Studies of mixing behavior of cationic surfactants

•Reporting thermodynamic properties of aqueous cationic surfactants and their mixtures.•Conductometric, fluorescence and NMR studies have been performed.•1H NMR studies help to interpret the relative arrangement of surfactant monomers in the mixed state.•The activity coefficients and cmc of mixed states clearly indicate nonideality.•The negative values of interaction parameter suggest the favorable interactions.

The behavior and properties of mixed surfactant systems are discussed in the context of experimental techniques. The aggregation behavior of tetradecyltrimethylammonium bromide (TTAB), hexadecyltrimethylammonium bromide (CTAB), and hexadecyltriphe-nylphosphonium bromide (HTPB) and their mixtures in aqueous medium was investigated using conductance, fluorescence and NMR techniques. The critical micelle concentration (cmc), counterion binding, thermodynamic parameters of micellization, aggregation number (Nagg) etc. have been quantitatively estimated. Results were analyzed using regular solution theory (RST) to obtain the composition of the mixed micelles and the interaction parameter, βm, to evaluate the type and strength of interactions of surfactants in the mixed micelle. Activity coefficients and excess free energy of mixing were also determined. 1H NMR studies suggested that when mixed with CTAB or TTAB, the bulky head group of HTPB induce steric hindrance. 1H NMR results suggest that CTAB + HTPB produced compact mixed micelles.