CLAYFF Force Field Research Articles

Influence of polypropylene fibers on the tensile mechanical properties of calcium silicate hydrate: molecular simulation.

This research assesses the influence of polypropylene (PP) fibers, both homopolymer and hydroxylated (PPOH), on the tensile properties of calcium silicate hydrate (C-S-H) composites through molecular dynamics (MD) simulations. Our models explore C-S-H matrices integrated with PP and PPOH fibers at varying polymerization degrees. The results demonstrate that both PP and PPOH fibers significantly influence the tensile strength and Young's modulus of the composites. Notably, PPOH fibers contribute to more substantial mechanical enhancements than PP, attributed to the increased polarity and enhanced intermolecular interactions from the hydroxyl groups. The study reveals a nonlinear relationship between polymer additive content and mechanical performance, with optimal properties at a polymerization degree of 20. Additionally, stress-strain analysis indicates that PPOH composites exhibit superior ductility and fracture energy, particularly at polymerization degrees of 20, showing enhanced ultimate strain and fracture energy by up to 9.6% and 13.9%, respectively, compared to PP counterparts. These results highlight the crucial role of tailored polymer additive composition and chemical modifications in maximizing the mechanical efficacy of C-S-H-based materials, enhancing their durability and structural performance. All MD simulations were conducted using LAMMPS. The models employed a combination of Clayff and Cvff force fields. During the entire tensile simulation, the system was configured under the NPT ensemble at 300K.

Molecular dynamics simulation of hydrocalumite as adsorbent for anionic radionuclides

Hydrocalumite, is a hydration product of aluminum-rich cements, and is known in cement chemistry as an AFm phase. Structurally, it belongs to the family of layered double hydroxides, or “anionic clays”, where positively charged crystal layers require the presence of negatively charged ions in the interlayer space. Therefore, AFm phases can serve as potential adsorbents for anionic radionuclides (e.g., 35Cl−, 125I−, 129I−, 131I−) from aqueous solutions. Here we use classical molecular dynamic simulations to analyze the structure and properties of AFm phases containing Cl− and I−. The classical ClayFF force field is used to quantitatively study the structure, energetics and mobility of anions and H2O molecules in the interlayers of these phases and at their interfaces with CsCl and CsI aqueous solutions. In this study we report that the basal (001) surfaces of AFm phases can strongly adsorb hydrated Cl− and I− anions due to the donated hydrogen bonds from the interfacial hydroxyls, but primarily due to their strong attraction to the structural Ca cations exposed at the surface. However, our simulations show that the adsorption of I− is weaker than that of Cl−, leading to the higher surface mobility of I− due to its stronger chaotropic effect. The interlayer diffusional mobility of the Cl− and I− anions in the AFm phases is also investigated by using the Eyring-Vineyard approach and is shown to be significantly lower than in larger nanopores. Hence, the most likely transport of such anionic radionuclides takes place through the nano- and micro-pores of hardened cement.

Molecular dynamics study of the mechanical properties of hydrated calcium silicate enhanced by functionalized carbon nanotubes.

The incorporation of functionalized carbon nanotubes can enhance the mechanical properties of cement-based materials. However, the types of functional groups and their roles in composite materials are not yet clear. In this study, molecular dynamics (MD) simulation methods were employed to investigate the mechanical performance of hybridized calcium silicate hydrate gel reinforced with pure carbon nanotubes, epoxy-coated carbon nanotubes, carboxylated carbon nanotubes, and hydroxylated carbon nanotubes. The results indicate that the addition of all four types of nanotubes can enhance the mechanical properties of hydrated calcium silicate gel compared to pure C-S-H. Tensile loading results show that carbon nanotubes can act as bridges for microcracks in the composite material, and functionalized nanotubes exhibit a better reinforcing effect than pure carbon nanotubes. Under tensile stress, hydroxylated nanotubes are more effective in increasing the toughness of the composite material, while carboxylated nanotubes tend to enhance the strength of the composite material. The compressive loading results indicate that the compressive strength of cement-based materials is higher than their tensile strength. Overall, carboxylated nanotubes show particularly remarkable performance in enhancing the mechanical properties of cement-based materials. Compared to pure C-S-H gel, the tensile and compressive elastic moduli of carboxylated nanotube/C-S-H composite material increased by 18.13% and 34.78%, respectively. Its tensile and compressive strengths also increased by 30.40% and 40.23%, respectively. All molecular dynamics simulations were performed on the classical computational simulation platform LAMMPS. In this paper, the parameters in the ClayFF force field are chosen to simulate calcium hydrated silicate (/C-S-H), and the ClayFF-CVFF combined force field is used to simulate the mechanical properties of the CNT/C-S-H molecular model structure.

