Structure and Energetics of Smectite Interlayer Hydration: Molecular Dynamics Investigations of Na- and Ca Hectorite

Molecular-scale interactions present at mineral-water interfaces and in clay interlayer galleries control numerous environmental processes, including chemical interactions in soils and transport of nutrients and pollutants through them.[1-4] Understanding these processes requires accurate knowledge of the structure, energetics, and dynamics of the interaction among the mineral substrate, ions, and water molecules.[5, 6] Challenges to this objective include experimental difficulties in probing these interfaces and interlayers at the molecular scale; fully characterizing the mineral substrate; and identifying how the mineral surface, ions, and water molecules each contribute to the overall structure, energetics, and dynamics of these systems.[6] Linked computational molecular dynamics (MD) simulations and experimental nuclear magnetic resonance (NMR) studies are particularly effective in addressing these issues.[7-9] Here we focus on MD studies of Na- and Ca-smectite (hectorite) interlayer galleries to provide a molecular-scale picture of the structure and dynamics of their hydration[9, 10] and to complement our earlier NMR investigations of these systems.[7-9] Classical MD simulations were undertaken in the NPT and NVT ensembles to determine the structural and energetic changes with increasing hydration with focus on the single- and double-layer hydrates. The results show substantial changes in the hydration of the interlayer cations, the orientations of the water molecules, the hydrogen bond network involving the water molecules and basal oxygen atoms, and the resulting potential energies as the interlayer gallery expands. [1] Scheidegger et al. (1996) Soil Science 161 813-831. [2] Stumm (1997) Colloids and Surfaces A-Physicochemical and Engineering Aspects 120 143-166. [3] O'Day (1999) Reviews of Geophysics 37 249-274. [4] Koretsky (2000) Journal of Hydrology 230 127-171. [5] Wang et al. (2001) Chemistry of Materials 13 145-150. [6] Wang et al. (2006) Geochimica et Cosmochimica Acta 70 562-582. [7] Bowers et al. (2008) Journal of Physical Chemistry C 112 6430-6438. [8] Bowers et al. (2011) Journal of Physical Chemistry C 115 23395-23407. [9] Bowers et al. (2012), unpublished. [10] Morrow et al. (2012) Journal of Physical Chemistry C, submitted

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

The carbon dioxide (CO₂) retention capacity and adsorption/desorption energetics of layered nanoporous oxide materials depend critically on the hydration level and the nature of molecular interactions among H₂O, CO₂, charge-balancing cations, and the oxide/hydroxide layers. Molecular-scale understanding of the structure, dynamics, and interfacial energetics of H₂O/CO₂ binary mixtures confined in the interlayer nanopores is paramount to geological CO₂ storage efforts in clay-rich materials. This Article investigates the effects of supercritical CO₂ (scCO₂) 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 H₂O and CO₂ coexisting in a single layer at the center of the interlayer. The water molecules in this central H₂O/CO₂ layer cluster around and hydrate Na⁺ ions desorbed from the basal surfaces, whereas CO₂–CO₂ hydrophobic interactions favor mutual clustering of CO₂ molecules. This arrangement results in dynamic percolation paths that facilitate single file-like anisotropic lateral diffusion of CO₂. The water clusters around the Na⁺ ions act as two-dimensional nanopores for the diffusion of Na⁺ between the basal surfaces and across the central H₂O/CO₂ layer, whereas the CO₂-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 CO₂ molecule. The activation energies for a H₂O and a CO₂ molecule to move out of the first coordination shell of a near-surface Na⁺ are ∼2.75 and ∼0.5 kcal/mol, respectively. The activation barriers for site-hopping of a H₂O molecule within the first coordination shell of near-surface and displaced Na⁺ ions are ∼1.6 kcal/mol whereas those for site-hopping of CO₂ around the near-surface and displaced Na⁺ ions are ∼1.8 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

Structure, energetics, and dynamics of smectite clay hydration: Molecular dynamics investigations of hectorite” (oral)

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NMR and computational molecular modeling studies of mineral surfaces and interlayer galleries: A review

International audienceThis paper reviews experimental nuclear magnetic resonance (NMR) and computational molecular dynamics (MD) investigations of the structural and dynamical behavior of cations, anions, H2O, and CO2 on the surfaces and in the interlayer galleries of layer-structure minerals and their composites with polymers and natural organic matter (NOM). The interaction among mineral surfaces, charge-balancing cations or anions, H2O, CO2, and NOM are dominated by Coulombic, H-bond, and van der Waals interactions leading to statically and dynamically disordered systems and molecular-scale processes with characteristic room-temperature frequencies varying from at least as small as 10(2) to >10(12) Hz. NMR spectroscopy provides local structural information about such systems through the chemical shift and quadrupolar interactions and dynamical information at frequencies from the sub-kilohertz to gigahertz ranges through the T-1 and T-2 relaxation rates and line shape analysis. It is often difficult to associate a specific structure or dynamical process to a given NMR observation, however, and computational molecular modeling is often effective in providing a much more detailed picture in this regard. The examples discussed here illustrate these capabilities of combining experimental NMR and computational modeling in mineralogically and geochemically important systems, including clay minerals and layered double hydroxides