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.