The atomic structure of dioctahedral 2:1 phyllosilicate edge surfaces was calculated using pseudopotential planewave density functional theory. Bulk structures of pyrophyllite and ferripyrophyllite were optimized using periodic boundary conditions, after which crystal chemical methods were used to obtain initial terminations for ideal (110)- and (010)-type edge surfaces. The edge surfaces were protonated using various schemes to neutralize the surface charge, and total minimized energies were compared to identify which schemes are the most energetically favorable. The calculations show that significant surface relaxation should occur on the (110)-type faces, as well as in response to different protonation schemes on both surface types. This result is consistent with atomic force microscopy observations of phyllosilicate dissolution behavior. Bond-valence methods incorporating bond lengths from calculated structures can be used to predict intrinsic acidity constants for surface functional groups on (110)- and (010)-type edge surfaces. However, the occurrence of surface relaxation poses problems for applying current bond-valence methods. An alternative method is proposed that considers bond relaxation, and accounts for the energetics of various protonation schemes on phyllosilicate edges.

Organic dyes such as methylene blue (MB) are often used in the characterization of clays and related minerals, but details of the adsorption mechanisms of such dyes are only partially understood from spectroscopic data, which indicate the presence of monomers, dimers, and higher aggregates for varying mineral surfaces. A combination of quantum (density functional theory) and classical molecular simulation methods was used to provide molecular detail of such adsorption processes, specifically the adsorption of MB onto kaolinite basal surfaces. Slab models with vacuum-terminated surfaces were used to obtain detailed structural properties and binding energies at both levels of theory, while classical molecular dynamics simulations of aqueous pores were used to characterize MB adsorption at infinite dilution and at higher concentration in which MB dimers and one-dimensional chains formed. Results for the neutral MB molecules are compared with those for the corresponding cation. Simulations of the aqueous pore indicate preferred adsorption on the hydrophobic siloxane surface, while charge-balancing chloride ions adsorb at the aluminol surface. At infinite dilution and in the gas-phase models, MB adsorbs with its primary molecular plane parallel to the siloxane surface to enhance hydrophobic interactions. Sandwiched dimers and chains are oriented perpendicular to the surface to facilitate the strong hydrophobic intermolecular interactions. Compared with quantum results, the hybrid force field predicts a weaker MB adsorption energy but a stronger dimerization energy. The structure and energetics of adsorbed MB at infinite dilution are consistent with the gas-phase binding results, which indicate that monomer adsorption is driven by strong interfacial forces rather than by the hydration properties of the dye. These results inform spectroscopic studies of MB adsorption on mineral surfaces while also revealing critical areas for development of improved hybrid force fields.

High temperature solid state sodium (23Na) magic angle spinning (MAS) NMR spin lattice relaxation times (T1) were evaluated for a series of NASICON (Na3Zr2PSi2O12) materials to directly determine Na jump rates. Simulations of the T1 temperature variations that incorporated distributions in Na jump activation energies, or distribution of jump rates, improved the agreement with experiment. The 23Na NMR T1 relaxation results revealed that distributions in the Na dynamics were present for all of the NASICON materials investigated here. The 23Na relaxation experiments also showed that small differences in material composition and/or changes in the processing conditions impacted the distributions in the Na dynamics. The extent of the distribution was related to the presence of a disordered or glassy phosphate phase present in these different sol-gel processed materials. The 23Na NMR T1 relaxation experiments are a powerful tool to directly probing Na jump dynamics and provide additional molecular level details that could impact transport phenomena.

A molecular-scale interpretation of interfacial processes is often downplayed in the analysis of traditional water treatment methods; however, such a fundamental approach is perhaps critical for the realization of enhanced performance in traditional desalination and related treatments, and in the development of novel water treatment technologies. Specifically, we examine in this article the molecular-scale processes that affect water and ion selectivity at the nanopore scale as inspired by nature, the behavior of a model polysaccharide as a biofilm, and the use of cluster-surfactant flocculants in viral sequestration.

