adsorption and aggregation of surfactants in solution

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Marcel Dekker, Inc. New York Basel

ADSORPTION AND AGGREGATION OF SURFACTANTS IN

SOLUTION

edited byK. L. Mittal

Hopewell Junction, New York, U.S.A.

Dinesh O. ShahUniversity of Florida

Gainesville, Florida, U.S.A.

Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.

ISBN: 0-8247-0843-1

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SURFACTANT SCIENCE SERIES

FOUNDING EDITOR

MARTIN J. SCHICK19181998

SERIES EDITOR

ARTHUR T. HUBBARDSanta Barbara Science Project

Santa Barbara, California

ADVISORY BOARD

DANIEL BLANKSCHTEINDepartment of Chemical EngineeringMassachusetts Institute of TechnologyCambridge, Massachusetts

S. KARABORNIShell International Petroleum

Company LimitedLondon, England

LISA B. QUENCERThe Dow Chemical CompanyMidland, Michigan

JOHN F. SCAMEHORNInstitute for Applied Surfactant ResearchUniversity of OklahomaNorman, Oklahoma

P. SOMASUNDARANHenry Krumb School of MinesColumbia UniversityNew York, New York

ERIC W. KALERDepartment of Chemical EngineeringUniversity of DelawareNewark, Delaware

CLARENCE MILLERDepartment of Chemical EngineeringRice UniversityHouston, Texas

DON RUBINGHThe Procter & Gamble CompanyCincinnati, Ohio

BEREND SMITShell International Oil Products B.V.Amsterdam, The Netherlands

JOHN TEXTERStrider Research CorporationRochester, New York

1. Nonionic Surfactants, edited by Martin J. Schick (see also Volumes 19, 23, and 60)2. Solvent Properties of Surfactant Solutions, edited by Kozo Shinoda (see Volume 55)3. Surfactant Biodegradation, R. D. Swisher (see Volume 18)4. Cationic Surfactants, edited by Eric Jungermann (see also Volumes 34, 37, and 53)5. Detergency: Theory and Test Methods (in three parts), edited by W. G. Cutler and R.

C. Davis (see also Volume 20)6. Emulsions and Emulsion Technology (in three parts), edited by Kenneth J. Lissant7. Anionic Surfactants (in two parts), edited by Warner M. Linfield (see Volume 56)8. Anionic Surfactants: Chemical Analysis, edited by John Cross9. Stabilization of Colloidal Dispersions by Polymer Adsorption, Tatsuo Sato and

Richard Ruch10. Anionic Surfactants: Biochemistry, Toxicology, Dermatology, edited by Christian

Gloxhuber (see Volume 43)11. Anionic Surfactants: Physical Chemistry of Surfactant Action, edited by E. H.

Lucassen-Reynders12. Amphoteric Surfactants, edited by B. R. Bluestein and Clifford L. Hilton (see Volume

59)13. Demulsification: Industrial Applications, Kenneth J. Lissant14. Surfactants in Textile Processing, Arved Datyner15. Electrical Phenomena at Interfaces: Fundamentals, Measurements, and Applications,

edited by Ayao Kitahara and Akira Watanabe16. Surfactants in Cosmetics, edited by Martin M. Rieger (see Volume 68)17. Interfacial Phenomena: Equilibrium and Dynamic Effects, Clarence A. Miller and P.

Neogi18. Surfactant Biodegradation: Second Edition, Revised and Expanded, R. D. Swisher19. Nonionic Surfactants: Chemical Analysis, edited by John Cross20. Detergency: Theory and Technology, edited by W. Gale Cutler and Erik Kissa21. Interfacial Phenomena in Apolar Media, edited by Hans-Friedrich Eicke and Geoffrey

D. Parfitt22. Surfactant Solutions: New Methods of Investigation, edited by Raoul Zana23. Nonionic Surfactants: Physical Chemistry, edited by Martin J. Schick24. Microemulsion Systems, edited by Henri L. Rosano and Marc Clausse25. Biosurfactants and Biotechnology, edited by Naim Kosaric, W. L. Cairns, and Neil C.

C. Gray26. Surfactants in Emerging Technologies, edited by Milton J. Rosen27. Reagents in Mineral Technology, edited by P. Somasundaran and Brij M. Moudgil28. Surfactants in Chemical/Process Engineering, edited by Darsh T. Wasan, Martin E.

Ginn, and Dinesh O. Shah29. Thin Liquid Films, edited by I. B. Ivanov30. Microemulsions and Related Systems: Formulation, Solvency, and Physical

Properties, edited by Maurice Bourrel and Robert S. Schechter31. Crystallization and Polymorphism of Fats and Fatty Acids, edited by Nissim Garti and

Kiyotaka Sato32. Interfacial Phenomena in Coal Technology, edited by Gregory D. Botsaris and Yuli M.

Glazman33. Surfactant-Based Separation Processes, edited by John F. Scamehorn and Jeffrey H.

Harwell34. Cationic Surfactants: Organic Chemistry, edited by James M. Richmond35. Alkylene Oxides and Their Polymers, F. E. Bailey, Jr., and Joseph V. Koleske36. Interfacial Phenomena in Petroleum Recovery, edited by Norman R. Morrow37. Cationic Surfactants: Physical Chemistry, edited by Donn N. Rubingh and Paul M.

Holland38. Kinetics and Catalysis in Microheterogeneous Systems, edited by M. Grtzel and K.

Kalyanasundaram39. Interfacial Phenomena in Biological Systems, edited by Max Bender40. Analysis of Surfactants, Thomas M. Schmitt (see Volume 96)

41. Light Scattering by Liquid Surfaces and Complementary Techniques, edited byDominique Langevin

42. Polymeric Surfactants, Irja Piirma43. Anionic Surfactants: Biochemistry, Toxicology, Dermatology. Second Edition, Revised

and Expanded, edited by Christian Gloxhuber and Klaus Knstler44. Organized Solutions: Surfactants in Science and Technology, edited by Stig E.

Friberg and Bjrn Lindman45. Defoaming: Theory and Industrial Applications, edited by P. R. Garrett46. Mixed Surfactant Systems, edited by Keizo Ogino and Masahiko Abe47. Coagulation and Flocculation: Theory and Applications, edited by Bohuslav Dobi48. Biosurfactants: Production Properties Applications, edited by Naim Kosaric49. Wettability, edited by John C. Berg50. Fluorinated Surfactants: Synthesis Properties Applications, Erik Kissa51. Surface and Colloid Chemistry in Advanced Ceramics Processing, edited by Robert

J. Pugh and Lennart Bergstrm52. Technological Applications of Dispersions, edited by Robert B. McKay53. Cationic Surfactants: Analytical and Biological Evaluation, edited by John Cross and

Edward J. Singer54. Surfactants in Agrochemicals, Tharwat F. Tadros55. Solubilization in Surfactant Aggregates, edited by Sherril D. Christian and John F.

Scamehorn56. Anionic Surfactants: Organic Chemistry, edited by Helmut W. Stache57. Foams: Theory, Measurements, and Applications, edited by Robert K. Prud'homme

and Saad A. Khan58. The Preparation of Dispersions in Liquids, H. N. Stein59. Amphoteric Surfactants: Second Edition, edited by Eric G. Lomax60. Nonionic Surfactants: Polyoxyalkylene Block Copolymers, edited by Vaughn M. Nace61. Emulsions and Emulsion Stability, edited by Johan Sjblom62. Vesicles, edited by Morton Rosoff63. Applied Surface Thermodynamics, edited by A. W. Neumann and Jan K. Spelt64. Surfactants in Solution, edited by Arun K. Chattopadhyay and K. L. Mittal65. Detergents in the Environment, edited by Milan Johann Schwuger66. Industrial Applications of Microemulsions, edited by Conxita Solans and Hironobu

Kunieda67. Liquid Detergents, edited by Kuo-Yann Lai68. Surfactants in Cosmetics: Second Edition, Revised and Expanded, edited by Martin M.

Rieger and Linda D. Rhein69. Enzymes in Detergency, edited by Jan H. van Ee, Onno Misset, and Erik J. Baas70. Structure-Performance Relationships in Surfactants, edited by Kunio Esumi and

Minoru Ueno71. Powdered Detergents, edited by Michael S. Showell72. Nonionic Surfactants: Organic Chemistry, edited by Nico M. van Os73. Anionic Surfactants: Analytical Chemistry, Second Edition, Revised and Expanded,

edited by John Cross74. Novel Surfactants: Preparation, Applications, and Biodegradability, edited by Krister

Holmberg75. Biopolymers at Interfaces, edited by Martin Malmsten76. Electrical Phenomena at Interfaces: Fundamentals, Measurements, and Applications,

Second Edition, Revised and Expanded, edited by Hiroyuki Ohshima and KunioFurusawa

77. Polymer-Surfactant Systems, edited by Jan C. T. Kwak78. Surfaces of Nanoparticles and Porous Materials, edited by James A. Schwarz and

Cristian I. Contescu79. Surface Chemistry and Electrochemistry of Membranes, edited by Torben Smith

Srensen80. Interfacial Phenomena in Chromatography, edited by Emile Pefferkorn

81. SolidLiquid Dispersions, Bohuslav Dobi, Xueping Qiu, and Wolfgang von Rybinski82. Handbook of Detergents, editor in chief: Uri Zoller

Part A: Properties, edited by Guy Broze83. Modern Characterization Methods of Surfactant Systems, edited by Bernard P. Binks84. Dispersions: Characterization, Testing, and Measurement, Erik Kissa85. Interfacial Forces and Fields: Theory and Applications, edited by Jyh-Ping Hsu86. Silicone Surfactants, edited by Randal M. Hill87. Surface Characterization Methods: Principles, Techniques, and Applications, edited

by Andrew J. Milling88. Interfacial Dynamics, edited by Nikola Kallay89. Computational Methods in Surface and Colloid Science, edited by Magorzata

Borwko90. Adsorption on Silica Surfaces, edited by Eugne Papirer91. Nonionic Surfactants: Alkyl Polyglucosides, edited by Dieter Balzer and Harald

Lders92. Fine Particles: Synthesis, Characterization, and Mechanisms of Growth, edited by

Tadao Sugimoto93. Thermal Behavior of Dispersed Systems, edited by Nissim Garti94. Surface Characteristics of Fibers and Textiles, edited by Christopher M. Pastore and

Paul Kiekens95. Liquid Interfaces in Chemical, Biological, and Pharmaceutical Applications, edited by

Alexander G. Volkov96. Analysis of Surfactants: Second Edition, Revised and Expanded, Thomas M. Schmitt97. Fluorinated Surfactants and Repellents: Second Edition, Revised and Expanded,

Erik Kissa98. Detergency of Specialty Surfactants, edited by Floyd E. Friedli99. Physical Chemistry of Polyelectrolytes, edited by Tsetska Radeva

100. Reactions and Synthesis in Surfactant Systems, edited by John Texter101. Protein-Based Surfactants: Synthesis, Physicochemical Properties, and Applications,

edited by Ifendu A. Nnanna and Jiding Xia102. Chemical Properties of Material Surfaces, Marek Kosmulski103. Oxide Surfaces, edited by James A. Wingrave104. Polymers in Particulate Systems: Properties and Applications, edited by Vincent A.

