Toward understanding microemulsion microstructure: A small-angle x-ray scattering study

The microemulsion phases formed in solutions of octane, commercial surfactant, and alcohol with various brines are examined with small-angle x-ray scattering (SAXS), electrical conductivity, and viscosity techniques. Models based on monodisperse populations of swollen micelles or microemulsion "droplets" adequately represent the SAXS data at low volume fractions of brine. Introduction of hard-sphere interactions with the Percus-Yevick approximation allows us to model the composition dependence of the radius of gyration and isothermal compressibility up to volume fractions of brine near a percolation threshold for electrical conductivity. For brine volume fractions above the percolation threshold, a mean field attractive interaction term is needed to model the variation of isothermal compressibility; however, the same theory fails to model the composition dependence of the apparent radius of gyration. But predictions from a model for a bicontinuous microemulsion structure that is geometrically irregular yet topologically ordered and that evolves continuously into swollen (inverted) micellar solutions at low volume fraction of water (oil) are in good agreement with the SAXS and electrical conductivity data over a wide range of brine volume fractions.