Atomistic Computer Simulations of Uranyl Adsorption on Hydrated Illite and Smectite Surfaces

A quantitative understanding of the molecular-scale mechanisms of radionuclide sorption on different clay minerals is crucial for the development and safe implementation of geological nuclear waste disposal technologies. We apply classical molecular dynamics (MD) computer simulations to study the adsorption of uranyl on the external basal surfaces of two typical clay models. In the illite model, negative charge is primarily localized in the tetrahedral sheets, while in the lower-charge smectite model, the isomorphic substitutions are introduced in the octahedral sheet. The comparison of atomic density distributions at the clay surfaces and adsorption-free energies profiles as a function of distance from these surfaces demonstrates that overall U behavior at the basal clay surface is quite similar for illite and smectite. Uranyl is sorbed as a mixture of outer-sphere aqua complexes [UO2(H2O)5]2+ and hydrolyzed aqua complexes [UO2(H2O)4–5OH]+ on both surfaces. The structural and compositional differences between the models do not greatly affect the uranyl’s nearest coordination environment and are mainly reflected in the specific localization and orientation of the uranyl ions at both surfaces and in the magnitude of the adsorption-free energies. The observed quantitative characteristics of uranyl interactions with illite and smectite surfaces will help to better understand U behavior during the sorption process on clay minerals for the entire range of mixed-layer illite–smectite structures. A comparison of two versions of the ClayFF force field in the simulations made it possible to more accurately and quantitatively evaluate some subtle features of the uranyl–clay interactions and to obtain a more precise composition of uranyl complex with the modified ClayFF force field (ClayFF-MOH).

Structure and Properties of Water in a New Model of the 10-Å Phase: Classical and Ab Initio Atomistic Computational Modeling

The 10-Å phase is an important member of the family of dense hydrous magnesium silicates (DHMSs) that play a major role in the water budget in the Earth’s upper mantle. Its nominal composition is usually written as Mg3Si4O10(OH)2·xH2O, and its structure is often described as layers of talc with some amount of water present in the interlayer space. However, its actual structure and composition and the detailed mechanisms of retaining H2O molecules within the mineral are not yet sufficiently known. In particular, more recent spectroscopic and diffraction data indicate the presence of Si vacancies in the tetrahedral silicate sheets of the 10-Å phase leading to the formation of Q2-type Si sites terminated by silanol groups. These silanols are, in turn, hydrogen bonded to interlayer H2O molecules. Here, we use classical and ab initio molecular dynamics (MD) simulations to compare the structures and properties of ideal and defect models of the 10-Å phase under ambient conditions. For classical MD simulations, the most recent modification of the ClayFF force field is used, which can accurately account for the bending of Mg–O–H and Si–O–H angles in the mineral layers, including the structural defects. The crystal lattice parameters, elastic constants, structure, and dynamics of the interlayer hydrogen bonding network for the model 10-Å phase are calculated and compared with available experimental data. The results demonstrate that the inclusion of Si vacancies leads to better agreement with crystallographic data, elastic constants, and bulk and shear moduli compared to a simpler model based on the idealized talc structure. The results also clearly illustrate the importance of the explicit inclusion of Mg–O–H and Si–O–H angular bending terms for accurate modeling of the 10-Å phase. In particular, the properly constrained orientation of the silanol groups promotes the formation of strong hydrogen bonds with the interlayer H2O molecules.