Hackley, P. Somasundaran, and Jennifer A. Lewis105. Colloid and Surface Properties of Clays and Related Minerals, Rossman F. Giese

and Carel J. van Oss106. Interfacial Electrokinetics and Electrophoresis, edited by ngel V. Delgado107. Adsorption: Theory, Modeling, and Analysis, edited by Jzsef Tth108. Interfacial Applications in Environmental Engineering, edited by Mark A. Keane109. Adsorption and Aggregation of Surfactants in Solution, edited by K. L. Mittal and

Dinesh O. Shah110. Biopolymers at Interfaces: Second Edition, Revised and Expanded, edited by Martin

Malmsten111. Biomolecular Films: Design, Function, and Applications, edited by James F. Rusling112. StructurePerformance Relationships in Surfactants: Second Edition, Revised and

Expanded, edited by Kunio Esumi and Minoru Ueno

ADDITIONAL VOLUMES IN PREPARATION

Liquid Interfacial Systems: Oscillations and Instability, Rudolph V. Birikh, Vladimir A.Briskman, Manuel G. Velarde, and Jean-Claude Legros

Novel Surfactants: Preparation, Applications, and Biodegradability: Second Edition,Revised and Expanded, edited by Krister Holmberg

Colloidal Polymers: Preparation and Biomedical Applications, edited by AbdelhamidElaissari

iii

Preface

This volume embodies, in part, the proceedings of the 13th InternationalSymposium on Surfactants in Solution (SIS) held in Gainesville, Florida,June 1116, 2000. The theme of this particular SIS was Surfactant Scienceand Technology for the New Millennium. The nal technical program com-prised 360 papers, including 96 poster presentations, which was a testimonialto the brisk research activity in the arena of surfactants in solution. In lightof the legion of papers, to chronicle the total account of this event wouldhave been impractical, so we decided to document only the plenary andinvited presentations. The contributors were asked to cover their topics in ageneral manner; concomitantly, this book reects many excellent reviews ofa number of important ramications of surfactants in solution.

Chapters 14 document the plenary lectures, including the written ac-count of the special Host Lecture by one of us (DOS) and Prof. BrijMoudgil. Chapters 532 embody the text of 28 invited presentations cov-ering many aspects of surfactants in solution. Among the topics covered are:surfactant-stabilized particles; solid particles at liquid interfaces; nanocap-sules; aggregation behavior of surfactants; micellar catalysis; vesicles andliposomes; the clouding phenomenon; viscoelasticity of micellar solutions;phase behavior of microemulsions; reactions in microemulsions; viscosityindex improvers; foams, foam lms, and monolayers; principles of emulsionformulation engineering; nano-emulsions; liposome gene delivery; poly-meric surfactants; and combinatorial surface chemistry.

As surfactants play an important role in many and diverse technologies,ranging from high-tech (microelectronics) to low-tech (washing clothes) ap-plications, an understanding of their behavior in solution is of paramount

iv Preface

importance. Also, as we learn more about surfactants and devise new sur-factant formulations, novel and exciting applications will emerge.

The present compendium of excellent overviews and research papers pro-vides a bounty of up-to-date information on the many and varied aspects ofsurfactants in solution. It also offers a commentary on current research ac-tivity regarding the behavior of surfactants in solution. We hope that anyoneinvolved or interestedcentrally or tangentiallyin surfactants will ndthis book useful. Further, we trust that both veteran researchers and thoseembarking on their maiden voyage in the wonderful world of surfactantswill nd this treatise valuable.

To put together a symposium of this magnitude and quality requires ded-ication and uninching help from a battalion of people, and now it is ourpleasure and duty to acknowledge those who helped in many and variedmanners in this endeavor. First and foremost, we express our heartfelt andmost sincere thanks to Prof. Brij Moudgil, Director of the Engineering Re-search Center for Particle Science and Technology, University of Florida,for helping in more ways than one. He wore many different hatsas co-chairman, as troubleshooter, as local hostand he was always ready andwilling to help with a smile. Next we are thankful to faculty members,postdoctoral associates, graduate students, and administrative staff of boththe Center for Surface Science and Engineering and the Engineering Re-search Center for Particle Science and Technology, University of Florida.

We acknowledge the generous support of the following organizations: theFlorida Institute of Phosphate Research, the National Science Foundation,and the University of Florida. Many individual industrial corporations helpedus by providing generous nancial support and we are grateful to them. Wealso thank the exhibitors of scientic instruments and books for their con-tribution and support.

We are grateful to the authors for their interest, enthusiasm, and contri-bution without which this book would not have seen the light of day. Last,we are appreciative of the efforts of the staff at Marcel Dekker, Inc. forgiving this book a body form.

K. L. MittalDinesh O. Shah

vContents

Preface iiiContributors ix

1. Highlights of Research on Molecular Interactions at Interfaces fromthe University of Florida 1Dinesh O. Shah and Brij M. Moudgil

2. Interaction Between Surfactant-Stabilized Particles:Dynamic Aspects 49J. Lyklema

3. Solid Particles at Liquid Interfaces, Including Their Effects onEmulsion and Foam Stability 61Robert Aveyard and John H. Clint

4. From Polymeric Films to Nanocapsules 91Helmuth Mohwald, Heinz Lichtenfeld, Sergio Moya, A. Voight,G. B. Sukhorukov, Stefano Leporatti, L. Dahne, Igor Radtchenko,Alexei A. Antipov, Changyou Gao, and Edwin Donath

5. Investigation of Amphiphilic Systems by Subzero TemperatureDifferential Scanning Calorimetry 105Shmaryahu Ezrahi, Abraham Aserin, and Nissim Garti

6. Aggregation Behavior of Dimeric and Gemini Surfactants inSolution: Raman, Selective Decoupling 13C NMR, andSANS Studies 133Hirofumi Okabayashi, Norikatsu Hattori, and Charmian J. OConnor

vi Contents

7. Snared by Trapping: Chemical Explorations of InterfacialCompositions of Cationic Micelles 149Laurence S. Romsted

8. Effect of Surfactants on Pregastric EnzymeCatalyzed Hydrolysis ofTriacylglycerols and Esters 171Charmian J. OConnor, Douglas T. Lai, and Cynthia Q. Sun

9. Effect of Benzyl Alcohol on the Properties of CTAB/KBrMicellar Systems 189Ganzuo Li, Weican Zhang, Li-Qiang Zheng, and Qiang Shen

10. Vesicle Formation by Chemical Reactions: Spontaneous VesicleFormation in Mixtures of Zwitterionic and CatanionicSurfactants 201Klaus Horbaschek, Michael Gradzielski, and Heinz Hoffmann

11. Mechanism of the Clouding Phenomenon in SurfactantSolutions 211C. Manohar

12. Atomic Force Microscopy of Adsorbed Surfactant Micelles 219William A. Ducker

13. A Simple Model to Predict Nonlinear Viscoelasticity and ShearBanding Flow of Wormlike Micellar Solutions 243J. E. Puig, F. Bautista, J. H. Perez-Lopez, J. F. A. Soltero, andOctavio Manero

14. Preparation and Stabilization of Silver Colloids in AqueousSurfactant Solutions 255Dae-Wook Kim, Seung-Il Shin, and Seong-Geun Oh

15. Silver and Palladium Nanoparticles Incorporated in LayerStructured Materials 269Rita Patakfalvi, Szilvia Papp, and Imre Dekany

16. Water-in-Carbon Dioxide Microemulsions Stabilized byFluorosurfactants 299Julian Eastoe, Alison Paul, David Steytler, Emily Rumsey,Richard K. Heenan, and Jeffrey Penfold

17. Organic Synthesis in Microemulsions: An Alternative or aComplement to Phase Transfer Catalysis 327Krister Holmberg and Maria Hager

Contents vii

18. Physicochemical Characterization of Nanoparticles Synthesizedin Microemulsions 343J. B.Nagy, L. Jeunieau, F. Debuigne, and I. Ravet-Bodart

19. Phase Behavior of Microemulsion Systems Based on OptimizedNonionic Surfactants 387Wolfgang von Rybinski and Matthias Wegener

20. Microemulsions in Foods: Challenges and Applications 407Anilkumar G. Gaonkar and Rahul Prabhakar Bagwe

21. Microemulsion-Based Viscosity Index Improvers 431Surekha Devi and Naveen Kumar Pokhriyal

22. Foams, Foam Films, and Monolayers 453Dominique Langevin

23. Role of Entry Barriers in Foam Destruction by Oil Drops 465Asen D. Hadjiiski, Nikolai D. Denkov, Slavka S. Tcholakova, andIvan B. Ivanov

24. Principles of Emulsion Formulation Engineering 501Jean-Louis Salager, Laura Marquez, Isabel Mira, Alejandro Pena,Eric Tyrode, and Noelia B. Zambrano

25. Nano-Emulsions: Formation, Properties, and Applications 525Conxita Solans, Jordi Esquena, Ana Maria Forgiarini, Nuria Uson,Daniel Morales, Paqui Izquierdo, Nuria Azemar, andMara Jose Garca-Celma

26. Surface Modications of Liposomes for Recognition and Responseto Environmental Stimuli 555Jong-Duk Kim, Soo Kyoung Bae, Jin-Chul Kim, and Eun-Ok Lee

