On the hydrogen bonding structure at the aqueous interface of ammonium-substituted mica: A molecular dynamics simulation

Molecular dynamics (MD) computer simulations were performed for an aqueous film of 3nm thickness adsorbed at the (001) surface of ammonium-substituted muscovite mica. The results provide a detailed picture of the near-surface structure and topological characteristics of the interfacial hydrogen bonding network. The effects of D/H isotopic substitution in N(H/D)4+ on the dynamics and consequently on the convergence of the structural properties have also been explored. Unlike many earlier simulations, a much larger surface area representing 72 crystallographic unit cells was used, which allowed for a more realistic representation of the substrate surface with a more disordered distribution of Al/Si isomorphic substitutions in muscovite. The results clearly demonstrate that under ambient conditions both interfacial ammonium ions and the very first layer of water molecules are H-bonded only to the basal surface of muscovite, but do not form H-bonds with each other. As the distance from the surface increases, the H-bonds donated to the surface by both N(H/D)4+ and H2O are gradually replaced by the H-bonds to the neighboring water molecules, with the ammonia ions experiencing one reorientational transition region, while the H2O molecules experiencing three such distinct consecutive transitions. The hydrated N(H/D)4+ ions adsorb almost exclusively as inner-sphere surface complexes with the preferential coordination to the basal bridging oxygen atoms surrounding the Al/Si substitutions

Molecular models of natural organic matter and its colloidal aggregation in aqueous solutions: Challenges and opportunities for computer simulations

Natural organic matter (NOM) is ubiquitous in soil and groundwater and its aqueous complexation with various inorganic and organic species can strongly affect the speciation, solubility and toxicity of many elements in the environment. Despite significant geochemical, environmental and industrial interest, the molecular-scale mechanisms of the physical and chemical processes involving NOM are not yet fully understood. Recent molecular dynamics (MD) simulations using relatively simple models of NOM fragments are used here to illustrate the challenges and opportunities for the application of computational molecular modeling techniques to the structural, dynamic, and energetic characterization of metal-NOM complexation and colloidal aggregation in aqueous solutions. The predictions from large-scale MD simulations are in good qualitative agreement with available experimental observations, but also point out to the need for simulations at much larger time and length scales with more complex NOM models in order to fully capture the diversity of molecular processes involving NOM

Computational molecular modeling of the multi-scale dynamics of water and ions at cement interfaces.

Structural and dynamic behavior of H2O molecules and aqueous at in-terfaces and in nanopores of model C-S-H binding phase (tobermorite) is quanti-fied on the basis of molecular dynamics computer simulations. At the (001) sur-face of tobermorite in contact with 0.25 M KCl aqueous solution, we can effectively distinguish water molecules that spend most of their time within chan-nels between the drierketten chains of silica on the tobermorite surface from the adsorbed molecules residing slightly above the interface. Within the channels, H2O molecules donate H-bonds to both the bridging and non-bridging oxygens of the Si-tetrahedra as well as to other H2O. Some of these molecules form very strong H-bonds persisting over 100 ps and longer, but many others undergo fre-quent librations and occasional diffusional jumps from one surface site to another. The average diffusion coefficients of the surface-associated H2O molecules that spend most of their time in the channels and those that lie above the nominal inter-face differ by about an order of magnitude (DH2O[internal]=5.0×10-11 m2/s and DH2O[external]=6.0×10-10 m2/s, respectively). The average diffusion coefficient for all surface-associated H2O molecules is about 1.0×10-10 m2/s. All of these values are significantly less than the value of 2.3×10-9 m2/s, characteristic of H2O self-diffusion in bulk liquid water, but they are in very good quantitative agreement with experimental data on the dynamics surface-associated water in similar ce-ment materials obtained be 1H NMR [1,2]

Computational molecular modeling of the multi-scale dynamics of water and ions at cement interfaces.

