THERMODYNAMICS OF MICELLIZATION AND SOLUBILIZATION IN SYSTEMS OF WATER SODIUM N-ALKYLCARBOXYLATES ALKOXYETHANOLS AT 25-DEGREES-C

The apparent molar volumes and adiabatic compressibilities, Y-phi,Y-S, of carboxylate surfactants, CnNa (n = 8, 10, 12), in aqueous solution s in the absence and presence of medium-chain-length alkoxyethanols, C 4EOX (EO = ethylene oxide group, X = 0-4), and of alkoxyethanols, Y-ph i,Y-A, in aqueous solutions in the absence and presence of surfactant, were determined at 25 degrees C from density and sound velocity measu rements as a function of both the surfactant and alcohol concentration s. The partial molar volumetric properties of CnNa and the transfer fu nctions of C4EOX from water to aqueous surfactant solutions were calcu lated from the apparent molar properties. Values of the thermodynamic parameters of micellization for CnNa, i.e., the critical micelle conce ntration, the partial molar property of the monomer at infinite diluti on, Y-S(0), and in the micellar state, Y-S(M), were obtained from simu lations of the experimental data, Y-phi,Y-S, using a mass-action model . As expected, these properties are strongly dependent on the surfacta nt chain length. The distribution coefficient of C4EOX between the mic elle and aqueous phases, K-D, and the change in the molar property of alcohols due to micellization, Y-A(M), extracted from fitting the tran sfer function data of C4EOX using a chemical equilibrium model, show t hat the solubilization of alkoxyethanols in carboxylate micelles is en hanced by increasing the surfactant chain length and the number of EO groups in the alcohol. The deeper penetration of C4EOX into the micell es of longer chain surfactants is associated with increasingly stronge r interactions between surfactant head groups and EO segments of the a lcohol on (or near) the micelle surface. Aggregation numbers of CnNa-C 4EOX mixed micelles show that addition of a small amount of C4EOX has little effect on the structure of the micelles formed from C8Na and C1 0Na, but leads to significant change in C12Na micelles.