27. Specic Partition of Surface-Modied Liposomes in AqueousPEO/Polysaccharide Two-Phase Systems 579Eui-Chul Kang, Kazunari Akiyoshi, and Junzo Sunamoto

28. Novel Cationic Transfection Lipids for Use in LiposomalGene Delivery 603Rajkumar Banerjee, Prasanta Kumar Das,Gollapudi Venkata Srilakshmi, Nalam Madhusudhana Rao, andArabinda Chaudhuri

29. Combinatorial Surface Chemistry: A Novel Concept for Langmuirand LangmuirBlodgett Films Research 619Qun Huo and Roger M. Leblanc

viii Contents

30. Oscillating Structural Forces Reecting the Organization of BulkSolutions and Surface Complexes 635Per M. Claesson and Vance Bergeron

31. Effect of Polymeric Surfactants on the Behavior of PolycrystallineMaterials with Special Reference to Ammonium Nitrate 655Arun Kumar Chattopadhyay

32. Surface Tension Measurements with Top-Loading Balances 675Brian Grady, Andrew R. Slagle, Linda Zhu, Edward E. Tucker,Sherril D. Christian, and John F. Scamehorn

Index 689

ix

Contributors

Kazunari Akiyoshi Department of Synthetic Chemistry and BiologicalChemistry, Kyoto University, Kyoto, Japan

Alexei A. Antipov Max-Planck-Institute of Colloids and Interfaces, Pots-dam, Germany

Abraham Aserin Casali Institute of Applied Chemistry, The HebrewUniversity of Jerusalem, Jerusalem, Israel

Robert Aveyard Department of Chemistry, Hull University, Hull, UnitedKingdom

Nuria Azemar Departament de Tecnologia de Tensioactius, InstitutdInvestigacions Qumiques i Ambientals de Barcelona, Barcelona, Spain

Soo Kyoung Bae Department of Chemical and Biomolecular Engineer-ing, KAIST, Daejeon, Korea

Rahul Prabhakar Bagwe Department of Chemical Engineering, Uni-versity of Florida, Gainesville, Florida, U.S.A.

Rajkumar Banerjee* Division of Lipid Science and Technology, IndianInstitute of Chemical Technology, Hyderabad, India

*Current afliation: University of Pittsburgh, Pittsburgh, Pennsylvania, U.S.A.

x Contributors

F. Bautista Departamento de Ingeniera Qumica, Universidad de Gua-dalajara, Guadalajara, Mexico

Vance Bergeron Ecole Normale Superieure, Paris, France

Arun Kumar Chattopadhyay Corporate Research and Development,United States Bronze Powders Group of Companies, Haskell, New Jersey,U.S.A.

Arabinda Chaudhuri Division of Lipid Science and Technology, IndianInstitute of Chemical Technology, Hyderabad, India

Sherril D. Christian Institute for Applied Surfactant Research, Univer-sity of Oklahoma, Norman, Oklahoma, U.S.A.

PerM.Claesson Department of Chemistry, Surface Chemistry, Royal In-stitute of Technology, and Institute for Surface Chemistry, Stockholm, Swe-den

John H. Clint Department of Chemistry, Hull University, Hull, UnitedKingdom

L. Dahne Max-Planck-Institute of Colloids and Interfaces, Potsdam, Ger-many

PrasantaKumarDas* Division of Lipid Science and Technology, IndianInstitute of Chemical Technology, Hyderabad, India

F. Debuigne Laboratoire de Resonance Magnetique Nucleaire, FacultesUniversitaires Notre-Dame de la Paix, Namur, Belgium

Imre Dekany Department of Colloid Chemistry, University of Szeged,Szeged, Hungary

Nikolai D. Denkov Laboratory of Chemical Physics and Engineering,Faculty of Chemistry, Soa University, Soa, Bulgaria

Surekha Devi Department of Chemistry, M. S. University of Baroda,Baroda, Gujarat, India

Deceased.*Current afliation: Massachusetts Institute of Technology, Cambridge, Massachusetts, U.S.A.

Contributors xi

Edwin Donath Department of Biophysics, Institute of Medical Physicsand Biophysics, University of Leipzig, Leipzig, Germany

WilliamA. Ducker Department of Chemistry, Virginia Tech, Blacksburg,Virginia, U.S.A.

Julian Eastoe School of Chemistry, University of Bristol, Bristol, UnitedKingdom

Jordi Esquena Departament de Tecnologia de Tensioactius, InstitutdInvestigacions Qumiques i Ambientals de Barcelona, Barcelona, Spain

Shmaryahu Ezrahi Technology and Development Division, IDF, TelHashomer, Israel

Ana Maria Forgiarini Departament de Tecnologia de Tensioactius, Insti-tut dInvestigacions Qumiques i Ambientals de Barcelona, Barcelona, Spain

Changyou Gao Department of Polymer Science and Engineering, Zhe-jiang University, Hangzhou, Peoples Republic of China

Anilkumar G. Gaonkar Research and Development, Kraft Foods, Inc.,Glenview, Illinois, U.S.A.

Mara Jose Garca-Celma Departament de Farma`cia, Facultad de Far-ma`cia, Universitat de Barcelona, Barcelona, Spain

NissimGarti Casali Institute of Applied Chemistry, The Hebrew Univer-sity of Jerusalem, Jerusalem, Israel

Brian Grady School of Chemical Engineering and Materials Science,University of Oklahoma, Norman, Oklahoma, U.S.A.

Michael Gradzielski Physical Chemistry I, University of Bayreuth, Bay-reuth, Germany

AsenD.Hadjiiski Laboratory of Chemical Physics and Engineering, Fac-ulty of Chemistry, Soa University, Soa, Bulgaria

Maria Hager Institute for Surface Chemistry, Stockholm, Sweden

xii Contributors

Norikatsu Hattori* Department of Applied Chemistry, Nagoya Instituteof Technology, Nagoya, Japan

Richard K. Heenan ISIS Facility, Rutherford Appleton Laboratory, Chil-ton, United Kingdom

Heinz Hoffmann Physical Chemistry I, University of Bayreuth, Bay-reuth, Germany

Krister Holmberg Department of Applied Surface Chemistry, ChalmersUniversity of Technology, Goteborg, Sweden

Klaus Horbaschek Physical Chemistry I, University of Bayreuth, Bay-reuth, Germany

QunHuo Department of Polymers and Coatings, North Dakota State Uni-versity, Fargo, North Dakota, U.S.A.

IvanB. Ivanov Laboratory of Chemical Physics and Engineering, Facultyof Chemistry, Soa University, Soa, Bulgaria

Paqui Izquierdo Departament de Tecnologia de Tensioactius, InstitutdInvestigacions Qumiques i Ambientals de Barcelona, Barcelona, Spain

L. Jeunieau Laboratoire de Resonance Magnetique Nucleaire, FacultesUniversitaires Notre-Dame de la Paix, Namur, Belgium

Eui-Chul Kang Department of Synthetic Chemistry and BiologicalChemistry, Kyoto University, Kyoto, Japan

Dae-Wook Kim Department of Chemical Engineering and CUPS, Han-yang University, Seoul, Korea

Jin-Chul Kim Department of Chemical and Biomolecular Engineering,KAIST, Daejeon, Korea

Jong-Duk Kim Department of Chemical and Biomolecular Engineering,KAIST, Daejeon, Korea

Douglas T. Lai Development Center for Biotechnology, Taipei, Taiwan

*Current afliation: Chisso Corporation, Tokyo, Japan.

Contributors xiii

Dominique Langevin Laboratoire de Physique des Solides, UniversiteParis Sud, Orsay, France

RogerM. Leblanc Department of Chemistry, University of Miami, CoralGables, Florida, U.S.A.

Eun-Ok Lee Department of Chemical and Biomolecular Engineering,KAIST, Daejeon, Korea

Stefano Leporatti Institute of Medical Physics and Biophysics, Univer-sity of Leipzig, Leipzig, Germany

Ganzuo Li Key Laboratory for Colloid and Interface Chemistry of StateEducation Ministry, Shandong University, Jinan, Peoples Republic of China

Heinz Lichtenfeld Max-Planck-Institute of Colloids and Interfaces, Pots-dam, Germany

J. Lyklema Physical Chemistry and Colloid Science, Wageningen Uni-versity, Wageningen, The Netherlands

Octavio Manero Instituto de Investigaciones en Materiales, UniversidadNacional Autonoma de Mexico, Mexico City, Mexico

C. Manohar Department of Chemical Engineering, Indian Institute ofTechnology, Mumbai, India

Laura Marquez Laboratory FIRP, School of Chemical Engineering, Uni-versity of the Andes, Merida, Venezuela

Isabel Mira* Laboratory FIRP, School of Chemical Engineering, Univer-sity of the Andes, Merida, Venezuela

Helmuth Mohwald Max-Planck-Institute of Colloids and Interfaces,Potsdam, Germany

Daniel Morales Departament de Tecnologia de Tensioactius, InstitutdInvestigacions Qumiques i Ambientals de Barcelona, Barcelona, Spain

*Current afliation: Institute for Surface Chemistry, Stockholm, Sweden.

xiv Contributors

Brij M. Moudgil Engineering Research Center, University of Florida,Gainesville, Florida, U.S.A.

Sergio Moya Max-Planck-Institute of Colloids and Interfaces, Potsdam,Germany

J. B.Nagy Laboratoire de Resonance Magnetique Nucleaire, FacultesUniversitaires Notre-Dame de la Paix, Namur, Belgium

Charmian J. OConnor Department of Chemistry, The University ofAuckland, Auckland, New Zealand

Seong-Geun Oh Department of Chemical Engineering and CUPS, Han-yang University, Seoul, Korea

Hirofumi Okabayashi Department of Applied Chemistry, Nagoya Insti-tute of Technology, Nagoya, Japan

Szilvia Papp Department of Colloid Chemistry and Nanostructured Ma-terials Research Group, Hungarian Academy of Sciences, University of Sze-ged, Szeged, Hungary

Rita Patakfalvi Department of Colloid Chemistry and NanostructuredMaterials Research Group, Hungarian Academy of Sciences, University ofSzeged, Szeged, Hungary

Alison Paul School of Chemistry, University of Bristol, Bristol, UnitedKingdom

Alejandro * Laboratory FIRP, School of Chemical Engineering,PenaUniversity of the Andes, Merida, Venezuela

Jeffrey Penfold ISIS Facility, Rutherford Appleton Laboratory, Chilton,United Kingdom

J. H. Perez-Lopez Departamento de Ingeniera Qumica, Universidad deGuadalajara, Guadalajara, Mexico

Naveen Kumar Pokhriyal Department of Chemistry, M. S. Universityof Baroda, Baroda, Gujarat, India

*Current afliation: Rice University, Houston, Texas, U.S.A.

