adsorption and aggregation of surf act ants in solution

ADSORPTION AND AGGREGATION OF SURFACTANTS IN SOLUTIONedited by K. L. MittalHopewell Junction, New York, U.S.A.

Dinesh O. ShahUniversity of Florida Gainesville, Florida, U.S.A.

Marcel Dekker, Inc.

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

ISBN: 0-8247-0843-1 This book is printed on acid-free paper. Headquarters Marcel Dekker, Inc. 270 Madison Avenue, New York, NY 10016 tel: 212-696-9000; fax: 212-685-4540 Eastern Hemisphere Distribution Marcel Dekker AG Hutgasse 4, Postfach 812, CH-4001 Basel, Switzerland tel: 41-61-260-6300; fax: 41-61-260-6333 World Wide Web http://www.dekker.com The publisher offers discounts on this book when ordered in bulk quantities. For more information, write to Special Sales/Professional Marketing at the headquarters address above. Copyright 2003 by Marcel Dekker, Inc. All Rights Reserved.

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SURFACTANT SCIENCE SERIES

FOUNDING EDITOR

MARTIN J. SCHICK 19181998SERIES EDITOR

ARTHUR T. HUBBARD Santa Barbara Science Project Santa Barbara, California

ADVISORY BOARD

DANIEL BLANKSCHTEIN Department of Chemical Engineering Massachusetts Institute of Technology Cambridge, Massachusetts S. KARABORNI Shell International Petroleum Company Limited London, England LISA B. QUENCER The Dow Chemical Company Midland, Michigan JOHN F. SCAMEHORN Institute for Applied Surfactant Research University of Oklahoma Norman, Oklahoma P. SOMASUNDARAN Henry Krumb School of Mines Columbia University New York, New York

ERIC W. KALER Department of Chemical Engineering University of Delaware Newark, Delaware CLARENCE MILLER Department of Chemical Engineering Rice University Houston, Texas DON RUBINGH The Procter & Gamble Company Cincinnati, Ohio BEREND SMIT Shell International Oil Products B.V. Amsterdam, The Netherlands JOHN TEXTER Strider Research Corporation Rochester, New York

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40.

Nonionic Surfactants, edited by Martin J. Schick (see also Volumes 19, 23, and 60) Solvent Properties of Surfactant Solutions, edited by Kozo Shinoda (see Volume 55) Surfactant Biodegradation, R. D. Swisher (see Volume 18) Cationic Surfactants, edited by Eric Jungermann (see also Volumes 34, 37, and 53) Detergency: Theory and Test Methods (in three parts), edited by W. G. Cutler and R. C. Davis (see also Volume 20) Emulsions and Emulsion Technology (in three parts), edited by Kenneth J. Lissant Anionic Surfactants (in two parts), edited by Warner M. Linfield (see Volume 56) Anionic Surfactants: Chemical Analysis, edited by John Cross Stabilization of Colloidal Dispersions by Polymer Adsorption, Tatsuo Sato and Richard Ruch Anionic Surfactants: Biochemistry, Toxicology, Dermatology, edited by Christian Gloxhuber (see Volume 43) Anionic Surfactants: Physical Chemistry of Surfactant Action, edited by E. H. Lucassen-Reynders Amphoteric Surfactants, edited by B. R. Bluestein and Clifford L. Hilton (see Volume 59) Demulsification: Industrial Applications, Kenneth J. Lissant Surfactants in Textile Processing, Arved Datyner Electrical Phenomena at Interfaces: Fundamentals, Measurements, and Applications, edited by Ayao Kitahara and Akira Watanabe Surfactants in Cosmetics, edited by Martin M. Rieger (see Volume 68) Interfacial Phenomena: Equilibrium and Dynamic Effects, Clarence A. Miller and P. Neogi Surfactant Biodegradation: Second Edition, Revised and Expanded, R. D. Swisher Nonionic Surfactants: Chemical Analysis, edited by John Cross Detergency: Theory and Technology, edited by W. Gale Cutler and Erik Kissa Interfacial Phenomena in Apolar Media, edited by Hans-Friedrich Eicke and Geoffrey D. Parfitt Surfactant Solutions: New Methods of Investigation, edited by Raoul Zana Nonionic Surfactants: Physical Chemistry, edited by Martin J. Schick Microemulsion Systems, edited by Henri L. Rosano and Marc Clausse Biosurfactants and Biotechnology, edited by Naim Kosaric, W. L. Cairns, and Neil C. C. Gray Surfactants in Emerging Technologies, edited by Milton J. Rosen Reagents in Mineral Technology, edited by P. Somasundaran and Brij M. Moudgil Surfactants in Chemical/Process Engineering, edited by Darsh T. Wasan, Martin E. Ginn, and Dinesh O. Shah Thin Liquid Films, edited by I. B. Ivanov Microemulsions and Related Systems: Formulation, Solvency, and Physical Properties, edited by Maurice Bourrel and Robert S. Schechter Crystallization and Polymorphism of Fats and Fatty Acids, edited by Nissim Garti and Kiyotaka Sato Interfacial Phenomena in Coal Technology, edited by Gregory D. Botsaris and Yuli M. Glazman Surfactant-Based Separation Processes, edited by John F. Scamehorn and Jeffrey H. Harwell Cationic Surfactants: Organic Chemistry, edited by James M. Richmond Alkylene Oxides and Their Polymers, F. E. Bailey, Jr., and Joseph V. Koleske Interfacial Phenomena in Petroleum Recovery, edited by Norman R. Morrow Cationic Surfactants: Physical Chemistry, edited by Donn N. Rubingh and Paul M. Holland Kinetics and Catalysis in Microheterogeneous Systems, edited by M. Grtzel and K. Kalyanasundaram Interfacial Phenomena in Biological Systems, edited by Max Bender Analysis of Surfactants, Thomas M. Schmitt (see Volume 96)

41. Light Scattering by Liquid Surfaces and Complementary Techniques, edited by Dominique Langevin 42. Polymeric Surfactants, Irja Piirma 43. Anionic Surfactants: Biochemistry, Toxicology, Dermatology. Second Edition, Revised and Expanded, edited by Christian Gloxhuber and Klaus Knstler 44. Organized Solutions: Surfactants in Science and Technology, edited by Stig E. Friberg and Bjrn Lindman 45. Defoaming: Theory and Industrial Applications, edited by P. R. Garrett 46. Mixed Surfactant Systems, edited by Keizo Ogino and Masahiko Abe 47. Coagulation and Flocculation: Theory and Applications, edited by Bohuslav Dobi 48. Biosurfactants: Production Properties Applications, edited by Naim Kosaric 49. Wettability, edited by John C. Berg 50. Fluorinated Surfactants: Synthesis Properties Applications, Erik Kissa 51. Surface and Colloid Chemistry in Advanced Ceramics Processing, edited by Robert J. Pugh and Lennart Bergstrm 52. Technological Applications of Dispersions, edited by Robert B. McKay 53. Cationic Surfactants: Analytical and Biological Evaluation, edited by John Cross and Edward J. Singer 54. Surfactants in Agrochemicals, Tharwat F. Tadros 55. Solubilization in Surfactant Aggregates, edited by Sherril D. Christian and John F. Scamehorn 56. Anionic Surfactants: Organic Chemistry, edited by Helmut W. Stache 57. Foams: Theory, Measurements, and Applications, edited by Robert K. Prud'homme and Saad A. Khan 58. The Preparation of Dispersions in Liquids, H. N. Stein 59. Amphoteric Surfactants: Second Edition, edited by Eric G. Lomax 60. Nonionic Surfactants: Polyoxyalkylene Block Copolymers, edited by Vaughn M. Nace 61. Emulsions and Emulsion Stability, edited by Johan Sjblom 62. Vesicles, edited by Morton Rosoff 63. Applied Surface Thermodynamics, edited by A. W. Neumann and Jan K. Spelt 64. Surfactants in Solution, edited by Arun K. Chattopadhyay and K. L. Mittal 65. Detergents in the Environment, edited by Milan Johann Schwuger 66. Industrial Applications of Microemulsions, edited by Conxita Solans and Hironobu Kunieda 67. Liquid Detergents, edited by Kuo-Yann Lai 68. Surfactants in Cosmetics: Second Edition, Revised and Expanded, edited by Martin M. Rieger and Linda D. Rhein 69. Enzymes in Detergency, edited by Jan H. van Ee, Onno Misset, and Erik J. Baas 70. Structure-Performance Relationships in Surfactants, edited by Kunio Esumi and Minoru Ueno 71. Powdered Detergents, edited by Michael S. Showell 72. Nonionic Surfactants: Organic Chemistry, edited by Nico M. van Os 73. Anionic Surfactants: Analytical Chemistry, Second Edition, Revised and Expanded, edited by John Cross 74. Novel Surfactants: Preparation, Applications, and Biodegradability, edited by Krister Holmberg 75. Biopolymers at Interfaces, edited by Martin Malmsten 76. Electrical Phenomena at Interfaces: Fundamentals, Measurements, and Applications, Second Edition, Revised and Expanded, edited by Hiroyuki Ohshima and Kunio Furusawa 77. Polymer-Surfactant Systems, edited by Jan C. T. Kwak 78. Surfaces of Nanoparticles and Porous Materials, edited by James A. Schwarz and Cristian I. Contescu 79. Surface Chemistry and Electrochemistry of Membranes, edited by Torben Smith Srensen 80. Interfacial Phenomena in Chromatography, edited by Emile Pefferkorn

