Transport properties of room-temperature ionic liquids from classical molecular dynamics

Impact of Dispersion Force Schemes on Liquid Systems: Comparing Efficiency and Drawbacks for Well-Targeted Test Cases

First-principles molecular dynamics (FPMD) calculations were performed on liquid GeSe₄ with the aim of inferring the impact of dispersion (van der Waals, vdW) forces on the structural properties. Different expressions for the dispersion forces were employed, allowing us to draw conclusions on their performances in a comparative fashion. These results supersede previous FPMD calculations obtained in smaller systems and shorter time trajectories by providing data of unprecedented accuracy. We obtained a substantial agreement with experiments for the structure factor regardless of the vdW scheme employed. This objective was achieved by using (in addition to FPMD with no dispersion forces) a selection of vdW schemes available within density functional theory. The first two are due to Grimme, D2 and D3, and the third one is devised within the so-called maximally localized Wannier functions approach (MLWF). D3 results feature a sizeable disagreement in real space with D2 and MLWF in terms of the partial and total pair correlation functions as well as the coordination numbers. More strikingly, total and partial structure factors calculated with D3 exhibit an unexpected sharp increase at low k. This peculiarity goes along with large void regions within the network, standing for a phase separation of indecipherable physical meaning. In view of these findings, further evidence of unconventional structural properties found by employing D3 is presented by relying on results obtained for a complex ionic liquid supported on a solid surface. The novelty of our study is multifold: new, reliable FPMD data for a prototypical disordered network system, convincing agreement with experimental data and assessment of the impact of dispersion forces, with emphasis on the intriguing behavior of one specific recipe and the discovery of common structural features shared by drastically dissimilar physical systems when the D3 vdW scheme is employed.

Dynamics of Ionic Liquid through Intrinsic Vibrational Probes Using the Dispersion-Corrected DFT Functionals

First principles molecular dynamics simulations have been utilized to study the spectral properties of the protic ionic liquid, methylammonium formate (MAF). All simulations were performed using density functional theory (DFT) and various van der Waals-corrected exchange-correlation functionals. We calculated the vibrational stretch frequency distributions, determined the time-frequency correlations of the intrinsic vibrational probes, the N-H and C-O modes in MAF, and the frequency-structure correlations. We also estimated the average hydrogen-bond lifetimes and orientation dynamics to capture the ultrafast spectral response. The spectroscopic signature of the N-H stretching vibrations using the Becke-Lee-Yang-Parr (BLYP) and Perdew-Burke-Ernzerhof (PBE) functionals displays a spectral shift in the lower frequency side, suggesting stronger hydrogen-bonding interactions represented by the gradient approximation functionals than the van der Waals (vdW)-corrected simulations. The carboxylate frequency profiles with the dispersion-corrected representations are almost similar without a significant difference in the normalized distributions. Besides, the COO stretching frequencies at the peak maxima positions of the PBE functionals exhibit a lesser deviation from the experimental data. Spectral diffusion dynamics of the intrinsic vibrational probes on the cationic and anionic sites of the ionic liquid proceed through a short time relaxation of the intact hydrogen bonds followed by an intermediate time constant and a longer time decay indicating the switchover of hydrogen bonds. Dispersion-corrected atom-centered one-electron potential (DCACP) correction added to the BLYP system slows down the picosecond time scales of frequency correlation and the time constants of rotational motion, lengthening the overall system dynamics. The observed trends in the time-dependent decays of frequency fluctuations and the orientation autocorrelation functions correlate with the structural interactions in liquid MAF and hydrogen-bond dynamics. In this study, we examine the predictions made by different density functional treatments comparing the results of the uncorrected BLYP and PBE representations with the semiempirical vdW methods of Grimme and matching our calculated data with the experimental observations.

Equilibrium Structure, Hydrogen Bonding, and Proton Conductivity in Half-Neutralized Diamine Ionic Liquids

Recent experiments on proton conducting ionic liquids point to half-neutralized diamine-triflate salts as promising candidates for applications in power generation and energy conversion electrochemical devices. Structural and dynamical properties of the simplest among these compounds are investigated by a combination of density functional theory (DFT) and molecular dynamics (MD) simulations based on an empirical force field. Three different cations have been considered, consisting of a pair of amine-ammonium terminations joined by a short aliphatic segment -(CH₂) _n- with n = 2, 3, and 4. First, the ground state structure, vibrational eigenstates, and hydrogen-bonding properties of single ions, neutral ion pairs, small neutral aggregates of up to eight ions, and molecularly thin hydrogen bonded wires have been investigated by DFT computations. Second, structural and dynamical properties of homogeneous liquid and amorphous phases are investigated by MD simulations over the temperature range of 200 ≤ T ≤ 440 K. Structure factors, radial distribution functions, diffusion coefficient, and electrical conductivity are computed and discussed, highlighting the inherent structural heterogeneity of these compounds. The core investigation, however, is the characterization of connected paths consisting of cation chains that could support proton transport via a Grotthuss-type mechanism. Since simulations are carried out using a force field of fixed bonding topology, this analysis is based on the equilibrium structure only, using geometrical criteria to identify potential paths for proton conduction. Paths of connected cations can reach a length of 80 cations and 30 Å, provided that bridging oxygen atoms from triflate anions are taken into account. The effects of water contamination at 1% weight concentration on the structure, dynamics, and paths for proton transport are discussed.

