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CRC PressTaylor & Francis Group6000 Broken Sound Parkway NW, Suite 300Boca Raton, FL 33487-2742

2009 by Taylor and Francis Group, LLCCRC Press is an imprint of Taylor & Francis Group, an Informa business

No claim to original U.S. Government works

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International Standard Book Number: 978-1-4200-5188-9 (Hardback)

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Library of Congress Cataloging-in-Publication Data

Kosmulski, Marek, 1956-Surface 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 2009541.335 --dc22 2009012179

Visit the Taylor & Francis Web site athttp://www.taylorandfrancis.comand the CRC Press Web site athttp://www.crcpress.com

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To my wife

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xiii

Contents

Preface ......................................................................................................... xxiii

Acknowledgments ..................................................................................... xxvii

1Chapter Introduction ................................................................................. 1

1.1 Nomenclature .................................................................... 81.2 Scope ............................................................................... 101.3 Inert Electrolytes ............................................................. 121.4 The Signifi cance of Parks Review ................................. 151.5 Structure of Adsorbents .................................................. 17

1.5.1 Alumina ........................................................... 171.5.2 Iron (Hydr)oxides ............................................. 191.5.3 Magnanese Oxides ........................................... 201.5.4 Silica ................................................................ 201.5.5 Titania .............................................................. 201.5.6 Clay Minerals .................................................. 201.5.7 Nitrides ............................................................ 21

1.6 Solubility ......................................................................... 211.6.1 Simple (Hydr)oxides ........................................ 211.6.2 Other Materials ................................................ 23

1.7 Solid Phase Transformation at Room Temperature in Contact with Solution ................................................. 241.7.1 Alumina ........................................................... 251.7.2 CdO .................................................................. 251.7.3 CuO .................................................................. 251.7.4 Iron (Hydr)oxides ............................................. 251.7.5 Other Systems .................................................. 25

1.8 Solid Phase Transformation on Heating ......................... 261.9 Kinetics ........................................................................... 26

1.9.1 Proton Adsorption ............................................ 271.9.2 Isotope Exchange ............................................. 281.9.3 Dissolution ....................................................... 29

1.10 Solution ChemistrypH Scale ....................................... 301.10.1 Problem 1: Concentration versus Activity ....... 321.10.2 Problem 2: Experiments at Constant

Ionic Strengths ................................................. 32

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1.10.3 Problem 3: Buffered versus Unbuffered System ... 331.10.4 Problem 4: Sodium Effect ................................ 331.10.5 Problem 5: Suspension Effect .......................... 331.10.6 Problem 6: Different pH Scales ....................... 341.10.7 Problem 7: Electrolysis .................................... 34

1.11 Very Dilute Solutions ...................................................... 341.12 Speciation in Solution ..................................................... 36

2Chapter Methods ..................................................................................... 39

2.1 Experimental Setup in Electrokinetic Measurements ...... 412.1.1 Electrophoresis ................................................ 412.1.2 Electro-Osmosis ............................................... 462.1.3 Streaming Potential ......................................... 472.1.4 Sedimentation Potential ................................... 482.1.5 Electroacoustic Methods .................................. 48

2.2 Experimental Conditions in Electrokinetic Measurements ................................................................. 51

2.3 CO2 and Silica Problem .................................................. 552.3.1 The CO2 Problem ............................................. 552.3.2 The Silica Problem .......................................... 57

2.4 Experimental Results: z Potential ................................... 592.4.1 Shapes of Individual Electrokinetic

Curves .............................................................. 602.4.2 Position of IEP ................................................. 612.4.3 Aging and Hysteresis ....................................... 612.4.4 Effect of Ionic Strength on the Numerical

Value of the z Potential .................................... 622.4.5 Effect of the Nature of the Salt on the

Numerical Values of the z Potential ................ 652.5 Experimental Conditions: Titration ................................ 66

2.5.1 The Choice of an Inert Electrolyte and the Range of Ionic Strengths ........................... 71

2.5.2 Solid-to-Liquid Ratio ....................................... 722.5.3 Other Titration Parameters .............................. 72

2.6 Results: Titration ............................................................. 742.6.1 Presence or Absence of CIP ............................ 742.6.2 Reproducibility and Reversibility .................... 762.6.3 Shape of Charging Curves and Typical

Values of s0 ...................................................... 772.6.4 Effect of Ionic Strength on Charging

Curves .............................................................. 792.6.5 Effect of the Nature of the Salt on