Contributors xv

J. E. Puig Departamento de Ingeniera Qumica, Universidad de Gua-dalajara, Guadalajara, Mexico

Igor Radtchenko Max-Planck-Institute of Colloids and Interfaces, Pots-dam, Germany

NalamMadhusudhanaRao Centre for Cellular and Molecular Biology,Hyderabad, India

I. Ravet-Bodart Laboratoire de Resonance Magnetique Nucleaire, Facul-tes Universitaires Notre-Dame de la Paix, Namur, Belgium

Laurence S. Romsted Department of Chemistry and Chemical Biology,Rutgers, The State University of New Jersey, New Brunswick, New Jersey,U.S.A.

Emily Rumsey School of Chemical Sciences, University of East Anglia,Norwich, United Kingdom

Jean-Louis Salager Laboratory FIRP, School of Chemical Engineering,University of the Andes, Merida, Venezuela

John F. Scamehorn Institute for Applied Surfactant Research, Univer-sity of Oklahoma, Norman, Oklahoma, U.S.A.

Dinesh O. Shah Departments of Chemical Engineering and Anesthesi-ology, Center for Surface Science and Engineering, University of Florida,Gainesville, Florida, U.S.A.

QiangShen Key Laboratory for Colloid and Interface Chemistry of StateEducation Ministry, Shandong University, Jinan, Peoples Republic of China

Seung-Il Shin Department of Chemical Engineering and CUPS, HanyangUniversity, Seoul, Korea

Andrew R. Slagle Institute for Applied Surfactant Research, Universityof Oklahoma, Norman, Oklahoma, U.S.A.

Conxita Solans Departament de Tecnologia de Tensioactius, InstitutdInvestigacions Qumiques i Ambientals de Barcelona, Barcelona, Spain

J. F. A. Soltero Departamento de Ingeniera Qumica, Universidad deGuadalajara, Guadalajara, Mexico

xvi Contributors

Gollapudi Venkata Srilakshmi Division of Lipid Science and Technol-ogy, Indian Institute of Chemical Technology, Hyderabad, India

David Steytler School of Chemical Sciences, University of East Anglia,Norwich, United Kingdom

G.B. Sukhorukov Max-Planck-Institute of Colloids and Interfaces, Pots-dam, Germany

Cynthia Q. Sun* Department of Chemistry, The University of Auckland,Auckland, New Zealand

Junzo Sunamoto Advanced Research and Technology Center, NiihamaNational College of Technology, Ehime, Japan

SlavkaS. Tcholakova Laboratory of Chemical Physics and Engineering,Faculty of Chemistry, Soa University, Soa, Bulgaria

Edward E. Tucker Institute for Applied Surfactant Research, Universityof Oklahoma, Norman, Oklahoma, U.S.A.

Eric Tyrode Laboratory FIRP, School of Chemical Engineering, Univer-sity of the Andes, Merida, Venezuela

Nuria Uson Departament de Tecnologia de Tensioactius, InstitutdInvestigacions Qumiques i Ambientals de Barcelona, Barcelona, Spain

A. Voight Max-Planck-Institute of Colloids and Interfaces, Potsdam, Ger-many

Wolfgang von Rybinski Corporate Research, Henkel KGaA, Dussel-dorf, Germany

MatthiasWegener Corporate Research, Henkel KGaA, Dusseldorf, Ger-many

Noelia B. Zambrano Laboratory FIRP, School of Chemical Engineer-ing, University of the Andes, Merida, Venezuela

Current afliation:*Hort Research, Auckland, New Zealand.Royal Institute of Technology (KTH), Stockholm, Sweden.M.W. Kellogg Ltd., Middlesex, United Kingdom.

Contributors xvii

WeicanZhang State Key Laboratory of Microbial Technology, ShandongUniversity, Jinan, Peoples Republic of China

Li-Qiang Zheng Department of Chemistry, Shandong University, Jinan,Peoples Republic of China

Linda Zhu Institute for Applied Surfactant Research, University ofOklahoma, Norman, Oklahoma, U.S.A.

11Highlights of Research onMolecular Interactions atInterfaces from the Universityof FloridaDINESH O. SHAH and BRIJ M. MOUDGIL University ofFlorida, Gainesville, Florida, U.S.A.

ABSTRACTAn overview of research highlights of the past three decades from the Uni-versity of Florida on molecular interactions at interfaces and in micelles ispresented. This overview includes work on (1) the kinetic stability of mi-celles in relation to technological processes, (2) molecular packing in mixedmonolayers and phase transition in monolayers, (3) microemulsions and theirtechnological applications including enhanced oil recovery (EOR) processesand preparation of nanoparticles of advanced materials, (4) adsorption ofpolymers at solidliquid interfaces and selective occulation, and (5) themechanical strength of surfactant lms at the solidliquid interface and itscorrelation with dispersion stability as well as the interfacial phenomena inchemicalmechanical polishing (CMP) of silicon wafers. Detailed resultsexplaining the role of molecular interactions at interfaces and in micelles aswell as pertinent references are given for each phenomenon discussed.

I. INTRODUCTIONIt is a great pleasure and privilege for us to summarize the highlights ofresearch on molecular interactions at interfaces from the University ofFlorida on this 13th International Symposium on Surfactants in Solution(SIS-2000). During the past 30 years at the University of Florida, we havehad an ongoing research program on fundamental aspects as well as tech-nological applications of interfacial processes. Specically, this overview

2 Shah and Moudgil

includes the kinetic stability of micelles in relation to technological pro-cesses, molecular packing in mixed monolayers and phase transition in mon-olayers, microemulsions and their technological applications including en-hanced oil recovery (EOR) processes and preparation of nanoparticles ofadvanced materials, adsorption of polymers at solidliquid interface andselective occulation, the mechanical strength of surfactant lms at thesolidliquid interface and dispersion stability, and the interfacial phenomenain chemical mechanical polishing (CMP) of silicon wafers.

II. MONOLAYERSDuring the past quarter century, considerable studies have been carried outon the reactions in monomolecular lms of surfactant, or monolayers. Figure1 shows the surface pressurearea curves for dioleoyl, soybean, egg, anddipalmitoyl lecithins [1]. For these four lecithins, the fatty acid compositionwas determined by gas chromatography. The dioleoyl lecithin has bothchains unsaturated, soybean lecithin has polyunsaturated fatty acid chains,egg lecithin has 50% saturated and 50% unsaturated chains, and dipalmitoyllecithin has both chains fully saturated. It is evident that, at any xed surfacepressure, the area per molecule is in the following order:

Dioleoyl lecithin > soybean lecithin > egg lecithin > dipalmitoyl lecithin

It can be assumed that the area per molecule represents the area of a squareat the interface. Thus, the square root of the area per molecule gives thelength of one side of the square, which represents the intermolecular dis-tance. Figure 2 schematically illustrates the area per molecule and inter-molecular distance in these four lecithins. The corresponding intermoleculardistances were calculated to be 9.5, 8.8, 7.1, and 6.5 A , respectively, at asurface pressure of 20 mN/m [2]. Thus, one can conclude that a change inthe saturation of the fatty acid chains produces subangstrom changes in theintermolecular distance in the monolayer.

In addition, it was desired to explore the effects that these small changesin intermolecular distance had on the enzymatic susceptibility of these lec-ithins to hydrolytic enzymes such as phospholipase A [35], a potent hy-drolytic enzyme found in cobra venom. Thus, microgram quantities of en-zyme were injected under this monolayer. By measuring the rate of changeof surface potential, one can indirectly measure the rate of reaction in themonolayer. It is assumed that these quantities [i.e., change in surface poten-tial (V) and the extent of reaction] are proportional to each other. Thekinetics of hydrolysis, as measured by a decrease in surface potential, werestudied for each lecithin monolayer as a function of initial surface pressureand are shown in Fig. 3 [6]. It was found that initially the reaction rate

Molecular Interactions at Interfaces 3

FIG. 1 Surface pressurearea curves of dipalmitoyl, egg, soybean, and dioleoyllecithins.

increased as the surface pressure increased. Subsequently, as the surfacepressure increased further, the reaction rate decreased until a critical surfacepressure was reached at which no reaction occurred. The critical surfacepressure required to block the hydrolysis of lecithin monolayer increasedwith the degree of unsaturation of fatty acid chains (Fig. 3). Thus, it appearsthat as the intermolecular distance increases because of the unsaturated fattyacid chains, a higher surface pressure is required to clock the penetration ofthe active site of the enzyme into the monolayer to cause hydrolysis. Thisalso led to a suggestion that subangstrom changes in the intermoleculardistance in the monolayer were signicant for the enzymatic hydrolysis ofthe monolayers.

4 Shah and Moudgil

FIG. 2 Schematic representation of the area per molecule and intermolecular dis-tance in dioleoyl, soybean, egg, and dipalmitoyl lecithin monolayers based on thedata plotted in Fig. 1.

In addition to hydrolysis reactions, the enzymatic synthesis in monolayerswas studied [7]. In this case, a steric acid monolayer was formed on anaqueous solution containing glycerol. After compression to a desired surfacepressure, a small amount of enzyme lipase was injected under the monolayer.The lipase facilitated the linkage of glycerol with fatty acid and producedmonoglycerides, diglycerides, and triglycerides in the monolayer [810],which could be detected by thin-layer chromatography (TLC) or high-per-formance liquid chromatography (HPLC).

Because the amount of product that can be synthesized using a monolayeris in microgram quantities, this method is not attractive for large-scale en-zymatic synthesis. Therefore, studies of enzymatic reactions in monolayerswere extended to studies of enzymatic reactions in a foam. A foam providesa large interfacial area, and by continuous aeration one can generate an evenlarger interfacial area. A soap bubble is stabilized by monolayers on bothinside and outside surfaces of the bubble (Fig. 4). The glycerol and enzymecan be added into the aqueous phase before producing the foam. Thus, itwas shown that almost 88% of free steric acid could be converted to di- andtriglycerides in 2 h by reactions in foams (Fig. 5). For surface-active sub-strates (or reactants) and enzymes, reactions in foams offer a very interestingpossibility to produce large-scale synthesis of biochemicals using a foam asa reactor.