81. SolidLiquid Dispersions, Bohuslav Dobi, Xueping Qiu, and Wolfgang von Rybinski 82. Handbook of Detergents, editor in chief: Uri Zoller Part A: Properties, edited by Guy Broze 83. Modern Characterization Methods of Surfactant Systems, edited by Bernard P. Binks 84. Dispersions: Characterization, Testing, and Measurement, Erik Kissa 85. Interfacial Forces and Fields: Theory and Applications, edited by Jyh-Ping Hsu 86. Silicone Surfactants, edited by Randal M. Hill 87. Surface Characterization Methods: Principles, Techniques, and Applications, edited by Andrew J. Milling 88. Interfacial Dynamics, edited by Nikola Kallay 89. Computational Methods in Surface and Colloid Science, edited by Magorzata Borwko 90. Adsorption on Silica Surfaces, edited by Eugne Papirer 91. Nonionic Surfactants: Alkyl Polyglucosides, edited by Dieter Balzer and Harald Lders 92. Fine Particles: Synthesis, Characterization, and Mechanisms of Growth, edited by Tadao Sugimoto 93. Thermal Behavior of Dispersed Systems, edited by Nissim Garti 94. Surface Characteristics of Fibers and Textiles, edited by Christopher M. Pastore and Paul Kiekens 95. Liquid Interfaces in Chemical, Biological, and Pharmaceutical Applications, edited by Alexander G. Volkov 96. Analysis of Surfactants: Second Edition, Revised and Expanded, Thomas M. Schmitt 97. Fluorinated Surfactants and Repellents: Second Edition, Revised and Expanded, Erik Kissa 98. Detergency of Specialty Surfactants, edited by Floyd E. Friedli 99. Physical Chemistry of Polyelectrolytes, edited by Tsetska Radeva 100. Reactions and Synthesis in Surfactant Systems, edited by John Texter 101. Protein-Based Surfactants: Synthesis, Physicochemical Properties, and Applications, edited by Ifendu A. Nnanna and Jiding Xia 102. Chemical Properties of Material Surfaces, Marek Kosmulski 103. Oxide Surfaces, edited by James A. Wingrave 104. Polymers in Particulate Systems: Properties and Applications, edited by Vincent A. Hackley, P. Somasundaran, and Jennifer A. Lewis 105. Colloid and Surface Properties of Clays and Related Minerals, Rossman F. Giese and Carel J. van Oss 106. Interfacial Electrokinetics and Electrophoresis, edited by ngel V. Delgado 107. Adsorption: Theory, Modeling, and Analysis, edited by Jzsef Tth 108. Interfacial Applications in Environmental Engineering, edited by Mark A. Keane 109. Adsorption and Aggregation of Surfactants in Solution, edited by K. L. Mittal and Dinesh O. Shah 110. Biopolymers at Interfaces: Second Edition, Revised and Expanded, edited by Martin Malmsten 111. Biomolecular Films: Design, Function, and Applications, edited by James F. Rusling 112. 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 Abdelhamid Elaissari

Preface

This volume embodies, in part, the proceedings of the 13th International Symposium on Surfactants in Solution (SIS) held in Gainesville, Florida, June 1116, 2000. The theme of this particular SIS was Surfactant Science and Technology for the New Millennium. The nal technical program comprised 360 papers, including 96 poster presentations, which was a testimonial to the brisk research activity in the arena of surfactants in solution. In light of the legion of papers, to chronicle the total account of this event would have been impractical, so we decided to document only the plenary and invited presentations. The contributors were asked to cover their topics in a general manner; concomitantly, this book reects many excellent reviews of a number of important ramications of surfactants in solution. Chapters 14 document the plenary lectures, including the written account of the special Host Lecture by one of us (DOS) and Prof. Brij Moudgil. Chapters 532 embody the text of 28 invited presentations covering many aspects of surfactants in solution. Among the topics covered are: surfactant-stabilized particles; solid particles at liquid interfaces; nanocapsules; aggregation behavior of surfactants; micellar catalysis; vesicles and liposomes; the clouding phenomenon; viscoelasticity of micellar solutions; phase behavior of microemulsions; reactions in microemulsions; viscosity index improvers; foams, foam lms, and monolayers; principles of emulsion formulation engineering; nano-emulsions; liposome gene delivery; polymeric 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) applications, an understanding of their behavior in solution is of paramountiii

iv

Preface

importance. Also, as we learn more about surfactants and devise new surfactant formulations, novel and exciting applications will emerge. The present compendium of excellent overviews and research papers provides a bounty of up-to-date information on the many and varied aspects of surfactants in solution. It also offers a commentary on current research activity regarding the behavior of surfactants in solution. We hope that anyone involved or interestedcentrally or tangentiallyin surfactants will nd this book useful. Further, we trust that both veteran researchers and those embarking on their maiden voyage in the wonderful world of surfactants will nd this treatise valuable. To put together a symposium of this magnitude and quality requires dedication and uninching help from a battalion of people, and now it is our pleasure and duty to acknowledge those who helped in many and varied manners in this endeavor. First and foremost, we express our heartfelt and most sincere thanks to Prof. Brij Moudgil, Director of the Engineering Research Center for Particle Science and Technology, University of Florida, for helping in more ways than one. He wore many different hatsas cochairman, as troubleshooter, as local hostand he was always ready and willing to help with a smile. Next we are thankful to faculty members, postdoctoral associates, graduate students, and administrative staff of both the Center for Surface Science and Engineering and the Engineering Research Center for Particle Science and Technology, University of Florida. We acknowledge the generous support of the following organizations: the Florida Institute of Phosphate Research, the National Science Foundation, and the University of Florida. Many individual industrial corporations helped us by providing generous nancial support and we are grateful to them. We also thank the exhibitors of scientic instruments and books for their contribution and support. We are grateful to the authors for their interest, enthusiasm, and contribution 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. for giving this book a body form. K. L. Mittal Dinesh O. Shah

Contents

Preface iii Contributors ix 1. Highlights of Research on Molecular Interactions at Interfaces from the University of Florida 1 Dinesh O. Shah and Brij M. Moudgil Interaction Between Surfactant-Stabilized Particles: Dynamic Aspects 49 J. Lyklema Solid Particles at Liquid Interfaces, Including Their Effects on Emulsion and Foam Stability 61 Robert Aveyard and John H. Clint From Polymeric Films to Nanocapsules 91 Helmuth Mohwald, Heinz Lichtenfeld, Sergio Moya, A. Voight, G. B. Sukhorukov, Stefano Leporatti, L. Dahne, Igor Radtchenko, Alexei A. Antipov, Changyou Gao, and Edwin Donath Investigation of Amphiphilic Systems by Subzero Temperature Differential Scanning Calorimetry 105 Shmaryahu Ezrahi, Abraham Aserin, and Nissim Garti Aggregation Behavior of Dimeric and Gemini Surfactants in Solution: Raman, Selective Decoupling 13C NMR, and SANS Studies 133 Hirofumi Okabayashi, Norikatsu Hattori, and Charmian J. OConnorv

2.

3.

4.

5.

6.

vi

Contents

7.

Snared by Trapping: Chemical Explorations of Interfacial Compositions of Cationic Micelles 149 Laurence S. Romsted Effect of Surfactants on Pregastric EnzymeCatalyzed Hydrolysis of Triacylglycerols and Esters 171 Charmian J. OConnor, Douglas T. Lai, and Cynthia Q. Sun Effect of Benzyl Alcohol on the Properties of CTAB/KBr Micellar Systems 189 Ganzuo Li, Weican Zhang, Li-Qiang Zheng, and Qiang Shen Vesicle Formation by Chemical Reactions: Spontaneous Vesicle Formation in Mixtures of Zwitterionic and Catanionic Surfactants 201 Klaus Horbaschek, Michael Gradzielski, and Heinz Hoffmann Mechanism of the Clouding Phenomenon in Surfactant Solutions 211 C. Manohar Atomic Force Microscopy of Adsorbed Surfactant Micelles William A. Ducker 219

8.

9.

10.

11.

12. 13.

A Simple Model to Predict Nonlinear Viscoelasticity and Shear Banding Flow of Wormlike Micellar Solutions 243 J. E. Puig, F. Bautista, J. H. Perez-Lopez, J. F. A. Soltero, and Octavio Manero Preparation and Stabilization of Silver Colloids in Aqueous Surfactant Solutions 255 Dae-Wook Kim, Seung-Il Shin, and Seong-Geun Oh Silver and Palladium Nanoparticles Incorporated in Layer Structured Materials 269 Rita Patakfalvi, Szilvia Papp, and Imre Dekany Water-in-Carbon Dioxide Microemulsions Stabilized by Fluorosurfactants 299 Julian Eastoe, Alison Paul, David Steytler, Emily Rumsey, Richard K. Heenan, and Jeffrey Penfold Organic Synthesis in Microemulsions: An Alternative or a Complement to Phase Transfer Catalysis 327 Krister Holmberg and Maria Hager

14.

15.

16.

17.

Contents

vii

18.

