Mechanism of aggregation of oil-in-water microemulsion droplets stabilized by dodecyl pentaoxyethylene alcohol, C12E5

Abstract

Modern experimental data allow us to analyse the mechanical and structural properties of a surfactant monolayer covering a microemulsion droplet. In this paper we have analysed theoretically data for oil/water microemulsions in the presence of the surfactant C₁₂E₅. We have determined the spontaneous area per polar head in the surfactant monolayer, a_s, and its dependence on the temperature, a_s(T), which is satisfactorily described by the same straight line for both types of microemulsion droplet containing heptane or tetradecane as oil. The results obtained allow us to conclude qualitatively that the spontaneous area per surfactant polar head is determined by the interaction between the polar heads. At the same time the spontaneous curvature of the monolayer is strongly affected by the penetration of oil molecules between the alkyl chains of the surfactant molecules.

We have proposed a mechanism for the aggregation of two microemulsion droplets during heating of the system. This mechanism is based on the relaxation of the elastic energy of area extension due to the breakdown of the surfactant monolayer during the aggregation process. The numerical evaluation of the corresponding change of the elastic energy supports the considered mechanism.