A Quantum Mechanically Derived Force Field To Predict CO₂ Adsorption on Calcite {10.4} in an Aqueous Environment

Density functional theory (DFT) with semiempirical dispersion corrections (DFT-D2) has been used to calculate the binding energy of a CO₂ molecule on the calcite {10.4} surface for different positions and orientations. This generated potential energy landscape was then used to parametrize a classical force field. From this, we used metadynamics (MTD) to derive free energy profiles at 300 and 350 K for CO₂ binding to calcite, CO₂ binding with Ca²⁺, and pairing of two CO₂ molecules, all for aqueous conditions. We subsequently performed classical molecular dynamics (MD) simulations of CO₂ and water on the {10.4} surface at pressures and temperatures relevant for CO₂ geological storage. Density profiles show characteristic structured water layering at the calcite surface and two distinct phases of water and CO₂. We have also calculated the densities of the CO₂-rich and water-rich phases and thereby determined the mutual solubilities. For all the pressures and temperatures in the studied range, CO₂ was unable to penetrate the ordered water layers and adsorb directly on the solid surface. This is further confirmed by the free energy profiles showing that in the presence of water there is neither direct adsorption to the {10.4} surface nor contact binding of CO₂ with Ca²⁺. Rather, we saw a weak affinity for the surface of the ordered water layers. At 5 MPa and 323 K, we observed the nucleation of a CO₂ droplet located above two structured water layers over the solid. It could not penetrate the structured water but remained bound to the second water layer for the first 10 ns of the simulation before eventually detaching and diffusing away.