Phase transition between microemulsion and lamellar phases in a C12E5/water/n-octane amphiphilic system

A ternary amphiphilic system consisting of water, n-octane and C12E5 has been studied by small angle X-ray scattering (SAXS) to elucidate the mechanism of phase transition between lamellar and microemulsion phases. SAXS profiles in the region between lamellar and microemulsion phases show two distinct peaks, which indicate that different structures coexist. These structures change continuously as transition goes on, which is in good agreement with the lever rule in phase diagram. The transition behavior has been explained as a result of the change of curvatures of the interfacial film as a function of temperature and subsequent change of bending elastic energy. A boundary of the coexistence region is determined by the double tangent construction for the free energies of the microemulsion and lamellar phases.