Enthalpy and entropy changes on molecular inclusion of 1,4-butanediol into α- and β-cyclodextrin cavities in aqueous solutions

To determine the enthalpies and entropies of inclusion, enthalpies of dilution of dilute aqueous 1,4-butanediol (BD) solutions and those of transfer of 1,4-BD from aqueous to aqueous α- or β-cyclodextrin (CD) solutions have been determined by microcalorimetry at various mole fractions at 298.15 K. Enthalpies of inclusion of 1,4-BD molecules into α- and β-CD cavities are determined, whose values are exothermic and small. However, stabilisation of gaseous 1,4-BD molecules in the CD cavities is large. 1,4-BD molecules must change their conformations to make the closest contacts with the atoms on the wall of α-CD cavities or those on the wall of β-CD cavities and the remaining water molecules in β-CD cavities.