Theoretical Calculation of the Thermodynamic Properties of 20 Amino Acid Ionic Liquids

The thermodynamic properties of gas-phase amino acid ionic liquids (AAILs) containing 20 amino acids ([AA]^-) and 1-ethyl-3-methylimidazolium ([Emim]⁺) are studied using a combination of the ab initio method, molecular dynamics simulations, Born-Haber (BH) cycle analysis, and isodesmic reactions. The M06-2X/TZVP method is used to explore the structure and dissociation enthalpies of [Emim][AA] by considering dispersion interaction, and the MP2/Aug-cc-pVTZ method is used to correct these enthalpies. The vaporization enthalpies of all 20 AAILs are calculated by molecular dynamics simulations, and the gas-phase formation enthalpies (Δ_f H) of the 20 [AA]^- anions and [Emim]⁺ cation are calculated by the density functional theory/M06-2X method and isodesmic reaction approaches. To obtain the Δ_f H of the AAILs, interconnections in the corresponding BH cycles are evaluated. A systematic study of the 20 [Emim][AA] ion pairs provides some initial factors contributing to the thermodynamic properties of AAILs: including length of the alkyl chain, interatomic electronic effects, steric repulsion from the cyclic group, and H-bonds formed by functional groups. Generally speaking, the results of this work provide insights into the structure-property relationships of not only ILs but also any ionic or molecular substance.

Molecular dynamics study of mesophase transitions upon annealing of imidazolium-based ionic liquids with long-alkyl chains

Molecular dynamics simulations are performed on a 1-dodecyl-3-methylimidazolium hexafluorophosphate ([C12mim][PF6]) ionic liquid using a united-atom model. The ionic liquid exhibits second step relaxation at temperatures below a crossover point, where the diffusion coefficient shows an Arrhenius to non-Arrhenius transition. Annealing below this crossover temperature makes an isotropic to mesophase transition, where the smectic A (SmA) phase or crystal-like smectic B (SmB) phase forms. Hundreds of nanoseconds are required for completing these transitions. A normal diffusion process is found for anions along the layer-normal and -lateral directions in the SmA phase, but only in the lateral directions in the SmB phase. We find a preserved orientational order for the imidazolium-ring rotational and the alkyl-chain reorientational dynamics in both of the smectic phases.

A molecular dynamics study of the ionic liquid, choline acetate

Structural and dynamic properties of the ionic liquid (IL) choline acetate are studied using molecular dynamics (MD) simulations. The hydroxyl group of choline shows significant hydrogen-bonding interactions with the oxygen atoms of acetate. Nearly all choline cations are found to form a hydrogen bond with acetate anions at 400 K, while about 67% of cations participate in hydrogen-bonding interactions at 600 K. At 400 K, subdiffusive and prominent non-Gaussian behavior persist for t > 10 ns. At 600 K, the usual diffusion regime is obtained after a few hundred ps of subdiffusive behavior. Analysis of reorientational motions of acetate ions, particularly those of their short axes, indicates a high degree of dynamic heterogeneity, in agreement with previous work on different IL systems.

Mapping the Free Energy of Lithium Solvation in the Protic Ionic Liquid Ethylammonuim Nitrate: A Metadynamics Study

Understanding lithium solvation and transport in ionic liquids is important due to their possible application in electrochemical devices. Using first-principles simulations aided by a metadynamics approach we study the free-energy landscape for lithium ions at infinite dilution in ethylammonium nitrate, a protic ionic liquid. We analyze the local structure of the liquid around the lithium cation and obtain a quantitative picture in agreement with experimental findings. Our simulations show that the lowest two free energy minima correspond to conformations with the lithium ion being solvated either by three or four nitrate ions with a transition barrier between them of 0.2 eV. Other less probable conformations having different solvation pattern are also investigated.

Room Temperature Ionic Liquids as Green Solvent Alternatives in the Metathesis of Oleochemical Feedstocks

One of the most important areas of green chemistry is the application of environmentally friendly solvents in catalysis and synthesis. Conventional organic solvents pose a threat to the environment due to the volatility, highly flammability, toxicity and carcinogenic properties they exhibit. The recently emerged room temperature ionic liquids (RTILs) are promising green solvent alternatives to the volatile organic solvents due to their ease of reuse, non-volatility, thermal stability and ability to dissolve a variety of organic and organometallic compounds. This review explores the use of RTILs as green solvent media in olefin metathesis for applications in the oleochemical industry.