Physicochemical Characterization of Nanoparticles Synthesized in Microemulsions 343 J. B.Nagy, L. Jeunieau, F. Debuigne, and I. Ravet-Bodart Phase Behavior of Microemulsion Systems Based on Optimized Nonionic Surfactants 387 Wolfgang von Rybinski and Matthias Wegener Microemulsions in Foods: Challenges and Applications Anilkumar G. Gaonkar and Rahul Prabhakar Bagwe Microemulsion-Based Viscosity Index Improvers Surekha Devi and Naveen Kumar Pokhriyal Foams, Foam Films, and Monolayers Dominique Langevin 453 431 407

19.

20. 21. 22. 23.

Role of Entry Barriers in Foam Destruction by Oil Drops 465 Asen D. Hadjiiski, Nikolai D. Denkov, Slavka S. Tcholakova, and Ivan B. Ivanov Principles of Emulsion Formulation Engineering 501 Jean-Louis Salager, Laura Marquez, Isabel Mira, Alejandro Pena, Eric Tyrode, and Noelia B. Zambrano Nano-Emulsions: Formation, Properties, and Applications 525 Conxita Solans, Jordi Esquena, Ana Maria Forgiarini, Nuria Uson, Daniel Morales, Paqui Izquierdo, Nuria Azemar, and Mara Jose Garca-Celma Surface Modications of Liposomes for Recognition and Response to Environmental Stimuli 555 Jong-Duk Kim, Soo Kyoung Bae, Jin-Chul Kim, and Eun-Ok Lee Specic Partition of Surface-Modied Liposomes in Aqueous PEO/Polysaccharide Two-Phase Systems 579 Eui-Chul Kang, Kazunari Akiyoshi, and Junzo Sunamoto Novel Cationic Transfection Lipids for Use in Liposomal Gene Delivery 603 Rajkumar Banerjee, Prasanta Kumar Das, Gollapudi Venkata Srilakshmi, Nalam Madhusudhana Rao, and Arabinda Chaudhuri Combinatorial Surface Chemistry: A Novel Concept for Langmuir and LangmuirBlodgett Films Research 619 Qun Huo and Roger M. Leblanc

24.

25.

26.

27.

28.

29.

viii

Contents

30.

Oscillating Structural Forces Reecting the Organization of Bulk Solutions and Surface Complexes 635 Per M. Claesson and Vance Bergeron Effect of Polymeric Surfactants on the Behavior of Polycrystalline Materials with Special Reference to Ammonium Nitrate 655 Arun Kumar Chattopadhyay Surface Tension Measurements with Top-Loading Balances 675 Brian Grady, Andrew R. Slagle, Linda Zhu, Edward E. Tucker, Sherril D. Christian, and John F. Scamehorn Index 689

31.

32.

Contributors

Kazunari Akiyoshi Department of Synthetic Chemistry and Biological Chemistry, Kyoto University, Kyoto, Japan Alexei A. Antipov dam, Germany Max-Planck-Institute of Colloids and Interfaces, Pots-

Abraham Aserin Casali Institute of Applied Chemistry, The Hebrew University of Jerusalem, Jerusalem, Israel Robert Aveyard Kingdom Department of Chemistry, Hull University, Hull, United

Nuria Azemar Departament de Tecnologia de Tensioactius, Institut dInvestigacions Qumiques i Ambientals de Barcelona, Barcelona, Spain Soo Kyoung Bae Department of Chemical and Biomolecular Engineering, KAIST, Daejeon, Korea Rahul Prabhakar Bagwe Department of Chemical Engineering, University of Florida, Gainesville, Florida, U.S.A. Rajkumar Banerjee* Division of Lipid Science and Technology, Indian Institute of Chemical Technology, Hyderabad, India*Current afliation: University of Pittsburgh, Pittsburgh, Pennsylvania, U.S.A.

ix

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, Indian Institute of Chemical Technology, Hyderabad, India Sherril D. Christian Institute for Applied Surfactant Research, University of Oklahoma, Norman, Oklahoma, U.S.A. Per M. Claesson Department of Chemistry, Surface Chemistry, Royal Institute of Technology, and Institute for Surface Chemistry, Stockholm, Sweden John H. Clint Kingdom L. Dahne many Department of Chemistry, Hull University, Hull, United

Max-Planck-Institute of Colloids and Interfaces, Potsdam, Ger-

Prasanta Kumar Das* Division of Lipid Science and Technology, Indian Institute of Chemical Technology, Hyderabad, India F. Debuigne Laboratoire de Resonance Magnetique Nucleaire, Facultes Universitaires 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 Physics and Biophysics, University of Leipzig, Leipzig, Germany William A. Ducker Virginia, U.S.A. Julian Eastoe Kingdom Department of Chemistry, Virginia Tech, Blacksburg,

School of Chemistry, University of Bristol, Bristol, United

Jordi Esquena Departament de Tecnologia de Tensioactius, Institut dInvestigacions Qumiques i Ambientals de Barcelona, Barcelona, Spain Shmaryahu Ezrahi Hashomer, Israel Technology and Development Division, IDF, Tel

Ana Maria Forgiarini Departament de Tecnologia de Tensioactius, Institut dInvestigacions Qumiques i Ambientals de Barcelona, Barcelona, Spain Changyou Gao Department of Polymer Science and Engineering, Zhejiang University, Hangzhou, Peoples Republic of China Anilkumar G. Gaonkar Glenview, Illinois, U.S.A. Research and Development, Kraft Foods, Inc.,

Mara Jose Garca-Celma Departament de Farmacia, Facultad de Far ` macia, Universitat de Barcelona, Barcelona, Spain ` Nissim Garti Casali Institute of Applied Chemistry, The Hebrew University of Jerusalem, Jerusalem, Israel Brian Grady School of Chemical Engineering and Materials Science, University of Oklahoma, Norman, Oklahoma, U.S.A. Michael Gradzielski reuth, Germany Physical Chemistry I, University of Bayreuth, Bay-

Asen D. Hadjiiski Laboratory of Chemical Physics and Engineering, Faculty of Chemistry, Soa University, Soa, Bulgaria Maria Hager Institute for Surface Chemistry, Stockholm, Sweden

xii

Contributors

Norikatsu Hattori* Department of Applied Chemistry, Nagoya Institute of Technology, Nagoya, Japan Richard K. Heenan ton, United Kingdom Heinz Hoffmann reuth, Germany ISIS Facility, Rutherford Appleton Laboratory, Chil-

Physical Chemistry I, University of Bayreuth, Bay-

Krister Holmberg Department of Applied Surface Chemistry, Chalmers University of Technology, Goteborg, Sweden Klaus Horbaschek reuth, Germany Physical Chemistry I, University of Bayreuth, Bay-

Qun Huo Department of Polymers and Coatings, North Dakota State University, Fargo, North Dakota, U.S.A. Ivan B. Ivanov Laboratory of Chemical Physics and Engineering, Faculty of Chemistry, Soa University, Soa, Bulgaria Paqui Izquierdo Departament de Tecnologia de Tensioactius, Institut dInvestigacions Qumiques i Ambientals de Barcelona, Barcelona, Spain L. Jeunieau Laboratoire de Resonance Magnetique Nucleaire, Facultes Universitaires Notre-Dame de la Paix, Namur, Belgium Eui-Chul Kang Department of Synthetic Chemistry and Biological Chemistry, Kyoto University, Kyoto, Japan Dae-Wook Kim Department of Chemical Engineering and CUPS, Hanyang 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, Universite Paris Sud, Orsay, France Roger M. Leblanc Department of Chemistry, University of Miami, Coral Gables, Florida, U.S.A. Eun-Ok Lee Department of Chemical and Biomolecular Engineering, KAIST, Daejeon, Korea Stefano Leporatti Institute of Medical Physics and Biophysics, University of Leipzig, Leipzig, Germany Ganzuo Li Key Laboratory for Colloid and Interface Chemistry of State Education Ministry, Shandong University, Jinan, Peoples Republic of China Heinz Lichtenfeld dam, Germany Max-Planck-Institute of Colloids and Interfaces, Pots-

J. Lyklema Physical Chemistry and Colloid Science, Wageningen University, Wageningen, The Netherlands Octavio Manero Instituto de Investigaciones en Materiales, Universidad Nacional Autonoma de Mexico, Mexico City, Mexico C. Manohar Department of Chemical Engineering, Indian Institute of Technology, 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, University of the Andes, Merida, Venezuela Helmuth Mohwald Potsdam, Germany Max-Planck-Institute of Colloids and Interfaces,

Daniel Morales Departament de Tecnologia de Tensioactius, Institut dInvestigacions 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 Germany Max-Planck-Institute of Colloids and Interfaces, Potsdam,

J. B.Nagy Laboratoire de Resonance Magnetique Nucleaire, Facultes Universitaires Notre-Dame de la Paix, Namur, Belgium Charmian J. OConnor Department of Chemistry, The University of Auckland, Auckland, New Zealand Seong-Geun Oh Department of Chemical Engineering and CUPS, Hanyang University, Seoul, Korea Hirofumi Okabayashi Department of Applied Chemistry, Nagoya Institute of Technology, Nagoya, Japan Szilvia Papp Department of Colloid Chemistry and Nanostructured Materials Research Group, Hungarian Academy of Sciences, University of Szeged, Szeged, Hungary Rita Patakfalvi Department of Colloid Chemistry and Nanostructured Materials Research Group, Hungarian Academy of Sciences, University of Szeged, Szeged, Hungary Alison Paul Kingdom School of Chemistry, University of Bristol, Bristol, United

