Transport Properties of Aqueous Solutions of Alkyltrimethylammonium Bromide Surfactants at 25°C

Abstract

Intradiffusion coefficients, D, of n-alkyltrimethylammonium bromides [CH3–(CH2)n−1–N(CH3)3Br, CnTAB] (n=6, 8, 10, 12) in mixtures with heavy water were measured by the PGSE–NMR technique at 25°C. The experimental data permitted evaluation of the influence of the alkyl chain length on the surfactant self-aggregation process. For all the surfactants considered, the D trend showed a slope change corresponding to the critical micellar composition (cmc). In the premicellar composition range, D decreased linearly with the square root of the surfactant molality. The D values extrapolated at infinite dilution were related to the limiting mutual diffusion coefficients, determined through the Taylor dispersion technique. In the micellar composition range, solubilized tetramethylsilane (TMS) molecules were used to determine the micelle intradiffusion coefficient, DM, from which the aggregate radii and the aggregation numbers were obtained. The decreasing trend of DM with increasing surfactant molality was interpreted in terms of interparticle electrostatic repulsion. DM values allowed evaluation of the Gouy–Chapman layer thickness. The solvent intradiffusion coefficient in the heavy water–CnTAB mixtures, Dw, was also measured. It decreased with increasing surfactant molality. For n=8, 10, 12 the Dw trend presented a slope change at the cmc, which could be ascribed to the strong decrease in hydration of surfactant molecules upon micellization. Because of its short hydrophobic tail, C6TAB exhibited peculiar aggregation behavior. Its cmc, which is poorly marked, is lower than the value predicted by extrapolating the cmc values obtained for the other terms of the series. The C6TAB aggregates do not solubilize TMS molecules; the estimated aggregation number is extremely low (∼3). Finally, no abrupt slope change in the solvent intradiffusion coefficient trend was detected. This evidence suggests that C6TAB molecules do not micellize in aqueous solution, but form trimers in which the surfactant hydrophobic tails are not hidden from contact with water molecules.