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 degreesC and 300 degreesC 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 degreesC but occur as pseudo-tetrahedral ZnCl(m)(H(2)O)(4-m) clusters at 300 degreesC. The ZnCl(3)(-) complex occurs as the pseudo-tetrahedral ZnCl(3)(H(2)O)(-) cluster at 25 and 300 degreesC. The tetrahedral ZnCl(4)(2-) complex, however, is the dominant Zn-Cl complex at 25 degreesC, 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.