Alejandro Pena* Laboratory FIRP, School of Chemical Engineering, University of the Andes, Merida, Venezuela Jeffrey Penfold United Kingdom ISIS Facility, Rutherford Appleton Laboratory, Chilton,

J. H. Perez-Lopez Departamento de Ingeniera Qumica, Universidad de Guadalajara, Guadalajara, Mexico Naveen Kumar Pokhriyal Department of Chemistry, M. S. University of 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 dam, Germany Max-Planck-Institute of Colloids and Interfaces, Pots-

Nalam Madhusudhana Rao Hyderabad, India

Centre for Cellular and Molecular Biology,

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, University of Oklahoma, Norman, Oklahoma, U.S.A. Dinesh O. Shah Departments of Chemical Engineering and Anesthesiology, Center for Surface Science and Engineering, University of Florida, Gainesville, Florida, U.S.A. Qiang Shen Key Laboratory for Colloid and Interface Chemistry of State Education Ministry, Shandong University, Jinan, Peoples Republic of China Seung-Il Shin Department of Chemical Engineering and CUPS, Hanyang University, Seoul, Korea Andrew R. Slagle Institute for Applied Surfactant Research, University of Oklahoma, Norman, Oklahoma, U.S.A. Conxita Solans Departament de Tecnologia de Tensioactius, Institut dInvestigacions Qumiques i Ambientals de Barcelona, Barcelona, Spain J. F. A. Soltero Departamento de Ingeniera Qumica, Universidad de Guadalajara, Guadalajara, Mexico

xvi

Contributors

Gollapudi Venkata Srilakshmi Division of Lipid Science and Technology, Indian Institute of Chemical Technology, Hyderabad, India David Steytler School of Chemical Sciences, University of East Anglia, Norwich, United Kingdom G. B. Sukhorukov dam, Germany Max-Planck-Institute of Colloids and Interfaces, Pots-

Cynthia Q. Sun* Department of Chemistry, The University of Auckland, Auckland, New Zealand Junzo Sunamoto Advanced Research and Technology Center, Niihama National College of Technology, Ehime, Japan Slavka S. Tcholakova Laboratory of Chemical Physics and Engineering, Faculty of Chemistry, Soa University, Soa, Bulgaria Edward E. Tucker Institute for Applied Surfactant Research, University of Oklahoma, Norman, Oklahoma, U.S.A. Eric Tyrode Laboratory FIRP, School of Chemical Engineering, University of the Andes, Merida, Venezuela Nuria Uson Departament de Tecnologia de Tensioactius, Institut dInvestigacions Qumiques i Ambientals de Barcelona, Barcelona, Spain A. Voight many Max-Planck-Institute of Colloids and Interfaces, Potsdam, Ger-

Wolfgang von Rybinski dorf, Germany Matthias Wegener many

Corporate Research, Henkel KGaA, Dussel

Corporate Research, Henkel KGaA, Dusseldorf, Ger

Noelia B. Zambrano Laboratory FIRP, School of Chemical Engineering, 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

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Weican Zhang State Key Laboratory of Microbial Technology, Shandong University, 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 of Oklahoma, Norman, Oklahoma, U.S.A.

1Highlights of Research on Molecular Interactions at Interfaces from the University of FloridaDINESH O. SHAH and BRIJ M. MOUDGIL Florida, Gainesville, Florida, U.S.A. University of

ABSTRACTAn overview of research highlights of the past three decades from the University of Florida on molecular interactions at interfaces and in micelles is presented. This overview includes work on (1) the kinetic stability of micelles in relation to technological processes, (2) molecular packing in mixed monolayers and phase transition in monolayers, (3) microemulsions and their technological applications including enhanced oil recovery (EOR) processes and preparation of nanoparticles of advanced materials, (4) adsorption of polymers at solidliquid interfaces and selective occulation, and (5) the mechanical strength of surfactant lms at the solidliquid interface and its correlation with dispersion stability as well as the interfacial phenomena in chemicalmechanical polishing (CMP) of silicon wafers. Detailed results explaining the role of molecular interactions at interfaces and in micelles as well as pertinent references are given for each phenomenon discussed.

I. INTRODUCTIONIt is a great pleasure and privilege for us to summarize the highlights of research on molecular interactions at interfaces from the University of Florida on this 13th International Symposium on Surfactants in Solution (SIS-2000). During the past 30 years at the University of Florida, we have had an ongoing research program on fundamental aspects as well as technological applications of interfacial processes. Specically, this overview1

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includes the kinetic stability of micelles in relation to technological processes, molecular packing in mixed monolayers and phase transition in monolayers, microemulsions and their technological applications including enhanced oil recovery (EOR) processes and preparation of nanoparticles of advanced materials, adsorption of polymers at solidliquid interface and selective occulation, the mechanical strength of surfactant lms at the solidliquid interface and dispersion stability, and the interfacial phenomena in chemical mechanical polishing (CMP) of silicon wafers.

II. MONOLAYERSDuring the past quarter century, considerable studies have been carried out on the reactions in monomolecular lms of surfactant, or monolayers. Figure 1 shows the surface pressurearea curves for dioleoyl, soybean, egg, and dipalmitoyl lecithins [1]. For these four lecithins, the fatty acid composition was determined by gas chromatography. The dioleoyl lecithin has both chains unsaturated, soybean lecithin has polyunsaturated fatty acid chains, egg lecithin has 50% saturated and 50% unsaturated chains, and dipalmitoyl lecithin has both chains fully saturated. It is evident that, at any xed surface pressure, 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 square at the interface. Thus, the square root of the area per molecule gives the length of one side of the square, which represents the intermolecular distance. Figure 2 schematically illustrates the area per molecule and intermolecular distance in these four lecithins. The corresponding intermolecular distances were calculated to be 9.5, 8.8, 7.1, and 6.5 A, respectively, at a surface pressure of 20 mN/m [2]. Thus, one can conclude that a change in the saturation of the fatty acid chains produces subangstrom changes in the intermolecular distance in the monolayer. In addition, it was desired to explore the effects that these small changes in intermolecular distance had on the enzymatic susceptibility of these lecithins to hydrolytic enzymes such as phospholipase A [35], a potent hydrolytic enzyme found in cobra venom. Thus, microgram quantities of enzyme were injected under this monolayer. By measuring the rate of change of surface potential, one can indirectly measure the rate of reaction in the monolayer. It is assumed that these quantities [i.e., change in surface potential ( V) and the extent of reaction] are proportional to each other. The kinetics of hydrolysis, as measured by a decrease in surface potential, were studied for each lecithin monolayer as a function of initial surface pressure and 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 dioleoyl lecithins.

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

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

In addition to hydrolysis reactions, the enzymatic synthesis in monolayers was studied [7]. In this case, a steric acid monolayer was formed on an aqueous solution containing glycerol. After compression to a desired surface pressure, a small amount of enzyme lipase was injected under the monolayer. The lipase facilitated the linkage of glycerol with fatty acid and produced monoglycerides, diglycerides, and triglycerides in the monolayer [810], which could be detected by thin-layer chromatography (TLC) or high-performance liquid chromatography (HPLC). Because the amount of product that can be synthesized using a monolayer is in microgram quantities, this method is not attractive for large-scale enzymatic synthesis. Therefore, studies of enzymatic reactions in monolayers were extended to studies of enzymatic reactions in a foam. A foam provides a large interfacial area, and by continuous aeration one can generate an even larger interfacial area. A soap bubble is stabilized by monolayers on both inside and outside surfaces of the bubble (Fig. 4). The glycerol and enzyme can be added into the aqueous phase before producing the foam. Thus, it was shown that almost 88% of free steric acid could be converted to di- and triglycerides in 2 h by reactions in foams (Fig. 5). For surface-active substrates (or reactants) and enzymes, reactions in foams offer a very interesting possibility to produce large-scale synthesis of biochemicals using a foam as a reactor.


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

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

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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 lipase in foam.


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

C20 alcohol monolayer allows only one third of the water loss related to evaporation of the pure cholesterol monolayer. This is presumably due to the fact that the C20 alcohol forms monolayers that are in the two-dimensional solid state. In contrast, the cholesterol monolayers are in the twodimensional liquid state. However, when cholesterol is incorporated into a C20 alcohol monolayer, the cholesterol mole fraction needs to be only about 20% to liquefy the solid monolayers of C20 alcohol. The abrupt increase in evaporation rate of water at 2025 mol% cholesterol illustrates the two-

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dimensional phase transition in the mixed monolayers from a solid state to a liquid state. After the cholesterol fraction reaches about 25 mol%, the monolayer remains in the two-dimensional liquid state and, hence, there is no further change in the rate of evaporation of water. Thus, one can utilize the evaporation of water through a lm as a very sensitive probe for observing the molecular packing in monolayers. The existence of a solid state or a liquid state for monolayers can be inferred from such experimental results. It has been shown that mixed monolayers of oleic acid and cholesterol exhibit the minimum rate of evaporation at a 1:3 molar ratio of oleic acid to cholesterol. This is shown in mixed monolayers of oleic acid and cholesterol in Fig. 7 as a function of surface pressure [11]. In has further been shown that a 1:3 molar ratio in mixed fatty acid and fatty alcohol monolayers, 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 welldened parameters. During the past 35 years of research, we have found the studies on monolayers to be rewarding in understanding the phenomena occurring at the gasliquid, liquidliquid, and solidliquid interfaces in relation to foams, emulsions, lubrication, and wetting processes.

