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 ∼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.