CLAYFF Force Field Research Articles

Toward Modeling Clay Mineral Nanoparticles: The Edge Surfaces of Pyrophyllite and Their Interaction with Water

The basal surfaces of phyllosilicate minerals have been widely studied, whereas the edge surfaces have received little attention. However, in order to simulate complete clay particles at the atomic level, the modeling of edge surfaces becomes crucially important, and such surfaces are likely to be far more active. We used a combination of quantum and potential based techniques to evaluate the structure of the edge surfaces of pyrophyllite and their interaction in an aqueous environment. These include {110}, {100}, {010}, {1̅10}, {130}, and {1̅30}. We found that the CLAYFF force field is an effective model for reproducing the DFT results. Furthermore, the results show that, for this notorious natural hydrophobic clay, all edge surfaces show hydrophilic behavior and that the precise structure of water above these surfaces is influenced by both the presence of hydroxyl groups and under-coordinated surface Al atoms; this will impact both geological processes where natural clays are involved and processes where such clays act as primary retention barriers to the dispersion of contaminants.

Propane simulated in silica pores: Adsorption isotherms, molecular structure, and mobility

Molecular dynamics simulations were conducted for bulk propane in contact with fully protonated slit-shaped silica pores. The effective pore width was of either 0.8 or 2.7nm. The temperature was set at 343, 368, and 373K. The TraPPE-UA and CLAYFF force fields were implemented to model propane and silica, respectively. Each individual simulation yields the density of confined fluid as a function of the bulk pressure. For a given temperature, adsorption isotherms were estimated by repeating the simulations at various bulk pressures. The results qualitatively agree with available experimental data; namely, at fixed temperature the excess sorption is found to show a maximum near the pressure at which the pores fill; at fixed pressure the excess adsorption is found to decrease as the temperature increases and as the pore width expands. At equilibrium, pronounced layering was observed for propane near the pore surface, especially in the narrower pore and at the highest densities considered. The propane molecules at contact with silica tend to lay with their CH3-CH3 vector parallel to the pore surface. The mean square displacement as a function of time was used to quantify the self-diffusion coefficient of confined propane as a function of temperature, pressure and pore width. These results will be useful for enhancing the interpretation of experimental data.

Aqueous dual-tailed surfactants simulated on the alumina surface.

Atomistic molecular dynamics (MD) simulations were used to compare the morphology of aqueous surfactant self-assembled aggregates on a flat alumina substrate. The substrate was modeled using the CLAYFF force field, and it was considered fully protonated. Three ionic surfactants were considered, all with a sulfate headgroup. The first surfactant was the single-tailed, widely studied sodium dodecyl sulfate (SDS), for which previous simulation results are available on several substrates. The results obtained for this surfactant were used for benchmarking the behavior of two dual-tailed surfactants. These latter surfactants have equal structure, except that in one case both linear tails are composed by seven fully protonated carbon atoms [CH3(CH2)6CHOSO3(CH2)6CH3(-), 2H7], whereas in the other, one tail is composed by seven fully protonated carbon atoms and the other tail is composed by seven fully fluorinated carbon atoms [CF3(CF2)6CHOSO3(CH2)6CH3(-), H7F7]. Our results suggest that preferential interactions lead to surfactant aggregates for H7F7 that differ compared to both those obtained for SDS and 2H7. Although molecular-level geometric structural differences can be invoked to explain differences between H7F7 and SDS aggregates, those between H7F7 and 2H7 aggregates can only be ascribed to atomic-scale interactions. Because as the adsorbed amount of surfactant increases, the self-assembled surfactant aggregates change, suggesting that the substrate on which adsorption occurs effectively evolves as adsorption progresses, compared to bare alumina. The morphological differences observed in our simulations coupled with molecular-level microphase separation might explain, in part, the unusual retrograde adsorption isotherm that has been observed experimentally for H7F7 surfactants on alumina.