III. MICELLE KINETICS AND TECHNOLOGICAL APPLICATIONSIt is well recognized that a surfactant solution has three components: surfactant monomers in the aqueous solution, micellar aggregates in solution, and monomers absorbed as a lm at the interface. The surfactant is in dynamic equilibrium among all of these components. From various theoretical considerations as well as experimental results, it can be assumed that micelles are dynamic structures whose stability is in the range of milliseconds to seconds. Thus, in an aqueous surfactant solution, micelles break and reform at a fairly rapid rate [1315]. Figure 8 shows the two characteristic relaxation times, 1 and 2, associated with micellar solutions. The shorter time, 1, generally of the order of microseconds, is related to the exchange of surfactant monomers between the bulk solution and the micelles, whereas the longer time, 2, generally of the order of milliseconds to seconds, is related to the formation or dissolution of a micelle after several molecular exchanges [13,14]. It has been proposed that the lifetime of a micelle can be approximated by n 2, 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.


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

Figure 9 shows the relaxation time 2 of micelles of sodium dodecyl sulfate (SDS) as a function of SDS concentration [13,16,17]. It is evident that the maximum relaxation time of micelles is observed at an SDS concentration of 200 mM. This implies that SDS micelles are most stable at this concentration. For several years researchers at the CSSE have tried to correlate the measured 2 with various equilibrium properties such as surface tension, surface viscosity, and others, but no correlation could be found. However, a strong correlation of 2 with various dynamic processes such as foaming ability, wetting time of textiles, bubble volume, emulsion droplet size, and solubilization of benzene in micellar solution was found [18].

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FIG. 8 Two relaxation times of micelles,

1

and

2

, and related molecular processes.

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

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


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FIG. 10 Various liquidgas system phenomena exhibiting minima or maxima at 200 mM SDS.

The currently accepted explanation for the effect of surfactant concentration on micellar stability was proposed by Aniansson and coworkers in the 1970s and expanded by Kahlweit and coworkers in the early 1980s [13 16]. Anniansons model [1315] nicely predicts micelle kinetics at a low surfactant concentration based on stepwise association of surfactant monomers. Hence, the major parameters in this model are the critical micelle concentration (cmc) and the total concentration of the surfactant in solution. At higher surfactant (and hence counterion) concentrations, experimental results begin to deviate from Anianssons model. Kahlweits fusionssion model [16] takes into account the concentration and ionic strength of the counterions in these solutions and proposes that as the counterion concentration increases, the charge-induced repulsion between micelles and submicellar aggregates decreases, leading to coagulation of these submicellar aggregates. In both of these models, the effects of intermicellar distance as well as the distance between submicellar aggregates have not been taken into ac-

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

count. Researchers at the CSSE have been attempting to introduce the effect of intermicellar distance into micellar kinetic theory. As the SDS concentration increases, the number of micelles increases, and thus the intermicellar distance decreases. By knowing the aggregation number of the micelles, the number of micelles present in the solution can be calculated. The solution can then be divided into cubes such that each cube contains one micelle. From this, the distance between the centers of the individual cubes can be taken as the intermicellar distance. Researchers at the CSSE determined that the concentration at which the SDS micelle was most stable (200 mM) coincided with an intermicellar distance of approximately one micellar diameter [1923]. At this concentration, one would expect a tremendous coulombic repulsion between the micelles at such a short distance. A possible explanation for the observed stability is that there is a rapid uptake of sodium as a counterion on the micellar surface at this concentration, making the micelles more stable. Thus, the coulombic repulsion between micelles with the concomitant uptake of


13

sodium ions allows the stabilization of micelles at this intermicellar distance and, hence, maximum 2 at 200 mM concentration. By introducing this so-called intermicellar coulombic repulsion model (ICRM) into existing theoretical models, better agreement between theoretically calculated and experimental 2 values may be attained. It should be mentioned that Per Ekwall proposed rst, second, and third critical micelle concentrations for sodium octanoate solutions [22]. At the second cmc, he showed sudden uptake or binding of sodium ions to the micellar surface and he proposed that at the second cmc there was tight packing of surfactant molecules in the micelle. Thus, our 200 mM SDS concentration could be equivalent to the second cmc as proposed by Ekwall. The phenomenon of a surfactant exhibiting a maximum 2 appears to be a general behavior, and perhaps other anionic or cationic surfactants may form tightly packed micelles at their own characteristic concentrations. As with the rst cmc, this critical concentration may also depend upon physical and chemical conditions such as temperature, pressure, pH, salt concentration, and other parameters in addition to the molecular structure of the surfactant [24,25]. Work is currently in progress at the CSSE on identifying a similar critical concentration for nonionic surfactants as well as mixed surfactant systems. In addition to this work, it has been shown that upon incorporation of a short-chain alcohol such as hexanol into the SDS micelles, the maximum 2 occurs at a lower SDS concentration [2628]. Thus, it appears that in a mixed surfactant system, one can produce the most stable micelle at a lower surfactant concentration upon incorporation of an appropriate cosurfactant [29]. Also investigated was the effect of long-chain alcohols on the micellar stability. Results were similar to those for short-chain alcohols for all but dodecanol, which showed a signicant increase in micellar stability over micelles containing only SDS because of the chain length compatibility effect [30]. Figure 12 shows the effect of coulombic attraction between oppositely charged polar groups as well as the chain length compatibility effect on the 2 or micellar stability of SDS plus alkyltrimethylammonium bromide (cationic surfactant) solutions. It shows that surface tension, surface viscosity, miscellar stability, foaming ability, and foam stability are all inuenced by the coulombic interaction as well as the chain length compatibility effect [30,31]. It should be noted that the ratio (weight basis) of SDS to alkyltrimethylammonium bromide was 95:5 in this mixed surfactant system. However, even at this low concentration, the oppositely charged surfactant dramatically changed the molecular packing of the resulting micelles as well as the surfactant lm absorbed at the interface.

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15

In view of the previous discussion, it is evident that micellar stability is of considerable importance to technological processes such as foaming, emulsication, wetting, solubilization, and detergency because a nely tuned detergent formulation can signicantly improve the cleaning efciency as well as reduce the washing time in a laundry machine, resulting in signicant energy savings at a national and global level. Micellar stability is thus a critical issue in any application in which surfactants are present as micelles, and the subsequent monomer ux is utilized in the application.

IV. MICROEMULSIONS IN ENHANCED OIL RECOVERY AND SYNTHESIS OF NANOPARTICLESAs early as 1943, Professor J. H. Schulman published reports on transparent emulsions [32]. From various experimental observations and intuitive reasoning, he concluded that such transparent systems were microemulsions. Figure 13 illustrates the transparent nature of a microemulsion in comparison with a macroemulsion. He also proposed the concept of a transient negative interfacial tension to induce the spontaneous emulsication in such systems. Considerable studies have been carried out on microemulsions during the past quarter-century, during which time it has been recognized that there are three types of microemulsions: lower-phase, middle-phase, and upper-phase microemulsions. The lower-phase microemulsion can remain in equilibrium with excess oil in the system, the upper-phase microemulsion can remain in equilibrium with excess water, and the middle-phase microemulsion can remain in equilibrium with both excess oil and water. As a result, the lowerphase microemulsion has been considered to be an oil-in-water microemulsion, the upper-phase microemulsion has been considered to be a water-in-oil microemulsion, whereas the middle-phase microemulsion has been the subject of much research and has been proposed to be composed of bicontinuous or phase-separated swollen micelles from the aqueous phase [3344]. Figure 14 shows the lower-, middle-, and upper-phase microemulsions, as represented by the darker liquid in each tube [4548]. The formation of lower-, middle-, and upper-phase microemulsions is related to the migration of surfactant from lower phase to middle phase to upper phase. Figure 15 illustrates that migration of the surfactant from the

< FIG. 12 Effect of coulombic attraction between polar groups of surfactants and chain length compatibility on 2 or micellar stability and other interfacial properties of mixed surfactant solutions. The dashed line represents the specic property of 100 mM pure SDS solution.

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FIG. 13 Illustration of the transparent nature of a microemulsion compared with a macroemulsion.

aqueous phase to the middle phase to the oil phase can be induced by changing a number of parameters, including adding salts (e.g., NaCl) to the system, decreasing the oil chain length, increasing the surfactant molecular weight, adding a cosurfactant, and decreasing the temperature [48,49]. However, it has been reported that in oil/water/nonionic surfactant systems, surfactant moves from lower phase to middle phase to upper phase as the temperature is increased [42,43], so each system must be carefully analyzed in order to determine the effects of certain parameters. Microemulsions exhibit ultralow interfacial tension with excess oil or water phases. Therefore, the middle-phase microemulsion is of special importance to the process of oil displacement from petroleum reservoirs.

