A novel united-atom force field for imidazolium-based ionic liquids

Entropy Drives Accelerated Ion Diffusion upon Carbon Dioxide Expansion of Electrolytes

Carbon dioxide-expanded liquids, organic solvents with high concentrations of soluble carbon dioxide (CO2) at mild pressures, have gained attention as green catalytic media due to their improved properties over traditional solvents. More recently, carbon dioxide-expanded electrolytes (CXEs) have demonstrated improved reaction rates in the electrochemical reduction of CO2, by increasing the rate of delivery of CO2 to the electrode while maintaining facile charge transport. However, recent studies indicate that the limiting behavior of CXEs at higher CO2 pressures is a decline in solution conductivity due to reduced polarity, leading to poorer charge screening and greater ion pairing. In this article, we employ molecular dynamics simulations to investigate the energetic driving forces behind the diffusive properties of an acetonitrile and tetrapropylammonium hexafluorophosphate (TPrAPF6) CXE with increasing CO2 concentration. Our results indicate that entropy drives solvent and electrolyte diffusion with increasing CO2 pressure. The activation energy of ion diffusion increases with higher concentrations of CO2, indicating that increasing the temperature may improve solution conductivity in these systems. This trend in the activation energies is traced to stronger cation-anion Coulombic interactions due to weaker solvent screening at high CO2 concentrations, suggesting that the choice of ion may provide a route to diminish this effect.

Temperature-dependent differential capacitance of an ionic liquid-graphene-based supercapacitor

One of the critical factors affecting the performance of supercapacitors is thermal management. The design of supercapacitors that operate across a broad temperature range and at high charge/discharge rates necessitates understanding the correlation of the molecular characteristics of the device (such as interfacial structure and inter-ionic and ion-electrode interactions) with its macroscopic properties. In this study, we use molecular dynamics (MD) simulations to investigate the influence of Joule heating on the structure and dynamics of the ionic liquid (IL)/graphite-based supercapacitors. The temperature-dependent electrical double layer (EDL) and differential capacitance-potential (CD-V) curves of two different ([Bmim][BF4] and [Bmim][PF6]) IL-graphene pairs were studied under various thermal gradients. For the [Bmim][BF4] system, the differential capacitance curves transition from 'U' to bell shape under an applied thermal gradient (∇T) in the range from 3.3 K nm-1 to 16.7 K nm-1. Whereas in [Bmim][PF6], we find a positive dependence of differential capacitance with ∇T with a U-shaped CD-V curve. We examine changes in the EDL structure and screening potential (ϕ(z)) as a function of ∇T and correlate them with the trends observed in the CD-V curve. The identified correlation between the interfacial charge density and differential capacitance with thermal gradient would be helpful for the molecular design of the IL-electrode interface in supercapacitors or other chemical engineering applications.

Insights into the Structure and Dynamics of Imidazolium Ionic Liquid and Tetraethylene Glycol Dimethyl Ether Cosolvent Mixtures: A Molecular Dynamics Approach

In this work, the effect of molecular cosolvents tetraethylene glycol dimethyl ether (TEGDME) on the structure and versatile nature of mixtures of these compounds with imidazolium-based ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF₆]) is analyzed and discussed at a molecular level by means of all-atom molecular dynamics (MD) simulations. In the whole concentration range of the binary mixtures, the structures and properties evolution was studied by means of systematic molecular dynamics simulations of the fraction of hydrogen bonds, the radial and spatial distribution functions for the various molecular ions and molecular species in the system, together with the snapshots visualization of equilibrated simulation boxes with a color-coding scheme and the rotational dynamics of coumarin 153 (C153) in the binary mixtures. The goal of the work is to provide a molecular-level understanding of significant improvement of ionic conductivity and self-diffusion with the presence of TEGDME as a cosolvent, which causes an enhancement to the ion translational motion and fluidity in the [bmim][PF₆] ionic liquids (ILs). Under a mixture concentration change, the microstructure changes of [bmim][PF₆] with the TEGDME molar fraction (X_TEG) above 0.50 show a slight difference from that of neat [bmim][PF₆] IL and concentrated [bmim][PF₆]/TEGDME mixture in terms of the radial and spatial distribution functions. The relative diffusivities of solvent molecules to cations as a function of concentration were found to depend on the solvent but not on the anion. A TEGDME increase is found to be advantageous to the dissipation of the polar regions as well as the nonpolar regions in the [bmim][PF₆] ionic liquids. These conclusions are consistent with the experimental results, which verified that the unique, complex, and versatile nature of [bmim][PF₆]/TEGDME mixture can be correctly modeled and discussed at a molecular level using MD simulation data.

