method” columns of the tables in Chapter 3. A few methods that give a PZC equivalent to that from the potentiometric titration method are described in Section 2.8.4.

2.5.1

THE CHOICE OF AN INERT ELECTROLYTE AND THE RANGE OF IONIC STRENGTHS

The standard procedure based on titrations at exactly three concentrations of the same salt (Figure 2.7) was used to obtain the CIP in numerous studies. Different inert electrolytes were selected by different authors, with NaCl, KCl, NaNO3, and KNO3 being more popular than other 1–1 salts. There is no fundamental reason to prefer one of these salts in surface charging studies. Usually, the lowest ionic strength was about 0.001M (difficulties in studies at lower ionic strengths are discussed in Section 1.10.2), the highest ionic strength was about 0.1M, and the difference between the highest and lowest electrolyte concentrations was a factor of 10 or more, with a few exceptions. In [536], the CIP of charging curves obtained in 0.1, 0.3, and 0.7M NaCl is reported. In [537], the difference between the highest (0.2M) and lowest (0.04M) ionic strengths was only a factor of 5; in [83] (two ionic strengths), it was only a factor of 3_13 . Not surprisingly, the difference in slopes between particular charging curves was not very significant. A difference in ionic strength by an order of magnitude leads to a substantial difference in the slopes of charging curves. The most popular (but entirely arbitrary) selections of electrolyte concentrations for surface charging studies are 0.001, 0.01, 0.1, and 1M. Usually, each dispersion for titration at a certain ionic strength is prepared from a separate portion of powder. Alternatively, one portion of powder can be used in consecutive titrations at increasing ionic strength; that is, the calculated amount of

72

Surface Charging and Points of Zero Charge

inert salt is added to the dispersion once the titration at a lower ionic strength is completed. This procedure was termed the “combined method” in [538] (backand-forth titration was followed by salt addition and a new titration), but, in the tables in the present book, “Combined Method” is considered as a variant of “Method” = “cip.” The results obtained with one portion of powder in all titrations are not affected by possible differences in properties between particular portions of powder. On the other hand, the error in Dn in Equation 2.11 increases with the total number of portions of acid or base added in a series of consecutive titrations of the same portion of powder. Another experimental procedure minimizes the error in Dn in Equation 2.11 but maximizes the error due to differences in properties between particular portions of powder. That is, each data point on a charging curve can be obtained with a separate portion of powder. In this approach, only one portion of acid or base is used to obtain a point on a charging curve, and propagation of errors resulting from addition of multiple portions of acid or base is avoided. Titrations at four ionic strengths are presented in [539,540], at five ionic strengths in [319,324,541–543], at six ionic strengths in [544], and in seven ionic strengths in [545]. Titrations at three concentrations of four salts are presented in [44], at four concentrations of one salt and one concentration of another salt in [546,547], at eight concentrations of five salts in [548], and at five or six concentrations of four salts in [549]. Titrations of rutile at two concentrations of five salts [327] and of hematite at three concentrations of three salts [328] have been carried out.

2.5.2

SOLID-TO-LIQUID RATIO

Typically, the solid-to-liquid ratios in titrations are fixed in a series of titrations (constant for all ionic strengths) and are relatively high (e.g., 1:10 by mass in [550]). Titrations at two different solid-to-liquid ratios, but only one ionic strength, are reported in [525]. Titrations at three different solid loads are reported in [551,552]. Titrations at different solid-to-liquid ratios are reported in [553]. The typical mass fraction of the solid in titrations is a few percent. Lowsurface-area powders require a high solid-to-liquid ratio. Reference [554] reports titrations at a solid load as low as 1% by mass, and [44] reports titrations at a solid load as low as 0.75% by mass. The solid-to-liquid ratios in other studies were higher, and the upper limit of the solid-to-liquid ratio is determined by the viscosity of the dispersion.

2.5.3

OTHER TITRATION PARAMETERS

The titration protocol (for a selected electrolyte at a certain concentration and a fixed solid-to-liquid ratio) defines the starting and end points of titration, the distances between the data points, and the rate of titration. The distances between data points can be defined by a fixed amount of titrant added per data point or by fixed differences in pH between data points. The rate of titration can be defined by a fixed time of equilibration or by an accepted rate of pH drift (the data point is taken once the rate drops below a pre-assumed value). Even in fast titrations,

Methods

73

long equilibration before taking the first data point is commonplace. For example 1 day’s equilibration before titration is reported in [554] and more than 8 hours’ equilibration of PbS in an acidic medium before base titration is reported in [13]. The titration can be carried out manually or automatically. A computer-controlled titrator was used in [555,556]. The results of titration performed according to a given experimental protocol are not necessarily representative of the system of interest. Even the reproducibility of titration with a fixed protocol is often limited. Apparently, in addition to the parameters discussed above, which are easy to control, the course of titration is also affected by factors that are difficult to control. Starting and end points of titration can be read from titration curves reported in the literature. In studies aimed at determining the PZC, the starting and end points should be selected so that the expected PZC lies between them. Specific solids have a limited pH range of low solubility, and titrations outside that range are not recommended. The direction of titration (from low to high or from high to low pH) is not always specified in scientific papers. Most papers report the results of titrations performed in one direction. Base titration starting at a low pH is often preferred, because of easier removal of CO2 from acidic than from basic solutions (see Section 2.3.1 for a discussion of the CO2 problem). Results of backand-forth titrations are reported in [45,112,119,336,530,557–566] (see Section 2.6.2 for more details). The typical distance between the data points in titrations, which can be read from titration curves reported in the literature, is 0.1–1 pH unit. The advantage of a substantial pH difference between consecutive points is that the volume of acid or base in Figure 2.6 is obtained as a sum of relatively few portions of the titrant, and the error in DV in Equation 2.10 is low. The advantage of a small pH difference between consecutive points is that a data point in the close neighborhood of the PZC is always available, and interpolation is not necessary to determine the CIP. The advantages of both strategies, that is, low error in DV and availability of data points near the PZC, are combined in the following titration program. One large portion of acid or base is added to almost reach (but not to exceed) the PZC. Then, titration is continued in small steps. In other words, the data points densely cover the narrow pH range on both sides of the PZC, but there are very few (or no) data points outside that range. Such a titration program is recommended in titrations aimed at determining the PZC rather than of the entire s0(pH) curve. An approximate position of the PZC can be determined in a preliminary experiment with a more uniform distribution of data points over a broad pH range. A broad range of titration rates has been reported. Fast titration with 3–5 minutes per data point was performed in [567]. Equilibration for a few days (until a constant pH is reached) was allowed in [553], 3 days’ equilibration was allowed in [568], and 1 day’s equilibration in [44]. In these studies, each data point was taken using a separate portion of dispersion. In [569], a data point was recorded when the drift was less than 0.1 mV/s (about 0.002 pH unit/s). A 10–48 hour equilibration period produced similar titration curves of kaolinite, but with 5 minutes’

74

Surface Charging and Points of Zero Charge

equilibration time, the titration curve was different, especially in the acidic range [140]. A study of the effect of equilibration time on charging curves is reported in [570]. The advantage of fast titrations is that the result is rather insensitive to slow processes such as dissolution and recrystallization of the solid phase. In fast titrations, the adsorption/desorption of protons may be not complete. The effect of an ultrasonic field on titration was studied in [571].

2.6 RESULTS: TITRATION PZCs obtained by titration are compiled in Chapter 3. Other aspects of titration curves will be discussed in this section. Figure 1.1 shows a typical set of s0(pH) curves obtained at three different concentrations of an inert electrolyte. Similar sets of curves have been obtained in numerous real systems. A few examples of less typical courses of surface charging curves reported in the literature will be discussed below.

2.6.1

PRESENCE OR ABSENCE OF CIP

Raw titration curves need a correction according to Equation 2.12, which is applicable only for sets of charging curves with a CIP. Therefore, the presence or absence of a CIP is essential. CIPs are usually observed for metal oxides, and sets of charging curves without a CIP are typical of silica (see Section 2.6.3) and clay minerals (see Section 2.6.6). The presence of a CIP implies that the absolute value of s0 increases with ionic strength on both sides of the PZC. Insignificant effects of the concentration of (C2H5)4NCl (0.001–0.1 M range) on the charging curves of silica [127], and of the concentration of NaNO3 (0.001–0.1 M range) on the charging curves of Fe2O3 [572], were observed over the entire pH range. Decreasing s0 with increasing ionic strength is reported in [573,574]. Irregular behavior (decreasing or increasing s0 with increasing ionic strength over different ranges of pH and/or ionic strength) was reported for alumina [575], hematite (normal behavior in the acidic range and abnormal behavior in the basic range) [576], and anatase [515]. The charging curves of hydrous manganese oxide reported in [577] merge at pH < 3; at pH > 3, the apparent charge density decreased in absolute value with increasing ionic strength (0.015–1.5 M). The basic branches of charging curves were almost insensitive to CsCl concentration, and s0 of red mud in the presence of NaCl showed an irregular ionic strength dependence [578]. In a few sets of charging curves, s0 was independent of ionic strength over a certain pH range, and it increased in absolute value with increasing ionic strength outside that range. Charging curves that merge at a low pH, and an increasing absolute value of s0 with increasing ionic strength at a high pH, were reported in [579] (hematite; merged at pH < 6 in the presence of two salts, but exhibited the usual behavior in the presence of a third salt), [580] (alumina), [581] (rutile; merged at pH < 5), [582] (titania coating on silica; merged at pH < 4), [39] (alumina; merged at pH < 5, three different electrolytes), [583] (rutile and cassiterite; merged

Methods

75

FIGURE 2.9 A set of charging curves at three electrolyte concentrations (c1 < c2 < c3) that merge at low pH.

at pH < 5), and [83] (manganite; merged at pH < 8). The charging curves of goethite presented in [584,585] merged at pH > 8.5, and the charging curves of FeOOH presented in [586] merged at pH > 10. The ionic strength in [586] was low (up to 0.002 M); at a high pH, the amount of base added to adjust the pH was high compared with the amount of salt. The charging curves of alumina presented in [587] had an intersection point at pH 8.2, but merged in the basic range. Charging curves that merge at one end of the pH scale are illustrated in Figures 2.9 and 2.10. The position of the pH axis (s0 = 0) is uncertain; it is indicated by the dashed lines in Figures 2.9 and 2.10. The upper or lower limit of the pH range in which the charging curves merge is a characteristic pH for a set of titration curves, but this

FIGURE 2.10 A set of charging curves at three electrolyte concentrations (c1 < c2 < c3) that merge at high pH.

76

Surface Charging and Points of Zero Charge

point is not sharp, and it is not recommended to identify that point with the PZC. More examples of merging s0(pH) curves are reported in the tables in Chapter 3 (“Merge” in the “Method” or “pH0” column). The charging curves of aluminum (hydr)oxides reported in [112] merge at pH 6–8 (no clear CIP), and they behave nearly as expected outside that pH range. The charging curves of goethite in [537] showed a CIP in the presence of NaNO3 and NaCl, but in the presence of Li salts there was no clear CIP. Other sets of charging curves without a clear CIP are reported in [160,588–590]. The charging curves obtained in a 0.005–0.06 M electrolyte in [319] had a CIP, but the charging curve obtained in a 0.3 M electrolyte failed to have a CIP with other curves. The charging curves of ZnO obtained at the lowest ionic strengths in [549] had a CIP, but the charging curves obtained in an electrolyte over 0.1 M did not cross the other curves at that point. For sets of charging curves without a clear CIP, the PZC cannot be independent of the ionic strength, irrespective of the value of the correction term in Equation 2.12. Thus, the concept of an inert electrolyte in the sense discussed in Section 1.3 is not applicable for the set of charging curves shown in Figure 2.8.

2.6.2

REPRODUCIBILITY AND REVERSIBILITY

The question of reproducibility of titrations has been addressed in a few publications (e.g., the results of repeated titrations are reported in [570]) but are ignored in many others. Poor reproducibility of silica titration curves at low ionic strength and good reproducibility at high ionic strength are reported in [591]. Poor reproducibility of charging curves of amorphous HFO is reported in [592]. Studies of reversibility (acid titration vs. base titration) are more numerous. Good reversibility (negligible hysteresis) of acid and base titrations was demonstrated for gibbsite (pH 5–10) [530,557], bayerite (pH 5–10) [557], goethite (pH 4–11 [558] and pH 5–10 [336]), and alumina (pH 6–10) [559]. Ceria nanoparticles pre-aged in different solutions produced reversible titration curves, but the CIP depended on the pH at which they were aged [593]. Negligible hysteresis for fast titration and more significant hysteresis for slower titration (20 minutes’ equilibration) were found in [560]. The hysteresis in titration curves of silica was negligible between pH 3 and 4.5, but when the titration direction was reversed at pH 8.5, substantial hysteresis was observed [561]. Significant hysteresis was observed in the titration of aluminas (pH 4–10) [112], gibbsite [562], hematite (slow titration) [119], goethite (at pH ª 7) [563], alumina-coated rutile [45], montmorillonite (from natural pH to pH 4 and then to pH 9) [564], another sample of montmorillonite [565], and carbon [566]. The first titration of humic acid produced different results, owing to incomplete dispersion, but the following titrations were fully reversible [594]. Storage of zirconia for 1 year in air induced a shift in the CIP by 1.4 pH units [595]. The CIP of a-alumina was insensitive to aging in water at 100∞C for 1 day [106]. The CIP of zirconia aged at pH 4.4 overnight was at pH 8, and that of the same zirconia aged at pH 9.4 overnight was at pH 7.2 [596].

77

Methods

In principle, the course of charging curves and the CIP should be independent of the solid-to-liquid ratio, and systematic studies in this direction are rare. The CIP in a yttria–NaClO4 system was found to shift to a high pH as the solid load increased [597].

2.6.3

SHAPE OF CHARGING CURVES AND TYPICAL VALUES OF s0

s0

Most charging curves reported in the literature are relatively smooth. A few studies resulted in irregular charging curves with local inflection points. For example, [598] reports smooth charging curves for Fe hydrous oxide and irregular charging curves for Al hydrous oxide. The irregular shape of the latter charging curves is probably due to the dissolution of the solid phase (which is difficult to control) accompanying surface charging or to incomplete dispersion or wetting rather than to the surface charging itself. The irregular shape of the charging curves has also been interpreted in terms of surface heterogeneity (see Section 2.9.3.2). All of the charging curves shown in Figures 1.1, 2.9, and 2.10 have similar shapes. Their slopes are negative and pH-dependent. The slope is more negative far from the PZC (on both sides) and less negative near the PZC; that is, a curve has its inflection point near the PZC. An extreme case of this type of behavior, namely an s0 of alumina equal to 0 at pH 5–7 (slope = 0), is presented in [39]. Usually, the difference in slopes between particular pH ranges is less spectacular. Lyklema and co-workers [599,600] demonstrated the similarity of charging curves of various metal oxides at a certain ionic strength and temperature when s0 was plotted against pH-PZC, and the only difference between the different materials was in the positions of their PZCs. The similarity of the shapes of the charging curves demonstrated for selected samples of hematite, rutile, and RuO2 was impressive, but the charging curves of other samples of oxides are not that similar.

PZC

0 c1 c2 c3 pH

FIGURE 2.11 A set of charging curves at three electrolyte concentrations (c1 < c2 < c3) with high absolute value of the slope near the PZC.

78

Surface Charging and Points of Zero Charge

s0

c1 c2

c3 pH

FIGURE 2.12 (c1 < c2 < c3).

A set of charging curves of silica at three electrolyte concentrations

In contrast with Figure 1.1, in a few studies, the slopes of the charging curves were less negative far from PZC and more negative near the PZC (Figure 2.11), or the slope constantly increased or decreased (no inflection point). The charging curve of maghemite reported in [601] is a straight line (constant slope) over the pH range 3–10. The charging curves of silica shown in Figure 2.12 have basic branches similar to those obtained for metal oxides (Figure 1.1). In the acidic range, the s0 of silica tends asymptotically to zero, positive s0 is not observed, and there is no CIP. A substantial positive s0 and an intersection point for quartz are reported in [602]. A substantial positive s0 of silica (but without a clear CIP) is reported in [603,604]. An unusual course of titration curves of silica, with a high slope at a low pH, is reported in [41]. The highest absolute values of s0 of metal oxides obtained by titration 2–3 pH units from the PZC are usually about 0.1 C/m2 for a 0.001 M electrolyte and 0.3 C/m2 for a 0.1 M electrolyte. These values are substantially higher than those of Hg and AgI, which are in the region of 0.05 C/m2 in the presence of a 0.1 M electrolyte [605]. Comparison of the s0 of mercury, of AgI, and of oxides can be made assuming that 1 pH or pI unit corresponds to 59 mV (Nernst equation). Silica (Figure 2.12) shows a relatively low s0 in the acidic and neutral ranges (as compared with metal oxides), and s0 of silica at pH 8 is in the same range as that of metal oxides. The ranges of s0 values reported in different publications for given materials are illustrated in numerous compilations. For example, charging curves of different aluminas in 1 M electrolyte solution are compiled in Figure 1 of [557]. Charging curves of common oxides in 0.001–1 M electrolyte solutions are compiled in [2]. Discrepancies by a factor of up to 3 in s0 values of certain oxides obtained by different authors (at the same pH and ionic strength) are commonplace, but discrepancies by an order of magnitude are rare.

79

Methods

The discrepancies in reported s0 values are to some degree due to the discrepancies in the specific surface areas reported in different publications for the same material. A few examples of surprisingly low or surprisingly high s0 can be found in the literature. A s0 as low as 0.01 C/m2 in 0.1 M electrolyte, 4 pH units below the PZC, was found for high-surface-area goethite in [569]. An absolute value of s0 of UO2 exceeding 1 C/m2 (on both sides of the PZC) in 0.1 M electrolyte is reported in [590].

2.6.4

EFFECT OF IONIC STRENGTH ON CHARGING CURVES

The effect of ionic strength on the charging curves can be quantified in a form of the Esin–Markov coefficient: Ê dpH ˆ b=Á Ë d log asalt ˜¯ s

(2.13) 0

which was originally introduced for the Hg electrode and was later adopted by Lyklema [606] for oxides. In typical charging curves, b = 0 at PZC, b Æ 1 for high positive s0, and b Æ -1 for high negative s0. In the vicinity of the PZC, b decreases smoothly from a maximum value (typically between 0.6 and 1) to a minimum value (typically between -0.6 and -1) [607]. A positive b on the negative branch and a negative b on the positive branch of charging curves correspond to a decreasing absolute value of s0 as the ionic strength increases. A few examples of such abnormal behavior are presented in Section 2.6.1. A very few studies (e.g., [579] for hematite/KNO3, in the basic range) report a maximum in the absolute value of b exceeding 1. Merging of charging curves (b = 0 over a broad pH range far from the PZC) is an extreme case of a low absolute value of b, and a few examples of such charging curves were discussed in Section 2.6.1. Charging curves of aged magnetite with a maximum b = 0.4 are reported in [158]. The same aged magnetite in the basic range and fresh magnetite showed absolute values of b in the usual range. Charging curves of hematite with a maximum b = 0.5 are reported in [608]. Charging curves of goethite with a maximum b = 0.4 and a minimum b = -0.2 are reported in [609]. The same paper reports charging curves of alumina with a minimum b = -0.2, while the b of the charging curves of titania (positive and negative branch) and of alumina (positive branch) was in the usual range. Charging curves of brucite with a maximum b = 0.1 are reported in [507].

2.6.5

EFFECT OF THE NATURE OF THE SALT ON NUMERICAL VALUES OF s0

The CIP is rather insensitive to the nature of an inert electrolyte. This was demonstrated for rutile (five salts) [327], hematite (three salts) [328], and many other systems. Numerical values of s0 far from the CIP are insensitive to the nature of the co-ion, but do depend on the nature of the counterion. Thus, the acidic branch of charging curves depends on the nature of the anion, and the basic branch of charging

80

Surface Charging and Points of Zero Charge

curves depends on the nature of the cation. Therefore, in a series of inert electrolytes with a common cation, the acidic branch is common for all salts, and the differences are observed in the basic range; and in a series of inert electrolytes with a common anion, the basic branch is common for all salts [610,611], and the differences are observed in the acidic range. Charging curves obtained at the same ionic strength in a series of salts with a common anion or common cation have a course analogous to the electrokinetic curves shown in Figures 2.4 and 2.5, except that the salt that produces the highest absolute value of the z potential gives the lowest absolute value of s0, and vice versa. The specific effects of anions and cations of inert electrolytes on surface charging will be discussed in more detail in Chapter 4. Numerous examples of the typical behavior described above can be found in the literature, and only a few examples of abnormal behavior have been reported. The papers reporting unusual effects of the nature of the electrolyte seldom discuss or attempt to explain that atypical behavior. A substantial effect of the nature of an inert electrolyte on the CIP was reported for alumina (KNO3 vs. KCl) [559], titania (CsCl vs. NaCl) [612], goethite [44], and b-FeOOH [613]. The charging curves of titania reported in [614] are insensitive to the nature of the electrolyte (0.1M NaCl, KCl, NaNO3, or KNO3). In contrast, the s0 of titania in the presence of NaCl reported in [615] was lower than in NaClO4 on both sides of the PZC.

