A study of α-cyclodextrin with a group of cationic gemini surfactants utilizing isothermal titration calorimetry and NMR

Isothermal titration calorimetry has been applied to determine the stability constants, stoichiometry, formation enthalpies, entropies, and Gibbs free energies for the complexes of α-cyclodextrin (α-CD) with a series of bis-quarternary ammonium surfactants, (CnN)2Cl2 (n = 12, 14, 16), in aqueous solutions at 293.15 K. The observed stability constants of the complexes are very large. For these quite stable inclusion complexes, the stoichiometry of most stable complexes changes from 2:1 to 6:1 as the number of methylenes (–CH2–) in each of the hydrophobic tail is increased from 12 to 16. According 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 characterized by both favorable enthalpy changes and unfavorable entropy changes. Chemical shift data of all protons in the CD molecule, induced by the formation of the (α-CD + (C12N)2Cl2) complexes have been determined by Proton NMR spectroscopy.