Thermodynamics of self-aggregation of mixed cationic gemini/sodium deoxycholate surfactant systems in aqueous solution: The cross-linking action of gemini surfactant

Thermodynamics of interaction between cationic gemini surfactants [C12H25(CH3)(2)N(CH2)(s)N(CH3)(2)C12H25]Br-2, (s=2, 6 or 10, assigned as C12CsC12Br2) and sodium deoxycholate (NaDC) in aqueous solution has been investigated by isothermal titration calorimetry (ITC), fluorescence and turbidity measurements. The critical micelle concentrations for NaDC-rich mixture (cmc(mix,12-s-12)) were determined by fluorescence method. The cmc(mix,12-s-12) values are much smaller than those for pure C12CsC12Br2 surfactants, and decrease as the spacer length s' increases. There is a strong synergistic effect between two surfactants in the NaDC-rich region. In the higher C12CsC12Br2 concentration range above cmc(mix,12-s-12), we did obtain from ITC and turbidity measurements the critical parameters for different events that occur as the C12CsC12Br2 concentration increases, including the critical concentrations (C-P, C-R, C-M) of formation and redissolution for LC of the mixed C12CsC12Br2/NaDC systems and transition to gemini-rich micelles. In comparison with the mixed C12CsC12Br2/NaC systems reported with sodium cholate (NaC), the mixed systems with NaDC have lower cmc(mix,12-s-12), C-P, C-R and C-M, and thus different self-aggregation processes are well controlled by surfactant concentration and molar fraction. Importantly, ITC allowed us to directly measure the enthalpy changes corresponding to different self-aggregation processesredissolution enthalpy (H-R) of LC of mixed surfactants and transition enthalpy (H-M) from LC to gemini-rich micelles. Eventually, we discussed the cross-linking effect on NaDC-rich micelles of the gemini surfactant (C12CsC12Br2) with double hydrophobic chains and cationic head groups linked through a methylene spacer.