Numerical Values of s0 .................................... 792.6.6 Surface Charging of Materials Other

than Metal Oxides ........................................... 80

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2.7 Relation of Results Obtained by Different Methods .......................................................................... 80

2.8 Other Methods ................................................................ 812.8.1 Methods Involving Nonaqueous Solvents ....... 812.8.2 Electrical Methods ........................................... 812.8.3 Sum Frequency Generation and Second-

Harmonic Generation ...................................... 822.8.4 Methods Equivalent to Titration ...................... 822.8.5 Force between Particles ................................... 872.8.6 Nonstandard Methods ...................................... 88

2.9 Adsorption Models .......................................................... 892.9.1 Density of Protonable Surface Groups ............ 892.9.2 Electrostatic Models ........................................ 922.9.3 Surface Acidity ................................................ 96

3Chapter Compilation of PZCs/IEPs ....................................................... 101

3.1 Simple Oxides ................................................................ 1013.1.1 Aluminum (Hydr)oxides ................................. 1013.1.2 Beryllium (Hydr)oxides ................................. 1933.1.3 Bi2O3 .............................................................. 1943.1.4 Ca(OH)2 ......................................................... 1953.1.5 Cadmium (Hydr)oxides ................................. 1953.1.6 Cerium (Hydr)oxides ..................................... 1973.1.7 Cobalt (Hydr)oxides ....................................... 2023.1.8 Chromium (Hydr)oxides ................................ 2073.1.9 Copper (Hydr)oxides ....................................... 2163.1.10 Dy2O3 ............................................................. 2213.1.11 Er2O3 .............................................................. 2213.1.12 Iron (Hydr)oxides ........................................... 2213.1.13 GeO2 .............................................................. 3213.1.14 Ga2O3 ............................................................. 3213.1.15 HfO2 ............................................................... 3223.1.16 HgO ................................................................ 3233.1.17 Indium (Hydr)oxides ...................................... 3233.1.18 IrO2 ................................................................ 3263.1.19 Hydroxides of Lanthanides ............................ 3273.1.20 La2O3 .............................................................. 3283.1.21 Magnesium (Hydr)oxides .............................. 3293.1.22 Manganese (Hydr)oxides ............................... 3333.1.23 Niobium (Hydr)oxides ................................... 3533.1.24 Neodymium (Hydr)oxides ............................. 3563.1.25 Nickel (Hydr)oxides ....................................... 3573.1.26 Lead (Hydr)oxides ......................................... 3643.1.27 PdO ................................................................ 3673.1.28 Praseodymium (Hydr)oxides ......................... 367

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3.1.29 PtO2 ................................................................ 3683.1.30 PuO2 ............................................................... 3693.1.31 Ruthenium (Hydr)oxides ................................ 3693.1.32 Sb2O5 .............................................................. 3733.1.33 Sc2O3 .............................................................. 3733.1.34 Samarium (Hydr)oxides ................................. 3733.1.35 Silica .............................................................. 3743.1.36 Tin (Hydr)oxides ............................................. 4313.1.37 Tantalum (Hydr)oxides .................................. 4403.1.38 Thorium (Hydr)oxides ................................... 4433.1.39 Titanium (Hydr)oxides ................................... 4453.1.40 Tl2O3 .............................................................. 5023.1.41 Uranium (Hydr)oxides ................................... 5033.1.42 Vanadium (Hydr)oxides ................................. 5063.1.43 Tungsten (Hydr)oxides ................................... 5073.1.44 Y2O3 ............................................................... 5083.1.45 Yb2O3 .............................................................. 5133.1.46 Zinc (Hydr)oxides ........................................... 5143.1.47 Zirconium (Hydr)oxides ................................ 524

3.2 Aluminosilicates, Phyllosilicates, Clays, and Clay Minerals ................................................................ 5483.2.1 Adularia ......................................................... 5483.2.2 Amelia Albite from Wards ............................ 5483.2.3 (Ca,Fe)2(Ln,Al,Fe)3Si3O12OH, Allanite

