Atomistic Simulation of the Absorption of Carbon Dioxide and Water in the Ionic Liquid 1-<i>n</i>-Hexyl-3-methylimidazolium Bis(trifluoromethylsulfonyl)imide ([hmim][Tf₂N]

The solubility of water and carbon dioxide in the ionic liquid 1-n-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([hmim][Tf2N]) is computed using atomistic Monte Carlo simulations. A newly developed biasing algorithm is used to enable complete isotherms to be computed. In addition, a recently developed pairwise damped electrostatic potential calculation procedure is used to speed the calculations. The computed isotherms, Henry's Law constants, and partial molar enthalpies of absorption are all in quantitative agreement with available experimental data. The simulations predict that the excess molar volume of CO2/ionic liquid mixtures is large and negative. Analysis of ionic liquid conformations shows that the CO2 does not perturb the underlying liquid structure until very high CO2 concentrations are reached. At the highest CO2 concentrations, the alkyl chain on the cation stretches out slightly, and the distance between cation and anion centers of mass increases by about 1 Å. Water/ionic liquid mixtures have excess molar volumes that are also negative but much smaller in magnitude than those for the case of CO2

Water-In-Salt LiTFSI Aqueous Electrolytes (2): Transport Properties and Li⁺ Dynamics Based on Molecular Dynamics Simulations

The transport properties of water-in-salt lithium bis­(trifluoromethane sulfonyl)­imide (LiTFSI) aqueous electrolytes were studied using classical molecular dynamics (MD) simulations. At high salt concentrations of 20 m, the calculated viscosity, self-diffusion coefficients, ionic conductivity, the inverse Haven ratio, and the Li+ apparent transference number all agree with previous experimental results quantitatively. Furthermore, analyses show that the high apparent transference number for Li+ is due to the fact that the dynamics of TFSI– decrease more quickly with increasing salt concentration than the dynamics of Li+ ions due to the formation of a TFSI– network. In addition, it was shown that the conduction of Li+ ions through the highly concentrated electrolyte occurs mainly via a hopping mechanism instead of a vehicular mechanism hypothesized in earlier studies of this system

A Force Field for 3,3,3-Fluoro-1-propenes, Including HFO-1234yf

The European Union (EU) legislation 2006/40/EC bans from January 2011 the cooperative marketing of new car types that use refrigerants in their heating, ventilation, and air conditioning (HVAC) systems with global warming potentials (GWP) higher than 150. Thus, the phase-out of the presently used tetrafluoroethane refrigerant R134a necessitates the adoption of alternative refrigerants. Fluoropropenes such as 2,3,3,3-tetrafluoro-1-propene (HFO-1234yf) are currently regarded as promising low GWP refrigerants, but the lack of experimental data on their thermophysical properties hampers independent studies on their performance in HVAC systems or in other technical applications. In principle, molecular modeling can be used to predict the relevant properties of refrigerants, but adequate intermolecular potential functions (“force fields”) are lacking for fluoropropenes. Thus, we developed a transferable force field for fluoropropenes composed of CF3−, −CF=, −CH=, CF2=, and CH2= groups and applied the force field to study 3,3,3 trifluoro-1-propene (HFO-1243zf), 2,3,3,3-tetrafluoro-1-propene (HFO-1234yf), and hexafluoro-1-propene (HFO-1216). We performed Gibbs ensemble simulations on these three fluoropropenes to compute the vapor pressure, saturated densities, and heats of vaporization. In addition, molecular dynamics simulations were conducted to provide predictions for the density, thermal expansivity, isobaric heat capacity, and transport properties of liquid HFO-1234yf in the temperature range from 263.15 to 310 K and pressures up to 2 MPa. Agreement between simulation results and experimental data and/or correlations (when available) was good, thereby validating the predictive ability of the force field

Molecular Simulation Study of Some Thermophysical and Transport Properties of Triazolium-Based Ionic Liquids

Results of a molecular dynamics study of several triazolium-based ionic liquids are reported. Triazolium cations include 1,2,4-triazolium, 1,2,3-triazolium, 4-amino-1,2,4-triazolium, and 1-methyl-4-amino-1,2,4-triazolium. Each cation was paired with a nitrate or perchlorate anion. These materials are part of a class of ionic compounds that have been synthesized recently but for which little physical property data are available. Properties of the more common ionic liquid, 1-n-butyl-3-methylimidazolium nitrate, are also computed and compared with the properties of the triazolium-based compounds. A molecular mechanics force field was developed for these materials using a mix of ab initio calculations and parameter fitting using the molecular compound 1H-1,2,4-triazole as a basis for the triazolium cations. Liquid-phase properties that were computed include heat capacities, cohesive energy densities, gravimetric densities/molar volumes as a function of temperature and pressure, self-diffusivities, rotational time constants, and various pair correlation functions. In the solid phase, heat capacities and lattice parameters were computed. Of all of these properties, only lattice parameters have been measured experimentally (and only for four of the triazolium compounds). The agreement with the experimental crystal structures was good. When compared with that of the imidazolium-based ionic liquid, the triazolium-based materials have much smaller molar volumes, higher cohesive energy densities, and larger specific heat capacities. They also tend to be less compressible, have a higher gravimetric density, and have faster rotational dynamics but similar translational dynamics

Rapid screening of gas solubility in ionic liquids using biased particle insertions with pre-sampled liquid trajectories

