The cavity elongation effect. Calorimetric studies of the complexes of long-chain carboxylic acids with methyl-a-cyclodextrin in aqueous solutions

The formation of complexes between methyl-a-cyclodextrin (MaCD) and monocarboxylic acids from C5 to C12 or cycloalkanols has been studied calorimetrically at 298 K in aq. phosphate buffer, pH 11.3. The forces involved in the assocn. process are discussed according to the signs and values of the thermodn. parameters obtained: assocn. enthalpy, assocn. const., Gibbs energy, and entropy. Methyl-a-cyclodextrin forms 1:1 inclusion complexes with monocarboxylic acids, characterized by a monotonic increase in the values of enthalpies and assocn. consts. at increasing alkyl chain length. Assocn. is characterized by neg. enthalpies and by pos. entropies, which dets. large assocn. consts. That behavior is compared to the unusual trend in the values of the assocn. consts. shown by the parent a-cyclodextrin interacting with the same monocarboxylic acids. The model proposed to rationalize the present data provides a cavity elongation effect. Namely, because of the presence of the Me groups on the outside, the cavity behaves as it were deeper than that of the parent cyclodextrin. The assocn. with cycloalkanols (cyclohexanol, cycloheptanol, cyclooctanol and 1-cyclohexyl-ethanol) is characterized by lower entropies, as detd. by the enhanced neg. contribution originating from the tighter fit of the guest into the cavity.