(orthite) from Kabuland, Norway .................. 5483.2.4 Amphiboles .................................................... 5483.2.5 Andalusite ...................................................... 5493.2.6 Andesine ........................................................ 5493.2.7 Anorthite ........................................................ 5493.2.8 Anorthoclase .................................................. 5493.2.9 Anthophyllite ................................................. 5493.2.10 Augite, (Al,Ca,Fe,Mg,Ti)2(Al,Si)2O6 .............. 5493.2.11 Beidellite, SBCa-1 .......................................... 5503.2.12 Bentonite ........................................................ 5503.2.13 Be3Al2Si6O18 Beryl from Hoggar, Algeria ..... 5503.2.14 Biotite K(Mg,Fe,Mn)3(OH,F)2

(Al,Fe,Ti)Si3O10 .............................................. 5503.2.15 Blazer from Huber Na2O Al2O3

2.8 SiO2 7 H2O .............................................. 5513.2.16 Bronzite from Kraubath .................................. 5513.2.17 Bytownite ........................................................ 5513.2.18 Chlorite (Mg,Al,Fe)12(Al,Si)8O20(OH)16 .......... 5513.2.19 Cleavelandite .................................................. 5523.2.20 Clinochlore .................................................... 552

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3.2.21 Clinoptilolite, Zeolite, Unit Cell: Na6(AlO2)6(SiO2)30 24H2O ........................... 552

3.2.22 Clinozoisite from Kirchham .......................... 5533.2.23 Cordierite 2MgO 2Al2O3 5SiO2 ................. 5533.2.24 Ca2(Fe,Al)Al2[O/OH/SiO4/Si2O7]

Epidote from Knappenwand .......................... 5533.2.25 Feldspar .......................................................... 5543.2.26 Garnets ........................................................... 5543.2.27 Halloysite-7 ................................................. 5553.2.28 Hornblende ..................................................... 5553.2.29 Illite ................................................................ 5553.2.30 Kaolinite and Kaolin Si2Al2O5(OH)4 ............. 5593.2.31 Labradorite ..................................................... 5723.2.32 Laponite Na0.8Mg5.4Li0.4Si8O20(OH)4

from Laporte .................................................. 5723.2.33 Mica ............................................................... 5723.2.34 Microcline ...................................................... 5743.2.35 Montmorillonite ............................................. 5753.2.36 MontmorilloniteAlumina Composite .......... 5833.2.37 Mordenite (Synthetic Zeolite) from Huber

NaAlSi5O12 3H2O ......................................... 5843.2.38 Muscovite ....................................................... 5843.2.39 Na3K(AlSiO4)4 Nephelin from

Skudesundskjaer ............................................ 5843.2.40 Oligoclase ...................................................... 5843.2.41 Olivine from Dreis ......................................... 5853.2.42 Orthoclase ...................................................... 5853.2.43 Palygorskite (Mg,Al)2Si4O10(OH) 4(H2O)

from Tunisia ................................................... 5853.2.44 Perlite from Cumaovasi, Turkey

(or from Izmir) ............................................... 5853.2.45 Pyrophyllite Al2(OH)2Si4O10 .......................... 5863.2.46 Rhomboporphyr ............................................. 5863.2.47 Ripidolite ....................................................... 5863.2.48 Sanidine ......................................................... 5873.2.49 Saponite ......................................................... 5873.2.50 Sapphirine ...................................................... 5873.2.51 Serpentine ...................................................... 5873.2.52 Smectite ......................................................... 5883.2.53 Tremolite ........................................................ 5883.2.54 Turmaline (drawite) NaMg3Al6B3Si6O27

(OH,F)4 .......................................................... 5893.2.55 Vermiculite from Clay Minerals Society

Repository ...................................................... 589

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3.2.56 Vesuvian from Solberg .................................. 5903.2.57 Zeolites .......................................................... 5903.2.58 Zinnwaldite ................................................... 591

3.3 Mixed Oxides ................................................................ 5913.3.1 Materials Containing Aluminum .................. 5923.3.2 BiTh Mixed Oxides ...................................... 6103.3.3 Materials Containing Ce ................................. 6113.3.4 Materials Containing Co ................................ 6113.3.5 Materials Containing Cr ................................. 6123.3.6 Materials Containing Fe ................................. 6133.3.7 InSn Mixed Oxides ...................................... 6243.3.8 Mixed Oxides Containing Mg ....................... 6253.3.9 Material Containing Nb ................................. 6263.3.10 Materials Containing Ni ................................ 6273.3.11 Materials Containing Pb ................................ 6283.3.12 Materials Containing Ru ............................... 6283.3.13 Silicates .......................................................... 6303.3.14 Materials Containing SnO2 ............................ 6473.3.15 Materials Containing TiO2 ............................ 6483.3.16 Materials Containing WO3 ............................ 6543.3.17 Materials Containing Zn ................................ 6553.3.18 Materials Containing Zirconia ...................... 656