Understanding Structural and Transport Properties of Dissolved Li2 S8 in Ionic Liquid Electrolytes through Molecular Dynamics Simulations

Lithium-sulfur batteries with high energy density are considered as one of the most promising future energy storage devices. However, the parasitic lithium polysulfides shuttle phenomenon severely hinders the commercialization of such batteries. Ionic liquids have been found to suppress the lithium polysulfides solubility, diminishing the shuttle effect effectively. Herein, we performed classical molecular dynamics simulations to explore the microscopic mechanism and transport behaviors of typical Li₂ S₈ species in ionic liquids and ionic liquid-based electrolyte systems. We found that the trifluoromethanesulfonate anions ([OTf]^- ) exhibit higher coordination strength with lithium ions compared with bis(trifluoromethanesulfonyl)imide anions ([TFSI]^- ) in static microstructures. However, the dynamical characteristics indicate that the presence of the [OTf]^- anions in ionic liquid electrolytes bring faster Li⁺ exchange rate and easier dissociation of Li⁺ solvation structures. Our simulation models offer a significant guidance to future studies on designing ionic liquid electrolytes for lithium-sulfur batteries.

Molecular dynamics simulation of imidazolium CnMIM-BF4 ionic liquids using a coarse grained force-field

Ionic liquids feature thermophysical properties that are of interest in solvents, energy storage materials and tunable lubrication applications. Here we use new Coarse Grained (CG) models to investigate the structure, dynamics and interfacial properties of the [C_2-8MIM][BF₄] family of ionic liquids (ILs). The simulated equation of state and diffusion coefficients are in good agreement with experimental data and with all-atom force-fields. We quantify the nano-structure and liquid-vapour interfacial properties of the ILs as a function of the size of the imidazolium cation. The computational efficiency of the CG models enables the simulation of very long time scales (100's of nanoseconds), which are needed to resolve the dynamic and interfacial properties of ILs containing cations with long aliphatic chains. For [C_>4MIM] [BF₄] the break in symmetry associated to the liquid-vapour interface induces nanostructuring of polar and non-polar domains in the direction perpendicular to the interface plane, with the inhomogeneous regions penetrating deep inside the bulk liquid, typically 5 nm for C₈MIM cations.

Current Status of AMOEBA-IL: A Multipolar/Polarizable Force Field for Ionic Liquids

Computational simulations of ionic liquid solutions have become a useful tool to investigate various physical, chemical and catalytic properties of systems involving these solvents. Classical molecular dynamics and hybrid quantum mechanical/molecular mechanical (QM/MM) calculations of IL systems have provided significant insights at the atomic level. Here, we present a review of the development and application of the multipolar and polarizable force field AMOEBA for ionic liquid systems, termed AMOEBA–IL. The parametrization approach for AMOEBA–IL relies on the reproduction of total quantum mechanical (QM) intermolecular interaction energies and QM energy decomposition analysis. This approach has been used to develop parameters for imidazolium– and pyrrolidinium–based ILs coupled with various inorganic anions. AMOEBA–IL has been used to investigate and predict the properties of a variety of systems including neat ILs and IL mixtures, water exchange reactions on lanthanide ions in IL mixtures, IL–based liquid–liquid extraction, and effects of ILs on an aniline protection reaction.

