A computational study toward understanding the separation of ions of potassium chloride microcrystal in water

Abstract

The dissolution phenomenon of potassium chloride microcrystal in water has been studied using DFT calculations and molecular dynamics studies. DFT study reveals the departure of Cl− to be more pronounced from the edge positions compared to the corner sites of the KCl [(KCl)6(H2O) n , n = 1–15] microcrystal lattice. The dissolution initiates through the movement of a Cl− from the edge of the crystal lattice (5.19 A) at n = 4 water molecules in agreement with the separation of ions from a single KCl molecule. This separation is more evident with the cluster of 6 water molecules (6.12 A). The characteristics of KCl dissolution dynamics, such as the sequential departure of ions from the crystal, the hydrated ions and the dynamical role of the water molecules, are further studied by classical molecular dynamics simulations employing GROMACS force field. Molecular dynamics calculations are performed with a larger crystal of KCl with {100} plane consisting of 108 K+ and 108 Cl− ions. The MD studies have been extended with relatively unstable planes of KCl {110} (consisting of 105 K+ and 105 Cl− ions) and {111} (consisting of 120 K+ and 120 Cl− ions). The simulations revealed that the dissolution of {110} and {111} planes is relatively faster than that of the stable {100} plane. A mean square displacement analysis also supported this observation. The dissolution of the ions generally occurs from the top layer of {100} surface, while other layers remain intact. However, such a definite pattern of dissolution is not noticed with {110} and {111} planes.