3.4 Salts ............................................................................... 6653.4.1 Aluminates and Haloaluminates ................... 6653.4.2 Borides and Borates ....................................... 6663.4.3 Carbides, Carbonates, and Salts of

Organic Acids ................................................ 6663.4.4 Chlorides ........................................................ 6963.4.5 Chromates ...................................................... 6973.4.6 LiCoO2 ........................................................... 6983.4.7 Fluorides ........................................................ 6983.4.8 Ba Ferrite from Aldrich ................................. 7013.4.9 AgI ................................................................. 7013.4.10 Manganates .................................................... 7013.4.11 Molybdates ..................................................... 7073.4.12 Sr1-xNbO3-d .................................................... 7083.4.13 Nitrides .......................................................... 7093.4.14 Niobates ......................................................... 7203.4.15 Phosphates and Apatites ................................ 7203.4.16 Silicates .......................................................... 7393.4.17 Sulfi des and Sulfates ...................................... 7403.4.18 Titanates ......................................................... 7693.4.19 Tungstates and Tungstophosphates ................ 7743.4.20 BaZrO3 ........................................................... 775

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3.5 Glasses .......................................................................... 7763.5.1 Commercial ................................................... 7763.5.2 Other .............................................................. 779

3.6 Carbon and Carbon-Rich Materials .............................. 7813.6.1 Diamond ........................................................ 7813.6.2 Graphite ......................................................... 7813.6.3 Fullerene C60 .................................................. 7813.6.4 Carbon Black, Activated Carbons,

and Related Products ..................................... 7823.6.5 Activated Carbon Cloths and Fibers .............. 8073.6.6 Composite Material ........................................ 811

3.7 Other Inorganic Materials .............................................. 8113.7.1 Silicon, >99.6% HQ Silgrain from

Elkem Materials .............................................. 8113.7.2 Sulfur .............................................................. 8113.7.3 Ice .................................................................. 8123.7.4 D2O Ice .......................................................... 8123.7.5 Gas Bubbles ................................................... 8123.7.6 Natural Inorganic Materials .......................... 813

3.8 Coatings ......................................................................... 8143.8.1 Alumina Coatings ........................................... 8143.8.2 Hydrous Chromia on Hematite ...................... 8203.8.3 Co Oxide on Stober Silica ............................. 8203.8.4 Iron (Hydr)oxide Coatings ............................. 8213.8.5 Germania on Silica ........................................ 8233.8.6 IrO2 on Stober Silica ...................................... 8243.8.7 Mn Compounds on Hematite ......................... 8243.8.8 Nickel (Hydr)oxide Coatings ......................... 8243.8.9 RuO2 on Silica ................................................ 8253.8.10 Silica Coatings ............................................... 8263.8.11 Sn(OH)4 on Hematite ..................................... 8283.8.12 Titania Coatings ............................................. 8293.8.13 Yttria on Hematite ......................................... 8333.8.14 Zr (Hydr)oxide Coatings ................................ 8343.8.15 Passive Layer on Ti6Al4V Alloy ................... 8353.8.16 Passive Films on Stainless Steels .................. 8353.8.17 NiCO3 Ni(OH)2 H2O on MnCO3 .................. 8353.8.18 YOHCO3 Coatings ......................................... 8353.8.19 Zr2O2(OH)2CO3 and Zr2 (OH)6 SO4

on Polystyrene ................................................ 8363.9 Well-Defi ned Low-Molecular-Weight Organic

Compounds ................................................................... 8373.9.1 Hydrocarbons ................................................. 8373.9.2 Bromododecane from Sigma-Aldrich ........... 839

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3.9.3 Fullerol ........................................................... 8393.9.4 Acids .............................................................. 8393.9.5 Cholesterol, 99+%, Alfa Aesar ...................... 841

3.10 Polymers (Macroscopic Specimens) ............................. 8413.10.1 Polyamides ..................................................... 8413.10.2 Polycarbonates ............................................... 8423.10.3 Polyetheretherketone, Victrex, Lite K,

from Lipp-Terler ............................................. 8423.10.4 Polyetherimide, Molecular Mass 89 100,

from Lipp-Terler ............................................. 8423.10.5 Polyethylene ................................................... 8433.10.6 Poly(ethylene imine) from