Interaction of Nitrite Ions with Hydrated Portlandite Surfaces: Atomistic Computer Simulation Study.

The nitrite admixtures in cement and concrete are used as corrosion inhibitors for steel reinforcement and also as anti-freezing agents. The characterization of the protective properties should account for the decrease in the concentration of free NO2- ions in the pores of cement concretes due to their adsorption. Here we applied the classical molecular dynamics computer simulation approach to quantitatively study the molecular scale mechanisms of nitrite adsorption from NaNO2 aqueous solution on a portlandite surface. We used a new parameterization to model the hydrated NO2- ions in combination with the recently upgraded ClayFF force field (ClayFF-MOH) for the structure of portlandite. The new NO2- parameterization makes it possible to reproduce the properties of hydrated NO2- ions in good agreement with experimental data. In addition, the ClayFF-MOH model improves the description of the portlandite structure by explicitly taking into account the bending of Ca-O-H angles in the crystal and on its surface. The simulations showed that despite the formation of a well-structured water layer on the portlandite (001) crystal surface, NO2- ions can be strongly adsorbed. The nitrite adsorption is primarily due to the formation of hydrogen bonds between the structural hydroxyls on the portlandite surface and both the nitrogen and oxygen atoms of the NO2- ions. Due to that, the ions do not form surface adsorption complexes with a single well-defined structure but can assume various local coordinations. However, in all cases, the adsorbed ions did not show significant surface diffusional mobility. Moreover, we demonstrated that the nitrite ions can be adsorbed both near the previously-adsorbed hydrated Na+ ions as surface ion pairs, but also separately from the cations.

Melting Behaviour under Pressure of Kaolinite Clay: A Nanoscale Study

In this study, the curves of variation of melting temperature as a function of pressure were determined for pressures up to 20 GPa using molecular dynamics (MD) calculations. The CLAYFF force field is used for the simulated PT curve of the clay kaolinite structure. For this purpose, we have adopted the Z-method to determine the melting point (Tm) and superheat limit temperature (TLS) for different densities in kaolinite clay. In addition, various quantities, such as the radial distribution function (RDF), the mean square displacement (MSD), and the diffusion coefficient were evaluated in order to ensure the solid behaviour at the superheat limit temperature and the liquid behaviour at the melting point for the equilibrated structure of kaolinite.

Equation of State, Compressibility, and Vibrational Properties of Brucite over Wide Pressure and Temperature Ranges: Atomistic Computer Simulations with the Modified ClayFF Classical Force Field

The behavior of brucite over wide ranges of temperatures and pressures is of great interest for fundamental geochemistry and geophysics. Brucite layers and their octahedral Mg(OH)6 structural units constitute an important structural part of layered dense magnesium hydrous silicates (DMHS), which play a major role in mineral equilibria controlling water balance in the subduction zones of the upper mantle. The ClayFF force field was originally developed for atomistic computer simulations of clays and other layered minerals and their hydrated interfaces. The crystallographic parameters of brucite at 25 °C and 1 bar were used, among several others, to develop the original ClayFF parametrization. Its new recent modification, ClayFF-MOH, can more accurately account for the bending of Mg–O–H angles in the brucite structure, and it was used here to test the applicability of this simple classical model over very wide ranges of temperature and pressure well beyond the range of its original implementation (up to 600 °C and 15 GPa). The pressure and temperature dependencies of brucite crystallographic parameters, the compressibility of the crystal lattice, the coefficients of thermal expansion, and the vibrational spectra were calculated in a series of classical molecular dynamics simulations using the ClayFF-MOH model and compared with a diverse set of available experimental data, including X-ray diffractometry, neutron scattering, IR and Raman spectroscopy. These new results demonstrated that ClayFF-MOH, as simple and approximate as it is, can be quite accurate in predicting many mineral properties at subduction zone conditions, which greatly expands the area of its applicability.

