Intermolecular Potential Model Research Articles

Intermolecular Potential Model Hamiltonians for Gas–Liquid Coexistence

Intermolecular Potential Model Hamiltonians for Gas–Liquid Coexistence

Transferable Ion Force Fields in Water from a Simultaneous Optimization of Ion Solvation and Ion-Ion Interaction.

The poor performance of many existing nonpolarizable ion force fields is typically blamed on either the lack of explicit polarizability, the absence of charge transfer, or the use of unreduced Coulomb interactions. However, this analysis disregards the large and mostly unexplored parameter range offered by the Lennard-Jones potential. We use a global optimization procedure to develop water-model-transferable force fields for the ions K+, Na+, Cl–, and Br– in the complete parameter space of all Lennard-Jones interactions using standard mixing rules. No extra-thermodynamic assumption is necessary for the simultaneous optimization of the four ion pairs. After an optimization with respect to the experimental solvation free energy and activity, the force fields reproduce the concentration-dependent density, ionic conductivity, and dielectric constant with high accuracy. The force field is fully transferable between simple point charge/extended and transferable intermolecular potential water models. Our results show that a thermodynamically consistent force field for these ions needs only Lennard-Jones and standard Coulomb interactions.

Wetting/non-wetting behaviour of quadrupolar molecules (N2, C2H4, CO2) on planar substrates

Molecular simulations were carried out to evaluate the performance of intermolecular potential models for the adsorbates: nitrogen, ethylene and carbon dioxide with leading quadrupoles. Models having only LJ interactions, are compared with the corresponding multi-site models with partial charges for their performance in describing the experimental data for vapour-liquid equilibrium (VLE), adsorption isotherms and isosteric heat of adsorption on planar adsorbent substrates. It is found that the models with partial charges outperform models using only LJ atoms for the description of adsorption, highlighting the important role of the quadrupole moment in a non-uniform adsorbed phase. These models were then used to construct the wetting maps of nitrogen, ethylene and carbon dioxide as a function of the temperature and the strength of the planar substrate to show the influence of the quadrupole moment on the behaviour of non-wetting/partial wetting/complete wetting at the coexistence pressure of the bulk phase.

Prediction of Surface Residual Stress on Titanium Alloy generated by Belt Grinding using Molecular System Dynamics

Abstract With the reason of its high strength, good corrosion resistance and high heat resistance, titanium alloy has been widely used in aeronautical field, especially in aero-engine design. Because of the characteristics of cold state and flexibility, belt grinding is one of the effective titanium alloy finishing. However, the variation of residual stress with grinding parameters in the belt grinding process are not clear, which greatly limits the further application of this method. In this paper, based on the knowledge of molecular system dynamics, a molecular system dynamics model of residual stress on the surface of abrasive belt grinding of titanium alloy is established. Based on the corresponding intermolecular potential function of titanium alloy material, a linear regression equation is established, and the expression of residual stress at molecular level on the surface of belt ground titanium alloy was obtained. The model is simulated with MATLAB, and the results are in good agreement with the intermolecular potential model. Through relevant experiments, the experimental results are compared with the simulation results of titanium alloy molecular system, the error is analyzed and the conclusion is given. When the grinding linear velocity is 8-24 m/s and the feed velocity is 20-60 m/ min, the residual stress of TC17 alloy belt grinding is - 169 to – 254 MPa, and the model prediction error is within 20%.

A simulation study of the low temperature phase diagram of the methane monolayer on graphite: a test of potential energy functions.

We have used molecular simulation with two intermolecular potential models, TraPPE-UA and TraPPE-EH, the latter of which accounts for the tetrahedral shape, to study the effects of shape on methane adsorption on graphite. Both models give good descriptions of the vapour-liquid equilibria in the bulk phase, but adsorption on graphite is better described by the TraPPE-EH model. Molecular configurations in the monolayer, show the variation with temperature of the registry sites for the carbon and hydrogen atoms of the methane molecules. At temperatures below 70 K, the centre of mass (COM) of the molecules is in registry with the centre of the carbon hexagons. For temperatures above 70 K, a commensurate monolayer is initially formed as at low temperatures, then as the loading is increased the first layer remains in registry, but the COM of the methane molecules in the first layer shifts to the top of the graphite carbon atoms with the C-H bond pointing to carbon atoms in the second shell of a C-hexagon. At temperatures above 93 K, the first adsorbate layer goes through these two commensurate states and then undergoes a transition to an incommensurate solid. Finally, for temperatures greater than 110 K methane behaves like a pseudo spherical molecule.

