Dissolution and nucleation phenomena of salts in water. Molecular dynamic approaches and supporting solution X-ray diffraction measurements

Molecular dynamics (MD) simulations have been demonstrated to elucidate the dissolution and nucleation phenomena at a molecular level for a variety of alkali halide salts in water at 25 "C. Within 12 to 20 ps anions in the LiCI, NaCl and CsF crystals have dissolved, but dissolution of cations has not been observed within the same time period. For KCI, NaF, and KF crystals which have cations and anions with similar size, no ions have dissolved within 20 ps, although the crystal lattice has significantly distorted. Thus, the rate of dissolution of ions is lower in crystals having the cations and anions with a similar size than in those with different sizes. In supersaturated aqueous solutions of NaCl and CsF ionic microclusters are formed which might be regarded as embryoes of nuclei of crystals. According to solution X-ray diffraction measurements of almost saturated aqueous solutions of NaCl (6.18 mol (kg H2O)-l) and KCI (4.56 mol (kg H20)-l), 1:l ion pairs are formed. On the other hand, ion clusters with the average coordination numbers of the cation-anion pairs, n ~ x , of 2.29 and 3.33 are formed in KF (16.15 mol (kg H20) l ) and CsF (31.96 mol (kg H2O)-l) solutions, respectively, the results being in good agreement with those obtained by the MD simulations.