The reactivity of caprock minerals in H₂O-bearing supercritical (sc) CO₂ fluids has received significant attention due to its relevance to the chemistry of carbon sequestration. In particular, divalent cation-rich silicate minerals such as olivines have a high potential for undergoing carbonation reactions and thus can strongly influence the long-term storage of carbon. In these conditions, water partitions onto olivine surfaces as quasi two-dimensionally confined films that are just Ångstroms to nanometers thick. Within these thin films, CO₂ reacts with H₂O to form carbonic acid, silicate minerals dissolve, and metal carbonates nucleate and grow. However, the H₂O-film thickness needs to exceed specific thresholds for these reactions to proceed. In this presentation, we will report classical molecular dynamics (CMD) simulations that provide a mechanistic understanding of carbonation thresholds. Potential of mean force (PMF) simulations were performed for a Mg²⁺ ion adsorbed at the Mg₂SiO₄ (010)/H₂O/scCO₂ interface for a range of H₂O-film thicknesses. One inner-sphere complex and three outer-sphere (OS) complexes were identified in the CMD simulations, and the PMF simulations showed that only the OS farthest from the Mg₂SiO₄ surface was laterally mobile. The free energy of this OS complex was extremely sensitive to the H₂O-film thickness. Consequently, the CMD-PMF simulations predicted that the onset of Mg²⁺ diffusion was due to an abrupt decrease in the free-energy barriers for lateral mobility of the outer-spherically adsorbed Mg²⁺ as the H₂O film reached a threshold thickness. The predicted H₂O-film thickness threshold was in good agreement with that obtained experimentally through infrared and electrical impedance spectroscopies.

John Loring

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