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

摘要

In order to understand and model both the macroscopic thermodynamic properties as well as theatomistic structure of high-pressure aqueous solutions in systems such as CaO-Si02-H20, new dataon 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 moleculardynamics simulations to study the Ca2+ ion in water at various temperatures and pressures, startingwith either Ca2+ or CaO in a reactive volume of 63 H20 molecules. In the case of aqueous Ca2+, theion is surrounded by six H20 molecules in the first hydration shell at 300 K / 0.3 GPa, with rareexchange between first and second hydrations shells. At 900 K / 0.9 GPa the coordination number inthe first hydration shell fluctuates between six and eight, the average being 7.0. CaO immediatelyreacts with the surrounding H20 molecules to form Ca2+ + 20H–. The hydroxyl ions form transientCa(OH)+ and Ca(OH)2 complexes and have a mean residence time in the first coordination shell ofCa `+ of 6 ± 4 ps at 500 K and 3 ± 3 ps at 900 K, respectively. At 500 K / 0.5 GPa the time-averagedrelative concentrations of the transient Ca2+, Ca(OH)+, and Ca(OH)2 species are 14, 55, and 29percent, while at 900 K / 0.9 GPa they are 2, 33, and 65 percent, respectively.