Birth of NaCl Crystals: Insights from Molecular Simulations

Molecular dynamics simulations are used to investigate the factors that influence the nucleation of NaCl crystals in a supersaturated aqueous solution. We describe a methodology for detecting solidlike NaCl clusters (potential nuclei) and following their evolution in time until they achieve nucleation (which is very rare) or dissolve back into solution. Through an analysis of cluster lifetimes and multiple nucleation events, we demonstrate that cluster size is not the only property that influences cluster stability and the probability of achieving nucleation. We introduce a parameter called cluster crystallinity, which is a measure of the solidlike order in a particular cluster. We show that cluster order (as measured by this parameter) has a strong influence on the lifetime and nucleation probability of clusters of equal sizes, with the lifetime and probability of nucleation increasing with increasing crystallinity. These observations remain true for clusters as small as six ions, showing that the structural factors are important even at the earliest stages of crystal birth.