Application of static charge transfer within an ionic-liquid force field and its effect on structure and dynamics

Investigating Biomolecules in Deep Eutectic Solvents with Molecular Dynamics Simulations: Current State, Challenges and Future Perspectives

Deep eutectic solvents (DESs) have recently gained increased attention for their potential in biotechnological applications. DESs are binary mixtures often consisting of a hydrogen bond acceptor and a hydrogen bond donor, which allows for tailoring their properties for particular applications. If produced from sustainable resources, they can provide a greener alternative to many traditional organic solvents for usage in various applications (e.g., as reaction environment, crystallization agent, or storage medium). To navigate this large design space, it is crucial to comprehend the behavior of biomolecules (e.g., enzymes, proteins, cofactors, and DNA) in DESs and the impact of their individual components. Molecular dynamics (MD) simulations offer a powerful tool for understanding thermodynamic and transport processes at the atomic level and offer insights into their fundamental phenomena, which may not be accessible through experiments. While the experimental investigation of DESs for various biotechnological applications is well progressed, a thorough investigation of biomolecules in DESs via MD simulations has only gained popularity in recent years. Within this work, we aim to provide an overview of the current state of modeling biomolecules with MD simulations in DESs and discuss future directions with a focus for optimizing the molecular simulations and increasing our fundamental knowledge.

Identification of apposite antagonist for androgen receptor in prostate cancer: an in silico study of fenugreek compounds

Benign Prostate Cancer (BPC), a prevalent condition predominantly affecting elderly males, manifests with voiding difficulties and urinary retention. A library of compounds from Trigonella foenum-graecum, commonly known as fenugreek was used in this study. We aimed to explore its potential anti-cancer effects by computationally assessing its inhibitory activity on the androgen receptor (AR). For in-silico drug assessment, we employed Maestro 12.8, part of the Schrödinger Suite, to identify the most promising candidates acting as androgen receptor antagonists in the treatment of BPC. Subsequently, 59 fenugreek compounds were retrieved from the PubChem database and subjected to molecular docking against the active site of the target protein, 1E3G. 100-nanosecond molecular dynamics (MD) simulations were performed to assess the stability and compactness of the AR-ligand complexes. Notably, the AR-kaempferol complex exhibited the least fluctuation within the AR active site throughout the simulation trajectory, followed by chlorogenic acid and the reference ligand, hydroxyflutamide. The MM/GBSA values revealed the compounds' maximum free binding energy (-103.3 ± 6, -87.4 ± 23, -68.5 ΔGbind) for chlorogenic acid, kaempferol, and hydroxyflutamide, respectively. These findings suggest their potential as promising leads for drug development. Further lead optimization and comprehensive studies on the top-ranked ligands identified in this investigation are warranted to advance their potential as therapeutic agents for BPC treatment.Communicated by Ramaswamy H. Sarma.

Accurate Calculation of Solvation Properties of Lithium Ions in Nonaqueous Solutions

We perform all-atom molecular dynamics simulations of lithium triflate in 1,2-dimethoxyethane using six different literature force fields. This system is representative of many experimental studies of lithium salts in solvents and polymers. We show that multiple historically common force fields for lithium ions give qualitatively incorrect results when compared with those from experiments and quantum chemistry calculations. We illustrate the importance of correctly selecting force field parameters and give recommendations on the force field choice for lithium electrolyte applications.

Charge Transport in Water-NaCl Electrolytes with Molecular Dynamics Simulations

A systematic description of microscopic mechanisms is necessary to understand mass transport in solid and liquid electrolytes. From Molecular Dynamics (MD) simulations, transport properties can be computed and provide a detailed view of the molecular and ionic motions. In this work, ionic conductivity and transport numbers in electrolyte systems are computed from equilibrium and nonequilibrium MD simulations. Results from the two methods are compared with experimental results, and we discuss the significance of the frame of reference when determining and comparing transport numbers. Two ways of computing ionic conductivity from equilibrium simulations are presented: the Nernst-Einstein approximation or the Onsager coefficients. The Onsager coefficients take ionic correlations into account and are found to be more suitable for concentrated electrolytes. Main features and differences between equilibrium and nonequilibrium simulations are discussed, and some potential anomalies and critical pitfalls of using nonequilibrium molecular dynamics to determine transport properties are highlighted.