Early stage of swelling process of dehydrated montmorillonite through molecular dynamics simulation

The early stage of the swelling process of dehydrated montmorillonite (MMT) is crucial to understand the mechanism of swelling. In this study, molecular dynamics simulations and analyses are presented to examine the conditions under which water molecules adsorb into the MMT interlayer in the early stages. Dehydrated (no interlayer water molecule - 0 W) and slightly hydrated (one and two water molecules per unit cell -1 W and 2 W) models of montmorillonite in a free water box are first built out with three different compensated cations (Ca2+, K+, or Li+). Simulations are then carried out at 298 K and 324 K using the CLAYFF force field. All results show that no water molecules can be adsorbed into dehydrated MMT interlayers at those temperatures. The MMT swelling process starts with existing initial interlayer water molecules, which allows more water molecules to adsorb into the interlayer to cause MMT swelling. The size and the valence of the compensated cations have a significant effect on the swelling behavior of MMT, and the Ca-MMT has a stronger swelling ability than K-MMT and Li-MMT. However, the swelling capacity of Ca-MMT is inhibited in saltwater compared to that in pure water.

Atomistic simulations of ettringite and its aqueous interfaces: Structure and properties revisited with the modified ClayFF force field

Ettringite, (Ca6[Al(OH)6]2[SO4]3·nH2O, n = 24–27), is one of the common phases of cement and plays an important role in cement chemistry as the primary cause of sulphate corrosion in Portland cement. Molecular dynamic computer simulations have already been applied earlier to model the crystal structure of ettringite and its interfaces with aqueous salt solutions. A recently developed version of the widely used ClayFF force field allows now to explicitly take into account the bending of M-O-H angles of (M = Al, Ca), leading to a much better agreement of the simulation results with available experimental data. The structure and dynamics of bulk ettringite crystal and its interfaces with NaCl and Na2SO4 aqueous solutions are quantitatively evaluated here for the new modified version of the force field, ClayFF-MOH, and compared with the results obtained with the earlier version, ClayFF-orig. The crystallographic parameters, elastic properties, the structure and dynamics of intracrystalline hydrogen bonding network and the vibrational spectra of ettringite are calculated by classical molecular dynamics simulations and quantitatively compared with available experimental data using both versions of ClayFF. Atomic density profiles for solution species at the ettringite surface, atomic distributions within the crystal-solution interface, and the interfacial diffusional mobility of the species are also calculated and compared. The results clearly demonstrate the importance of the explicit inclusion of M-O-H angular bending terms for accurate modeling of the mineral systems containing structural and interfacial hydroxide groups. The simulation results also show that the application of the new more accurate ClayFF-MOH version of the force field leads to the formation of a stronger hydrogen bonding network structure in the intercolumnar space of the ettringite crystal and at its surface, resulting in a stronger immobilization of the water molecules involved, as well as the ions. The ionic adsorption at the ettringite surface is also generally stronger than it was predicted by the earlier model.

Molecular dynamics insight into the adsorption and distribution of bitumen subfractions on Na-montmorillonite surface

