DODECYLTRIMETHYLAMMONIUM BROMIDE IN WATER-UREA MIXTURES - VOLUMES, HEAT-CAPACITIES, AND CONDUCTIVITIES

Densities, heat capacities, and conductivities of dodecyltrimethylammonium bromide (DTAB)-water-urea mixtures over a wide range of urea and surfactant concentrations were measured at 298 K. The partial molar volumes (V2) and heat capacities (C(p2)) of DTAB in urea solutions at fixed compositions were derived. The profiles of C(p2) as a function of DTAB concentration show anomalies at about 0.30-0.35 mol kg-1, depending on the urea molality (m(u)), whereas those of V2 do not. According to literature findings, this peculiarity has been attributed to a micelle structural transition. The partial molar volume of monomeric surfactant at the cmc as well as that at infinite dilution increases linearly with m(u) while the heat capacity decreases in a nonlinear manner. As far as the surfactant in the micellized form is concerned, on increasing m(u) the partial molar heat capacity increases linearly while the partial molar volume increases with a curvature. Consequently, both volume and heat capacity of micellization, evaluated on the basis of the pseudophase transition model, do not change linearly with m(u). The effect of added urea on DTAB micellization is due to the change of the physicochemical properties of the solvent mixture since urea does not solubilize in the micellar phase. The cmc and the degree of counterion dissociation (beta) of DTAB micelles were derived from conductivity measurements in a large range of urea concentration (up to 10m(u)). The cmc increases linearly with m(u) while beta-changes in a nonlinear manner.