Binding Forces Contributing to the Association of Cyclodextrin with Alcohol in an Aqueous Solution

Abstract

Abstract The association constants and thermodynamic parameters were determined for complexes of α- and β-cyclodextrins (CD) with a variety of alcohols (ROH) by the spectrophotometric examination of the inhibitory effect of ROH on the association of CD with azo dyes, and they were analyzed in connection with the partition coefficients of ROH in a diethyl ether–water solvent system in order to elucidate the binding forces contributing to the association of CD with an organic substrate. Hydrophobic and van der Waals interactions were of primary importance in the complexation. Among them, the latter was preferential in α-CD–1-alkanol adducts, while the former, in β-CD–1-alkanol adducts. As the bulkiness of ROH increased, the stability of the α-CD adduct was lowered owing to van der Waals repulsion between ROH and the relatively small α-CD cavity, whereas that of the β-CD adduct was enhanced by the attainment of the close van der Waals contact of ROH with the relatively large β-CD cavity.