Energetics of Water/Cyclodextrins Interactions

The heat capacities of solid γ-CD, 8.1 H2O and α-CD, 6.0 H2O have been measured between 10 and 300 K by adiabatic calorimetry. Using earlier results obtained in similar experiments with anhydrous cyclodextrins and with β-CD, 9.7 H2O, a comparative analysis has been developed. The energetic behaviours of anhydrous and hydrated cyclodextrins (CDs) have been compared in order to investigate the role of water molecules in the stabilization of the cyclodextrin's rings and on their reactivities. Calculations, based on the additivity of thermodynamic properties, provide the energetic and entropic average contributions of water molecules in each cyclodextrin. From these results, we assumed that the water–water and water–CD interactions are rather different according to the cyclodextrin. In the (β-CD, 9.7 H2O) structure, the water molecules seem to be better organised in a relatively independent network. Concerning hydrated α-CD and γ-CD, stronger water–CD interactions probably prevent an optimal organisation of the water–water bonds network. Differential scanning calorimetry was also used to follow the evolution of the thermal behaviour of γ-CD, nH2O versus hydration ratio between 170 and 300 K. Our results indicate that the γ-CD ring needs at least 1.6 water molecules to be stabilized in the solid state.