Polysciences ................................................... 8433.10.7 PMMA ........................................................... 8433.10.8 Polypropylene from E-Plas ............................ 8433.10.9 Polystyrene ..................................................... 8443.10.10 PTFE .............................................................. 8443.10.11 Polyurethane .................................................. 8443.10.12 Polymers, Fibers ............................................ 8453.10.13 Polymers, Powders ......................................... 845

3.11 Latexes .......................................................................... 8453.11.1 Commercial ................................................... 8453.11.2 Synthetic ........................................................ 8483.11.3 Origin Unknown ............................................ 851

3.12 Natural High-Molecular-Weight Organic Substances ..... 8523.12.1 Humic and Fulvic Acid .................................. 8523.12.2 Marine Colloidal Organic Matter .................. 8553.12.3 Suspended Particulate Matter from

River Mersey in NW England ....................... 8563.12.4 Cellulose ........................................................ 8563.12.5 Dextrin ........................................................... 8573.12.6 b-Casein ........................................................ 8573.12.7 Lysozyme ....................................................... 8573.12.8 Chitosan ......................................................... 8583.12.9 ChitosanPolymethacrylic Acid

Composites ..................................................... 8583.12.10 Asphaltene ..................................................... 858

3.13 Microorganisms ............................................................ 8593.13.1 Bacterium Bacillus subtilis ............................ 8593.13.2 Bacterium Corynebacterium xerosis ............. 8593.13.3 Cell Walls of Bacterium Rhodococcus

erythropolis .................................................... 8603.13.4 Bacterium Rhodococcus opacus from

Fundacao Tropical de Pesquisas e Tecnologia Andre Tosello, Sao Paulo ............ 860

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3.13.5 Bacterium Shewanella putrefaciens .............. 8603.13.6 MS2 Bacteriophages ...................................... 860

3.14 Metals ............................................................................ 8613.15 Literature Intentionally Ignored .................................... 861

3.15.1 PZCs/IEPs Not Reported or Not Found ........ 8613.15.2 Secondary Sources ........................................ 8613.15.3 The Electrolyte Is Not Inert ........................... 8623.15.4 Mechanical Mixtures and Complex and

Ill-Defi ned Materials ..................................... 8633.15.5 Nonstandard, Incorrect, or Undefi ned

Method, and Nonstandard Terminology ........ 8643.15.6 Wrong Citations ............................................. 866

3.16 Temperature Effect ....................................................... 8663.17 Pressure Effect .............................................................. 8683.18 Compilations of PZC of Various Materials .................. 8693.19 Correlations ................................................................... 8703.20 Mixed WaterOrganic Solvents .................................... 8733.21 Nonaqueous Solvents .................................................... 874

3.21.1 Allegedly Pure Solvents ................................. 8753.21.2 Effect of Water ............................................... 8753.21.3 Effect of Inorganic Electrolytes ..................... 8763.21.4 Effect of pH ................................................... 876

3.22. Conclusion ..................................................................... 876

4 Chapter Ion Specifi city .......................................................................... 879

4.1 Affi nity Series ............................................................... 8794.1.1 Aluminas ........................................................ 8804.1.2 Iron (Hydr)oxides ........................................... 8804.1.3 MnO2 .............................................................. 8814.1.4 Hydrous Niobia .............................................. 8814.1.5 Silica .............................................................. 8814.1.6 SnO2 ............................................................... 8824.1.7 Thoria ............................................................. 8834.1.8 Titania ............................................................ 8834.1.9 UO2 ................................................................ 8834.1.10 WO3 ................................................................ 8834.1.11 Zirconia .......................................................... 8834.1.12 Mica ............................................................... 8844.1.13 Na-Montmorillonite ....................................... 8844.1.14 Red Mud ........................................................ 8844.1.15 Alkali Metal-Substituted Manganese Oxides .... 8844.1.16 d-MnO2 .......................................................... 8844.1.17 Si3N4 ............................................................... 8844.1.18 Chrisotile ....................................................... 884

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4.1.19 Controlled Pore Glasses ................................. 8844.1.20 Diamond ........................................................ 884

