Thermodynamics of micelle formation of some alkylpyridinium bromides

Publisher Website
Google Scholar
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.

This publication has 19 references indexed in Scilit: