SWOLLEN MICELLAR MODELS FOR SOLUBILIZATION

Two models are examined from a thermodynamic point of view when a single surfactant is the solubilizing agent. The solubilized system is assumed to contain either swollen micelles with a core of solubilized molecules or swollen-hydrated micelles having water both in the central core and in the large polar head groups. An equation is established for the free energy of the system which includes, in addition to the terms due to interfacial tension and to the chemical potentials of the components in the dispersed and continuous phases, the free energy due to the entropy of dispersion of the globules in the continuous phase. The minimum of the free energy with respect to the radius provides an equation for the radius. The domain of existence of thermodynamically stable solubilized systems is identified by deriving equations for its boundaries. One of them, the solubilization curve, is obtained from the equilibrium condition between the solubilized system and solubilized molecules, whereas the other boundary, the cloud (haze) point curve, is obtained from the equilibrium between the solubilized system and surfactant. The main conclusions are: (a) The entropy of dispersion of the globules in the continuous phase plays a major role in determining the solubilization curve, (b) the solubilization of water in oil by nonionic surfactants with polyoxyethylene head groups can be interpreted by the swollen-hydrated micelle model, and (c) for oil solubilized in water, a different model is needed. In this case, the molecules of oil can probably be found between the hydrocarbon chains of the amphiphiles. © 1979.