Microemulsions: a Landau-Ginzburg theory

We present a Landau-Ginzburg theory for oil-water-surfactant mixtures with a scalar order parameter ψ for the oil-water subsystem and a vector τ for the surfactant part. A physical interpretation of the parameters in terms of single-membrane energetics is given. A detailed density-functional calculation yields three-phase coexistence between oil-rich, water-rich, and microemulsion phases, and modulated phases including lamellar, columnar, and cubic phases. The computed structure factor for water in the microemulsion phase shows a variety of trends in qualitative accord with experiments