2.6.6

SURFACE CHARGING OF MATERIALS OTHER THAN METAL OXIDES

The surface charging behavior of materials other than metal oxides reported in the literature is less consistent than that of metal oxides. A few studies suggest similarity to metal oxides (see Figure 1.1), but other studies suggest completely different behavior. The presence or absence of a CIP of charging curves obtained at different ionic strengths is an example of a property for which contradictory results have been reported. A clear CIP (for three or more ionic strengths) has been reported for kaolinite [616], hydroxyapatite [617], synthetic hydroxyapatite [618], various activated carbons [619,620], and bacterial cell walls [621]. An intersection point of titration curves obtained at two ionic strengths has been reported for kaolinite and monomorillonite [125] and for activated carbon [622]. Mixed evidence (the presence of a CIP for certain specimens or at certain conditions, and the absence of a CIP for other specimens or at different conditions) has been reported for homoionic illite [623], silicon nitride [624], and Al- and Ti-doped MCM-41 [625]. Ionic-strengthindependent charging curves (and thus the absence of a CIP) have been reported for glass [626], bentonite [36], kaolinite [100,627], sepiolite [628], and humic acid [629,630]. Nearly parallel charging curves (and thus the absence of a CIP) have been reported for kaolinite [631,632], montmorillonite [564,632], and humic and fulvic acids [633]. Sets of charging curves without a CIP have also been reported for kaolinite [189], fulvic acid [634], humic acid [594], and bacteria [635].

2.7 RELATIONS OF RESULTS OBTAINED BY DIFFERENT METHODS For numerous samples of pure sparingly soluble metal oxides, the IEPs and PZCs (CIPs) obtained by titration matched or nearly matched. A matching PZC obtained

Methods

81

by different methods and under different experimental conditions is more significant as a characteristic quantity for a given material than a PZC obtained by one method at specific conditions. A few publications report substantial discrepancies between the PZC and IEP [636,637]. In common models of the electrical double layer at a metal oxide–inert electrolyte interface (see Section 2.9), the PZC and IEP must be equal; thus, a discrepancy between these quantities is considered as an indication of the presence of impurities or of experimental error. Interpretation of potentiometric titration data in terms of s0 requires numerous assumptions, which are often not verified. For example, water-soluble species, which may contribute to the consumption of acid or base in titration, must be absent or properly corrected for. An analysis of supernatants for products of dissolution was carried out in [127], but in most publications the consumption of an acid or base is entirely attributed to surface charging. It is often the case that different size fractions are used in titrations on the one hand and electrophoresis on the other. These fractions may have somewhat different properties, but a large difference between the PZC and IEP is not expected. Several models allow differences between the PZC, CIP, and IEP, and the equality of these quantities has been challenged in a few publications [638–641], even for sparingly soluble metal oxides. Rejection of the assumption that CIP = PZC implies that the correction for an acid or base associated with solid particles illustrated in Figure 2.7 is not applicable, and the correction term in Equation 2.12 has to be determined by another means. With materials other than sparingly soluble metal oxides, the charging curves obtained at different ionic strengths often do not exhibit a CIP, and even if they do, it is not necessarily equal to the IEP. Discrepancies between CIP and IEP have been reported for various activated carbons [619].

2.8 OTHER METHODS 2.8.1

METHODS INVOLVING NONAQUEOUS SOLVENTS

An acid–base scale for solids based on titrations with 1-butylamine or trichloroacetic acid in benzene in the presence of indicators was described in [642]. In [643], 0.2 g of material was mixed with 3 g of heptane and 0.01 cm3 of pH indicator solution. A series of indicators was used, and the apparent pH detected by these indicators was identified with the PZC. These methods are reported as historical curiosities, and they are not recommended by the present author.

2.8.2

ELECTRICAL METHODS

The surface charging of materials that show a certain degree of electric conductivity can be measured directly by electrical methods. Such measurements are not used to determine the PZC. The potential of a hematite electrode prepared as a coating on Pt [243] was Nernstian (59 mV/pH unit) at 20∞C in 0.005 M KCl. Also, a monocrystalline hematite electrode [644] had a nearly Nernstian potential in the acidic range in 0.0005 M NaNO3, but the slope was lower in the basic range and

82

Surface Charging and Points of Zero Charge

in 0.0068 M NaNO3. Hysteresis (acid vs. base titration) was observed in the potential of a monocrystalline hematite electrode [170]. A monocrystalline anatase electrode showed 82% of Nernstian slope in 0.001 M NaClO4, and 65% of Nernstian slope in 0.001 M NaCl [615], and even lower slopes are reported in [645]. The properties of an anatase electrode formed as a layer on the surface of metallic Ti were studied in [646]. The open-circuit potential of TiO2 films on Ti was studied in [647]. The slope was about 41 mV/pH unit in the pH range 5–10. A monocrystalline pyrite electrode was studied in [648]. The PZC was estimated from capacitance–voltage characteristics in electrolyte–insulator–semiconductor structures in [649], but this method is not recommended by the present author.

2.8.3

SUM FREQUENCY GENERATION AND SECOND-HARMONIC GENERATION

The orientation of water dipoles at a charged surface affects light reflected from that surface. This phenomenon can be used to determine pH0. A few efforts in this direction have been published. Selective sum frequency vibrational spectroscopy was proposed as a tool to determine pH0 [650]. Several publications are devoted to second-harmonic generation, in which two photons of the same frequency are combined. The experimental setup is shown in [651,652]. Monochromatic (w) and polarized laser light is reflected at a corundum–liquid interface (from the solution side or from the solid side in a total internal reflection geometry), and the intensity of the second-harmonic frequency (2w) is measured in the reflected light. In [651], the second-harmonic field strength increased when the pH increased, and the pH effect became more significant when the ionic strength decreased. The curves obtained at different ionic strengths intersected at pH 5–6, which was taken to be the PZC. The experiments were carried out with three sodium salts, and the intersection points were consistent. However, the course of the second-harmonic field strength versus pH curves at pH > 5 was strongly anion-dependent. This result does not support the hypothesis that the intensity of the second-harmonic frequency is solely dependent of the surface potential, as postulated in [651]. Namely, at pH > pH0, the surface charge and surface potential are rather insensitive to the nature of the anion (see Figure 2.4). Qualitatively similar results were obtained in [652], but only one salt was used. Different crystallographic faces of corundum produced different intersection points in the pH range 4–5. Possible sources of discrepancies between the pH0 obtained by the methods discussed in this section on the one hand and by standard methods on the other are discussed in Section 2.3.2.

2.8.4

METHODS EQUIVALENT TO TITRATION

A few methods may produce PZCs equivalent to those obtained by potentiometric titration (with or without correction for acid or base associated with solid particles). There are a limited number of such methods, although some of them have been “re-invented” several times, and given different names. Only a few of these names are used in the tables in Chapter 3. The methods that produce results

Methods

83

equivalent to potentiometric titration without correction for acid or base associated with solid particles (natural pH of dispersion) are referred to as “pH” in the “Method” columns of those tables. The methods that produce results equivalent to the CIP are referred to as “cip.” The terms “batch equilibration” [653], “pH drift” method [654], “addition method” [552], “solid addition method” [655], “powder addition method” (cited in [656] after [654]), “potentiometric titration” [234] (“sic”—in the present book, the term “potentiometric titration” is reserved for a different method, described in Section 2.5), and “salt addition” [573] (“sic”—in the present book, the term “salt addition” is reserved for a different method, described later in this section) refer to the same method, which is now described. A series of solutions of different pHs is prepared and their pHs are recorded. Then, the powder is added and the final pH is recorded. The addition of a solid induces a shift in the pH in the direction of the PZC. The pH at which the addition of powder does not induce a pH shift is taken to be the PZC. Alternatively, the PZC is determined as the plateau in the pHfinal(pHinitial) curve. The method assumes that the powder is absolutely pure (free of acid, base, or any other surface-active substance), which is seldom the case. Even with very pure powders, the above method is not recommended for materials that have a PZC at a nearly neutral pH. Namely, the method requires accurate values of the initial pH, which is the pH of an unbuffered solution. The display of a pH meter in unbuffered solutions in the nearly neutral pH range is very unstable, and the readings are not particularly reliable. The problem with pH measurements of solutions is less significant at strongly acidic or strongly basic pHs (see Section 1.10.3). The above method (under different names) became quite popular, and the results are referred to as “pH” in the “Method” columns in the tables in Chapter 3. The experimental conditions in the above method (solid-to-liquid ratio, time of equilibration, and nature and concentration of electrolyte) can vary, but little attention has been paid to the possible effects of the experimental conditions on the apparent PZC. The plateau in the pHfinal(pHinitial) curve for apatite shifted by 2 pH units as the solid-to-liquid ratio increased from 1:500 to 1:100 [653]. Thus, the apparent PZC is a function of the solid-to-liquid ratio. The “potentiometric mass titration” method [657,658] produces results equivalent to those of the “drift method” described above. The same amount of base is added to three dispersions with different solid-to-liquid ratios and a constant ionic strength. The dispersions are titrated with acid, and the pH is recorded as a function of the amount of acid added. The intersection point of the obtained curves is taken as the PZC. In other words, the PZC is identified with the pH at which solid addition does not induce a change in pH. The “drift method” and mass titration are based on the same principle, the difference being that in “potentiometric mass titration,” the reagents are added in a different order. Potentiometric mass titration is affected by the acid or base associated with the powder in the same way as in the “drift method” and mass titration. The advantage of potentiometric mass titration over the “drift method” is that in the former the pH is measured only in buffered systems.

84

Surface Charging and Points of Zero Charge

In the set of charging curves shown in Figure 1.1, the pH of a dispersion equilibrated at a certain pH shifts in the direction of the CIP on the addition of an inert electrolyte. This property is used in the salt addition method, which dates back to the 1960s [659]. This method was termed “salt addition” in [660] and “salt titration” in [661]. In the present book, the term “salt titration” is reserved for a different method, described below. In a classical salt addition method, a series of dispersions of constant solid-to-liquid ratio and different pH is prepared, and DpH (the shift in pH induced by an increase in the KNO3 concentration from 0.01 to 0.1 M) is plotted as a function of pH. The PZC is determined as the intersection of the curve with the pH axis. Such a PZC is equivalent to the intersection of the charging curves obtained in 0.01 and 0.1 M KNO3. Different salts and different concentrations produce different values of DpH, but similar PZCs. The advantage of the salt addition method is that deviations in the solid-to-liquid ratio, in the amount of dispersion, or in the salt concentration before and after salt addition have limited effects on the results. These deviations may affect the value of DpH, but the sign is always positive below the PZC and negative above it. The pH in the salt addition method is measured in a wellbuffered system; thus, the reading of a pH meter is stable and reliable. The method is insensitive to the acid or base associated with the solid particles. The disadvantage of the salt addition method is that it gives only the PZC, but not s0. The salt addition method is based on the assumption that the intersection of the charging curves at two ionic strengths represents the CIP, which in real systems is not always the case (see Section 2.6.1). Therefore, the salt addition method is not recommended for materials other than sparingly soluble metal oxides. Certainly, the choice of salt in the salt addition method is limited to an inert electrolyte. The idea of using a similar procedure with non-inert electrolytes [662,663] has also appeared in the literature, but such a method does not produce a pristine PZC. For spinels [664], the salt addition method gave reproducible results with KCl but irreproducible results with KNO3. The salt titration method [666] is a modification of the salt addition method. A portion of salt is added to a dispersion, and the DpH is recorded. When DpH > 0, base is added to shift the pH to an even higher value. When DpH < 0, acid is added to shift the pH to an even lower value. Once a constant pH has been established, a new portion of salt is added, and DpH is recorded again. The series of salt additions followed by acid or base additions is continued until DpH = 0. The advantage of the salt titration method as compared with the classical salt addition method is that only one portion of dispersion is used. Thus, PZC determination requires a smaller amount of solid and only one reaction vessel. Moreover, a series of measurements (e.g., at different temperatures or at different concentrations of a nonaqueous co-solvent) can be carried out with the same portion of solid, and effects due to a difference in surface properties between different portions of solid are avoided. The number of consecutive salt additions in the salt titration method is limited, because the sensitivity of DpH to an addition of the same amount of salt decreases as the initial salt concentration increases.

Methods

85

In the mass titration method, the PZC is determined as the natural pH of a concentrated dispersion. A detailed description of the experimental procedure can be found in [667]. Mass titration become popular in the late 1980s [668,669], but the same method was already known in the 1960s as the “pH drift” method [183]. Usually, a series of natural pH values of dispersions with increasing solid loads is reported, but only the natural pH of the most concentrated dispersion is actually used. The only role of the data points obtained at lower solid loads is to confirm that a plateau was reached in pH as a function of solid load; that is, a further increase in the solid load is unlikely to bring about a change in pH. The mass titration method is based on the assumption that the solid does not contain acid, base, or other surface-active impurities. This is seldom the case, thus mass titration often produces erroneous PZCs. In this respect mass titration is similar to the potentiometric titration without correction illustrated in Figure 2.7, only the solid-to-liquid ratio is different. The experimental conditions in mass titration (solid-to-liquid ratio, time of equilibration, nature and concentration of electrolyte, and initial pH) can vary, but little attention has been paid to the possible effects of experimental conditions on the apparent PZC. The effect of an acid or base associated with solid particles on the course of mass titration was studied in [670]. To this end, a series of “artificially contaminated samples” was prepared by the addition of an acid or base to a commercial powder. The apparent PZC of silicon nitride obtained in [671] by mass titration varied from 4.2 (extrapolated to zero time of equilibration) to 8.2 for time of equilibration longer than 20 days. The method termed “mass titration” was used in [672], but it was different from the method discussed above. A potentiometric titration curve often has an inflection point at the PZC (Section 2.6.3). This property has been proposed as a method to determine the PZC [673]. The “inflection point” method gained some popularity after a publication by Zalac and Kallay [670]. Also, the “differential potentiometric titration” described in [674] is equivalent to the inflection point method. This method is not recommended by the present author as a standalone method to determine the PZC, but a few results obtained by the “inflection point” method, usually in combination with other methods, are reported in the tables in Chapter 3 (as “Inflection” in the “Methods” columns). In [675], the potentiometric titration curve of one sample had two inflections, and the inflection at the lower pH was assumed to be the PZC. The potentiometric titration curves of other samples had one inflection each. Reference [676] reports an inflection point in the titration curve of niobia at pH 8, which is far from the pH0 reported in the literature. A few examples of charging curves without an inflection point or with multiple inflection points are discussed in Section 2.6.3. In classical potentiometric titration, soluble species produced by dissolution of the solid may contribute to the consumption of an acid or base and induce an error in the calculated s0. This error is not corrected for in back titrations of an inert electrolyte (Figure 2.6). The back-titration method was designed to avoid the error induced by the dissolution of the solid phase, and it has been used for solids that show appreciable solubility. For solids that are practically insoluble, the

86

Surface Charging and Points of Zero Charge

back-titration method would probably produce the same results as classical titration, although this has not been confirmed experimentally. In the back-titration method, which gained popularity among soil scientists, a series of dispersions with different final pHs is prepared, and each supernatant is titrated. In [451], the initial pH values of solutions (before powder addition) were recorded, and each supernatant was titrated back to the respective initial pH. The amount of acid/base consumed in each back titration was used in Equation 2.10 to calculate s0. In [677], each supernatant was titrated to pH 7, and s0 was calculated from the difference in the amount of acid/base used to adjust the pH of the dispersion on the one hand and in the back titration of the supernatant to pH 7 on the other. Typically, the equilibration time in the back-titration method is long (about 1 day), but in principle the same procedures can be followed with shorter equilibration times. Back titration at one ionic strength has the same disadvantage as classical titration at one ionic strength, namely, an acid or base associated with solid particles induces an error in s0. This error can be corrected for by performing back titration at three or more ionic strengths, and the correction is calculated in the same way as in classical titration (Figure 2.7). Back titration has a serious drawback, which has been overlooked by its adherents. Equilibration of alumina (taken as an example of a solid that shows appreciable solubility) at a high or low pH produces water-soluble aluminum species. In back titration, these species form a colloid of high surface area. The nucleation process is rather unpredictable and difficult to control. Thus, the properties of colloidal particles are irreproducible. The acid or base is used up not only in neutralizing the charged solution species, but also in charging the colloidal particles produced in the titration process. Owing to the unpredictable properties of colloidal particles, the later process is difficult to account for, and when the surface charging of the precipitate is ignored in the interpretation of back-titration results, the calculated surface charge is erroneous. The error caused by surface charging of colloidal particles is less significant at high solid-to-liquid ratios, and it can be eliminated when the supernatant is titrated to the pH of the PZC of the material of interest (about 9 for alumina). Then, an acid or base is not used to charge the colloid. However, the exact position of the PZC is unknown before the titration is completed. Certainly, the back-titration method produces correct results when the solubility of the powder is negligible, but in such cases classical titration will give similar results. The above criticism is not aimed at the idea of correction for soluble species in solution—it only points out that back titration (in the form described in the literature) does not solve the problem. The charging curves of alumina obtained by means of back titration [677] were not smooth (there were many local minima and maxima and inflection points), and the pH0 (CIP) was substantially lower than typical values observed for alumina. An original approach to determining the PZC was presented by Pechenyuk et al. [678,679]. They precipitated series of materials from the same precursor at various pH values, and studied the natural pH of the dispersions of the precipitate at different ionic strengths. Not surprisingly, the natural pH of these dispersions (of fresh or aged precipitates) was similar to the pH of precipitation due to an acid or base associated with solid particles. The pH of precipitation, which was equal

Methods

87

to the natural pH of the dispersion of the precipitate, was identified with the PZC. Indeed, the PZC obtained by means of the method described above matched the PZC obtained by well-established methods. The nomenclature used by Pechenyuk et al. does not meet the standard recommended in the present book. The method of Pechenyuk et al. is applicable to a series of materials precipitated from the same precursor at various pH values, but is not applicable to specific materials that might have been obtained by methods other than precipitation. A coulometric method to determine s0 (with OH- ions being generated electrochemically) was suggested in [680].

2.8.5

FORCE BETWEEN PARTICLES

The force between particles is the sum of a pH-independent van der Waals component, which is always attractive, and a pH-dependent electrostatic component, which can be attractive or repulsive. In Derjaguin–Landau–Verwey–Overbeek (DLVO) theory, the z potential is used to calculate the interaction force or energy as a function of the distance between the particles. Atomic force microscopy (AFM) makes it possible to directly measure the force between the particles as a function of the distance, and commercial instruments are available to perform such measurements. Different approaches have been proposed to utilize the results obtained by AFM to determine the pH0. The quantity obtained by AFM corresponds to the IEP rather than the PZC. AFM was used to measure the force between SiO2 (negative z potential over the entire studied pH range) and Si3N4 (IEP to be determined) in [681]. The pH at which the force at a distance of 17 nm was equal to zero was identified with the IEP. The van der Waals forces are negligible at such a distance, and the force is governed by an electrostatic interaction. The experimental results were consistent with DLVO theory. AFM was used to measure the force between an Si3N4 tip and Al2O3, SiO2, and SnO2 surfaces in [682]. The force changed its sign at pH0 of the tip or of the surface. Reference [683] reports a pH0 determined by scanning force microscopy. In spite of the sign reversal of the electrostatic force, there is no abrupt change in the total interaction force at the pH0, and the pH0 can rather be roughly estimated than exactly determined from the AFM results. AFM is not usually employed as a standalone method to determine the pH0, but rather to verify the IEP obtained by other methods. A few examples are given in the tables in Chapter 3. Coagulation behavior is a direct consequence of the interaction force, and a maximum coagulation rate, maximum aggregate size, and maximum sedimentation rate coincide with the IEP. Different techniques have been used to quantify coagulation behavior, and different physical quantities (e.g., intensity of transmitted or of scattered light, and mass of sediment) have been directly measured. A more or less sharp minimum or maximum of the measured quantity was observed at the IEP. Sharp extrema are observed at low ionic strengths. Coagulation is not usually employed as a standalone method to determine the pH0, but rather to verify the IEP obtained by other methods. A few examples of such studies are reported in the tables in Chapter 3. Studies reporting IEPs based on coagulation

88

Surface Charging and Points of Zero Charge

behavior that were not verified against other methods [481,684,685] are ignored in Chapter 3. The pH dependence of the coagulation behavior of silica is not fully understood, and is very different from that observed for metal oxides [686].

