Transferable Potentials For Phase Equilibria Research Articles

Surface Tension of Organophosphorus Compounds: Sarin and its Surrogates

While the production and stockpiling of organophosphorus chemical warfare agents (CWAs), such as sarin, was banned three decades ago, CWAs have remained a threat. New approaches for decontamination and destruction of CWAs require detailed knowledge of their various physicochemical properties. In particular, surface tension is needed to describe the formation and evolution of hazardous aerosols when CWA liquids are dispersed in the air. Due to the extreme toxicity of sarin, most experimental studies are carried out using its surrogates─organophosphorus compounds which, while having similar structures, are much less toxic, e.g., dimethyl methylphosphonate (DMMP) and diisopropyl methylphosphonate (DIMP). However, not only for sarin, but also for its surrogates, literature data on the surface tension are scarce. Here we present experimental measurements and computational predictions of the surface tension of DMMP and DIMP. Classical molecular dynamics simulations using the Transferable Potentials for Phase Equilibria (TraPPE) force field produced an excellent agreement with the experimental results in the temperature range from 3 to 60 °C, validating the predictive capability of TraPPE. Consequently, we applied the TraPPE force field to sarin. Our modeled values for the sarin surface tension cover the range of temperatures from 0 to 85 °C, and the four experimental data points from the literature measured between 20 and 35 °C agree perfectly with our predictions. The temperature-dependent surface tension values for sarin and its surrogates obtained in our study can be used in models predicting the formation and evolution of aerosols made of these chemicals. Furthermore, our results justify the use of the TraPPE force field to derive the thermodynamic properties of other organophosphorus compounds with structures similar to the ones studied here.

Molecular simulation on carbon dioxide capture performance for carbons doped with various elements

Among the different types of CO2 capture technologies for post-combustion, sorption CO2 capture technology with carbon-based sorbents have been extensively explored with the purpose of enhancing their sorption performance by doping hetero elements due to the rapid reaction kinetics and low costs. Herein, sorption capacity and selectivity for CO2 and N2 on carbon-based sorbents doped with elements such as nitrogen, sulfur, phosphorus, and boron, are evaluated and compared using the grand canonical Monte Carlo (GCMC) method, the universal force field (UFF), and transferable potentials for phase equilibria (TraPPE). The sorption capacities of N-doped porous carbons (PCs) at 50 °C were 76.1%, 70.7%, 50.6%, and 35.7% higher than those of pure PCs, S-doped PCs, P-doped PCs, and B-doped PCs, respectively. Its sorption selectivity at 50 °C was approximately 14.0, nearly twice that of pure PCs or other hetero-element-doped PCs. The N-doped PCs showed the largest sorption heat at 50 °C among all the PCs, approximately 20.6 kJ·mol−1, which was 9.7%−25.5% higher than that of the pure PCs under post-combustion conditions. Additionally, with the product purity of 41.7 vol.%−75.9 vol.% for vacuum pressure swing sorption, and 53.4 vol.%−83.6 vol.% for temperature swing sorption, the latter is more suitable for post-combustion conditions than pressure-swing sorption.

Molecular Simulations of Vapor-Liquid Equilibrium of Isocyanates.

The wide range of applications of the isocyanates across multiple industries sparks the interest in the study of their phase behavior. A molecular simulation is a powerful tool that can go beyond experimental investigations relying on a molecular structure of a chemical. The success of a molecular simulation relies on a description of the system, namely, force field, and its parameterization on reproducing properties of interest. In this work, we propose a united-atom force field based on the transferable potentials for phase equilibria (TraPPE) to model the vapor-liquid phase behavior of isocyanates. With Monte Carlo and molecular dynamics simulation methods and the introduced force field, we modeled vapor-liquid equilibrium for a family of linear mono-isocyanates, from methyl isocyanate to hexyl isocyanate, and hexamethylene diisocyanate. Additionally, we performed similar calculations for methyl, ethyl, and butyl isocyanates based on the all-atom GAFF-IC force field available in the literature for modeling isocyanate viscosities. We showed that the developed TraPPE-based force field generally overperformed the GAFF-IC force field and overall showed excellent performance in modeling phase behavior of isocyanates. Based on the simulated vapor pressures for the considered compounds, we estimated the Antoine equation parameters to calculate the vapor pressure in a range of temperatures. The predictions are of particular use in the investigation of thermodynamic properties for those isocyanates lacking experimental vapor pressure data. Results can also be employed in modeling the phase behavior of isocyanate mixtures to investigate their sensing and capturing. Furthermore, from the vapor-liquid equilibrium binodals, we predicted the critical properties of isocyanates which can be used in thermodynamic models based on an equation of state.

