Molecular Dynamics Study of Aqueous NaCl Solutions: Flash Crystallization Caused by Solution Phase Change

Abstract

Solutions under sub- or supercritical conditions receive much attention because of their significance in geology and industrial applications. One of the most important properties is their very low salt solubility, which leads to rapid crystallization. The morphology of sodium chloride crystals formed in supercritical fluids has been shown to be affected by the presence of the high-concentration liquid phase and low-concentration vapor phase [Armellini and Tester, J. Supercritical Fluid 4, 254–264 (1991)]. However, because of the short time scales involved, experimental observation of the underlying mechanism is difficult. In the present study, microsecond-scale molecular dynamics, which provide insight with picosecond resolution, were conducted for the NaCl salt–solution interfacial system at sub- and super-critical conditions. We propose the utilization of the correlation between the number density of ions and water to parameterize the solution phase. This correlation and the two-dimensional number densities provide insight into flash crystallization at the atomic scale. Vapor–liquid phase coexistence was found at high pressure and the diffusive high-concentration liquid phase could transport sodium and chloride ions to form a compact solid phase. In contrast, an isolated crystal forms at low pressure owing to the rapid volume expansion of the vapor phase.