FIG. 3 Hydrolysis rate (as measured by surface potential) versus initial surfacepressure of various lecithin monolayers.

Another interesting investigation at the Center for Surface Science andEngineering (CSSE) was focused on the possible existence of phase tran-sitions in mixed monolayers of surfactants. Figure 6 shows the rate of evap-oration from pure and mixed monolayers of cholesterol and arachidyl (C20)alcohol, as well as their mixed monolayers [11]. It is evident that the pure

6 Shah and Moudgil

FIG. 4 Schematic diagram of a lipase-catalyzed reaction in a foam vessel.

FIG. 5 Decrease in free fatty acids and synthesis of di- and triglycerides by lipasein foam.


FIG. 6 Rate of evaporation from pure and mixed monolayers of cholesterol andarachidyl (C20) alcohol.

C20 alcohol monolayer allows only one third of the water loss related toevaporation of the pure cholesterol monolayer. This is presumably due tothe fact that the C20 alcohol forms monolayers that are in the two-dimen-sional solid state. In contrast, the cholesterol monolayers are in the two-dimensional liquid state. However, when cholesterol is incorporated into aC20 alcohol monolayer, the cholesterol mole fraction needs to be only about20% to liquefy the solid monolayers of C20 alcohol. The abrupt increase inevaporation rate of water at 2025 mol% cholesterol illustrates the two-

8 Shah and Moudgil

dimensional phase transition in the mixed monolayers from a solid state toa liquid state. After the cholesterol fraction reaches about 25 mol%, themonolayer remains in the two-dimensional liquid state and, hence, there isno further change in the rate of evaporation of water. Thus, one can utilizethe evaporation of water through a lm as a very sensitive probe for ob-serving the molecular packing in monolayers. The existence of a solid stateor a liquid state for monolayers can be inferred from such experimentalresults.

It has been shown that mixed monolayers of oleic acid and cholesterolexhibit the minimum rate of evaporation at a 1:3 molar ratio of oleic acidto cholesterol. This is shown in mixed monolayers of oleic acid and cho-lesterol in Fig. 7 as a function of surface pressure [11]. In has further beenshown that a 1:3 molar ratio in mixed fatty acid and fatty alcohol monolay-ers, one observes the maximum foam stability, minimum rate of evaporation,and maximum surface viscosity in these systems [12].

Monolayers are fascinating systems with extreme simplicity and well-dened parameters. During the past 35 years of research, we have found thestudies on monolayers to be rewarding in understanding the phenomenaoccurring at the gasliquid, liquidliquid, and solidliquid interfaces inrelation to foams, emulsions, lubrication, and wetting processes.

III. MICELLE KINETICS ANDTECHNOLOGICAL APPLICATIONS

It is well recognized that a surfactant solution has three components: sur-factant monomers in the aqueous solution, micellar aggregates in solution,and monomers absorbed as a lm at the interface. The surfactant is in dy-namic equilibrium among all of these components. From various theoreticalconsiderations as well as experimental results, it can be assumed that mi-celles are dynamic structures whose stability is in the range of millisecondsto seconds. Thus, in an aqueous surfactant solution, micelles break and re-form at a fairly rapid rate [1315]. Figure 8 shows the two characteristicrelaxation times, 1 and 2, associated with micellar solutions. The shortertime, 1, generally of the order of microseconds, is related to the exchangeof surfactant monomers between the bulk solution and the micelles, whereasthe longer time, 2, generally of the order of milliseconds to seconds, isrelated to the formation or dissolution of a micelle after several molecularexchanges [13,14]. It has been proposed that the lifetime of a micelle canbe approximated by n2, where n is the aggregation number of the micelle[15]. Thus, relaxation time 2 is proportional to the lifetime of the micelle.A large value of 2 represents high stability of the micellar structure.


FIG. 7 Minimum rate of evaporation at a 1:3 molar ratio in mixed monolayers ofoleic acid and cholesterol.

Figure 9 shows the relaxation time 2 of micelles of sodium dodecylsulfate (SDS) as a function of SDS concentration [13,16,17]. It is evidentthat the maximum relaxation time of micelles is observed at an SDS con-centration of 200 mM. This implies that SDS micelles are most stable atthis concentration. For several years researchers at the CSSE have tried tocorrelate the measured 2 with various equilibrium properties such as surfacetension, surface viscosity, and others, but no correlation could be found.However, a strong correlation of 2 with various dynamic processes such asfoaming ability, wetting time of textiles, bubble volume, emulsion dropletsize, and solubilization of benzene in micellar solution was found [18].

10 Shah and Moudgil

FIG. 8 Two relaxation times of micelles, 1 and 2, and related molecular processes.

FIG. 9 Relaxation time, 2, of SDS micelles as a function of SDS concentration.Maximum 2 found at 200 mM (vertical line).

Figures 10 and 11 summarize the effects of SDS concentration on thephenomena mentioned as well as on other related phenomena. Figure 10shows typical phenomena in liquidgas systems, and Fig. 11 shows typicalphenomena in liquidliquid and solidliquid systems. It is evident that eachof these phenomena exhibits a maximum or minimum at 200 mM SDS,depending on the molecular process involved. Thus the take-home mes-sage emerging from our extensive studies of the past decades is that mi-cellar stability can be the rate-controlling factor in the performance of var-ious technological processes such as foaming, emulsication, wetting,bubbling, and solubilization [19].


FIG. 10 Various liquidgas system phenomena exhibiting minima or maxima at200 mM SDS.

The currently accepted explanation for the effect of surfactant concentra-tion on micellar stability was proposed by Aniansson and coworkers in the1970s and expanded by Kahlweit and coworkers in the early 1980s [1316]. Anniansons model [1315] nicely predicts micelle kinetics at a lowsurfactant concentration based on stepwise association of surfactant mono-mers. Hence, the major parameters in this model are the critical micelleconcentration (cmc) and the total concentration of the surfactant in solution.

At higher surfactant (and hence counterion) concentrations, experimentalresults begin to deviate from Anianssons model. Kahlweits fusionssionmodel [16] takes into account the concentration and ionic strength of thecounterions in these solutions and proposes that as the counterion concen-tration increases, the charge-induced repulsion between micelles and sub-micellar aggregates decreases, leading to coagulation of these submicellaraggregates.

In both of these models, the effects of intermicellar distance as well asthe distance between submicellar aggregates have not been taken into ac-

12 Shah and Moudgil

FIG. 11 Various liquidliquid and solidliquid system phenomena exhibiting min-ima or maxima at 200 mM SDS.

count. Researchers at the CSSE have been attempting to introduce the effectof intermicellar distance into micellar kinetic theory. As the SDS concentra-tion increases, the number of micelles increases, and thus the intermicellardistance decreases. By knowing the aggregation number of the micelles, thenumber of micelles present in the solution can be calculated. The solutioncan then be divided into cubes such that each cube contains one micelle.From this, the distance between the centers of the individual cubes can betaken as the intermicellar distance.

Researchers at the CSSE determined that the concentration at which theSDS micelle was most stable (200 mM) coincided with an intermicellardistance of approximately one micellar diameter [1923]. At this concen-tration, one would expect a tremendous coulombic repulsion between themicelles at such a short distance. A possible explanation for the observedstability is that there is a rapid uptake of sodium as a counterion on themicellar surface at this concentration, making the micelles more stable. Thus,the coulombic repulsion between micelles with the concomitant uptake of


sodium ions allows the stabilization of micelles at this intermicellar distanceand, hence, maximum 2 at 200 mM concentration.

By introducing this so-called intermicellar coulombic repulsion model(ICRM) into existing theoretical models, better agreement between theoret-ically calculated and experimental 2 values may be attained.

It should be mentioned that Per Ekwall proposed rst, second, and thirdcritical micelle concentrations for sodium octanoate solutions [22]. At thesecond cmc, he showed sudden uptake or binding of sodium ions to themicellar surface and he proposed that at the second cmc there was tightpacking of surfactant molecules in the micelle. Thus, our 200 mM SDSconcentration could be equivalent to the second cmc as proposed by Ekwall.

The phenomenon of a surfactant exhibiting a maximum 2 appears to bea general behavior, and perhaps other anionic or cationic surfactants mayform tightly packed micelles at their own characteristic concentrations. Aswith the rst cmc, this critical concentration may also depend upon physicaland chemical conditions such as temperature, pressure, pH, salt concentra-tion, and other parameters in addition to the molecular structure of the sur-factant [24,25]. Work is currently in progress at the CSSE on identifying asimilar critical concentration for nonionic surfactants as well as mixed sur-factant systems.

In addition to this work, it has been shown that upon incorporation of ashort-chain alcohol such as hexanol into the SDS micelles, the maximum 2occurs at a lower SDS concentration [2628]. Thus, it appears that in amixed surfactant system, one can produce the most stable micelle at a lowersurfactant concentration upon incorporation of an appropriate cosurfactant[29]. Also investigated was the effect of long-chain alcohols on the micellarstability. Results were similar to those for short-chain alcohols for all butdodecanol, which showed a signicant increase in micellar stability overmicelles containing only SDS because of the chain length compatibility ef-fect [30].

Figure 12 shows the effect of coulombic attraction between oppositelycharged polar groups as well as the chain length compatibility effect on the2 or micellar stability of SDS plus alkyltrimethylammonium bromide (cat-ionic surfactant) solutions. It shows that surface tension, surface viscosity,miscellar stability, foaming ability, and foam stability are all inuenced bythe coulombic interaction as well as the chain length compatibility effect[30,31]. It should be noted that the ratio (weight basis) of SDS to alkyltri-methylammonium bromide was 95:5 in this mixed surfactant system. How-ever, even at this low concentration, the oppositely charged surfactant dra-matically changed the molecular packing of the resulting micelles as wellas the surfactant lm absorbed at the interface.