2.8.6

NONSTANDARD METHODS

A few nonstandard methods to determine the PZC have been proposed. These methods are not recommended as replacements for standard methods, but they may be useful in certain systems for which standard methods are not applicable. The PZC of titania was obtained in [687] as the pH at which the rate constant of reduction of methyl viologen in the presence of titania was independent of ionic strength. Adhesion of positively and negatively charged latex was used to estimate the IEP of stainless steel (original and heated in air for 2 hours at 1000∞C) [688]. Adhesion occurs when the signs of the charge of the studied surface and of the latex are opposite. The same method has been used for other metals [689]. The nonzero electrical conductivity of metals excludes measurements of their IEP by means of standard methods. A different version of the adhesion method was used in [690]. An amidinegrafted polystyrene latex with IEP at pH 6 (electrophoresis) was used. IEPs of thin films of iridium and tungsten oxides (among other materials) were determined, and the results complied with standard methods. Coincidence between the maximum in contact angle as a function of pH and IEP has been claimed [691–693]. A method of determining the IEP based on measurements of the contact angle would be beneficial for studies of thin films and of single faces in monocrystals. To be considered as a reliable method, it has to be supported by a theory, and to pass extensive tests against materials with well-established IEPs. Some IEPs reported by adherents of the contact angle method matched those determined by standard methods, but others did not. The contact angles of kaolinite, pyrophyllite, and illite as functions of pH are reported in [60]. The angles were rather insensitive to pH, and did not show a clear maximum at the IEP. A chromatographic method to determine the s0 of materials with low surface area was proposed in [694]. s0 was calculated from the diffuse front of a breakthrough curve. Measurements at pH ª 7 are not possible in this method, since the solution is unbuffered and it is difficult to obtain a solution of well-defined pH. As is classical potentiometric titration, the charging curves usually require a correction for an acid or base associated with solid particles. The uptake of anions of a 1-1 electrolyte by materials that show pH-dependent surface charging decreases with increasing pH and the uptake of cations of 1-1 electrolyte increases. At a certain pH, the uptake of both ions of a 1-1 electrolyte is equal, and such a pH has been taken as the PZC, especially in soil chemistry [26]. The present author does not consider measurements of uptake of ions of a 1-1 electrolyte as a reliable method to determine the PZC. In many theoretical models, the point of equal uptake coincides with the PZC, but this coincidence is

Methods

89

of limited practical importance. The uptake of ions of an inert electrolyte near the PZC is usually low (see Section 2.9.3.3 and Chapter 4 for more detailed discussion) and rather insensitive to pH. Then there is a broad pH range in which the uptake of the anion and cation of an inert electrolyte are almost equal, and the apparent PZC (point of equal uptake) is very sensitive to accidental scatter of results and to experimental errors. The points of equal uptake of ions of 1-1 electrolytes reported in the literature are ignored in the tables in Chapter 3, although they have been quoted in secondary sources as the PZC. A few PZCs reported in [1] (and quoted after [1]) were obtained by methods other than titration or electrokinetic methods. The principles of these methods were not described in detail, but names such as “suspension effect,” “by adsorption of Zn and Co,” and “minimum gelation rate” suggest that these PZCs do not meet the standards recommended in the present book, and such results are ignored in Chapter 3. On the other hand, a few PZCs obtained by means of the “drift method” cited after [1] are cited as such in Chapter 3. The pH of the minimum of solubility of calcite was termed the “PZC” or “IEP” in [695,696]. The same authors report PZCs obtained from flotation by anionic and cationic collectors. These “PZCs” (which have been quoted in numerous papers) do not meet the standards recommended in the present book and are ignored in the tables in Chapter 3. The rate of suction in filter paper in contact with a dispersion as a function of pH was observed [697] in a specially designed apparatus, originally described and illustrated in [698], and a shallow minimum in suction time was observed at the IEP. A method of determination of the PZC from the filtration rate was proposed in [699]. However, it is difficult to point to any advantage of methods based on filtration rate other than originality.

2.9 ADSORPTION MODELS The present discussion is focused on models of primary surface charging, that is, of adsorption of protons in the presence of inert electrolytes. These models are elements of more general models, which describe adsorptions of all kinds of species. Basically, the models discussed in this section apply to metal oxides, but similar models have been used for other materials. For example, a model of proton and heavy metal binding by humic acid described in [700] is similar to models used for oxides.

2.9.1

DENSITY OF PROTONABLE SURFACE GROUPS

In the absence of surface-active species, the surface charge of metal oxides is due solely to the adsorption/desorption of protons. The ions of an inert electrolyte remain at some distance from the surface and affect the surface charging indirectly. In most models, proton adsorption and desorption is interpreted as the protonation and deprotonation of discrete surface sites. Direct spectroscopic observation of these surface sites is difficult, and it does not give clear and

90

Surface Charging and Points of Zero Charge

unequivocal information about the nature and density of surface sites. The surface of titania at various pH values was studied by internal reflection spectroscopy in the infrared range in [701], and different wavenumbers were assigned to different degrees of protonation of surface TiOH groups. An X-ray photoelectron spectroscopy (XPS) study [702] indicated the presence of ⬅SiOH2+ and of two other surface species on the surface of silica. In view of the limited success of spectroscopic methods, indirect information is often used to gain information about surface sites. A common approach is that in certain materials the number of surface sites is proportional to the surface area, which is the product of mass and specific surface area. Specific surface area is an important element of characterization of adsorbents, and varies widely from one sample to another. Available data for particular samples are reported in Chapter 3. The values of specific surface areas reported for certain commercial materials in different publications are usually consistent, but serious discrepancies are also common. These discrepancies may reflect actual differences between particular lots of material with the same trade name. The specific surface areas reported in Chapter 3 often refer to purified or ground materials, which may differ substantially from the original material. Most specific surface areas reported in the literature have been obtained by means of the Brunauer–Emmett–Teller (BET) method, and numerous types of commercial instruments for such measurements are offered. Many publications report specific surface areas of commercial materials provided by manufacturers. Various techniques, differing in the number of data points, the range of pressures, and even the nature of the gas adsorbed, are termed BET techniques (more precisely, techniques based on the BET equation for the adsorption isotherm), and experimental details are seldom reported. Nitrogen is the gas most commonly used in the BET method, but krypton BET has also been used [703]. A few studies report comparisons of results obtained by the BET method in different laboratories or under different conditions. A perfect match between nitrogen and krypton specific surface area is reported in [704]. Argon and nitrogen specific surface areas of three powders reported in [705] matched within 5%, but over 20% difference was obtained with the fourth powder. Specific surface areas of five materials from three laboratories reported in [538] show differences by a factor up to 2. There was no regular trend; that is, low and high values were found among results from each laboratory. These and other problems with BET surface area are discussed in [706]. Techniques other than BET can give substantially different specific surface areas. For example, ethylene glycol monomethyl ester (EGME) penetrates the interlammelar spaces in clay minerals, and with these materials it produces a substantially higher specific surface area than the BET method. Nitrogen and H2O specific surface areas for a series of minerals are reported in [707], and a good correlation was found between specific surface area and anion adsorption capacity. Specific surface areas and particle radii determined by different methods are compared in [708]. The surface area given by the manufacturer was re-determined by three different methods in [709]. The three methods were used to determine the specific surface area in [144], and the results nearly matched. The surface area was

Methods

91

determined by means of room temperature water vapor adsorption in [710,711] and by the glycerol method or by paranitrophenol adsorption from xylene in [711]. Phosphate adsorption was used to determine the specific surface area in [229]. In monodispersed colloids, the specific surface area can be calculated from the specific density of particles and their geometrical dimensions. The concentration of surface sites is usually reported as site density per unit surface area. Different approaches to the nature of surface sites responsible for primary surface charging can be found in the literature. Different definitions of surface sites are discussed in [712]. Not surprisingly, different approaches lead to different site densities. In older publications, the surface sites were identified with the surface OH groups that are formed by surface reactions of sparingly soluble oxides with liquid water or with atmospheric water (dissociative adsorption). Densities of surface OH groups of a few common materials obtained by infrared spectroscopy, thermogravimetric analysis (TGA), and LiAlH4 titration are reported in [713], and the results are rather consistent: about 3 nm-2 for silicas and about 5 nm-2 for titania. Concentrations of surface OH groups in various materials obtained by different methods are reported in [8,714] and, for goethite, in [715]. BET specific surface areas and surface densities of OH groups in 21 materials obtained by means of the Grignard method are summarized in [716]. Site densities of 44 sites/nm2 for Al(OH)3 and 5–11 sites/nm2 for silica, Fe(OH)3, and for Al(OH)3-coated silica were obtained by deuterium exchange in [717]. The surface density of exchangeable hydrogen associated with a rutile surface outgased at different temperatures was studied by deuterium exchange in [718]. The concentration of surface OH groups was determined by reaction with KI in dimethylformamide (DMF) (liberation of iodine) in [720]. Site densities in different faces of common oxides determined from crystallographic data in [721] were about 2 sites/ nm2 for all faces in all materials. The above examples refer to the determination of total site density, irrespective of the variable proton affinities of particular surface sites. In models of a heterogeneous surface, certain (but not all) types of surface oxygen atoms are considered as surface sites. Namely, only a fraction of surface oxygen atoms undergo protonation and deprotonation over the pH range of interest and thus contribute to the pH-dependent surface charge. The surface concentrations of different types of oxygen atoms (singly, doubly, and triply coordinated) in various faces of hematite and of goethite calculated from crystallographic data in [722] were in the range 1.5–14 nm-2. The surface site density obtained by means of the aforementioned independent methods can be used to interpret potentiometric titration data. Alternatively, the titration data can be used to calculate the best-fit surface site density (or densities of various types of sites) as parameter(s) of a certain model. In such a calculation, knowledge about the nature of the surface sites is not required. The site densities have also been calculated (e.g., in [723]) as parameters of adsorption isotherms of various adsorbates (usually small ions). The problem with such site densities is that protons behave differently from other adsorbates, and sites that are capable of binding protons are not necessarily capable of binding other species, and vice versa.

92

Surface Charging and Points of Zero Charge

2.9.1.1 Model without Surface Sites The concept of protonable surface sites does not explain the experimentally observed (see Chapter 3) pH-dependent surface charging of gas bubbles or of droplets of hydrocarbons. The dispersed phase does not provide oxygen atoms, which may be considered as surface sites, and the surface charging is due to the protonation/deprotonation of the surface layer of water [724]. This phenomenon is rather insensitive to the nature of the dispersed phase, and indeed many materials that do not have protonable hydroxyl groups on their surfaces show IEPs at pH ª 4, which seems to be the IEP of a water/inert gas interface. Preferential adsorption of hydroxide ions (with respect to protons) at a water/hydrophobic interface was found in a recent molecular dynamics study [725]. Protons are more hydrophilic than hydroxyl ions, and at neutral pH there is an excess of protons in bulk solution and an excess of hydroxyl ions in the interfacial layer, and a bubble is negatively charged. In other words, the activity coefficient of protons in the surface layer is very much greater than 1, and the activity coefficient of hydroxyl anions is very much lower than 1. When the pH decreases, the concentration of hydroxyl ions in the interfacial layer decreases and the concentration of protons in the interfacial layer increases, and, at sufficiently low pH, the sign of the surface charge is reversed to positive. This result is in line with the difference in Gibbs energy of solvation, which is more negative for protons than for hydroxyl ions [726]. A spontaneous increase in the concentration of hydroxyl ions in the interfacial layer at a neutral pH may contribute to the overall surface charge of all materials, including those that acquire their surface charge chiefly owing to a protonation of surface sites, with the relative significance of the two mechanisms of surface charging depending on the density of surface sites. A mixed mechanism of surface charging is also responsible for discrepancies in the PZCs of materials that have surface sites with the same proton affinities, with only their concentrations being different. The discrepancies in the PZC are more significant for materials that have PZCs far from 4 (e.g., alumina) and less significant for materials that have PZCs close to 4 (e.g., titania).

2.9.2

ELECTROSTATIC MODELS

The protonation and deprotonation of a surface site in contact with solution ⬅S-x + H+  ⬅SH1-x

(2.14)

where -x (0 < x < 1) is the charge of deprotonated site, is governed by an expression similar to the law of mass action: Ka =

{∫ SH1- x } Ê ey ˆ exp Á 0 ˜ -x + Ë kT ¯ {∫S }{H }

(2.15)

where {} denote activities of surface species and of protons, and y0 is the surface potential, which cannot be measured directly. The fractional (rather than

93

Methods

integer) charge of protonated and deprotonated surface sites, which seems counterintuitive at first glance, will be explained later in this chapter. In this approach, the surface concentrations of various species rather than their surface excesses (adsorption) are used. The exponential factor in Equation 2.15 reflects the electrostatic work accompanying the transfer of an ion (proton) from the bulk solution to the surface, which has an electric potential y0 with respect to the bulk solution. The surface potential y0 is related to s0, which in turn depends on the number of protonated and deprotonated sites. The relationship between s0 and y0 is a part of a model of the charged interface called an electrostatic model, and various such models have been proposed. An extensive set of simulations of surface charging within various electrostatic models can be found in Chapter 5 of [2]. 2.9.2.1 Non-Electrostatic Model In the simplest model, called the non-electrostatic model [727,728], the exponential factor in Equation 2.15 is set equal to 1, that is, the electrostatic work is neglected, and Equation 2.15 reduces to a simple mass-law expression. The nonelectrostatic model is simple, but unphysical, and it overestimates the effect of pH on s0. It does not explain the effect of ionic strength on s0 unless reactions other than Equation 2.14 contribute to s0. An ion exchange approach to adsorption of ions from solution was popular in the old literature, but has seldom been used in recent work [729]. Studies of the effect of ionic strength on the specific adsorption of ions provide a strong argument against the ion exchange approach. Namely, the specific adsorption of ions is often insensitive to ionic strength, and sometimes it even increases with ionic strength. This apparent paradox can be quantitatively explained in terms of the electrosorption models discussed below (see [2] for specific examples). 2.9.2.2 Nernst Equation The other simple expression for y0 is the Nernst equation:

y0 =

kT (pH 0 - pH) e

(2.16)

The Nernst equation reflects the fact that adsorption of positively charged species (protons) produces a positive surface potential, which prevents further adsorption of positively charged species. The surface potential calculated from Equation 2.16 changes by 59 mV per 1 pH unit at 25∞C. The applicability of the Nernst equation to metal oxide surfaces is discussed in [730], where it is concluded that oxide surfaces are Nernstian, at least near the PZC. A few results presented in Section 2.8.2 suggest the validity of the Nernst equation for metal oxides, and a few other results suggest that the absolute value of the surface potential changes by less than 59 mV per 1 pH unit. Thus, Equation 2.16 sets the upper limit of |dy0/dpH|.

94

Surface Charging and Points of Zero Charge

Combination of Equations 2.15 and 2.16 gives pH- and ionic-strengthindependent {⬅SH1-x}/{⬅S-x}. When the activities of surface species are proportional to their concentrations, pH- and ionic-strength-independent {⬅SH1-x}/ {⬅S-x} are equivalent to pH- and ionic-strength-independent s0, which is in obvious contradiction with experimental facts. The contradiction disappears when the activities of surface species in Equation 2.15 are not proportional to their concentrations [731,732] or when reactions other than Reaction 2.14 contribute to s0. A model with Nernstian surface potential and activity coefficients of charged surface groups calculated from the Debye–Hückel equation (with the site density and the permittivity of the interfacial region as adjustable parameters) quantitatively reflects the experimentally observed effect of ionic strength and pH on s0. 2.9.2.3 Constant Capacitance Model The constant capacitance model is based on the expression s0 = Cy0

(2.17)

where the capacitance C of the electrical double layer is independent of pH, but depends on the nature and concentration of the electrolyte. The capacitance of the electrical double layer reflects the fact that the adsorption of each cation (proton) leaves one excess anion (e.g., chloride ion) in solution. That anion is attracted by a positively charged surface, but its hydration shell sets the limit of closest approach. An analogous explanation applies to a negatively charged surface. A version of the constant capacitance model with two different values of C (below and above pH0) has also been used. The capacitance of a flat capacitor is equal to e/d, where d is the thickness of the capacitor, which in the case of a double layer reflects the size of the hydrated counterion. With d = 0.3 nm and e equal to the permittivity of water, a capacitance C of the electrical double layer of 2.3 F/m2 is expected. The best-fit C reported in the literature [2] is usually about 1 F/m2. The constant capacitance model is able to reproduce existing experimental data, but it has a limited ability to predict the course of charging curves at ionic strengths for which experimental data are not available. 2.9.2.4 Diffuse Layer Model The diffuse layer model is based on the expression sd =

ekkT [exp(ey d /2 kT ) - exp( - ey d /2 kT )] e

(2.18)

where the subscript d refers to the diffuse layer, and the reciprocal Debye length k is defined by k2 =

N A e2 Âi ci zi 2 ekT

(2.19)

Methods

95

in which ci is the concentration (in mol/m3). Equation 2.18 reflects the interaction between adsorbed protons and excessive anions in solution, which is more complex than in the constant capacitance model. The distance between an anion in solution and a positively charged surface is limited not only by the thickness of the hydration shell of the anion, but also by the mutual repulsion of the anions in the interfacial region. Therefore, the electrical double layer is thicker than the distance of closest approach, especially at low ionic strengths. Equations 2.18 and 2.19 contain only physical constants and measurable quantities; thus the diffuse layer model does not require any adjustable parameters. The reciprocal Debye length increases with the electrolyte concentration; thus, the diffuse layer model predicts an increase in sd at constant yd when the ionic strength increases. 2.9.2.5 Stern Model The purely electrostatic diffuse layer model often underestimates the affinity of the counterions to the surface. In the Stern model, the surface charge is partially balanced by chemisorbed counterions (the Stern layer), and the rest of the surface charge is balanced by a diffuse layer. In the Stern model, the interface is modeled as two capacitors in series. One capacitor has a constant capacitance (independent of pH and ionic strength), which represents the affinity of the surface to chemisorbed counterions, and which is an adjustable parameter; the relationship between sd and yd in the other capacitor (the diffuse layer) is expressed by Equation 2.18. A version of the Stern model with two different values of C (below and above pH0) has also been used. The capacitance of the Stern layer reflects the size of the hydrated counterion and varies from one salt to another. The correlation between cation size and Stern layer thickness was studied for a silica–alkali chloride system in [733]. Ion specificity of adsorption on titania was discussed in terms of differential capacity as a function of pH in [545]. The Stern model with the shear plane set at the end of the diffuse layer overestimated the absolute values of the z potential of titania [734]. A better fit was obtained with the location of the shear plane as an additional adjustable parameter (fitted separately for each ionic strength). Chemisorption of counterions can also be quantified within the chemical model in terms of expressions similar to the mass law (Section 2.9.3.3). 2.9.2.6 More Complex Electrostatic Models The electrostatic models discussed in Sections 2.9.2.1 through 2.9.2.5 apply to a simple chemical model involving one reaction (Reaction 2.14) of the transfer of one species (proton) from the solution to the surface. More complex chemical models involving the transfer of two or more species from the solution to the surface and/or allowing various distances of the adsorbed solution species from the surface require more complex electrostatic models. Usually, three or more capacitors in series are considered. All the capacitors but one have constant capacitances, and the relationship between sd and yd in one capacitor (diffuse layer) is expressed by Equation 2.18.

