Microemulsions and the flexibility of oil/water interfaces

Abstract

The purpose of this review is to understand why a random structure of this type does not collapse into an ordered phase. The interface saturated by surfactant has a nearly vanishing surface tension; one essential parameter is then the elastic constant K describing the curvature elasticity of the interface. The persistence length xi/sub K/ of the interface increases exponentially with K. This should have some important effects. (1) When K is above a certain value K/sub c/ the interfaces tend to stack or, more generally, to build up a periodic, stable, phase. (2) When K is below K/sub c/ the interface can become extremely wrinkled and the resulting gain in entropy is larger than the loss of energy due to the departure from a periodic array. This case K < K/sub c/ would correspond to microemulsions. In this picture one major effect of cosurfactants (additives which favor the mocroemulsion phase) is to increase the flexibility of the layers. 10 figures.