Properties of supercritical N2, O2, CO2, and NH3 mixtures from the virial equation of state

AbstractWe report virial coefficients up to sixth order in density for N2, O2, NH3, and CO2, covering temperatures from 50 to 1,000 K. The reported values include coefficients and their first three temperature derivatives, for the pure species as well as all of those needed to evaluate full composition dependence of mixtures formed from any or all of these compounds. The values are obtained by calculation of appropriate cluster integrals using Mayer sampling Monte Carlo, with intermolecular interactions described by the Transferable Potential for Phase Equilibria (TraPPE) force field. All coefficients are fit as a function of temperature, yielding a thermodynamic model with analytic dependence on temperature, density, and composition. The coefficients and properties computed from them are compared to experimental data where available.

Molecular Mechanism for Azeotrope Formation in Ethanol/Benzene Binary Mixtures through Gibbs Ensemble Monte Carlo Simulation.

Azeotropes have been studied for decades due to the challenges they impose on separation processes but fundamental understanding at the molecular level remains limited. Although molecular simulation has demonstrated its capability of predicting mixture vapor-liquid equilibrium (VLE) behaviors, including azeotropes, its potential for mechanistic investigation has not been fully exploited. In this study, we use the united atom transferable potentials for phase equilibria (TraPPE-UA) force field to model the ethanol/benzene mixture, which displays a positive azeotrope. Gibbs ensemble Monte Carlo (GEMC) simulation is performed to predict the VLE phase diagram, including an azeotrope point. The results accurately agree with experimental measurements. We argue that the molecular mechanism of azeotrope formation cannot be fully understood by studying the mixture liquid-state stability at the azeotrope point alone. Rather, azeotrope occurrence is only a reflection of the changing relative volatility between the two components over a much wider composition range. A thermodynamic criterion is thus proposed on the basis of the comparison of partial excess Gibbs energy between the components. In the ethanol/benzene system, molecular energetics shows that with increasing ethanol mole fraction, its volatility initially decreases but later plateaus, while benzene volatility is initially nearly constant and only starts to decrease when its mole fraction is low. Analysis of the mixture liquid structure, including a detailed investigation of ethanol hydrogen-bonding configurations at different composition levels, reveals the underlying molecular mechanism for the changing volatilities responsible for the azeotrope.

Effect of fluorination on the partitioning of alcohols

ABSTRACTIn order to understand the role of fluorination on the interactions and partitioning of alcohols in aqueous and organic environments, isobaric-isothermal ensemble Monte Carlo simulations are used to determine environmental predictors, such as free energies of hydration and solvation in 1-octanol and n-hexadecane. Calculations are performed with the united-atom Transferable Potentials for Phase Equilibria (TraPPE) force field and compared against available experimental data. TraPPE was found to provide reliable qualitative predictions of trends with respect to the effect of fluorination on partitioning. Investigation of the local solvation environment around the hydroxyl group reveals that fluorination of carbons closest to the hydroxyl group has the greatest effect on solvation free energies for alcohols in water, 1-octanol and n-hexadecane.

