Thermodynamics of self-aggregation of mixed cationic gemini/sodium deoxycholate surfactant systems in aqueous solution

Thermodynamics of interaction between cationic gemini surfactants [C 12 H 25 (CH 3 ) 2 N(CH 2 ) s N(CH 3 ) 2 C 12 H 25 ]Br 2 , ( s  = 2, 6 or 10, assigned as C 12 C s C 12 Br 2 ) 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 C 12 C s C 12 Br 2 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 C 12 C s C 12 Br 2 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 C 12 C s C 12 Br 2 concentration increases, including the critical concentrations ( C P , C R , C M ) of formation and redissolution for LC of the mixed C 12 C s C 12 Br 2 /NaDC systems and transition to gemini-rich micelles. In comparison with the mixed C 12 C s C 12 Br 2 /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 processes—redissolution 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 (C 12 C s C 12 Br 2 ) with double hydrophobic chains and cationic head groups linked through a methylene spacer.