Phase Behavior and Structure of Microemulsions

Abstract

Various pathways can be employed to prepare microemulsions. One of them starts from micellar solutions. Surfactants dissolved in water form, above the critical micelle concentration, a large number of micelles. Hydrocarbon molecules, though sparingly soluble in water, can be solubilized in the hydrophobic core of the micellar aggregates. The solubilized molecules are usually located among the hydrocarbon tails of the micelles. For some surfactants or, more generally, when a medium chain length alcohol (a cosurfactant) is also present, the solubilized molecules can form a core covered by a layer of surfactant and alcohol molecules. Of course, some hydrocarbon molecules and part of such molecules remain located among the hydrocarbon tails of the interfacial layer of surfactant and cosurfactant. A dispersion containing the latter kind of microstructures is called a microemulsion. Micellar aggregates as well as solubilized micellar aggregates containing hydrocarbon molecules among the hydrocarbon tails of the surfactant molecules are thermodynamically stable. This suggests, by extrapolation, that the microemulsions can also constitute thermodynamically stable dispersions.