THE CRYSTALLIZATION OF ALKALI-HALIDES FROM AQUEOUS-SOLUTION - AN APPLICATION OF DENSITY-FUNCTIONAL THEORY

Density-functional theory is applied to the problem of salt crystallization from solution and explicit results are given for model aqueous alkali-halide systems. Both direct- and Fourier-space methods of calculation are considered and it is found that only the direct (i.e., r space) method converges sufficiently rapidly to provide reliable results for ionic crystals at 25-degrees-C. It is shown that the density-functional method is capable of predicting crystallization, but that the solid-state parameters and, for some salts, the crystal structures obtained are in poor agreement with experiment or computer simulations. The calculated crystal/solution coexistence concentrations are found to be extremely sensitive to the short-range part of the interionic pair potentials. This is consistent with earlier observations that the activity coefficients of model aqueous alkali-halide solutions are very strongly dependent upon the short-range ion-ion interactions. Therefore, we do not believe that this sensitivity to details of the short-range interionic potentials is an artifact of theoretical approximations, but rather a real effect significantly influencing crystallization.