Computer modeling of melting of ionized ice microcrystals

Ionized water clusters, OH-(H2O)(N) and H3O+(H2O)(N), of different sizes (N=19 and 26) have been studied at temperatures ranging from 10 to 200 K using molecular dynamics simulations. The solid-liquid phase transitions are investigated to estimate the effects of the presence of an ion on the melting temperature of the clusters. It was found that the behavior of the aggregates during the melting is determined mainly by water-water interactions. Compared to corresponding pure water clusters, the observed changes in the melting temperature, T-m, are small and within the statistical uncertainty of the simulations. A weak trend can be observed with the hydroxyl ion reducing the T-m, while there is a slight tendency for an increase of T-m for clusters containing the hydronium ion. In general, the ions disrupt the hydrogen bond network and at the same time, the formation of a strong hydration shell contributes to a decrease of the mobility of the molecules. These two phenomena affect the solid-liquid phase transition temperature to opposite directions and seem to largely compensate each other in the simulated clusters. (C) 2003 American Institute of Physics.