Phenomena in Mixed Surfactant Systems In Phenomena in Mixed Surfactant Systems; Scamehorn, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986. In Phenomena in Mixed Surfactant Systems; Scamehorn, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986. ACS S Y M P O S I U M SERIES 311 Phenomena in Mixed Surfactant Systems John F. Scamehorn, EDITOR University of Oklahoma Developed from a symposium sponsored by the ACS Division of Colloid and Surface Chemistry at the 59th Colloid and Surface Science Symposium and the 5th International Conference on Surface and Colloid Science, Potsdam, New York, June 24-28, 1985 American Chemical Society, Washington, DC 1986 In Phenomena in Mixed Surfactant Systems; Scamehorn, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986. Library of Congress Cataloging-in-Publication Data Phenomena in mixed surfactant systems. (ACS symposium series; 311) Bibliography: p. Includes index. 1. Surface active agents—Congresses. I. Scamehorn, John F., 1953 . II. American Chemical Society. Division of Colloid and Surface Chemistry. III. Colloid and Surfac Symposium (59th: Potsdam, N.Y. IV. International Conference on Science (5th: Potsdam, N.Y.: 1985) V. Series. TP994.P48 1986 541.3'45 86-8062 ISBN 0-8412-0975-8 Copyright © 1986 American Chemical Society All Rights Reserved. The appearance of the code at the bottom of the first page of each chapter in this volume indicates the copyright owner's consent that reprographic copies of the chapter may be made for personal or internal use or for the personal or internal use of specific clients. This consent is given on the condition, however, that the copier pay the stated per copy fee through the Copyright Clearance Center, Inc., 27 Congress Street, Salem, MA 01970, for copying beyond that permitted by Sections 107 or 108 of the U.S. Copyright Law. 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McGinniss C. Grant Willson Battelle Columbus Laboratories IBM Research Department In Phenomena in Mixed Surfactant Systems; Scamehorn, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986. FOREWORD The ACS SYMPOSIUM SERIES was founded in 1974 to provide a medium for publishing symposia quickly in book form. The format of the Series parallels that of the continuing ADVANCES IN CHEMISTRY SERIES except that, in order to save time, the papers are not typese by the authors in camera-read the supervision of the Editors with the assistance of the Series Advisory Board and are selected to maintain the integrity of the symposia; however, verbatim reproductions of previously pub lished papers are not accepted. Both reviews and reports of research are acceptable, because symposia may embrace both types of presentation. In Phenomena in Mixed Surfactant Systems; Scamehorn, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986. PREFACE ^N^IXED SURFACTANT SYSTEMS are of scientific interest as well as technological value. Surfactants are used in numerous applications, including detergents, flotation, enhanced oil recovery, surface-wetting modification, foaming control, emulsification, controlled-release, and surfactant-based separation processes. Commercial surfactants are almost always composed of mixtures of surfactants. The production of single-component surfactants is generally expensive, an surfactants are rarely bette may exhibit superior behavior compared to the pure surfactant components. It is an enormous challenge to understand the interactions between different surfactant components in the various applications in which surfactants are used. This book presents chapters that discuss research on surfactant mixture behavior from a variety of active researchers around the world. I am grateful to the organizing committee of the symposium on which this book is based for allowing me to organize and chair sessions on this topic and especially to Josip Kratohvil for organizing the logistics of the program in such an efficient fashion. I thank departmental staff Polly Dvorak, Sherry Childress, and Rick Wheeler for their efficient help with correspondence associated with the book. Kevin Stellner and Jim Rathman helped me by proofreading the first draft of the manuscripts. Cuong Nguyen provided the figure upon which the drawing on the cover of this book is based. I would also like to thank the authors who participated in this effort and the reviewers who must remain anonymous. Finally, I would like to thank my colleagues and graduate students at the University of Oklahoma for stimulating interactions and for helping to keep my perspective fresh and my interest in surfactants high. JOHN F. SCAMEHORN School of Chemical Engineering and Materials Science University of Oklahoma Norman, OK 73019 January 6, 1986 ix In Phenomena in Mixed Surfactant Systems; Scamehorn, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986. 1 An Overview of Phenomena Involving Surfactant Mixtures John F. Scamehorn School of Chemical Engineering and Materials Science, University of Oklahoma, Norman, OK 73019 The effect o on micelle formation, monolayer formation, solubilization, adsorption, precipitation, and cloud point phenomena is discussed. Mechanisms of surfactant interaction and some models useful in describing these phenomena are outlined. The use of surfactant mixtures to solve technological problems is also considered. Surfactants used in practical applications essentially always consist of a mixture of surface-active compounds. Isomerically pure surfactants are often expensive to produce and generally have only a small potential advantage in performance over the less expensive surfactant mixtures. In many applications, mixtures of dissimilar surfactants can have superior properties to those of the individual surfactant components involved. These synergistic properties of surfactant mixtures have provided impetus for much of the research on interactions between surfactants. Individual surfactants vary 'in their tendency to form aggregated structures. Examples of such aggregates are micelles, precipitate, and monolayers. In solutions containing mixtures of surfactants, the tendency to form aggregated structures can be substantially different than in solutions containing only the pure surfactants involved. For example, precipitation may not occur in a surfactant mixture whose components individually precipitate when present as single components. The tendency for components to distribute themselves between the unaggregated state and an aggregate may vary from component to component for mixtures. Therefore, for 0097-6156/ 86/ 0311 -0001 $08.00/ 0 © 1986 American Chemical Society In Phenomena in Mixed Surfactant Systems; Scamehorn, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986. 