Computational Insights into Mg2+ Dehydration in the Presence of Carbonate

Water exchange around a free magnesium ion and magnesium paired with carbonate in aqueous solution was studied using free energy methods. Both a rigid-ion and a polarizable force field based on the AMOEBA model were examined. The parameters were adjusted to accurately reproduce the hydration structures of magnesium and carbonate in aqueous solution. The magnesium carbonate ion pairing free energies calculated with both force fields were found to be in excellent agreement with experimental data. Metadynamics simulations of the water exchange conducted with both models revealed that the formation of a contact magnesium carbonate ion pair significantly decreases the energy barrier for water exchange relative to the free magnesium ion in solution. This finding suggests that the presence of carbonate could accelerate the water exchange around magnesium and constitutes a first step toward a better understanding of the atomic-scale mechanisms involved in the nucleation of magnesium-bearing carbonate minerals.