Cationic gemini and sodium cholate – Following the interaction of oppositely charged surfactants by calorimetry, turbidity and conductivity

The thermodynamics of the process of self-assembly of cationic gemini surfactants, [C 12 H 25 (CH 3 ) 2 N(CH 2 ) S N(CH 3 ) 2 C 12 H 25 ]Br 2 , (the spacer S being 2, 6 or 10, assigned as C 12 C S C 12 Br 2 ) and the system of oppositely charged mixture of surfactants formed by C 12 C S C 12 Br 2 and sodium cholate (NaCA) in aqueous solution has been investigated by isothermal titration calorimetry (ITC), conductivity and turbidity measurements. The critical micelle concentration values (cmc) for the gemini surfactants C 12 C S C 12 Br 2 obtained from calorimetry and conductivity were found to be consistent with values reported in the literature. The enthalpies of micellization (Δ H mic ) of C 12 C S C 12 Br 2 are all exothermic, presenting a strong negative minimum at S   =   6, corresponding to the maximum in the cmc values. For the mixed system of oppositely charged surfactants (C 12 C S C 12 Br 2 ( S   =   2,   6,   10)/NaCA), we did obtain from ITC the critical parameters for different events that take place as the concentration of gemini surfactant increases, such as the formation of NaCA-rich mixed micelles (cmc mix ,   Δ H mic-mix ), the formation of a precipitate (immiscible liquid crystalline (LC) phase) ( C P ,   Δ H P ) and its re-dissolution ( C R ,   Δ H R ), and finally the formation of positive charge-rich mixed micelles ( C M ,   Δ H M ). It should be stressed that the values of cmc mix (gemini) are much smaller than those for pure gemini and pure NaCA. These results also show that there is a stronger synergistic effect between the two surfactants in the NaCA-rich region. The turbidity measurements proved valuable to the determination of the region of immiscible LC phase. The ITC results are combined with those obtained by conductivity and turbidity leading to a thorough discussion of the effect of the gemini spacer length on the aggregation behavior of the mixed systems.