96

2.9.3

Surface Charging and Points of Zero Charge

SURFACE ACIDITY

Different chemical models can be combined with either of the electrostatic models. The chemical models quantify the uptake of protons and of the ions of an inert electrolyte in terms of expressions similar to the mass law. 2.9.3.1 1-pK versus 2-pK The model based on a single reaction (Reaction 2.14) was originally introduced in [735] with x = ½ and ⬅S = ⬅AlOH, and is called the 1-pK model. The equilibrium constant of Reaction 2.14 defined by Equation 2.15 can be calculated directly from the experimentally determined PZC [2], Ê1 - xˆ log K a (Reaction 2.14 ) = PZC - log Á Ë x ˜¯

(2.20)

and, in the special case of x = ½, log Ka (Reaction 2.14) = PZC. One equilibrium constant is sufficient to describe surface protonation and deprotonation, and, at the PZC, the charge of negatively charged sites is balanced by the charge of positively charged sites. The 1-pK model can be considered as a refinement of the 2-pK model [736], in which two-step protonation and deprotonation of one type of site is considered: ⬅SOH2+  ⬅SOH + H+

Ka1

(2.21)

⬅SOH  ⬅SOH- + H+

Ka2

(2.22)

where Ka1 and Ka2 are defined by equations analogous to Equation 2.15. It can easily be shown that pKa1 + pKa2 = 2PZC

(2.23)

More precisely, the PZC calculated from the best-fit pKa1 and pKa2 in a 2-pK model by means of Equation 2.23 matches the experimental PZC when the acidic and basic branches of the charging curves are nearly symmetrical. Otherwise, the PZC in the best-fit model curve calculated from Equation 2.23 may deviate substantially from the experimental PZC. Several PZC values reported in Chapter 3 were calculated by means of Equation 2.23. Namely, several publications report the best-fit acidity constants rather than the experimental PZC. In the classical version of the 2-pK model, sites at three different degrees of protonation coexist in the vicinity of the PZC. Coexistence of solution species at three different degrees of protonation seldom occurs in small molecules, and this has been used as an argument against the 2-pK model and in favor of the 1-pK model. Namely, similarity between protonation of surface species and of analogous solution species is expected. To avoid three different degrees of protonation

Methods

97

of the same site, the 2-pK model has been interpreted in terms of pairs of neighboring sites [737], of which one can be only protonated and another can be only deprotonated. Not necessarily all oxide surfaces are amphoteric. Koopal [738,739] defined silica-type (deprotonation-only) and gibbsite-type (amphoteric) surface charging behavior. 2.9.3.2 Multisite Approach The models discussed above can be refined by allowing several types of surface sites, which differ in their proton affinities. A two-site model was used in 1971 [740] to interpret the proton and alkali metal affinities of silica (ion exchange approach). The multisite approach has a sound physical basis, namely, not all surface oxygen atoms have the same coordination environment. In this respect, the “overall” acidity constants discussed above can be considered as weighted averages of acidity constants of various types of sites. In several publications, the parameters of the distribution function of acidity constants (discrete or continuous) have been considered as adjustable parameters. Certainly, by increasing the number of adjustable parameters, one can obtain a better fit. On the other hand, only a limited number of adjustable parameters can be unequivocally derived from experimental data; that is, with too many adjustable parameters, multiple different sets of parameters produce similar model curves. Most distributions of acidity constants derived from experimental charging curves by data fitting are not unique solutions, although this problem is seldom addressed in publications presenting such models. The pKa of five or six types of discrete sites were calculated from charging curves of alumina and of HFO at one ionic strength in [741], and the continuous distributions of acidity constants were considered as sums of Gaussian-type distributions around particular discrete values. A continuous proton affinity distribution of alumina calculated from a titration curve had four broad maxima, which were assigned to pKa values of certain types of surface sites [742]. Similar proton affinity distributions were obtained [743] for various ionic strength and at various temperatures, but each charging curve was analyzed separately. A substantial effect of the ionic strength on the shape of the apparent distribution curves indicated their limited significance. Parameters of two-parameter Freundlich-type isotherms were fitted to the proton adsorption data in [744]. Three different Freundlich-type isotherms have been tested. The MUSIC (multisite complexation) model was originally introduced in [446,745] and later modified [746–748]. In contrast with the curve-fitting approach discussed above, the site densities and acidity constants of particular types of surface groups in the MUSIC model are derived from crystallographic data and from the correlation between the coordination environment of a surface oxygen atom and its proton affinity. Different types of such correlations are discussed in [749]. Prediction of surface acidity constants from bond valence was discussed in [58]. Each crystallographic face has different surface densities of oxygen atoms of a particular type, a different s0, and a different PZC. The s0 of the edge plane of gibbsite and the 100, 010, and 001 planes of goethite were calculated in [446]. The s0 of different faces of a-alumina was calculated in [750]. The overall s0 of

98

Surface Charging and Points of Zero Charge

a powder is obtained as a weighted average of the contributions of particular crystallographic planes and depends on crystal morphology. This effect was studied for two goethite crystals of two different morphologies [77]. Only certain types of surface oxygen atoms contribute to s0. Other types of surface oxygen atoms, and even entire crystallographic planes, may remain uncharged over the entire pH range of interest. For example, the 001 face of hematite was found to be uncharged at pH 3–9 [751]. Different types of surface oxygen atoms may show very similar proton affinities, in spite of different coordination environments. Therefore, only a few types of surface oxygen atoms have to be distinguished (in terms of their proton affinity) in the calculations of s0, even in crystals of complex morphology. For example, six types of surface sites were considered in MUSIC calculations for gibbsite [752]. 2.9.3.3 Binding of Inert Electrolyte Ions The triple-layer model (TLM) [753] considers surface protonation and deprotonation according to Reactions 2.21 and 2.22 (2-pK model) and two additional types of surface species: +

⬅SOH2+ + X-  ⬅SOH2 ◊ X

-

+ ⬅SOH- + Y+  ⬅SOH ◊ Y

(2.24)

(2.25)

which contribute to s0. The surface species on the right-hand sides of Reactions 2.24 and 2.25 are analogs of ion pairs, which occur in concentrated electrolyte solutions. Reactions 2.24 and 2.25 are governed by mass-action expressions similar to Equation 2.15, in which the electrostatic potential of the ions of the inert electrolyte involved in the species on the right-hand sides of Reactions 2.24 and 2.25 is taken into account. The inert electrolyte ions are adsorbed at a certain distance from the surface (called the b-plane), which is equal for the cation and anion of an inert electrolyte in the original TLM. The TLM uses a special electrostatic model that involves two capacitances. The capacitance between the surface and the b-plane and the capacitance between the b-plane and the d-plane (the center of the charge of the diffuse layer) are assumed to be constant and are adjustable parameters of the model. The equilibrium constants of Reactions 2.24 and 2.25 are related; that is, only one of them is an adjustable parameter. Thus, the possibility of explaining the effect of the nature of the counterion on s0 (without a shift in the PZC) in terms of different equilibrium constants of Reactions 2.24 and 2.25 for particular ions is limited. Six parameters are effectively adjusted in the TLM, and this can be comfortably done by means of FITEQL [754,755] or by other speciation programs. One set of TLM parameters can reproduce the course of charging curves obtained at multiple ionic strengths in many systems, in which the models discussed above (the diffuse layer and Stern models) fail. It should be emphasized that the TLM parameters produced by speciation programs are not unique; that is, multiple sets of parameters of the TLM produce practically

99

Methods

identical model curves [609,756,757]. In this respect, the values of the TLM parameters reported in the literature, and the correlations between the best-fit TLM parameters and physical constants (e.g., the dielectric constant of the solid particles [641]), are of limited significance. In a four-layer model [758], the same types of surface species are considered as in the TLM, but the anions and cations of the inert electrolyte are adsorbed at different distances from the surface rather than in a common b-plane. The fourlayer model requires an additional capacitance as an adjustable parameter. A similar effect may be achieved by using two different capacitances for the positive and negative branches of the charging curves in the classical TLM. Different affinities of particular anions and cations of 1-1 electrolytes to goethite can be also explained in terms of a charge distribution (CD) model [537]. Different electrostatic positions of an adsorbed ion are quantified as fractions of the charge of that ion attributed to each of two planes at different distances from the surface. The CD model was originally designed for specific adsorption, but was later adopted for adsorption of inert counterions. Reactions analogous to Reactions 2.24 and 2.25 can be used to modify the 1-pK model [744], but such a combination has been less popular than the TLM. Counterion binding and surface heterogeneity can be combined in one model. Equal affinities for ion pair formation (KNa1 = KNa2 and KCl1 = KCl2) have been assumed for two types of surface groups [734] in the following two-site MUSICtype model with counterion binding: ⬅TiOH-1/3 + Na +  ⬅TiOH-1/3 – Na + (⬅Ti)2OH-2/3 + Na +  (⬅Ti)2OH-2/3 – Na +

KNa1

KNa2

(2.26)

(2.27)

⬅TiOH2+2/3 + Cl-  ⬅TiOH2+2/3 – Cl-

KCl1

(2.28)

(⬅Ti)2OH+1/3 + Cl-  (⬅Ti)2OH+1/3 –Cl-

KCl2

(2.29)

Different ion binding constants were allowed for particular monovalent anions and cations. The MUSIC model with counterion binding has been tested against a large set of data: two types of goethite, at different concentrations of NaCl and NaNO3 [759]. The counterion and proton affinities of particular types of surface sites may be considered as independent variables, which may be correlated or uncorrelated [640]. 2.9.3.4 Materials Other than Oxides The above models were designed in principle for oxide surfaces. Models of surface charging of clay minerals (with permanent charge) are discussed in [760].

100

Surface Charging and Points of Zero Charge

The basal planes and edges in clay minerals have different acid–base properties. PZCs of the edge (at pH 8) and of the Al layer (at pH 6) of kaolinite are reported in [761]. The IEP of the edge surface of Na-kaolinite at pH 7.3 was obtained [762] as the intersection point of Bingham yield stress (pH) curves at various ionic strengths. A three-site 2-pK model for primary surface charging of montmorillonite was used in [763,764]. Site densities and acidity constants of 27 types of sites responsible for surface charging of montmorillonite are reported in [765]. A model of surface charging of bentonite [766] involves silanol and aluminol sites (two-step protonation and counterion binding) and ion exchange sites. A model of surface charging of montmorillonite (edge and basal planes) involved surface heterogeneity [767]. Models used for surface charging of Na-montmorillonite are summarized in [768]. Different authors have used a 1-pKa or 2-pKa TLM with one or two types of sites (in a few models attributed to Al and Si) with or without additional ion exchange (permanent charge) sites. A multisite model of surface charging of illite is discussed in [769]. Multisite models analogous to those discussed in this section have been applied to humic acid [633,770,771]. Also, charging curves of ZnS have been interpreted in terms of protonation/deprotonation of multiple surface species [772].

3

Compilation of PZCs/IEPs

The PZCs/IEPs of materials are presented in this chapter in tabular form. The organization of the tables and the abbreviations therein are explained in detail in Chapters 1 and 2. Note that, throughout the tables, T is in °C.

3.1 3.1.1

SIMPLE OXIDES ALUMINUM (HYDR)OXIDES

Aluminum has only one stable oxidation state (+3) within the electrochemical window of water, but it forms numerous relatively stable compounds with oxygen and hydrogen, which differ in their degree of hydration and in their crystallographic structure. The nominal degree of hydration indicated by a chemical name/formula reported in the literature does not necessarily reflect the actual degree of hydration. PZCs/IEPs of aluminum (hydr)oxides are presented in Tables 3.1 through 3.263. PZCs/IEPs of aluminum oxide and of gibbsite are compiled in [773]. Acid– base properties of the Keggin A13 polymer are discussed in [774]. 3.1.1.1 Aluminum Oxide PZCs/IEPs of aluminum oxides (nominally Al2O3) are presented in Tables 3.1 through 3.181. Previous compilations of PZCs/IEPs of aluminum oxides were published in [54,775–778]. A previous compilation of IEPs of aluminum oxides was published in [519] (powders and single crystals). 3.1.1.1.1 Commercial PZCs/IEPs of aluminum oxide from different commercial sources are presented in Tables 3.1 through 3.148. 3.1.1.1.1.1 g-Alumina from Akzo Properties: BET specific surface area 208 m2/g [779], 265 m2/g [780], 270 m2/g (product code DRUM 1696) [781]. 101

102

Surface Charging and Points of Zero Charge

TABLE 3.1 PZC/IEP of γ-Alumina from Akzo Electrolyte

T

Method

Instrument

pH0

Reference

0.1 M NaNO3

25

pH

Two solid-to-liquid ratios

7.5

[781]

3.1.1.1.1.2 RA45E from Alcan Properties: a-form, 99.8% pure, XRD pattern, EDXRF spectrum available [105].

TABLE 3.2 PZC/IEP of RA45E from Alcan Description

Electrolyte

T

As received

0.001–0.1 M KNO3

Method

Instrument

Mass titration

pH0

Reference

7.6

[105]

3.1.1.1.1.3 Aluminas from Alcoa 3.1.1.1.1.3.1 A 11 Properties: a-form, BET specific surface area 8.8 m2/g [373].

TABLE 3.3 PZC/IEP of A 11 from Alcoa Electrolyte

T

Method

Instrument

pH0

Reference

iep

Zeta-Meter 3.0

9.2a

[373]

0.001 M KNO3 a

Arbitrary interpolation.

3.1.1.1.1.3.2 A 12 Properties: a–form [389,458], detailed analysis available [458], BET specific surface area 10.6 m2/g, average particle size 300 nm [389].

TABLE 3.4 PZC/IEP of A 12 from Alcoa Electrolyte 0.001 M KNO3 a

T

Method iep

Subjective interpolation.

Instrument Malvern Zetasizer 3000

pH0 9.2

a

Reference [389]

103

Compilation of PZCs/IEPs

3.1.1.1.1.3.3 A 14 Properties: a-form, 0.04% Si, 0.01% Ga, 0.03% Fe, 0.02% Ca, 0.001% Mg, 0.02% Na, 0.005% Ti [458].

TABLE 3.5 PZC/IEP of A 14 from Alcoa Description Stored for 10 d in borosilicate glass HF-treated a

Electrolyte

T

Method

Instrument

pH0

Reference

0.001 M NaClO4

20

iep

Zeta-Meter

8.7a

[458]

8.9a

Subjective interpolation.

3.1.1.1.1.3.4 A-16 Obtained by the Bayer process. Properties: a-form [151,782], 99.8% pure [784,785], 510 ppm Na2O, 60 ppm CaO, 360 ppm MgO, 290 ppm SiO2, 130 ppm Fe2O3 [151], BET specific surface area 9.3 m2/g [784–787], 10.3 m2/g [788], specific surface area 9 m2/g [151], mean particle size 1.15 mm [788], d10 = 0.24 mm, d50 = 0.4 μm [784,785], average diameter 0.38 mm [786]. TABLE 3.6 PZC/IEP of A 16 from Alcoa Electrolyte 0.01 M NaNO3 0.01 M NaCl None 0.01 M NaNO3 0.001 M KNO3 0.001 M NaNO3 a

b

c d

T

Room

Method iep iep iep iep iep iep

Instrument Zeta-Meter 2.0 Acoustosizer Zeta-Meter 3.0+ Zeta-Meter 2.0 Zeta-Meter 3.0 ESA 8000

pH0 a

7.6 8.2 8.6 8.8b 9.2c 9.9

Reference [787] [307] [782] [784d,785] [788] [786]

Subjective interpolation. IEP roughly matches the maximum in the yield stress in a 25 vol% dispersion. The maximum in the yield stress and the minimum in the stability roughly match the IEP from the electrophoretic measurements. Subjective interpolation. Only value, no data points.

3.1.1.1.1.3.5 A 16-SG Obtained by the Bayer process. Properties: a-form [267,789–792,798], 99.9% pure [791,796], >99.8% [793], 99.8% Al2O3 [794], 99.7% [792], 99% pure [790], contains MgO [795], 0.08% Na2O, 0.025% SiO2, 0.01% Fe2O3, 0.02% K2O [792], 0.08% Na2O, 0.1% CaO, 0.05% SiO2, 0.5% Fe2O3 [789], 0.05% Na2O, 0.02% CaO, 0.02% SiO2, 0.01% Fe2O3, 0.01%

104

Surface Charging and Points of Zero Charge

MgO [796], 0.08% Na2O [794], BET specific surface area 11 m 2/g [796,797], 10 m 2/g [795], 10.2 m 2/g [794], 9.6 m 2/g [792], 9.5 m 2/g [798], 9 m 2/g [793], specific surface area 8.4 m 2/g [789], 8–10 m 2/g (according to the manufacturer) [589], 9 m 2/g [790], size range 0.1–1 mm [792], average size 0.2 mm [267], mean diameter 350 nm [789], 2 μm [794], particle size up to 10 mm [790], average particle size 500 nm [793], d50 = 300 nm [798], average particle size 600 nm [320], 700 nm [791], particle size 400–500 nm (manufacturer) [589], TEM image available [267], AFM and TEM images available [589], XRD pattern available [798].

TABLE 3.7 PZC/IEP of A 16-SG from Alcoa Description

Electrolyte

As received

0.001 M KCl 0.001 M NaCl 0.01 M NaCl

T

20b

0.001–0.1 M KNO3

Method iep iep iep pH iep

Instrument Malvern Zetasizer 4 Acoustosizer ESA 8000 Matec Malvern Zetasizer 4

Titration None Washed in 0.01, 0.1 M boiling water NaCl 2.5 5 10 15 vol%

0.01 M KCl

0.001 M KCl 0.001 0.01 0.1 M KCl a b c d e f

25

iep iep

Malvern Zetasizer MK 11 Rank Brothers Mark II

25 25

iep iep

ESA 8050 Matec Acustosizer

25 iep 20d pH

Rank Brothers Mark II

pH0

Reference

8.1 8.3a 8.5 8.5 8.6c

[796,797] [589] [793] [790]e [320]

8.7

Reference 6 in [798] [799] [789]

8.7 8.8 9 9 8.7 8.3 8.3 9.2 9.9 10 10.1

[791]e [792]f

[794] [795]

The charging curves did not show a clear CIP (merge at pH 7–9). Also 10 and 40°C. IEP matches the maximum in viscosity of 75 mass% dispersion. Also 40 and 60°C. Only value, no data points. IEP roughly matches the maximum in viscosity and yield stress of 25 vol% dispersion.

3.1.1.1.1.3.6 A-152 Obtained by the Bayer process. Properties: a-form, 380 ppm Na2O, 110 ppm CaO, 45 ppm MgO, 390 ppm SiO2, 235 ppm Fe2O3, specific surface area 4.6 m2/g [151].

105

Compilation of PZCs/IEPs

TABLE 3.8 PZC/IEP of A-152 from Alcoa Description

Electrolyte

T

Instrument

pH0

a

As received Washed a

Method pH/iep

9.3/9.5 9.8

Reference [151]

Only values, no data points. Similar results are reported for two lots of A-152.

3.1.1.1.1.3.7 A-1000 SG Obtained by the Bayer process. Properties: 99.8% pure, 0.1% Na2O, D50 = 400 nm, specific surface area 9 m2/g [800].

TABLE 3.9 PZC/IEP of A-1000 SG from Alcoa Electrolyte

T

KOH + HNO3 (acid titration)

Method

Instrument

pH0

Reference

iep

DT 1200

10

[800]

3.1.1.1.1.3.8 CL2500SG Properties: a-form, specific surface area 1.5 m2/g, particle diameter 2.3 μm, >99.8% Al2O3, 0.02% SiO2, 0.02% Fe2O3, 0.05% Na2O, 0.02% CaO by mass [801].

TABLE 3.10 PZC/IEP of CL2500SG from Alcoa Electrolyte

T

Method

Instrument

pH0

Reference

0.001 M KCl

25

iep

AcoustoSizer

8.1

[801]

3.1.1.1.1.3.9 3.5 μm [791].

CL3000SG Properties: a-form, 99.8% pure, particle diameter

TABLE 3.11 PZC/IEP of CL3000SG from Alcoa Electrolyte

a

T

Method

Instrument

pH0

Reference

25

iep

ESA 8050 Matec

9a

[791]

Only value, no data points.

106

Surface Charging and Points of Zero Charge

Properties: a-form, 99.7% pure, particle diame-

3.1.1.1.1.3.10 CT1200SG ter 1.3 μm [791].

TABLE 3.12 PZC/IEP of CT1200SG from Alcoa Electrolyte

a

T

Method

Instrument

pH0

Reference

25

iep

ESA 8050 Matec

9a

[791]

Only value, no data points.