Vibrational Analysis of Brucite Surfaces and the Development of an Improved Force Field for Molecular Simulation of Interfaces.

We introduce a nonbonded three-body harmonic potential energy term for Mg–O–H interactions for improved edge surface stability in molecular simulations. The new potential term is compatible with the Clayff force field and is applied here to brucite, a layered magnesium hydroxide mineral. Comparisons of normal mode frequencies from classical and density functional theory calculations are used to verify a suitable spring constant (k parameter) for the Mg–O–H bending motion. Vibrational analysis of hydroxyl librations at two brucite surfaces indicates that surface Mg–O–H modes are shifted to frequencies lower than the corresponding bulk modes. A comparison of DFT and classical normal modes validates this new potential term. The methodology for parameter development can be applied to other clay mineral components (e.g., Al, Si) to improve the modeling of edge surface stability, resulting in expanded applicability to clay mineral applications.

Atomic-scale modelling of elastic and failure properties of clays

The elastic and failure properties of a typical clay, illite, are investigated using molecular simulation. We employ a reactive (ReaxFF) and a non-reactive (ClayFF) force field to assess the elastic properties of the clay. As far as failure is concerned, ReaxFF was used throughout the study; however, some calculations were also performed with ClayFF. A crack parallel to the clay layers is found to have low fracture resistance when submitted to a tensile loading perpendicular to the crack. The mechanism of both yield and fracture failures is decohesion in the interlayer space. In contrast, under shear loading, the nanoscale failure mechanism is a stick-slip between clay layers. No fracture propagation is observed as the clay layers slide on top of each other. The low fracture resistance in mode I and the stick-slip failure in mode II are both the consequence of the lack of chemical bonds between clay layers where the cohesion is provided by non-covalent interactions. This work, which provides a description of the failure of clays at the microscopic scale, is the first step towards describing the failure of clays at a larger scale where the polycrystalline distribution of clay grains must be taken into account.

Nanoscale simulation of Na-Montmorillonite hydrate under basin conditions, application of CLAYFF force field in parallel GCMC

The swelling of Na-Montmorillonite is simulated by Monte Carlo molecular simulations in NPzzT and μVT ensembles, in two sedimentary basin conditions of 423 K at 750 atm and 483 K at 1050 atm and by using the CLAYFF force field [J. Phys. Chem. B 108, 1255–1266 (2004)]. The CLAYFF force field predicts a one, two-layer stable hydrate under 423 K at 750 atm and a one-layer stable hydrate under 483 K at 1050 atm with the stable basal spacings of 12.54 and 12.44 Å. The adsorbed water molecules are 45.22 and 41.92 per clay layer, with water densities of 0.28 and 0.26 g/cm3 and total energy of 2.11 and 3.65 kcal/mol of water. The predicted structure of Na-Montmorillonite are consistent with those reported in literature for high temperature and pressure conditions, however the results did not indicate a one-layer hydrate as the stable phase under basin conditions of 423 K at 750 atm [Clays Clay Miner. 34, 385 (1986)]. This work allows addressing the issue of the effect of basin condition on the structure of Na-Montmorillonite hydrate.

Behind adhesion of uranyl onto montmorillonite surface: A molecular dynamics study

We have performed molecular dynamics simulations to investigate the adsorption of radionuclide elements species onto substituted Montmorillonite (001) surface in the presence of different counterions. The structure and the dynamics of uranyl ion as well as its aquo, chloride ion, and carbonate complexes are analyzed. In addition, we have studied the surface energy between layered Montmorillonite sheets and the work of adhesion between radionuclide and charged Montmorillonite. The clay model used here is a Wyoming-type Montmorillonite with 0.75e negative charge per unit cell resulting from substitutions in Octahedral and Tetrahedral sheets. The system model was constructed based on CLAYFF force field potential model. To evaluate the thermodynamic work of adhesion, each surface and clay layer regions are converted to a thin film model. One and two species of radionuclide elements (UO2(H2O)5,UO2CO3(H2O)5, and UO2Cl2(H2O)5) were deposited near the clay surface in a pseudo-two-dimensional periodic cell. Analysis shows that the uranyl ion structure is preserved with two axial oxygen atoms detected at 1.8Å. Radial distribution functions results indicate that average UOw distances are 2.45–2.61Å, and 2.29–2.40Å for UOc distance. Average UCl distances are 2.78–3.08Å, which is relatively larger than that of Uranium atom-Oxygen atom because of electrostatic factors.

