Thermodynamic investigation of water evaporation process from α-cyclodextrin hydrates

Abstract

The pressure of saturated water vapor over α-CD·nH2O hydrates has been measured by static method with membrane-gauge manometer. The temperature dependences of water vapor pressure were obtained in the wide range of temperatures (298–404 K), pressures (0.2–119.7 kPa) and water content (2.61 ≤ n ≤ 4.61) under condition of a quasi-constant composition of the α-CD·nH2O samples. It was observed that water vapor pressure increases proportionally with growth of water content in the α-CD·nH2O hydrate samples. The phase transition was revealed at temperature T = 344 K and its thermodynamic characteristics were determined. The values of enthalpy and entropy of water evaporation process from α-CD nH2O (n = 1) were calculated. The analytical expressions for the lnp – 1/T dependences obtained by the approximation of the experimental data enable to calculate the vapor pressure of the α-CD·nH2O hydrates within the studied temperature range for any n from 2.61 to 6.10. Experimental data were used to obtain changes in the Gibbs energy when water molecules bind to the α-CD framework. The thermodynamic characteristics obtained in this work are new and valuable data not only for fundamental areal, but also for practical applies.