Study of the effects of cosolvent on the complexation of b-cyclodextrin with alkanols by calorimetry at 298 K

The formation of complexes between b-cyclodextrin and 1-alkanols has been studied calorimetrically at 298 K in water and in concd. aq. solns. of ethanol or urea. When a complex is formed, calorimetry enables the calcn. of both the enthalpy and the assocn. const., from which the free energy and the entropy of the process can be obtained. The effects of ethanol and urea on the hydration co-spheres of the interacting substances have been investigated through the study of the binary solns. of the involved solutes in water and in the mixed solvents. The findings obtained are, then, related to the consequent changes in the assocn. parameters. The forces involved in the assocn. process are discussed in the light of the signs and values of the thermodn. parameters obtained. The most important features coming out from this study are:(i) assocn. in water is an entropy-driven process; (ii) in concd. aq. solns. of cosolvent, the enthalpic term contributes significantly to the Gibbs energy, while the entropic contribution is smaller; (iii) for every solvent medium employed, the invariance of the entropic contribution with increasing alkyl chain length of the alkanol is an indication that the relaxation of water mols. from the cavity of the macrocycle mainly dets. the assocn. process.