RANDOM INTERFACE MODEL OF SPONGE PHASES

We present a coherent structural and thermodynamic approach to sponge phases of surfactant solutions based on the curvature energy of the surfactant film characterized by the bending moduli kappa, (kappa) over bar, the surfactant concentration phi(S) and the inside/outside ratio phi. We find that an entropy-driven transition between symmetric (S) and asymmetric (A) sponge phases with a critical phi(B) similar to kappa(-1/3) is possible. Film and bulk scattering are given consistently, in terms of the correlation length xi(kappa, (kappa) over bar, phi(S)) and the structural wave vector k(0)(kappa, (kappa) over bar, phi(S)). The film scattering at small wave vectors has approximately an arctangent form and-depending on the value of k(0) xi- more or less pronounced irregularity at about 2k(0).