Phase coexistence and dynamic properties of water in nanopores

The dynamical properties of a confined fluid depend strongly on the (spatially varying) density. Its knowledge is therefore an important prerequisite for molecular-dynamics (MD) simulations and the analysis of experimental data. In a mixed Gibbs ensemble Monte Carlo (GEMC)/MD simulation approach we first apply the GEMC method to find possible phase states of water in hydrophilic and hydrophobic nanopores. The obtained phase diagrams evidence that a two-phase state is the most probable state of a fluid in incompletely filled pores in a wide range of temperature and level of pore filling. Pronounced variations of the average and local densities are observed. Subsequently, we apply constant-volume MD simulations to obtain water diffusion coefficients and to study their spatial variation along the pore radius. In general, water diffusivity slightly decreases in a hydrophilic pore and noticeably increases in a hydrophobic pore (up to about 40% with respect to the bulk value). In the range of gradual density variations the local diffusivity essentially follows the inverse density and the water binding energy. The diffusivity in the quasi-two-dimensional water layers near the hydrophilic wall decreases by 10 to 20% with respect to the bulk value. The average diffusivity of water in incompletely filled pore is discussed on the basis of the water diffusivities in the coexisting phases.Received: 1 January 2003, Published online: 14 October 2003PACS: 61.20.Ja Computer simulation of liquid structure - 64.70.Fx Liquid-vapor transitions     

Residue-specific NH exchange rates studied by NMR diffusion experiments

We present a novel approach to the investigation of rapid (>2s(-1)) NH exchange rates in proteins, based on residue-specific diffusion measurements. (1)H, (15)N-DOSY-HSQC spectra are recorded in order to observe resolved amide proton signals for most residues of the protein. Human ubiquitin was used to demonstrate the proposed method. Exchange rates are derived directly from the decay data of the diffusion experiment by applying a model deduced from the assumption of a two-site exchange with water and the "pure" diffusion coefficients of water and protein. The "pure" diffusion coefficient of the protein is determined in an experiment with selective excitation of the amide protons in order to suppress the influence of magnetization transfer from water to amide protons on the decay data. For rapidly exchanging residues a comparison of our results with the exchange rates obtained in a MEXICO experiment showed good agreement. Molecular dynamics (MD) and quantum mechanical calculations were performed to find molecular parameters correlating with the exchangeability of the NH protons. The RMS fluctuations of the amide protons, obtained from the MD simulations, together with the NH coupling constants provide a bilinear model which shows a good correlation with the experimental NH exchange rates.     

Interdiffusion and thermotransport in Ni–Al liquid alloys

In this paper, we present extensive self-consistent results of molecular dynamics (MD) simulations of diffusion and thermotransport properties of Ni–Al liquid alloys. We develop a new formalism that allows easy connection between results of the MD simulations and the real experiments. In addition, this formalism can be extended to the case of ternary and higher component liquid alloys. We focus on the temperature and composition dependence of the self-diffusion coefficients, interdiffusion coefficients, thermodynamic factor, Manning factor and the reduced heat of transport. The two latter quantities both represent measures of the off-diagonal Onsager phenomenological coefficients. The Manning factor and the reduced heat of transport can be related to experimentally obtainable quantities provided the thermodynamic factor is available. The simulation results for the reduced heat of transport show that for all compositions, in the presence of a temperature gradient, Ni tends to migrate to the cold end. This is in agreement with an available experimental study for a Ni_21.5Al_78.5 melt (only qualitative result is available so far).     

Application of electron spin echo ESR-tomography to study radical diffusion in porous media

ESR-tomography based on electron spin echo was used to study the translational diffusion of stable radical 2,2,6,6-tetramethyl-4-pipyridinoxyl in the solutions filling the pore space of silica gel. The values of the efficient diffusion coefficients were measured for the solution of radicals 2,2,6,6-tetramethyl-4-pipyridinoxyl in methanol and 2-methyl-tetrahydrofuran, depending on the mean size of the silica gel pore. It was shown that the value of the diffusion coefficient decreased as pore size decreased, and furthermore depended on the solvent type. A linear relation between the diffusion coefficient and the spin exchange constant was established within the scope of the quasihomogeneous diffusion model.     