How bitumen works with clay minerals is an essential matter for the application of nano-clay minerals (e. g., montmorillonite, vermiculite) in bitumen modification. In this paper, the combination of ClayFF and consistent-valence force field (CVFF) is developed to characterize the bitumen-clay interaction. The adsorption configuration, distribution pattern of bitumen subfractions on the Na-montmorillonite surface and thermal effect are systematacially investigated by examining the evolution of bitumen model at different temperatures based on the molecular dynamics simulation. For comparision, 12 isolated bitumen monomers and bitumen SARA fractions (saturates, aromatics, resins, asphaltenes) are attached to Na-montmorillonite surface respectively. The results show that asphaltenes have preference for parallel adsorption near bitumen-clay interface, but the tightly adsorbed configuration can be disrupted by their self-aggregation and light fractions. The distribution pattern is highly influenced by the adsorption strength and molecular diffusion, despite non-negligible spatial effect. Herein, the interaction energy of isolated monomers with clay platelet shows the adsorption strength of SARA fractions in the order of asphaltenes > resins > aromatics > saturates, while the diffusion can be ranked as: Saturates > Resins > Aromatics > Asphaltenes. The distribution pattern of bitumen inferred from the synergy of interaction strength and diffusion suggests that the bitumen exhibits stratified property on clay surface, the asphaltenes and resins content is likely to be higher in the range (1.0～1.5 nm) close to the surface. As for the thermal effect, it is observed that the asphaltenes and resins desorb from the clay surface as the temperature rises as aromatics dominates the adsorption zone, also supported by the finding that asphaltenes dimers are formed and the interaction energy of bitumen with clay platelet decreases to 72.98% at elevated temperature, which implies the thermal effect can reverse the distribution by inconsistently increasing the diffusion of bitumen subfractions. Overall, these findings are helpful for understanding bitumen-clay interaction, optimizing processing temperature and screening surfactant to improve compatibility of nano-clay modifiers with bitumen.

Understanding adsorption of amine surfactants on the solvated quartz (1 0 1) surface by a jointed Dreiding-ClayFF force field

In this study, ClayFF and Dreiding force fields are jointed to model the adsorption of amine surfactants or their hydrolysate species on the solvated quartz surface under a wide range of pH values. As two case studies, cationic surfactant DDA (dodecylamine) and amphoteric surfactant LAO (N,N-Dimethylaminopropyldodecylamide oxide) are respectively selected as typical surfactants in molecular dynamic simulations. The simulation results demonstrate that DDA and LAO have excellent adsorption under natural conditions, but they have different adsorption mechanisms owing to their different polar groups. The polar group (-RN+H3) of hydrolysis DDA can penetrate the first hydration layer and perpendicularly adsorb on the quartz surface via strong electrostatic attraction and hydrogen bonds. Compared with DDA, the hydrolysis LAO has a larger polar group consisting of R2N+→OH and -C(=O)NH– and its polar group lay horizontally around the first hydration layer rather than penetrate it. Besides, the adsorption of amine surfactants on quartz is inferred as multilayer based on the partial density distributions (PDD) analysis results. Subsequently, we determined the adsorption amounts, adsorption configurations and interaction energy that agreed with the quartz recovery in flotation test with a wide range of pH values.

Structural and dynamical properties of H2O molecules confined within albite-quartz system under cyclic thermal loading: insights from molecular dynamic simulation

The motivation behind this work was to make a first attempt at gaining molecular-level insights into the structural and dynamical properties of H2O molecules confined within albite-quartz system under cyclic thermal loading, where CLAYFF force field was introduced into Forcite module. Derived from the distance variations of the tetrahedral atoms surrounding the Na position and RDF, the movement tendency of Na atom to be near Al atom and a stronger interaction between albite and H2O molecules can be observed. Accepting H-bonds from the H2O molecules near-surface are important for the development of continuous H-bond networks across the interfacial region. The discrepancy of the orientation order of H2O molecules and the H-bond configurations resulted from long-term thermal cycling are largely responsible for the structure and dynamics of near-surface H2O molecules.

Molecular dynamics study of confined water in the periclase-brucite system under conditions of reaction-induced fracturing

