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

The micellization and phase equilibria in aqueous solutions of cationi c quaternary ammonium bromide surfactants of the type alkyltrimethyl-, -ethyl-, -propy-, -butyl-, and -pentylammonium bromide have been inve stigated as a function of alkyl chain length, electrolyte concentratio n, and temperature. A large isotropic (micellar) phase is observed in triethyl- and tripropylammonium surfactants, which demixes into two co njugate phases on warming in the case of the alkyltributylammonium bro mides. Liquid-liquid phase separation is also observed for tetradecylt ripentylammonium bromide. Critical micelle concentrations are found to decrease with increasing hydrophobicity of the surfactant ion, but th e free energy of micellization of tetradecyltributylammonium bromide i s the same as that of tetradecyltrimethylammonium bromide over a range of temperatures. Some evidence is found for the formation of premicel lar aggregates in tributylammonium surfactants at elevated temperature s, consistent with significant polydispersity and changing monomer con centrations 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.