2 PHENOMENA IN MIXED SURFACTANT SYSTEMS example, the surfactant composition of a micelle may differ greatly from that of the surfactant monomer with which i t i s in equilibrium. This i s important because the processes of interest may depend only on either monomer composition or on aggregate composition. For example, adsorption of surfactant on solids such as minerals depends only on monomer composition and concentration, not micellar properties. On the other hand, solubilization of compounds into micelles depends only on mi cellar composition. The surfactant technologist needs to be able to predict and manipulate the tendency for surfactant mixtures to form aggregates, the properties of these aggregates, and the distribution of surfactant components between monomer and aggregate. A central theme of this paper i s that mixtures of surfactants can achieve great synergisms in variou relative tendency t Often, the formatio o a certai aggregate inhibit the formation of a less desirable aggregate. For example, addition of nonionic surfactants to anionic surfactants enhances the formation of micelles, resulting in a reduced tendency for the anionic surfactant to precipitate. This overview will outline surfactant mixture properties and behavior in selected phenomena. Because of space limitations, not a l l of the many physical processes involving surfactant mixtures can be considered here, but some which are important and illustrative will be discussed: these are micelle formation, monolayer formation, solubilization, surfactant precipitation, surfactant adsorption on solids, and cloud point phenomena. Mechanisms of surfactant interaction will be discussed, as well as mathematical models which have been shown to be useful in describing these systems. Practical applications will be covered as part of the consideration of individual phenomena. This overview will attempt to outline the state of current knowledge, without much comment on the areas in which further research i s needed, the direction the field is taking, and the impact of the other chapters in this book. These are reserved for the Future Perspectives Chapter (last chapter of the book). Micelle Formation The structure and thermodynamics of formation of mixed micelles i s of great theoretical interest. Micelles are also present and often integrally involved in practical processes. For example, in a small pore volume surfactant flooding process (sometimes called micellar flooding), the solution injected into an o i l field generally contains 5-12 weight V, surfactant <1) and the surfactant i s predominately in micellar form in the reservoir water. In detergency, solubilization can be In Phenomena in Mixed Surfactant Systems; Scamehorn, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986. 1. SCAMEHORN Overview of Phenomena 3 important (2), so micelles are generally present in the detergent solution. In mi eellar-enhanced ultra-filtration, a separation technique to remove dissolved organic from water (3), micelles effect the separation. Monomer-Micelle Equilibria. The distribution of surfactant components between micelles and monomeric state in aqueous solutions depends on surfactant structures as well as on overall solution composition. For example, for a binary system of surfactants A and Β in solution, the micelle may contain 50 mole 7, A/50 7» Β while the monomer may be 90 7, A/10 7. B. Since either the monomer or the micelle composition may be crucial to behavior of the system, the ability to predict the relative distribution of surfactant components between monomer and micelle composition, i s an importan Except for some anionic/cationic surfactant mixtures which form ion pairs, in a typical surfactant solution, the concentration of the surfactant components as monomeric species i s so dilute that no significant interactions between surfactant monomers occur. Therefore, the monomer—micelle equilibria i s dictated by the tendency of the surfactant components to form micelles and the interaction between surfactants in the micelle. Prediction of monomer-micelle equilibria reduces to modeling of the thermodynamics of mixed micelle formation. The behavior of mixed micelles i s commonly approximated by using the pseudo-phase separation model (4). This considers the micelles to be a separate thermodynamic phase in equilibrium with the monomer. Monomer—micelle equilibria then becomes analogous to vapor-liquid equilibria; i.e., in both cases a dilute phase with little intermolecular interaction i s in equilibrium with a concentrated phase in which intercomponent interaction can be significant. The vast Array of solution thermodynamic models developed for mixing in macroscopic phases can be utilized for mixed micelles using the pseudo-phase approach. The pseudo- phase separation model i s a good approximation when the mi cellar aggregation number i s greater than about 50 (5), which i s commonly the case for surfactants of commercial importance. Ideal Mixed Micelles. The Critical Micelle Concentration (CMC) i s the lowest surfactant concentration at which micelles form; the lower the CMC, the greater the tendency of a system to form micelles. When the total surfactant concentration equals the CMC, an infintesimal fraction of surfactant i s present as micelles; therefore, the CMC i s equal to the total monomer concentration in equilibrium with the micellar pseudo-phase. The CMC for monomer-mieelle equilibrium i s analogous to the dew point in vapor-liquid equilibrium. In Phenomena in Mixed Surfactant Systems; Scamehorn, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.