Evidence that crystal nucleation in aqueous NaCl solution Occurs by the two-step mechanism

Molecular dynamics simulations are used to investigate the mechanism of crystal nucleation in NaCl solution. Our observations support a two-step mechanism, similar to that recently proposed for other systems [P.G. Vekilov, Nanoscale 2 (2010) 2346]. The two sequential steps consist of a concentration fluctuation resulting in a dense ion “patch” followed by spatial ordering. The second, spatial-ordering step is rate determining, consistent with some protein solutions. The “size” of the critical NaCl nucleus is ∼ 1 nm, including ∼ 75 ions at ambient temperatures. Given that evidence for the two-step mechanism is found for solutes as different as NaCl and proteins, it appears to be an important mechanism of homogeneous crystal nucleation.