RANDOM BILAYER PHASES OF DILUTE SURFACTANT SOLUTIONS

Surfactant molecules in dilute solution may aggregate reversibly into extended structures. For suitably chosen molecules, the preferred packing involves a locally flat bilayer, which tends to wander entropically at large distances. At low temperatures (and/or high concentrations) the system forms a stack of flat sheets with one-dimensional quasi-long range order (a smectic liquid crystal), but at high temperatures or low concentrations, the stack can melt into a random surface structure that resembles a multiply connected labyrinth or 'sponge' or bilayer in a sea of solvent. Recent theoretical and experimental progress in understanding the properties of the sponge is reviewed. We argue that the sponge phase may provide a good system for the study of various liquid-state critical phenomena.