EFFECT OF HEADGROUP SIZE ON MICELLIZATION AND PHASE-BEHAVIOR IN QUATERNARY AMMONIUM SURFACTANT SYSTEMS

The micellization and phase equilibria in aqueous solutions of cationic quaternary ammonium bromide surfactants of the type alkyltrimethyl-, -ethyl-, -propy-, -butyl-, and -pentylammonium bromide have been investigated as a function of alkyl chain length, electrolyte concentration, and temperature. A large isotropic (micellar) phase is observed in triethyl- and tripropylammonium surfactants, which demixes into two conjugate phases on warming in the case of the alkyltributylammonium bromides. Liquid-liquid phase separation is also observed for tetradecyltripentylammonium bromide. Critical micelle concentrations are found to decrease with increasing hydrophobicity of the surfactant ion, but the free energy of micellization of tetradecyltributylammonium bromide is the same as that of tetradecyltrimethylammonium bromide over a range of temperatures. Some evidence is found for the formation of premicellar aggregates in tributylammonium surfactants at elevated temperatures, consistent with significant polydispersity and changing monomer concentrations observed previously. The results are discussed in terms of the hydrophobic and electrostatic contributions to the free energy of micellization, and some inferences are drawn regarding the nature of the interactions which give rise to phase separation.