Thermodynamic, structural and dynamic properties of ionic liquids [C4mim][CF3COO], [C4mim][Br] in the condensed phase, using molecular simulations

In this work a series of thermodynamic, structural, and dynamical properties for the 1-butyl-3-methylimidazolium trifluoroacetate ([C4mim][CF3COO]) and 1-butyl-3-methylimidazolium bromide, ([C4mim][Br]) ionic liquids (ILs) were calculated using Non-polarizable Force Fields (FF), parameterized using a methodology developed previously within the research group, for condensed phase applications. Properties such as the Vapor-Liquid Equilibrium (VLE) curve, critical points (ρ c, T c), Radial, Spatial and Combined Distribution Functions and self-diffusion coefficients were calculated using Equilibrium Molecular Dynamics simulations (EMD); other properties such as shear viscosities and thermal conductivities were calculated using Non-Equilibrium Molecular Dynamics simulations (NEMD). The results obtained in this work indicated that the calculated critical points are comparable with those available in the literature. The calculated structural information for these two ILs indicated that the anions interact mainly with hydrogen atoms from both the imidazolium ring and the methyl chain; the bromide anion displays twice the hydrogen coordination number than the oxygen atoms from the trifluoroacetate anion. Furthermore, Non-Covalent interactions (NCI index), determined by DFT calculations, revealed that some hydrogen bonds in the [C4mim][Br] IL displayed similar strength to those in the [C4mim][CF3COO] IL, in spite of the shorter O--H distances found in the latter IL. The majority of the calculated transport properties presented reasonable agreement with the experimental available data. Nonetheless, the self-diffusion coefficients determined in this work are under-estimated with respect to experimental values; however, by escalating the electrostatic atomic charges for the anion and cation to ±0.8e, only for this property, a remarkable improvement was obtained. Experimental evidence was recovered for most of the calculated properties and to the best of our knowledge, some new predictions were done mainly in thermodynamic states where data are not available. To validate the FF, developed previously within the research group, dynamic properties were also evaluated for a series of ILs such as [C4mim][PF6], [C4mim][BF4], [C4mim][OMs], and [C4mim][NTf2] ILs.

Green chemical engineering in China

In China, the rapid development greatly promotes the national economic power and living standard but also inevitably brings a series of environmental problems. In order to resolve these problems fundamentally, Chinese scientists have been undertaking research in the area of green chemical engineering (GCE) for many years and achieved great progresses. In this paper, we reviewed the research progresses related to GCE in China and screened four typical topics related to the Chinese resources characteristics and environmental requirements, i.e. ionic liquids and their applications, biomass utilization and bio-based materials/products, green solvent-mediated extraction technologies, and cold plasmas for coal conversion. Afterwards, the perspectives and development tendencies of GCE were proposed, and the challenges which will be faced while developing available industrial technologies in China were mentioned.

In Situ Tracking of Organic Reactions at the Vapor/Liquid Interfaces of Ionic Liquids

The molecular structures of ionic liquids at interfaces play a crucial role in determining their chemical activities in applications. In situ X-ray photoelectron spectroscopy (XPS) was used to track the evolution of X-ray irradiation-induced chemical reactions in a series of ionic liquids ([C_n mim][AuCl₄ ]; n=4, 6, 8, 10) on the Si (111) single-crystal surface. Analyses of microstructure and chemical bonding based on the XPS results indicated that reactions occurred at the vapor/liquid interfaces of the ionic liquids. The time-resolved XPS spectra revealed that with increasing irradiation time, the intensity of the peak corresponding to trivalent Au anion decreased for the four ionic liquids as Au was continually reduced to a lower chemical state and finally converted to gold nanoparticles. The rate and conversion of the reaction were associated with the length of the alkyl chain of the ionic liquids cation. Molecular dynamics simulations further revealed that the alkyl chain of the cation in the ionic liquids was oriented towards the vacuum environment at the vapor/liquid interface. Our results provide a real-time atomic-scale experimental evidence of organic reactions at the vapor/liquid interfaces of ionic liquids. The findings are important for understanding the roles of ionic liquids in catalysis, separation, electrochemistry, functional materials, and so on.

