Thermodynamic Study of Inclusion Interactions between Gemini Surfactants and β-Cyclodextrin
The inclusion complexes of gemini surfactants, (CnN)2C12 (where n=12, 14), and β-cyclodextrin (β-CD) in aqueous solutions were investigated by an isothermal titration calorimetric method and 1H NMR spectroscopy at 293.15 K. The stability constants, stoichiometry, formation enthalpies, entropies and Gibbs energies for the complexes in aqueous solution have been derived from the calorimetric data. β-CDin aqueous (C12N)2Cl2/(C14N)2Cl2 solutions forms 2:1 mole ratio host-guest complexes. The large values of the stability constants indicate that these complexes are stable in the aqueous solutions. The negative Gibbs energy changes indicate that formation of the host-guest complexes is generally a spontaneous process. Both the formation enthalpy and formation entropy evidently decrease as the number of methylenes in each of the hydrophobic tails increase. The thermodynamic parameters are discussed in the light of the different structures of the host and guest molecules. Chemical shift data of protons in the CD molecule, induced by the formation of the complexes have been determined by 1H NMR spectroscopy.
KEYWORDS: Gemini surfactant, ;-cyclodextrin, isothermal titration calorimetry, 1H NMR.
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