Uranyl adsorption on (001) surfaces of kaolinite: A molecular dynamics study

The aim of the present paper is to investigate the adsorption of uranyl species (UO2)2+(H2O)5 onto kaolinite (001) surfaces. To this end we have employed molecular dynamic simulations based on CLAYFF force field potential. Various types of surface model for inner-sphere adsorption complexes and one model for outer-sphere adsorption complexes were optimized. In order to have a neutral structure, the uranyl (UO2)2+(H2O)5 or the kaolinite was deprotonated to form the outer-sphere or inner-sphere adsorption complexes. Both singly protonated and partially deprotonated states of the Al(0) kaolinite surface were considered for adsorption in the model of inner-sphere complexes. The first uranyl coordination shell exhibits pentagonal bi-pyramidal symmetry with the pentagonal formed by 5 water molecules. We show that the average U–OW distances are between 2.49 and 2.57Å for water molecules. The bond of uranyl with deprotonated O− center is always short because of the charge attraction. The obtained results agree well with density functional calculations and EXAFS measurements, and show how and why the adsorption of uranyl appears on the surface of kaolinite.

Liquid Ethanol Simulated on Crystalline Alpha Alumina

Equilibrium molecular dynamics simulations were employed to investigate the structural properties of interfacial liquid ethanol on two alumina surfaces-the (0001) and the (11[overline]02) terminations of α-Al2O3-both described using the CLAYFF force field. The resultant atomic density profiles show that ethanol molecules in the first monolayer are well ordered and that their distribution depends on the surface OH groups. A pronounced dipolar orientation is found for the ethanol molecules in the first layer and also, although to a smaller extent, among the ethanol molecules in the second molecular layer. The orientational distribution predicted for the methyl group of ethanol on α-Al2O3 (11[overline]02) is consistent with that observed experimentally by Shen and co-workers. The orientation of molecules in the second layer is opposite compared to that found in the first adsorbed molecular layer. Our simulations show long residence times and slowly decaying reorientation autocorrelation functions for ethanol molecules in the first adsorbed layer, suggesting that within the first adsorbed layer ethanol molecules are strongly coordinated with both α-Al2O3 (0001) and α-Al2O3 (11[overline]02) surfaces. Analysis of the hydrogen bond network confirms that preferential surface-ethanol interactions are responsible for such observations.

A molecular dynamics study of the swelling patterns of Na/Cs-montmorillonites and the hydration of interlayer cations

We report on a molecular dynamics study of the swelling patterns of an Na-rich/Cs-poor montomorillonite and a Cs-montomorillonite. The recently developed CLAYFF force field is used to predict the basal spacing as a function of the water content in the interlayer. The simulations reproduce the swelling patterns of the Na and Cs-montomorillonite, suggesting a mechanism of its hydration different from that of the montomorillonite. In addition, we find that the differences in size and hydration energy of Na and Cs ions have strong implications for the structure and the internal energy of interlayer water. In particular, our results indicate that the hydrate difference in the presence of coexistent Na and Cs has a larger influence on the behavior of a clay—water system. For Na-rich/Cs-poor montomorillonite, the hydration energy values of Na ions and water molecules each have a dramatic increase compared with those in Na-montomorillonite on the interlayer spacing, and the hydration energy values of Cs ions and water molecules decrease somewhat compared with those in Cs-montomorillonite.

