Role of Cations in CO2 Adsorption, Dynamics, and Hydration in Smectite Clays under in Situ Supercritical CO2 Conditions

This paper explores the molecular-scale interactions between CO2 and the representative smectite mineral hectorite under supercritical conditions (90 bar, 50 degrees C) using novel in situ X-ray diffraction (XRD), infrared (IR) spectroscopy, and magic angle spinning (MAS) nuclear magnetic resonance (NMR) techniques. Particular emphasis is placed on understanding the roles of the smectite charge balancing cation (CBC) and H2O in these interactions. The data show that supercritical CO2 (scCO(2)) can be adsorbed on external surfaces and in the confined interlayer spaces of hectorite at 50 degrees C and 90 bar, with the uptake of CO2 into the interlayer favored at low H2O content and when the basal spacing is similar to a monolayer hydrate of hectorite (1WL, similar to 12.5 angstrom). These results are in agreement with published spectroscopic and molecular modeling data for the related smectite Na-montmorillonite. Charge balancing cations with small radii, large hydration energies, and low polarizabilities tend to scavenge H2O from humid scCO(2) or retain the H2O they held before scCO(2) exposure, swelling spontaneously to a bilayer hydrate (2WL) dominated state that largely prevents CO2-ion interactions and influences the extent of CO2 intercalation into the interlayer. In contrast, ions with large radii, low hydration energies, and large polarizabilities more readily form close associations with CO2 with the energetics enabling coexistence of CO2 and H2O in the interlayer over a wide range of scCO(2) humidities. Integrating our results with those from molecular dynamics simulations of wet CO2-bearing montmorillonites suggest that adsorbed CO2 in 1WL-type interlayers is oriented with its long axis parallel to the clay sheets and experiences dynamics dominated by anisotropic rotation about the axis perpendicular to the CO2 long axis at rates of at least similar to 10(5) Hz. If appreciable CO2 is adsorbed in 2WL-type interlayers, it must experience a mean orientation and dynamic averaging affects that mimic the 1WL-type adsorption environment. External surface adsorbed CO2 is dynamically similar to the 1WL case, but the CO2 long axis samples a larger range of orientations with respect to the smectite surface and adopts a different mean angle between the long axis and the smectite surface. Our data also suggest that equilibrating hectorite with a large volume of scCO(2) at 50 degrees C and 90 bar leads to interlayer dehydration, with the extent of dehydration correlating with the hydrophilicity of the CBC.