Fick Diffusion Coefficients of the Gaseous CH4–CO2 System from Molecular Dynamics Simulations Using TraPPE Force Fields at 101.325, 506.625, 1013.25, 2533.12, and 5066.25 kPa

Gaseous Fick diffusion coefficients are calculated for the CH4–CO2 system using molecular dynamics. CH4 was modeled using the united-atom transferable potentials for phase equilibria (TraPPE) force field developed by Martin and Siepmann [J. Phys. Chem B. 1998, 102, 2569], and CO2 was modeled using the rigid TraPPE force field developed by Potoff and Siepmann [AIChE J. 2001 47, 1676]. They can adequately predict the compressibility factor of the CH4–CO2 system over a substantial temperature, pressure, and composition range. However, deviations from experimental values are observed at conditions for which the ideal gas mixture assumption is not valid. It is shown that reasonable calculations of the Fick diffusion coefficient are obtained when compared to experimental data, and a correlation is found in the literature at conditions for which the ideal gas mixture behavior is observed. Calculations are also performed where the mixture does not behave as an ideal gas and are compared to calculations from the aforementioned correlation. The methods yield different results, however the lack of experimental data at these conditions does not allow one to say which are more accurate.

Prediction of phase equilibria and Gibbs free energies of transfer using molecular exchange Monte Carlo in the Gibbs ensemble

The molecular exchange Monte Carlo (MEMC) method is extended to Gibbs ensemble Monte Carlo (GEMC). The utility of the MEMC method is demonstrated through the calculation of the free energies of transfer of n-alkanes from vapor into liquid 1-octanol, n-hexadecane, and 2,2,4-trimethylpentane using isobaric-isothermal GEMC simulations. Calculations are performed for both the Transferable Potentials for Phase Equilibria (TraPPE) and Mie potentials. For all solutes, the Mie potentials predict free energies of transfer that are within 0.1 kcal/mol of experiment for solvation in n-hexadecane and 2,2,4-trimethylpentane. For TraPPE, solutes shorter than n-butane show similar agreement with experiment, but free energies for n-octane in n-hexadencane and 2,2,4-trimethylpentane are over-predicted by approximately 0.25 and 0.5 kcal/mol, respectively.

Improvements and limitations of Mie λ-6 potential for prediction of saturated and compressed liquid viscosity

Over the past decade, the Mie λ-6 (generalized Lennard-Jones) potential has grown in popularity due to its improved accuracy for predicting vapor-liquid coexistence densities and pressure compared to the traditional Lennard-Jones 12-6 potential. This manuscript explores the hypothesis that greater accuracy in characterizing the coexistence properties may lead to greater accuracy for viscosity predictions. Four united-atom force fields are considered in detail: the Transferable Potentials for Phase Equilibria (TraPPE-UA and the recently developed TraPPE-2) model of Siepmann and coworkers, the Transferable Anisotropic Mie (TAMie) model of Gross and coworkers, the fourth generation anisotropic-united-atom (AUA4) model of Ungerer and coworkers, and the model of Potoff and coworkers.Equilibrium molecular dynamics simulations are analyzed using the Green-Kubo method for viscosity characterization. Simulations are performed for linear alkanes with two to twenty-two carbons and branched alkanes with four to eight carbons. Simulation conditions follow the saturated liquid from reduced temperatures of 0.5–0.85 and along the 293 K isotherm in the dense liquid region.In general, the more accurate force fields for coexistence properties do indeed predict viscosity more accurately. For saturated liquids, both Mie-based potential models (Potoff and TAMie) provide roughly 10% accuracy for linear alkanes, while deviations are between 20 and 50% for TraPPE-UA. For branched alkanes, the performance is slightly diminished, but Potoff still provides roughly 15–20% accuracy, while the TAMie force field results in deviations of 20–40%, and TraPPE-UA has deviations of approximately 25–60%. The AUA4 deviations are 10–20% for ethane and 30–60% for 2,2-dimethylpropane, the only compounds tested with the AUA4 force field. The TraPPE-2 deviations for ethane are similar to those using the original TraPPE force field, namely, between 10 and 20%. The percent deviations for each compound and force field tend to increase with decreasing temperature, with the exception of the Potoff deviations for propane, which are nearly constant to the triple point temperature.For compressed liquids, the Mie-based potential models perform better once again than the Lennard-Jones-based force fields, but tend to over estimate the viscosity at very high densities. As the Potoff and TAMie models also tend to over estimate the pressure at high densities, a fortuitous cancellation of errors leads to predictions of viscosity with respect to pressure that are accurate to within about 10%. The comparison with experimental viscosity data is limited to pressures below 200 MPa for most normal and branched alkanes. However, accurate predictions are obtained for propane near 1000 MPa with the Potoff force field.