Nanoscale Elastic Properties of Montmorillonite upon Water Adsorption

Smectites are an important group of clay minerals that experience swelling upon water adsorption. This paper uses molecular dynamics with the CLAYFF force field to simulate isothermal isobaric water adsorption of interlayer Wyoming Na-montmorillonite, a member of the smectite group. Nanoscale elastic properties of the clay-interlayer water system are calculated from the potential energy of the model system. The transverse isotropic symmetry of the elastic constant matrix was assessed by calculating Euclidean and Riemannian distance metrics. Simulated elastic constants of the clay mineral are compared with available results from acoustic and nanoindentation measurements.

Molecular Structure and Dynamics in Thin Water Films at Metal Oxide Surfaces: Magnesium, Aluminum, and Silicon Oxide Surfaces

All-atom equilibrium molecular dynamics simulations were employed to investigate the structural and dynamical properties of interfacial water on the magnesium oxide surface. The solid support was modeled utilizing two different formalisms, both based on the CLAYFF force field. In one case, the atoms in the MgO substrate are allowed to vibrate, whereas in the other they are maintained rigid. The properties of water within the thin film are assessed in terms of density profiles in the direction perpendicular to the substrate as well as along planes parallel to the substrate, in-plane radial distribution functions, density of hydrogen bonds, residence times in contact with the substrate, and orientation distribution of interfacial water molecules. The contact angle for a small droplet on various substrates (MgO, SiO2, Al2O3) was also calculated and compared with experimental observations. On MgO, the substrate in which the atoms are maintained fixed is the one that most closely reproduces experimental contac...

Phase diagram of kaolinite from Molecular Dynamics calculations

Structural and thermal behaviors of uncharged 1:1 phyllosilicates kaolinite were investigated from molecular dynamics simulations based on the CLAYFF force field. The focus is on the variation of structural properties including bulk modulus with pressure from 0 to 20GPa under various range of temperature. The largest bulk modulus between the pressures of 200 and 800MPa varies from 80GPa at 298K to 50GPa at 1473K. The obtained value of Cp varies between 7.8 and 13.6Kcalmol−1K−1 in the pressure range of 0.1MPa–20GPa. Besides, a huge difference was noticed regarding the computed properties at the superheating point. Finally, we show the relationship between superheating point temperature and pressure leading to a phase diagram of kaolinite.

Thermal behaviour and superheating temperature of Kaolinite from molecular dynamics

The aim of the present work is the investigation of the thermodynamics properties of kaolinite by means of molecular dynamics calculations based on the CLAYFF force field. The obtained results give various thermodynamical quantities, not yet estimated for kaolinite, such as the linear thermal expansion coefficient in the three directions, heat capacities Cp and Cv in the temperature range between 100 and 2088K and enthalpy. We focussed our efforts, in particular, on the calculation of the superheating point, which is estimated at 1572K. This value was deduced from the variation of the lattice parameters, volume and enthalpy with temperature. In addition, the solid–liquid transformation was also seen from the flattening of the atomic density profiles and radial distribution functions, where the order is lost in the bulk structure.

Investigating the Quartz (101̅0)/Water Interface using Classical and Ab Initio Molecular Dynamics

