ABSTRACT
Due to their amphiphilic nature, surfactants solubilize water and oil and form a wide variety of thermodynamically stable phases. Indeed, mixtures of amphiphiles, water, and oil may give rise to either isotropic phases (microemulsion, sponge) or lyotropic liquid crystalline phases (lamellar phase, hexagonal phase) [1–4]. The term “microemulsion” introduced in 1959 by Schulman et al. [5] in its most general use connotes a thermodynamically stable, fluid oil-water-surfactant mixture in which the oil and water regions are well separated and the surfactant molecules are organized as monolayers at the internal water/oil interfaces. Microemulsions often exist over a wide range of water and oil concentrations and can be formed with a few percent of surfactant. Many also contain alcohol and salt. The configuration of the oil and water domains varies with composition; for small fractions of oil in water (O/W) or of water in oil (W/O), the structure is that of swollen micelles. When the volume fractions of oil and water are comparable, a random bicontinuous structure is found. The first suggestion of this type of structure came from Scriven [6]. The oil regions have the structure of a connected random network, and the same property also holds for the water regions, the surfactant molecules making a continuous film between the oil and water domains with no long-range order such as that found in lyotropic liquid crystals.
