Supercritical Carbon Dioxide at Smectite Mineral–Water Interfaces: Molecular Dynamics and Adaptive Biasing Force Investigation of CO₂/H₂O Mixtures Nanoconfined in Na-Montmorillonite

Abstract

The carbon dioxide (CO 2 ) retention capacity and adsorption/desorption energetics of layered nano-porous oxide materials depend critically on the hydration level and the nature of molecular interactions among H 2 O, CO 2 , charge-balancing cations and the oxide/hydroxide layers. Molecular-scale understanding of the structure, dynamics and interfacial energetics of H 2 O/CO 2 binary mixtures confined in the interlayer nano-pores is paramount to geological CO 2 storage efforts in clay-rich materials. This Article investigates the effects of supercritical CO 2 (scCO 2 ) in the hydrated interlayer galleries of the hydrophilic smectite mineral (Na-montmorillonite) under geochemically relevant conditions using classical molecular dynamics simulations and enhanced sampling free energy methods. For the compositions investigated, the interactions among the cations, intercalated fluid species, and the basal surfaces result in structures with H 2 O and CO 2 coexisting in a single layer at the center of the interlayer. The water molecules in this central H 2 O/CO 2 layer cluster around and hydrate Na + ions desorbed from the basal surfaces, whereas CO 2 -CO 2 hydrophobic interactions favor mutual clustering of CO 2 molecules. This arrangement results in dynamic percolation paths that facilitate single file-like anisotropic lateral diffusion of ---2 CO 2 . The water clusters around the Na + ions act as two-dimensional nano-pores for the diffusion of Na + between the basal surfaces and across the central H 2 O/CO 2 layer, whereas the CO 2 -rich regions are not permeable to Na + . The near-surface Na + ions occur in two distinct types of coordination environments with distinct NMR spectral fingerprints. Type-I near-surface Na + ions are coordinated by two basal oxygen atoms and four water molecules, whereas for type-II one of the coordinating water molecules is replaced by a CO 2 molecule. The activation energies for a H 2 O and a CO 2 molecule to move out of the first coordination shell of a near-surface Na + are ~2.75 kcal/mol and ~0.5 kcal/mol, respectively. The activation barriers for site-hopping of a H 2 O molecule within the first coordination shell of near-surface and displaced Na + ions are ~1.6 kcal/mol whereas those for site-hopping of CO 2 around the near-surface and displaced Na + ions are ~1.8 kcal/mol and ~3.5 kcal/mol, respectively. The results provide a detailed picture of the interlayer structure and energetics of diffusional motion of cations and intercalates.---3 I. INTRODUCTIONThe interaction of water with hydrophilic surfaces is driven by water-water and watersurface hydrogen bonding (H-bonding) and the ion-water-substrate interactions involving the charge-balancing, exchangeable surface ions. 1-6 On external hydrophilic surfaces and in twodimensional nano-confinement between surfaces, the interplay of these molecular interactions minimizes the water-surface interfacial energy and maximizes the contact area leading to welldefined layers of water molecules with densities greater than in bulk water. 1-8 In ...

Competitive Adsorption of H2O and CO2 in 2-Dimensional Nanoconfinement: GCMD Simulations of Cs- and Ca-Hectorites

“…The OCO2-OCO2 RCN is ~4.6, and their mutual arrangement is dominated by a slipped parallel arrangement (parallel CO2 with slight offset) with only a small fraction of T-shaped arrangements (Figure 3b). 38 The rapid reorientation of CO2 perpendicular to the O-C-O molecular axis is well illustrated by the dispersed contours for OCO2 in the 1L structure ( Figure S2).…”

Clay Swelling in Dry Supercritical Carbon Dioxide: Effects of Interlayer Cations on the Structure, Dynamics, and Energetics of CO2 Intercalation Probed by XRD, NMR, and GCMD Simulations

“…Recent experimental and molecular simulation studies at conditions relevant to subsurface injection of supercritical CO2 [9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][40][41][42][43][44][45][46][47][48] have shown that the intercalation and retention of CO2 in smectite interlayers is possible but depends strongly on the amount and thermodynamic properties of water in the system. For example, Giesting et al 11,12 and Schaef et al 17,19 have shown that the H2O saturation state (relative humidity, R.H.) of supercritical CO2 (scCO2) controls CO2 intercalation in Na-, Mg-and Ca-montmorillonite at 323 K and 90 bar.…”

Tipping Point for Expansion of Layered Aluminosilicates in Weakly Polar Solvents: Supercritical CO2

“…A striking observation was that H 2 O and CO 2 intercalation was both synergistic and competitive: fully collapsed Na-montmorillonite neither expanded nor sorbed CO 2 into the interlayer region when exposed to dry scCO 2 . 26,[31][32][33] However, small additions of H 2 O to scCO 2 promoted CO 2 intercalation up to a critical H 2 O concentration beyond which H 2 O expelled CO 2 from the interlayer region. An intuitive explanation is that because the free energy of solvation for H 2 O-Na + in the interlayer is expectedly larger than that for Na-CO 2 , even at relatively low H 2 O concentrations hydrated or partially hydrated Na + essentially props the interlayers open, allowing CO 2 to enter as secondary outcome.…”