Systematic thermodynamics of hydration (and of solvation) of inorganic solids.

The thermodynamics of the formation of solid and liquid inorganic hydrates and ammoniates is examined. In earlier studies, average values of the Gibbs energy of reaction, Delta(r)G, assuming a constant additivity per mole of bound water, have been obtained and have suggested that hydration is always marginally thermodynamically favorable. More detailed consideration now demonstrates that the mean value of Delta(r)G per mole of water, from anhydrous parent to hydrate within a sequence, increases consistently toward zero, becoming progressively less favorable as the degree of hydration, n, increases, and broadly independent of any structural features of the materials. Furthermore, the consistent behavior suggests that missing intermediate hydrates in hydrate sequences are likely to be thermodynamically stable, even if difficult to prepare, isolate, or measure.The behavior of ammoniates is similar but less regular, the irregularity being ascribed to a wider range of interactions within the solid ammoniates than in the hydrates. The "Ostwald Rule of Stages" suggests that the first precipitate from a supersaturated solution is usually a metastable phase, having an intermediate value of the Gibbs energy between that of the anhydrous parent and of the thermodynamically stable phase, then progressing to the stable phase with the lowest Gibbs energy, implying kinetic rather than thermodynamic control of the sequence of precipitation. The implications for hydrate formation are briefly considered.