Thermodynamic and Interfacial Adsorption Studies on the Micellar Solutions of Alkyltrimethylammonium Bromides in Ethylene Glycol (1) + Water (2) Mixed Solvent Media

Micellization behavior of three alkyltrimethylammonium bromides, viz., hexadecyl-, tetradecyl-, and dodecyltrimethylammonium bromide (CTAB, TTAB, and DTAB, respectively) in ethylene glycol (EG) (1) + water (2) mixed solvent media with varying mass fractions of EG (w(1)) has been studied by means of electrical conductivity and surface tension measurements. Temperature dependence of the critical micelle concentrations was also investigated to understand the micellar thermodynamics of these systems. The mass action model of micelle formation was applied to obtain the thermodynamic parameters of micellization. Only small differences have been observed in the standard molar Gibbs energies of micellization over the temperature range investigated. The enthalpy of micellization was found to be negative in all cases, whereas the entropic contributions were always positive, indicating that an enthalpy-entropy compensation effect is the governing factor for micellization. Surface tension measurements also provide information about the effects of the addition of ethylene glycol to water on the excess surface concentration, minimum area per surfactant molecule, the surface pressure at the critical micelle concentrations, and standard Gibbs energy of adsorption. The Gordon parameters for the EG (1) + water (2) mixtures were also estimated by means of surface tension measurements to assess the ability of EG (1) + water (2) mixtures to bring about the self-association of the surfactants investigated.