Melting, freezing and nucleation in nanoclusters of potassium chloride - I - Molecular dynamics simulation

Molecular dynamics simulations of the melting, freezing and nucleation are presented for unconstrained nanoclusters of KCl with a number of ions between 512 and 10648. The maximum extent of the probed liquid supercooling is analysed to the light of theoretical predictions and compared with experimental data. The fraction of the solid-like ions in the supercooled liquid is used as an indicator of heterogeneities within the liquid. Induced nucleation by seeding the supercooled liquid indicates that solid-liquid coexistence is stable, and sustained during the lifetime of the clusters, relatively to the supercooled liquid. A phenomenological analysis on the relaxation times of the crystal growth process is made. Critical nuclei sizes computed from the effectiveness of the seeds in the heterogeneous nucleation of the supercooled liquid, and from the residual crystallites in clusters not totally melted, are presented as a function of the temperature. The behavior of the systems is followed through various properties such as liquid and solid molar fractions, enthalpies of melting, heat capacities, self-diffusion coefficients and relaxation times related to the freezing process. The consistency of the simulation results for the heterogeneous nucleation is assessed by means of a classical nucleation model, from which an estimate of the interfacial surface tension is also worked out and compared with experimental data.