Atomistic Mechanism of NaCl Nucleation from an Aqueous Solution

Despite great technological relevance, the initial steps of nucleation and crystal growth from solution are still poorly understood. While experimentally difficult to access, simulations in principle may provide insight at the atomic level. However, in most cases the computational demand dramatically exceeds the scope of current hardware. Since crystallization usually occurs on time scales much larger than the few ns of a molecular dynamics simulation, special techniques for the study of rare events are of particular interest. In the present work the nucleation of sodium chloride aggregates from aqueous solution is investigated from path sampling molecular dynamics simulation. The introduced simulation schemes appear to be widely applicable.

Figure 1

Formation of a $[{\mathrm{N}\mathrm{a}}_x{\mathrm{C}\mathrm{l}}_y]^{x-y}{}_{\mathrm{a}\mathrm{q}}$ aggregate in water (not shown) as observed in a 150 ps trajectory obtained from path sampling. Sodium and chloride ions are colored blue and green, respectively. Entirely nonhydrated ions are highlighted in purple. Snapshots are taken in intervals of 25 ps.

Figure 2

(a) Configuration of a selected $[{\mathrm{N}\mathrm{a}}_x{\mathrm{C}\mathrm{l}}_y]^{x-y}{}_{\mathrm{a}\mathrm{q}}$ aggregate at the end of the 1 ns simulation run. Sodium and chloride ions are represented as blue and green balls, respectively. The water molecules of the first coordination sphere of the displayed ions are shown as sticks; all other molecules were cut. (b) Configuration of a selected $[{\mathrm{N}\mathrm{a}}_x{\mathrm{C}\mathrm{l}}_y]^{x-y}{}_{\mathrm{a}\mathrm{q}}$ aggregate at the end of the 1 ns simulation run. As observed in the majority of our simulation runs, one sodium ion (purple) is entirely free of hydration water molecules. The coordination by chloride ions is similar to those in the crystal structure, though the Cl-Na-Cl angles may deviate by $\pm 10°$ from the ideal $90°$. The Na-Cl distances vary from 2.8 to $3.1\AA$ and thus tend to be larger than in the crystal structure. (c) Configuration of one out of two $[{\mathrm{N}\mathrm{a}}_x{\mathrm{C}\mathrm{l}}_y]^{x-y}{}_{\mathrm{a}\mathrm{q}}$ aggregates, exhibiting two nonhydrated sodium ions (purple) at the end of the 1 ns simulation run. The distance of these sodium ions is $0.1\AA$ larger than in the crystal structure. The ${\mathrm{C}\mathrm{l}}^{-}$ octahedra coordinating two nonhydrated sodium ions are strongly distorted. The corresponding Cl-Na-Cl angles range from $75°$ to $110°$.