Complexation forces in aqueous solution. Calorimetric studies of the association of 2-hydroxypropyl-b-cyclodextrin with monocarboxylic acids or cycloalkanols.

The formation of complexes between 2-hydroxypropyl-b-cyclodextrin and monocarboxylic acids or cycloalkanols has been studied calorimetrically at 298 K in phosphate buffer, pH 11.3. The forces involved in the assocn. process are discussed in the light of the signs and values of the thermodn. parameters obtained: assocn. enthalpy, binding const., Gibbs free energy, and entropy. For monocarboxylic acids, hydrophobic interactions are the primary force detg. complexation, as indicated by the small enthalpies and by the high and pos. entropies. For the cycloalkanols, instead, enthalpies are neg. and entropies pos. or neg., depending on the solvent medium employed, namely water or phosphate buffer. The most important requirement for the formation of the complex is a good spatial fit between the two interacting mols. A cavity elongation effect occurs because of the presence of the hydroxypropyl groups on the rim of the macrocycle. The relative contribution of hydrophobic and van der Waals interactions varies with the dimensions of the guest mols. A linear correlation exists between enthalpy and entropy of complexation, underlying that inclusion is a process dominated by hydration phenomena and ascribed to the modifications experienced by the solvent in the hydration shells of the interacting substances.