ROLE OF THE FUNCTIONAL-GROUP IN THE FORMATION OF THE COMPLEXES BETWEEN ALPHA-CYCLODEXTRIN AND ALKANOLS OR MONOCARBOXYLIC ACIDS IN AQUEOUS-SOLUTIONS - A CALORIMETRIC STUDY AT 25-DEGREES-C

The interaction of alpha-cyclodextrin with 1-alkanols, monocarboxylic acids and alpha,omega-diols has been studied calorimetrically at 25 de grees C in water, in phosphoric acid, pH 1.3, and in phosphate buffer, pH 11.3. When a complex is formed, calorimetry enables the calculatio n of both the enthalpy and the association constant, from which the fr ee energy and the entropy of the process can be obtained. Inclusion co mplexes are formed by 1-alkanols and monocarboxylic acids. For alkanol s, a model is proposed to explain the unusual trend of the association constants at increasing alkyl chain length. The association occurs th rough the insertion of the guest's alkyl chain into the host's cavity. However, for terms longer than C-6, two forms of the guest can exist, each one associating to alpha CD with a different constant and enthal py. alpha,omega-Diols associate through a mechanism which involves pre vailingly the exterior of alpha-cyclodextrin. For terms longer than C- 7, another mechanism is proposed which provides the inclusion of the a lkyl chain, with the hydroxyl groups both laying outside the cavity. T he main role played by the different functional groups, and the forces involved in the association process are discussed in the light of the analysis of the signs and values of the thermodynamic parameters obta ined. (C) 1998 Elsevier Science Ltd. All rights reserved.