Summary. The formation of a so-called water/oil (W/O)-microemulsion (AOT/isooctane/water) was followed by light scattering and ultracentrifuge measurements. With increasing weighed-in water concentrations the microemulsion is stabilized by repeated aggregational processes of micelles containing water due to a decrease of the free intcrfacial enthalpy.This process conforms very satisfactorily to a model describing an adsorption of surfactants at the water/hydrocarbon interface resulting from dipole-image dipole interactions.While there exists already a large number of experimental results on so-called microemulsions formed by nonionic and ionic surfactants in aqueous and nonaqueous media (e.g. [l-41) relatively little is known quantitatively with regard to the mode of origin of these microemulsions. In a preceding paper details of the solubilization process and micellar aggregation phenomena have been investigated with the help of dielectric increment measurements [5]. More recently the exchange of water and aqueous electrolyte solutions between micelles in apolar media was followed by fluorescence quenching experiments. Such an exchange was to be expected as a necessary prerequisite with respect to the experimentally observed considerable inonodispersity of lipophilic micelles containing polar liquids [6]. Finally, the stabilizing effect of polar liquids on lipophilic micelles was predicted theoretically on the basis of dipole-image dipole interactions between the solubilized water 'pool' and the surfactant molecule [7].In order to check the predictions of this theoretical approach and to investigate details of the micellar aggregation process and thus of the formation of a so-called W/O-microemulsion a micellar system with a particular high solubilization ratio (water/surfactant) was selected.Finally, it is believed that the present investigation may contribute towards a clarification as to the thermodynamic stability of these systems.Materials and Methods. -Aerosol OT (= AOT = Sodium di-2-ethylhexylsulfosuccinate, Mo1.-Wt. = 444.56) was used as a suitable representative model of an anionic surfactant because of its pronounced solubilizing capacity in some nonpolar organic solvents. The compound, a commercial product (Fluka AG, Buchs/SG) of pharmaceutical quality, had been purified and dried additionally before use. The procedure is described elsewhere [ 8 ] .According to preliminary investigations isooctanc was chosen as the most suitable solvcnt. The commercial product (Fluka AG, Buchs/SG) of high grade purity was distilled over 'Siccon' followed by a continous distillation over sodium/potassium alloy. The residual water content of the isooctane was determined by the Karl-Fischer-method to bc 0.0052%. Bidistilled water was used as solubilizate.

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