How Crystals Nucleate and Grow in Aqueous NaCl Solution

Large-scale molecular dynamics simulations (64 000 particles) are used to examine the microscopic mechanism of crystal nucleation and growth in a slightly supersaturated solution of NaCl in water at 300 K and 1 atm. Early-stage nucleation is observed, and the growth of a single crystal is followed for similar to 140 ns. It is shown that the nucleation and growth process is better described by Ostwald's rule of stages than by classical nucleation theory. Crystal nucleation originates in a region where the local salt concentration exceeds that of the bulk solution. The early-stage nucleus is a loosely ordered arrangement of ions that retains a significant amount of water. The residual water is slowly removed as the crystal grows and evolves toward its stable anhydrous state.