THERMODYNAMICS OF MICELLE FORMATION OF SOME ALKYLPYRIDINIUM BROMIDES

PVC membrane electrodes selective to surfactant ions have been made and used to determine mean activity coefficients of undecyl-, tridecyl-, tetradecyl-, and pentadecyl-pyridinium bromides in aqueous solutions as well as their c.m.c.s in water, 0.01, 0.05 and 0.1 mol kg-1 NaBr. At concentrations below the c.m.c. the measured gamma+/-values showed good agreement with those predicted by the Debye-Huckel theory. The standard Gibbs free energies of micellization, DELTAG0, were determined at 25-degrees-C using the mass action approach, taking into account the surfactant mean activity coefficients. The corresponding heats of micelle formation, DELTAH0, were determined at 25-degrees-C in water, 0.01 and 0.1 mol kg-1 NaBr by direct calorimetric measurements. In water, the DELTAH0 values were calculated also from the variation of c.m.c. with temperature, and satisfactory agreement with the calorimetric DELTAH0 values was observed. The small, exothermic DELTAH0 values were combined with the corresponding DELTAG0 values to obtain the standard entropies of micellization, DELTAS0, which are large and positive. The micellization of alkylpyridinium bromides can thus be considered as an entropy-driven process in which the observed entropy gain is mainly due to the breakdown of the hydrophobically structured water around the monomer as it enters the micelle.