The effect of ions on solid-liquid phase transition in small water clusters. A molecular dynamics simulation study

Small water clusters, containing ions, have been studied using molecular dynamics simulations at temperatures ranging from 0 to 250 K. The simulations are carried out systematically by varying the ion size, shape, and charge as well as the cluster size and the initial configuration. Transitions between solid and liquid phases are followed to study the effects of the ions on the cluster melting temperature, compared to pure water clusters of the same size. The effect of the ion on the ice-cluster melting appears to be a complicated process which depends simultaneously on a variety of factors, such as the initial cluster configuration and the ion position inside the cluster as well as the ion mass, size and its charge. In the case of monovalent cations the most important characteristics for the cluster evolution is the ion mass, while for divalent cations the ion charge is the most dominant factor. In the case of negatively charged ions the main factor of the system evolution is the ion size. Two principally different types of cluster structures can be observed from the simulations: The peripheral structure where the ion takes up a position, preferably on the cluster surface and the interior structure where the ion prefers the center of the system. The peripheral structure is typical for clusters containing the small monovalent Li+ cation but also for those containing the large Cl- anion, while divalent cations, large monovalent Na+ cation and small F- anions gave rise to the interior type of structure. Generally, an increase of the ion size changes the cluster structure making the peripheral variant more preferable. (C) 2003 American Institute of Physics.