Prediction of drop size of natural gas condensates using molecular simulation and Young growth model

This paper describes the development and implementation of a molecular simulation model to predict the nucleation process during the condensation of heavy components of the gas natural mixtures of linear alkane (C1 - C6 and C9) at transport conditions (10-40 bar). Specifically, it was used the Monte Carlo method with configurational-bias, the united-atom force field known as “Transferable Potentials for Phase Equilibria (TraPPE-UA)," and the Umbrella sampling technique. The growth of the droplets was evaluated with the model of Young considering numbers of Knudsen below 0.1. The simulation results obtained for the droplet nucleation and growth were compared with experimental data reported in the literature with the aim of validating the implemented models. The simulations predict a droplets size of 2.09 μm which is in good agreement with the experimental results.

Transferable Potentials for Chloroethenes: Insights into Nonideal Solution Behavior of Environmental Contaminants.

Predicting the nonideal phase behavior of binary and multicomponent systems remains a significant challenge for particle-based simulations. Here, we develop a transferable force field for chloroethenes, common environmental contaminants, that can accurately model the vapor liquid phase equilibria including azeotrope formation. The new all-atom force field reproduces saturated liquid densities, saturated vapor pressures, boiling points, and critical properties within 1, 10, 1, and 1% of the experiment data, respectively. Furthermore, the vapor liquid equilibria of trichloroethylene and 1-propanol binary mixture, which forms a minimum boiling point azeotrope, is predicted with a reasonable accuracy. The microstructure of neat and binary systems is explored using pair correlation functions and spatial distribution functions. As the new force field is consistent with transferable potentials for phase equilibria (TraPPE) force field, it expands the applicability of TraPPE force field to chloroethenes.

Surface tension data of n-propane, n-octane and n-dodecane from nucleation simulations

The Classical Nucleation Theory (CNT) has been a dominant model in understanding the self-assembly of new thermodynamic phases. CNT provides significant explanations to processes such as aerosol formation and cloud condensation. In this work, we generated the nucleation free energy profiles of normal alkanes (n-propane, n-octane and n-dodecane) at five different temperatures using the grand-canonical version of the nucleation algorithm. From these free energy profiles, characteristic () values were obtained. Using the density, values from United-Atom Transferable Potentials for Phase Equilibria (TraPPE-UA) force field and the obtained values, we calculated the corresponding surface tension, values of these n-alkane systems at different temperatures. Values obtained are within reasonable agreement with experimental data.

Viscosity of heavy n-alkanes and diffusion of gases therein based on molecular dynamics simulations and empirical correlations

The viscosity of pure n-alkanes and n-alkane mixtures was studied by molecular dynamics (MD) simulations using the Green–Kubo method. n-Alkane molecules were modeled based on the Transferable Potential for Phase Equilibria (TraPPE) united atom force field. MD simulations at constant number of molecules or particles, volume and temperature (NVT) were performed for n-C8 up to n-C96 at different temperatures as well as for binary and six-component n-alkane mixtures which are considered as prototypes for the hydrocarbon wax produced during the Gas-To-Liquid (GTL) Fischer–Tropsch process. For the pure n-alkanes, good agreement between our simulated viscosities and existing experimental data was observed. In the case of the n-alkane mixtures, the composition dependence of viscosity was examined. The simulated viscosity results were compared with literature empirical correlations. Moreover, a new macroscopic empirical correlation for the calculation of self-diffusion coefficients of hydrogen, carbon monoxide, and water in n-alkanes and mixtures of n-alkanes was developed by combining viscosity and self-diffusion coefficient values in n-alkanes. The correlation was compared with the simulation data and an average absolute deviation (AAD) of 11.3% for pure n-alkanes and 14.3% for n-alkane mixtures was obtained.

