Thermodynamic Modeling of the L3 (Sponge) Phase with Electrostatic Contribution to the Bending Energy in a Ternary System (Aerosol OT/NaCl/Water)

The L 3 (sponge) phase is a disordered fluid membrane phase where an infinite topologically complex bilayer separates the solvent into two separate domains. In this work we have extended a previous description of the phase equilibria in terms of the flexible surface model to also account for the concentration-dependent electrostatic interactions. We consider a ternary system surfactant/salt/water where all components are explicitly described. The contributions to the free energy are separated into an electrostatic and a nonelectrostatic part, and we calculate the phase equlibria modeling in addition to the sponge phase also the lamellar phase and an isotropic solution, L 1, dilute in surfactant. The theoretical results have been compared with experimental data for the Aerosol OT/NaCl/water system. The model captures most of the typical features of the experimental phase diagram: the narrowness of the sponge one phase region, the characteristic sequences of phases, and the variation in the electrolyte concentration of the L 1 solution in equilibrium with the L 3 phase. There is a discrepancy between theory and experiment with respect to the width of the two phase area separating the sponge and lamellar phases.