The fate of chemical and radioactive wastes in the environment is related to the ability of natural phases to attenuate and immobilize contaminants through chemical sorption and precipitation processes. Our understanding of these complex processes at the atomic level is provided by a few experimental and analytical methods such as X-ray absorption and NMR spectroscopies. However, due to complexities in the structure and composition of clay and other hydrated minerals, and the inherent uncertainties of the experimental methods, it is important to apply theoretical molecular models for a fundamental atomic-level understanding, interpretation, and prediction of these phenomena. In this effort, we have developed a general force field, CLAYFF, suitable for the simulation of hydrated and multicomponent mineral systems and their interfaces with aqueous solutions. Interatomic potentials were derived from parametrizations incorporating structural and spectroscopic data for a variety of simple hydrated compounds. A flexible SPC-based water model is used to describe the water and hydroxyl behavior. Metal-oxygen interactions are described by a LennardJones function and a Coulombic term with partial charges derived by Mulliken and ESP analysis of DFT results. Bulk structures, relaxed surface structures, and intercalation processes are evaluated and compared to experimental and spectroscopic findings for validation. Our approach differs from most others in that we treat most interatomic interactions as nonbonded. This allows us to effectively use the force field for a wide variety of phases and to properly account for energy and momentum transfer between the fluid phase and the solid, while keeping the number of parameters small enough to allow modeling of relatively large and highly disordered systems. Simulations of clay, hydroxide, and oxyhydroxide phases and their interfaces with aqueous solutions combine energy minimization and molecular dynamics methods to describe the structure and behavior of water, hydroxyl, surface species, and intercalates in these systems. The results obtained to date demonstrate that CLAYFF shows good promise to evolve into a widely adaptable and broadly effective force field for molecular simulations of fluid interfaces with clays and other clay-related phases, as well as other inorganic materials characterized by complex, disordered, and often ill-determined structure and composition.

Molecular dynamics (MD) computer simulations of liquid water adsorbed on the muscovite (001) surface provide a greatly increased, atomistically detailed understanding of surface-related effects on the spatial variation in the structural and orientational ordering, hydrogen bond (H-bond) organization, and local density of H2O molecules at this important model phyllosilicate surface. MD simulations at constant temperature and volume (statistical NVT ensemble) were performed for a series of model systems consisting of a two-layer muscovite slab (representing 8 crystallographic surface unit cells of the substrate) and 0 to 319 adsorbed H2O molecules, probing the atomistic structure and dynamics of surface aqueous films up to 3 nm in thickness. The results do not demonstrate a completely liquid-like behavior, as otherwise suggested from the interpretation of X-ray reflectivity measurements and earlier Monte Carlo simulations. Instead, a more structurally and orientationally restricted behavior of surface H2O molecules is observed, and this structural ordering extends to larger distances from the surface than previously expected. Even at the largest surface water coverage studied, over 20% of H2O molecules are associated with specific adsorption sites, and another 50% maintain strongly preferred orientations relative to the surface. This partially ordered structure is also different from the well-ordered 2-dimensional ice-like structure predicted by ab initio MD simulations for a system with a complete monolayer water coverage. However, consistent with these ab initio results, our simulations do predict that a full molecular monolayer surface water coverage represents a relatively stable surface structure in terms of the lowest diffusional mobility of H2O molecules along the surface. Calculated energies of water adsorption are in good agreement with available experimental data.

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

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Molecular dynamics computer simulations are performed to study the structure and dynamical behavior of chloride and associated cations at the interfaces between aqueous solutions and portlandite (Ca(OH) 2 ), Friedel's salt ([Ca 2 Al(OH) 6 ]Cl‚2H 2 O), tobermorite (Ca 5 -Si 6 O 16 (OH) 2 ), and ettringite (Ca 6 [Al(OH) 6 ] 2 [SO 4 ] 3 ‚26H 2 O). These phases are important in calcium silicate and calcium aluminate cements and are models of important poorly crystalline cement phases. They are also representative of many hydrous hydroxide, aluminate, and silicate materials stable near room temperature and pressure. The MD simulations use a recently developed semiempirical force field and take into account the flexibility of surface OH groups and allow for energy and momentum transfer between the solid and solution to effectively simulate the sorption. The principal focus is on the structure at and near the solution/solid interfaces and on the molecular mechanisms of adsorption of aqueous Cl -, Na + , and Cs + ions on a neutral portlandite surface and comparison to the Clsorption behavior on the positively charged surface of Friedel's salt. Power spectra of molecular motions for bulk and surface species, diffusion coefficients for Cl -, Na + , and Cs + ions in different surface-related environments, and mean residence times on surface sites are calculated. Relative to the diffusion coefficients in bulk solution, those of Clin an innersphere surface complex are reduced about an order of magnitude, those in outer-sphere complexes are reduced less, and for both types the coeffcients are reduced more for Friedel's salt than for portlandite. No Cladsorption was observed on tobermotite, and little, on ettringite. The simulation results are in good qualitative agreement with experimental sorption and 35 Cl NMR studies. The MD results provide further confirmation that chloride binding on C-S-H, which is the most abundant phase in many cements, can be thought of as due to sorption on surface sites similar to those on portlandite.