3.1.1.1.1.3.11 CT3000SG Properties: 99.9% of a-form [802,803], 99.7% of a-form [804,805], 99.85% pure [806], 0.02% Na2O, 0.05% SiO2, 0.03% Fe2O3 [803], BET specific surface area 7 m2/g [802,803], specific surface area 9 m2/g [806], 6 m2/g [804,805], D50 = 500–800 nm [806], d50 = 600 nm [804,805], average particle size 0.7 μm [802,803]. Properties are also presented in Reference 20 of [493]. TABLE 3.13 PZC/IEP of CT3000SG from Alcoa Electrolyte

T

Method

Instrument

pH0

0.01 M KCl + KOH or NaOH 0.01 M KNO3 None

25

iep

AcoustoSizer

8a

[493]

pH iep

PCD, Mutek

8.3 9.1b

[804,805] [802] [803]

a

b

Reference

Various IEP in a range 7.9–8.9 were obtained for different solid-to-liquid ratios, bases (NaOH, KOH, NH3) and directions of titration. Data points closest to IEP: pH 8.4, 9.1, and 11.1. The axis label in Figure 1 in [803] suggests that s0 was measured rather than z potential.

3.1.1.1.1.3.12 CT3000SE Properties: a-form, 99.7% pure, XRD pattern, EDXRF spectrum available [105].

TABLE 3.14 PZC/IEP of CT3000SE from Alcoa Description

Electrolyte

As received

0.001–0.1 M KNO3

T

Method Mass titration pH

Instrument

pH0

Reference

8.4 6.9

[105]

107

Compilation of PZCs/IEPs

3.1.1.1.1.3.13 F 1 Properties: g-form [807], g-form, with small amounts of gibbsite and boehmite [522], BET specific surface area 250 m2/g (manufacturer) [807], 287 m2/g [522], specific surface area 210 m2/g [808,809].

TABLE 3.15 PZC/IEP of F 1 from Alcoa Description

Electrolyte

T

HCl- and waterwashed (different sequences) NaOH-washed 0.05, 0.5 M NaClO4 NaOH-washed 0.001–0.1 M NaNO3 a

Method a

iep

Instrument Streaming potential

Intersection cip 48 h equilibration

20

pH0

Reference

6.2 7.3 8.9 7.4 8.1

[809]

[522] [807]

Only values, no data points.

3.1.1.1.1.3.14 T-60

TABLE 3.16 PZC/IEP of T-60 from Alcoa Description

Electrolyte

T

Method

Instrument

Acid-washed

KNO3

25

iep

Streaming potential

a

pH0 Reference 7.5a

[288]

Arbitrary interpolation.

3.1.1.1.1.3.15

XA-139 Properties: a-form [810].

TABLE 3.17 PZC/IEP of XA-139 from Alcoa Electrolyte

T

Method Titration

a

Only value, no data points.

Instrument

pH0 a

8.8

Reference [810]

108

Surface Charging and Points of Zero Charge

3.1.1.1.1.3.16 g-Alumina

TABLE 3.18 PZC/IEP of g-Alumina from Alcoa Description

Electrolyte

Washed

0.1 M KNO3

T

Method

Instrument

pH0

Reference

8.2

[741]

pH

3.1.1.1.1.3.17 Alcoa, Type Unknown Properties of several aluminas from Alcoa are reported in Sections 3.1.1.1.1.3.1 through 3.1.1.1.1.3.16 and in [806]. Properties of A-39 are reported in [151]. Properties of A-3000 FL and of EK8R are reported in [800].

TABLE 3.19 PZC/IEP of Unspecified Aluminas from Alcoa Description

Electrolyte

a form, average size 0.001 M NaCl 2 μm, >99% pure 800 nm particles 0.001 M KNO3 a

T

Method

Instrument

pH0

Reference

25

iep

Streaming potential

7.6

[811]

iep iep

Matec ESA 9800 Laser ZeeMeter

7.8 8.7a

[433] [812]

Maximum in viscosity at pH 9.

3.1.1.1.1.4 Aluminas from Aldrich 3.1.1.1.1.4.1 a-Alumina Properties: 0.6 m2/g [813], >99.7% [502], 99% [814], BET specific surface area 7.3 m2/g [502,814].

TABLE 3.20 PZC/IEP of a-Alumina from Aldrich Description Washed and calcined at 700°C

Electrolyte

T

Method

Instrument

pH0

Reference

0–0.005 M NaCl

25

iep

Malvern Zetasizer 3000 HS

6.7

[502]

3.1.1.1.1.4.2 Brockmann I Properties: g-form, BET specific surface area 159 m2/g (measured), 155 m2/g (manufacturer), pore diameter 5.8 nm (manufacturer) [815].

109

Compilation of PZCs/IEPs

TABLE 3.21 PZC/IEP of Brockmann I from Aldrich Electrolyte

T

0.1 M NaCl

Method

Instrument

pH

pH0

Reference

5.3

[815]

3.1.1.1.1.4.3 Mesoporous Properties: g-form and amorphous material, pore size 6.5 nm, TEM image available, specific surface area 284 m2/g [816].

TABLE 3.22 PZC/IEP of Mesoporous Alumina from Aldrich Electrolyte

T

0–0.1 M NaCl

3.1.1.1.1.4.4 [817,818].

Method

Instrument

cip

pH0

Reference

9.1

[816]

Other Properties: g-form, specific surface area 47.7 m2/g

TABLE 3.23 PZC/IEP of Unspecified Alumina from Aldrich Description Washed

Electrolyte

T

Method

Instrument

pH0

Reference

0.001–0.1 M NaClO4/NaCl

20

iep cip

Pen Kem Laser ZeeMeter 501

7.7/7.5 7.8/8.1

[518]

3.1.1.1.1.5 Alumina from Alfa Aesar Properties: a-form [819], 99.99% of a-form [428], 99,99% pure [819,820], BET specific surface area 12.8 m2/g [428], particle size 1 mm [819], 99.99% pure, specific surface area 4.6 m2/g [835], BET specific surface area 4 m2/g [833], d90 = 4.5 μm, d50 = 650 nm, d10 = 350 nm [835], average particle size 600 nm [833]. TABLE 3.38 PZC/IEP of HPA 1.0 from Ceralox or from Condea Vista Electrolyte

T

Method

Instrument

pH0

Reference

iep

Matec ESA

10

[833]

HNO3 + KOH

Properties of SPA-TMXX3 from Ceralox are reported in [833]. 3.1.1.1.1.17 Alumina from Chlorovinyl Another sample studied by the same research group is described in Section 3.1.1.1.1.36. Properties: Amorphous (80%) + g-form (20%), BET specific surface area 60 and 140 m2/g, IR spectrum available [836]. TABLE 3.39 PZC/IEP of Alumina from Chlorovinyl Electrolyte

T

Method

Instrument

pH0

Reference

iep

ZetaPlus Brookhaven

9.8

[836,837]

HCl + NaOH

3.1.1.1.1.18 Aluminas from Condea 3.1.1.1.1.18.1 HPA05 without MgO See also Section 3.1.1.1.1.16.2. Properties: a-form, d50 = 350 nm, specific surface area 9.5 m2/g [838]. TABLE 3.40 PZC/IEP of HPA05 without MgO from Condea Electrolyte

T

Method

Instrument

pH0

Reference

iep

Zeta-Meter 3.0+

9a

[838]

0.01 M KCl a

Arbitrary interpolation.

115

Compilation of PZCs/IEPs

3.1.1.1.1.18.2 Condea, Type Unknown Properties: g-form [839,840], BET specific surface area 206 m 2/g [840], 119 m 2/g (material pretreated in air at 1048 K for 1 day) [839]. TABLE 3.41 PZC/IEP of Unspecified Aluminas from Condea Electrolyte

T

0.1 M NaNO3 γ + 5–10% δ

Method

Instrument

Titration cip

b

Reference

8 8.25

γ + 56% δ a

pH0 a

[840] [841]b

no cip

Only acidity constants reported. Also composites with 5% of silica, Zr, or La. Only values, no data points.

3.1.1.1.1.19 Alumina(s) from Cyanamid Properties: g-form [668,669,742, 743,842–844], Cu 100 ppm, Fe 70 ppm, Na 40 ppm, S and Pd 10 ppm, Mo and As 1 ppm [742,743,842], BET specific surface area 140 m2/g [742,743,842], 190 m2/g [668,843], 195 m2/g, [669], 205 m2/g [844], particle size 60 mm [668], 225 mm [843], pore volume 0.85 cm3/g [742,743,842], 0.484 cm3/g [669]. TABLE 3.42 PZC/IEP of Alumina(s) from Cyanamid (Including SN 7053) Description SN 7053 calcined at 600°C for 16 h SN 7053

Electrolyte

b c

Instrument

pH0

Reference

pH

6.8

[842]

0.0001–0.1 M NaNO3

cip

7

[742]

pH Mass titration cip

7.8 7.2 7.4

[843]

cip/ 24 h Mass equilibration titration Mass titration iep ZM-3

7.5/7.4 7.8/7.8

[668]

23a 0.001–0.1 M NaNO3 0.001–0.1 M NaCl

None 0.001 M KCl a

Method

0.1 M NaNO3

Ground Calcined at 600°C for 6 h Original Calcined at 500°C for 16 h

T

Also 23–82°C. Also 10–50°C. +12 mV at pH 7.8, −5 mV at pH 9.

25b

8.3 99.5% pure [437], >97% pure [854], 97% pure [875], 99% pure [864], 99.7% from Fluka Description

Electrolyte

T

Method

Instrument

pH0

Reference

Washed, then calcined at 700∞C

0–0.001 M

25

iep

Malvern Zetasizer 3000 HS

7.2

[323]

3.1.1.1.1.27.4 Fluka, Type Unknown Properties: BET specific surface area 150 m2/g [875], specific surface area 155 m2/g (manufacturer) [909,910].

TABLE 3.56 PZC/IEP of Unspecified Alumina from Fluka Electrolyte 0–1 M KCl

T

Method cip EMF

Instrument

pH0

Reference

7.8

[875] [909]

123

Compilation of PZCs/IEPs

3.1.1.1.1.28 q-Alumina Powder from Forever Chemical, Taiwan TABLE 3.57 PZC/IEP of θ-Alumina Powder from Forever Chemical Electrolyte

T

Method

HCl + NH4OH a

Instrument

iep

pH0

Reference

a

Malvern Zetasozer NS

9.2

[414]

Matches the maximum in average particle size.

3.1.1.1.1.29 T-126 from Girdler Properties: g-form [911,912,470], BET specific surface area 188 m2/g [911]. TABLE 3.58 PZC/IEP of T-126 from Girdler Electrolyte 0.001 M KCl 0.001 M KCl 0.001 M NaCl 0.001 M KNO3 0.001 M NH4Cl

T

Method

Instrument

pH0

Reference

22.5

iep iep

Zm-77 Zeta-Meter ZM-77

8.8 8.8 8.8 8.6 8.6

[911] [912] [470]

3.1.1.1.1.30 g-Alumina from Goodfellow specific surface area 142 m2/g [315].

Properties: 99.995% pure, BET

TABLE 3.59 PZC/IEP of γ-Alumina from Goodfellow Description Dialyzed

3.1.1.1.1.31

Electrolyte

T

Method

Instrument

pH0

Reference

0.001–0.01 M KCl

25

iep cip

Malvern Zetasizer 2000

9.2

[315]

Flat Plates from Harrick Properties: a-form, 0.7% fluoride [463].

TABLE 3.60 PZC/IEP of Alumina Flat Plates from Harrick Description Multistep cleaning procedure 0.001 M KCl a

Electrolyte 0.0001, 0.001 M KNO3

Confirmed by AFM results.

T

Method iep AFM

Instrument

pH0

Reference

Streaming potential

4.2a

[463]

9.3

[819]

124

Surface Charging and Points of Zero Charge

3.1.1.1.1.32 Alumina from Harshaw Properties: g-form [913–915], specific surface area 170 m2/g [913], 180 m2/g [914,915]. TABLE 3.61 PZC/IEP of Alumina from Harshaw Description

Electrolyte

Calcined at 500∞C

HNO3 + NH3 0.1 M NaCl

a

T

Method

Instrument

pH Mass titration

Overnight equilibration

pH0

Reference

7.6a

[913]

7.8a

[914]

Only value, no data points.

3.1.1.1.1.33 Alumina from Houdry 3.1.1.1.1.33.1 Alumina 415 (or Ho 415), 100–150 mesh Properties: g-form [580,588,916–919], 0.23 mmol Na/g [588], 0.05% SiO2, 0.7% Na2O by mass [917], specific surface area 123 m2/g [580,588,916–921,924] (probably BET, crushed, calcined at 600∞C), 190 m2/g [922]. TABLE 3.62 PZC/IEP of Alumina 415 (Ho 415) from Houdry Description Original 0.31 0.39 0.62 0.98 1.6 2.5 mmol Na/ga

Crushed, calcined at 600∞C for 12 h

a

b c

d

Electrolyte

T

Method

0.001–0.1 M KNO3

25 cip

0.001–0.1 M KNO3

25 cip Titration Intersection

0.001 and 0.1 M KNO3 0.01 M NH4NO4

25 iep

Instrument

Rank Brothers Mark II

pH0

Reference

5.3 9.6 9.7 9.8 9.3 9.8 10.1 5.3 5.6 5.9

[588] [921]

[580b,917d] [920,923] [922]

7 8.3a 8.3b

Reference [931] [927] [930]

Only value, no data points. Only acidity constants, no data points.

3.1.1.1.1.38 ALO-4 (from JRC) Properties: g-form [932], 1.5 ppm Na, 0.5 ppm Fe2O3, 1.9 ppm TiO2, 0.3 ppm CaO, specific surface area 177 m2/g [933], BET specific surface area 155 m2/g [932,934].

127

Compilation of PZCs/IEPs

TABLE 3.69 PZC/IEP of ALO-4 from JRC Description Washed

Electrolyte

T

Method

0.1 M NaNO3

25

pH

Instrument

pH0

Reference

8.4

[932]

3.1.1.1.1.39 RP-1 from Japan Fine Ceramics Center (JFCC) Properties: a-form, specific surface area 1.9 m2/g, d10 = 0.63 μm, d50 = 1.85 μm, d90 = 4.48 μm [935]. TABLE 3.70 PZC/IEP of RP-1 from JFCC Electrolyte

T

0.001 M NH4NO3

a

Method

Instrument

iep

Matec ESA 8000 Mutek PCD

pH0 7.7

Reference

a

[935]

Different solid-to-liquid ratios.

3.1.1.1.1.40 Alumina Identical with Material of Membranes from Jiangsu Jiusi High-Tech Co. Ltd China Properties: a-form [936]. TABLE 3.71 PZC/IEP of Alumina Identical with Material of Membranes from Jiangsu Jiusi High-Tech Co. Ltd Electrolyte

T

0.001–0.1 M NaCl

Method

Instrument

pH0

Reference

iep

Electro-osmosis

5.7

[936]

3.1.1.1.1.41 Aluminas from Johnson Matthey 3.1.1.1.1.41.1 g-Alumina Properties: g form [205,937,938], 99.999% pure [205,937], 99.99% pure [938], BET specific surface area 55 m2/g (manufacturer) [205], specific surface area 79 m2/g [937], 115 m2/g (manufacturer) [938], mean particle diameter 1 mm [205], mean diameter 10 nm [937]. TABLE 3.72 PZC/IEP of γ-Alumina from Johnson Matthey Description

Electrolyte

T

Method

Instrument

pH0

Reference

As obtained

0.001 M NaNO3

23

pH

1 d equilibration

4

[205]

128

Surface Charging and Points of Zero Charge

3.1.1.1.1.41.2 a-Alumina Properties: 99.99% pure, a-form, BET specific surface area 10.9 m2/g [939,940]. TABLE 3.73 PZC/IEP of α-Alumina from Johnson Matthey Description

Electrolyte

10% HNO3- and 10% NaOH-washed a

T

Method a

0.01–1 M NaNO3

25

Instrument

pH0

Reference

9.1

[940]

cip

Also at 30–70∞C.

3.1.1.1.1.42 Grade A Alumina from Konig Keramik Properties: Particle diameter calculated from specific surface area 60 nm [242]. TABLE 3.74 PZC/IEP of Grade A Alumina from Konig Keramik Electrolyte

T

0.01 M NaNO3 a

Method

Instrument

iep

pH0

PCD Mutek

Reference

7.2–9.3

a

[242]

Hysteresis; the result depends on the solid-to-liquid ratio.

3.1.1.1.1.43 Aluminas from LaRouche 3.1.1.1.1.43.1 a-Alumina Properties: BET specific surface area 30 m2/g [14]. TABLE 3.75 PZC/IEP of α-Alumina from LaRouche Description

Electrolyte

Calcined at 1050∞C for 4 h

T

0.1 M NaNO3

Method

Instrument

Mass titration

pH0

Reference

8.5

[14]

3.1.1.1.1.43.2 g-Alumina Properties: BET specific surface area 138 m2/g [14]. TABLE 3.76 PZC/IEP of γ-Alumina from LaRouche Electrolyte 0.1 M NaNO3

T

Method Mass titration

Instrument

pH0

Reference

8

[14]

129

Compilation of PZCs/IEPs

3.1.1.1.1.44 Aluminas from Linde 3.1.1.1.1.44.1 Linde A (Linde A from Union Carbide or from Praxair) See also Sections 3.1.1.1.1.13 and 3.1.1.1.1.78. Obtained by controlled calcination of pure ammonium alum. Properties: 90% a + 10% g [941], large particles (230 nm) are made up of highly crystalline a-form, and smaller particles (17 nm) are predominantly g-form [942], a-form [703,943,944], relative surface area 40% a-form, 60% g-form [945], high-purity [703,947], impurities (ppm): Si 200, Ga 30, Fe 30, Ca 7, Cu 7, Mg 5 [944], BET specific surface area 15 m2/g [941], [703] (krypton), [943,944,946,947], specific surface area 14 m2/g [948], mean diameter 0.3 mm [941], average diameter 100 nm [944], TEM image available [942,945], electron diffraction patterns available [945]. TABLE 3.77 PZC/IEP of Linde A Electrolyte

T

Method

None

iep

0.002 M NaCl

23b

pH

0.002 M NaCl

23

iep

0.001–0.1 M KCl, KNO3, KClO4

25

cip iep

Instrument Laser Zee Meter, Pen Kem 501

pH0 >8a

Reference [949]

8.8

[703]

Zeta-Meter

9.1

[943]

Zeta-Meter

9.1

[944]

Zeta-Meter

7.5a Pen Kem 501 Laser Zee-Meter 8.9 Zeta-Meter 3.0 9 Acoustosizer Matec 9b

Reference [1008] [369] [978] [1006] [1010]

+5 mV at pH 7.5, −10 mV at pH 9. Matches the maximum in yield stress of 30 vol% dispersion.

3.1.1.1.1.72.7 AKP 20 Properties: a-form [1010,1012], purity 99.99% [1012], >99.99% pure [1010], BET specific surface area 4.5 m2/g [1012], 4.3 m2/g [1010], mean particle diameter 0.56 mm, range 0.2–1.8 mm [1012], mean particle size 540 nm [1010].

TABLE 3.129 PZC/IEP of AKP 20 from Sumitomo Electrolyte

T

Method

Instrument

pH0

Reference

0, 0.01M NaCl 0.001 M KNO3

25

iep iep

Zeta-Meter 3.0 Acoustosizer Matec

8.2 9a

[1012] [1010]

a

Yield stress of 30 vol% dispersions in 0.01 M KNO3 peaks at pH 9.

3.1.1.1.1.72.8 AKP 30 (from Sumitomo or from Mandoval) Properties: a-form [242,1008,1010,1013–1017] and [521] (from Mandoval), purity: >99.995% [1014], >99.99% [793,835,1010,1015], 99.99% [521], impurities (ppm): Fe 20, Si 50, Cu 10, Mg 10, Na 10 (manufacturer) [521], Si 8, Na 2, Mg 2, Cu 1, Fe 5 [1018], density 3970 kg/m3 [1015,1018], BET specific surface area 5.9 m2/g [1014], 6.5 m2/g [667] (the column headings in Table 1 of [667] were probably swapped) [1013], 7 m2/g [835,1015,1019,1020], 7.1 m2/g [1016], 7.5 m2/g [1017,1018], 10 m2/g [793], specific surface area 6.8 m2/g [443,1010], mean diameter 0.3 μm [1015,1018,1019], 360 nm [1010], average particle size 390 nm [1013], 400 nm [1008], average particle diameter 0.3–0.5 μm [1016], 0.37 μm [1014], average

146

Surface Charging and Points of Zero Charge

particle size 0.4 μm [793], 300 nm [1017], d90 = 610 nm, d50 = 350 nm, d10 = 200 nm [835], d50 = 400 nm [1020], median diameter 310 nm [242], volume mean diameter 400 nm, number mean diameter 340 nm [521], mean diameter 270 nm [443], oblong particles, aspect ratio < 2:1 [1018,1019].

