Microstructural response of variably hydrated ca-rich montmorillonite to supercritical CO₂

First-principles molecular dynamics simulations were carried out to explore the mechanistic and thermodynamic ramifications of the exposure of variably hydrated Ca-rich montmorillonites to supercritical CO₂ and CO ₂-SO₂ mixtures under geologic storage conditions. In sub- to single-hydrated systems (≤1W), CO₂ intercalation causes interlamellar expansion of 8-12%, while systems transitioning to 2W may undergo contraction (≈7%) or remain almost unchanged. When compared to ≈2W hydration state, structural analysis of the ≤1W systems, reveals more Ca-CO₂ contacts and partial transition to vertically confined CO ₂ molecules. Infrared spectra and projected vibrational frequency analysis imply that intercalated Ca-bound CO₂ are vibrationally constrained and contribute to the higher frequencies of the asymmetric stretch band. Reduced diffusion coefficients of intercalated H₂O and CO ₂ (10^-6-10^-7 cm²/s) indicate that Ca-montmorillonites in ≈1W hydration states can be more efficient in capturing CO₂. Simulations including SO₂ imply that ≈0.66 mmol SO₂/g clay can be intercalated without other significant structural changes. SO₂ is likely to divert H ₂O away from the cations, promoting Ca-CO₂ interactions and CO₂ capture by further reducing CO₂ diffusion (10 ^-8 cm²/s). Vibrational bands at ≈1267 or 1155 cm ^-1 may be used to identify the chemical state (oxidation states +4 or +6, respectively) and the fate of sulfur contaminants.