Entropy scaling of viscosity for molecular models of molten salts

Entropy scaling relates dynamic and thermodynamic properties by reducing the viscosity to a function of only the residual entropy. Molecular simulations are used to investigate the entropy scaling of the viscosity of three models of sodium chloride and five monovalent salts. Even though the correlation between the potential energy and the virial is weak, entropy scaling applies at liquid densities for all models and salts investigated. At lower densities, entropy scaling breaks down due to the formation of ion pairs and chains. Entropy scaling can be used to develop more extendable correlations for the dynamic properties of molten salts.

Treating Polarization Effects in Charged and Polar Bio-Molecules Through Variable Electrostatic Parameters

Polarization plays important roles in charged and hydrogen bonding containing systems. Much effort ranging from the construction of physics-based models to quantum mechanism (QM)-based and machine learning (ML)-assisted models have been devoted to incorporating the polarization effect into the conventional force fields at different levels, such as atomic and coarse grained (CG). The application of polarizable force fields or polarization models was limited by two aspects, namely, computational cost and transferability. Different from physics-based models, no predetermining parameters were required in the QM-based approaches. Taking advantage of both the accuracy of QM calculations and efficiency of molecular mechanism (MM) and ML, polarization effects could be treated more efficiently while maintaining the QM accuracy. The computational cost could be reduced with variable electrostatic parameters, such as the charge, dipole, and electronic dielectric constant with the help of linear scaling fragmentation-based QM calculations and ML models. Polarization and entropy effects on the prediction of partition coefficient of druglike molecules are demonstrated by using both explicit or implicit all-atom molecular dynamics simulations and machine learning-assisted models. Directions and challenges for future development are also envisioned.

Rapid Water Dynamics Structures the OH-Stretching Spectra of Solitary Water in Ionic Liquids and Dipolar Solvents

In a recent study [ J. Phys. Chem. B 126, 4584 (2022)] we used infrared spectroscopy to investigate the solvation and dynamics of solitary water in ionic liquids and dipolar solvents. Complex shapes observed for water OH-stretching bands common to all high-polarity solvents were assigned to water in several solvation states. In the present study, classical molecular dynamics simulations of a single water molecule in four ionic liquids and three dipolar solvents were used to test and refine this interpretation. Consistent with past assignments, simulations show solitary water usually donates two hydrogen bonds to distinct solvent molecules. Such symmetrically solvated water produces the primary pair of peaks identified in the OH spectra of water in nearly all solvents. We had further proposed that additional features flanking this main peak are due to asymmetric solvation states, states in which only one OH group makes a hydrogen bond to solvent. Such states were found in significant concentrations in all of the systems simulated. Simulations of the OH stretching spectra using a semiclassical description and the vibrational map developed by Auer and Skinner [ J. Chem. Phys. 128, 224511 (2008)] provided semi-quantitative agreement with experiment. Analysis of species-specific spectra also confirmed assignment of the additional features in the experimental spectra to asymmetrically solvated water. The simulations also showed that rapid water motions cause a marked motional narrowing compared to the inhomogeneous limit, and that this narrowing is largely responsible for making the additional features due to minority solvation states manifest in the spectra.

Dynamical Ion Association and Transport Properties in PEO-LiTFSI Electrolytes: Effect of Salt Concentration

The association of ions describes the formation of ion species in electrolyte solutions and is strongly related to the salt concentration. However, the discussion of ion species and their transport is ambiguous in some studies on electrolyte materials due to the assumption of ideal solutions. Accordingly, in this work, molecular dynamics simulations are used to study ion association and transport properties of poly(ethylene)oxide (PEO)-lithium bis(trifluoromethanesulfonyl)imide electrolytes over a range of salt concentrations (r = [Li]/[EO]) from 0.01 to 0.20. Based on the analysis of the solvation environment and ion species, it is revealed that the distinct ion-ion correlations exist in two different characteristic areas, with a salt concentration of 0.10 as the limit. Below the critical concentration, the dynamic equilibrium between free ions and ion pairs is the most important process affecting the transport properties of electrolytes, but the process may have a minor influence on the applicability of the Nernst-Einstein relation. In concentrated solutions, a large number of ion pairs, triplets, and so forth appear in the electrolytes. The high-order ion clusters, with an average size of 3.95 at r = 0.20, are the main stable structures for transporting Li⁺, but the trapped free ions are the most abundant ion species. Meanwhile, the effect of salt concentrations on the average transport of ion clusters is to increase their average lifetime, but their transport rates remain unchanged. In addition, the coupling dynamics between ions and polymers is also discussed in order to gain a complete insight into the importance of salt concentrations.