Water in porous glasses. A computer simulation study

We report molecular dynamics simulations of water confined in a cylindrical silica pore. The pore geometry and size is similar to that of typical pores in porous Vycor glass. In the present study we focus on the dependence of microscopical structural and dynamical properties on the degree of hydration of the pore. We have performed five simulations of systems between 19 and 96 % hydration. In all cases, water adsorbs strongly on the pore surface, clearly demonstrating the hydrophilic nature of the Vycor surface. Two layers of water molecules are affected strongly by the interactions with the glass surface. With decreasing degree of hydration an increasing volume in the center of the pore is devoid of water molecules. At 96 % hydration the center is a continuous and homogeneous region that has, however, a lower density than bulk water at ambient conditions. A well-pronounced mobility profile exists, where molecules in the center of the pores have substantially higher self diffusion coefficients than molecules on the pore surface. The spectral densities of center of mass and hydrogen atom motion show the signature of confinement for the molecules close to the pore surface, while the spectral densities in the center of the pore are similar to those in bulk water. The molecular dynamics results are in good agreement with recent experiments. Our data indicate that the dependence of experimental data on the level of hydration of the Vycor sample is due to the different relative contribution of molecules adsorbed on the pore surface and bulk-like molecules in the interior of the pore to the experimental averages.     

Predicted diffusion rates on fcc (001) metal surfaces for adsorbate/substrate combinations of Ni, Cu, Rh, Pd, Ag, Pt, Au

We investigate the diffusion of a single metal atom on the surface of a fcc (001) metal. Two points concerning the application of kinetic models to diffusion were considered. First, we test the assumption of kinetic models that diffusion occurs via a sequence of uncorrelated jumps. Second, when kinetic models are applicable we predict reasonable values of the kinetic rate constants.

Direct molecular dynamics (MD) simulations were performed for Ag on Ag(001) and Rh on Rh(001) systems. Diffusion was found to obey an Arrhenius-type dependence on temperature in both systems. The barriers and prefactors extracted from the MD results agree with estimates made from transition state theory (TST) and the experimental values for the Rh system. We conclude that kinetic models are applicable to diffusion on fcc (001) surfaces.

Transition state theory was then used to estimate diffusion parameters for all other adsorbate/ substrate combinations of the metals Ni, Cu, Rh, Pd, Ag, Pt, and Au. These results indicate that the characteristics of diffusion are primarily a property of the adsorbate. We also predict Ag atoms to have an anomalously low diffusion barrier on all of the substrates in this study. We use the accurate many-body density functional based MD/MC-CEM potential energy surface which allows us to consistently treat these multi-component systems.     

Peculiar diffusion behavior of AlCl_4 intercalated in graphite from nanosecond-long molecular dynamics simulations

The diffusion property of the intercalated species in the graphite materials is at the heart of the rate performance of graphite-based metal-ion secondary battery. Here we study the diffusion process of a AlCl_4 molecule within graphite — a key component of a recently reported aluminum ion battery with excellent performance — via molecular dynamics(MD) simulations. Both ab-initio MD(AIMD) and semiempirical tight-binding MD simulations show that the diffusion process of the intercalated AlCl_4 molecule becomes rather inhomogeneous, when the simulation time exceeds approximately 100 picoseconds. Specifically, during its migration in between graphene layers, the intercalated AlCl_4 molecule may become stagnant occasionally, and then recovers its normal(fast) diffusion behavior after halting for a while. When this phenomenon occurs, the linear relationship of the mean squared displacement(MSD) versus the duration time is not fulfilled. We interpret this peculiar behavior as a manifestation of inadequate sampling of rare event(the stagnation of the molecule), which does not yet appear in short-time MD simulations. We further check the influence of strains present in graphite intercalated compounds(GIC) on the diffusion properties of AlCl_4, and find that their presence in general slows down the diffusion of the intercalated molecule, and is detrimental to the rate performance of the GIC-based battery.     

Self-diffusion coefficients and orientational correlation times in aqueous NaCl solutions: Complementarity with structural investigations

The dynamical properties of pure water and aqueous NaCl solutions over a wide range of salt concentrations (0-6 m) at ambient conditions are characterized by molecular dynamics (MD) simulations. MD simulations are performed with a flexible SPC water model as a solvent, while the ions are treated as charged Lennard-Jones particles. In this paper, attention has been focused on the self-diffusion coefficients (D_i) of ions and water molecules and on orientational correlation time of water molecules. It is found that the self-diffusion coefficients decrease with ion concentration. Moreover, the self diffusion coefficients of sodium and chloride at higher salt concentrations are very comparable which may be due to the formation of clusters of these ions. The deduced rotational dynamics speeds up as the salt concentration increases. Some complementarities between dynamical properties and structural ones, recently obtained, are carried out.     

页岩气中CH4-C2H6吸附-扩散分子动力学研究

本文采用巨正则蒙特卡洛(GCMC)和分子动力学(MD)模拟方法,对比分析了不同温度、压力和孔径对二元气体(CH4-C2H6)在K-伊利石中的吸附-扩散的影响.结果表明,在低压条件下,K-伊利石对C2H6的吸附能力大于CH4, C2H6优先吸附在K-伊利石孔隙表面.热力学因子随着孔径的增加而减小,C2H6的热力学因子大于...     

