Microemulsions

Abstract

AbstractMicroemulsions are true, thermodynamically equilibrated liquids that contain a surfactant and/or other amphiphile(s); one or more nonpolar components, such as hydrocarbons; water; and, frequently, inorganic salt (s). The surfactant(s) may be nonionic, ionic, or amphoteric; but most ionic surfactants also require a nonionic cosurfactant or amphiphile to suppress formation of liquid crystals. Microemulsions are distinct from (macro) emulsions, which are thermodynamically unstable dispersions of macroscopic droplets of one or more liquid and/or liquid crystalline phases in a continuous liquid. However, a microemulsion may be one of the phases of an emulsion. The microemulsion may appear as one of three phase, eg, a middle phase in simultaneous equilibrium with a top or upper oleic phase, and a bottom or lower aqueous phase. Because the three phases are close to a tricritical point (where all three phases simultaneously become a single phase), the interfacial tensions among the three phases are several orders of magnitude less than the tensions produced by conventional micellar solutions. Moreover, by suitable changes of temperature, salinity, or other compositional variable, the composition of the middle phase can be made to vary continuously between that of the bottom and top phases. Current or potential industrial uses of microemulsions include agrochemicals, biotechnology, catalysis, cosmetics, detergency, dyeing, enhanced oil recovery, foods, pharmaceuticals,in situremediation of pollutants, separations, and synthesis of nanomaterials. Annual U.S. consumption of surfactants is ∼3.4 billion kg.