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

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 L-3 "sponge" state at 27 degrees C and a stacked lamellar L-alpha monophase 55 degrees C. The prominent scattering features of these phases are correlation peaks due to the mean passage size of the L-3 sponge and the L-alpha stacking separation. This ratio of the monophase peak positions Q(alpha)/Q(3)approximate to 1.3, is consistent with previous observations in this and similar systems. In the present study we tracked this system through the intermediate L-3 + L-alpha biphasic miscibility gap. There the initial appearance of the L-alpha peak at 33.25 degrees C was at a scattering vector some 23% higher than the final high temperature monophase value. During coexistence both L-3 and L-alpha phase peak positions decreased linearly with increasing temperature maintaining a roughly constant ratio Q(alpha)/Q(3) similar to 1.6-1.7. Two phase fits to the scattering data and application of scaling law predictions allow us to obtain local L-3 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.