Speciation of Ca~(2+) in water as a function of temperature, pressure and pH: Ab-initio molecular dynamics simulation study

摘要

In order to understand and model both the macroscopic thermodynamic properties as well as the atomistic structure of high-pressure aqueous solutions in systems such as CaO-SiO_2-H_2O, new data on the speciation of dissolved Ca are required (e.g., Adeagbo and Doltsinis, 2007; Adeagbo et al., submitted; Doltsinis et al., 2007a, 2007b). In this work, we have used first principles molecular dynamics simulations to study the Ca~(2+) ion in water at various temperatures and pressures, starting with either Ca~(2+) or CaO in a reactive volume of 63 H_2O molecules. In the case of aqueous Ca~(2+), the ion is surrounded by six H_2O 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 H_2O molecules to form Ca~(2+) + 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 Ca~(2+) 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 Ca~(2+), Ca(OH)~+, and Ca(OH)_2 species are 14, 55, and 29 percent, while at 900 K / 0.9 GPa they are 2, 33, and 65 percent, respectively.