A Monte Carlo simulation study of the liquid–liquid equilibria for binary dodecane/ethanol and ternary dodecane/ethanol/water mixtures

Obtaining knowledge of the mutual miscibility of alkanes, surfactants, and water over a wide range of temperatures and pressures is of great relevance for the energy industry. Molecular simulations using advanced sampling techniques and transferable force fields can serve as a predictive method to study a variety of phase equilibria at different temperatures, pressures, and overall compositions. In this work, configurational-bias Monte Carlo simulations in the NpT Gibbs ensemble with the Transferable Potentials for Phase Equilibria (TraPPE) force field are applied to investigate the binary mixture of n-dodecane and ethanol at pressures of 0.1 and 100MPa and the ternary mixture of these compounds with water at one state point. The simulations qualitatively reproduce the increase of the upper critical solution temperature (UCST) with increasing pressure, but the miscibility gap is overestimated at both pressures and the UCSTs are shifted up by about 50K compared to the experimental values. The TraPPE model correctly shows a gradual decrease for the ethanol solubility in the n-dodecane-rich phase upon addition of water to the mixture.

The effect of fluorination on the physical properties and the free energies of hydration of 1-alcohols

Because of the widespread use of fluorinated compounds and their subsequent release into the environment, these chemicals have been subjected to a great deal of scrutiny in the past years to determine the extent of their toxicity. In order to better understand the interactions of perfluorinated alcohols and fluorotelomer alcohols in aqueous environments, molecular dynamics simulations are utilized to determine environmental predictors, such as the free energy of hydration. In this work, the predictions of two different force fields, the united-atom Transferable Potentials for Phase Equilibria (TraPPE) and the all-atom Optimized Potentials for Liquid Simulations (OPLS) are compared against available experimental data. Both force fields yield reliable predictions for liquid densities, heats of vaporization and hydration free energies. Investigation of the local solvation environment around the hydroxyl group reveals that fluorination of carbons closest to the hydroxyl group has the greatest effect on the overall hydration free energy.

Atomistic molecular dynamics simulations of H2O diffusivity in liquid and supercritical CO2

Molecular dynamics simulations were employed for the calculation of diffusion coefficients of pure CO2 and of H2O in CO2 over a wide range of temperatures (298.15 K < T < 523.15 K) and pressures (5.0 MPa < P < 100.0 MPa), that are of interest to CO2 capture-and-sequestration processes. Various combinations of existing fixed-point-charge force-fields for H2O (TIP4P/2005 and Exponential-6) and CO2 (elementary physical model 2 [EPM2], transferable potentials for phase equilibria [TraPPE], and Exponential-6) were tested. All force-field combinations qualitatively reproduce the trends of the experimental data for infinitely diluted H2O in CO2; however, TIP4P/2005–EPM2, TIP4P/2005–TraPPE and Exponential-6–Exponential-6 were found to be the most consistent. Additionally, for H2O compositions ranging from infinite dilution to , the Maxwell–Stefan diffusion coefficient is shown to have a weak non-linear composition dependence.