Effect of drying technique on the physicochemical properties of sodium silicate-based mesoporous precipitated silica

The conventional drying (oven drying) method used for the preparation of precipitated mesoporous silica with low surface area (>300 m²/g) and small pore volume is often associated with a high production cost and a time consuming process. Therefore, the main goal of this study was to develop a cost-effective and fast drying process for the production of precipitated mesoporous silica using inexpensive industrial grade sodium silicate and spray drying of the precipitated wet-gel silica slurry. The precipitated wet-gel silica slurry was prepared from an aqueous sodium silicate solution through the drop-wise addition of sulfuric acid. Mesoporous precipitated silica powder was prepared by drying the wet-gel slurry with different drying techniques. The effects of the oven drying (OD), microwave drying (MD), and spray drying (SD) techniques on the physical (oil, water absorption, and tapping density), and textural properties (specific BET surface area, pore volume, pore size, and % porosity) of the precipitated mesoporous silica powder were studied. The dried precipitated mesoporous silica powders were characterized with field-emission scanning electron microscopy; Brunauer, Emmett and Teller and BJH nitrogen gas adsorption/desorption methods; Fourier-transform infrared spectroscopy; thermogravimetric and differential analysis; N₂ physisorption isotherm; pore size distribution and particle size analysis. There was a significant effect of drying technique on the textural properties, such as specific surface area, pore size distribution and cumulative pore volume of the mesoporous silica powder. Additionally, the effect of the microwave-drying period on the physicochemical properties of the precipitated mesoporous silica powder was investigated and discussed.     

Zero modes of various graphene configurations from the index theorem

The diffusion of carbon dioxide in both NaX and NaY Faujasite systems is investigated by combining Quasi-Elastic Neutron Scattering (QENS) and Molecular Dynamics (MD) simulations. The transport diffusivity evaluated experimentally increases with the loading whereas the simulated self diffusivity decreases. This general behaviour is in good agreement with those previously reported in the literature for different gases in similar zeolites systems. It was also shown that the corrected diffusivity exhibits a significant concentration dependence. At low loading, the activation energies for diffusion derived from QENS and MD simulations are in agreement. They increase from NaY to NaX due to a stronger interaction between the CO₂ molecules and the extra-framework cations. The extrapolation of the transport and self diffusivities to zero coverage allowed us to emphasize a good agreement between experiment and simulation.     

一种描述金属界面原子扩散的加速分子动力学方法

传统分子动力学(MD)的纳秒级时间尺度限制了对固体界面原子的深层扩散、渗透以及相形成等长时动力学性质的模拟研究.在Voter的超动力学框架内,提出了一种更为简单的偏移势的构建方法.该方法通过在偏移势中引入一个加速因子,抬高了原势阱,从而加速了原子的跃迁,将MD模拟的时间尺度提高了若干个数量级.更为重要的是,该方法不需要预知体系势能的势阱及鞍点分布,还能够将原势能曲面的特性完全保留.以Mg/Zn界面扩散为例,选取简单的Lennard-Jones双体势,考察了不同加速因子对界面原子扩散速度的影响.结果显示,该 关键词： 超动力学 加速因子 原子扩散 金属界面     

Molecular-Dynamics Simulation of Grain-Boundary Diffusion Creep

Molecular-dynamics (MD) simulations are used, for the first time, to study grain-boundary diffusion creep of a model polycrystalline silicon microstructure. Our fully dense model microstructures, with a grain size of up to 7.5 nm, were grown by MD simulations of a melt into which small, randomly oriented crystalline seeds were inserted. In order to prevent grain growth and thus to enable steady-state diffusion creep to be observed on a time scale accessible to MD simulations (of typically 10^-9s), our input microstructures were tailored to (i) have a uniform grain shape and a uniform grain size of nm dimensions and (ii) contain only high-energy grain boundaries which are known to exhibit rather fast, liquid-like self-diffusion. Our simulations reveal that under relatively high tensile stresses these microstructures, indeed, exhibit steady-state diffusion creep that is homogenous (i.e., involving no grain sliding), with a strain rate that agrees quantitatively with that given by the Coble-creep formula.     