Behaviour of rarified steam at very high temperature an orientation-averaged interaction potential approach towards its accurate description

We propose an orientation-averaged intermolecular potential model for the accurate computation of relevant thermodynamic and transport properties of steam at conditions typically encountered in extreme environments such as rocky exoplanetary atmospheres, aircraft and rocket engines, high temperature steam reforming and electrochemical reactors, as well as advanced ultra-supercritical steam power generators. We assess the reliability of the potential model to describe accurately the temperature dependence of relevant thermodynamic properties of rarified steam up to and beyond 3000K. Moreover, we discuss some fundamental issues including (i) the conditions at which an orientation-averaged intermolecular potential for a molecular fluid becomes an accurate representation of the fluid behaviour in extreme environments, (ii) the temperature range of validity of the approximations underlying an orientation averaging approach and, (iii) the methodology of force-field parameterisation to obtain an optimised representation, as well as the concomitant modelling of the thermodynamics and transport properties of rarified fluids. Finally, we illustrate how the simultaneous accurate description of the temperature dependence of the fluid’s collision integrals and second virial coefficient is crucial to the successful modelling of rarified steam in extreme environments.

Charge Distributions of Nitro Groups Within Organic Explosive Crystals: Effects on Sensitivity and Modeling

The charge distribution of NO2 groups within the crystalline polymorphs of energetic materials strongly affects their explosive properties. We use the recently introduced basis-space iterated stockholder atom partitioning of high-quality charge distributions to examine the approximations that can be made in modeling polymorphs and their physical properties, using 1,3,5-trinitroperhydro-1,3,5-triazine, trinitrotoluene, 1-3-5-trinitrobenzene, and hexanitrobenzene as exemplars. The NO2 charge distribution is strongly affected by the neighboring atoms, the rest of the molecules, and also significantly by the NO2 torsion angle within the possible variations found in observed crystal structures. Thus, the proposed correlations between the molecular electrostatic properties, such as trigger-bond potential or maxima in the electrostatic potential, and impact sensitivity will be affected by the changes in conformation that occur on crystallization. We establish the relationship between the NO2 torsion angle and the likelihood of occurrence in observed crystal structures, the conformational energy, and the charge and dipole magnitude on each atom, and how this varies with the neighboring groups. We examine the effect of analytically rotating the atomic multipole moments to model changes in torsion angle and establish that this is a viable approach for crystal structures but is not accurate enough to model the relative lattice energies. This establishes the basis of transferability of the NO2 charge distribution for realistic nonempirical model intermolecular potentials for simulating energetic materials.

Estimating Al2O3–CO2 nanofluid viscosity: a molecular dynamics approach

High-viscosity CO2 is of interest to the oil and gas industry in enhanced oil recovery and well-fracturing applications. Dispersing nanoparticles in CO2 is one way of achieving increased viscosity. However, parametric studies on viscosity estimation of CO2 nanofluids is not found in the open literature. A comparison of various interatomic potentials for their accuracy in predicting viscosity is also missing. In this work, we studied Al2O3 nanoparticles in CO2 base fluid. We screened the inter-molecular interaction potential models available for CO2–CO2 interactions and found that the TraPPE-flexible model (with MORSE potential) to be most suitable for conditions used in this work. We estimated the CO2–Al2O3 interaction potential using quantum mechanical simulations. Using this combination for CO2–CO2 and CO2–Al2O3 interactions, we explored the effects of temperature and nanoparticle size on viscosity using molecular dynamics simulations (MD). We predicted that the viscosity would increase with increase in temperature and particle size. We also calculated the base fluid self-diffusion coefficient to investigate the effect of Brownian motion and its contribution to changes in viscosity. We found that it decreases with increase in particle size and temperature, thereby indicating that Brownian motion does not contribute to the increased viscosity. Further, the nanolayer formed at the Al2O3–CO2 interface is studied through density distributions around the nanoparticle; the thickness of this nanolayer is found to increase with nanoparticle diameter. Finally, we examined the structures of CO2 fluid in presence of nanoparticles at different thermodynamic states through radial distribution functions. The current work sheds light on the viscosity enhancement by the addition of nanoparticles; it is hoped that such studies will lead to tools that help tailor fluid properties to specific requirements.

