Thermodynamics of micellization of decyldimethylbenzylammonium bromide in aqueous solution

The specific conductivity, κ, of decyldimethylbenzylammonium bromide (C10BBr) in aqueous solution was measured as a function of molality in the 5–50 °C temperature range at 1 °C intervals. The κη against temperature plots (where η is the viscosity of water at the corresponding temperature) exhibit three, molality dependent regions. In the premicellar concentration range the plots are linear with a small negative slope. A concavity on the plots, due to change in the degree of ionization of the micelles and the U-shaped dependence of the critical micelle concentration, cmc, on temperature, is observed in the immediate post-micellar concentration range. At high concentration of the surfactant a monotonic increase of the slope of the plots can be observed. The critical micelle concentration, cmc, of C10BBr and ionization degree of the micelles, β, were estimated from the intersection of two extrapolated linear fragments of the conductivity against molality plots and the ratio of the slopes of the two lines, respectively. Comparison with literature data indicates that the minimum on the cmc against temperature plots, T*, is shifted to lower temperatures with increase in length of the alkyl chain of the surfactant and with increase in hydrophobicity of the counterion. The temperature dependence of ln χcmc (χcmc being the cmc expressed as mol fractions) was fitted to an equation suggested by Muller, which was modified by taking into account the temperature dependencies of both of β and of change in heat capacity upon micellization. From the fitting parameters, Gibbs free energies, enthalpies and entropies of micellization of C10BBr as a function of temperature were estimated. Enthalpies and entropies were found to be strongly correlated.