Structural and dynamic behavior of H2O molecules and aqueous at in-terfaces and in nanopores of model C-S-H binding phase (tobermorite) is quanti-fied on the basis of molecular dynamics computer simulations. At the (001) sur-face of tobermorite in contact with 0.25 M KCl aqueous solution, we can effectively distinguish water molecules that spend most of their time within chan-nels between the drierketten chains of silica on the tobermorite surface from the adsorbed molecules residing slightly above the interface. Within the channels, H2O molecules donate H-bonds to both the bridging and non-bridging oxygens of the Si-tetrahedra as well as to other H2O. Some of these molecules form very strong H-bonds persisting over 100 ps and longer, but many others undergo fre-quent librations and occasional diffusional jumps from one surface site to another. The average diffusion coefficients of the surface-associated H2O molecules that spend most of their time in the channels and those that lie above the nominal inter-face differ by about an order of magnitude (DH2O[internal]=5.0×10-11 m2/s and DH2O[external]=6.0×10-10 m2/s, respectively). The average diffusion coefficient for all surface-associated H2O molecules is about 1.0×10-10 m2/s. All of these values are significantly less than the value of 2.3×10-9 m2/s, characteristic of H2O self-diffusion in bulk liquid water, but they are in very good quantitative agreement with experimental data on the dynamics surface-associated water in similar ce-ment materials obtained be 1H NMR [1,2]

Cation and Water Structure, Dynamics, and Energetics in Smectite Clays: A Molecular Dynamics Study of Ca-Hectorite

The incorporation of Ca2+ into smectite minerals is well-known to have a significant effect on the swelling behavior and mechanical properties of this environmentally and technologically important group of materials. Relative to common alkali cations such as Na+, K+, and Cs+, Ca2+ has a larger charge/ionic radius ratio and thus interacts very differently with interlayer water molecules and the oxygens of the clay basal surface. Recent 2H and 43Ca NMR studies of the smectite mineral, hectorite, show that the molecular scale interlayer dynamics is quite different with Ca2+ than with alkali cations. Classical molecular dynamics (MD) simulations presented here use a newly developed hectorite model with a disordered distribution of Li+/Mg2+ substitutions in the octahedral sheet and provide new insight into the origin of the effects of Ca2+ on the structure, dynamics, and energetics of smectite interlayers. The computed basal spacings and thermodynamic properties suggest the potential for formation of stable monolayer hydrates that have partial and complete water contents, a bilayer hydrate, and possible expansion to higher hydration states. The system hydration energies are comparable to those previously calculated for Ca–montmorillonite and are more negative than for Cs– and Na–hectorite due to the higher hydration energy of Ca2+. The coordination environments of Ca2+ change significantly with increasing interlayer hydration, with the extent of coordination to basal oxygens decreasing as the number of interlayer molecules increases. On external (001) surfaces, the H2O molecules closest to the surface are adsorbed at the centers of ditrigonal cavities and bridge Ca2+ to the surface. The Ca2+ ions on the external surface are all in outer-sphere coordination with the basal oxygens of the surface, and the proximity-restricted region with a significant number of Ca2+ is approximately 6 Å thick. Quantification of these interactions provides a basis for understanding intercalation of Ca2+ by organic species and smectite minerals

Hydrogen bonding and coordination in normal and supercritical water from X-ray inelastic scattering

A direct measure of hydrogen bonding in water under conditions ranging from the normal state to the supercritical regime is derived from the Compton scattering of inelastically-scattered X-rays. First, we show that a measure of the number of electrons $n_e$ involved in hydrogen bonding at varying thermodynamic conditions can be directly obtained from Compton profile differences. Then, we use first-principles simulations to provide a connection between $n_e$ and the number of hydrogen bonds $n_{HB}$. Our study shows that over the broad range studied the relationship between $n_e$ and $n_{HB}$ is linear, allowing for a direct experimental measure of bonding and coordination in water. In particular, the transition to supercritical state is characterized by a sharp increase in the number of water monomers, but also displays a significant number of residual dimers and trimers.Comment: 14 pages, 5 figures, 1 tabl

