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Abstract

Water and oil can be made completely miscible by adding a sufficient amount of an amphiphilic compound, such as soap or a detergent. For historical reasons, such stable homogeneous solutions are called "microemulsions." In this article the term microemulsion is used in a more restrictive manner; at low concentrations of the amphiphile, mixtures of water, nonpolar solvents, and amphiphiles may separate into three coexisting liquid layers, namely, an aqueous phase, an amphiphile-rich phase, and an oil-rich phase. In the amphiphile-rich phase, which is the microemulsion in the narrower sense, one finds for thermodynamic reasons a maximum of the mutual solubility between water and oil, combined with a minimum of the interfacial tension between the aqueous and the oil-rich phase, properties that are of interest for both theory and application. The present state of art in this rapidly growing field of science is reviewed.