This paper presents a classical molecular dynamics (MD) and metadynamics investigation of the relationships between the structure, energetics, and dynamics of Na-hydroxyhectorite and serves to provide additional, molecular-scale insight into the interlayer hydration of this mineral. The computational results support a model for interlayer H2O structure and dynamics based on 2H NMR spectroscopy and indicate that H2O molecules undergo simultaneous fast librational motions about the H2O C2 symmetry axis and site hopping with C3 symmetry with respect to the surface normal. Hydration energy minima occur at one-, one-and-one-half-, and two-water-layer hydrates, which for the composition modeled correspond to 3, 5.5, and 10 H2O/Na+, respectively. Na+ ions are coordinated by basal O atoms (OMIN) at the lowest hydration levels and by H2O molecules (OH2O) in the two-layer hydrate, and H2O molecules have an average of three H-bonds at the greatest hydration levels. The metadynamics calculations yield activation energies for site hopping of H2O molecules of 6.0 kJ/mol for the one-layer structure and 3.3 kJ/mol for hopping between layers in the two-layer structure. Computed diffusion coefficients for water and Na+ are substantially less than in bulk liquid water, as expected in a nanoconfined environment, and are in good agreement with previous results

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 ...

The reactions of aluminosilicate clusters with water are investigated using ab initio calculations. There are several reaction sites on a mineral surface, and, in the case of aluminosilicates, the dissolution chemistry is dictated by chemically distinct surface termination sites: Al and Si. Environmental factors such as pH determine the protonation state and configuration around these terminal sites. The dissolution mechanisms for Al- and Si-terminated sites in protonated, neutral, and deprotonated states are determined using density functional theory calculations. In all protonation states, Si are tetra-coordinated; however, the ability of Al to exist in tetra-, penta-, and hexa-coordination states makes the dissolution mechanisms for the two types of terminal sites fundamentally different. The calculated barrier heights for Al-terminated sites are predicted to be lower than those for Si-terminated sites, a trend that has been observed in experimental studies. The sensitivity of the calculations on the choice of density functionals and basis sets is tested using three functionals: B3LYP, PBE1PBE, and M05-2X, in combination with the 6-311+G(d,p) and MG3S basis sets. For all these calculations, the geometries of the stationary points along the reaction path and the barrier heights are presented.

This paper reviews experimental nuclear magnetic resonance (NMR) and computational molecular dynamics (MD) investigations of the structural and dynamical behavior of cations, anions, H 2 O, and CO 2 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, chargebalancing cations or anions, H 2 O, CO 2 , 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 subkilohertz 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.