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

• ITC and conductivity provide critical concentrations for these complex systems.
• OD helps the understanding of their structural changes.
• ITC provides the enthalpies accompanying the various processes in the mixed systems.
• The cmcmix are smaller than cmc for pure surfactants indicating a synergistic effect.
• The gemini spacer length influences its aggregation behavior with NaCA.

The thermodynamics of the process of self-assembly of cationic gemini surfactants, [C12H25(CH3)2N(CH2)SN(CH3)2C12H25]Br2, (the spacer S being 2, 6 or 10, assigned as C12CSC12Br2) and the system of oppositely charged mixture of surfactants formed by C12CSC12Br2 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 C12CSC12Br2 obtained from calorimetry and conductivity were found to be consistent with values reported in the literature. The enthalpies of micellization (ΔHmic) of C12CSC12Br2 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 (C12CSC12Br2(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 (cmcmix, ΔHmic-mix), the formation of a precipitate (immiscible liquid crystalline (LC) phase) (CP, ΔHP) and its re-dissolution (CR, ΔHR), and finally the formation of positive charge-rich mixed micelles (CM, ΔHM). It should be stressed that the values of cmcmix (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.

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