Nucleation of NaCl from Aqueous Solution: Critical Sizes, Ion-Attachment Kinetics, and Rates

Nucleation
and crystal growth are important in material synthesis,
climate modeling, biomineralization, and pharmaceutical formulation.
Despite tremendous efforts, the mechanisms and kinetics of nucleation
remain elusive to both theory and experiment. Here we investigate
sodium chloride (NaCl) nucleation from supersaturated brines using
seeded atomistic simulations, polymorph-specific order parameters,
and elements of classical nucleation theory. We find that NaCl nucleates
via the common rock salt structure. Ion desolvationnot diffusionis
identified as the limiting resistance to attachment. Two different
analyses give approximately consistent attachment kinetics: diffusion
along the nucleus size coordinate and reaction-diffusion analysis
of approach-to-coexistence simulation data from Aragones et al. (J. Chem. Phys. 2012, 136, 244508). Our simulations were performed at realistic
supersaturations to enable the first direct comparison to experimental
nucleation rates for this system. The computed and measured rates
converge to a common upper limit at extremely high supersaturation.
However, our rate predictions are between 15 and 30 orders of magnitude
too fast. We comment on possible origins of the large discrepancy.