The Low Concentration aggregation of sodium oleate- sodium linoleato aqueous mixtures

Abstract

Sodium oleate (NaOL, C18H33O2Na)-sodium linoleate (NaLin, C18H31O2Na) mixtures were studied in the micellar and in the air/water interface states at 298.15 K. Three aggregation steps were found: a premicellar aggregation, the critical micelle concentration (CMC), and a structural change of micelles. Micelles, both at the CMC and at the structural change concentration, are richer in oleate than the overall mixture composition. Micelles are strongly non-ideal and the interaction is repulsive. The non-ideal behavior and the dependence of the micelle ionization degree with micelle composition are explained on the basis of the interaction of the π electrons of the surfactants' chains with water at the hydrocarbon/water micellar interface. The air/solution adsorbed monolayer is also non-ideal, but the interaction is attractive and there is a preferential composition with a mole fraction of sodium oleate of about 0.7. The surface pseudophase behaves as if oleate were the solvent and linoleate a strongly soluble solute. This behavior and the dependence of the average area per adsorbed molecule were explained on the basis of the interaction of the double bonds with water. At the air/solution interface, the linoleate molecule area was similar to that of a heterogemini surfactant having a spacer with seven carbon atoms.