14 Shah and Moudgil


> 1. So it depends on the system andon the rate of approach of the particles which case prevails. Polystyrenelattices, which carry covalently bound sulfate groups, and clay minerals,which exhibit a negative plate charge because of isomorphic substitution,are clearly constant-charge examples unless counterion adsorption can takeplace very rapidly. On the other hand, silver halides and oxides, for which 0 is determined by adsorption and desorption of charge-determining ions(Ag, I and H, OH, respectively) are constant-potential candidates pro-vided De for the sorption processes is small enough. The distinction is notabsolute: if we are capable of shooting AgI particles onto each other at sucha high rate that the sorption processes cannot relax, we attain the constant-

52 Lyklema

charge limit. Likewise, the charge inside clay particles can relax by solid-state diffusion over geological periods.

It is concluded that the distinction between the two cases is not absolute.Intermediate situations with partial relaxation may also occur; then the workto be done may be partly chemical, partly electrostatic. The analysis of theserelaxations takes us to the domain of double-layer dynamics, and the pre-vailing issues are the identication of the relevant processes and the assess-ment of their rates.

III. RELAXATIONOF SURFACTANT ADSORBATESLet us next consider the more complicated case of an adsorbed ionic sur-factant layer, as in Fig. 1. To keep things simple, we assume that the particlesurface itself does not carry a charge. When two particles approach eachother, for each adsorbate three relaxation mechanisms may be envisaged.

1. Desorption of surfactants2. Relaxation of Stern ions3. Relaxation of diffuse ions

Can we estimate the rates of these processes and compare them with therate of interaction? For the interaction time (int) we estimate

1 5(int) 10 s (2)2Dfor colloidal particles approaching each other by diffusion over a distanceof order 1. Here, D is the diffusion coefcient of the particles.

Let us rst discuss relaxation mechanism 3 because relatively simple rulescan be given. For relaxation of a space charge by conduction, accordingto Maxwell

t/ (diff)(t) = (t = 0)e (3)with the relaxation time obeying

L(diff) = /K (4)0where 0 is the permittivity of free space and is the relative dielectricconstant of the solution and KL its conductivity. For a derivation of Eq. (3)see Ref. 3. For typical electrolyte solutions (103101 M for monovalentsalts) (diff) O(109107 s), so that the diffuse double-layer part isalways relaxed during interaction. Hence,

Interaction of Surfactant-Stabilized Particles 53

FIG. 3 Estimation of the order of magnitude of the van der Waals attraction be-tween two surfactant-covered surfaces, expressed as Gibbs energies per molecule ofsurfactant.

(diff)De(diff) =

54 Lyklema

surfactant was 2.5 molecules nm2 to express the expelling Gibbs energyper molecule. This Gibbs energy must be compared with the molecularGibbs energy of desorption, G(des), which for a surfactant with 1015CH2 groups in the chain is about 1218 kT, according to Traubes rule.From Fig. 3 we see that the driving force for desorption of single surfactantsis marginal under the chosen conditions. The surfaces must come very closebefore substantial desorption sets in.

Even if the driving force is high enough, it is questionable whether therate of desorption is fast enough to keep up with the rate of particle ap-proach. For nonionic surfactants (where electrostatics does not play a role)desorption relaxation times from cellulose monolayers of the order of 104103 s have been reported [5,6]. In these experiments, desorption was ini-tiated by diluting the solution. If we may use these data as representativefor the particle interaction case, considering Eq. (2) we nd

2De(des) 1010 (pair collision) (7)meaning that desorption would not take place. The issue deserves furtherconsideration. Questions to be addressed include the following: (1) Do themolecules desorb individually or collectively? (2) How do we account forthe acceleration of particle approach because of particle attraction and de-celeration because of hydrodynamic drag? (3) How do desorbed surfactantmolecules leave the narrow gap between the approaching particles?

For the present purpose, we assume that no desorption will take placeduring particle encounter. However, under shelf stability conditions

De(des) >1, we now address mechanism 2, the relaxation ofStern ions. The issue of lateral mobility of ions that are closely associatedwith the head groups is related to electrokinetic problems and the dynamicsof pair interaction in the absence of surfactants. As before, we must establishthe driving forces and estimate the rates. These two processes are interde-pendent: the driving force depends on the extent of disequilibration of thedouble layer, but this extent depends on the rate of lateral transport and,hence, on the driving force.


Let us redene the problem. We start from the premises that the diffusepart of the double layer is fully relaxed but that the surfactant adsorption isnot relaxed at all. In other words, the surfactant charge surf is xed. Weassume it to be negative, as in Fig. 1. Let us say that Na ions are thecounterions in the Stern layer, and let their surface charge density in thislayer be Then the question is what happens to the sumNa .

i surf Na (9)of the surfactant charge and the sodium ion charge if the particles approacheach other. We have the following limiting conditions

(Na ) iDe >> 1 constant (10a)(int)

(Na ) iDe

56 Lyklema

FIG. 4 Extent of variation of the sum of the surface charge and the Stern chargeas a function of the dimensionless particle distance at various Deborah numbers forthe Stern counterions.

ion and the counterions in the Stern layer. This sum can change with h onlybecause the Stern charge varies; the charge of the surfactants remains con-stant because surfactants do not desorb.

However, as the theory leading to Fig. 4 is general, this gure would alsoapply when surfactant desorption was rate determining [after replacing iby surf and De by De(des)].

This discussion indicates that the interaction dynamics can be phenom-enologically elaborated with the residence times of Stern ions as the param-eter. How can we measure these?

V. IONIC MOBILITIES IN STERN LAYERSOver the past decade it has become possible to measure the lateral mobilityof Stern ions and, in this way, establish their residence times. Basically, theprocedure for obtaining these mobilities consists of measuring the surfaceconduction in that layer, Ki, and the Stern charge i to obtain the requiredmobility of ion i in that layer, fromiu ,i

i i iK = u (12)iThis equation holds for a Stern layer with only one ionic species, i, in it.Surface conduction in the Stern layer takes place parallel to that in thediffuse layer. Hence, the total surface conductivity is

d iK = K K (13)where Kd and Ki refer to the surface conductivity in the diffuse and the


Stern layer, respectively. Surface conductivities are measurable quantities.For instance, one can prepare a porous plug of the particles to be studiedand measure its conductivity K(plug) as a function of the salt concentration.The K(plug) consists of two parts: a part determined by the interparticleuid (which is proportional to csalt) and a part contributed by surface con-duction. The latter can be found from the intercept of K(plug) versus csaltplots for csalt 0. For Kd a theory is available (by Bikerman); this quantityis determined by the electrokinetic potential and csalt. If is available, sayfrom additional electrophoresis measurements, Kd can be computed andsubtracted from K to obtain Ki. One of the assumptions in this analysis isthat the diffuse part of the double layer coincides with the part that is beyondthe slip plane. The value of i can be established as the difference betweenthe surface charge and the diffuse charge, the latter being equated to theelectrokinetic charge. For details of this and other procedures for establishingKi and hence see the review in Ref. 8. That review also gives someiuifeeling for the validity of the various assumptions that have to be made.

Lateral mobilities are now available for Stern layers on inorganic colloidsand lattices. As far as the author is aware, such data have not yet beenreported for surfactant-covered particles, although the required experimentsare not difcult and might produce interesting information. The closest in-formation we now have refers to phospholipid vesicles, where the (constant!)surfactant charge is caused by dissociation of the phosphate groups andwhere several counterions have been investigated [9]. In this study, electro-phoresis experiments were combined with dielectric spectroscopy. For tech-nical details we refer to the original paper, but it is very likely that theresults are correct because they agree with independent data obtained by aquite different approach, namely from the difference between the isoelectricand isoconductic points [10]. Moreover, the trends are very similar to thoseon latices and inorganic surfaces, so the results will probably not be toodifferent for surfactant adsorbates.

It is observed that

1. For monovalent counterions the mobility is, within experimental error,identical to that in the bulk.

2. The mobility ratio decreases drastically with the valence of the coun-terion.

3. Between counterions of the same valence there is no detectable ionicspecicity.

These rules give rise to two considerations. First, if the dynamics ofsurfactant-stabilized particles are elaborated on the GouyStern level, onecan substitute bulk mobilities for those in the Stern layer, at least if thecounterions are monovalent. For bivalent ones the average ratio is about

58 Lyklema

2/3, averaged over all systems studied so far. This conclusion, of course,also applies to such studies in the absence of surfactants.

The second consideration is of a more academic nature. Why is the mo-bility of, say, Na ions in the Stern layer almost identical to that in the bulkwhereas the viscosity in that layer is very high? Why is this ratio nonspe-cic? (On negatively charged polystyrene latices we found ratios of aboutunity even for protons, which migrate by a very different mechanism [11]).And why is there such a pronounced valence inuence?

We are now beginning to get some grip on these matters by invokinghelp from molecular dynamics simulations for large numbers of moleculesand ions [12]. Let us briey summarize the main ndings of these simula-tions.1. It is generally known, and our simulations conrm, that liquids in con-

tact with a hard surface exhibit density oscillations over the rst fewmolecular layers, which can be described by the distribution function(z), where z is the distance from the surface. This function has a pro-nounced maximum not far from the sum of the radii of the surface atomsand those of the liquid molecules. After this maximum there is a min-imum, then a more shallow second maximum, etc., the oscillations pe-tering out after a few molecular cross sections.

2. The stagnant layer in electrokinetics, that is, the layer with an apparentinnitely high viscosity, is closely related to these density oscillations.Hence, the thickness of this layer is determined by the extension ofthese oscillations into the solution.

3. The electrokinetically bound ions also accumulate in this layer. At thesame time, these ions are more or less identical to the Stern ions indouble-layer electrostatics.

4. As we are talking about a general phenomenon, stagnant layers shouldbe ubiquitous for all hard surfaces, be they charged or uncharged, hy-drophilic or hydrophobic. They should also exist on the surfaces ofsurfactant-stabilized particles. However, they should not be observed forliquidvapor interfaces because vapors are not hard. In fact, for suchinterfaces (z) follows a hyperbolic tangent prole rather than exhibitingoscillations.

5. When ions translate tangentially along hard surfaces, they reside for partof their time inside the stagnant layer and then can jump out, to movetangentially to the surface in the bulklike liquid, to return later to thestagnant layer, or to remain denitively in the diffuse part of the doublelayer after exchange against another ion. In other words, ionic conduc-tion in the stagnant layer is short-circuited through the diffuse part.