English

A new formula for mathematics is derived which gives the upper half of the series expansion of the expression (ay+b)2n, where n, is a natural number. A proof of the new formula is given followed by a simple example to test its accuracy. This formula is helpful whenever n is large. Key words: Series Expansion, fluid models, Intermolecular Potential model

Electron-Hole Theory of the Effect of Quantum Nuclei on the X-Ray Absorption Spectra of Liquid Water.

Electron-hole excitation theory is used to unveil the role of nuclear quantum effects on the x-ray absorption spectral signatures of water, whose structure is computed via path-integral molecular dynamics with the MB-pol intermolecular potential model. Compared to spectra generated from the classically modeled water, quantum nuclei introduce important effects on the spectra in terms of both the energies and line shapes. Fluctuations due to delocalized protons influence the short-range ordering of the hydrogen bond network via changes in the intramolecular covalence, which broaden the preedge spectra. For intermediate-range and long-range ordering, quantum nuclei approach the neighboring oxygen atoms more closely than classical protons, promoting an "icelike" spectral feature with the intensities shifted from the main edge to the postedge. Computed spectra are in nearly quantitative agreement with the available experimental data.

Pressure broadening coefficients in the ν6 band of CH3F in collision with He

In this study we report the first measurements of the pressure line broadening coefficients of 219 lines of CH3F in the PP, RP, RQ, PQ, and RR sub-branches of the ν6 band (8.5 µm-spectral region) at room temperature using a high-resolution Fourier transform spectrometer. The collisional line widths are obtained from the non-linear least-square multi-pressure fitting techniques. A Voigt line shape model was used in the present analysis. Measured values are compared with the results of theoretical calculation based on semi-classical formalism of Robert-Bonamy (RB) model with exact isotropic trajectories. Even with a Smith-Giraud-Cooper intermolecular potential model the calculated values show a quite good agreement with experimental results.

The validity of the potential model in predicting the structural, dynamical, thermodynamic properties of the unary and binary mixture of water-alcohol: Methanol-water case

Two intermolecular potential models of methanol (TraPPE-UA and OPLS-AA) have been used in order to examine their validity in reproducing the selected structural, dynamical, and thermodynamic properties in the unary and binary systems. These two models are combined with two water models (SPC/E and TIP4P). The temperature dependence of density, surface tension, diffusion and structural properties for the unary system has been computed over specific range of temperatures (200-300K). The very good performance of the TraPPE-UA potential model in predicting surface tension, diffusion, structure, and density of the unary system led us to examine its accuracy and performance in its aqueous solution. In the binary system the same properties were examined, using different mole fractions of methanol. The TraPPE-UA model combined with TIP4P-water shows a very good agreement with the experimental results for density and surface tension properties; whereas the OPLS-AA combined with SPCE-water shows a very agreement with experimental results regarding the diffusion coefficients. Two different approaches have been used in calculating the diffusion coefficient in the mixture, namely the Einstein equation (EE) and Green-Kubo (GK) method. Our results show the advantageous of applying GK over EE in reproducing the experimental results and in saving computer time.

Thermophysical properties of binary mixtures containing oxygen and noble gases

This work deals with the determination of reliable transport and equilibrium thermophysical properties of binary mixtures of O2 and the noble gases Ar, Kr, and Xe. The second virial coefficients B(T), the viscosities η(T), and the binary diffusion coefficients D12(T) were calculated for each mixture in the temperature range between 200 K and 1000 K using a a temperature dependent (n-6) Lennard-Jones intermolecular interaction potential model. The properties of the mixtures were obtained by using the Lorentz-Berthelot and Hohm-Zarkova-Damyanova mixing rules. Our results were compared to other theoretical and experimental data and acceptable agreement was found in all cases. Fitting formulae for the calculation of the intermolecular potential parameters and equilibrium and transport properties of the examined mixtures are recommended proposed.

