Understanding hydration of Zn(2+) in hydrothermal fluids with ab initio molecular dynamics

With ab initio molecular dynamics simulations, the free-energy profiles of hydrated Zn2+ are calculated for both gaseous and aqueous systems from ambient to supercritical conditions, and from the derived free-energy information, the speciation of hydrated Zn2+ has been revealed. It is shown that the 4, 5 and 6 fold complexes coexist in both phases at ambient condition. As T-P increases a clear trend is found for both phases: the 6 fold states gradually become unstable and cannot exist stably any more over 620 K, but in contrast, the 4 and 5 fold states can hold up to 1000 K with similar probabilities. It is found that the stability of the 4 and 5 fold states has an entropic origin. This study constitutes a relatively complete speciation picture for aqua-zinc complexes over a broad T-P range.