Computational Insights into Materials and Interfaces for Capacitive Energy Storage

Supercapacitors such as electric double-layer capacitors (EDLCs) and pseudocapacitors are becoming increasingly important in the field of electrical energy storage. Theoretical study of energy storage in EDLCs focuses on solving for the electric double-layer structure in different electrode geometries and electrolyte components, which can be achieved by molecular simulations such as classical molecular dynamics (MD), classical density functional theory (classical DFT), and Monte-Carlo (MC) methods. In recent years, combining first-principles and classical simulations to investigate the carbon-based EDLCs has shed light on the importance of quantum capacitance in graphene-like 2D systems. More recently, the development of joint density functional theory (JDFT) enables self-consistent electronic-structure calculation for an electrode being solvated by an electrolyte. In contrast with the large amount of theoretical and computational effort on EDLCs, theoretical understanding of pseudocapacitance is very limited. In this review, we first introduce popular modeling methods and then focus on several important aspects of EDLCs including nanoconfinement, quantum capacitance, dielectric screening, and novel 2D electrode design; we also briefly touch upon pseudocapactive mechanism in RuO2. We summarize and conclude with an outlook for the future of materials simulation and design for capacitive energy storage.

The role of errors related to DFT methods in calculations involving ion pairs of ionic liquids

Ionic liquids (ILs) play a key role in many chemical applications. As regards the theoretical approach, ILs show added difficulties in calculations due to the composition of the ion pair and to the fact that they are liquids. Although density functional theory (DFT) can treat this kind of systems to predict physico-chemical properties, common versions of these methods fail to perform accurate predictions of geometries, interaction energies, dipole moments, and other properties related to the molecular structure. In these cases, dispersion and self-interaction error (SIE) corrections need to be introduced to improve DFT calculations involving ILs. We show that the inclusion of dispersion is needed to obtain good geometries and accurate interaction energies. SIE needs to be corrected to describe the charges and dipoles in the ion pair correctly. The use of range-separated functionals allows us to obtain interaction energies close to the CCSD(T) level. © 2017 Wiley Periodicals, Inc.

Understanding the Solubility of Acetaminophen in 1-n-Alkyl-3-methylimidazolium-Based Ionic Liquids Using Molecular Simulation

During the manufacturing of pharmaceutical compounds, solvent mixtures are commonly used, where the addition of a cosolvent allows for the tuning of the intermolecular interactions present in the system. Here we demonstrate how a similar effect can be accomplished using a room temperature ionic liquid. The pharmaceutical compound acetaminophen is studied in 21 common ionic liquids composed of a 1-n-alkyl-3-methylimidazolium cation with 1 of 7 anions. Using the acetate anion, we predict a large enhancement in solubility of acetaminophen relative to water. We show how this is caused by a synergistic effect of favorable interactions between the ionic liquid and the phenyl, hydroxyl and amide groups of acetaminophen, demonstrating how the ionic liquid cation and anion may be chosen to preferentially solvate different functional groups of complex pharmaceutical compounds. Additionally, while the use of charge scaling in ionic liquid force fields has previously been found to have a minute effect on ionic liquid structural properties, we find it appreciably affects the computed solvation free energy of acetaminophen, which in turn affects the predicted solubility.

Monte Carlo simulation and SAFT modeling study of the solvation thermodynamics of dimethylformamide, dimethylsulfoxide, ethanol and 1-propanol in the ionic liquid trimethylbutylammonium bis(trifluoromethylsulfonyl)imide

Molecular simulations and equation of state modelling enable accurate prediction of solute–ionic liquid mixing thermodynamics.

Multiscale modeling of the trihexyltetradecylphosphonium chloride ionic liquid

Hierarchical trihexyltetradecylphosphonium cationic and chloride anionic models.

Computational approaches to understanding reaction outcomes of organic processes in ionic liquids

The utility of using a combined experimental and computational approach for understanding ionic liquid media, and their effect on reaction outcome, is highlighted through a number of case studies.

Amphiphilic interactions of ionic liquids with lipid biomembranes: a molecular simulation study

