Randall T. Cygan

Molecular models and simulations of layered materials

The micro- to nano-sized nature of layered materials, particularly characteristic of naturally occurring clay minerals, limits our ability to fully interrogate their atomic dispositions and crystal structures. The low symmetry, multicomponent compositions, defects, and disorder phenomena of clays and related phases necessitate the use of molecular models and modern simulation methods. Computational chemistry tools based on classical force fields and quantum-chemical methods of electronic structure calculations provide a practical approach to evaluate structure and dynamics of the materials on an atomic scale. Combined with classical energy minimization, molecular dynamics, and Monte Carlo techniques, quantum methods provide accurate models of layered materials such as clay minerals, layered double hydroxides, and clay–polymer nanocomposites.

Ab Initio Determination of Edge Surface Structures for Dioctahedral 2 : 1 Phyllosilicates: Implications for Acid-Base Reactivity

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.

Molecular dynamics simulation of uranyl(VI) adsorption equilibria onto an external montmorillonite surface

We used molecular dynamics simulations to study the adsorption of aqueous uranyl species (UO(2)(2+)) onto clay mineral surfaces in the presence of sodium counterions and carbonato ligands. The large system size (10,000 atoms) and long simulation times (10 ns) allowed us to investigate the thermodynamics of ion adsorption, and the atomistic detail provided clues for the observed adsorption behavior. The model system consisted of the basal surface of a low-charge Na-montmorillonite clay in contact with aqueous uranyl carbonate solutions with concentrations of 0.027 M, 0.081 M, and 0.162 M. Periodic boundary conditions were used in the simulations to better represent an aqueous solution interacting with an external clay surface. Uranyl adsorption tendency was found to decrease as the aqueous uranyl carbonate concentration was increased, while sodium adsorption remained constant. The observed behavior is explained by physical and chemical effects. As the ionic strength of the aqueous solution was increased, electrostatic factors prevented further uranyl adsorption once the surface charge had been neutralized. Additionally, the formation of aqueous uranyl carbonate complexes, including uranyl carbonato oligomers, contributed to the decreased uranyl adsorption tendency.

133Cs NMR study of cesium on the surfaces of kaolinite and illite

Abstract 133Cs MAS NMR of Cs-exchanged illite, kaolinite, boehmite, and silica gel is shown to be a powerful tool to investigate the adsorption sites and atomic dynamics of Cs on mineral surfaces. Cesium is adsorbed on these mineral surfaces in primarily two ways: at sites relatively tightly bonded to the surface (Stern layer, Cs1) and at more loosely bonded sites in the diffuse (Gouy) layer (Cs2). For illite, both edge sites and crystallite basal surfaces are important adsorption sites. For kaolinite, edge sites, expandable layers, and probably crystallite basal surfaces are important. The 133Cs NMR chemical shifts for the Cs1 site become more shielded (more negative) as the Si Al ratio of the substrate phase increases, paralleling the chemical shift variations of other cations and consistent with this site being relatively tightly bonded to the surface. The 133Cs NMR chemical shifts of Cs2 do not vary systematically with solid composition due to the larger distance of these sites from the surface and weaker electrostatic attraction to the surface compared to Cs1. Rather, the Cs2 chemical shifts are significantly influenced by relative humidity (R. H.) and Cs population ( Cs H 2 O ratio) on the surface. The Cs1 chemical shifts vary less with these parameters. Cs2 is removed by washing with 1–5 mL of deionized water due to its weak attraction to the surface. The Cs1 chemical shifts become less shielded after washing and with decreasing solution concentration due to a decrease in the Cs surface density. At 100% R. H., Cs in the two sites undergoes motional averaging at frequencies > 100 kHz. With decreasing R. H., peaks for Cs on the two sites are resolved due to decreasing exchange frequencies related to a decreasing number of adsorbed water layers. Motional averaging at 100% R. H. is verified by low temperature experiments with illite.