A. Microemulsions in Enhanced Oil RecoveryFigure 16 shows a schematic view of a petroleum reservoir as well as the process of water or chemical ooding by an inverted ve-spot pattern [33]. Several thousand feet below the ground, oil is found in tightly packed sand or sandstones in the presence of water as well as natural gas. During the primary and secondary recovery processes (water injection method), about 35% of the available oil is recovered. Hence, approximately 65% is left in the petroleum reservoir. This oil remains trapped because of the high interfacial tension (2025 mN/m) between the crude oil and reservoir brine. It is known that if the interfacial tension between crude oil and brine can be reduced to around 10 3 mN/m, one can mobilize a substantial fraction of


17

FIG. 14 Samples of (a) lower-, (b) middle-, and (c) upper-phase microemulsions in equilibrium with excess oil, excess water and oil, or excess water, respectively.

the residual oil in the porous media in which it is trapped. Once mobilized by an ultralow interfacial tension, the oil ganglia must coalesce to form a continuous oil bank. The coalescence of oil droplets has been shown to be enhanced by a very low interfacial viscosity in the system. The incorporation of these two critical factors into a suitable surfactant system for oil recovery was crucial in developing the surfactantpolymer ooding process for enhanced oil recovery from petroleum reservoirs. Conceptually, one injects a surfactant formulation in the porous media in the petroleum reservoir so that upon mixing with the reservoir brine and oil it produces the middle-phase microemulsion in situ. This middle-phase microemulsion, which is in equilibrium with excess oil and excess brine, propagates throughout the petroleum reservoir. The design of the process is such that the oil bank maintains

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FIG. 15 The transition from lower- to middle- to upper-phase microemulsions can be brought about by the addition of salts or by varying other parameters. The transition from lower to middle to upper phase (I m u) occurs by (1) increasing salinity, (2) decreasing oil chain length, (3) increasing alcohol concentration (C4, C5, C6), (4) decreasing temperature (for ionic surfactants), (5) increasing total surfactant concentration (for high-molecular-weight anionic surfactants), (6) increasing brine/ oil ratio (for high-molecular-weight anionic surfactants), (7) increasing surfactant solution/oil ratio (for high-molecular-weight anionic surfactants), and (8) increasing molecular weight of surfactant.

ultralow interfacial tension with reservoir brine until it arrives at the production wells. One parameter that has been discovered to be crucially important in the successful implementation of the surfactantpolymer ooding process is the salinity of the aqueous phase. As discussed previously, addition of salt to the microemulsion system induces the change from lower- to middle- to upper-phase microemulsion (Fig. 15) [33]. It was found that at a particular salt concentration, deemed the optimal salinity, a number of important parameters were optimized for the oil recovery process. The optimal salinity was found to occur when equal amounts of oil and brine were solubilized by the middle-phase microemulsion [50]. Figure 17 summarizes the various parameters that are important in the surfactantpolymer ooding process as a function of salt concentration [33,5154]. It is evident that all of these parameters exhibit a maximum or a minimum at the optimal salinity. Thus, it appears that all of these processes are interrelated for the oil displacement in porous media by the surfactant polymer ooding process. It also appears that the optimal salinity value is a crucial parameter for consideration of a system to be used in this process.

B. Formation of Nanoparticles Using MicroemulsionsAnother very interesting use of microemulsions that has been investigated in our laboratory over the past decade is in the production of nanoparticles.


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FIG. 16 Schematic view of a petroleum reservoir and the process of water or chemical ooding (ve-spot pattern).

Figure 18 schematically illustrates the formation of nanoparticles using water-in-oil microemulsions. For this process, two identical water-in-oil microemulsions are produced, the only difference between the microemulsions being the nature of the aqueous phase, into which the two water-soluble reactants, A and B, are dissolved separately. Upon mixing the two nearly identical microemulsions, the water droplets collide and coalesce, allowing the mixing of the reactants to produce the precipitate AB. Ultimately, these droplets again disintegrate into two aqueous droplets, one containing the nanoparticle AB and the other containing just the aqueous phase [5557]. Thus, a precipitation reaction can be carried out in the aqueous cores of water-in-oil microemulsions using the dispersed water droplets as nanoreactors. The size of the particles formed is physically limited by the reactant concentration as well as the size of the water droplets. In this way,

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FIG. 17 Various phenomena occurring at the optimal salinity in the surfactant polymer ooding process for enhanced oil recovery.

monodisperse particles in the range 210 nm in diameter can be produced. The production of nanoparticles with homogeneity of particle size (i.e., small size range) has inherently been a problem with other conventional methods. This method of nanoparticle synthesis is an improvement over other methods for applications that require the production of monodisperse nanoparticles [5860]. Superconducting nanoparticles have also been produced in our laboratory using the microemulsion method. Table 1 shows the composition of the two microemulsions used for synthesizing nanoparticles of YBCO (Yttrium Bar-


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FIG. 18 Formation of nanoparticles using microemulsions (water-in-oil) as nanoreactors. The water droplets continually collide, coalesce, and break up upon mixing of two microemulsions containing reactants.

ium Copper Oxide) superconductor [6163]. In this case, water-soluble salts of yttrium, barium, and copper were dissolved in the aqueous cores of one microemulsion and ammonium oxalate was dissolved in the aqueous cores of the other microemulsion. Upon mixing the two microemulsions, precursor nanoparticles of metal oxalates were formed. The nanoparticles were centrifuged and then washed with chloroform, methanol, or acetone to remove the surfactants and oil. These nanopowders were then calcined at the appropriate temperature to convert the oxalate precursors into oxides of these materials. The oxides were then compressed into a pellet and sintered at 860 C for 24 h. The pellet was cooled, and the critical temperature of zero resistance was measured. It was found that this critical temperature did not show any change from crit-

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TABLE 1 Composition of Two Microemulsions for Synthesizing Nanoparticles of YBCO Superconductors Surfactant phase Microemulsion I CTAB 1-butanol Hydrocarbon phase n-Octane Aqueous phase (Y, Ba, Cu)nitrate solution, total metal concentration = 0.3 N Ammonium oxalate solution, 0.45 N 11.33%

Microemulsion II Weight fraction (for both I and II)

CTAB

1-butanol

n-Octane 59.42%

29.25%

ical temperatures of superconductors produced by the traditional coprecipitation method. However, the fraction of the ideal Meissner shielding was strikingly different for the two samples prepared by different methods. It is the Meissner effect that is related to the levitational effect of the superconducting pellet on a magnetic eld. Thus, it appears that the leakage of magnetic ux from the conventionally prepared sample was greater than that from the sample produced by microemulsion-derived nanoparticles. Figure 19 shows scanning electron microscope (SEM) images of sintered pellets produced by the two methods. It is evident that the pellets prepared from the nanoparticles produced by the microemulsion method showed 30100 times larger grain size, less porosity, and higher density as compared with the samples prepared by conventional precipitation of aqueous solutions of these salts. A possible explanation for these effects is that nanoparticles, because of their extremely small size and large surface area, can disintegrate very quickly and allow diffusion of atoms to the site of the growing grains to support the growth process of the grains. Therefore, samples prepared from nanoparticles exhibit large grain size and low porosity [64]. Thus, it appears that these nanoparticles may be useful to produce high-density ceramics. Since the pioneering work of the rst 20 years on the formation of nanoparticles of heavy metals by the microemulsion method [65], we have added to the understanding of the mechanism and control of the reaction kinetics in microemulsions by controlling the interfacial rigidity of the microemulsion droplet. We have introduced the concept of a chain length compatibility effect observed for the reactions in microemulsions, in which the interfacial rigidity is maximized by a certain chain length combination of the surfactant, oil, and cosurfactant alcohol, causing a decreased reaction rate [66]. All of


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FIG. 19 SEM images of superconducting pellets prepared from nanopowders (a and b) using microemulsions and conventionally prepared powders (c and d) (by precipitation of aqueous solutions).

these contributions have led to a greater understanding of the method of nanoparticle production using microemulsion media.

V. CONTROL OF POLYMER ADSORPTION AT PARTICLE SURFACESPolymers at particle surfaces play an important role in a range of technologies such as paints, polishing, ltration, separations, enhanced oil recovery, and lubrication. In order to optimize these technologies, it is important to understand and control the adsorption, conformation, and role of surface molecular architecture in selective polymer adsorption. For a particular polymer functionality, the adsorption depends on the nature and energetics of the adsorption sites that are present on the surface. The adsorption of polymers via electrostatics, chemical bonding, and hydrophobic interaction is relatively well understood, and most of the unexpected adsorption behavior is attributed to hydrogen bonding, which is ubiquitous

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in nature. Research carried out at the University of Florida Engineering Research Center for Particle Science and Technology (UF-ERC) has focused on the role of surface chemistry and surface molecular architecture in hydrogen bonding of polymers to particle surfaces. In addition, practical applications of controlled polymer adsorption have been investigated for solidsolid separations via selective occulation technology.