Recent Developments in Polarizable Molecular Dynamics Simulations of Electrolyte Solutions

Polarizable molecular dynamics simulations are a fast progressing field in the scientific research of ionic liquids. The fundamentals of polarizable simulations, as well as their application to ionic liquids, were summarized in a review [Bedrov, D.; Piquemal, J.-P.; Borodin, O.; MacKerell, Jr., A. D.; Roux, B.; Schröder, C. Molecular Dynamics Simulations of Ionic Liquids and Electrolytes Using Polarizable Force Fields. Chem. Rev. 2019, 119, 7940–7995] in 2019. Since then, new methods to treat intermolecular interaction of induced dipoles in these highly charged systems were developed. This concerns the damping of these interactions and additional charge transfer as well as the prediction of ionic materials with ultrahigh refractive indices. In addition to the progress of the polarizable force fields, also thermostats and barostats for polarizable simulations evolved recently.

Charge transfer and polarisability in ionic liquids: a case study

The practical use of ionic liquids (ILs) is benefiting from a growing understanding of the underpinning structural and dynamic properties, facilitated through classical molecular dynamics (MD) simulations. The predictive and explanatory power of a classical MD simulation is inextricably linked to the underlying force field. A key aspect of the forcefield for ILs is the ability to recover charge based interactions. Our focus in this paper is on the description and recovery of charge transfer and polarisability effects, demonstrated through MD simulations of the widely used 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide [C₄C₁im][NTf₂] IL. We study the charge distributions generated by a range of ab initio methods, and present an interpolation method for determining atom-wise scaled partial charges. Two novel methods for determining the mean field (total) charge transfer from anion to cation are presented. The impact of using different charge models and different partial charge scaling (unscaled, uniformly scaled, atom-wise scaled) are compared to fully polarisable simulations. We study a range of Drude particle explicitly polarisable potentials and shed light on the performance of current approaches to counter known problems. It is demonstrated that small changes in the charge description and MD methodology can have a significant impact; biasing some properties, while leaving others unaffected within the structural and dynamic domains.

Solvent-Assisted Li-Ion Transport and Structural Heterogeneity in Fluorinated Battery Electrolytes

The electrolytes diluted with fluorinated solvents show promising properties toward better battery technology than existing ones. The transport of Li ions in these fluorinated electrolytes is essential to access the performance of a battery. It is believed that the transport of the Li ion in these electrolytes occurs through polar solvents in the matrix of nonpolar solvent molecules. The atomistic details of this mechanism are yet to be proved using the dynamics of these mixtures. In this study, we performed classical molecular dynamics simulations at various temperatures to probe this mechanism through the structure and dynamics of electrolytes at the atomic level. Here, we have shown that the polar fluorinated solvents assist the Li-ion transport in a region of nonpolar solvent. Highly polar molecules also solvate the Li ion at a lower temperature. The nonpolar solvent solvates the Li ion weakly as compared to others. The calculated values of the ionic conductivity from the Green-Kubo relation provide a better match than that from an experimental conductivity meter. Furthermore, we probed the heterogeneity in the dynamics of the electrolytes by calculating the non-Gaussian parameter. We also show that the transport mechanism of the Li ion in diluted concentrated electrolytes is different than a few of the other reported electrolytes. We have also calculated the ion-pair and ion-cage lifetimes to see the most and least lived ion/ion-solvent pairs. The mechanism given from the present study may help to design the fluorinated electrolytes for Li-ion batteries.

Comparison and Validation of Force Fields for Deep Eutectic Solvents in Combination with Water and Alcohol Dehydrogenase

Deep eutectic solvents (DESs) have become popular as environmental-friendly solvents for biocatalysis. Molecular dynamics (MD) simulations offer an in-depth analysis of enzymes in DESs, but their performance depends on the force field chosen. Here, we present a comprehensive validation of three biomolecular force fields (CHARMM, Amber, and OPLS) for simulations of alcohol dehydrogenase (ADH) in DESs composed of choline chloride and glycerol/ethylene glycol with varying water contents. Different properties (e.g., protein structure and flexibility, solvation layer, and H-bonds) were used for validation. For two properties (viscosity and water activity) also experiments were performed. The viscosity was calculated with the periodic perturbation method, whereby its parameter dependency is disclosed. A modification of Amber was identified as the best-performing model for low water contents, whereas CHARMM outperforms the other models at larger water concentrations. An analysis of ADH's structure and interactions with the DESs revealed similar predictions for Amber and CHARMM.

