A study on complexation between α-cyclodextrin and bis-quaternary ammonium surfactants

The inclusion interaction between a-cyclodextrin (alpha-CD)with three cationic gemini surfactants, (CnN)(2)Cl-2(n=12, 14, 16), in aqueous solutions has been investigated by isothermal titration calorimetry, combining with H-1 NMR at 293.15 K. The results show that these complexes are quite stable. As the number of carbon atoms in each of the hydrophobic tail, CnH2n+1, increases, the stoichiometry of stable complexes changes from 2:1 to 6:1. Corresponding to the same change of the hydrophobic chain, both formation enthalpy and formation entropy evidently decrease. The results also indicate that the association processes are enthalpy driven ones. The data of proton NMR spectroscopy indicate that chemical shift data of all protons in the alpha-CD molecule move to high field in the presence of the (C12H25N)(2)Cl-2, which can be regarded as a microscopic evidence of the occurrence of inclusion interaction.