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 similar to 1 nm, including similar to 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. (C) 2013 Elsevier B. V. All rights reserved.