Aqueous sodium oleate–sodium dehydrocholate mixtures at low concentration

The aqueous mixed system sodium dehydrocholate (NaDHC)–sodium oleate (NaOL) was studied by several methods to determine the influence of the hydrophobic structure of both surfactants in the mixed micellization and the formation of the mixed monolayer adsorbed at the air–water interface. The molecular area at the critical micelle concentration in pure surfactant solutions suggests that the adsorbed oleate chain was folded to allow the double bond in the middle of the molecule to remain in contact with water, and that the NaDHC molecule was situated with its plane laying parallel to the water surface, allowing the three carbonyl groups in the hydrocarbon backbone to form hydrogen bonds with water. The interaction was repulsive at the surface, and in the mixed monolayer some molecules must move away the less hydrophilic groups from water (double bond of NaOL, carbonyl groups of NaDHC). The interaction in mixed micelles was strongly attractive, showing a preferential composition roughly equimolar. The hydrolysis in mixed micelles was augmented in comparison with pure surfactants systems, which could be explained by assuming the existence of a more hydrophobic mixed micelle core. The mixed micelle degree of ionization was below that of the pure micelles, thus indicating a high surface charge density.