Complexation of natural and methylated b-cyclodextrin with long-chain carboxylic acids in aqueous solutions. Calorimetric studies at 25°C

The formation of complexes between b-cyclodextrin or methyl-b-cyclodextrin and monocarboxylic acids from C5 to C12 has been studied calorimetrically at 298 K in aq. buffer phosphate, pH 11.3. When a complex forms, calorimetry enables the calcn. of both the enthalpy and the assocn. const., from which the Gibbs free energy and the entropy of the process can be obtained. The forces involved in the assocn. process are then discussed in the light of the signs and values of the thermodn. parameters obtained. For b-cyclodextrin, 1:1 inclusion complexes form, characterized by an irregular variation in the values of enthalpies, entropies and assocn. consts. at increasing alkyl chain length. A model is proposed to explain this unusual behavior for acids longer than ten carbon atoms. The assocn. involving methyl-b-cyclodextrin shows, instead, a regular variation of the thermodn. parameters up to the C12 term. An elongation of the cavity effect is discussed: the cavity behaves as it were deeper than that of the natural cyclodextrins. Assocn. is characterized prevailingly by hydrophobic interactions; for the longer terms, the high and almost invariant entropic contribution dets. the large assocn. consts., notwithstanding the pos., unfavorable enthalpic contribution. This is an indication that the relaxation of water mols. from the hydrophobic hydration shells of the external Me groups is the contribution that mainly dets. the assocn. process.