Improving wettability and preventing Li-ion batteries from thermal runaway using microchannels

Effects of using microchannels on the wettability of the porous electrodes and preventing the battery cell from thermal runaway have been studied. Two-dimensional Lattice Boltzmann Method (LBM) simulation was carried out to simulate the effects of embedded microchannels inside the electrodes on the electrolyte transport in the porous electrodes as well as their effects to lead the generated gases during thermal runaway out of the battery cell. The two-phase intermolecular potential model was utilized to investigate the microscopic behavior of the electrolyte flow in the porous electrodes. The results showed that embedding microchannels inside the electrodes significantly improves the wettability of the electrodes, especially for the electrodes with lower porosities. Furthermore, the use of microchannels inside the electrodes could considerably reduce possibility of occurrence of the thermal runaway. During thermal runaway, the electrodes with higher number of smaller microchannels could drive the generated gases out of the battery cell much quicker than the electrodes with lower number of larger microchannels.

Selective Removal of Hydrogen Sulphide from Industrial Gas Mixtures Using Zeolite NaA

Abstract Hydrogen sulphide removal from simple gas mixtures using a highly polar zeolite was studied by molecular simulation. The equilibrium adsorption properties of hydrogen sulphide, hydrogen, methane and their mixtures on dehydrated zeolite NaA were computed by Grand Canonical Monte Carlo simulations. Existing all-atom intermolecular potential models were optimized to reproduce the adsorption isotherms of the pure substances. The adsorption results of the mixture, also confirmed by IAST calculations, showed very high selectivities of hydrogen sulphide to the investigated non-polar gases, predicting an outstanding performance of zeolite NaA in technological applications that target hydrogen sulphide capture.

Determination of structure factor and effective intermolecular parameters of liquid alkali metals using mean spherical approximation theory

The structural properties of liquid cesium (Cs) were investigated in the temperature range of 400–2000 K by application of the effective hard-core attractive Yukawa (H-Y) potential function to the mean spherical approximation (MSA) theory. An analytical equation of state was applied to calculate the molecular parameters of a potential function at any thermodynamic state. The proposed approach was used to predict the behavior of the structure factor, S(k), at a wide range of wave vectors (k). The results obtained were compared with the available experimental measurements, which revealed a good agreement over the whole liquid range for Cs. The structure factor at a long wavelength limit, S(0), was also calculated and compared with the experimental data, which showed the accuracy of the Ornstein-Zernike (OZ) behavior of S(k) at the thermodynamic states below the boiling temperature (Tb). For the case of Cs, the presented intermolecular potential model was very much responsive to the thermodynamic states and led to the indication of the two distinct isotherm ranges T <Tb and T >Tb. Two analytical expressions were obtained for the temperature dependency of the potential well-depth (Keff) over the temperature ranges of T <Tb and T >Tb. Furthermore, the proposed approach showed several metal-nonmetal transitions for Cs at about 500, 1100, 1350, and 1650 K. Using the Keff relations for the other liquid alkali metals Na, K, and Rb provided the S(k) results that were more accurate in comparison with the experimental data.

Membrane separation study for methane-hydrogen gas mixtures by molecular simulations

Direct simulation results for stationary gas transport through pure silica zeolite membranes (MFI, LTA and DDR types) are presented using a hybrid, non-equilibrium molecular dynamics simulation methodology introduced recently. The intermolecular potential models for the investigated CH$_{4}$ and H$_{2}$ gases were taken from literature. For different zeolites, the same atomic (Si and O) interaction parameters were used, and the membranes were constructed according to their real (MFI, LTA, or DDR) crystal structures. A realistic nature of the applied potential parameters was tested by performing equilibrium adsorption simulations and by comparing the calculated results with the data of experimental adsorption isotherms. The results of transport simulations carried out at 25$^0$C and 125$^0$C, and at 2.5, 5 or 10 bar clearly show that the permeation selectivities of CH$_{4}$ are higher than the corresponding permeability ratios of pure components, and significantly differ from the equilibrium selectivities in mixture adsorptions. We experienced a transport selectivity in favor of CH$_{4}$ in only one case. A large discrepancy between different types of selectivity data can be attributed to dissimilar mobilities of the components in a membrane, their dependence on the loading of a membrane, and the unlike adsorption preferences of the gas molecules.

Hydrogen-Bonding Polarizable Intermolecular Potential Model for Water.

