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 CO 2 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 CO 2 –H 2 O–Na + complex in the montmorillonite clay interlayer. Simulation results show that stable monolayer hydrates (1W) with a basal spacing around 12 Å are formed at RH = 30–60%. As RH is increased to 70% and above, bilayer CO 2 –H 2 O mixtures with a basal spacing around 15–16 Å (2W) are more stable. In general, the CO 2 intercalation process is strongly influenced by RH. While a high relative humidity facilitates water molecules entering the clay interlayer, it nonetheless decreases CO 2 intercalations. The sorbed H 2 O 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 CO 2 concentrations from our simulations are higher than the IR experimental results. We attribute these discrepancies in both sorbed H 2 O and CO 2 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 CO 2 molecules is changed from the partial hydration in 1W to the full hydration in 2W with the increase in RH, and CO 2 dimers are frequently seen in both 1W and 2W hydration states. CO 2 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 CO 2 molecules hardly migrate into the first hydration shell of sodium ions. The overall diffusion coefficients of CO 2 molecules are larger than those of water molecules and sodium ions. This comparably high mobility of CO 2 molecules in the clay interlayer, together with the low probability of CO 2 participation in the first hydration shell of Na + ions, essentially prevents CO 2 and Na + from direct interactions in clay interlayers.