Effects of added DOTAB on the cmc and enthalpy of micelle formation at 298.2 K for CTAB(aq)

In a titration calorimetric study an aqueous solution held in a syringe and containing hexadecyltrimethylammonium bromide (CTAB; 15.4x10(-3) mol dm(-3)) is injected in aliquots (5x10(-3) dm(3)) into a sample cell containing initially water. Analysis of the data shows that the cme equals 0.97x10(-3) dm(-3) and the enthalpy of micelle formation equals -10.3 kJ mol(-1). When the solution in the syringe is replaced by a mixed surfactant solution, CTAB + dodecyltrimethylammonium bromide, at the same total concentration of surfactant, the cme of CTAB decreases gradually with increasing mole fraction of DOTAB but the enthalpy of CTAB micelle formation is hardly affected. We conclude, therefore, that incorporation of DOTAB monomers into the CTAB micelles stabilizes entropically the CTAB micelles.