A. Control of Hydrogen BondingThe surfaces of most oxides and minerals have two different kinds of acid sites, Bronsted and Lewis, on which hydrogen-bonding polymers can adsorb. Bronsted acids are dened as proton donors, such as the M OH sites on oxide surfaces. The more electron withdrawing the underlying substrate, the greater is the Bronsted acidity. Lewis acid sites are dened as electron de cient or as having the ability to accept electrons. Examples of Lewis acid sites include M (OH)2 groups on oxide surfaces. For a hydrogen-bonding polymer such as poly(ethylene oxide) (PEO), whose ether oxygen linkage acts as a Lewis base, it was illustrated that not only did the number of hydrogen-bonding sites differ from one substrate to another but also the energy of the hydrogen-bonding sites varied [67]. Based on adsorption studies of PEO on silica and other oxides [67], it was determined that the amount of adsorbed polymer depended on the nature of the Bronsted acid (proton donor) sites on the particles. Hence, the more electron withdrawing the underlying substrate, the greater the Bronsted acidity, and thus the lower the point of zero charge (pzc) of the material. It is seen from Fig. 20 that SiO2, MoO3, and V2O5 strongly adsorb PEO whereas oxides with pzc greater than that of silica, such as TiO2, Fe2O3, Al2O3, and MgO, did not exhibit signicant adsorption of PEO. However, within a single system (silica) it has been found that PEO will adsorb onto sol-gelderived silica but not onto glass or quartz at a pH of 9.5. This suggests that the strength of Bronsted acid sites (higher ability to donate protons), as deter mined by the surface molecular architecture, also inuences the adsorption process. It was also shown that the nature and energetics of the surface sites could be modied by surface modication techniques such as calcination and rehydroxylation [68], Upon calcination of a silica surface to 800 C, the number of isolated surface hydroxyl groups [determined from Fourier transform infrared (FTIR) spectroscopy] and three-membered silicate rings (determined from Raman spectroscopy) increased, resulting in higher surface acidity [determined from solid-state nuclear magnetic resonance (NMR) spectroscopy using triethyl phosphine oxide probe]. These changes led to higher adsorption of the PEO polymer (Fig. 21) [68]. Based on the polymer functionality, it may be possible to predict the surface molecular architecture or the surface sites that are required for the


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FIG. 20 Effect of surface Bronsted acidity on the adsorption of a hydrogen-bonding polymer, poly(ethylene oxide) (PEO). (From Ref. 31.)

FIG. 21 Adsorption of PEO on sol-gel silica with different treatments (calcined 800 C, rehydroxylated). (From Ref. 32.)

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polymer to adsorb. Studies of the adsorption of PEO, PAM (nonionic polyacrylamide), PAA [nonionic poly(acrylic acid)], PAH (polyallylamine hydrochloride), and PVA (poly (vinyl alcohol)) onto silica, alumina, and hematite were carried out, and a master table correlating functional group active surface site relationships was developed (Table 2) [69]. Strong adsorption of PEO onto silica was observed, with none onto alumina and hematite, in agreement with the earlier results [67]. No adsorption of PAM onto silica was observed; however, PAM was found to adsorb onto hematite and alumina at pH 3.0. At pH 9.5, there was no adsorption of PAM onto alumina. Given the lack of adsorption of PAM onto silica, which has strong Bronsted acid sites, it was concluded that PAM

TABLE 2 Polymer PEO PVA

Correlation of Polymer Functionality with the Surface Adsorption Sites Repeat unit Functionality Ether Hydroxyl Adsorbs onto SiO2 SiO2 Adsorption sites Bronsted Bronsted

PAA

Carboxylic acid

Fe2O3 Al2O3 TiO2

Lewis

PAM

Amide

Fe2O3 Al2O3 TiO2

Lewis

PAH

Amine

SiO2

Bronsted

PEO, poly(ethylene oxide); PVA, poly(vinyl alcohol); PAA, poly(acrylic acid); PAM, poly(acrylamide) (nonionic); PAH, polyallylamine. Source: Ref. 33.


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adsorbed on Lewis acid sites. The lack of adsorption of PAM onto alumina at pH 9.5 was explained by poisoning of active Lewis sites due to preferential adsorption of hydroxide ions at pH values below the isoelectric point of alumina (pHiep = 8.8). Nonionic PAA was found to adsorb onto hematite and alumina but not onto silica at pH 3. Adsorption experiments were not conducted above pH 3 because PAA becomes ionized and the adsorption is dominated by electrostatic interactions. Having developed the fundamental knowledge base on the role of surface molecular architecture in polymer adsorption and the surface sitepolymer functional group correlation, the next logical step was to use these fundamentals in real particulate systems, which contain heterogeneous surfaces with impurities that in some cases may foil selective adsorption schemes.

B. Control of Polymer Adsorption for Selective SeparationFlocculation of ne particles using polymeric materials (occulants) and separation of such aggregates from particles of the other component(s) in the dispersed phase is known as selective occulation [67]. The competition between different surfaces for the occulant must be controlled in order to achieve adsorption on the targeted component(s). The aggregates of the polymer-coated particles, or ocs, thus formed are separated from the suspension by either sedimentationelutriation or oc otation. The major barrier to further commercialization of the selective occulation technology is the poor success in extension of single-component successes to mixed-component systems. The selectivity observed in single-component tests is often lost in mixed-component or natural systems. One of the signicant reasons for this loss in selectivity is heteroocculation, wherein a small amount of polymer adsorption on the inert material leads to coocculation with the active material. One of the major advances at the University of Florida (UF) has been the development of the site-blocking agent (SBA) concept [70,71] to overcome heteroocculation, thus achieving selectivity in particle separation. The SBA concept is illustrated schematically in Fig. 22. The concept involves blocking all the active sites for polymer adsorption on the inert material (component of the particle mixture not to be occulated) by addition of the SBA. After the addition of the SBA, when the occulant is added, it adsorbs only onto the active component (material intended to be occulated), resulting in selectivity of separation. A lower molecular weight fraction of the same or a similar occulant, which on its own is incapable of inducing occulation in the active or the oc-

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FIG. 22 Schematic illustrating the site-blocking agent (SBA) concept. (From Ref. 31.)

culating material, was successfully used as an SBA to minimize heteroocculation. The concept has since been commercialized by Engelhard Corporation for removal of titania impurities from kaolin clays [72].

VI. SURFACTANT SELF-ASSEMBLY AT THE SOLIDLIQUID INTERFACESurfactants at the solidliquid interface are used in various industrial processes ranging from ore otation and paint technology to enhanced oil recovery [73]. Apart from the traditional uses of surfactants, surfactant structures are increasingly being investigated as organic templates to synthesize mesoscopic inorganic materials with controlled nanoscale porosity, which are expected to have applications in electronics, optics, magnetism, and catalysis [74]. Surfactant structures at the solidliquid interface have also been utilized to stabilize particulate dispersions [7577]. Recent work carried out at UR-ERC has shown that self-assembled surfactants can be utilized to


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prepare particulate dispersions under extreme conditions [78,79] in which traditionally used dispersing methods, such as electrostatics (surface charge), inorganic dispersants (sodium silicate, sodium hexametaphosphate), and polymers, may not result in a completely dispersed suspension. Figure 23 depicts both the suspension turbiditya measure of stability and the surface forces present between the atomic force microscope (AFM) tip and silica substrate as a function of dodecyltrimethylammonium bromide (C12TAB) concentration at pH 4 and 0.1 M NaCl. Under these conditions, the silica suspension without a dispersant is unstable. As the surfactant concentration is increased, the suspension remains unstable until a surfactant concentration of about 8 mM. Between surfactant concentrations of 8 and 10 mM, a sharp transition in the stability (unstable to stable) and forces (no repulsion to repulsion) is observed. A good correlation exists between the suspension stability and repulsive forces due to self-assembled surfactant aggregates. The repulsive force is an order of magnitude higher than electrostatic forces alone, indicating that the repulsion is steric in origin. It was proposed that the dominant repulsion mechanism was the steric repulsion due to the elastic deformation of the self-assembled aggregates when two surfaces approached each other. The adsorption, zeta potential, and contact angle measurements on silica surfaces in 0.1 M NaCl at pH 4.0 as a function of solution C12TAB concen-

FIG. 23 Turbidity of silica particles after 60 min in a solution of 0.1 M NaCl at pH 4 as a function of C12TAB concentration, and the measured interaction forces between an AFM tip and silica substrate under identical solution conditions. (From Ref. 42.)

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tration were measured and are presented in Fig. 24. Based on interface properties, the entire self-assembly process was divided into six stages, marked AF in Fig. 24. At low concentrations (below 0.007 mM, region A in Fig. 24), individual surfactant adsorption takes place. The next structural transition in this system was the formation of hemimicelles, which is evidenced by a signicant effect on both the zeta potential and hydrophobicity of the surface. At approximately 0.1 mM in region B, the sign of the zeta potential reverses but the contact angle continues to increase, indicating that the reversal in zeta potential is not due to formation of bilayers as suggested in the past [73,7577]. This reversal in zeta potential while the hydrophobicity continues to increase was attributed either to hydrophobic association between the surfactant tails, resulting in formation of hemimicelles, or to some kind of specic adsorption. In regions B and C, contact angle continues to increase, indicating increasing concentration of hemimicelles at the interface. Beyond a certain concentration (approximately 2.3 mM), in region D the hydrophobicity decreases, accompanied by a sharp increase in the zeta potential. This indicates the formation of structures with an increasing number of polar heads oriented toward the solution. The sharp increase in zeta potential and a corresponding decrease in contact angle were attributed to the transition of surfactant structure from hemimicelles to either bilayers, spherical aggregates (imaged at surfaces using AFM), or structures having semispheres on top of perfect monolayers (compact monolayer covering the entire surface), as suggested by Johnson and Nagarajan [80]. At higher surfactant concentrations beyond

FIG. 24 Adsorption isotherm (squares), zeta potential (triangles), and contact angle (spheres) of silica surfaces in 0.1 M NaCl at pH 4.0 as a function of solution C12TAB concentration. (From Refs. 42 and 43.)