Two different terminations of the (1010) surface of quartz (α and β) interacting with water are simulated by classical (CMD) (using two different force fields) and ab initio molecular dynamics (AIMD) and compared with previously published X-ray reflectivity (XR) experiments. Radial distribution functions between hydroxyl and water show good agreement between AIMD and CMD using the ClayFF force field for both terminations. The Lopes et al. (Lopes, P. E. M.; Murashov, V.; Tazi, M.; Demchuk, E.; MacKerell, A. D. J. Phys. Chem. B2006, 110, 2782-2792) force field (LFF), however, underestimates the extent of hydroxyl-water hydrogen bonding. The β termination is found to contain hydroxyl-hydroxyl hydrogen bonds; the quartz surface hydroxyl hydrogens and oxygens that hydrogen bond with each other exhibit greatly reduced hydrogen bonding to water. Conversely, the hydroxyl hydrogen and oxygens that are not hydrogen bonded to other surface hydroxyls but are connected to those that are show a considerable amount of hydrogen bonding to water. The electron density distribution of an annealed surface of quartz (1010) obtained by XR is in qualitative agreement with electron densities calculated by CMD and AIMD. In all simulation methods, the interfacial water peak appears farther from the surface than observed by XR. Agreement among AIMD, LFF, and XR is observed for the relaxation of the near-surface atoms; however, ClayFF shows a larger discrepancy. Overall, results show that for both terminations of (1010), LFF treats the near-surface structure more accurately whereas ClayFF treats the interfacial water structure more accurately. It is shown that the number of hydroxyl and water hydrogen bonds to the bridging Si-O-Si oxygens connecting the surface silica groups to the rest of the crystal is much greater for the α than the β termination. It is suggested that this may play a role in the greater resistance to dissolution of the β termination than that of the α termination.

Interfacial water on crystalline silica: a comparative molecular dynamics simulation study

Understanding the properties of interfacial water at solid–liquid interfaces is important in a wide range of applications. Molecular dynamics is becoming a widespread tool for this purpose. Unfortunately, however, the results of such studies are known to strongly depend on the selection of force fields. It is, therefore, of interest to assess the extent by which the implemented force fields can affect the predicted properties of interfacial water. Two silica surfaces, with low and high surface hydroxyl density, respectively, were simulated implementing four force fields. These force fields yield different orientation and flexibility of surface hydrogen atoms, and also different interaction potentials with water molecules. The properties for interfacial water were quantified by calculating contact angles, atomic density profiles, surface density distributions, hydrogen bond density profiles and residence times for water near the solid substrates. We found that at low surface density of hydroxyl groups, the force field strongly affects the predicted contact angle, while at high density of hydroxyl groups, water wets all surfaces considered. From a molecular-level point of view, our results show that the position and intensity of peaks observed from oxygen and hydrogen atomic density profiles are quite different when different force fields are implemented, even when the simulated contact angles are similar. Particularly, the surfaces simulated by the CLAYFF force field appear to attract water more strongly than those simulated by the Bródka and Zerda force field. It was found that the surface density distributions for water strongly depend on the orientation of surface hydrogen atoms. In all cases, we found an elevated number of hydrogen bonds formed between interfacial water molecules. The hydrogen bond density profile does not depend strongly on the force field implemented to simulate the substrate, suggesting that interfacial water assumes the necessary orientation to maximise the number of water–water hydrogen bonds irrespectively of surface properties. Conversely, the residence time for water molecules near the interface strongly depends on the force field and on the flexibility of surface hydroxyl groups. Specifically, water molecules reside for longer times at contact with rigid substrates with high density of hydroxyl groups. These results should be considered when comparisons between simulated and experimental data are attempted.

Elucidating the Crystal Face- and Hydration-Dependent Catalytic Activity of Hydrotalcites in Biodiesel Production

Hydrotalcites (HTs) are an important class of heterogeneous catalysts for a wide variety of traditionally base-catalyzed organic transformations and have been considered as an alternative to homogeneous catalysts in the transesterification of fats/oils in biodiesel production. Previous experimental observations show that HT-like materials possess a crystal face-dependent activity that varies between related organic transformations, as well as a profound influence for HT hydration on its catalytic activity. We use a combination of molecular dynamics simulations and periodic plane-wave density functional theory to elucidate both the crystal face- and hydration-dependent activity of Mg−Al HT. We develop a new force field for HT, based on the existing ClayFF force field, and utilize this model to examine the interaction of a model ester reactant with HT in the presence of differing solvents, representative of conditions for common transesterification and hydrolysis reactions. Our results suggest that the variations in the activity of the HT crystal faces can be explained in terms of dramatically different adsorption free energies of the ester onto the various faces, changing with the nature of the solvent. We further conclude that the observed hydration dependence of the HT activity is due to the deprotonation of interfacial acidic Mg(Al)−O−H hydroxyl hydrogen by interlayer hydroxide counterions in the absence of interlayer hydration, leading to a loss of Brønsted basicity.

