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

Area of Science:

• Computational chemistry
• Physical chemistry
• Materials science

Background:

• Understanding the behavior of calcium ions and calcium oxide in aqueous solutions is crucial for various chemical and geological processes.
• Previous studies have explored hydration shells and reactions, but detailed molecular-level insights under varying conditions are needed.

Purpose of the Study:

• To investigate the hydration structure and dynamics of calcium ions (Ca2+) and the reaction of calcium oxide (CaO) with water using first principles molecular dynamics.
• To elucidate the formation and stability of transient calcium hydroxide complexes under different temperature and pressure conditions.

Main Methods:

• First principles molecular dynamics (FPMD) simulations were performed.
• Simulations involved varying temperatures (300 K to 900 K) and pressures (0.3 GPa to 0.9 GPa).
• Systems contained 63 H2O molecules with either Ca2+ or CaO as the starting species.

Main Results:

• Aqueous Ca2+ exhibits a first hydration shell of approximately six water molecules at 300 K/0.3 GPa, with limited exchange.
• At 900 K/0.9 GPa, the coordination number of Ca2+ fluctuates between six and eight, averaging 7.0.
• CaO reacts with water to form Ca2+ and hydroxide ions (OH-), leading to transient Ca(OH)+ and Ca(OH)2 complexes. The residence time of OH- in the Ca2+ coordination shell decreases with increasing temperature.
• The relative concentrations of Ca2+, Ca(OH)+, and Ca(OH)2 species shift towards Ca(OH)2 at higher temperatures (900 K/0.9 GPa).

Conclusions:

• The hydration shell of Ca2+ is dynamic and sensitive to temperature and pressure.
• CaO readily reacts with water, and the resulting hydroxide ions form stable complexes with Ca2+.
• The speciation of calcium in aqueous solutions is strongly influenced by temperature and pressure, favoring less hydrated forms at elevated conditions.