Microemulsions: a qualitative thermodynamic approach

Microemulsions, that is, stable colloidal dispersions of water and nonpolar solvents stabilized by amphiphiles, are of growing interest in research and industry.The phase behavior of the multicomponent mixture is essentially determined by the features of corresponding binary mixtures.The efficiency of an amphiphile in solubilizing the solvents reaches its maximum in the temperature interval in which the mixture separates into three coexisting liquid phases.The domain size of the dispersion is determined by the interfacial tension between the aqueous and the oil-rich phase in the presence of a saturated monolayer.Because the interfacial tension reaches its minimum in the three-phase interval and, furthermore, decreases with increasing amphiphilicity, the transition from weakly structured solutions to microemulsions is gradual.It is, therefore, suggested that microemulsions be defined as stable colloidal dispersions of domains sufficiently large for the dispersed solvent to exhibit the properties as, e.g., the dielectric number of a bulk phase.