Ca2+ solvation as a function of p, T, and pH from ab initio simulation

First principles molecular dynamics simulations have been carried out at various temperatures and pressures starting with either Ca2+ or CaO in a reactive volume of 63 H2O molecules. In the case of aqueous Ca2+, the ion is surrounded by six H2O molecules in the first hydration shell at 300 K/0.3 GPa, with rare exchange between first and second hydrations shells. At 900 K/0.9 GPa, the coordination number in the first hydration shell fluctuates between six and eight, the average being 7.0. CaO immediately reacts with the surrounding H2O molecules to form Ca2+ + 2OH(-). The hydroxyl ions form transient Ca(OH)(+) and Ca(OH)(2) complexes and have a mean residence time in the first coordination shell of Ca2+ of 6 +/- 4 ps at 500 K and 3 +/- 3 ps at 900 K, respectively. At 500 K/0.5 GPa, the time-averaged relative concentrations of the transient Ca2+, Ca(OH)(+), and Ca(OH)(2) species are 14%, 55%, and 29%, while at 900 K/0.9 GPa, they are 2%, 34%, and 63%. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4754129]