Properties of the intrinsic surface of liquid acetone, as seen from computer simulations

Molecular dynamics simulations of the liquid–vapour interface of acetone have been performed in the canonical (N, V, T) ensemble at 298 K with two different molecular models, belonging to the transferable potential for phase equilibria (TraPPE) and Chemistry at Harvard Molecular Mechanics 27 (CHARMM27) force fields, respectively. The first three molecular layers of the liquid phase beneath its surface have been identified by means of the identification of the truly interfacial molecules (ITIM) method. The results obtained reveal that the vicinity of the vapour phase affects only the first molecular layer of the liquid phase, and its effect vanishes in every respect already in the second molecular layer. The surface acetone molecules exhibit a dual orientational preference. In their dominant orientation, the molecules stay perpendicular to the macroscopic plane of the liquid surface, the C˭O bond lying almost parallel, whilst the line connecting the two CH3 groups staying almost perpendicular to this plane, declining only by about 5° from the perfectly parallel and perpendicular alignments, respectively. This finding is also in a clear accordance with results of some of the former experimental studies of this system. Besides the above orientation, another alignment is also found to be preferred, in particular, at the tips of the crests of the molecularly wavy liquid surface. In this orientation, the acetone molecule lays almost parallel with the surface plane, declining by about 10° from it, pointing the O atom slightly toward the liquid, whilst the two CH3 groups slightly toward the vapour phase.

Prediction of Vapor–Liquid Coexistence Properties and Critical Points of Polychlorinated Biphenyls from Monte Carlo Simulations with the TraPPE–EH Force Field

Gibbs ensemble Monte Carlo simulations using the explicit-hydrogen version of the transferable potentials for phase equilibria (TraPPE–EH) force field were carried out to predict the vapor–liquid coexistence and critical properties of biphenyl, monochlorinated biphenyls, and of 16 polychlorinated biphenyls. The predictions are in very good agreement with the limited experimental data. Transferring the TraPPE–EH Lennard-Jones parameters from benzene to construct biphenyl yields predicted critical properties and normal boiling temperature with an average deviation of less than 1 %. The saturated vapor pressures for biphenyl, 2-chorobiphenyl, and 4-chlorobiphenyl fall within 10 % of the experimental data. Overall, the critical temperatures increase nearly linearly with the number of chlorine substituents and are correlated with the dipole moment for the monochlorinated isomers. In contrast, 4,4′-dichlorobiphenyl, the most elongated compound, exhibits the highest critical temperature among the disubstituted biphenyls.

An online parameter and property database for the TraPPE force field

The transferable potentials for phase equilibria (TraPPE) force field aims to be accurate, computationally efficient and applicable to a wide range of chemical compounds, state points and thermophysical properties. When new users wish to implement TraPPE models into their chosen simulation program, they face several obstacles: the TraPPE models are dispersed over many separate publications and misinterpretations of the primary literature are possible; the TraPPE force field makes specific choices for standard conventions that may require non-trivial code modifications for some simulation software. Therefore, the TraPPE developers report here a resource website and online searchable parameter and property database designed to provide new and experienced users with tools for successful implementation and validation (http://www.chem.umn.edu/groups/siepmann/trappe/).

Adapting SAFT-γ perturbation theory to site-based molecular dynamics simulation. I. Homogeneous fluids

In this work, we aim to develop a version of the Statistical Associating Fluid Theory (SAFT)-γ equation of state (EOS) that is compatible with united-atom force fields, rather than experimental data. We rely on the accuracy of the force fields to provide the relation to experimental data. Although, our objective is a transferable theory of interfacial properties for soft and fused heteronuclear chains, we first clarify the details of the SAFT-γ approach in terms of site-based simulations for homogeneous fluids. We show that a direct comparison of Helmholtz free energy to molecular simulation, in the framework of a third order Weeks-Chandler-Andersen perturbation theory, leads to an EOS that takes force field parameters as input and reproduces simulation results for Vapor-Liquid Equilibria (VLE) calculations. For example, saturated liquid density and vapor pressure of n-alkanes ranging from methane to dodecane deviate from those of the Transferable Potential for Phase Equilibria (TraPPE) force field by about 0.8% and 4%, respectively. Similar agreement between simulation and theory is obtained for critical properties and second virial coefficient. The EOS also reproduces simulation data of mixtures with about 5% deviation in bubble point pressure. Extension to inhomogeneous systems and united-atom site types beyond those used in description of n-alkanes will be addressed in succeeding papers.