Dissolution of an Nacl crystal with the (111) and (−1−1−1) faces

Abstract

The dissolution process of an NaCI crystal with the (111) and (−1−1−1) faces, the former consisting of only chloride ions and the latter only sodium ions, as well as the {10 0} faces, in water has been demonstrated by means of molecular dynamics simulation. Ion-ion, ion-water and water-water interactions are assumed to be described in terms of the Tosi-Fumi, Popkie-Kistenmacher-Clementi and Matsuoka-Clementi-Yoshimine potentials, respectively. Twenty-eight sodium ions, twenty-eight chloride ions and 189 water molecules were placed in a box having the side-length of 2000 pm. Collision of water molecules with the walls of the box was assumed to be completely elastic. The temperature of the system was kept at 298 K during the simulation procedure, which was carried out for 7 ps (the time step At — 1.0 x 10-15s, the total steps performed were 7000) after starting dissolution of the NaCI crystal. The first, second and third ions dissolved are chloride ions at the corners of the crystal, as have been found in the previous work (ref. 1). The fourth one liberated was also a chloride ion on the (1 1 1) face. As we have seen in the previous simulation using another NaCI crystal with the {1 0 0} faces (ref. 1), no sodium ion was removed within 7 ps even from the (−1−1−1) face which was exposed to the bulk water phase. Repulsive forces arising between the chloride ions and water molecules which tend to hydrate sodium ions around the chloride ions are the force for separating the chloride ions from the crystal. © 1989 IUPAC.