Entropy Changes in the Inclusion Complex Formation of α-Cyclodextrin with Alcohols as Studied by the Titration Calorimetry

The inclusion complex formation of α-cyclodextrin(α-CD) with various alcohols has been studied by means of titration calorimetry. The titration curve was analyzed successfully by the formation of a 1:1 hostguest complex, and the thermodynamic parameters were determined. The entropies of reaction are estimated from the statistical thermodynamics of molecules. However, the estimated values are found to deviate by an order of magnitude from the experimental ones. To resolve such a discrepancy, α-CD in solution is assumed to include a definite number(n) of water molecules which are replaced by a guest molecule with the progress of reaction. The entropies thus estimated depend extensively on the value of n, and a value of n is determined for each alcohol so that it reproduces just the experimental entropy. The resulting n values are nearly constant for various alcohols studied and are within a reasonable range expected from the data reported in crystal, supporting the model of interpretation.