Efficient Parametrization of Force Field for the Quantitative Prediction of the Physical Properties of Ionic Liquid Electrolytes

The prediction of transport properties of room-temperature ionic liquids from nonpolarizable force field-based simulations has long been a challenge. The uniform charge scaling method has been widely used to improve the agreement with the experiment by incorporating the polarizability and charge transfer effects in an effective manner. While this method improves the performance of the force fields, this prescription is ad hoc in character; further, a quantitative prediction is still not guaranteed. In such cases, the nonbonded interaction parameters too need to be refined, which requires significant effort. In this work, we propose a three-step semiautomated refinement procedure based on (1) atomic site charges obtained from quantum calculations of the bulk condensed phase; (2) quenched Monte Carlo optimizer to shortlist suitable force field candidates, which are then tested using pilot simulations; and (3) manual refinement to further improve the accuracy of the force field. The strategy is designed in a sequential manner with each step improving the accuracy over the previous step, allowing the users to invest the effort commensurate with the desired accuracy of the refined force field. The refinement procedure is applied on N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium bis(trifluoromethanesulfonyl)imide (DEME-TFSI), a front-runner as an electrolyte for electric double-layer capacitors and single-molecule-based devices. The transferability of the refined force field is tested on N,N-dimethyl-N-ethyl-N-methoxyethoxyethylammonium bis(trifluoromethanesulfonyl)imide (N_112,2O2O1-TFSI). The refined force field is found to be better at predicting both structural and transport properties compared to the uniform charge scaling procedure, which showed a discrepancy in the X-ray structure factor. The refined force field showed quantitative agreement with structural (density and X-ray structure factor) and transport properties-diffusion coefficients, ionic conductivity, and shear viscosity over a wide temperature range, building a case for the wide adoption of the procedure.

Hydrogen Bond Kinetics, Ionic Dynamics, and Voids in the Binary Mixtures of Protic Ionic Liquids with Alkanolamines

Classical molecular dynamics simulations were used to investigate the structural and dynamical properties of the mixtures of ionic liquids (ILs) with the conjugate forms of the cation in a 1:1 molar ratio. The experimental studies suggested the combination of ethanolamines and ILs as novel absorbents for acidic gases such as CO₂ and H₂S, which provide the advantage of efficient absorption of gases at low pressures. However, the microscopic properties of the ionic mixtures are not studied. From our computational investigations, the densities of mixtures are reported and compared with the experimental results. The structural evolution of mixtures is reported by radial distribution functions, coordination numbers, void analysis, and spatial distribution functions. The mixtures' dynamic properties were studied by analyzing the hydrogen bond, ion-pair, and ion-cage lifetimes of the system. Monoethanolammonium and triethanolammonium ILs show different types of spatial distribution functions. The cations have lesser effect on dynamics compared with anions. The charge on the anion greatly affects the dynamics of mixtures. The dianion mixtures show slower dynamics than the monoanionic mixtures. The hydrogen bonding between cations and anions is stronger than that between cations and neutral molecules due to strong coulombic attractive forces. The cations spend more time around the dianions as compared to monoanions. The distributions of voids show that the void sizes are smaller in triethanolamine-based mixtures. The sulfobenzoate-based mixtures show voids smaller than those of pyridine-3-carboxylate-based mixtures due to more available free space between the entities, which facilitates the overall dynamics.

Thermodynamic, transport, and structural properties of hydrophobic deep eutectic solvents composed of tetraalkylammonium chloride and decanoic acid

