A quantum mechanically derived force field to predict CO2Adsorption on calcite {10.4} in an aqueous environment

Abstract

© 2017 American Chemical Society. Density functional theory (DFT) with semiempirical dispersion corrections (DFT-D2) has been used to calculate the binding energy of a CO 2 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 CO 2 binding to calcite, CO 2 binding with Ca 2+ , and pairing of two CO 2 molecules, all for aqueous conditions. We subsequently performed classical molecular dynamics (MD) simulations of CO 2 and water on the {10.4} surface at pressures and temperatures relevant for CO 2 geological storage. Density profiles show characteristic structured water layering at the calcite surface and two distinct phases of water and CO 2 . We have also calculated the densities of the CO 2 -rich and water-rich phases and thereby determined the mutual solubilities. For all the pressures and temperatures in the studied range, CO 2 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 CO 2 with Ca 2+ . 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 CO 2 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.