Molecular dynamics simulation of nucleation in the freezing of molten potassium iodide clusters

Molecular dynamics simulations have been carried out to study the effects of cluster size and temperature on the nucleation rate in potassium iodide clusters in the temperature range of 400-580 K. The clusters (KI)(108), (KI)(256), and (KI)(500) were studied. The rate at which nuclei materialized per unit volume appeared to decrease with increasing cluster size. This size dependence could be accounted for by the agency of three roughly equal factors, namely, the Laplace pressure, the larger coefficients of diffusion for the smaller clusters, and the area-to-volume ratio. The latter factor arises because nucleation invariably occurs at the surface rather than in the interior of the clusters. Sizes of critical nuclei obtained by direct counting of ordered molecules were considerably larger than predicted by the classical nucleation theory unless the outer layer of ordered molecules was subtracted from the total count. The rationale for such a subtraction is that the outer layer corresponds to the ordered liquid layer predicted by Turnbull. Consistent with Kashchiev's criterion, the number of nuclei per cluster increased with cluster size and with increased supercooling. Nucleation time lags decreased as the degree of supercooling increased. The temperature dependence of the kinetic parameters of the interfacial free energy between the solid and the liquid and the Granasy interface thickness derived from the nucleation rates are discussed. A pictorial account of nucleation and subsequent crystal growth in a typical (KI)(500) cluster is presented.