A shell model for the simulation of rhombohedral carbonate minerals and their point defects

Abstract The electronic polarization of oxygen ions has been explicitly incorporated in a shell model to better simulate the structure of calcite and related rhombohedral carbonate minerals. Pair-potentials for Ca2+ ions and C and O comprising the carbonate molecular ion were simultaneously fitted to experimental lattice, elastic, dielectric, and vibrational data for calcite, and the structure and elastic properties of aragonite. The resulting potential parameters for the CO32- group were then transferred to models for the structures and bulk moduli of the carbonate minerals incorporating Mn, Fe, Mg, Ni, Zn, Co, Cd, and thus a fully consistent set of interaction parameters for calculating the properties of the carbonate minerals was obtained. Defect energies for doping the divalent cations into the calcite structure, and for calcium and carbonate ion vacancies were calculated. In addition, various disorder types for dolomite, including anti-site defects, stacking defects, and the energy related to increasing the Ca/Mg ratio in the dolomite structure were simulated. The theoretical enthalpy for dolomite ordering (34.4 kJ/mol) compares very well with experimental measurements.

Molecular modeling of the structure and dynamics of the interlayer and surface species of mixed-metal layered hydroxides: Chloride and water in hydrocalumite (Friedel's salt)

Abstract The dynamical behavior of Cl- and H2O molecules in the interlayer and on the (001) surface of the Ca-aluminate hydrate hydrocalumite (Friedel’s salt) over a range of temperatures from -100 to 300 °C was studied using isothermal-isobaric molecular dynamics computer simulations. This phase is currently the best available model compound for other, typically more disordered, mixed-metal layered hydroxides. The computed crystallographic parameters and density are in good agreement with available X-ray diffraction data and the force field developed for these simulations preserves the structure and density to within less than 2% of their measured values. In contrast to the highly ordered arrangement of the interlayer water molecules interpreted from the X-ray data, the simulations reveal significant dynamic disorder in water orientations. At all simulated temperatures, the interlayer water molecules undergo rapid librations (hindered hopping rotations) around an axis essentially perpendicular to the layers. This results in breaking and reformation of hydrogen bonds with the neighboring Cl- anions and in a time-averaged nearly uniaxial symmetry at Cl-, in good agreement with recent 35Cl NMR measurements. Power spectra of translational, librational, and vibrational motions of interlayer and surface Cl- and H2O were calculated as Fourier transforms of the atomic velocity autocorrelation functions and compared with the corresponding spectra and dynamics for a bulk aqueous solution. The ordered interlayer space has significant effects on the motions. Strong electrostatic attraction between interlayer water molecules and Ca atoms in the principal layer makes the Ca···OH2 bond direction the preferred axis for interlayer water librations. The calculated diffusion coefficient of Cl- as an outer-sphere surface complex is almost three times that of inner-sphere Cl-, but is still about an order of magnitude less than that of Cl- in bulk aqueous solution at the same temperature.

Diffusion of Ca and Mg in Calcite

Abstract The self-diffusion of Ca and the tracer diffusion of Mg in calcite have been measured experimentally using isotopic tracers of 25Mg and 44Ca. Natural single crystals of calcite were coated with a thermally sputtered oxide thin film and then annealed in a CO2 gas at 1 atm total pressure and temperatures from 550 to 800 °C. Diffusion coefficient values were derived from the depth profiles obtained by ion microprobe analysis. The resultant activation energies for Mg tracer diffusion and Ca self-diffusion are, respectively: Ea(Mg) = 284 ± 74 kJ/mol and Ea(Ca) = 271 ± 80 kJ/mol. For the temperature ranges in these experiments, the diffusion of Mg is faster than Ca. The results are generally consistent in magnitude with divalent cation diffusion rates obtained in previous studies, and provide a means of interpreting the thermal histories of carbonate minerals, the mechanism of dolomitization, and other diffusion-controlled processes. The results indicate that cation diffusion in calcite is relatively slow and cations are the rate-limiting diffusing species for the deformation of calcite and carbonate rocks. Application of the calcite-dolomite geothermometer to metamorphic assemblages will be constrained by cation diffusion and cooling rates. The direct measurement of Mg tracer diffusion in calcite indicates that dolomitization is unlikely to be accomplished by Mg diffusion in the solid state but by a recrystallization process.