Molecular dynamics computer simulations of Mg/Al hydrotalcite with interlayer Cl- were performed to better understand the structure of layered double hydroxides and their hydration behavior. A set of models with variable numbers of interlayer water molecules was investigated, with the assumption of no constraints on the movements of any atoms or on the geometry of the simulation supercells. Crystallographic parameters and two components of the hydration energy were calculated. One of these components is related to the interaction of water molecules with the rest of the structure and is controlled primarily by formation of a hydrogen-bonding network in the interlayer. The other is related to expansion of the host structure itself and reflects decreasing electrostatic interactions as the c-axis expands upon swelling. The dependence of these two energy components on the degree of hydration provides useful insight into the nature of hydrotalcite swelling behavior. There are two stable hydration states with c-axis dimensions of 23.9 and 21.7 Å, corresponding to hydrotalcite with 2 water molecules per each chloride in the interlayer, and dehydrated hydrotalcite, respectively. The first state is observed experimentally under ambient atmospheric conditions. The simulations also reveal a distorted octahedral structure of the hydroxide layer similar to that of hydrocalumite, the related Ca/Al phase.

Natural organic matter (NOM, or humic substance) has a known tendency to form colloidal aggregates in aqueous environments, with the composition and concentration of cationic species in solution, pH, temperature, and the composition of the NOM itself playing important roles. Strong interaction of carboxylic groups of NOM with dissolved metal cations is thought to be the leading chemical interaction in NOM supramolecular aggregation.Computational molecular dynamics (MD) study of the interactions of Na + , Mg 2+ , and Ca 2+ with the carboxylic groups of a model NOM fragment and acetate anions in aqueous solutions provides new quantitative insight into the structure, energetics, and dynamics of the interactions of carboxylic groups with metal cations, their association and the effects of cations on the colloidal aggregation of NOM molecules. Potentials of mean force and the equilibrium constants describing overall ion association and the distribution of metal cations between contact ion pairs and solvent separated ions pairs were computed from free MD simulations and restrained umbrella sampling calculations. The results provide insight into the local structural environments of metal-carboxylate association and the dynamics of exchange among these sites. All three cations prefer contact ion pair to solvent separated ion pair coordination, and Na + and Ca 2+ show a strong preference for bidentate contact ion pair formation. The average residence time of a Ca 2+ ion in a contact ion pair with the carboxylic groups is of the order of 0.5 ns, whereas the corresponding residence time of a Na + ion is only between 0.02 and 0.05 ns. The average residence times of a Ca 2+ ion in a bidentate coordinated contact ion pair vs. a monodentate coordinated contact ion pair are about 0.5 ns and about 0.08 ns, respectively. On the 10-ns time scale of our simulations, aggregation of the NOM molecules occurs in the presence of Ca 2+ but not Na + or Mg 2+ . These results agree with previous experimental observations and are explained 3 by both Ca 2+ ion-bridging between NOM molecules and decreased repulsion between the NOM molecules due to the reduced net charge of the NOM-metal complexes. Simulations on a larger scale are needed to further explore the relative importance of the different aggregation mechanisms and the stability of NOM aggregates.4

Three new structural models of montmorillonite with differently distributed Al/Si and Mg/Al substitutions in the tetrahedral and octahedral clay layers are systematically developed and studied by means of MD simulations to quantify the possible effects of such substitutional disorder on the swelling behavior, the interlayer structure, and mobility of aqueous species. A very wide range of water content, from 0 to 700 mg water /g clay is explored to derive the swelling properties of Cs−montmorillonite. The determined layer spacing does not differ much depending on the clay model. However, at low water contents up to 1-layer hydrate (∼100 mg water /g clay ) the variation of specific locations of the tetrahedral and octahedral substitutions in the two TOT clay layers slightly but noticeably affects the total hydration energy of the system. Using atom−atom radial distribution functions and the respective atomic coordination numbers we have identified for the three clay models not only the previously observed binding sites for Cs + on the clay surface but also new ones that are correlated with the position of tetrahedral substitution in the structure. The mobility of Cs + ions and H 2 O diffusion coefficients, as expected, gradually increase both with increasing water content and with increasing distance from the clay surface, but they still remain 2 to 4 times lower than the corresponding bulk values. Only small differences were observed between the three Cs− montmorillonite models, but these differences are predicted to increase in the case of higher charge density of the clay layers and/or interlayer cations.