On the Thermodynamics of Reverse Micelles: Effect of Water on Micellization

Abstract

Water may be involved in reverse micelle formation either in the form of a hydration shell or as a solubilisate. Both cases have been analyzed within the framework of a theory based on the mass action law and the definition of the critical micelle concentration (CMC) via the constant of this law. In the first case, it is supposed that a surfactant is primordially hydrated, and water is a priori incorporated into the surfactant. Its amount is determined from the volume of the hydrated core of a micelle, while the aggregation number of the surfactant is calculated from the packing conditions for spherical, cylindrical, and platelike micelles. The CMC is, in turn, found from its dependence on the aggregation number, with this dependence being determined by the general theory. In the second case, water is taken into account as a separate component of a micelle, but in the form of a solubilisate (hydration water is, as before, introduced into the chemical formula of the surfactant). Here, the CMC appears to depend on not only the overall concentration of water (with no regard to its hydration form), but also on the value of the critical degree of water micellization. The effect of water on micellization has been analyzed at both its preset chemical potential and its constant amount. All versions of the theory lead to the conclusion that CMC decreases with an increase in water content, thereby confirming the prediction put forward by Eicke and Christen (Eicke, H-F. and Christen, H., Helv. Chim. Acta, 1978, vol. 61, p. 2258).