Thermodynamic Properties of Alkali Halides. II. Enthalpies of Dilution and Heat Capacities in Water at 25 degrees C

The enthalpies of dilution and volumetric specific heats of most alkali halides were measured in water at 25 degrees C with flow microcalorimeters in the concentration range 0.01 to 1 m. Apparent molal relative enthalpies phi(L), derived from the enthalpies of dilution, can be represented by a parametric equation in molality. Combining phi(L) with osmotic data, excess entropies can be calculated. Excess free energies, enthalpies, and entropies are compared at 0.5 m, and the observed trends are consistent with a model of structural interactions in aqueous alkali halide solutions. The apparent molal heat capacities phi(C) were fitted with the equation phi(C) = phi degrees(C)+ A(C)(d(0)m)(1/2)+ B(C)m. The phi degrees(C) are, in general, additive to better than 1 J-K-1-mole(-1) and reflect mostly the structural part of ion-solvent interactions. Taking phi degrees(C)( H+) = 0, conventional ionic phi degrees(C) are obtained. The parameter B-C for different pairs of ions follows approximately the same trends as the corresponding parameter B-V for apparent molal volumes and seems to reflect structural interactions between the ions.