Thermodynamic Study of the Inclusion Interaction between Gemini Surfactants and Cyclodextrins by Isothermal Titration Microcalorimetry

The inclusion complexes of a series of bis-quarternary ammonium surfactants, (CnN)2Cl2 (where n = 12, 14, 16) and sodium bis(2-ethylhexyl) sulfosuccinae (AOT), with α-cyclodextrin (α-CD), β-cyclodextrin (β-CD) and γ-cyclodextrin (γ-CD) in aqueous solutions were investigated by using isothermal titration calorimetry (ITC) at 298.15 K. The stability constants, stoichiometry, and formation enthalpies, entropies and Gibbs energies for the complexes in aqueous solutions have been derived from the calorimetric data. The values of the binding constant, K∘i, are very large, which indicates that these complexes are quite stable in their aqueous solutions. The enthalpy changes (ΔH∘) for all of the inclusion processes are negative, showing that the complex process is enthalpy driven. The entropy effect (TΔS∘) is negative, so the inclusion process is entropically unfavorable. The large negative Gibbs energy changes indicate that formation of host-guest inclusion complexes is generally a spontaneous process. The thermodynamic parameters are discussed in the light of the different structures of the host and guest molecules.