With the emergence of hydrophobic deep eutectic solvents (DESs), the scope of applications of DESs has been expanded to include situations in which miscibility with water is undesirable. Whereas most studies have focused on the applications of hydrophobic DESs from a practical standpoint, few theoretical works exist that investigate the structural and thermodynamic properties at the nanoscale. In this study, Molecular Dynamics (MD) simulations have been performed to model DESs composed of tetraalkylammonium chloride hydrogen bond acceptor and decanoic acid hydrogen bond donor (HBD) at a molar ratio of 1:2, with three different cation chain lengths (4, 7, and 8). After fine-tuning force field parameters, densities, viscosities, self-diffusivities, and ionic conductivities of the DESs were computed over a wide temperature range. The liquid structure was examined using radial distribution functions (RDFs) and hydrogen bond analysis. The MD simulations reproduced the experimental density and viscosity data from the literature reasonably well and were used to predict diffusivities and ionic conductivities, for which experimental data are scarce or unavailable. It was found that although an increase in the cation chain length considerably affected the density and transport properties of the DESs (i.e., yielding smaller densities and slower dynamics), no significant influence was observed on the RDFs and the hydrogen bonds. The self-diffusivities showed the following order for the mobility of the various components: HBD > anion > cation. Strong hydrogen bonds between the hydroxyl and carbonyl groups of decanoic acid and between the hydroxyl group of decanoic acid and chloride were observed to dominate the intermolecular interactions.

Spatial-Decomposition Analysis of Electrical Conductivity in Mixtures of Ionic Liquid and Sodium Salt for Sodium-Ion Battery Electrolytes

Ionic liquid (IL)-based electrolytes are a promising material for the development of sodium-ion batteries, and their performance can be quantified by electrical conductivity. In this highly concentrated ionic system, the correlated motions of ion pairs are influential on the ionic transport properties. Herein, all-atom analyses are conducted through molecular dynamics simulations to bridge the macroscopically observable electrical conductivity with the molecular pictures of correlated motion of ion pairs. The analysis is applied to three mixtures of IL with sodium salt that are relevant to the electrolyte for a sodium-ion battery: [1-ethyl-3-methylimidazolium, Na][bis(fluorosulfonyl)amide] ([C₂C₁im, Na][FSA]), [N-methyl-N-propylpyrrolidinium, Na][FSA] ([C₃C₁pyrr, Na][FSA]), and [K, Na][FSA]. The computational results on electrical conductivities are in agreement with the experimental reports, and their dependency on temperature and sodium-ion composition is reproduced well. The overall contributions from cross-correlated motions are found to be negative in all the IL mixtures; thus, the total conductivities are less than their Nernst-Einstein estimates. The spatial view of cross-correlated motions is further obtained by decomposing the time correlation functions of velocities according to the distances between ion pairs. It is observed that ion pairs are moving in the same direction for ∼0.3 ps when they were initially within the first coordination shell, followed by motions toward opposite directions. The cross-correlation terms are also dissected into local and nonlocal components, and it is commonly seen for all the ion pairs that the local component is negative for cation-anion pairs and is positive for cation-cation and anion-anion pairs. The motions of ion pairs are accompanied by a "backflow" that manifests in the form of the nonlocal component whose sign is opposite to the corresponding local component. In fact, the contributions of the correlated motions of ions to the electrical conductivity are not localized to contact pairs and extend spatially beyond the first coordination shell of the cation-anion pairs.

Optimized Atomic Partial Charges and Radii Defined by Radical Voronoi Tessellation of Bulk Phase Simulations

We present a novel method for the computation of well-defined optimized atomic partial charges and radii from the total electron density. Our method is based on a two-step radical Voronoi tessellation of the (possibly periodic) system and subsequent integration of the total electron density within each Voronoi cell. First, the total electron density is partitioned into the contributions of each molecule, and subsequently the electron density within each molecule is assigned to the individual atoms using a second set of atomic radii for the radical Voronoi tessellation. The radii are optimized on-the-fly to minimize the fluctuation (variance) of molecular and atomic charges. Therefore, our method is completely free of empirical parameters. As a by-product, two sets of optimized atomic radii are produced in each run, which take into account many specific properties of the system investigated. The application of an on-the-fly interpolation scheme reduces discretization noise in the Voronoi integration. The approach is particularly well suited for the calculation of partial charges in periodic bulk phase systems. We apply the method to five exemplary liquid phase simulations and show how the optimized charges can help to understand the interactions in the systems. Well-known effects such as reduced ion charges below unity in ionic liquid systems are correctly predicted without any tuning, empiricism, or rescaling. We show that the basis set dependence of our method is very small. Only the total electron density is evaluated, and thus, the approach can be combined with any electronic structure method that provides volumetric total electron densities-it is not limited to Hartree-Fock or density functional theory (DFT). We have implemented the method into our open-source software tool TRAVIS.

