The effects of increasing NaCl concentration on the stability of inclusion complexes in aqueous solution

Equilibrium constants and standard molar enthalpies of reaction were determined by titration calorimetry for the reaction of 1-butanol with 2-hydroxypropyl-beta-cyclodextrin (HP-beta-CD) in aqueous solution at different concentrations of NaCl (0-1.9 M). The standard molar free energy and entropy changes associated with the complexation were calculated from the corresponding equilibrium constants, K, and standard enthalpies determined experimentally. In NaCl solutions the inclusion complexes ButOH/HP-beta-CD are more stable than in water and their stability increases at increasing NaCl concentration; otherwise, the standard molar enthalpy associated to the formation of the complexes does not change with the increasing of salt concentration. The dependence of K on NaCl concentration were used to evaluate the number of water molecules displaced from the hydration shells of HP-beta-CD and ButOH in forming complexes.