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

Abstract

Thermodynamics of interaction between cationic gemini surfactants [C12H25(CH3)2N(CH2)sN(CH3)2C12H25]Br2, (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 (cmcmix,12-s-12) were determined by fluorescence method. The cmcmix,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 cmcmix,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 (CP, CR, CM) 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 cmcmix,12-s-12, CP, CR and CM, 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 (∆HR) of LC of mixed surfactants and transition enthalpy (∆HM) 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.