Thermodynamic Properties of Alkali Halides. III. Volumes and Heat Capacities of Transfer from H2O to D2O at 25 degrees C

The volumetric specific heats and densities of alkali fluorides, alkali bromides, and sodium halides were measured in D2O at 25 degrees C in the concentration range 0.05 to 1 aquamolal. The results can be combined with data in H2O to give the corresponding standard and excess transfer functions from H2O to D2O. The volumes and heat capacities of transfer are both negative but, contrary to the hydration functions, show little dependence on ionic size and sign. Also, while heats, entropies, and volumes of transfer are usually small compared with the hydration functions, the heat capacity of transfer is of comparable magnitude. These observations, when interpreted with the Frank and Wen model, suggest that the total number of water molecules in the hydration cosphere is approximately constant for all alkali halides and that heat capacities are more sensitive to structural interactions than volumes and enthalpies. The sign of the excess transfer functions is consistent with the presence of structural ion-ion interactions, but no systematic trend with ionic size can be detected in view of the large experimental uncertainty.