Self-assembly of tetradecyltrimethylammonium bromide in glycerol aqueous mixtures: A thermodynamic and structural study

Abstract

In this paper, we have studied the effect of glycerol on the micelle formation of tetradecyltrimethylammonium bromide. Changes in both the critical micelle concentration and the degree of counterion binding of the surfactant upon the addition of glycerol across a temperature range (20–40 °C) were examined by using the conductance method. The equilibrium model of micelle formation was applied to obtain the thermodynamic parameters of micellization. An enthalpy–entropy compensation effect was observed in all the solvent systems, but whereas the micellization of the surfactant in the medium with 20% glycerol occurs under the same structural conditions as in pure water, in glycerol rich mixtures the results suggest that the lower aggregation in these media is due to the minor cohesive energy of the solvent system in relation to water. It was also observed that the micellar aggregation number, as obtained by the static quenching method, decreases with the glycerol content. This fact was attributed to an increase in the surface area per headgroup of the surfactant as a consequence of an enhanced solvation, probably induced by the incorporation of some glycerol molecules in the micellar solvation layer. Although the pyrene 1:3 ratio index does not indicate significant changes in the micropolarity at the micelle–bulk interface, the data of fluorescence anisotropy of coumarin 6 and fluorescein are compatible with the formation of a more compact solvation layer.