Critical micelle concentrations of aqueous hexadecytrimethylammonium bromide-sodium oleate mixtures

The aqueous hexadecyltrimethylammonium bromide (which is also known as cetyltrimethylammonium bromide and commonly abbreviated CTAB)-sodium oleate (NaOL) system was studied by several methods. The system showed three critical concentrations at high water content. The lowest concentration corresponds to premicellar association, followed by spherical or nearly spherical micelle formation (CMC) and a transformation to anisometric micelles. Except for the equimolar mixture, the micelle degree of ionization α at the CMC was estimated. The value of α increases in mixtures with respect to the pure component micelles and it depends on the composition. The CMC was studied by the regular solution theory and the interaction parameter; the activity coefficient and the activity of components in mixed micelles were determined. The micelle composition at the CMC was also computed and it was found that the mixed micelles were systematically richer in CTAB than the nominal proportion. The thermodynamic results indicate that there is a preference for micelles having 25% of NaOL. The possible explanation is based on the effect of the double bond in the oleate chain and its affinity with water, which affects the curvature of the micelle-water interface. In NaOL-rich systems, the hydrolysis of the soap is similar to that in common soap solutions. In equimolecular or CTAB-rich systems, there is a fundamental change: above the CMC, the hydroxide ion concentration is constant and very low, indicating that almost all oleate ions are in the micelles and do not undergo hydrolysis.