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the bulk cmc (regions E and F), based on AFM imaging, spherical aggregates or composite semispheres on top of perfect monolayers (it is not possible to distinguish between these structures on the basis of just AFM imaging) are known to exist [81]. Thus, based on the adsorption, zeta potential, and contact angle results, several plausible surfactant structures were proposed at the interface at different concentrations of the surfactant. To illustrate the exact structural transitions taking place at the interface, the FTIRATR (with polarized IR beam) technique, which can probe the adsorbed structures directly, was employed. The FTIRATR technique relies on the fact that individual surfactant molecules, hemimicelles, monolayers, bilayers, and spherical or cylindrical aggregates at the interface will have different average orientations of the alkyl chains with respect to the surface normal. Different average orientations result in different absorptions of the plane-polarized IR beam and can thus be used to identify the surfactant structures at the interface [79]. Based on the FTIRATR study, the proposed surfactant structures in the different regions were veried. In region D, it was found that spherical aggregates were formed directly from hemimicelles, without the formation of bilayers. In fact, no evidence of bilayer formation was seen in this system even at very high surfactant concentrations. Based on the contact angle FTIR, zeta potential, and adsorption results, the preceding structural transitions are summarized by the schematic shown in Fig. 25, which illustrates the structures present at the interface in the concentration regions AF in Fig. 24.

A. Control of the Repulsion Barrier Using CosurfactantsIn bulk micellization processes, it has been proposed that oppositely charged surfactant incorporates itself into micelles and by reducing the repulsion between the ionic groups increases stability and lowers the bulk cmc [82,83]. A similar process can be expected to occur at the solidliquid interface. As depicted in Fig. 26, very small additions of SDS were observed to have a dramatic effect on the formation of the surfactant surface structures. Figure 26 shows the correlation of suspension stability of silica particles with the maximum repulsive force measured against a silica plate in the presence of 3 mM C12TAB and 0.1 M NaCl at pH 4 as a function of addition of SDS. At 5 m SDS addition, no repulsive force is observed between the surfaces. However, at 10 M SDS, strong repulsive force has developed and continues to increase with increase in SDS concentration. Correspondingly, over an identical range of SDS concentration, the initially unstable suspension becomes stabilized.

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FIG. 25 Schematic representation of the proposed self-assembled surfactant lms at concentrations corresponding to AF in Fig. 24. (a) Individual surfactant adsorption, (b) low concentration of hemimicelles on the surface, (c) higher concentration of hemimicelles on the surface, (d) hemimicelles and spherical surfactant aggregates formed due to increased surfactant adsorption and transition of some hemimicelles to spherical aggregates, (e) randomly oriented spherical aggregates at onset of steric repulsive forces, and (f) surface fully covered with randomly oriented spherical aggregates. (From Ref. 43.)

The abnormally low concentration of SDS needed to form the surface surfactant structures may be explained by the preferential adsorption of SDS into self-assembled C12TAB aggregates at the silica surface. Adsorption experiments, using total organic carbon analysis to determine the total amount of adsorbed surfactant and inductively coupled plasma spectroscopy to determine the SDS concentration through the sulfur emission line, have shown that nearly all the SDS added adsorbed at the solidliquid interface. Hence, the molecular ratio at the interface was estimated to be on the order of 1:10 instead of 1:100 in bulk solution. This is particularly interesting because


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FIG. 26 Turbidity of a 0.02 vol% suspension of sol-gelderived 250-nm silica particles after 60 min in a solution of 0.1 M NaCl at pH 4 with 3 mM C12TAB as a function of SDS addition and the measured interaction forces between an AFM tip and silica substrate under identical solution conditions. (From Ref. 42.)

it was found that no measurable quantity of SDS adsorbed onto silica in the absence of C12TAB. In addition, because little SDS is present in solution, the system is far below the bulk cmc and yet strong repulsive forces are once again observed. The use of cosurfactants or other coadsorbing reagents is a critical factor in the utility of a surfactant dispersant in industrial processes. Not only can the concentrations for effective stabilization be reduced, but also many other options can become available to control the overall dispersion of single- and multicomponent suspensions. Availability of these engineered dispersant systems can enhance the processing of nanoparticulate suspensions for emerging specialized end uses, such as chemicalmechanical polishing of silicon wafers in microelectronics manufacturing.

VII. STABILIZATION OF CHEMICALMECHANICAL POLISHING SLURRIES UTILIZING SURFACE-ACTIVE AGENTSChemicalmechanical polishing (CMP) is a widely used technique in microelectronic device manufacturing to achieve multilevel metallization (Fig. 27). In the CMP process, the wafer surface (on which the microelectronic devices are built) is planarized by using a polymeric pad and a slurry com-

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FIG. 27 (Top) Schematic representation of chemicalmechanical polishing (CMP) process. (From http://www.el.utwente.nl/tdm/mmd/projects/polish/index.html.) (Bottom) Review of tungsten CMP: (a) silica (interlayer dielectric) is etched, (b) tungsten is deposited onto silica ILD, and (c) CMP is applied to remove excessive tungsten layer and other levels are built on this level (multilevel metallization). (From Ref. 90.)

posed of submicrometer-size particles and chemical. The ultimate goal of CMP is to achieve an optimal material removal rate while creating an atomically smooth surface nish with a minimal number of defects. This can be accomplished by the combined effect of the chemical and mechanical components of the process. The mechanical action in CMP is mostly provided by the submicrometer-size abrasive particles contained in the slurries as they ow between the pad and the wafer surface under the applied pressure. The chemical effect, on the other hand, is provided by the addition of pH reg-


35

ulators, oxidizers, or stabilizers depending on the type of the CMP operation, which makes it easy for the reacted surface to be removed by abrasive particles. As the rapid advances in the microelectronics industry demand a continuous decrease in the sizes of the microelectronic devices, removal of a very thin layer of material with atomically at and clean surfaces has to be achieved during manufacturing [84]. These trends necessitate improved control of the CMP process by analyzing the slurry particle size distribution and stability effects on polishing. Past investigations suggest the use of monosized particles for the CMP slurries to achieve a planarized surface and to minimize the surface deformation [85]. However, in practical applications there may be oversize particles in the slurries in the form of hard-core larger particles (hard agglomerates) or agglomerates of the primary slurry particles because of slurry instability (soft agglomerates). Polishing tests conducted in the presence of hard agglomerates in the CMP slurries veried signicant degradation in the polishing performance [86]. To remove the hard agglomerates, ltration of slurries is commonly practiced in industrial CMP operations. Nevertheless, even after ltering the slurries, the defect counts on the polished surfaces have been observed to be higher than desired [87]. This observation suggested the possibility of formation of soft agglomerates during the polishing operations. Indeed, it was reported that the commercial CMP slurries tended to coagulate and partially disperse during polishing [88]. Figure 28 shows AFM images of silica wafers polished with soft agglomerated baseline silica slurries of 0.2 m monosize (at pH 10.5). It was observed that even the soft agglomerates resulted in signicant surface deformations [89], indicating that the CMP slurries must remain stable to obtain optimal polishing performance.

A. Stabilization of Alumina Slurries Using Mixed Surfactant Systems for Tungsten ChemicalMechanical PolishingIn CMP processes, polishing slurries have to be stabilized in extreme environments of pH, ionic strength, pressure, and temperature. In tungsten CMP, high concentrations of potassium ferricyanide are used to enhance surface oxidation of tungsten. These species reduce the screening length between the alumina particles of the CMP slurries to near zero, allowing for rapid coagulation of particles and destabilization of dispersions. It has been shown by Palla [90] that addition of a mixture of ionic (SDS) and nonionic (Tween 80) surfactants can stabilize alumina particles in the presence of high concentrations of charged species. Figure 29 illustrates schematically the mechanism of stabilization, which can be explained as enhanced ad-

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FIG. 28 Surface quality response of the silica wafers polished with (a) baseline 0.2- m size, 12 wt% slurry; (b) dry aggregated slurry; (c) PEO occulated slurry; (d) NaCl coagulated slurry. The inverted triangles in the AFM images (left) show the locations corresponding to the sample roughness plots (right). (From Ref. 89.)


37

FIG. 29 Mechanisms of slurry stabilization with SDS (anionic) and Tween 80 (C18PEO, nonionic) surfactants. (From Ref. 90.)

sorption of nonionic surfactant using the strongly adsorbing ionic surfactant as a binding agent. The stabilizing ability of the surfactant system was found to increase with increasing hydrophobicity of both the nonionic and ionic surfactants. The effect of surfactant concentration on stability is shown to have an optimal concentration range for a number of surfactants [90]. As shown in Fig. 30a and b, when the tungsten polishing was conducted using slurries stabilized by the described mixed surfactant system, 30% less material removal was obtained compared with the baseline slurry; however, much better surface quality was obtained [91].

B. Stabilization of Silica CMP Slurries Utilizing Self-Assembled Surfactant Aggregates: Role of ParticleParticle and ParticleSurface Interactions in CMPAs discussed in the previous section, self-assembled C12TAB, a cationic surfactant, provided stability to silica suspensions at high ionic strengths by introducing a strong repulsive force barrier [78,79]. This novel concept has

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FIG. 30 (a) Material removal rate response of the tungsten CMP slurries stabilized with mixed surfactant systems (SDS and Tween 80). (b) Surface roughness response of the tungsten CMP slurries stabilized with mixed surfactant systems (SDS and Tween 80). (From Ref. 91.)

Molecular Intera

adsorption and aggregation of surf act ants in solution

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