The First Rideal Lecture. Microemulsions, a field at the border between lyophobic and lyophilic colloids

Abstract

Microemulsions can be regarded as thermodynamically stable dispersions of droplets of one phase in another phase. There are continuous transitions to swollen micelles, when the droplets become extremely small, and to coarse emulsions when the droplets are large. The mechanism leading to the formation of microemulsions is the tendency to extend the interfacial area until the concentrations of surfactants are sufficiently low that non-negative interfacial tension is achieved. The role of the combination of a surfactant and a cosurfactant is seen in their leading to a decrease in the interfacial tension to potentially negative values. An expression is given for the Gibbs energy of the whole system, for the special case of an ionized surfactant and a non-ionized cosurfactant. The minimization of this Gibbs energy leads to the stability conditions for the microemulsion. The factors which promote O/W and W/O emulsions, respectively, are briefly discussed. The electrical contribution to the interfacial tension depends on the particle size. This leads to a narrow size distribution. It is finally pointed out that, in principle, fine dispersions of solids in a liquid may be thermodynamically stable in the presence of large amounts of easily adsorbed compounds.