Structure, Energetics, and Dynamics of Cs+ and H2O in Hectorite: Molecular Dynamics Simulations with an Unconstrained Substrate Surface

Classical molecular dynamics simulations were performed for the smectite clay hectorite with charge-balancing Cs+ cations using a newly developed structural model with a disordered distribution of Li/Mg substitutions in the octahedral sheet and the fully flexible CLAYFF force field. Calculations for systems with interlayer galleries containing 0–19 H2O/Cs+ suggest that the monolayer hydrate is the only stable state at all relative humidities at ambient pressure and temperature, in agreement with experimental results and previous molecular calculations. The basal spacing of this structure is also in good agreement with experimental values. In contrast to previous molecular modeling results, however, the new simulations show that interlayer Cs+ occurs on 2 different inner sphere adsorption sites: above the center of ditrigonal cavities and above Si tetrahedra. Unlike previous simulations, which employed a rigid clay model and fixed orientations of the structural −OH groups, the present results are obtained for an unconstrained clay substrate structure, where the structural −OH groups are able to assume various orientations, including being nearly parallel to the clay layers. This flexibility allows the Cs+ ions to approach the surface more closely above the centers of the hexagonal rings. In this structural arrangement, Cs+ ions are not hydrated by the H2O molecules which share the same interlayer plane, but rather by the H2O molecules coordinated to the opposite surface. In contrast, on the external basal surface, a significant fraction of H2O molecules are adsorbed above the centers of ditrigonal cavities adjacent to adsorbed Cs+ ions. For these H2O molecules, both HH2O atoms coordinate and H-bond to Ob surface oxygen atoms. The mean residence times for the Cs+–H2O, Cs+–Ob, and H2O–Ob coordination pairs show that Cs+ ions are more strongly coordinated with Ob atoms than H2O molecules. This result is the opposite of the behavior in Ca-hectorite, due to the much smaller hydration energy of Cs+ compared to that of Ca2+

Percolation transition of hydration water at hydrophilic surfaces

An analysis of water clustering is used to study the quasi-2D percolation transition of water adsorbed at planar hydrophilic surfaces. Above the critical temperature of the layering transition (quasi-2D liquid-vapor phase transition of adsorbed molecules) a percolation transition occurs at some threshold surface coverage, which increases with increasing temperature. The location of the percolation line is consistent with the existence of a percolation transition at the critical point. The percolation threshold at a planar surface is weakly sensitive to the size of the system when its lateral dimension increases from 80 to 150 A. The size distribution of the largest water cluster shows a specific two-peaks structure in a wide range of surface coverage : the lower- and higher-size peaks represent contributions from non-spanning and spanning clusters, respectively. The ratio of the average sizes of spanning and non-spanning largest clusters is about 1.8 for all studied planes. The two-peak structure becomes more pronounced with decreasing size of the planar surface and strongly enhances at spherical surfaces.Comment: 17 pages, 11 figure

Effects of Ca2+ on supramolecular aggregation of natural organic matter in aqueous solutions: A comparison of molecular modeling approaches

Natural organic matter (NOM) represents a complex molecular system that cannot be fully characterized compositionally or structurally in full atomistic detail. This makes the application of molecular modeling approaches very difficult and significantly hinders quantitative investigation of NOM properties and behavior by these otherwise very efficient computational techniques. Here we report and analyze three molecular dynamics (MD) simulations of Ca2+ complexation with NOM in aqueous solutions in an attempt to quantitatively assess possible effects of model- and system size-dependence in such simulations. Despite some obvious variations in the computed results that depend on the size of the simulated system and on the parameters of the force field models used, all three simulations are quite robust and consistent. They show Ca2+ ions associated with 35-50% of the NOM carboxylic groups at near-neutral pH and point to a strong preference for the stability of bidentate-coordinated contact ion pairs. The degree and potential mechanisms of NOM supramolecular aggregation in the presence of Ca2+ ions in solution are also assessed on a semi-quantitative level from two larger-scale MD simulations