The volume-increase associated with hydration reactions in rocks may lead to reaction-induced fracturing, but requires a stable water film to be present at reactive grain boundaries even when subject to compressive stress. Hydration of periclase to brucite is associated with a solid volume increase of ca. 110%. Recent experiments on the periclase-brucite system observed that when the effective mean stress exceeds 30 MPa, the reaction rate slows down dramatically. We hypothesize that for the brucite forming reaction to progress, the fluid film between grains must remain stable. If the applied pressure becomes larger than the hydration force, the fluid film will collapse and be squeezed out of the grain contacts. To quantify this effect, we study the behavior of a water film confined between periclase or brucite surfaces subject to compressive stress, by performing molecular dynamics simulations. The simulations are carried out using the ClayFF force field and the single point charge (SPC) water model in the molecular dynamics simulations program LAMMPS. The setup consists of two interfaces of either periclase or brucite surrounded by water. Our simulations show that when the pressure reaches a few tens of MPa, the water film collapses and reduces the water film to one or two water layers, while the self-diffusion coefficient of water molecules by a factor of eight. A water film thickness below two water layers is thinner than the size of the hydration shell around Mg2+-ions, which will limit ion-transport. The observed collapse of the water film to a single layer at a normal pressure of 25–30 MPa might explain the observed slow-down of reaction-induced fracturing in the periclase-brucite system.

Coupling between clay swelling/collapse and cationic partition

The semipermeability of clay aggregates in mudstones, shales, and marine sediments gives rise to geological ultrafiltration of ions and water. This behavior has been numerically analyzed based on the theory of the electric double layer (EDL). The model of EDL is suited for describing ionic partitions in clay pore with a width of a few nanometers. However, the interlayer space of smectite might be hardly described by the EDL model. An explicit atomic-scale description is more appropriate for the interlayer space. Besides, clay swelling or collapse corresponding to a variation of environmental conditions, and its impact on partitions of water and ions between interlayers and environment, have not been taken into account numerically. With atomic-scale molecular dynamics simulations and thermodynamic integrations based on them, the coupling between montmorillonite swelling/collapse and partitions of water and cations under different aqueous activity conditions is systematically disclosed here. With the combination of the Horinek’s force field to describe cations and ClayFF force field for atoms in solid layers, consistent swelling/collapse of Na- and K-montmorillonite with experimental observations as a function of water activity is shown. The swelling/collapse is connected to the transition between monolayer and bilayer hydration states. Decomposition of the swelling/collapse free energy shows both energy and entropy contributions are important in determining the thermodynamically stable hydration state. Taking environmental water activity and Na+/K+ ionic activity ratio as variables, a picture of cationic partition in response to those variables is established. It shows, under all circumstances, K+ ions are enriched in montmorillonite, which might explain K+ depletion in pore fluids of sediments. A low water activity improves K+ enrichment in montmorillonite. In addition, cationic partition accompanies variations of hydration states and alters the ionic concentration of the aqueous environment. These results can be applied to explain water and ionic partitions during subsurface water flow and the diagenesis processes of sediments.

Effect of porosity and temperature on thermal conductivity of jennite: A molecular dynamics study

Despite growing interest of cement materials in design and construction of green buildings, their thermal properties are not well understood at atomic scale. In this work, we reported the thermal conductivity of porous jennite, the major component of cement paste, and its porosity and temperature dependences, using molecular dynamics simulations with ClayFF force field. With molecular modeling for the porous jennite with different porosities by removing atoms within the sphere with a certain radius, we mostly applied the non-equilibrium Müller-Plathe method to the porous jennite models, using the sufficiently extended simulation boxes along the heat flux direction and divided into numbers of plane slices. Our calculations revealed that the volumetric thermal conductivity of porous jennite decreases from 1.141 to 0.144 W/m⋅K as increasing the porosity from 0 to 72% at room temperature, which follows the empirical coherent potential model when the pore is filled with air. Moreover, as temperature increases, their thermal conductivities were observed to increase first and gradually decrease after certain temperature. We compared the calculation data with our measurement and other available experiments, confirming reasonable agreement between our calculation and experiment.

