Conductance study of the thermodynamics of micellization of 1-hexadecylpyridinium bromide in (water plus cosolvent)

The conductivity of (cosolvent + water) in the presence of increasing concentration of 1-hexadecylpyridinium bromide was measured at various temperatures. Acetonitrile, dimethylformamide, dimethylsulfoxide, dioxane and ethylene glycol were used as cosolvents. From the conductivity data, the critical micelle concentration c.m.c., and the effective degree of counter ion dissociation LY, were obtained at various temperatures. In all the cases studied, a linear relationship between log ([c.m.c]/mol . dm(-3)) and the mass fraction of cosolvent in solvent mixtures was observed. The thermodynamic properties Delta H-m(o) and Delta S-m(o) were evaluated from the temperature dependence of the equilibrium constants for micellization of the surfactant. While the micellization process in pure water is both enthalpy and entropy stabilized, it becomes entropy destabilized in all solvent mixtures used; the values of Delta S-m(o) being more negative with increase in the cosolvent content of the solvent mixtures. The resulting Delta H-m(o) against T Delta S-m(o) plot showed a fairly good linear correlation, indicating the existence of an enthalpy-entropy compensation in the micellization process. (C) 2000 Academic Press.