Role of the functional group in the formation of the complexes between α-cyclodextrin and alkanols or monocarboxylic acids in aqueous solutions. A calorimetric study at 25 °C

The interaction of α-cyclodextrin with 1-alkanols, monocarboxylic acids and α,ω-diols has been studied calorimetrically at 25 °C in water, in phosphoric acid, pH 1.3, and in phosphate buffer, pH 11.3. When a complex is formed, calorimetry enables the calculation of both the enthalpy and the association constant, from which the free energy and the entropy of the process can be obtained. Inclusion complexes are formed by 1-alkanols and monocarboxylic acids. For alkanols, a model is proposed to explain the unusual trend of the association constants at increasing alkyl chain length. The association occurs through the insertion of the guestʹs alkyl chain into the hostʹs cavity. However, for terms longer than C6, two forms of the guest can exist, each one associating to αCD with a different constant and enthalpy. α,ω-Diols associate through a mechanism which involves prevailingly the exterior of α-cyclodextrin. For terms longer than C7, another mechanism is proposed which provides the inclusion of the alkyl chain, with the hydroxyl groups both laying outside the cavity. The 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 obtained.