TABLE 3.130 PZC/IEP of AKP 30 from Sumitomo or from Mandoval Description As obtained

Electrolyte 0.001 M KNO3

Soxhlet washed with water KNO3 Mandoval

T

Method

25 iep Mass titration iep iep

0.0001–0.1 M NaCl, KCl, NaNO3 0.01 M NaCl 0.01, 0.1 M NaCl 0.006 M NaCl 0.01 M NH4Cl 0.01 M NaNO3

iep Intersection 22 iep iep 22 iep

0.001 M KNO3

25 iep

0.01, 0.05 M NaCl

20 iep pH 25 iep

0.001, 0.01 M KNO3, Na and K trichloroacetate, trifluoroacetate and trifluoromethanesulfonate 0.01 M KCl

0.01–1 M NaNO3, KNO3, CsNO3, KCl, KBr, KI 0.01 M LiNO3

Instrument Rank Brothers Mark II

Brookhaven Zeta PALS Malvern Zetasizer II, Zetamaster ESA 8000 Matec

pH0

Reference

3.7

[1008]

7.9

[667]

8a

[59]

8c

[521]

8.7 8.7 Acustosizer 8.7b Sugiura 2 VD 9 Malvern Zetasizer 9d 3000 HS, Matec ESA 8000 and 9800 PCD Mutek DT 1200 Acoustosizer 9e Matec ESA 8000 Matec 9 7.6 Malvern Nano ZP 9.2 DT 1200 Acoustosizer II

[1017] [1014] [1016] [1013] [242]

iep

Acoustosizer Matec

9.4

[443]

25 iep

Acoustosizer

9.4–9.5 [1015,1018, 1019e,1021]

[1010] [793f,835g, 1020] [40]

continued

147

Compilation of PZCs/IEPs

TABLE 3.130 (continued) Description

Electrolyte

Original and washed

0.01–1 M KNO3

a b

c

d

e f g

T

Method

Instrument

25 cip Malvern Nano ZP iep DT 1200 Salt titration Acoustosizer

pH0

Reference

9.6 9.4 9.2

[492]

Arbitrary interpolation. IEP roughly matches maxima in viscosity, in apparent particle size and in sedimentation volume in a 2 vol% dispersion. IEP obtained in different electrolytes at different concentrations showed substantial scatter (7.5–8.2), but no systematic trend. For volume fractions of 5% and 10%. IEP observed with 1% volume fraction was substantially higher. PCD produced lower IEP than other instruments and a substantial hysteresis. Matches maximum in yield stress of 20–30 vol% dispersion. Also 10 and 40°C. Only value, no data points.

3.1.1.1.1.72.9 AKP 50 or LAKP-50 Obtained by hydrolysis of very pure organometallic precursors, followed by calcination at >1100°C and ball-milling [106]. Properties: a-form [106,344,371,509,519,978,1006,1009–1011,1022,1023 (LAKP-50), 1024–1026,1028,1029,1033], purity >99.99% [1010], 99.99% [106,1011], >99.9% [1006], 99.9% [1028], >99.9% [509], 99.995% [371,1029,1030], high purity (LAKP-50) [1023], impurities: 8 ppm Si, 8 ppm Fe, 3 ppm Mg, Cu, and Na (according to manufacturer) [371], 15 ppm Si, 4 ppm Fe, 2 ppm Mg [106], specific density 3970 kg/m3 [509], 3940 kg/m3 [1009], BET specific surface area 7 m2/g [1031], 10–12 m2/g [509], 9.9 m2/g [1009], 10 m2/g [106], 11.5 m2/g [1026], 10.9 m2/g [978], 10.8 m2/g [1022], 9.7 m2/g [371], specific surface area 10.5 m2/g [1010], 9.5 m2/g [1029], 11.5 m2/g [1027], 10 m2/g [1028], mean particle size 0.3 mm [1028], 180 nm [1010], 200 nm [978], average particle size 0.2 mm [1032], average diameter 200 nm [1009,1033], 220 nm [1027], particle diameter 100–300 nm [106], particle size 100–300 nm [371], 220 nm [1026], mean diameter 200 nm [1022], 210 nm [1029], particles consist of 2–3 smaller (80 nm) particles [106], mean particle diameter 0.2 mm [509], d50 = 250 nm [344,519,1024,1025], 230 nm [1006], spherical particles [1026], TEM image available [106].

TABLE 3.131 PZC/IEP of AKP-50 or LAKP-50 from Sumitomo Description

Electrolyte KCl CH3COOH + (CH3)4NOH

T

Method

Instrument

pH0

Reference

iep iep

Malvern Nano ZS Electrophoresis, Matec ESA 8000

7.9 8

[1031] [1027]

continued

148

Surface Charging and Points of Zero Charge

TABLE 3.131 (continued) Description

Electrolyte

T

Method

0.01 M NaCl

iep

0.005 M KCl 0.01–0.02 M NH4NO3, NaNO3 0.01 M NH4Cl

iep iep

Zetasizer MKII, Malvern Pen Kem 501 ESA 8000, Matec

iep

Zeta-Meter 3.0

0.03 M NaCl

iep

0.005 M KNO3 0.001 M KNO3

iep iep

0.001 M KNO3

25 iep

0.01 M KNO3 0.005 M KNO3 0.01 M KBr, KNO3 Original and 0.0027–0.102 M after aging in NaNO3 water at 100°C 0.001 M KNO3 for 1 d 1 M HNO30–0.1 M NaNO3 washed Calcined at 0.025, 0.1 M 1200°C for 5 h NaNO3 0.01 M NH4Cl

b c d e f g

pH0

Reference

8.1

[371]

8.2 8.7

[1023] [1030] [1032]a [1011] [978] [1022]c [1025] [1028] [1024]c

iep iep iep iep

8.7 9b Pen Kem 501 Laser 8.9 Zee Meter Zeta-Meter 3.0 9 Zeta-Meter 3.0 9 Pen Kem 501 Malvern Zetasizer 3 MBS 8000 Matec Acoustosizer 9d Matec 9.1 Zeta-Meter 3.0 9.3 Zeta-Meter 3.0 9.3 Matec Acoustosizer 9.3–9.5e

cip iep

Coulter Delsa 440 SX

[106]

9.3 9

cip

9.5

Intersection

9.2f

iep

25 iep 0.01, 0.1 M NaNO3, NaCl, NaClO4, NaBrO3 a

Instrument

[1010] [1033] [1006] [344] [519]

Zeta-Meter 3.0

9.5

[773]

Acoustosizer

9.6g

[509]

Arbitrary interpolation. Broad stability minimum around IEP. Only value, no data points. Matches maximum in yield stress of 30 vol% dispersion. A misleading horizontal line in Figure 7 in [519] corresponds to ζ ≈ 15 mV. The charging curves obtained at three ionic strengths (0, 0.025, 0.1 M NaNO3) do not have a CIP. Maximum of the yield stress of dispersions (0.25 volume fraction) matches IEP.

149

Compilation of PZCs/IEPs

3.1.1.1.1.72.10 AKP HP40 Properties: a-form, BET specific surface area 5.3 m2/g, mean particle diameter 0.45 mm [567].

TABLE 3.132 PZC/IEP of AKP HP40 from Sumitomo Electrolyte

T

Method

0.001, 0.01 M KNO3 20 (text) 25 (figures)

Instrument

Intersection iep Stability

Zetasizer 3, Malvern Zee Meter 501, Pen Kem

pH0

Reference

9 9 9

[567]

3.1.1.1.1.72.11 AKP-G015 Properties: g-form, BET specific surface area 140 m2/g [1034]. Similar material identified as g-alumina from Sumitomo. Properties: 99.995%, BET specific surface area 140 m2/g [150,1035–1037], 152 m2/g [1038].

TABLE 3.133 PZC/IEP of AKP-G015 from Sumitomo Description

Electrolyte

T

Method

Aged

0.1 M NaCl 0.1 M NaCl

25 25

pH pH

a

Instrument

pH0

Reference

8.2 8.4a

[1035] [1034]

Only acidity constants reported, no data points.

3.1.1.1.1.72.12 a-Alumina Properties: Purity: 99.9% [1039], >99.99% [1040,1041], BET specific surface area 9.7 m2/g [1039], specific surface area 10.5 m2/g [1040], particle size 100–300 nm [1039].

TABLE 3.134 PZC/IEP of α-Alumina from Sumitomo Electrolyte KOH + HCl 0.0001–0.01 M NaCl

a

T

Method

25

Only value reported, no data points.

Instrument

pH0

Reference

iep iep

Zeta-Meter Pen Kem 7000

8 8.3a

[1040] [1041]

cip iep

Rank Brothers II

8.5 8.7

[1039]

150

Surface Charging and Points of Zero Charge

3.1.1.1.1.73 Aluminas from Tamei (or Taimei) 3.1.1.1.1.73.1 TM-DAR Properties: a-form [1042–1044], 99.99% pure [832], >99.99% pure [1042–1044], detailed analysis available [1042], BET specific surface area: 13.4 m2/g [1042–1044], 14.3 m2/g [832], d50 = 116 nm [1042–1044], average size 220 nm [832], SEM image available [1042]. A sample calcined for 4 hours at 400°C was studied in [1042–1044]. TABLE 3.135 PZC/IEP of TM-DAR from Taimei Aging

Electrolyte

16 h 1h 16 h

T

Method

Instrument

pH0

Reference

25 ± 2

iep iep/pH

DT 1200 DT 1200

9.1 9.3/7.2 9/7.2

[1043,1044] [1042]

3.1.1.1.1.73.2 Alumina from Tamei (or Taimei), Type Unknown Properties: a-form [1045,1046], 99.99% pure [1046], BET specific surface area 14.7 m2/g [1045], 17.4 m2/g [1046], particle size 210 nm [1045]. TABLE 3.136 PZC/IEP of Unidentified Alumina from Tamei (or Taimei) Electrolyte 0.01 M NaNO3 a

T

Method

Instrument

pH0

Reference

20a

iep

Zeta-Meter

9.1

[1046]

Also studied at 10–40∞C.

3.1.1.1.1.74 Microfiltration Membrane from Terronic (Czech Republic) Crushed and milled. Properties: a-form [418,1047]. TABLE 3.137 PZC/IEP of Microfiltration Membrane from Terronic Electrolyte None 0.001 M NaCl 0.005 M NaCl None 0.001 M NaCl 0.01 M NaCl a

T

25

Arbitrary interpolation.

Method

Instrument

pH0

Reference

iep

Zeta PALS Brookhaven

[1047]

iep

Zeta PALS Brookhaven

3.5 4.5 4.5a 3.8 3.8 4.1

[418]

151

Compilation of PZCs/IEPs

3.1.1.1.1.75 Alumina from Thiokol Properties: BET specific surface area 0.14 m2/g [30,1048].

TABLE 3.138 PZC/IEP of Alumina from Thiokol Description

Electrolyte

Washed with hot HCl

3.1.1.1.1.76

T

0.1, 1 M NaCl 0.001 M NaCl

Method

Instrument

pH0

Reference

pH iep

Streaming potential

8.2 8.5

[30] [1048]

Sapphire Single Crystal from Tyco

TABLE 3.139 PZC/IEP of Sapphire Single Crystal from Tyco Electrolyte 0.01, 0.05 M NaBr 0.001 M NaCl

a

T 20–25

Method iep iep

Instrument

pH0

Reference a

Electro-osmosis Electro-osmosis

3.1–3.5 3.3

[1049] [278]

The same study reports σ0 (pH) curves calculated from combined electrokinetic and radiotracer measurements (Na and Br adsorption) for two NaBr concentrations.

3.1.1.1.1.77 Alumina from UCAR Properties: a-form, specific density 3.92 g/cm3 [1050].

TABLE 3.140 PZC/IEP of Alumina from UCAR Electrolyte 0.001 M KCl

T

Method

Instrument

pH0

Reference

iep

Rank Brothers

7.5

[1050]

3.1.1.1.1.78 Aluminas from Union Carbide (Praxair) 3.1.1.1.1.78.1 Linde A See Sections 3.1.1.1.1.13 and 3.1.1.1.1.44.1. 3.1.1.1.1.78.2 3.1.1.1.1.78.3 14 m2/g [1051].

Linde B See Section 3.1.1.1.1.44.2. Nonporous a-Alumina Properties: Specific surface area

152

Surface Charging and Points of Zero Charge

TABLE 3.141 PZC/IEP of Nonporous α-Alumina from Union Carbide Electrolyte

T

Method

0.03 M NaCl

43

pH

Instrument

pH0

Reference

8

[1051]

3.1.1.1.1.79 Chromatographic Alumina from Veb Lab Apolda Properties: BET specific surface area 67.3 m2/g, particle diameter 2–5 mm [1052]. TABLE 3.142 PZC/IEP of Chromatographic Alumina from Veb Lab Description As received a

Electrolyte

T

0.0001–1 M KCl

Method

25

cip

Instrument

a

pH0

Reference

8.9

[1052]

Sample was also studied by means of electrophoresis. The ζ potential at pH < 8 was positive. No results at pH > cip were reported.

3.1.1.1.1.80 Aluminas from Whitfield and Sons 3.1.1.1.1.80.1 SDK 160 Properties are reported in [1053]. TABLE 3.143 PZC/IEP of SDK 160 from Whitfield and Sons Electrolyte None 0.001 M NaCl, KCl 0.05 M NaCl, KCl 0.01 M KCl + KOH or NaOH a b

T

25

Method

Instrument

pH0

Reference

iep

AcoustoSizer

[307]

iep

AcoustoSizer

7.4 7.9a 8.2a 8.1b

[493]

Base titration. Acid titration produces IEP higher by about 0.2 pH unit. Various IEP in a range 8.1–9.1 were obtained for different bases (NaOH, KOH, NH3) and directions of titration.

3.1.1.1.1.80.2 SDK 161

Properties are reported in [1053].

TABLE 3.144 PZC/IEP of SDK 161 from Whitfield and Sons Electrolyte

T

Method

Instrument

pH0

Reference

0.01 M KCl + KOH or NaOH

25

iep

AcoustoSizer

7.6a

[493]

a

Various IEP in a range 7.5–8.2 were obtained for different bases (NaOH, KOH, NH3) and directions of titration.

153

Compilation of PZCs/IEPs

3.1.1.1.1.81 Alumina 200 from Woelm Pharma Properties: BET specific surface area 143.2 m2/g, average particle size 3.8 mm, average pore radius 3.6 nm [1054].

TABLE 3.145 PZC/IEP of Alumina 200 from Woelm Pharma Description

Electrolyte

T

Method

Acid-washed

0.001–0.1 M KNO3

25

cip

Instrument

pH0

Reference

8.7

[1054]

3.1.1.1.1.82 Alumina from Wusong Fertilizer Factory Properties: a-form [407,1055,1056], BET specific surface area 4.4 m2/g [1055], 26.3 m2/g [407, 1056].

TABLE 3.146 PZC/IEP of Alumina from Wusong Electrolyte

T

0.001 M NaCl 0.001 M KCl a

Method

Instrument

pH0

Reference

iep iep

ZetaPlus Brookhaven ZetaPlus Brookhaven

8.2a 8.3

[407,1056] [1055]

Based on arbitrary interpolation.

3.1.1.1.1.83 Alumina from Wu-Xi Chemical Reagent Factory, Shanghai Properties: g-form, BET specific surface area 117 m2/g [167].

TABLE 3.147 PZC/IEP of Alumina from Wu-Xi Description

Electrolyte

T

Method

Water-washed

0.01–1 M NaNO3

30

cip

a

Also 17 and 45°C.

Instrument

pH0

Reference

7.7

[167,1057a]

154

3.1.1.1.1.84

Surface Charging and Points of Zero Charge

Origin Unknown

TABLE 3.148 PZC/IEP of Aluminas from Unknown Commercial Sources Description

Electrolyte

T

NaOH + HCl α, molten at 2010– 2050°C, then crushed, washed in 6% HCl at 60°C 1 M KCl α, reagent grade γ, chromatographic, 180 m2/g, calcined for 1 d at 800°C Chromatographic grade, activated α, high purity, 280 m2/g Chromatographic, 115 m2/g

a

b c d e

iep

Electroosmosis

pH0

Reference [1058]

pH

7.1d

[1059]

0.0005–0.01 M NaCl

pH

7.3a

[575]

0.001 M NaCl

iep

8b

[1060]

0.0001 M KNO3, KCl

iep

8.3

[825]

0.1 M KCl 0.1 M KBr 0.1 M KNO3

iep

8.3

[1061]

8.5

[1062,1063]

iep

α, 29.8 m2/g, crystallite 0.001–0.1 M KNO3 size 191 nm, SEM image available 0.01 M NaCl α, reagent grade, acid- and basewashed, 9.3 m2/gc Analytical grade, 0.01 M KNO3 99.5% pure, Si 500 ppm, Ca 700 ppm, Fe 270 ppm, 220 m2/g

Chromatographic, as received

Instrument

6.7

α, >99.9% pure, 720 nm average diameter, 9.1 m2/g

α, ground membrane

Method

0.001–0.1 M KCl

Streaming potential Streaming potential

PCD Mutek

25

cip

8.9

[112]

25

pH

9

[1064]

30

iep

Zeta-Meter

9.1

[1065]e

iep

Pen Kem 3000

9.5–10 [1066]e

Titration Drift

10 9.8

[1067]e

Charging curves obtained at different ionic strengths do not show a clear CIP, and the ionic strength effect on σ0 is irregular. Arbitrary interpolation. Cited after [533], in which γ-alumina was studied. From Table 2. Fig. 1 suggests a higher PZC. Only value/range, data points not reported.

155

Compilation of PZCs/IEPs

3.1.1.1.2 Synthetic Aluminas PZCs/IEPs of home-synthesized aluminum oxides are presented in Tables 3.149 through 3.179. 3.1.1.1.2.1 Obtained by Hydrolysis of Alkoxides 3.1.1.1.2.1.1 From Aluminum Isopropoxide, Calcined for 1 day at 873K Properties: BET specific surface area 174 m2/g [1068].

TABLE 3.149 PZC/IEP of Alumina from Aluminum Isopropoxide, Calcined for 1 d at 873K Electrolyte

T

Instrument

pH0 a

Mass titration cip

0.001–0.1 M NaNO3 a

Method

8.2 8.5

Reference [1068]

Only value, data points not reported.

3.1.1.1.2.1.2 From Aluminum sec-Butoxide, Recipe from [416] Aluminum sec-butoxide was mixed with 100 molar parts of water. The mixture was heated to 90°C and 0.07 mol of HCl was added per mole of aluminum sec-butoxide. The sol was dried at room temperature and calcined. Properties: mean particle size, BET specific surface area, and structure are given in Table 3.150 [416].

TABLE 3.150 PZC/IEP of Alumina from Aluminum sec-Butoxide, Recipe from [416] Calcination Size BET (°C) (µm) (m2/g) Structure Electrolyte T Method Instrument pH0a 25 300 500 800 1100 1400 a

80°C with stirring. 1.5 dm3 of water was used per mole of butoxide. Then 0.07 mol of HNO3 was added per mole of butoxide. The mixture was boiled until butanol evaporated, and then it was refluxed for 16 h. Properties: g-form, specific surface area 250 m2/g [312].

TABLE 3.154 PZC/IEP of Alumina from Aluminum sec-Butoxide, Recipe from [1072] Electrolyte

T

Method

Instrument

pH0

Reference

0.001–0.1 M NaCl

25

iep

Malvern Zetasizer 3000 HSa

8.3

[312]

3.1.1.1.2.1.7 Obtained by Heating of a Hydroxide Properties: g form [1074].

TABLE 3.155 PZC/IEP of Alumina Obtained by Heating of a Hydroxide Electrolyte

T

NaOH a

20

Method iep

Instrument Electrophoresis

pH0 8

Reference

a

[1074]

Only value, data points not reported.

3.1.1.1.2.2 Obtained by Hydrolysis of Inorganic Precursors at Room Temperature 3.1.1.1.2.2.1 From Nitrate and Ammonia, Recipe from [658] Ammonia was added to Al(NO3)3 at pH 10. The dispersion was aged at 25°C for 5 h, and filtered. The solid was dried at 110°C for 1 day and calcined in air for 5 h at 500°C. Properties: g-form [658], specific surface area 230 m2/g [674].