4.2 Uptake of 1-1 Electrolyte Ions at or Near the PZC ....... 8844.2.1 Alumina ......................................................... 8854.2.2 Gibbsite .......................................................... 8864.2.3 CdO ................................................................ 8864.2.4 Co3O4 ............................................................. 8864.2.5 Magnetite (containing 2.4% of Silica) ........... 8864.2.6 Hematite ......................................................... 8864.2.7 Goethite ......................................................... 8864.2.8 HfO2 ............................................................... 8864.2.9 Niobia ............................................................. 8874.2.10 Silica .............................................................. 8874.2.11 Hydrous Tin Oxide ........................................ 8874.2.12 ThO2 ............................................................... 8874.2.13 Titania ............................................................ 8874.2.14 Zirconia .......................................................... 8884.2.15 AluminaSilica Mixed Oxides ...................... 8884.2.16 SilicaTitania and AluminaSilica

Titania Mixed Oxides .................................... 8884.2.17 TitaniaZirconia Mixed Oxides .................... 8884.2.18 d-MnO2 .......................................................... 8884.2.19 Porous Glasses ............................................... 889

4.3 High Ionic Strength ....................................................... 8894.3.1 Ions in Solution .............................................. 8894.3.2 Experimental Methods .................................. 8904.3.3 Electroacoustic Method ................................. 891

Appendix .................................................................................................... 893

References .................................................................................................. 911

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

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xxiii

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), refl ecting the change in its research profi le 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 refl ected by the number of citations. Yet my colleagues were not entirely satisfi ed. Parks reports data only for a limited number of materials. Moreover, my colleagues were concerned about the signifi cance 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., insuffi cient purity), and it is very diffi cult 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-defi ned specimens of materials are sorted by trade name and manufacturer (for commercial materials), location (for natural materials), or specifi c 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 classifi cation of materials according to the above criteria was more diffi -cult than originally expected. Detailed sample information is often missing or

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xxiv Preface

incomplete in scientifi c publications. Often, literature references are given instead of specifi c data. Spelling errors in trade and manufacturers names are common. Even complete trade name and manufacturer information is not suffi cient for cor-rect classifi cation. The results reported in this book were taken from papers pub-lished 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 distin-guished in this book is probably much larger than the number of signifi cantly different products. On the other hand, it may very well be that the recipe for a commercial product may have been modifi ed 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 diffi cult. 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 calcinationrehydration sequences on surface charging. In fact, a new material is produced when the original sample is calcined at suffi ciently 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 modifi es 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 impuri-ties 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 diffi cult to assess the nature of the changes induced by washing.

In the present book, all results obtained for the same original commercial materialuncalcined and calcined, unwashed and washed in different wayshave 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

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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 fi lters) and the chemicals (manufacturer and purity) are usually omitted. For several specimens, a literature reference is given instead of a specifi c recipe. Several recipes are reported only in theses, internal reports, and other diffi cult-to-access sources. Usually, the original papers reporting synthetic recipes analyze a broad spectrum of experimental conditions. A literature reference is then not suf-fi cient to identify a specifi c recipe. Problems with identifi cation of specifi c recipes are explicitly stated in this book when appropriate. Most studies reporting PZCs/IEPs of home-synthesized materials refer to a specifi c 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 specifi c 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-syn-thesized 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

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xxvii

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 Prchniak and Teresa Chlebik from the Department of Electrochemistry, Lublin University of Tech nology 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 bo Akademi (Finland) and the European Institute of Transuranium Elements (Karlsruhe, Germany). Professors Jarl B. Rosenholm and Thomas Fanghnel are acknowledged for their hospitality.

Marek Kosmulski

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11 Introduction

Introductory descriptions of surface charging and the electrical double layer can be found in numerous handbooks of surface and colloid chemistry (e.g., [47]), in other books (e.g., [810]), and in review articles (e.g., [11]). The reader of the pres-ent 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 concentra-tions 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 suffi cient 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 fi gures 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 repre-sents 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

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2 Surface Charging and Points of Zero Charge

through these points gives pH0. Actual systems are more complicated. Figures 1.4 through 1.12 show typical problems that make determination of pH0 diffi cult. Only one problem is illustrated in each fi gure, 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 con-centration of the 1-1 electrolyte and of impurities present in the system, are usu-ally 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 scientifi c paper (e.g., the tempera-ture limits) is not necessarily realistic. Different measurable quantities, for exam-ple s0 from titration and z potential from electrokinetic measurements, represent the state of the charged surface. In principle, each of these quantities can reverse

0

pH

c3

IEP

z

c2

c1

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.

