Aqueous Hexadecyltrimethylammonium Acetate Solutions: pH and Critical Micelle Concentration Evidence for Dependence of the Degree of Micelle Ionic Dissociation on Acetate Ion Concentration

Abstract

The counterion binding parameters (β) of aqueous hexadecyltrimethylammonium acetate (C16TAOAc) are estimated from pH data at various C16TAOAc−AcOH concentrations with or without added sodium acetate buffer. In contrast to what is usually reported for ionic amphiphiles associated with less hydrophilic counterions, β varies from ca. 0.4 to ca. 0.7 and depends upon the acetate ion concentration in the aqueous phase. The critical micelle concentrations (cmc) for C16TAOAc, C16TAOAc−NaOAc, C16TAOAc−AcOH, and C16TAOAc−NaOAc−AcOH solutions, measured from extinction data for fluorescence of acridine, are also examined in terms of AcO- concentration dependence. The plot of the logarithm of the amphiphile activity at cmc vs the logarithm of acetate ion activity is markedly curved, as expected for significant variations in β.