SPONGE PHASES IN SURFACTANT SOLUTIONS

In surfactant solutions the amphiphilic molecules can self-assemble reversibly into a variety of structures. In many cases the most stable structural unit is a bilayer which, although locally flat, tends to wander entropically at large distances. This can result in a stable isotropic phase, known as L3, which consists of a spongelike "random surface" of bilayer that divides space into two interpenetrating solvent labyrinths. This phase has unusual thermodynamics: in particular, it can support a second-order phase transition of Ising type from a symmetric state, in which the two solvent regions have equivalent statistics, to an asymmetric state in which they differ. In this article we survey the present understanding of this phase in relation to the theory of elasticity for two-dimensional fluid films. We then focus more closely on the thermodynamics of the system, which we develop within the framework of Landau-Ginzburg model with two coupled order parameters. This reproduces correctly several interesting features of the phase diagram and also explains the very unusual correlation functions of the sponge phase that are measured in static light scattering. We present evidence for a line of second-order phase transitions between symmetric and asymmetric states in a sponge system and show that the correlation functions are different on the two sides of the transition, in accordance with theory. It is argued that sponge phases may provide an ideal testing ground for theories of tricritical points and higher order critical behavior.