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

PZC

0

pH

c1c2c3

s 0

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Introduction 3

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 differ-ent. 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 experi-mental errors and impurities or whether they may also occur in properly con-ducted 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

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

pH

pH0

0

XYZ

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.

pH0 (T2)

pH0 (T1)pH0 (T3)

T1

0

XYZ

T2T3

pH

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4 Surface Charging and Points of Zero Charge

specimens (>1 pH unit) are due to impurities. In principle, different crystallo-graphic 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 (Y )

pH

X, Y

, or Z

(sca

led)

0

XYZ

pH0 (Z)

pH0 (X)

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 com-pared with z potential.

pH0 (specimen 3)

pH0 (specimen 1)pH0 (specimen 2)

Specimen 1Specimen 2Specimen 3

pH

0

XYZ

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.

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Introduction 5

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.

Possible range of pH0

0

pH

XYZ

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 uncer-tain over a range of pH, which depends on the error in XYZ.

Possible range of pH0

0

XYZ

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.

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6 Surface Charging and Points of Zero Charge

Slow titrationXYZ

0

Fast titration

pH

pH0 (base titration)pH0 (acid titration)

Base titrationAcid titration

pH

XYZ

FIGURE 1.9 Real situation: hysteresis. The loop narrows as the titration rate decreases, but it is often diffi cult 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 diffi cult 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 carbon-ates is complex; for example, metal silicates and carbonates are often more stable (in terms of G0 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 inter-polations look reasonable, but they produce very different pH0 values.

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Introduction 7

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 deter-mined 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 specifi c value of pH0.

Data points

pH

pH0 (interpolation 1)

pH0 (interpolation 2)

XYZ

0

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.

XYZ

0

pH0 (CO2/SiO2 present)

pH0 (CO2/SiO2 removed)

CO2/SiO2 removedCO2/SiO2 present

pH

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

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8 Surface Charging and Points of Zero Charge

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 defi ned as the conditions at which the sur-face charge density equals zero; for metal oxides and related materials, it is deter-mined 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 defi ned as the conditions at which the electro-kinetic charge density and thus the electrokinetic (z) potential equals zero; it is determined by electrokinetic methods (see [17] for measurement and interpreta-tion of electrokinetic phenomena).

Different versions of these abbreviationslower- and upper-case, with or without periodsare 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 poten-tial, 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 abbre-viation 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

pH0 (extrapolation 2)

pH0 (extrapolation 1)

0

Data points

pH

XYZ

FIGURE 1.12 pH0 falls outside the range of data points, and is determined by extrapola-tion. 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.

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Introduction 9

quantities is suffi cient, 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 mea-surements, 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 concentra-tions 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 mate-rials 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 chiefl y to electro-phoresis 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 loga-rithm of iodide concentrationand 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 defi ned above, has been used in a popular textbook [5] and in several other publications (e.g., [23]). The term point of zero zeta poten-tial (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 numeri-cally 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) sur-face charge, and the methods and defi nitions 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

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10 Surface Charging and Points of Zero Charge

obtained at various ionic strengths; thus, the above defi nition that identifi es the PZC with the CIP is not applicable. For example, [26] defi nes fi ve 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 meth-ods (designed for materials having variable surface charge). Sposito argued that particles with local positive and negative charge may show substantial electropho-retic mobility when their net electrokinetic charge is zero.

1.2 SCOPE

The present book reviews PZC/IEP data reported for well-defi ned, homogeneous materials without surface coating; that is, ill-defi ned 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 stud-ies (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 sur-face-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 fi rst), and then by degree of hydration (lower degree of hydration fi rst).

2. Aluminosilicates and clay minerals. Within this class, compounds are sorted alphabetically by their names. There are numerous, often multi-level, classifi cations of clay minerals, and different names are often assigned to the same material or to very similar materials. Such materi-als are listed under the names used in the original publications and inter-connected 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

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Introduction 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 compo-nents. 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 com-prises 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].

4. 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).

5. Glasses. 6. Carbon. Data are given for natural diamond, graphite, and fullerene, and

then for commercial and home-made activated carbons. 7. Other well-defi ned inorganic materials. These include sulfur, ice, and

air bubbles. 8. Natural inorganic materials. 9. 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 14 above.

10. Well-defi ned low-molecular-weight organic compounds and their mixtures.

11. Synthetic polymers (macroscopic specimens). These are sorted alpha-betically by chemical names.

12. Latexes. Commercial products are sorted by manufacturers name and trade name as the primary identifi ers (irrespective of the chemical nature of the monomers). These are followed by home-made latexes.

