ABSTRACT
Emulsions are thermodynamically unstable in the sense that complete separation reduces inter-facial area and hence interfacial free energy. The thermodynamic stability of microemulsions, much like that of surfactant micelles, stems from removal of hydrocarbon chains of the surfactant molecules from contact with water and from the large entropy of dispersion of many small entities. Emulsions can be formed by either mechanically dispersing one phase in the other or creating conditions in one liquid where drops of another are formed as a result of chemical reaction or supersaturation. Another approach for producing oil-in-water nanoemulsions is rapid dilution with water of a properly chosen oil–surfactant mixture. Sedimentation/creaming, flocculation, and coalescence are not the only phenomena that can produce changes in emulsion properties. Knowledge of emulsions can generally be applied to nanoemulsions, but the magnitude of some important effects is very different. Much higher shear rates are required to generate nanoemulsions than ordinary emulsions by mechanical means.
