Neutron scattering study of a membrane phase miscibility gap: Coexistence of L3 "sponge" and Lα Lamellar phases

Abstract

We report on a small angle neutron scattering (SANS) study of a temperature driven first order phase transition in a 25wt% solution of the surfactant AOT (Sodium Di-2-ethylhexyl Sulfosuccinate) in 1.5wt% heavy brine between an isotropic L3 "sponge" state at 27°C and a stacked lamellar Lα monophase 55°C. The prominent scattering features of these phases are correlation peaks due to the mean passage size of the L3 sponge and the Lα stacking separation. This ratio of the monophase peak positions Qα/Q3≈1.3, is consistent with previous observations in this and similar systems. In the present study we tracked this system through the intermediate L3 +Lα biphasic miscibility gap. There the initial appearance of the Lα peak at 33.25°C was at a scattering vector some 23% higher than the final high temperature monophase value. During coexistence both L3 and Lα phase peak positions decreased linearly with increasing temperature maintaining a roughly constant ratio Qα/Q3 ∼1.6-1.7. Two phase fits to the scattering data and application of scaling law predictions allow us to obtain local L3 phase volume fractions in the biphasic region and make preliminary determinations of the structural accomodations necessitated by phase coexistence in this system's miscibility gap.