Molecular Structure and Thermodynamics of CO₂ and Water Adsorption on Mica

Abstract

The adsorption of CO2 and water on clay surfaces plays a key role in applications such as gas storage in saline aquifers and depleted hydrocarbon reservoirs but is not yet fully understood. Here, the adsorption of CO2 and water vapor is studied using Grand Canonical Monte Carlo and molecular dynamics simulations. At a bulk pressure of 100 bar, pure CO2 adsorbs strongly on mica and forms extensive layers next to it. CO2 adsorption is lowered substantially if introducing water vapor above mica and is largely eliminated when the relative humidity (RH) approaches about 60%. When pure water vapor is introduced above a mica surface, a sub-nm thick liquid water film develops on it to form apparent liquid-solid and liquid-vapor interfaces simultaneously. Using the ITIM (Identification of the Truly Interfacial Molecules) analysis, how individual water layers develops in this film as RH increases is delineated. It was discovered that the water film is spatially heterogeneous, and the true liquid-vapor interface emerges only at an RH of 60-80%. Introducing 100-bar CO2 into the water vapor above the mica surface modulates water adsorption nonlinearly: at RH = 0.01%, the water adsorption is reduced by ≈30%; as RH increases, the reduction is weakened, and eventually, enhancement of water adsorption by about 7% occurs at RH = 90%. These variations are attributed to the interplay of film thinning by high-pressure CO2, competition of mica surface sites by CO2 molecules, and the energetic and entropic stabilization of interfacial water by CO2 molecules. This work has important implications for subsurface energy and environmental technologies. For example, it indicates that, in depleted unconventional reservoirs, assuming a completely dry environment can grossly overestimate the CO2 storage contributed by adsorption on mineral surfaces. Furthermore, it suggests that the swelling of clay in very low relative humidity can be suppressed by the displacement of interstitial water by high-pressure CO2, which can compromise the mechanical integrity of caprocks in underground hydrogen storage sites employing CO2 as cushion gas. These implications warrant experimental studies in the future.