Membrane osmometry of solubilized systems

Abstract

Abstract The solubilization of decane, ethyl p-hydroxybenzoate, methyl anisate and p-hydroxybenzoic acid by aqueous micellar solutions of cetomacrogol 1000 has been examined by membrane osmometry and viscosity techniques. The effect of the solubilizates on the number-average micellar molecular weight, Mn, has been related to their site of incorporation in the micelle. Decane and methyl anisate are solubilized in the hydrocarbon core of the micelles and both compounds produce an increase in Mn up to a maximum value of 2·0 times 105, at a solubilizate concentration of approximately 80% of the saturation limit for each compound. This increase is shown to result from an increase in the number of molecules of both solubilizate and of surfactant per micelle. Further addition of solubilizate, to produce a saturation level in excess of 80%, results in a decrease in micellar size in both systems. The solubilization of ethyl p-hydroxybenzoate and p-hydroxybenzoic acid is thought to involve the oxyethylene region of the micelle and both solubilizates cause an increase in Mn owing to the inclusion of solubilizate into the micelle, the number of molecules of surfactant per micelle being unaffected by the solubilization process. Viscosity studies on the decane-ceto-macrogol-water system are interpreted as indicating no change in either micellar symmetry or hydration as a result of solubilization. A spherical model for the micelles is shown to be consistent with the experimental data.