THERMODYNAMICS OF THE L(3) (SPONGE) PHASE IN THE FLEXIBLE SURFACE MODEL

The thermodynamics of the L(3) (sponge) phase is investigated within t he flexible surface model. The well-established leading-order phi(3) s caling for the free energy density (where phi is the surfactant volume fraction) requires modification in order to describe the narrow chara cter of the phase and the observed sequence of phase transitions. We f ind that higher order contributions to the free energy density from th e local curvature energy provide a straightforward mechanism to accoun t for these features. The phase equilibria with the dilute solution on one side of the phase and the lamellar phase on the other are evaluat ed for a model binary (surfactant-solvent) system, and calculated phas e diagrams are presented. Experimental evidence in support of the mode l comes from static light scattering experiments on the pseudobinary s ystem AOT-NaNO3-water. We calculate the forward scattering intensity a long chosen dilution lines within the phase using the model, and predi ct that the scattering intensity varies significantly as we move from one phase boundary to the other at constant phi. The experimental resu lts confirm these predictions, and provide strong evidence in support of the proposed model.