Different States of water in α-Cyclodextrin hydrates

The pressure of saturated and unsaturated water vapor over α-CD·nH₂O hydrates has been measured by static method with membrane-gauge manometer under conditions of complete loss of water. The temperature dependences of water vapor pressure have been obtained in the wide range of temperatures (287–469 K), pressures (0.04–34.95 kPa) and water content (0.17 ≤ n ≤ 6.0).

The data obtained indicate the presence of water molecules with different volatility in α-CD·nH₂O hydrates. Water molecules with lower volatility can be considered “external”, included in the network of intermolecular hydrogen bonds, while water molecules with higher volatility, discovered in our previous study, are considered “internal”, presumably located in the α-CD cavity. For each composition, the amount of “external” and “internal” water has been determined and the equations described the dependence of the content of each type of water on n in α-CD·nH₂O have been obtained. The analytical expressions for the lnp – 1/T dependences as well as the values of enthalpy and entropy of processes studied have been calculated. Experimental data were used to obtain the Gibbs energy changes when “external” water molecules bind to the α-CD framework. The values obtained are differed from the values for “internal” water given in our previous study. The phase transition in α-CD·nH₂O revealed in our previous study is also observed in this work and its thermodynamic characteristics are in good agreement with the results obtained earlier. The findings have been confirmed by ¹H NMR studies.