Molecular dynamics simulations employing large systems (up to 100,000 particles) are used to investigate the mechanisms of crystal nucleation and dissolution in aqueous NaCl solutions. It is shown that crystal nucleation occurs via a two-step mechanism, similar to that which has been proposed for some other solutes such as proteins. The two sequential steps consist of a concentration fluctuation which results in a local region of high ion concentration, followed by spatial ordering. The first step occurs much more frequently than the second, such that spatial ordering determines the nucleation rate. We estimate that the critical NaCl nucleus contains 70-80 ions at ambient temperatures. The fact that the two-step mechanism applies to solutes as different as NaCl and proteins suggests that it is an important mechanism of homogeneous crystal nucleation. The dissolution of NaCl nanocrystals of varying size (up to 1400 ion pairs) and shape (cubes, spheres, rods, and tablets) is also investigated. The dissolution rate and mechanism is shown to strongly depend on these physical variables. Interestingly, however, in some cases conventional macroscopic rate laws are shown to apply until the crystal size is reduced to a few hundred ion pairs.

Nucleation and dissolution of NaCl crystals in aqueous solution

248th National Meeting (2014)

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