Switching between Proton Vacancy and Excess Proton Transfer Pathways in the Reaction between 7-Hydroxyquinoline and Formate

Bifunctional or amphoteric photoacids simultaneously present donor (acidic) and acceptor (basic) properties making them useful tools to analyze proton transfer reactions. In protic solvents, the proton exchange between the acid and the base is controlled by the acidity or basicity strength and typically occurs on two different pathways known as protolysis and hydrolysis. We report here how the addition of a formate base will alter the relative importance of the possible reaction pathways of the bifunctional photoacid 7-hydroxyquinoline (7HQ), which has been recently understood to predominantly involve a hydroxide/methoxide transport mechanism between the basic proton-accepting quinoline nitrogen site toward the proton-donating OH group with a time constant of 360 ps in deuterated methanol (CD₃OD). We follow the reaction dynamics by probing the IR-active marker modes of the different charged forms of photoexcited 7HQ, and of formic acid (HCOOD) in CD₃OD solution. A comparison of the transient IR spectra as a function of formate concentration, and classical molecular dynamics simulations enables us to identify distinct contributions of “tight” (meaning “contact”) and “loose” (i.e., “solvent-separated”) 7HQ–formate reaction pairs in our data. Our results suggest that depending on the orientation of the OH group with respect to the quinoline aromatic ring system, the presence of the formate molecule in a proton relay pathway facilitates a net proton transfer from the proton-donating OH group of 7HQ-N* via the methanol/formate bridge toward the quinoline N site.

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.

Dissolving Cellulose in 1,2,3-Triazolium- and Imidazolium-Based Ionic Liquids with Aromatic Anions

We present 1,2,3-triazolium- and imidazolium-based ionic liquids (ILs) with aromatic anions as a new class of cellulose solvents. The two anions in our study, benzoate and salicylate, possess a lower basicity when compared to acetate and therefore should lead to a lower amount of N-heterocyclic carbenes (NHCs) in the ILs. We characterize their physicochemical properties and find that all of them are liquids at room temperature. By applying force field molecular dynamics (MD) simulations, we investigate the structure and dynamics of the liquids and find strong and long-lived hydrogen bonds, as well as significant π-π stacking between the aromatic anion and cation. Our ILs dissolve up to 8.5 wt.-% cellulose. Via NMR spectroscopy of the solution, we rule out chain degradation or derivatization, even after several weeks at elevated temperature. Based on our MD simulations, we estimate the enthalpy of solvation and derive a simple model for semi-quantitative prediction of cellulose solubility in ILs. With the help of Sankey diagrams, we illustrate the hydrogen bond network topology of the solutions, which is characterized by competing hydrogen bond donors and acceptors. The hydrogen bonds between cellulose and the anions possess average lifetimes in the nanosecond range, which is longer than found in common pure ILs.

Molecular simulation of liquid-vapor coexistence for NaCl: Full-charge vs scaled-charge interaction models

Scaled-charge models have been recently introduced for molecular simulations of electrolyte solutions and molten salts to attempt to implicitly represent polarizability. Although these models have been found to accurately predict electrolyte solution dynamic properties, they have not been tested for coexistence properties, such as the vapor pressure of the melt. In this work, we evaluate the vapor pressure of a scaled-charge sodium chloride (NaCl) force field and compare the results against experiments and a non-polarizable full-charge force field. The scaled-charge force field predicts a higher vapor pressure than found in experiments, due to its overprediction of the liquid-phase chemical potential. Reanalyzing the trajectories generated from the scaled-charge model with full charges improves the estimation of the liquid-phase chemical potential but not the vapor pressure.

The Effect of Phenyl Substitutions on Microstructures and Dynamics of Tetraalkylphosphonium Bis(trifluoro- methylsulfonyl)imide Ionic Liquids

Extensive atomistic simulations demonstrated that a gradual substitution of hexyl chains with phenyl groups in tetraalkylphosphonium cations results in remarkable changes in hydrogen bonding interactions, liquid structures and scattering structural functions, and rotational dynamics of hexyl chains and phenyl groups in tetraalkylphosphonium bis(trifluoromethylsulfonyl)imide ionic liquids. Hydrogen donor sites in hexyl chains present competitive characteristics with those in phenyl groups in coordinating anions, as well as their continuous and intermittent hydrogen bonding dynamics. Cation-cation and anion-anion spatial correlations show concomitant shift to short distances with decreased peak intensities with variations of cation structures, whereas cation-anion correlations have a distinct shift to large radial distances due to decreased associations of anions with neighboring cations. These microstructural changes are qualitatively manifested in shifts of prominent peaks for prevalent charge alternations and adjacency correlations between ion species in scattering structural functions. Meanwhile, rotational dynamics of hexyl chains speed up, which, in turn, slow down rotations of phenyl groups, whereas anions exhibit imperceptible changes in their rotational dynamics. These computational results are intrinsically correlated with conformational flexibilities, molecular sizes, and steric hindrance effects of phenyl groups in comparison with hexyl chains, and constrained distributions of anions around cations in heterogeneous ionic environments.