TABLE 3.156 PZC/IEP of Alumina from Nitrate and Ammonia, Recipe from [658] Electrolyte

T

Method

Instrument

pH0

Reference

0.001–0.1 M KNO3

25

cip iep

Malvern Zetasizer 5000

8.6 8.2

[657b,658,674]a

a b

The results of mass titration and inflection point are also reported. Only value, data points not reported.

158

Surface Charging and Points of Zero Charge

3.1.1.1.2.2.2 From Nitrate and Ammonia, Recipe from [1075] 1:1 ammonia was added dropwise to 0.1 M Al(NO3)3 with stirring up to pH 8. The gel was aged for 1 day, water-washed, and dried for 4 days at 120°C. It was ground to 250 mesh, and calcined at 450°C for 5 h in air. Properties: g-form, BET specific surface area 214 m2/g [1075].

TABLE 3.157 PZC/IEP of Alumina from Nitrate and Ammonia, Recipe from [1075] Electrolyte

T

HCl + KOH

Method

Instrument

pH0

Reference

iep

Rank Mark II

8.2

[1075]

3.1.1.1.2.2.3 From Chloride and NaOH, Modified Recipe from [1076] 10 cm3 of 0.35 M NaOH was slowly added to 10 cm3 of 0.13 M AlCl3, shaken for 15 min and centrifuged. Properties: Traces of gibbsite and bayerite, median diameter 160 nm, size distribution available [353].

TABLE 3.158 PZC/IEP of Alumina from Chloride and NaOH, Modified Recipe from [1076] Electrolyte 0.001 M NaCl a

T

Method

Instrument

pH0

Reference

iep

Zeta-Meter 3.0

9.5a

[353]

Only value, data points not reported. The minimum in CCC matches the IEP (only acidic branch available).

3.1.1.1.2.2.4 From Nitrate and Ammonia, Recipe from [1077] Solution of nitrate was adjusted to pH 2 with HNO3, and then treated with excess of ammonia. The precipitate was washed with water, calcined at 600°C for 5 h, washed with water again, calcined at 600–1400°C for 3 h, and washed with water again. Properties: g-structure below 800°C, partial transition to a at 1000°C, and complete transition at >1200°C [1078], lattice spacing data [1077] and specific surface area available [1078]. Recipe from [33]. Hydroxide was obtained from 1 M Al(NO3)3 and a 10% excess of 3 M ammonia, washed with water, and calcined at 600°C for 6 h. The oxide was washed with water until constant conductivity. Properties: 20 nm in diameter [33].

159

Compilation of PZCs/IEPs

TABLE 3.159 PZC/IEP of Alumina from Nitrate and Ammonia, Recipe from [1077] Second Calcination Temperature (°C) None 1200 600 800 1000 1200 1400

Electrolyte

T

0.001 M NaCl 25 0.001 M NaCl 25 0.001 M NaCl 25

Method

Instrument

pH iep iep

Streaming potential Streaming potential Streaming potential

pH0 Uncrushed/ Crushed Reference 9.2 9.6 11.4/11.2 11.2 9.7/11 9.6/9.7 9.6/9.6

[33] [1077] [1078]

3.1.1.1.2.2.5 From Nitrate and Ammonia, Recipe from [250] Dilute ammonia was added to Al(NO3)3. The precipitate was washed and ignited. Properties: Ignition at 1000°C produced a-Al2O3. The lower the temperature, the stronger the gibbsite lines [250].

TABLE 3.160 PZC/IEP of Alumina from Nitrate and Ammonia, Recipe from [250] Heated at (°C) 300 600 1200

Electrolyte 0–0.005 M KCl, KNO3, KClO4

T

Method

Instrument

pH0

Reference

30

iep

Electrophoresis

9.2 8.2–8.5 6.8–7.7

[250]

3.1.1.1.2.2.6 Obtained from AlCl3 and NaOH (Cited by [1] and by Many Others as IEP of Amorphous Al2O3 Properties: Cl/Al2O3 mole ratio 0.005 in solid [1079].

Table 3.161 PZC/IEP of a Precipitate Obtained from AlCl3 and NaOH (Cited by [1] and by Many Others as IEP of Amorphous Al2O3) Electrolyte NaCl

T

Method

Instrument

pH0

Reference

iep

Electrophoresis

8.1

[1079,2226]

160

Surface Charging and Points of Zero Charge

3.1.1.1.2.2.7 From Aluminum Dross Tailings Chemical Waste Leaching with NaOH and precipitation with different agents. Calcination of the precipitate at 600°C for 3 h. Properties: g-form [1080]. TABLE 3.162 PZC/IEP of Alumina from Aluminum Dross Tailings Chemical Waste Precipitation Agent

BET (m2/g)

H2O2 NH4Al(SO4)2 Al(OH)3 seeds

243 169 176

a

Electrolyte

T

Method

Instrument

pH0a

Reference

HCl + KOH

Room

iep

Zeta-Meter 3.0

8.5 8.6 8.4

[1080]

Arbitrary interpolation.

3.1.1.1.2.2.8 From Natural Bauxite from India Leaching with NaOH and precipitation with different agents. Calcination of the precipitate at 600°C for 3 h. Properties: g-form [1080]. TABLE 3.163 PZC/IEP of Alumina from Natural Bauxite from India Precipitation agent

BET m2/g

H2O2 NH4Al(SO4)2 Al(OH)3

244 170 158

a

Electrolyte

T

Method

Instrument

pH0a

Reference

HCl + KOH

Room

iep

Zeta-Meter 3.0

8.5 8.6 9

[1080]

Arbitrary interpolation.

3.1.1.1.2.2.9 From Chloride and NaOH, Recipe from [1081] 400 cm3 of 2 M NaOH was slowly added to 200 cm3 of 1.5 M AlCl3. The precipitate was washed with water and with 95% ethanol, and dried at 70°C for 1 day. Properties: Amorphous, BET specific surface area 209 m2/g [381].

TABLE 3.164 PZC/IEP of Alumina Obtained from Chloride and NaOH, Recipe from [1081] Description

Electrolyte 0.01 M NaCl

T

Method

Instrument

pH0

Reference

iep

Zeta-Meter 3.0

9.4

[381]

161

Compilation of PZCs/IEPs

3.1.1.1.2.3 Hydrolysis of Inorganic Precursor at Elevated Temperature Solution 0.2 M in Al(NO3)3 and 50 g/dm3 in urea was heated at 95°C for 100 min.

TABLE 3.165 PZC/IEP of Alumina Obtained by Hydrolysis of Nitrate at 95°C Electrolyte

T

Method

Instrument

pH0

Reference

iep

ZetaPlus Brookhaven

8.2

[1082]

0.001 M KNO3

3.1.1.1.2.4 Controlled Dehydration of Hydroxides 3.1.1.1.2.4.1 Thermal Decomposition of Gibbsite Properties: a-form [225], g-form in samples decomposed at lower temperatures, a-form in sample calcined at 1350°C. In sample calcined at 1000°C, a-form was found with admixture of g-form [1083], g-form, BET specific surface area 26 m2/g, crystallite size 10 nm, SEM image, XRD pattern available [112].

TABLE 3.166 PZC/IEP of Alumina Obtained by Thermal Decomposition of Gibbsite Decomposition Temperature (°C) 1200 550, 1.5 h

Electrolyte

T

Method

NaOH + HCl 0.001–0.1 M KNO3

iep cip

Electrophoresis

25

3a 7b

[225] [112]

iep

Streaming potential

7.9 8.6 7.4 9.1 9.2

[1083]

400 600 800 1000 1350 a b

Instrument

pH0

Reference

Arbitrary interpolation. Reported in text. The curves shown in Figure 6 merge at pH 5–8.

3.1.1.1.2.4.2 Obtained by Calcination of Baker Al(OH)3 for 2 days at 1400°C Properties: a-form, BET specific surface area 3.7 m2/g [189].

TABLE 3.167 PZC/IEP of Alumina Obtained by Calcination of Baker Al(OH)3 Electrolyte

T

Method

0.001, 0.1 M NaNO3

25

Intersection

Instrument

pH0

Reference

8.4

[189]

162

Surface Charging and Points of Zero Charge

3.1.1.1.2.4.3 for 4 h

a-Al2O3 Obtained by Calcination of g-AlOOH at 1150°C

TABLE 3.168 PZC/IEP of α-Al2O3 Obtained by Calcination of γ-AlOOH at 1150°C for 4 h Electrolyte

T

Method cipa

0.005–0.5 M NaClO4 a

Instrument

pH0

Reference

9.4

[3215]

Only value, no data points.

3.1.1.1.2.4.4 Calcination of Diaspore at 500°C specific surface area given in Table 3.169 [1084].

Properties: a-form,

TABLE 3.169 PZC/IEP of Alumina Obtained by Calcination of Diaspore at 500°C BET (m2/g) 28 32 89 130

Electrolyte

T

HCl + NaOH

Method

Instrument

pH0

Reference

pH

1 h, 1 d equilibration

8.8 8.4 8.9 9.2

[1084]

3.1.1.1.2.4.5 Calcination of Boehmite at 800°C for 16 hours in Air Properties: g-form, specific surface area 270 m2/g [989].

TABLE 3.170 PZC/IEP of Alumina Obtained by Calcination of Boehmite at 800°C for 16 h in Air Electrolyte

T

Method Titration

a

Instrument

pH0 a

8.2

Reference [989]

Only value, data points not reported.

3.1.1.1.2.4.6 Calcination of Spherical Boehmite at 1200°C for 30 Minutes Properties: a-form, BET specific surface area 6 m2/g [1085].

163

Compilation of PZCs/IEPs

TABLE 3.171 PZC/IEP of Alumina Obtained by Calcination of Spherical Boehmite at 1200°C for 30 min Electrolyte

T

Method

Instrument

pH0

Reference

iep AFM

Malvern Zetamaster

5.6a 5

[1085]

NaCl, different concentrations a

Only value, data points not reported.

3.1.1.1.2.5 High-Temperature Reactions 3.1.1.1.2.5.1 Oxidation of Aluminum sec-Butoxide in Air at 1400°C Properties: Specific surface area 220 m2/g, XRD pattern available [1086]. TABLE 3.172 PZC/IEP of Alumina Obtained by Oxidation of Aluminum sec-Butoxide Electrolyte

T

Method

Instrument

pH0

Reference

iep

Malvern Zetasizer 3000

8.8

[1086]

NaOH + HCl

3.1.1.1.2.5.2 High-Temperature Hydrolysis of AlCl3 in Hydrogen Flame Properties: g-form, larger particles with admixture of cubic phase in the surface layer, particle size 10–50 nm, specific surface area 144 m2/g [1087]. Table 3.173 PZC/IEP of Alumina Obtained by High-Temperature Hydrolysis of AlCl3 Electrolyte

T

Method

Instrument

pH0

Reference

iep

Electrophoresis

7

[1087]

3.1.1.1.2.5.3 Low-Temperature Plasma Synthesis Properties: >99.9% pure, spherical particles 78 nm in diameter, a-form, specific surface area 43 m2/g [587]. TABLE 3.174 PZC/IEP of Alumina Obtained by Low-Temperature Plasma Synthesis Electrolyte

T

0.01, 0.1 M NaNO3

a

Arbitrary interpolation.

Method

Instrument

pH0

Reference

Intersection iep

Electrophoresis

8.2 7.8a

[587]

164

Surface Charging and Points of Zero Charge

3.1.1.1.2.5.4 a-form [1088].

Flame-Fused Ground in mullite ware, not cleaned. Properties:

TABLE 3.175 PZC/IEP of Flame-Fused Alumina Description

Electrolyte

T

Aged for 99.9% pure, 100 nm average size, 8.5 m2/g, TEM image available a 37 μm, 122 m2/g g

Method

Titration iep iep

114 m2/g a, 99.2% pure, 770 nm g, 100 m2/g a

T

Titration iep Titration pH

Electrophoresis Matec ESA 8000 1 h, 1 d equilibration

iep Coagulation iep Rank Brothers Mark II iep Malvern Zetasizer 4

8.5 8.8 7.4 8.8 8.8 8.8 8.6 9.4 8.9

[1100]

[1105]b [1106]b [1084]

[1107]

9

[1108]

9

[1109]

pH Titration

9 9b

[1110]b [1111]

Titration

9.1

[1112]c

Arbitrary interpolation. Only value, no data points. Only acidity constants, data points not reported.

167

Compilation of PZCs/IEPs

3.1.1.2 Aluminum Oxide Hydroxide PZCs/IEPs of aluminum oxide hydroxides (nominally AlOOH) are presented in Tables 3.182 through 3.209. 3.1.1.2.1 Commercial Boehmite PZCs/IEPs of boehmite from various commercial sources are presented in Tables 3.182 through 3.190. 3.1.1.2.1.1 Boehmite from BA Chemical Properties: BET specific surface area 175 m2/g, 260 nm in diameter [1113].

TABLE 3.182 PZC/IEP of Boehmite from BA Chemical Electrolyte

T

Method

Instrument

pH0

Reference

iep

Malvern ZetaMaster S

9.2

[1113]

3.1.1.2.1.2 Boehmites from Condea 3.1.1.2.1.2.1 Dispersal (see also Micrographs available [497].

Section

3.1.1.2.1.6) Properties:

TABLE 3.183 PZC/IEP of Dispersal from Condea Electrolyte

T

Method

KOH + HNO3

Instrument

iep

pH0

Reference

9

[497]

3.1.1.2.1.2.2 Boehmite from Condea (Condea Vista), Type Unknown Properties: Purity 99.6% [525,1036], 99.2% [1034,1114], BET specific surface area 180 m2/g [525,1034,1036,1115], 180 m2/g (original), 183 m2/g (after 4 months in aqueous dispersion) [1114], 270 m2/g [114].

TABLE 3.184 PZC/IEP of Unspecified Boehmite from Condea Electrolyte

T

Method

0.1 M NaCl

25

pH

Instrument

pH0

Reference

8.6

[525,1034,1114]

3.1.1.2.1.3 Boehmite from Electronic Space Product Industries Properties: 98% boehmite, 7

Reference [1127]

No data points for pH > 7.

3.1.1.2.2.2 Hydrolysis of Inorganic Precursor at Room Temperature 3.1.1.2.2.2.1 From Chloride, Recipe from [52] 820 cm3 of 2 M CO2-free NH4OH was added to a solution of 50 g of AlCl3 in 200 cm3 of water at 18°C. The precipitate was washed, dialyzed, and dried at 105°C. Properties: Admixture of bayerite [52].

TABLE 3.193 PZC/IEP of Boehmite Obtained from Chloride, Recipe from [52] Description

Electrolyte NaOH + HCl

a

T

Method

Instrument

pH0

Reference

iep

Electrophoresis

8.8a

[52]

Only IEP reported (no experimental data points).

3.1.1.2.2.2.2 From Na3AlO3, Recipe from [52] Air was passed through a Na3AlO3 solution of specific gravity 1150 kg/m3 (sample B) or through the same solution diluted 1:100 (sample C). The precipitate was washed, dialyzed, and dried at 105°C.

171

Compilation of PZCs/IEPs

TABLE 3.194 PZC/IEP of Boehmite Obtained from Na3AlO3, Recipe from [52] Sample B C a

Electrolyte

T

Method

NaOH + HCl

Instrument

iep

pH0

Reference

a

Electrophoresis

7.6 6.5a

[52]

Only IEP reported (no experimental data points).

3.1.1.2.2.3 Hydrolysis of Inorganic Precursor at Elevated Temperature 3.1.1.2.2.3.1 From Chloride, Recipe from [1128] 0.1 M AlCl3 was adjusted to pH 11 by slow addition of NaOH with stirring. It was aged with stirring for 10 min at room temperature and heated in an autoclave at 160°C for different times. Properties: TEM image, FTIR spectra, XRD pattern available [1128].

TABLE 3.195 PZC/IEP of Boehmite Obtained from Chloride, Recipe from [1128] Time of Heating 2h 1d

Electrolyte

T

HCl + NaOH

Method

Instrument

pH0

Reference

iep

Pen Kem S3000

9.3 9

[1128]

3.1.1.2.2.3.2 Pseudoboehmite (Poorly Crystalline Boehmite), Recipe from [1129] A solution 0.6 M in NaOH and 4 M in NaCl was added dropwise to a solution 0.4 M in AlCl3 and 4 M in NaCl (OH:Al ratio 2.95). It was centrifuged, aged for 20 h at 60°C in plastic bottles, washed, and dried at 60°C. Properties: BET specific surface area 287 m2/g [1129].

TABLE 3.196 PZC/IEP of Pseudoboehmite, Recipe from [1129] Electrolyte

T

Method

Instrument

pH a

pH0 a

9.3

Reference [1129]

Only acidity constants, data points not reported.

3.1.1.2.2.3.3 From Chloride and Perchlorate, Recipes from [1130] 0.0001–0.01 M AlCl3 containing HCl (Cl:Al mole ratio 3–4.5) was aged at 125°C

172

Surface Charging and Points of Zero Charge

for different times, and then cooled in ice–water. Analogous experiments were carried out with Al(ClO4)3–HClO4 mixtures. Properties: Boehmite, TEM and SEM images available [1130].

TABLE 3.197 PZC/IEP of Boehmites Obtained from Chloride and Perchlorate According to Recipes from [1130] Description

Electrolyte

T

0.005 M AlCl3, 3 days 0.005 M Al(ClO4)3, 3 days a

Method

Instrument

iep

pH0 Reference

Rank Brothers

8a 9.3

[1130]

Arbitrary interpolation.

3.1.1.2.2.3.4 Recipe from [1131] 1 M Al(NO3)3 or AlCl3 was adjusted at room temperature to pH 6–10 with 1–10 M ammonia. The precipitate was washed and autoclaved for 6 h at 100–300°C. Properties: XRD results, TEM image available [1132], BET specific surface area 203 m2/g [1132,1133].

TABLE 3.198 PZC/IEP of Boehmite Obtained According to Recipe from [1131] Electrolyte

T

Method

0.001–1 M KNO3, KCl, KBr, KI

20

pH

Instrument

pH0

Reference

>7

[1132]

3.1.1.2.2.4 Controlled Dehydration of Hydroxide 3.1.1.2.2.4.1 Calcination of Gibbsite from Prolabo Properties: (calcination at 450°C for 5 h) BET specific surface area 246 m2/g, XRD image available [1134], (calcination at 350°C for 16 h) well-crystallized, specific surface area 318 m2/g [989].

TABLE 3.199 PZC/IEP of Boehmite Obtained by Calcination of Gibbsite from Prolabo Electrolyte

T

Method Titration

a

Only value, data points not reported.

Instrument

pH0

Reference

7.7a

[989]

173

Compilation of PZCs/IEPs

3.1.1.2.2.4.2

Calcination of Gibbsite at 300°C

TABLE 3.200 PZC/IEP of Boehmite Obtained by Calcination of Gibbsite at 300°C Electrolyte

T

Method

Instrument

pH0

Reference

iep

Streaming potential

7.7

[1083]

3.1.1.2.2.4.3 Aging of Amorphous Hydroxide

TABLE 3.201 PZC/IEP of Boehmite Obtained by Aging of Amorphous Hydroxide Electrolyte

T

Method

Instrument

pH0

Reference

NaOH

20

iep

Electrophoresis

9.4

[1074,1142]

3.1.1.2.2.5 Other Recipes 3.1.1.2.2.5.1 Recipe from [1135] using Perchlorate Three different recipes are reported in [1135]. Properties: BET specific surface area 41.2 m2/g [1136].

TABLE 3.202 PZC/IEP of Boehmite Obtained According to Recipe from [1135] Electrolyte 0.0001–0.01 M NaClO4

T

Method

Instrument

pH0

Reference

iep

Zeta-Meter

10.4

[1136]

3.1.1.2.2.5.2 Oxidation of Aluminum Powder in the Presence of HgCl2 Aluminum powder, 100 mesh, was treated with 0.0005 M HgCl2 for 10 min, then washed, and then treated with an excess of 1:4 aqueous acetic acid at 75°C. The dispersion was cooled to room temperature and aged overnight. It was then boiled at constant volume for 8 h (water added) to remove the excess acetic acid, following which, it was heated at 170°C for 8 h in a pressure vessel. The sediment was freeze-dried, resuspended in 30% H2O2, and boiled at constant volume for 6 h (water added). It was freeze-dried again. Properties: Fibers, 400–600 nm long and 50 nm in diameter, BET specific surface area 187.2 m2/g [529]. (Slightly modified procedure): boehmite, BET specific surface area 216 m2/g [1137].

