Stability and phase behavior of mixed surfactant vesicles

Abstract

Although large, spherical surfactant vesicles are generally unstable to either lamellar or micellar phases, mixtures of two surfactants can lead to spontaneous vesicle formation. We show theoretically how the energetic stabilization of mixed vesicles can occur by considering the curvature elasticity of the surfactant bilayer. Interactions between the two species (of the proper sign and magnitude) are crucial to stabilizing these vesicles. These interactions lead to composition asymmetries and effective spontaneous curvatures of the inner and outer layers that are of equal and opposite signs. The vesicles have a Gaussian distribution about an average size determined by the effective spontaneous curvature; the width of the distribution is calculated as a function of concentration. The stability of these vesicles with respect to a flat lamellar phase is estimated. 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) are in qualitative agreement with recent experiments.