Current bottlenecks in the large-scale commercial use of many ionic liquids (ILs) include their high costs, low biodegradability, and often unknown toxicities. As a proactive effort to better understand the molecular mechanisms of ionic liquid toxicities, the work herein presents a comprehensive molecular simulation study on the interactions of 1-n-alkyl-3-methylimidazolium-based ILs with a phosphatidylcholine (PC) lipid bilayer. We explore the effects of increasing alkyl chain length (n = 4, 8, and 12) in the cation and anion hydrophobicity on the interactions with the lipid bilayer. Bulk atomistic molecular dynamics (MD) simulations performed at millimolar (mM) IL concentrations show spontaneous insertion of cations into the lipid bilayer regardless of the alkyl chain length and a favorable orientational preference once a cation is inserted. Cations also exhibit the ability to "flip" inside the lipid bilayer (as is common for amphiphiles) if partially inserted with an unfavorable orientation. Moreover, structural analysis of the lipid bilayer show that cationic insertion induces roughening of the bilayer surface, which may be a precursor to bilayer disruption. To overcome the limitation in the timescale of our simulations, free energies for a single IL cation and anion insertion have been determined based on potential of mean force calculations. These results show a decrease in free energy in response to both short and long alkyl chain IL cation insertion, and likewise for a single hydrophobic anion insertion, but an increase in free energy for the insertion of a hydrophilic chloride anion. Both bulk MD simulations and free energy calculations suggest that toxicity mechanisms toward biological systems are likely caused by ILs behaving as ionic surfactants. [Yoo et al., Soft Matter, 2014].

Distributed polarizability models for imidazolium-based ionic liquids

Quantum chemical calculations are used to derive distributed polarizability models sufficiently accurate and compact to be used in classical molecular dynamics simulations of imidazolium-based room temperature ionic liquids. Two distributed polarizability models are fitted to reproduce the induction energy of three imidazolium cations (1,3-dimethyl-, 1-ethyl-3-methyl-, and 1-butyl-3-methylimidazolium) and four anions (tetrafluoroborate, hexafluorophosphate, nitrate, and thiocyanate) polarized by a point charge located successively on a grid of surrounding points. The first model includes charge-flow polarizabilities between first-neighbor atoms and isotropic dipolar polarizability on all atoms (except H), while the second model includes anisotropic dipolar polarizabilities on all atoms (except H). For the imidazolium cations, particular attention is given to the transferability of the distributed polarizability sets. The molecular polarizability and its anisotropy rebuilt by the distributed models are found to be in good agreement with the exact ab initio values for the three cations and 23 additional conformers of 1-ethyl-3-methyl-, 1-butyl-3-methyl-, 1-pentyl-3-methyl-, and 1-hexyl-3-methylimidazolium cations.

Quantitative prediction of physical properties of imidazolium based room temperature ionic liquids through determination of condensed phase site charges: a refined force field

Quantitative prediction of physical properties of room temperature ionic liquids through nonpolarizable force field based molecular dynamics simulations is a challenging task. The challenge lies in the fact that mean ion charges in the condensed phase can be less than unity due to polarization and charge transfer effects whose magnitude cannot be fully captured through quantum chemical calculations conducted in the gas phase. The present work employed the density-derived electrostatic and chemical (DDEC/c3) charge partitioning method to calculate site charges of ions using electronic charge densities obtained from periodic density functional theory (DFT) calculations of their crystalline phases. The total ion charges obtained thus range between -0.6e for chloride and -0.8e for the PF6 ion. The mean value of the ion charges obtained from DFT calculations of an ionic liquid closely matches that obtained from the corresponding crystal thus confirming the suitability of using crystal site charges in simulations of liquids. These partial charges were deployed within the well-established force field developed by Lopes et al., and consequently, parameters of its nonbonded and torsional interactions were refined to ensure that they reproduced quantum potential energy scans for ion pairs in the gas phase. The refined force field was employed in simulations of seven ionic liquids with six different anions. Nearly quantitative agreement with experimental measurements was obtained for the density, surface tension, enthalpy of vaporization, and ion diffusion coefficients.

Influence of hydroxyl group position and temperature on thermophysical properties of tetraalkylammonium hydroxide ionic liquids with alcohols

