Interlayer Cation Polarizability Affects Supercritical Carbon Dioxide Adsorption by Swelling Clays

Several strategies for mitigating the build-up of atmospheric carbon dioxide (CO₂) bring wet supercritical CO₂ (scCO₂) in contact with phyllosilicates such as illites and smectites. While some work has examined the role of the charge-balancing cation and smectite framework features on CO₂/smectite interactions, to our knowledge no one has examined how the polarizability of the charge-balancing cation influences these behaviors. In this paper, the scCO₂ adsorption properties of Pb²⁺, Rb⁺, and NH₄⁺ saturated smectite clays at variable relative humidity are studied by integrating in situ high-pressure X-ray diffraction (XRD), infrared spectroscopic titrations, and magic angle spinning nuclear magnetic resonance (MAS NMR) methods. The results are combined with previously published data for Na⁺, Cs⁺, and Ca²⁺ saturated versions of the same smectites to isolate the roles of the charge-balancing cations and perform two independent tests of the role of charge-balancing cation polarizability in determining the interlayer fluid properties and smectite expansion. Independent correlations developed for (i) San Bernardino hectorite (SHCa-1) and (ii) Wyoming montmorillonite (SWy-2) both show that cation polarizability is important in predicting the interlayer composition (mol% CO₂ in the interlayer fluid and CO₂/cation ratio in interlayer) and the expansion behavior for smectites in contact with wet and dry scCO₂. In particular, this study shows that the charge-balancing cation polarizability is the most important cation-associated parameter in determining the expansion of the trioctahedral smectite, hectorite, when in contact with dry scCO₂. While both independent tests show that cation polarizability is an important factor in smectite-scCO₂ systems, the correlations for hectorite are different from those determined for montmorillonite. The root of these differences is likely associated with the roles of the smectite framework on adsorption, warranting follow-up studies with a larger number of unique smectite frameworks. Overall, the results show that the polarizability of the charge-balancing cation should be considered when preparing smectite clays (or industrial processes involving smectites) to capture CO₂ and in predicting the behavior of caprocks over time.