Thermodynamics of micellization and solubilization in systems of water – sodium n-alkylcarboxylates – alkoxyethanols at 25 °C

Abstract

The apparent molar volumes and adiabatic compressibilities, [Formula: see text] of carboxylate surfactants, CnNa (n = 8, 10, 12), in aqueous solutions in the absence and presence of medium-chain-length alkoxyethanols, C4EOX (EO = ethylene oxide group, X = 0–4), and of alkoxyethanols, [Formula: see text] in aqueous solutions in the absence and presence of surfactant, were determined at 25 °C from density and sound velocity measurements as a function of both the surfactant and alcohol concentrations. The partial molar volumetric properties of CnNa and the transfer functions of C4EOX from water to aqueous surfactant solutions were calculated from the apparent molar properties. Values of the thermodynamic parameters of micellization for CnNa, i.e., the critical micelle concentration, the partial molar property of the monomer at infinite dilution, [Formula: see text] and in the micellar state, [Formula: see text] were obtained from simulations of the experimental data, [Formula: see text] using a mass-action model. As expected, these properties are strongly dependent on the surfactant chain length. The distribution coefficient of C4EOX between the micelle and aqueous phases, KD, and the change in the molar property of alcohols due to micellization, [Formula: see text] extracted from fitting the transfer function data of C4EOX using a chemical equilibrium model, show that the solubilization of alkoxyethanols in carboxylate micelles is enhanced by increasing the surfactant chain length and the number of EO groups in the alcohol. The deeper penetration of C4EOX into the micelles of longer chain surfactants is associated with increasingly stronger interactions between surfactant head groups and EO segments of the alcohol on (or near) the micelle surface. Aggregation numbers of CnNa–C4EOX mixed micelles show that addition of a small amount of C4EOX has little effect on the structure of the micelles formed from C8Na and C10Na, but leads to significant change in C12Na micelles. Keywords: sodium carboxylate salts, alkoxyethanols, partial molar volume and compressibility, transfer functions, distribution coefficient, mean aggregation number.