Abstract
Using small angle X-ray scattering technique, we study the remarkable stability of the sponge phase domain which extends continuously from the brine rich corner to the concentrated side of the phase diagram of a cationic surfactant+alcohol+brine system. The scattering data show the persistence of the elementary bilayer structure, over a very wide range of dilution, while a swelling behaviour is observed independent of sensitive changes of salinity and temperature. We discuss this swelling behaviour on the basis of three different dilution laws which were proposed for a symmetric sponge phase.