6. The measured tangential mobility in the stagnant layer [Eq. (12) andiuiTable 1] is the average of the fractions of time spent in the stagnant


TABLE 1 Ratios R Between Ionic Mobilities in the Stern Layer oniu /u ()i iVesicles and the Corresponding Bulk Mobilities for Liposome Vesicles

Reference

Counterion

Na, Cs Ca2 Cd2, Cu2 La3

Barchini et al. [9] 1.0 0.6 0.6Verbich et al. [10] 1.0 0.8 0.07

FIG. 5 Comparison of the lateral displacement of ions and solvent molecules instagnant layers.

(bound) and diffuse (free) states. At present we are attempting to analyzethese fractions by molecular dynamics simulations. It is at least in linewith the observation (see points 13 in this section) that the ratio R isclose to unity if the ions are readily exchangeable against the bulk. Formonovalent counterions this is apparently the case. Such ions are readilyshort-circuited, or shunted, by the diffuse part. This observation is alsoin line with the absence of ion specicity. We do not yet have enoughquantitative information for counterions of higher valence, but the factthat they are more strongly electrostatically bound to the surface maywell help to explain their lower mobility ratio R.

7. An essential element in the analysis that we have learned to appreciateis the difference between self-diffusion of individual molecules and thecollective diffusion in large collections of molecules. Figure 5 illustrateswhat we mean. The ion transport refers to the trajectories of individualspecies, whereas the displacement of the solvent reects the viscosity,as obtained from the time correlation of the pressure tensor. See Ref.

60 Lyklema

12 for details. Near the surfaces, this tensor is anisotropic; this surfaceviscosity is rather difcult to obtain, but because of the very large num-ber of molecules that we could simulate, we were able to estimate thecomponents of the tensor. The uid transport simulated in this waytypically represents the movement of large amounts of molecules, inwhich all molecular motions at any position are correlated.

In terms of phenomenological physical chemistry, the slopes of the twolines in Fig. 5 are measures of the ionic mobility in and the viscosity of thestagnant layer. The former is very much larger than the latter, meaning thations do move in a hydrodynamically virtually stagnant layer.

VI. CONCLUSIONBy analyzing the dynamics of surfactant-stabilized particles, a link was es-tablished with electrokinetics. In this way a cross-fertilization between twodifferent domains of interfacial science was established. The potentials ofthis nding deserve further elaboration.

REFERENCES1. EJW Verwey, JThG Overbeek. Theory of the Stability of Lyophobic Colloids.

Amsterdam: Elsevier, 1948.2. BV Derjaguin (=Deryagin). Acta Physicochim URSS 14:633, 1942.3. For a derivation, see, e.g., J Lyklema. Fundamentals of Interface and Colloid

Science. Vol I. 2nd printing. London: Academic Press, 1993, sec 6.6c.4. J Lyklema, HP van Leeuwen, M Minor. Adv Colloid Interface Sci 83:33, 1999.5. BV Zhmud, F Tiberg, J Kizling. Langmuir 16:2557, 2000.6. LH Torn. PhD thesis, Wageningen University, 2000.7. SYu Shulepov, SS Dukhin, J Lyklema. J Colloid Interface Sci 171:340, 1995.8. J Lyklema. In: A Delgado, ed. Interfacial Electrokinetics and Electrophoresis.

New York: Marcel Dekker, Chapter 3, p. 87, (2002).9. R Barchini, HP van Leeuwen, J Lyklema. Langmuir 16:8238, 2000.

10. SV Verbich, SS Dukhin, H Matsumura. J Dispersion Sci Technol 20:83, 1999;H Matsumura, SV Verbich, SS Dukhin. Colloids Surf A 159:271, 1999.

11. M Minor, AJ van der Linde, J Lyklema. J Colloid Interface Sci 203:177, 1998.12. S Rovillard, J de Coninck, J Lyklema. Langmuir 14:5659, 1998.

61

3Solid Particles at LiquidInterfaces, Including Their Effectson Emulsion and Foam StabilityROBERT AVEYARD and JOHN H. CLINT Hull University, Hull,United Kingdom

ABSTRACTBoth hydrophilic and hydrophobic particles are surface active at air/water(a/w) and oil/water (o/w) interfaces. The free energy of attachment to aninterface is simply related to the contact angle, , that the interface makeswith the particle surface. Particles are most strongly anchored to a liquidsurface when = 90.

Isolated hydrophobic particles resting at one surface of a thin aqueouslm can cause the lm to rupture (depending on ) when the second surfaceengages the particle, causing the particle to bridge the lm. For this reason,small hydrophobic particles can act as antifoams or foam breakers. Com-mercially, antifoams are often formulations in which hydrophobic particlesare dispersed in a mineral or a silicone oil. Particles adsorbed in close-packed layers on droplet interfaces in an emulsion can stabilize the emulsioneven in the absence of surfactant. Indeed, in some respects monolayers ofparticles behave similarly to adsorbed surfactants. Thus, hydrophobic par-ticles [which are akin to surfactants with low hydrophilelipophile balance(HLB)] can stabilize water-in-oil emulsions, whereas hydrophilic particlescan be expected to stabilize oil-in-water emulsions (just as high-HLB sur-factants do). In the rst part of the chapter aspects of the current state ofunderstanding of the effects that adsorbed particles have on foam and emul-sion stability are reviewed.

The second part of the chapter describes some of our recent work on (1)possible effects of line tension on the wettability of particles at uid inter-faces and (2) lateral interactions between (charged) polystyrene particles inmonolayers at a/w and o/w interfaces and the consequences for monolayer

62 Aveyard and Clint

structure. It is shown how the contact angle inuences the lateral inter-action between particles in monolayers.

I. INTRODUCTIONWhen present in emulsions or foams, small solid particles can drasticallyinuence the stability of the thin liquid lms present and hence the stabilityof the whole system. Liquid droplets present in foams can have a similareffect [113]. For this to happen, the particles or droplets must be able toenter the surfaces of the lms [14]. It is well known that once located withina liquid surface, a solid particle, for example, is usually strongly attached[1517]. Thus, small particles are in some respects akin to surfactant andother surface-active molecules.

Studies of foam breaking by solid particles have usually focused on thebehavior of individual particles at liquid interfaces. On the other hand, theability of small particles to stabilize (Pickering) emulsions in the absence ofa surfactant arises from processes involving particle assemblies (monolayers)at droplet surfaces [15]. Particle monolayers, like insoluble molecular mono-layers [18], can be studied using a Langmuir trough (e.g., Refs. 1924).Adsorbed particles exert a surface pressure, i.e., lower the interfacialtension, to an extent dependent on the surface concentration of particles.Monolayers can be compressed without particle loss to contiguous bulkphases because the energy of attachment to the surface is high.

The behavior of particle monolayers has not been widely studied, al-though signicant work has been done, particularly over the last two decades[2534]. We present here some of our own recent ndings on the structureand interactions within particle monolayers and on the mode of collapse asthe lateral pressure is increased [2224]. The mode of collapse is of directrelevance to the measurement of particle wettability at interfaces, as will beseen. It is particle wettability that determines the ability or otherwise ofparticles to rupture thin liquid lms and inuence the stability of emulsionsand foams.

In what follows we explain why particles are usually surface active atliquid interfaces. We also discuss mechanisms by which particles can rupturethin lms in foams and emulsions. Because antifoam formulations are fre-quently dispersions of particle-containing oil droplets in an aqueous phase,we also allude to the way in which oil droplets can break foam lms andto the synergy that exists between the oil and particles in this process. Fi-nally, we illustrate some of our own work on the behavior of particlemonolayers at (mainly) oil/water interfaces. Particularly important is the ob-servation that very long range electrical repulsion between charge-bearingparticles can occur through nonpolar oils at the oil/water interface.

Solid Particles at Liquid Interfaces 63

FIG. 1 Contact angles of water with solids in air or under oil. Top diagrams rep-resent spherical particles in a plane liquid/uid interface, and bottom diagrams showthe equivalent angles for a drop of water resting on a plane solid surface. For anglesless than 90 we refer to the solid as being hydrophilic in character and for anglesgreater than 90 we refer to the solid as being hydrophobic.

II. ADSORPTIONOF SOLID PARTICLES ATLIQUID INTERFACES

For simplicity, we consider spherical rather than irregularly shaped particles;general principles can be introduced in this way, and much of our ownexperimental work has involved the use of spherical polystyrene particles.Ideas introduced for solid particles apply to liquid droplets as well, but withthe added complication that a droplet in bulk is deformed to a lens when itenters a liquid surface [15,35].

The wettability of a particle at a liquid interface, which is related to theenergy of attachment to that interface, can be usefully expressed in terms ofthe contact angle that the liquid interface makes with the solid particle(Fig. 1). Here, we take to be the angle measured through the aqueousphase. Also, for simplicity we assume the liquid interface remains planar upto contact with the particle. Because we are concerned here with small par-ticles (usually with diameters of a few m), the latter assumption is notunreasonable. In Fig. 1 we show spherical particles immersed in an air/wateror oil/water interface and similar systems where the solid surface (with thesame surface characteristics as the particles) is planar and the liquid surface


FIG. 2 Uptake of a particle, originally in the -phase, at the / interface. An areaA of solid/ interface is lost, as is an area S of / interface. An area A of solid/interface is gained and a three-phase contact line is formed.

curved. The latter conguration is often the one used in the measurementof contact angles, which are then supposed to apply equally to the particlesystems. Later, we discuss alternative methods for measurement of contactangles using particles directly.

In a general way we will refer to solids with contact angles of over 90with the aqueous solutions as being hydrophobic; for angles < 90 we willcall the solid hydrophilic. We note that solids having > 90 with watermay well have < 90 with a surfactant solution. That is, a solid that ishydrophobic with respect to water may well be hydrophilic with respect toa surfactant solution.

The process of uptake of a particle, radius R, by a uid/uid (/) inter-face is illustrated in Fig. 2. Initially, the particle is completely immersed inthe -phase. The lower area A of the particle/ interface will, on uptake ofthe particle by the interface, be immersed in the -phase (producing area Aof solid/ interface) and a three-phase (//solid) contact line is formedaround the particle. Crucially, an area (designated S in Fig. 2) of plane /interface is lost. The various areas are each associated with an interfacialtension () and the contact line with a line tension, , It is readily shown[16] that the free energy of attachment, G, of the particle to, e.g., an oil/water (o/w) interface with tension ow, is given by

22 2G = R (1 cos ) (1 cos ) (1)ow R sin owThe free energy G is that of the system with the particle at equilibrium at


FIG. 3 (a) Spherical particle resting at a planar liquid/uid interface. (b) Schematicdiagram of the free energy of attachment (G ) of the particle to the interface as afunction of the equilibrium contact angle .

the interface relative to the free energy of the system in which the particleis fully immersed in the more wetting bulk phase.