Swelling of K+, Na+ and Ca2+-montmorillonites and hydration of interlayer cations: a molecular dynamics simulation

This paper performs molecular dynamics simulations to investigate the role of the monovalent cations K, Na and the divalent cation Ca on the stability and swelling of montmorillonite. The recently developed CLAYFF force field is used to predict the basal spacing as a function of the water content in the interlayer. The simulations reproduced the swelling pattern of these montmorillonites, suggesting a mechanism of their hydration different (K+ < Na+ < Ca2+) from that of K+-, Na+-, and Ca2+-montmorillonites. In particular, these results indicate that the valence of the cations has the larger impact on the behaviour of clay-water systems. It also finds that the differences in size and hydration energy of K+, Na+ and Ca2+ ions have strong implications for the structure of interlayer. This leads to the differences in the layer spacings of the simulated K+-, Na+-, and Ca2+-montmorillonites. Furthermore, these simulations show that the K cations interact strongly with the clay sheets for the dehydrated clay sheets, but for the hydrated clays the Ca cations interact clearly strongly with the clay sheets.

Implementation of a Morse potential to model hydroxyl behavior in phyllosilicates

The accurate molecular simulation of many hydrated chemical systems, including clay minerals and other phyllosilicates and their interfaces with aqueous solutions, requires improved classical force field potentials to better describe structure and vibrational behavior. Classical and ab initio molecular dynamics simulations of the bulk structure of pyrophyllite, talc, and Na-montmorillonite clay phases exhibit dissimilar behavior in the hydroxyl stretch region of power spectra derived from atomic trajectories. The classical simulations, using the CLAYFF force field, include either a standard harmonic potential or a new Morse potential parametrized for both dioctahedral and trioctahedral phases for the O-H bond stretch. Comparisons of classical results with experimental values and with ab initio molecular dynamics simulations indicate improvements in the simulation of hydroxyl orientation relative to the clay octahedral sheet and in the O-H bond stretch in the high frequency region of the power spectrum.

Molecular Dynamics Simulation on Structure of Water Molecules in a Kaolinite-Water System

Using Hendricks′s model as initial structure,a molecular dynamics simulation without crystallographic restrictions was used to investigate the kaolinite-water system with the CLAYFF force field. Results show that interlayer water molecules have three types:type Ⅰ is similar to Costanzo′s"hole water"molecule and its HH vector(a vector from one hydrogen atom pointing to another hydrogen atom in water molecule) is parallel to the(001) plane while its C2-axis is slightly sloped and is nearly normal to the(001) plane;type Ⅱ is similar to"associated water"where one O—H bond points to its neighboring layer tetrahedral oxygen and its hydrogen forms hydrogen bonds with oxygen while the other O—H bond is approximately parallel to the(001) plane;type Ⅲ water molecule approximates a vertical form where one hydrogen forms a bond with tetrahedral oxygen and the other forms a hydrogen bond with a hydroxyl oxygen in the opposite clay layer. The concentration curve of the hydroxyl hydrogen of kaolinite along the normal line of the(001) plane shows that a portion of the hydroxyls change their orientation to be nearly parallel to the(001) plane. The corresponding oxygen atoms in the hydroxyl can therefore be exposed and form a hydrogen bond with interlayer water. Furthermore,some oxygen atoms of Type Ⅱ water molecule deviate from their average position in the interlayer space and are closer to the tetrahedral layer and this result is consistent with Costanzo′s result and may be the reason for the weak(002) peak observed in X-ray diffraction.