All-atom ab initio energy minimization of the kaolinite crystal structure

Abstract Calculations that minimize the energy and optimize the geometry of all atomic coordinates for two proposed kaolinite crystal structures were performed using a first-principles, quantum chemical code based on local density functional theory. All calculations were performed using published unit-cell parameters. Inner- and interlayer H atom positions agree well with those determined by Bish (1993) from neutron diffraction data and confirm a unit cell with C1 symmetry.

Ca self-diffusion in grossular garnet

Abstract Use of a thin-film technique and an ion microprobe make it possible to conduct cation self-diffusion experiments of 44Ca in near end-member grossular garnet at temperatures of 800-1000 °C. The experiments were conducted at 1 atm and under quartz + fayalite + magnetlte fO2 conditions. The resulting activation energy (Ea = 155 ± 10 kJ/mol) and the frequency factor (Do = 7.2 × 10-16 m2/s; log Do = -15.1 ± 0.4) were obtained from the temperature dependence of the diffusion data. Comparison of these data with a comparable study of 25Mg self-diffusion coefficients in pyrope confirmed that Ca diffuses more slowly through garnet than Mg under identical conditions.

Gibbsite growth kinetics on gibbsite, kaolinite, and muscovite substrates: atomic force microscopy evidence for epitaxy and an assessment of reactive surface area

Abstract New experimental data for gibbsite growth on powdered kaolinite and single crystal muscovite and published data for gibbsite growth on gibbsite powders at 80°C in pH3 solutions show that all growth rates obey the same linear function of saturation state provided that reactive surface area is evaluated for each mineral substrate. Growth rate (mol m−2 s−1) is expressed by Rateppt = (1.9 ± 0.2) × 10−10|ΔGr|/RT(0.90±0.01), which applies to the range of saturation states from ΔGr = 0 to 8.8 kJ mol−1, where ΔGr = RT[ln(Q/K)] for the reaction Al3+ + 3H2O = Al(OH)3 + 3H+, and equilibrium defined as ΔGr = 0 was previously determined. Identification of the growth phase as gibbsite was confirmed by rotating anode powder x-ray diffraction. Rates on kaolinite were determined using steady-state measured changes between inlet and outlet solutions in single-pass stirred-flow experiments. Rates on muscovite were determined by measuring the volume of precipitated crystals in images obtained by Tapping Mode™ atomic force microscopy (TMAFM). In deriving the single growth rate law, reactive surface area was evaluated for each substrate mineral. Total BET surface area was used for normalizing rates of gibbsite growth onto powdered gibbsite. Eight percent of the BET surface area, representing the approximate amount occupied by the aluminum octahedral sheet exposed at crystal edges, was used for powdered kaolinite. The x - y area of the TMAFM images of the basal surface was used for single crystal muscovite. Linearity of growth rates with saturation state suggests that the dominant nucleation and growth mechanisms are two dimensional. Such mechanisms are supported by observations of the morphologies of gibbsite crystals grown on muscovite at ΔGr = 8.8 kJ mol−1. The morphologies include (1) apparent epitaxial films as determined by hexagonal outlines of edges and thicknesses of 30 to 40 A, (2) elongate crystals 30 to 40 A thick aligned with the structure of the distorted Si-tetrahedral sheet of the 2M1 muscovite, and (3) micrometer-scale three-dimensional clumps of intergrown crystals. Reactive surface area as defined now for heterogeneous crystal growth in reactive-transport models must be modified to include substrates other than that of the growing mineral and to account for the role of structural and chemical controls on epitaxial nucleation and growth.