Thermodynamics of micellization of tetradecyltrimethylammonium bromide in ethylene glycol-water binary mixtures

The aggregation behaviour of tetradecyltrimethylammonium bromide in ethylen e glycol-water mixtures across a range of temperatures has been investigate d by electrical conductivity measurements. The critical micelle concentrati on (cmc) and the degree of counterion dissociation of micelles were obtaine d at each temperature from plots of differential conductivity, (partial der ivative k/partial derivative c)(T,P), versus the square root of the total c oncentration of the surfactant. This procedure not only enables us to deter mine the cmc values more precisely than the conventional method, based on p lots of conductivity against total concentration of surfactant, but also al lows straightforward determination of the limiting molar conductance and th e molar conductance of micellar species. The equilibrium model of micelle f ormation was applied to obtain the thermodynamics parameters of micellizati on. Only small differences have been observed in the standard molar Gibbs f ree energies of micellization over the temperature range investigated. The enthalpy of micellization was found to be negative in all cases, and it sho wed a strong dependence on temperature in the ethylene glycol poor solvent system. An enthalpy-entropy compensation effect was observed for all the sy stems, but whereas the micellization of the surfactant in the solvent syste m with 20 wt% ethylene glycol seems to occur under the same structural cond itions as in pure water, in ethylene glycol rich mixtures the results sugge st that the lower aggregation of the surfactant is due to the minor cohesiv e energy of the solvent system in relation to water.