Thermodynamics of micelle formation of some alkylpyridinium bromides

Abstract

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 γ_± values showed good agreement with those predicted by the Debye–Hückel theory. The standard Gibbs free energies of micellization, ΔG⁰, were determined at 25 °C using the mass action approach, taking into account the surfactant mean activity coefficients. The corresponding heats of micelle formation, ΔH⁰, were determined at 25 °C in water, 0.01 and 0.1 mol kg^–1 NaBr by direct calorimetric measurements. In water, the ΔH⁰ values were calculated also from the variation of c.m.c. with temperature, and satisfactory agreement with the calorimetric ΔH⁰ values was observed. The small, exothermic ΔH⁰ values were combined with the corresponding ΔG⁰ values to obtain the standard entropies of micellization, ΔS⁰, 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.