Stability and Phase Behavior of Mixed-Surfactant Vesicles
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STABILITY AND PHASE BEHAVIOR OF MIXED-SURFACTANT VESICLES F. C. MacKintosh* and S. A. Safran*" "Department of Physics, University of Michigan, Ann Arbor, MI 48109-1120 "-Department of Materials and Interfaces, Weizmann Institute, Rehovot, Israel, 76100 ABSTRACT While ideal fluid membranes are characterized solely by curvature elasticity, many structural ("external") properties of real membranes are strongly influenced by "internal degrees of freedom" . In particular, recent experimental reports of stable vesicles in surfactant mixtures seem to illustrate the important interplay between composition and curvature in bilayer membranes. Such stable vesicles are unexpected in single-surfactant bilayers. We show theoretically how the energetic stabilization of mixed vesicles can occur in mixed-surfactant systems. This is illustrated by a microscopic model of mixed-surfactant bilayers, which is treated within both the random mixing and strongly interacting limits. The predictions of the ranges of stability of the various phases as a function of the three concentrations (solvent - e.g., water - and the two amphiphiles) is in qualitative agreement with recent experiments. In addition, we consider the effect of phase separation in a mixed-surfactant system. In contrast with a bulk system, we find that for a binary mixture in a two dimensional fluid bilayer membrane, phase separation determines a length scale for the formation of stable vesicles. This results in a stable, one phase vesicle region near the critical composition, with a simple dependence of the vesicle size on composition. We also find regions of coexistence of vesicles with lamellae. 1. INTRODUCTION Two-dimensional fluid membranes composed of amphiphilic molecules are of importance in a variety of biological, chemical as well as physical contexts. In contrast with ordinary three-dimensional solids and liquids, these membranes can be regarded as incompressible two-dimensional fluids, which have the ability, to bend into the third dimension. Biological cell membranes provide the most important natural examples of such fluid membranes. There are also many examples of synthetic fluid membranes, such as oil-water-surfactant microemulsions and surfactant bilayer membranes in water. Among the most important applications of synthetic bilayer membranes are simple, closed vesicles. Unilamellar vesicles are of broad interest as models of biological membranes as well as for various applications involving microencapsulation. Large, stable vesicles of controlled size are most desirable for such applications. The formation of vesicles in single surfactant systems, however, often requires the input of mechanical energy into the system, for example, by sonication. The resulting vesicles are only metastable, or are stable only at very low volume fractions. Equilibrium, or spontaneous vesicles have been reported, however, in mixed surfactant systems [1]. In particular, Kaler et al. [2] recently reported a very general method of mixing anionic and cationic surfactants, which form
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