Polarizable MD simulations of ionic liquids: How does additional charge transfer change the dynamics?

A new model for treating charge transfer in ionic liquids is developed and applied to 1-ethyl-3-methylimidazolium tetrafluoroborate. The model allows for us to examine the roles of charge transfer, polarizability, and charge scaling effects on the dynamics of ionic liquids.

Solvate ionic liquids based on lithium bis(trifluoromethanesulfonyl)imide–glyme systems: coordination in MD simulations with scaled charges

Charge scaling in molecular dynamics simulations of lithium bis(trifluoromethanesulfonyl)imide–glyme solvate ionic liquids yields better agreement with experiments.

Molecular Dynamics Simulations of Ionic Liquids and Electrolytes Using Polarizable Force Fields

Many applications in chemistry, biology, and energy storage/conversion research rely on molecular simulations to provide fundamental insight into structural and transport properties of materials with high ionic concentrations. Whether the system is comprised entirely of ions, like ionic liquids, or is a mixture of a polar solvent with a salt, e.g., liquid electrolytes for battery applications, the presence of ions in these materials results in strong local electric fields polarizing solvent molecules and large ions. To predict properties of such systems from molecular simulations often requires either explicit or mean-field inclusion of the influence of polarization on electrostatic interactions. In this manuscript, we review the pros and cons of different treatments of polarization ranging from the mean-field approaches to the most popular explicit polarization models in molecular dynamics simulations of ionic materials. For each method, we discuss their advantages and disadvantages and emphasize key assumptions as well as their adjustable parameters. Strategies for the development of polarizable models are presented with a specific focus on extracting atomic polarizabilities. Finally, we compare simulations using polarizable and nonpolarizable models for several classes of ionic systems, discussing the underlying physics that each approach includes or ignores, implications for implementation and computational efficiency, and the accuracy of properties predicted by these methods compared to experiments.

Coupled hydroxyl and ether functionalisation in EAN derivatives: the effect of hydrogen bond donor/acceptor groups on the structural heterogeneity studied with X-ray diffractions and fixed charge/polarizable simulations

We present a study by energy-dispersive X-ray diffraction of liquid 2-(2-hydroxyethoxy)ethan-1-ammonium nitrate, NH₃CH₂CH₂(OCH₂CH₂OH)⁺NO₃^- (22HHEAN). This ionic liquid is derived from the parent ethylammonium nitrate (EAN) with an ether link in the chain and a hydroxyl group in the terminal position. The absence of peaks at low-q values in the experimental diffraction curve indicates that the added polar groups and the high conformational isomerism of the cations alter strongly the nanosegregation of the parent EAN liquid. Aggregation between ionic species may involve hydrogen bonding between cations and anions and a variety of intermolecular hydrogen bonds between cations. Diffraction patterns are compared with the results of molecular dynamics simulations with two different force fields: the fixed point charge force field (GAFF) with different charge scaling protocols and the polarizable AMOEBA force field. Most point charge models lead to the appearance of a quite evident low q-peak which decreases gradually, when the percentage and type of the scaling (uniform vs. non-uniform) are increased. In the polarisable model and in the model where only anion charges are scaled to 20%, instead, the pre-peak is absent in agreement with our experiments.

Permeabilities of CO2, H2S and CH4 through Choline-Based Ionic Liquids: Atomistic-Scale Simulations

Molecular dynamics simulations are used to study the transport of CO2, H2S and CH4 molecules across environmentally friendly choline-benzoate and choline-lactate ionic liquids (ILs). The permeability coefficients of the considered molecules are calculated using the free energy and diffusion rate profiles. Both systems show the largest resistance to CH4, whereas more than 5 orders of magnitude larger permeability coefficients are obtained for the other two gas molecules. The CO2/CH4 and H2S/CH4 selectivity was estimated to be more than 104 and 105, respectively. These results indicate the great potential of the considered ILs for greenhouse gas control.