The dependence of G on the equilibrium contact angle is illustrated inFig. 3; it is assumed here that the effects of line tension are absent (seelater). Clearly, a particle of given radius is most strongly held to the interfacewhen = 90. The dashed lines in the gure represent the free energy atthe interface relative to that with the particle in the less wetting bulk phase.The changes in the free energy of a system as a particle is pushed verticallythrough the interface are shown in Fig. 4 [16,17]. The depth of immersionof a particle below the liquid interface is designated h so that the quantityh/2R varies from zero (particle completely in the upper phase) to 1 (com-pletely in the lower phase). The lower curve, for = 0, exhibits a singleminimum, corresponding to the equilibrium depth of immersion. This arisesas a result of the way in which the areas A and S (Fig. 2) vary with thedepth of immersion h.

In the cases where line tension is important, the curves of G versus h/2Rshow two maxima and a single minimum. The maxima arise because of theway in which the contact line length varies with the extent of immersion.In the middle curve in Fig. 4, which is for an intermediate value of (positive), the minimum lies below zero and there is an equilibrium congurationfor the particle in the interface. For higher values of positive , however,the minimum can occur for G > 0. The minimum here corresponds to a


FIG. 4 Free energy as a function of depth of immersion of a particle through aninterface. The areas of the particle/bulk phase contacts and area of liquid/uid in-terface lost change with depth of immersion, as does the length of the three-phasecontact line. The lowest curve on the graph corresponds to zero line tension. Thetwo upper lines are for systems with positive line tensions (see text). The minimain the curves correspond to the equilibrium positions of the particles in the interface.

metastable state. The free energy of a system with the particle immersed inthe more wetting bulk phase is lower than that where the particle is in theminimum energy state within the interface. For high enough values of , theminimum disappears and no stable or metastable state exists for a particlein the interface.

From what has been said, it can be appreciated that there are two specialvalues of ; there are a critical value, c, for which G is zero and a maximumvalue, m, above which no equilibrium or metastable state can exist for aparticle in the interface. This situation is reected in the curves of contactangle and free energy against assumed (positive) values of , shown in Fig.5. For a given < m there are two values of contact angle, one of whichrepresents an unstable conguration. Similarly, for the free energy there aretwo values, one of which is for a nonequilibrium system.

Some of the current interest in the effects of line tension arises from theuncertainty of the magnitude of line tension. First, we mention that, unlikethe case for interfacial tensions, can be positive or negative. Positive values(considered earlier), which are normally observed for liquid/uid/solid con-


FIG. 5 Effects of line tension on contact angles and stability of particles at uid/uid interfaces. (a) Effect of increasing positive line tension on a system with >90; increase in causes to rise. (b) This effect is shown together with regions ofstability, metastability, and instability. (c) Corresponding effects of on the freeenergy G (see text).

tact lines, correspond to a tendency of a contact line to shrink in size (justas positive values of interfacial tension are associated with a tendency of asurface to contract). Such line tensions tend to push the contact angle awayfrom 90, forcing the particle toward the more wetting bulk phase. For agiven line tension, the effects are greater the smaller the particle size [seeEq. (1)]. Negative line tensions, on the other hand, correspond to contactlines that tend to expand; this forces the contact angle toward 90. Themagnitudes of positive reported in the literature vary widely, from around1012 N (theoretical values) to the largest of experimental values in the range106 to 105 N (see Ref. 17). These large values are expected to preventeven large particles (say 1 mm in diameter) from entering a liquid interface[17]. Whatever the subtleties of the sign, magnitude, and effects of linetension may be, we are of the opinion that because particles with diametersless than 10 m can be readily incorporated into liquid surfaces, the highvalues of line tension reported are probably in error.


FIG. 6 (a) Foam volume (or height, h) as a function of time; initial volumeis a measure of foamability and the change with time a measure of foam stability.(b) A foaming column for creation of foam and measurement of its volume (seetext).

III. EFFECTS OF SMALL PARTICLES AND OIL DROPSON AQUEOUS FOAMS

A. Introductory RemarksMuch of the interest in the way in which small particles and oil droplets,separately or in combination, affect the stability of thin liquid lms hasarisen as a result of the use of formulations containing oil and particles asaqueous antifoams. Unwanted foaming in many industrial systems and pro-cesses is an important problem [1].

Some simple experiments can be carried out to study the way in whichantifoams operate (Fig. 6). Probably the simplest is to form a foam by ag-itation (in a reproducible fashion) of an appropriate solution and observe (1)the initial volume of foam formed (a measure of foamability) and (2) theway in which foam volume subsequently falls with time (to measure foamstability). Alternatively, a foam can be formed in a foaming column (Fig.6b). Clean gas, say nitrogen, is passed through a glass frit and foaming liquidto form a foam, the volume of which grows with time. The maximum rateof growth is obviously equal to the gas ow rate. Both of the methods


mentioned measure the total volume of gas trapped within a foam and giveno direct information on the rupture of lms within the bulk of the foam.

B. Some Experimental ResultsThe effectiveness of particles as aqueous foam breakers depends stronglyon the wettability of the particles by the foaming liquid. In general, particlesneed to be hydrophobic (see earlier) in order to be effective. To illustratethis, we show in Fig. 7a the percent reduction of the initial foam volume of0.2 mM aqueous cetyltrimethylammonium bromide (CTAB) solutionscaused by the presence of spherical ballotini beads [7]. The beads weretreated to varying extents with octadecyltrichlorosilane to yield a range ofcontact angles with the CTAB solution. It is clear that the higher the contactangle, the greater the initial foam reduction.

Contact angles can change as a result of variation of surfactant concen-tration as well as surface treatment of solid. The results shown in Fig. 7brelate to foam breaking of aqueous CTAB solutions by particles of ethylene-bis-stearamide (EBS), a waxy solid used in commercial antifoams [8]. Inthis case, changes in result entirely from changes in CTAB concentrationin the foaming solutions. As in the case of surface treatment discussed be-fore, the percent foam reduction increases with increase in contact angle(decrease in CTAB concentration). We note that the foamability of the CTABsolutions did not vary with surfactant concentration in the absence of theparticles.

Commercial antifoams often rely on the synergistic effects of particlesand water-insoluble oil droplets (e.g., hydrocarbon or silicone oil) in reduc-ing foam volume. This is illustrated in Fig. 7b, where it is seen that EBSparticles in combination with dodecane are much more effective in causingfoam reduction than EBS particles alone.

The kind of results obtained using a foaming column, such as that illus-trated in Fig. 6, can be seen in Fig. 8 [36]. Here, foam volume is plotted asa function of time for foams produced by passing nitrogen through 1 wt%aqueous solutions of the nonionic surfactant C12H25(OCH2CH2)5OH(C12E5).The surfactant solutions exhibit a cloud point (i.e., they phase separate onheating) at about 31C. The phases formed are an aqueous phase rich insurfactant (the droplet phase) and a surfactant-lean aqueous phase. It is seenin Fig. 8 that foam volumes at a given time fall with increasing temperature.The gas ow rate used was the same for all temperatures. The inset showsthe foam volume after 30 min as a function of temperature. Clearly, thevolume falls rapidly up to about the cloud point (31C) and then levels off.This may result from the antifoam effect of the phase-separated surfactant-rich droplets.


FIG. 7 (a) Foam reduction (see test) of 0.2 mM cetyltrimethylammonium bromide(CTAB) solutions caused by hydrophobized glass beads (average diameter 45 m).The hydrophobicity of the beads and of glass plates (used for contact angle mea-surements) was adjusted by treatment with octadecyltrichlorosilane. (b) Foam re-duction of aqueous CTAB solutions caused by ethylene-bis-stearamide (EBS) par-ticles as a function of the contact angle of the surfactant solutions with atEBS-coated plates in air. Open circles represent systems without dispersed dodecanedrops and lled circles systems with dodecane. Here, changes in contact angle arebrought about by changing the concentration of the surfactant solution.


FIG. 8 Foam volume (in the foaming column) as a function of time for 1 wt%aqueous C12E5 over a range of temperature spanning the cloud point, which is around31C. The inset shows the foam volume formed after 30 min as a function of tem-perature. It is seen that the volume drops up to the cloud point and then becomesindependent of temperature up to at least 38C, the highest temperature studied.

C. Mechanisms of Foam BreakingThere is now a reasonable consensus concerning the modes of operation ofhydrophobic particles, alone or in combination with oil droplets, in reducingfoam volume [113]. With reference to Fig. 9a, consider a particle restingon one of the surfaces of a thin liquid foam lm, exhibiting a contact angle> 90. Films within foams drain and thin with time due to gravity and tosuction of liquid into plateau borders (see later). Ultimately, the lower sur-face touches the surface of the particle (Fig. 9b) and the liquid rapidlydewets the solid surface. The resulting curvature of the lm in the vicinityof the particles causes a local increase in pressure within the lm. Thisexcess of pressure (i.e., the Laplace pressure) forces the liquid away fromthe particle (as shown by the arrows in the curved liquid surfaces within theplateau borders).* When the two contact lines around the particle meet, the

*As a result of the action of surface tension, , there exists across a curved liquid surface apressure difference, p, that depends on the curvature of the surface. The pressure is greateron the inside of the surface. If the principal radii of curvature of the surface are r1 and r2,then the Laplace equation can be written p = [(1/r1) (1/r2)].


FIG. 9 Effect of spherical particles on the stability of thin liquid lms. (a) Particleresting at equilibrium at the air/water surface. (b) As the lm thins through drainage,the lower surface of the lm comes in contact with the particle surface and theparticle dewets. The lower contact line approaches the upper contact line, and thelm ruptures when the contact lines meet. The arrows show the liquid ow generatedby the Laplace pressure. (c) A particle at the surface of an oil droplet immersed inan aqueous lm. (d) A magnied picture showing the two contact angles aw andow; bas

adsorption and aggregation of surfactants in solution

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