A polarizable intermolecular potential model with short-range directional hydrogen-bonding interactions was developed for water. The model has a rigid geometry, with bond lengths and angles set to experimental gas-phase values. Dispersion interactions are represented by the Buckingham potential assigned to the oxygen atom, whereas electrostatic interactions are modeled by Gaussian charges. Polarization is handled by a Drude oscillator site, using a negative Gaussian charge attached to the oxygen atom by a harmonic spring. An explicit hydrogen-bonding term is included in the model to account for the effects of charge transfer. The model parameters were optimized to density, configurational energy, pair correlation function, and the dielectric constant of water under ambient conditions, as well as the minimum gas-phase dimer energy. Molecular dynamics and Gibbs ensemble Monte Carlo simulations were performed to evaluate the new model with respect to the thermodynamic and transport properties over a wide range of temperature and pressure conditions. Good agreement between model predictions and experimental data was found for most of the properties studied. The new model yields better performance relative to the majority of existing models and outperforms the BK3 model, which is one of the best polarizable models, for vapor-liquid equilibrium properties, whereas the new model is not better than the BK3 model for representation of other properties. The model can be efficiently simulated with the thermalized Drude oscillator algorithm, resulting in computational costs only 3 times higher than those of the nonpolarizable TIP4P/2005 model, whereas having significantly improved properties. Because it involves only a single Drude oscillator site, the new model is significantly faster than polarizable models with multiple sites. With the explicit inclusion of hydrogen-bond interactions, the model may provide a better description of the phase behavior of aqueous mixtures.

Development of intermolecular potential models for electrolyte solutions using an electrolyte SAFT-VR Mie equation of state

ABSTRACTWe present a theoretical framework and parameterisation of intermolecular potentials for aqueous electrolyte solutions using the statistical associating fluid theory based on the Mie interaction potential (SAFT-VR Mie), coupled with the primitive, non-restricted mean-spherical approximation (MSA) for electrolytes. In common with other SAFT approaches, water is modelled as a spherical molecule with four off-centre association sites to represent the hydrogen-bonding interactions; the repulsive and dispersive interactions between the molecular cores are represented with a potential of the Mie (generalised Lennard-Jones) form. The ionic species are modelled as fully dissociated, and each ion is treated as spherical: Coulombic ion–ion interactions are included at the centre of a Mie core; the ion–water interactions are also modelled with a Mie potential without an explicit treatment of ion–dipole interaction. A Born contribution to the Helmholtz free energy of the system is included to account for the process of charging the ions in the aqueous dielectric medium. The parameterisation of the ion potential models is simplified by representing the ion–ion dispersive interaction energies with a modified version of the London theory for the unlike attractions. By combining the Shannon estimates of the size of the ionic species with the Born cavity size reported by Rashin and Honig, the parameterisation of the model is reduced to the determination of a single ion–solvent attractive interaction parameter. The resulting SAFT-VRE Mie parameter sets allow one to accurately reproduce the densities, vapour pressures, and osmotic coefficients for a broad variety of aqueous electrolyte solutions; the activity coefficients of the ions, which are not used in the parameterisation of the models, are also found to be in good agreement with the experimental data. The models are shown to be reliable beyond the molality range considered during parameter estimation. The inclusion of the Born free-energy contribution, together with appropriate estimates for the size of the ionic cavity, allows for accurate predictions of the Gibbs free energy of solvation of the ionic species considered. The solubility limits are also predicted for a number of salts; in cases where reliable reference data are available the predictions are in good agreement with experiment.

Gaussian-Charge Polarizable and Nonpolarizable Models for CO2.

A polarizable intermolecular potential model using three classical Drude oscillators on the atomic sites has been developed for CO2. The model is rigid with bond lengths and molecular geometries set to their experimental values. Electrostatic interactions are represented by three Gaussian charges connected to the molecular frame by harmonic springs. Nonelectrostatic interactions are represented by the Buckingham exponential-6 potential, with potential parameters optimized to vapor-liquid equilibria (VLE) data. A nonpolarizable CO2 model that shares the other ingredients of the polarizable model was also developed and optimized to VLE data. Gibbs ensemble Monte Carlo and molecular dynamics simulations were used to evaluate the two models with respect to a variety of thermodynamic and transport properties, including the enthalpy of vaporization, second virial coefficient, density in the one-phase fluid region, isobaric and isochoric heat capacities, radial distribution functions, self-diffusion coefficient, and shear viscosity. Excellent agreement between model predictions and experimental data was found for all properties studied. The polarizable and nonpolarizable models provide a similar representation of CO2 properties, which indicates that the properties of pure CO2 fluid are not strongly affected by polarization. The polarizable model, which has an order of magnitude higher computational cost than the nonpolarizable model, will likely be useful for the study of a mixture of CO2 and polar components for which polarization is important.