Mechanism of microemulsion formation in systems with low interfacial tension: Occurrence, properties, and behavior of microemulsions

Abstract

The mechanism of microemulsion formation is developed. It is shown that the process is controlled by a dynamic equilibrium, in which the rate of self-emulsification from one or both of the liquid-liquid boundaries is equal to the rate of coalescence within the microemulsion. The theory gives an understanding as to which of the liquid-liquid interfaces is responsible for spontaneous emulsification, and as to the nature of the “internal” phase. The methodology of experimental investigation of microemulsions is worked out. This methodology establishes the possibility of predicting dynamic and equilibrium characteristics. The experimental quantities that are needed can be measured directly; and in some cases, the necessary data are already available. The equation that was derived has no adjustable constants. It predicts the dependence of the radii of droplets on their concentration in the microemulsion as well as on interfacial tension and temperature. The analysis of experimental data shows that the size of nascent droplets, and average equilibrium drop dimensions, increase with decrease of interfacial tension. This trend agrees with published data. The theory gives an explanation of “phase inversion” in microemulsions. Existing data was analyzed and the dimensions of droplets near the interface were calculated, where they are freshly formed (i.e., nascent droplets) as well as the equilibrium drop dimensions. There is an excellent agreement between predictions and experimental results.