Molecular dynamics studies of the kinetics of freezing of (NaCl)108 clusters

Molecular dynamics simulations adopting the Born-Mayer-Huggins potential function were performed on a series of (NaCl)(108) clusters to observe their properties during heating and cooling and to monitor their nucleation events during freezing. Melting was found to be considerably sharper than that of clusters of softer materials whose solid form, unlike that of salt, is well-wetted by the melt. The resulting liquid clusters were observed to be greatly distorted from a spherical shape by capillary waves. The freezing of highly supercooled liquid clusters was found to be in qualitative accord with the classical theory of homogeneous nucleation. Moreover, the interfacial free energy parameter sigma(sl) inferred via nucleation theory, namely, 117.7, 117.6, and 119.6 mJ/m(2) at 500, 525, and 550 K, respectively, was close to the value predicted by the empirical Turnbull relation and in crude agreement with an experimental value determined at a much lower degree of supercooling and derived on a very different basis. Nevertheless, the thickness of the interface between the liquid and solid implied by Granasy's diffuse interface theory and by density functional calculations was so large in comparison with the radius of the critical nucleus according to the classical theory as to cast considerable doubt about the quantitative applicability of that theory to the present clusters. On the other hand, the MD nucleation rates, taken together with the prior and somewhat speculative experimental result, indicate that the classical theory performs better than the diffuse interface theory. Calculations and possible experiments to clarify the situation are outlined.