Nucleation is scientifically and technologically important, but the mechanisms and kinetics remain elusive to both theory and experiment. In this contribution, we provide insights into sodium chloride (NaCl) nucleation from supersaturated brines using seeded atomistic simulations and elements of classical nucleation theory.<br/>NaCl is appealing because force fields exist that reliably reproduce experimental data on key properties: the solution chemical potential and the solubility limit. With such a model, we find that ion depletion was low enough in our seeded simulations so that the driving force did not changed significantly. For a given supersaturation, the critical nucleus size, n*, is determined from a series of simulations in which the seed size was varied, by finding the transition from nucleus shrinkage to growth.<br/>We test two models of attachment kinetics to the critical nucleus, which reflect diffusion limitation and ion-desolvation limitation, respectively. The diffusion analysis required the calculation of mutual electrolyte diffusivities at varying concentration, which agree well with experimental results from interferometry. The diffusion model grossly overestimates the attachment frequency compared to an analysis of the mean square change in nucleus size. Therefore, ion-desolvation is identified as the limiting resistance to attachment. A second independent analysis, based on approach-to-coexistence data, confirms the negligible influence of diffusion.<br/>We considered three different supersaturations for our seeded simulations based on available nucleation rate measurements in literature. That permits the first direct comparison to experiments for this system. The computed and measured rates converge to a common upper limit at extremely high supersaturation, which is rationalized with an upper bound estimate from ion-association times. Our rate predictions from seeded simulations are up to 30 orders of magnitude faster than experiments. We conclude by discussing possible short-comings in both the simulation and experimental procedure.

Nucleation of NaCl from aqueous solution: critical sizes, ion-attachment kinetics, and rates

251st National Meeting (2016)

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