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 CO₂–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 CO₂ 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 CO₂ (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 H₂O 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