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 su rfactant 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 kap pa(-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 arc tangent form and-depending on the value of k(0) xi- more or less prono unced irregularity at about 2k(0).