Atomistic simulation methods have been used to model the structure of the (1014) surfaces of calcite, dolomite, and magnesite under dry and wet conditions. The potential parameters for the carbonate and water species contain shell terms to model the polarizability of the oxygen atoms. These static calculations show that the surfaces undergo relaxation leading to the rotation and distortion of the carbonate groups with associated movement of cations. The dry surface energies are 0.322, 0.247, and 0.256 Jm−2 for calcite, dolomite, and magnesite respectively. The influence of water on the surface structure and energies has been investigated for monolayer coverage. When fully hydrated with a monolayer of water, the surface energy for calcite is reduced indicating a stabilization of the surface with hydration. The extent of carbonate group distortion is greater for the dry surfaces compared to the hydrated surfaces, and for the dry calcite relative to that for dry magnesite.

A series of molecular dynamics computer simulations of the self-diffusion of lithium in pure and several doped lithium-manganese spinel materials has been completed. The theoretical approach is part of an effort to understand the mechanisms and rates of lithium diffusion, and to evaluate the structural control of the cathode materials upon lithium intercalation (charge-discharge) process. The molecular dynamics approach employs a fully ionic forcefield that accounts for electrostatic, repulsive, and dispersion interactions among all ions. A reference unit cell comprised of 56 ions (Li8Mn3+8Mn4+8O32) is used to perform the simulations under constant volume and constant pressure constraints. All atomic positions are allowed to vary during the simulation. Simulations were completed for the undoped and doped LiMn2O4 at various levels of lithium content (based on the number of lithium ions per unit cell and manganese oxidation state). The molecular dynamics results indicate an activation energy of approximately 97 kJ/mole for self-diffusion of lithium in the undoped material. Lithium ion trajectories from the simulations provide diffusion coefficients that decrease by a factor of ten as the cathode accumulates lithium ions during discharge. Molecular dynamics results for the doped spinel suggest a decrease in the diffusion rate with increasing dopant ion.

Deterioration of monuments, buildings, and works of art constructed of carbonate-based stone potentially can be arrested by applying a combination of chemical passivants and consolidants that prevent hydrolytic attack and mechanical weakening. We used molecular modeling and laboratory synthesis to develop an improved passivating agent for the calcite mineral surface based on binding strength and molecular packing density. The effectiveness of the passivating agent with and without a linked outer layer of consolidant against chemical weathering was determined through leaching tests conducted with a pH-stat apparatus at pH 5 and 25°C. For the range of molecules considered, modeling results indicate that the strongest-binding passivant is the trimethoxy dianionic form of silylalkylaminocarboxylate (SAAC). The same form of silylal-kylphosphonate (SAP) is the second strongest binder and the trisilanol neutral form of aminoethylaminopropylsilane (AEAPS) is ranked third. Short-term leaching tests on calcite powders coated with the trisilanol derivative of SAAC, the triethoxy neutral form of SAP, and the trimethoxy neutral form of AEAPS show that the passivant alone does not significantly slow the dissolution rate. However, all passivants when linked to the sol consolidant result in decreased rates. Combined AEAPS plus consolidant results in a coating that performs better than the commercial product Conservare® OH and at least as well as Conservare® H. The modeling results indicate that there may be a threshold binding energy for the passivant above which the dissolution rate of calcite is actually enhanced. More strongly-binding passivants may aid in the dissolution mechanism or dissociate in aqueous solution exposing the calcite surface to water.

In order to understand and evaluate materials for use in lithium ion rechargeable battery electrodes, we have modeled the crystal structures of various manganese oxide and lithium manganese oxide compounds. We have modeled the MnO2 polymorphs and several spinels with intermediate compositions based on the amount of lithium inserted into the tetrahedral site. Three-dimensional representations of the structures provide a basis for identifying site occupancies, coordinations, manganese valence, order-disorder, and potentially new dopants for enhanced cathode behavior. X-ray diffraction simulations of the crystal structures provide good agreement with observed patterns for synthesized samples. Ionic modeling of these materials consists of an energy minimization approach using Coulombic, repulsive, and van der Waals interactions. Modeling using electronic polarizabilities (shell model) allows a systematic analysis of changes in lattice energy, cell volume, and the relative stability of doped structures using ions such as aluminum, titanium, nickel, and cobalt.