174

Surface Charging and Points of Zero Charge

TABLE 3.203 PZC/IEP of Boehmite Obtained by Oxidation of Al Powder in the Presence of HgCl2 Electrolyte

T

0.1 M KCl 0.001 M KCl

Method

Instrument

pH0

Reference

pH iep

Pen Kem Laser Zee Meter 501

7.2 9.4

[529]

3.1.1.2.2.5.3 Oxidation of Aluminum Foil in the Presence of HgCl2 Aluminum foil (99.99% pure) was amalgamated with 0.025 M HgCl2, and then reacted with moist air. It was then exposed to boiling water for 3 min. Then purified by flotation, the precipitate was dried in vacuum, ground, and dried at 90°C. Properties: Pseudoboehmite, 160 ppm Hg, 10. This may be because, at such a high pH, the amount of base used to adjust the pH became comparable to the salt concentration.

3.1.12.5.3.3 Aging of Acidified 0.1 M FeCl3 at 40°C The description is not clear. Properties: BET specific surface area 116.4 m2/g [332,1417].

TABLE 3.519 PZC/IEP of Akageneite Obtained by Aging of Acidified 0.1 M FeCl3 at 40°C Electrolyte 0.01 M NaNO3 0.01 M KNO3 a

T

Method

Instrument

pH0

Reference

iepa

Malvern Zetasizer 3000HSa

6.6 >6.7

[332] [1417]

Only value, data points not reported.

298

Surface Charging and Points of Zero Charge

3.1.12.5.3.4 Refluxing of 0.2 M FeCl3 for 1 day Properties: BET specific surface area 31 m2/g [707].

TABLE 3.520 PZC/IEP of Akageneite Obtained by Refluxing of 0.2 M FeCl3 for 1 day Electrolyte

T

NaCl a

Method

Instrument

Salt titration

pH0

Reference

7.2a

[707]

Only value, data points not reported.

3.1.12.5.3.5 Refluxing of FeCl3 Solution Containing Urea for 45 Minutes Equal volumes of 0.037 M FeCl3 and 0.75 M urea were mixed and refluxed at 95–100°C for 45 min. Properties: total iron 58.1%, b-FeOOH structure confirmed by XRD [661], specific surface area 51 m2/g (original and stored) [568], 51.6 m2/g [1319], wheatgrain shape [661], TEM image available [661]. TABLE 3.521 PZC/IEP of Akageneite Obtained by Refluxing of FeCl3 Solution Containing Urea for 45 min Description

Stored for 2 years

Electrolyte

T

0.001–0.1 M KNO3

27

0.001–0.5 M NaCl

25 25

Method Salt titration cip iep cip iep

Instrument Mark II Rank Brothers 3 d equilibration Mark II Rank Brothers

pH0 Reference 7 7.2 7.2 7 7.2

[661]

[568]

3.1.12.5.3.6 Aging of FeCl3 Solution Containing Urea for 6.5 h at 100°C 2 dm3 of solution containing 50 g of Fe(iii) and 50g* of urea was adjusted to pH 1.52* with ammonia. Then the mixture was aged for 6.5h* at 100°C*. It was then aged overnight at room temperature. The precipitate was filtered, washed with water, and dried for 5 d at 50°C. The quantities marked with * were variable. Properties: b-form, BET specific surface area 77.8 m2/g [1527]. Reference [1623] reports properties of 10 samples obtained in a similar way (quantities marked with * were different from those in the above recipe), but the surface areas of all samples studied in [1623] were different from that of a sample used in [1527].

299

Compilation of PZCs/IEPs

TABLE 3.522 PZC/IEP of Akageneite Obtained by Aging of FeCl3 Solution Containing Urea for 6.5 h at 100°C Electrolyte

T

0.001–0.1 M KNO3 a

Method

Instrument

Titration

pH0

Reference

7.9a

[1527]

Only value, data points not reported.

3.1.12.5.3.7 Aging of FeCl3 Solution Containing Urea for 13 h at 100°C 2 dm3 of solution containing 50 g of Fe(iii) (as chloride) and 200 g of urea was adjusted to pH 0.6 with HCl, and agitated at 100°C for 13 h. It was aged overnight, filtered, and the precipitate was dried at 50°C for 5 d. Properties: Akageneite, BET specific surface area 8.6 m2/g [613].

TABLE 3.523 PZC/IEP of Akageneite Obtained by Aging of FeCl3 Solution Containing Urea for 13 h at 100°C Electrolyte

T

Method

0.001–0.1 M KNO3 0.001–0.1 M KCl 0.001–0.1 M LiNO3 0.001–0.1 M NaNO3

3.1.12.5.4

Instrument

cip

pH0 Reference 7.6 8.2 7.5 7.5

[613]

Synthetic Feroxyhite (Feroxyhyte)

3.1.12.5.4.1 Hydrolysis and Oxidation of FeCl2 Simultaneous addition of NaOH and H2O2 to FeCl2 solution at room temperature. The precipitate was washed and dried at 65°C. Properties: BET specific surface area 235 m2/g [707].

TABLE 3.524 PZC/IEP of Feroxyhite Obtained by Hydrolysis and Oxidation of FeCl2 Electrolyte NaCl a

T

Method Salt titration

Only value, data points not reported.

Instrument

pH0 a

7.5

Reference [707]

300

Surface Charging and Points of Zero Charge

3.1.12.5.4.2 Recipe from [1624] Four various recipes are reported in [1624]. Properties: d-form, BET specific surface area 117 m2/g [1527]. TABLE 3.525 PZC/IEP of Feroxyhite Obtained According to Recipe from [1624] Electrolyte

T

Method

0.001–0.1 M KNO3 a

Instrument

pH0

Reference

a

Titration

8

[1527]

Only value, data points not reported.

3.1.12.5.5

FeOOH, Structure Not Specified

3.1.12.5.5.1

Nanocrystalline, Recipe from [1625]

TABLE 3.526 PZC/IEP of Nanocrystalline FeOOH Obtained According to Recipe from [1625] Electrolyte

T

0.001 M a

25

Method

Instrument

pH0

Reference

iep

Riddick type II UVA cell

9.2

[1626]

a

Reported in text. A figure suggests 22.5°C.

3.1.12.5.5.2 Natural FeOOH, Structure and Origin Unknown Water-washed, and dried at 110°C. TABLE 3.527 PZC/IEP of Natural FeOOH of Unknown Structure and Origin Electrolyte

T

Method

Instrument

pH0

Reference

0, 0.001 M KCl

25

iep

Rank Mark II

6.6

[1213]

3.1.12.5.5.3

Other Properties: Specific surface area 22 m2/g [1101].

TABLE 3.528 PZC/IEP of FeOOH of Unknown Structure and Origin Electrolyte

T

Method Titration iep

a

Instrument

pH0

Reference

7.2 3.1

[1101]a

Only values, data points not reported. Similar results from Ref. [1618] are reported in Section 3.1.12.5.2.4.

301

Compilation of PZCs/IEPs

3.1.12.6 Limonite Limonite is a rock that contains goethite, lepidocrocite, and other minerals. Its empirical formula is Fe2O3·nH2O (n > 1). PZCs/IEPs of limonites are presented in Tables 3.529 and 3.530. 3.1.12.6.1 Limonite from China TABLE 3.529 PZC/IEP of Limonite from China Electrolyte

T

Method a

0.01 M NaCl a

iep

Instrument

pH0

Reference

Pen Kem 300

6.6

[1317]

Arbitrary interpolation.

3.1.12.6.2 Limonite from the Netherlands FeOOH, “Limonite” from Rhederoord, Netherlands, ground, dialyzed, and dried. TABLE 3.530 PZC/IEP of Limonite from the Netherlands Electrolyte

T

Method

Instrument

iep

Electrophoresis

NaOH + HCl a

pH0 Reference 3.6a

[225]

Arbitrary interpolation.

3.1.12.7 Fe5HO8 · 4H2O, Ferrihydrite PZCs/IEPs of ferrihydrites (nominally Fe5HO8 · 4H2O) are presented in Tables 3.531 through 3.542. 3.1.12.7.1 Synthetic 3.1.12.7.1.1 Aging of Precipitate Obtained at pH 8 from 0.2 M Fe(NO3)3 and 1 M NaOH 0.2 M Fe(NO3)3 was titrated to pH 8 with 1 M NaOH. The gel was dialyzed for 10 d. It was converted into an aggregate by freezing and thawing. Properties: Two-line ferrihydrite [539,1510], BET specific surface area 327 m2/g [1510], aggregates 235 μm in diameter [539]. TABLE 3.531 PZC/IEP of Ferrihydrite Obtained by Aging of Precipitate Obtained at pH 8 from 0.2 M Fe(NO3)3 and 1 M NaOH Electrolyte

T

Method

0.01–0.3 M NaNO3 0.01–1 M NaCl

25 20

cip Titration

a

Only value, data points not reported.

Instrument

pH0

Reference

8.7 8.7a

[539] [1510]

302

Surface Charging and Points of Zero Charge

3.1.12.7.1.2 Aging of Precipitate Obtained at pH 10 from 0.05 M Fe(NO3)3 and KOH 0.05 M Fe(NO3)3 was titrated with KOH to pH 10, and the precipitate was cleaned by dialysis. Properties: Two-line ferrihydrite [1627], BET specific surface area 254.2 m2/g [1628].

TABLE 3.532 PZC/IEP of Ferrihydrite Obtained by Aging of Precipitate Obtained at pH 10 from 0.05 M Fe(NO3)3 and KOH Electrolyte

T

Method

0.001 M KCl

a

iep

Instrument Coulter Delsa 440

pH0 8.3

a

Reference [1627] [1628]

Only value, data points not reported.

3.1.12.7.1.3 Precipitated at pH 7.5 from Fe(NO3)3 and 1 M KOH 500 cm3 of solution containing 40 g of Fe(NO3)3 · 9 H2O was titrated with 310 cm3 of 1 M KOH at 100 cm3/min. The pH was adjusted to 7.5 with 1 M KOH. The precipitate was washed with 0.1 M NaCl and stored at 2°C. Properties: Two-line ferrihydrite [157,1629], structure confirmed by XRD, three-point BET specific surface area 202 m2/g [1630].

TABLE 3.533 PZC/IEP of Precipitate Obtained at pH 7.5 from Fe(NO3)3 and 1 M KOH Electrolyte 0.001–0.1 M NaCl NaCl a

T

Method

Instrument

pH0

Reference

cip iep

Malvern Zetasizer 3000

8.5 7.6

[1630]a [157] [1629]

Only value, data points not reported.

3.1.12.7.1.4 Neutralization of Acidified Fe(NO3)3 Solution Recipe from [1631]. Fe(NO3)3 was dissolved in 0.1 M HNO3, and the pH was adjusted to 8 with NaOH. Properties: XRD results available, BET specific surface area 176 m2/g [1611].

303

Compilation of PZCs/IEPs

TABLE 3.534 PZC/IEP of Precipitate Obtained by Neutralization of Acidified Fe(NO3)3 Solution Electrolyte

a

T

Method

Instrument

pH0

Reference

iep

Malvern Zeta Master

7a

[1611]

Only value, data points not reported.

3.1.12.7.1.5 Rapid Titration to pH 6–6.5 then Slow Titration to pH 7 CO2-free 0.02 M Fe(NO3)3 was titrated rapidly to pH 6–6.5 and then slowly to pH 7 with 1 M NaOH. The precipitate was washed, freeze-dried, and stored in darkness. Properties: Two-line ferrihydrite, XRD pattern available, BET specific surface area 300 m2/g [1632]. TABLE 3.535 PZC/IEP of Precipitate Obtained by Rapid Titration to pH 6–6.5 then Slow Titration to pH 7 Electrolyte

T

0.01 M NaClO4

Method

Instrument

pH0

Reference

iep

Brookhaven Zeta PALS

7.1

[1632]

3.1.12.7.1.6 Rapid Hydrolysis of FeCl3 at pH 7–8 by KOH Properties: Onepoint BET specific surface area 269 m2/g, two-line ferrihydrite structure confirmed by XRD [173]. TABLE 3.536 PZC/IEP of Precipitate Obtained by Rapid Hydrolysis of FeCl3 at pH 7–8 by KOH Electrolyte

T

Method

0.001–0.1 M NaClO4

20

cip

Instrument

pH0

Reference

8

[173]

3.1.12.7.1.7 Recipe(s) from Schwertmann and Cornell Examples of specific recipes. A: 150 cm3 of solution containing 0.3 g of Fe(NO3)3 · 9H2O was titrated with 0.1 M NaOH to pH 7.

304

Surface Charging and Points of Zero Charge

B: 4 or 0.4 g of Fe(NO3)3 was dissolved in 50 cm3 of water and saturated with N2. It was then titrated with 0.1 M NaOH to pH 7. Properties: Two-line ferrihydrite structure confirmed by XRD [637], six-line [332,1417], BET specific surface area 360 m2/g [637], 193.3 m2/g [332,1417]. Modified recipe. Properties: Two-line ferrihydrite [741,1633,1634], BET specific surface area 245 m2/g, X-ray micrographs at different levels of preparation available [1633].

TABLE 3.537 PZC/IEP of Ferrihydrites Obtained According to Recipe(s) from Schwertmann and Cornell Electrolyte 0.01 M NaNO3 0.01 M KNO3 0.1 M KNO3 0.001–0.1 M NaClO4 0.001–0.1 M NaNO3 0.1 M KNO3 a b c d

T

Method

Instrument

a

25

25

iep

Malvern Zetasizer 3000HSa

pH cip iep cip pH

Electrophoresis

pH0

Reference

6.8 6.9 7 7.8 8.7a 7.9 8, 7.2d

[332,1417] [1417] [1635]b [637] [570c,1634] [741]

Only value, data points not reported. [741] cited for recipe. [1633] cited for modified recipe, but no detailed recipe could be found there. Two specimens, recipe B.

3.1.12.7.1.8 Recipe from [460] Two recipes (for two- and six-line ferridydrite) are reported in [460]. The PZC was reported for one of these products.

TABLE 3.538 PZC/IEP of Ferrihydrite Obtained According to Recipe from [460] Electrolyte

T

Method

0.01–1 M KNO3

25

cip

Instrument

pH0

Reference

8

[460]

3.1.12.7.1.9 Short-Range-Ordered Iron Oxide (Ferrihydrite?) 3.4 dm3 of 0.118 M Fe(NO3)3 was titrated at 100 cm3/h with 3 M NaOH with stirring until pH 5. The solution was diluted to 4 dm3, aged for 4 d at 23°C, and centrifuged. The precipitate was washed with 0.2 M NaNO3, freeze-dried, and ground. Properties: EGME specific surface area 344 m2/g [1636].

305

Compilation of PZCs/IEPs

TABLE 3.539 PZC/IEP of Short-Range Ordered Iron Oxide Electrolyte

T

Method

Instrument

cip

0.001–0.1 M NaNO3

pH0 Reference 9.2

[1636]

3.1.12.7.1.10 Neutralization of 0.2 M Fe(ClO4)3 with Stoichiometric Amount of NaOH Filtered 0.2 M Fe(ClO4)3 was mixed with NaOH (OH:Fe = 3:1) under nitrogen. The dilute dispersion was aged for 21 d. Properties: BET specific surface area 244 m2/g [1637].

TABLE 3.540 PZC/IEP of Ferrihydrite Obtained by Neutralization of 0.2 M Fe(ClO4)3 with Stoichiometric Amount of NaOH

a

Electrolyte

T

Method

0.3 M NaClO4

25

pH

Instrument

pH0

Reference

8.2

[1637]a

Also cited in [1405] as Spandini, personal communication, and PZC at pH 8.3 is reported.

3.1.12.7.2 Natural Ferrihydrites from Finland Properties: In Table 3.541 [460].

TABLE 3.541 PZC/IEP of Natural Ferrihydrites from Finland Si/Fe + Si (molar)

Specific Surface C% Area (m2/g)

0.1 0.11 0.27 0.24 0.21 0.16

1.3 5 2.7 2.5 1.7 3.3

279 433 325 366 498 425

Electrolyte

T

0.01–1 M KNO3 25

Method cip

Instrument

pH0 Reference 7.5 6.3 5.3 5.5 5.4 6.1

[460]

3.1.12.7.3 Recipe from [1638] Cited in [1527] as recipe for ferrihydrite, but originally published as recipe for amorphous iron oxyhydroxide [1638]. Carbonate-free NaOH was added dropwise

306

Surface Charging and Points of Zero Charge

to Fe(NO3)3 solution. The dispersion was aged for 3 h at pH 7 and then for 1 h at the pH of interest (no washing). Properties: BET specific surface area 225 m2/g [1527].

TABLE 3.542 PZC/IEP of Precipitate Obtained According to Recipe from [1638] Electrolyte

T

Method

a

Instrument

titration

0.001–0.1 M KNO3

pH0

Reference

7.9a

[1527]

Only value, no data points reported.

3.1.12.8 Fe(OH)3 PZC of iron hydroxides, and their physical properties and preparation methods are reviewed in [10]. PZCs/IEPs of iron hydroxides (nominally Fe(OH)3) are presented in Tables 3.543 through 3.578. 3.1.12.8.1 From Nitrate 3.1.12.8.1.1 Neutralization to pH 7 with NaOH in Polypropylene Beaker Fe(NO3)3 solution was neutralized to pH 7 with NaOH in a polypropylene beaker within 15 min. The dispersion was aged at pH 7 for 1 h, and diluted before use. Properties: Amorphous [1151].

TABLE 3.543 PZC/IEP of Fe(OH)3 Obtained by Neutralization of Fe(NO3)3 to pH 7 with NaOH in Polypropylene Beaker Electrolyte NaNO3 a

T

Method Salt addition

Instrument

pH0

Reference

8.1

[1151,1188a]

Only value, no data points reported.

3.1.12.8.1.2 Neutralization of 0.4 M Fe(NO3)3 Solution to pH 7 with 1 M NaOH Properties: Amorphous, BET specific surface area 280 m2/g [1639].

307

Compilation of PZCs/IEPs

TABLE 3.544 PZC/IEP of Fe(OH)3 Obtained by Neutralization of 0.4 M Fe(NO3)3 Solution to pH 7 with 1 M NaOH Electrolyte NaClO4 a

T

Method

Room

cip

Instrument

pH0

Reference

8.1a

[1639]

Only value, no data points reported.

3.1.12.8.1.3 Dropwise Neutralization of Fe(NO3)3 with 0.1 M NaOH to pH 8 Properties: Amorphous [1640]. TABLE 3.545 PZC/IEP of Fe(OH)3 Obtained by Dropwise Neutralization of Fe(NO3)3 with 0.1 M NaOH to pH 8 Electrolyte

T

Method

Instrument

pH0

Reference

iep

Photal ELS-800 from Otsuka

8

[1640]

0.1 M NaNO3

3.1.12.8.1.4 Dropwise Neutralization of Fe(NO3)3 with NaOH to pH 8 An acidified solution of Fe(NO3)3 was adjusted to pH 8 by dropwise addition of carbonate-free NaOH. The pH was maintained at 7.5 for 4 h. Properties: Amorphous, BET specific surface area 182 m2/g [1641]. TABLE 3.546 PZC/IEP of Fe(OH)3 Obtained by Dropwise Neutralization of Fe(NO3)3 with NaOH to pH 8 Electrolyte

T

Method

0.001–0.1 M NaNO3

25

cip Salt titration

a

Instrument

pH0

Reference

7.9

[1641]a [1642]a

Only value, no data points reported.

3.1.12.8.1.5 Dropwise Neutralization of Fe(NO3)3 with NaOH to pH 8.5 Prepared under nitrogen. Aged for 4 h.

308

Surface Charging and Points of Zero Charge

TABLE 3.547 PZC/IEP of Fe(OH)3 Obtained by Dropwise Neutralization of Fe(NO3)3 with NaOH to pH 8.5 Electrolyte 0.01 M NaNO3 a

T

Method

22–25

pH

Instrument

pH0

Reference

8

[1643]a

Only value, data points not reported.

3.1.12.8.1.6 From 0.02 M Fe(NO3)3 and 0.5 M NaOH (Added Dropwise), 40 Minutes Aged TABLE 3.548 PZC/IEP of Fe(OH)3 Obtained from 0.02 M Fe(NO3)3 and 0.5 M NaOH Electrolyte

T

0.3 M NH4NO3

Method

Instrument

pH0

Reference

iep

Zeta-Meter