Molecular Understanding of CO2 and H2O in a Montmorillonite Clay Interlayer under CO2 Geological Sequestration Conditions

Grand canonical Monte Carlo (GCMC) simulations are carried out to investigate a supercritical carbon dioxide (scCO(2))-water mixture in the Na-montmorillonite clay interlayer under typical CO2 geological sequestration conditions (T = 323 K, P = 90 bar and T = 348 K, P = 130 bar). The stable clay interlayer distances at different relative humidity (RH) are determined based on the normal pressure and free energy curves of the CO2-H2O-Na+ complex in the montmorillonite clay interlayer. Simulation results show that stable monolayer hydrates (1W) with a basal spacing around 12 angstrom are formed at RH = 30-60%. As RH is increased to 70% and above, bilayer CO2-H2O mixtures with a basal spacing around 15-16 angstrom (2W) are more stable. In general, the CO2 intercalation process is strongly influenced by RH. While a high relative humidity facilitates water molecules entering the clay interlayer, it nonetheless decreases CO2 intercalations. The sorbed H2O concentrations from our simulations compare remarkably well with the in situ infrared (IR) spectroscopy experimental data by Loring et al. [Langmuir, 2014, 30, 6120-6128], if the continuous experimental curve is considered as the "smear-out" of the stepwise curve from our simulations. However, the overall sorbed CO2 concentrations from our simulations are higher than the IR experimental results. We attribute these discrepancies in both sorbed H2O and CO2 concentrations (measured from experiments and simulations) to the complexity of hydrated clay particles in the IR spectroscopy experiment, to which the hydration-heterogeneity model could provide a reasonable interpretation. Molecular dynamics (MD) simulations show that the hydration state of CO2 molecules is changed from the partial hydration in 1W to the full hydration in 2W with the increase in RH, and CO2 dimers are frequently seen in both 1W and 2W hydration states. CO2 dimers largely take the slipped parallel configurations, while the remaining dimers take the perpendicular T-shaped geometry. Further, sodium ions in the interlayer tend to be fully hydrated by water molecules due to their relatively large hydration energy. Moreover, we find that CO2 molecules hardly migrate into the first hydration shell of sodium ions. The overall diffusion coefficients of CO, molecules are larger than those of water molecules and sodium ions. This comparably high mobility of CO2 molecules in the clay interlayer, together with the low probability of CO2 participation in the first hydration shell of Na+ ions, essentially prevents CO2 and Na+ from direct interactions in clay interlayers.