Ab Initio Simulations and Electronic Structure of Lithium-Doped Ionic Liquids: Structure, Transport, and Electrochemical Stability

Density functional theory (DFT), density functional theory molecular dynamics (DFT-MD), and classical molecular dynamics using polarizable force fields (PFF-MD) are employed to evaluate the influence of Li(+) on the structure, transport, and electrochemical stability of three potential ionic liquid electrolytes: N-methyl-N-butylpyrrolidinium bis(trifluoromethanesulfonyl)imide ([pyr14][TFSI]), N-methyl-N-propylpyrrolidinium bis(fluorosulfonyl)imide ([pyr13][FSI]), and 1-ethyl-3-methylimidazolium boron tetrafluoride ([EMIM][BF4]). We characterize the Li(+) solvation shell through DFT computations of [Li(Anion)n]((n-1)-) clusters, DFT-MD simulations of isolated Li(+) in small ionic liquid systems, and PFF-MD simulations with high Li-doping levels in large ionic liquid systems. At low levels of Li-salt doping, highly stable solvation shells having two to three anions are seen in both [pyr14][TFSI] and [pyr13][FSI], whereas solvation shells with four anions dominate in [EMIM][BF4]. At higher levels of doping, we find the formation of complex Li-network structures that increase the frequency of four anion-coordinated solvation shells. A comparison of computational and experimental Raman spectra for a wide range of [Li(Anion)n]((n-1)-) clusters shows that our proposed structures are consistent with experiment. We then compute the ion diffusion coefficients and find measures from small-cell DFT-MD simulations to be the correct order of magnitude, but influenced by small system size and short simulation length. Correcting for these errors with complementary PFF-MD simulations, we find DFT-MD measures to be in close agreement with experiment. Finally, we compute electrochemical windows from DFT computations on isolated ions, interacting cation/anion pairs, and liquid-phase systems with Li-doping. For the molecular-level computations, we generally find the difference between ionization energy and electron affinity from isolated ions and interacting cation/anion pairs to provide upper and lower bounds, respectively, to experiment. In the liquid phase, we find the difference between the lowest unoccupied and highest occupied electronic levels in pure and hybrid functionals to provide lower and upper bounds, respectively, to experiment. Li-doping in the liquid-phase systems results in electrochemical windows little changed from the neat systems.

Structure and Dynamics of Ionic Liquids Confined in Amorphous Porous Chalcogenides

Besides the abundant literature on ionic liquids in porous silica and carbon, the confinement of such intriguing liquids in porous chalcogenides has received very little attention. Here, molecular simulation is employed to study the structural and dynamical properties of a typical ionic liquid confined in a realistic molecular model of amorphous chalcogenide with various pore sizes and surface chemistries. Using molecular dynamics in the isobaric-isothermal (NPT) ensemble, we consider confinement conditions relevant to real samples. Both the structure and self-dynamics of the confined phase are found to depend on the surface-to-volume ratio of the host confining material. Consequently, most properties of the confined ionic liquid can be written as a linear combination of surface and bulk-like contributions, arising from the ions in contact with the surface and the ions in the pore center, respectively. On the other hand, collective dynamical properties such as the ionic conductivity remain close to their bulk counterpart and almost insensitive to pore size and surface chemistry. These results, which are in fair agreement with available experimental data, provide a basis for the development of novel applications using hybrid organic-inorganic solids consisting of ionic liquids confined in porous chalcogenides.

Mechanism of bismuth telluride exfoliation in an ionic liquid solvent

Bismuth telluride (Bi2Te3) is a well-known thermoelectric material that has a layered crystal structure. Exfoliating Bi2Te3 to produce two-dimensional (2D) nanosheets is extremely important because the exfoliated nanosheets possess unique properties, which can potentially revolutionize several material technologies such as thermoelectrics, heterogeneous catalysts, and infrared detectors. In this work, ionic liquid (IL) 1-butyl-3-methylimidazolium chloride ([C4mim]Cl) is used to exfoliate Bi2Te3 nanoplatelets. In both experiments and in molecular dynamics (MD) simulations, the Bi2Te3 nanoplatelets yield a stable dispersion of 2D nanosheets in the IL solvent, and our MD simulations provide molecular-level insight into the kinetics and thermodynamics of the exfoliation process. An analysis of the dynamics of Bi2Te3 during exfoliation indicates that the relative translation (sliding apart) of adjacent layers caused by IL-induced forces plays an important role in the process. Moreover, an evaluation of the MD trajectories and electrostatic interactions indicates that the [C4mim](+) cation is primarily responsible for initiating Bi2Te3 layer sliding and separation, while the Cl(-) anion is less active. Overall, our combined experimental and computational investigation highlights the effectiveness of IL-assisted exfoliation, and the underlying molecular-level insights should accelerate the development of future exfoliation techniques for producing 2D chalcogenide materials.

Efficient and convenient oxidation of sulfides to sulfoxides with molecular oxygen catalyzed by Mn(OAc)2 in ionic liquid [C12mim][NO3]

A simple, efficient, and eco-friendly procedure for aerobic oxidation of sulfides catalyzed by Mn(OAc)₂/[C₁₂mim][NO₃] has been developed.