13. 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 manufacturers/retailers name and/or trade name. When manufacturers/retailers 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 identi-fi er. Cross-references are provided between categories that are likely to represent the same material.

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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 accord-ing to the method, precursor, etc.

C. Natural materials. These are organized alphabetically by country of origin.

Most scientifi c papers report suffi cient information to assign the material of interest to one of subclasses AC and then to a smaller subclasses. Several speci-mens could not be classifi ed because of insuffi cient 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 mate-rials are referred to as origin unknown.

1.3 INERT ELECTROLYTES

The idea of an inert (indifferent) electrolyte was coined in the context of electro-capillary studies using the Hg electrode. Grahame [34] found a series of electro-lytes that produced the same PZC (determined as the electrocapillary maximum) irrespective of the nature or concentration of the electrolyte. Such behavior sug-gests 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 con-centration of the electrolyte. A shift in the PZC suggests that ions of these elec-trolytes can be positively adsorbed in spite of electrostatic repulsion; that is, they interact with the surface by a noncoulombic force. This phenomenon is termed specifi c adsorption. Thus, an inert electrolyte does not show specifi c adsorption of either ion.

The above terminology (inert vs. specifi c) was adopted for studies of the surface charging of colloids. Different experimental methods are used and differ-ent 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 charg-ing 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 solu-tion 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

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Introduction 13

electrolytes that are inert with respect to Hg show specifi c 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 cri-terion for the presence or absence of specifi c adsorption. Increasing the concen-tration of an inert electrolyte at constant pH induces an asymptotic decrease in the absolute value of the z potential without sign reversal. In contrast, increas-ing the concentration of specifi cally 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 aque-ous solution. In mixed and nonaqueous solvents, 1-1 electrolytes that are inert in water show specifi c adsorption of cations, which induces shifts in the IEP to high pH [37].

In principle, specifi c adsorption of anions induces an increase in the negative electrokinetic charge and a shift in the IEP to low pH, and specifi c 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 fl uorides is usually specifi c. Reference [16] discusses F- as an inert ion, and specifi c adsorption of F- on alumina is considered as an exception. Reference [39] describes the adsorption of nitrate, perchlorate, and chloride as nonspecifi c on quartz, titania, and alumina, but as specifi c on zirconia and thoria. With SnO2 and Fe2O3, adsorption of chloride was found to be specifi c, and adsorp-tion of nitrate and perchlorate to be nonspecifi c. Further examples of such excep-tions are discussed in Chapter 2. Reference [40] shows that sodium and potassium trichloroacetate, trifl uoroacetate, and trifl uoromethanesulfonate 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].

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14 Surface Charging and Points of Zero Charge

The usual approach to inert electrolytes assumes that a broad pH range is cov-ered, 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/specifi c 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 pres-ence 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 electro-lyte 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 addi-tion; thus the specifi c/nonspecifi c 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 tita-nia at three KCl concentrations, and the fi gure caption claims that these curves do not intersect at a common point, suggesting that Cl is specifi cally 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 specifi c 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 hypoth-esis that an electrolyte is inert.

Different research groups use different terminology in describing surface charging behavior in the presence of specifi c adsorption. Specifi cally 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 specifi c adsorption. Mixed evidence is found in the literature regarding specifi c/nonspecifi c 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 specifi c 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 defi nition) independent of the nature and concentration of the electrolyte, and these details are often omitted in

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Introduction 15

scientifi c publications. Otherwise, in the presence of specifi cally adsorbing ions, pH0 refers to a specifi c nature and concentration of electrolyte, which should be clearly indicated, for example, in the caption or key of a fi gure 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 disper-sant (phosphate), although this was not indicated on the fi gure 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 electrolyte-background-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 signifi cant 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 fi eld 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 well- established physical quantities such as the bond valence and the degree of oxi-dation 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 fi eld 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 chiefl y based upon that

120

100

80

Cita

tions

60

40

20

01965 1970 1975 1980 1985 1990 1995 2000 2005

FIGURE 1.13 Citations of Parks review [1].

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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 origi-nally published in [1]. Also, the present compilation is in some senses a continu-ation 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 scientifi c papers cited in [1] report results that do not represent PZCs/IEPs by todays 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 iso-electric 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 scientifi c 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 confi rm 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.

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Introduction 17

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 poly-hedra, 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 graphi-cal 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

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.)

(a)

(010) (001)

(b)

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