Forward Flux Sampling Calculation of Homogeneous Nucleation Rates from Aqueous NaCl Solutions

TL;DR

This study employs molecular dynamics and forward flux sampling to calculate homogeneous nucleation rates of NaCl from supersaturated solutions, revealing lower rates than experiments and emphasizing the role of crystallinity in nucleation.

Contribution

It introduces a non-classical approach to nucleation rate calculation and highlights the importance of crystallinity in the nucleation process of NaCl from aqueous solutions.

Findings

Nucleation rates are significantly lower than experimental data.

Classical nucleation theory better describes the observed pathways.

Crystallinity influences nucleus growth and stability.

Abstract

We used molecular dynamics simulations and the path sampling technique known as forward flux sampling to study homogeneous nucleation of NaCl crystals from supersaturated aqueous solutions at 298 K and 1 bar. Nucleation rates were obtained for a range of salt concentrations for the Joung-Cheatham NaCl force field combined with the SPC/E water model. The calculated nucleation rates are significantly lower than available experimental measurements. The estimates for the nucleation rates in this work do not rely on classical nucleation theory, but the pathways observed in the simulations suggest that the nucleation process is better described by classical nucleation theory than an alternative interpretation based on Ostwald's step rule, in contrast to some prior simulations of related models. In addition to the size of NaCl nucleus, we find that the crystallinity of a nascent cluster plays an important role in the nucleation process. Nuclei with high crystallinity were found to have higher growth probability and longer lifetimes, possibly because they are less exposed to hydration water.