THEORETICAL UNDERSTANDING OF MORPHOLOGICAL DISTRIBUTION OF HYDROGEN IN AMORPHOUS CALCIUM SILICATE HYDRATE

Determining the morphological distribution of hydrogen atoms in calcium silicate hydrate phase (C-S-H) is of great concern but challenging. In this work, based on the stoichiometric composition of (CaO)1.7(SiO2)(H2O)1.8, a series of amorphous Ca1.7•Si•(O)3.7-x•(OH)2x•(H2O)1.8-x models are constructed using Monte Carlo method, where x represents the molar number of dissociated H2O molecules (x∈0~1.8). A molecular dynamics simulation has been performed with the ClayFF force field. The analysis of Qn, radial distribution function (RDF), coordinate number (CN), mean chain length (MCL), mean square displacement (MSD), elastic modulus (M), and Young’s modulus (E) are used to explore the microstructure, hydrogen atom distribution and mechanical properties. The results demonstrate that the proportions of hydrogen atoms in Ca-OH, Si-OH, and H2O are about 26%~45%, 13%~22%, and 61%~33%, respectively. The calculated microstructure parameters and mechanical properties, especially for C-S-H models with x of 0.7~1.2, are in good agreement with those reported previously in theoretical and experimental works, indicative of the excellent rationality of our theoretical C-S-H models. The simulated results of this work provides a new strategy of constructing theoretical C-S-H model, which might be suitable for studying the doping behavior of metal oxides or metal salts in C-S-H.

A combined first principles and classical molecular dynamics study of clay-soil organic matters (SOMs) interactions

In this study, clay-soil organic matters (SOMs) interactions were investigated by combining first principles molecular dynamics (FPMD) and classical molecular dynamics (CMD) techniques. FPMD was employed to quantify the binding mechanisms of reactive SOMs groups on clay surfaces and to derive parameters for the bonding of the reactive groups on edge surfaces. By integrating the derived parameters with CVFF–CLAYFF force fields, CMD simulations were carried out to investigate the structures of large models of clay-SOMs associations. All possible reactive SOMs groups, including carboxylate, phosphate, quinone species and ammonium groups were taken into account. FPMD results showed that all these groups are important to the complexation of SOMs on clay surfaces under dry conditions but only chemical bonding of carboxylate and phosphate and cation bridging are of significance under wet conditions. pH dependence of carboxylate and phosphate and Eh-pH dependence of quinone species are presented. CMD simulations showed that SOMs bound on clay surfaces via direct bonding of carboxylate and Ca2+ bridging and revealed the effect of water on structures of the clay-SOMs association. Based on the computational results, a procedure was proposed for constructing realistic molecular models for soils. Possible applications and further improvements of these models are discussed.

Predicting the Effects of Temperature on the Water Wettability of the Mg/MgO Surface through Atomistic Simulations

Based on the SPC/E and the CLAYFF force field, the effects of temperature on the water wettability of the MgO surface have been studied using molecular dynamics simulations. Surprisingly, the conta...

Simulations of water structure and the possibility of ice nucleation on selected crystal planes of K-feldspar.

Molecular dynamics simulations are employed to investigate the structure of supercooled water (230 K) in contact with the (001), (010), and (100) surfaces of potassium feldspar (K-feldspar) in the microcline phase. Experimentally, K-feldspar and other feldspar minerals are known to be good ice-nucleating agents, which play a significant role in atmospheric science. Therefore, a principal purpose of this work is to evaluate the possibility that the K-feldspar surfaces considered could serve as likely sites for ice nucleation. The (001) and (010) surfaces were selected for study because they are perfect cleavage planes of feldspar, with (001) also being an easy cleavage plane. The (100) surface is considered because some experiments have suggested that it is involved in ice nucleation. Feldspar is modeled with the widely used CLAYFF force field, and the TIP4P/Ice model is employed for water. We do not observe ice nucleation on any of the K-feldspar surfaces considered; moreover, the density profiles and the structure of water near these surfaces do not exhibit any particularly icelike features. Our simulations indicate that these surfaces of K-feldspar are likely not responsible for its excellent ice nucleating ability. This suggests that one must look elsewhere, possibly at water-induced surface rearrangements or some other "defect" structure, for an explanation of ice nucleation by K-feldspar.