In this work, we have explored the thermophysical properties of tetraalkylammonium hydroxide ionic liquids (ILs) such as tetrapropylammonium hydroxide (TPAH) and tetrabutylammonium hydroxide (TBAH) with isomers of butanol (1-butanol, 2-butanol and 2-methyl-2-propanol) within the temperature range 293.15-313.15 K, with interval of 5 K and over the varied concentration range of ILs. The molecular interactions between ILs and butanol isomers are essential for understanding the function of ILs in related measures and excess functions are sensitive probe for the molecular interactions. Therefore, we calculated the excess molar volume (V(E) ) and the deviation in isentropic compressibility (Δκs ) using the experimental values such as densities (ρ) and ultrasonic sound velocities (u) that are measured over the whole compositions range at five different temperatures (293.15, 298.15, 303.15, 308.15 and 313.15 K) and atmospheric pressure. These excess functions were adequately correlated by using the Redlich-Kister polynomial equation. It was observed that for all studied systems, the V(E) and Δκs values are negative for the whole composition range at 293.15 K. And, the excess function follows the sequence: 2-butanol>1-butanol>2-methyl-2-propanol, which reveals that (primary or secondary or tertiary) position of hydroxyl group influence the magnitude of interactions with ILs. The negative values of excess functions are contributions from the ion-dipole interaction, hydrogen bonding and packing efficiency between the ILs and butanol isomers. Hence, the position of hydroxyl group plays an important role in the interactions with ILs. The hydrogen bonding features between ILs and alcohols were analysed using molecular modelling program by using HyperChem 7.

Evaluation of thermophysical properties of ionic liquids with polar solvent: a comparable study of two families of ionic liquids with various ions

In this work, we explore and compare the role of the ion effect on the thermophysical properties of two families of ionic liquids (ILs), namely, tetra-alkyl ammonium cation [R4N](+) with hydroxide [OH](-) anion and 1-alkyl-3-methyl imidazolium cation [amim](+) with different anions (chloride, methyl sulfate, and tetrafluoroborate), with polar solvent such as dimethylsulfoxide (DMSO) in the temperature range from 25 to 40 °C and over the whole concentration range of ILs. Two families of ILs, namely, tetramethyl ammonium hydroxide [(CH3)4N][OH] (TMAH), tetraethyl ammonium hydroxide [(C2H5)4N][OH] (TEAH), tetrapropyl ammonium hydroxide [(C3H7)4N][OH] (TPAH), and tetrabutyl ammonium hydroxide [(C4H9)4N][OH] (TBAH) from ammonium-based ILs and 1-ethyl-3-methylimidazolium chloride [Emim][Cl], 1-ethyl-3-methylimidazolium methylsulfate [Emim][MeSO4], 1-butyl-3-methylimidazolium tetrafluoroborate [Bmim][BF4], and 1-butyl-3-methylimidazolium chloride ([Bmim][Cl]) from imidazolium family of ILs, are used in the present study. To address the molecular interactions of ILs with DMSO, densities (ρ), ultrasonic sound velocities (u), and viscosities (η) have been measured over the entire composition range and at four temperatures, 25, 30, 35, and 40 °C, under atmospheric pressure. From these experimental data, the excess molar volume (V(E)), the deviation in isentropic compressibility (Δκs), and the deviation in viscosity (Δη) were calculated and were adequately correlated by using the Redlich-Kister polynomial equation. The measured and predicted data were interpreted on the basis of intermolecular interactions and structural effects between like and unlike molecules upon mixing. The hydrogen-bonding features between ammonium-based ILs and DMSO were analyzed using molecular modeling program by HyperChem 7.

Influence of solvent on ion aggregation and transport in PY15TFSI ionic liquid-aprotic solvent mixtures

Molecular dynamics (MD) simulations using a many-body polarizable APPLE&P force field have been performed on mixtures of the N-methyl-N-pentylpyrrolidinium bis(trifluoromethanesulfonyl)imide (PY15TFSI) ionic liquid (IL) with three molecular solvents: propylene carbonate (PC), dimethyl carbonate (DMC), and acetonitrile (AN). The MD simulations predict density, viscosity, and ionic conductivity values that agree well with the experimental results. In the solvent-rich regime, the ionic conductivity of the PY15TFSI-AN mixtures was found to be significantly higher than the conductivity of the corresponding -PC and -DMC mixtures, despite the similar viscosity values obtained from both the MD simulations and experiments for the -DMC and -AN mixtures. The significantly lower conductivity of the PY15TFSI-DMC mixtures, as compared to those for PY15TFSI-AN, in the solvent-rich regime was attributed to the more extensive ion aggregation observed for the -DMC mixtures. The PY15TFSI-DMC mixtures present an interesting case where the addition of the organic solvent to the IL results in an increase in the cation-anion correlations, in contrast to what is found for the mixtures with PC and AN, where ion motion became increasingly uncorrelated with addition of solvent. A combination of pfg-NMR and conductivity measurements confirmed the MD simulation predictions. Further insight into the molecular interactions and properties was also obtained using the MD simulations by examining the solvent distribution in the IL-solvent mixtures and the mixture excess properties.