Computer simulation investigation of diffusion selectivity in graphite slit pores

Equilibrium molecular dynamics simulations are reported of oxygen and nitrogen molecules confined in graphite slit pores. Self- and collective diffusion coefficients have been calculated as a function of pore width, temperature and density for each pure component in the pore space. The aim of this study was to elucidate the mechanism by which oxygen and nitrogen are kinetically separated when air is passed over an adsorbent bed consisting of molecular sieving carbon in the commercial production of oxygen. It was found that a critical pore width exists for each species at which there is a sharp drop in the rate of diffusion (both self- and collective diffusion) of each fluid. The critical pore width is one for which the individual molecules are prevented from rotating freely about one of their axes. The greater length of a nitrogen molecule means that the critical pore width is higher for this species than for oxygen. Consequently, oxygen molecules diffuse substantially faster than nitrogen molecules in the vicinity of the nitrogen critical pore width. From an analysis of correlation functions and their corresponding power spectra it is shown that the restricted rotations, which occur at or below the critical pore width, cause a decoupling of translational and rotational modes, with the net result being a lowering of translational diffusion. The nitrogen critical pore width lies within the range of the mean pore size of most commercial molecular sieving carbons, and so this mechanism may help to explain the high oxygen selectivities reported in the literature.     

Supercooled confined water and the mode coupling crossover temperature

We present a molecular dynamics study of the single-particle dynamics of supercooled water confined in a silica pore. Two dynamical regimes are found. Close to the hydrophilic substrate molecules are below the mode coupling crossover temperature, T(C), already at ambient temperature. The water closer to the center of the pore (free water) approaches upon supercooling T(C) as predicted by mode coupling theories. For free water the crossover temperature and crossover exponent gamma are extracted from power-law fits to both the diffusion coefficient and the relaxation time of the late alpha region.     

Molecular dynamic simulations and global equation of state of square-well fluids with the well-widths from λ = 1.1 to 2.1

We present the results of extensive new molecular dynamic (MD) simulations in the one-phase region for square well fluids with well widths λ?=?1.10, 1.15, 1.20, 1.25, 1.375, 1.50, 1.75, 1.90, 2.0, and 2.10. These data have been used in developing a crossover equation of state (CR EOS) for square-well fluids with well widths 1.1?≤?λ?≤?2.1. The CR EOS incorporates non-analytic scaling laws in the critical region, and in the limit of low densities yields the exact second and third virial coefficients. Also in the high-temperature region, it reproduces first-order perturbation theory results. The CR EOS was tested against our new MD simulations, and earlier MD and Monte-Carlo (MC) simulations reported by other authors as well. Excellent agreement between calculated values and simulation data for all SW fluids is observed. In combination with the density-functional theory, the CR EOS is also capable of reproducing surface tension simulations with high accuracy. Application of the CR EOS for solid–liquid equilibrium calculations in combination with the Lennard–Jones and Devonshire cell model for the solid phase, is also discussed.     

Test of the Gouy-Chapman theory for a charged lipid membrane against explicit-solvent molecular dynamics simulations

A wealth of experimental data has verified the applicability of the Gouy-Chapman (GC) theory to charged lipid membranes. Surprisingly, a validation of GC by molecular dynamics (MD) simulations has been elusive. Here, we report a test of GC against extensive MD simulations of an anionic lipid bilayer solvated by water at different concentrations of NaCl or KCl. We demonstrate that the ion distributions from the simulations agree remarkably well with GC predictions when information on the adsorption of counterions to the bilayer is incorporated.     

Simulation study of polymorphism and diffusion anomaly for SiO2 and GeO2 liquid

We report molecular dynamic (MD) simulations of a silica and germania liquid. We show for the first time that the liquid models constructed of the BKS and Ocffner-Elliott potential are composed of D₄, D₅ and D₆ domains. These domains of different liquids (identical chemical composition but different density) are structurally identical. The liquid densification occurs gradually with variation in the volume of the D₄, D₅ and D₆ domains. The void radius distributions (VRD) have been computed and they are well fitted to a Gaussian form. Both the average void radii and width of the VRD decrease with pressure. The diffusivity of Si/Ge or O depends linearly on the fraction of TO_x (x = 4, 5 and 6; T is Ge or Si). The diffusion anomaly results from the very high mobility of silicon in the D₅ domain compared to domains D₄ and D₆.     

NMR study of the vapor phase contribution to diffusion in partially filled silica glasses with nanometer and micrometer pores

The contribution of the vapor phase to molecular diffusion in porous silica glasses with nanometer (Vycor) and micrometer (VitraPor#5) pores partially filled with water (polar) or cyclohexane (nonpolar) was investigated with the aid of field-gradient NMR diffusometry. Due to the vapor phase, the effective diffusion coefficient of cyclohexane filling micrometer pores (VitraPor#5) increased up to 10 times relative to the value in bulk liquid upon reduction of the pore space filling factor. On the other hand, the effective diffusion coefficient of water first decreases and then increases when the liquid content is reduced. The dependence of the effective diffusion coefficient on the pore filling factor is strongly related to the pore dimension. A general two-phase exchange model is presented that is well accounting for all experimental diffusion features.