X-ray and neutron-scattering study of the lamellar and L₃ phases of the system aerosol-OT-water: effect of NaCl and decane

Abstract

The phase diagrams of the system AOT-H₂O(D₂O)-NaCl (at 20° C) with and without decane is determined in the water-rich region in order to look at the effect of adding salt and/or oil on theL _α andL ₃ phases. It is seen that salt and oil play the same role that the alcohol to surfactant ratio in the systems surfactant-alcohol-brine. The local structure of these phases is obtained by smallangle x-ray and neutron-scattering experiment. It consists of infinite membranes composed of simple bilayers of AOT in the ternary system, or bilayers swollen by oil in the quaternary system. Assuming that salt and oil act on the spontaneous curvature of the AOT monolayers constituting the bilayers, these results are compatible with a description in which the transitionL _α toL ₃ is controlled (at constant volume fraction of bilayers) by the Gaussian curvature rigidity.