Multiscale Simulation of Ion Migration for Battery Systems

In this paper we describe a multi-scale approach to ion migration processes, which involves a bridging from the atomic scale to the macroscopic scale. To this end, the diffusion coefficient of a material i.e. a macroscopic physical quantity, will be appropriately determined from molecular dynamics simulations on the microscale. This way, performance predictions become possible prior to material synthesis. However, standard methods produce in general wrong results for ensemble setups which correspond to battery or capacitor applications.

We introduce a novel method to derive correct values also for such problems. These values are then used in a macroscopic system of partial differential equation (Poisson-Nernst-Planck system) for the numerical simulation of ion migration in a battery.

Molecular dynamic simulation of dicationic ionic liquids: effects of anions and alkyl chain length on liquid structure and diffusion

Structures and dynamics of nine geminal dicationic ionic liquids (DILs) Cn(mim)2X2, where n = 3, 6, and 9 and X = PF6(-), BF4(-), and Br(-), were studied by molecular dynamic simulations (J. Phys. Chem.B2004, 108, 2038-2047). A force field with a minor modification for C3(mim)2 × 2 was adopted for the simulations. Densities, detailed microscopic structures, mean-square displacements (MSD), and self-diffusivities for various ion pairs from MD simulations have been presented. The calculated densities for C9(mim)2X2 (X = Br(-) and BF4(-)) agreed well with the experimental values. The calculated RDFs show that anions are well organized around the imidazolium rings. The calculated RDFs indicate that, unlike the mono cationic ILs, the anions and cations in DILs distribute homogeneously. Enthalpies of vaporization were calculated and correlated with the structural features of DILs. The local structure of C9(mim)2X2 (X = Br, PF6) was examined by the spatial distribution function (SDF). The calculated SDFs show that similar trends were found by other groups for mono cationic ionic liquids (ILs). The highest probability densities are located around the imidazolium ring hydrogens. The calculated diffusion coefficients show that the ion diffusivities are 1 order of magnitude smaller than that of the mono cationic ionic liquids. The effects of alkyl chain length and anion type on the diffusion coefficient were also studied. The dynamics of the imidazolium rings and the alkyl chain in different time scales have also discussed. The calculated transference numbers show that the anions have the major role in carrying the electric current in a DIL.

A simple AIMD approach to derive atomic charges for condensed phase simulation of ionic liquids

The atomic charges for two ionic liquids (ILs), 1-n-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF6]) and 1-ethyl-3-methylimidazolium hexafluorophosphate ([EMIM][PF6]), were derived from periodic crystal phase calculations with density functional theory (DFT) and plane wave basis sets (denoted as "AIMD-c charge"). For both ILs, the total charge was found to be ±0.8 e for the cation and anion, respectively, due to the charge transfer between ions and polarization caused by the environment. These atomic charges were used in a force field developed within the general Amber force field framework. Using this force field, static, dynamic, and thermodynamic properties were computed for the two ILs using molecular dynamics simulation. The results were compared against results obtained using the same Amber force field but four different sets of partial charges, denoted as full charge, scaled charge, AIMD-l charge, and AIMD-b charge, respectively. The full charge was derived from quantum chemistry calculation of isolated ions in a vacuum and resulted in a total charge of unity on each ion. The scaled charge was obtained by uniformly scaling the full charge by 0.8. AIMD-l and AIMD-b charges were derived from liquid phase ab initio molecular dynamics simulations. The scaled charges have the same total charge on the ions as the AIMD-c charge but different distributions. It was found that simulation results not only depend on the total charge of each ion, but they are also sensitive to the charge distribution within an ion, especially for dynamic and thermodynamic properties. Overall, for the two ILs under study, the AIMD-c charge was found to predict experimental results better than the other four sets of charges, indicating that fitting charges from crystal phase DFT calculations, instead of extensive sampling of the liquid phase configurations, is a simple and reliable way to derive atomic charges for condensed phase ionic liquid simulations.