We present an efficient, general-purpose variant of the Widom test particle insertion method for computing chemical potentials of gaseous solutes in fluids or porous solids. The method is implemented in the Monte Carlo molecular simulation engine Cassandra, but receiving phase configurations are independent of this process and may be pre-sampled by other molecular simulation engines such as molecular dynamics codes. Efficiency enhancements present in this method include configurational biasing and accelerated atomic overlap detection. When applied to the estimation of Henry's law constants of atomistic difluoromethane and pentafluoroethane in ionic liquids, the accelerated overlap detection results in a speedup of more than an order of magnitude compared to conventional methods without sacrificing accuracy. We found good agreement between this method and Hamiltonian replica exchange (HREX) for Henry's law constant and absorption isotherm estimation. This embarrassingly parallel method is especially well suited for screening Henry's law constants of many small gases in the same solvents, since a liquid trajectory can be reused for as many solutes as desired.</p

GAFF-Based Polarizable Force Field Development and Validation for Ionic Liquids

Ionic liquids (ILs) have been used in many applications, including gas separations, electrochemistry, lubrication, and catalysis. Understanding how the different properties of ILs are related to their chemical structure and composition is crucial for these applications. Experimental investigations often provide limited insights and can be tedious in exploring a range of state points. Therefore, molecular simulations have emerged as a powerful tool that not only offers a microscopic perspective but also enables rapid screening and prediction of physical properties. The accuracy of these predictions, however, depends on the quality of the intermolecular potentials (force fields) used. The widely used classical fixed charge models, such as GAFF, OPLS, and CL&P, are popular due to their simplicity and computational efficiency. However, it has been shown that the use of integer charges with these classical models leads to sluggish dynamics. The use of scaled charge models can improve the dynamics, but these mean-field approaches are unable to account for polarization effects explicitly. Several different approaches have been proposed to include polarizability in IL force fields. In this work, we follow the protocol of the CL&Pol model to develop a Drude oscillator model based on the GAFF force field (Goloviznina, K., et al. J. Chem. Theory Comput. 2019, 15, 5858). We compare the performance of the model for eight imidazolium- and pyrrolidinium-based ILs against that of other models. We find that the new model provides reasonable estimations of density, self-diffusivity, and structural properties for these ILs and suggests a relatively simple way of extending the general GAFF model to more ILs

Vapor–Liquid Coexistence and Critical Behavior of Ionic Liquids via Molecular Simulations

Vapor–liquid coexistence curves and critical points are of great practical and fundamental importance. Our understanding of these phenomena is well-developed for most fluids but is severely lacking for ionic liquids, a class of salts that are liquid near ambient temperatures. Thermal stability limitations virtually eliminate direct experimental determination of these properties. In this Letter, we report the first vapor–liquid phase diagrams and critical points for ionic liquids obtained in silico with an atomistic force field. We show that within a homologous series of imidazolium-based ionic liquids, the critical temperature, critical density, critical pressure, boiling point, and enthalpy of vaporization all decrease with increasing length of the cation alkyl chain, while the saturation pressure increases with chain length. These trends are opposite to what is observed for alkanes and other nonionic polar compounds such as alcohols. In the vapor phase, we find that ions are distributed across clusters of different sizes with neutral ion pairs being the predominant aggregation state

Molecular Modeling of the Vapor−Liquid Equilibrium Properties of the Alternative Refrigerant 2,3,3,3-Tetrafluoro-1-propene (HFO-1234yf)

The European Union legislation 2006/40/EC results in a phase-out of the presently used tetrafluoroethane refrigerant R134a from automotive heating ventilation and air conditioning systems. This necessitates the adoption of alternative refrigerants, and 2,3,3,3-tetrafluoro-1-propene (HFO-1234yf) is currently regarded as the most promising alternative refrigerant. However, the lack of experimental data hampers independent studies on its performance in technical applications. We have developed a force field for HFO-1234yf that enables reliable predictions of its thermophysical properties via molecular simulation. The simulation results complement experimental data and provide a molecular-level perspective of the fluid behavior. In this letter we present the force field and its validation using Gibbs ensemble simulations on its vapor liquid equilibria

Direct Correlation between Ionic Liquid Transport Properties and Ion Pair Lifetimes: A Molecular Dynamics Study

Self-diffusivities as a function of temperature were computed for 29 different ionic liquids (ILs) covering a wide variety of cation and anion classes. Ideal ionic conductivities (σ_NE) were estimated from the self-diffusivities via the Nernst–Einstein relation. The ion pair (IP) lifetimes (τ_IP) and ion cage (IC) lifetimes (τ_IC) of each IL were also computed. A linear relationship between the calculated self-diffusivities and the inverse of IP or IC lifetimes was observed. A similar inverse linear relationship was also observed for ideal ionic conductivity. These relationships were found to be independent of temperature and the nature of the IL. These observations connect macroscopic dynamic properties with local atomic-level motions and strongly suggest that the dynamics of ILs are governed by a universal IP or IC forming and breaking mechanism. Thus, in order to design an ionic liquid with enhanced dynamics, one should consider how to minimize IP or IC lifetimes

Computing the Liquidus of Binary Monatomic Salt Mixtures with Direct Simulation and Alchemical Free Energy Methods

We describe and validate a free-energy-based method for computing the liquidus for binary solid–liquid phase diagrams in molecular simulations of monatomic salts. The method is demonstrated by calculating the liquidus for LiCl–KCl and MgCl2–KCl salt mixtures with the polarizable ion model (PIM). The free-energy-based method is cross-validated with direct coexistence simulations. Both techniques show excellent agreement with one another. Though the predictions of the PIM disagree with experiments, we use our free-energy-based approach to decouple the contributions of liquid mixture nonidealities and pure component solid–liquid equilibrium to the phase diagram. In both mixtures, the PIM accurately reproduces the liquid phase nonidealities but fails to predict the liquidus because it does not accurately predict the pure component melting temperature of LiCl or MgCl2