Zinc Complexation in Hydrothermal Chloride Brines: Results from ab Initio Molecular Dynamics Calculations

We determined the coordination environment of Zn 2+ in aqueous Cl - brines at 25 °C and 300 °C using ab initio molecular dynamics simulations. The ZnCl + and ZnCl 2 complexes exist as pseudo-octahedral ZnCl m (H 2 O) 6 - m clusters at 25 °C but occur as pseudo-tetrahedral ZnCl m (H 2 O) 4 - m clusters at 300 °C. The ZnCl 3 - complex occurs as the pseudo-tetrahedral ZnCl 3 (H 2 O) - cluster at 25 and 300 °C. The tetrahedral ZnCl 4 2- complex, however, is the dominant Zn−Cl complex at 25 °C, at least in highly concentrated (7.4 m) Cl - brines. The change in hydration number with temperature for the ZnCl + and ZnCl 2 complexes will complicate extrapolations of solvation energies to hydrothermal conditions using a Born-model-based equation of state.