Ionic Dynamics of Hydroxylammonium Ionic Liquids: A Classical Molecular Dynamics Simulation Study

Predicting Gaseous Solute Diffusion in Viscous Multivalent Ionic Liquid Solvents

Calculating solute diffusion in dense, viscous solvents can be particularly challenging in molecular dynamics simulations due to the long time scales involved. Here, a new scaling approach is developed for predicting solute diffusion based on analyses of CO2 and SO2 diffusion in two different multivalent ionic liquid solvents. Various scaling approaches are initially evaluated, including single and separate thermostats for the solute and solvent, as well as the application of the Arrhenius relationship and the Speedy-Angell power law. A very strong logarithmic correlation is established between the solvent-accessible surface area and solute diffusion. This relationship, reflecting Danckwerts' surface renewal theory and the Vrentas-Duda free volume model, presents a valuable method for estimating diffusion behavior from short simulation trajectories at elevated temperatures. The approach may be beneficial for enhancing predictive modeling in similar challenging systems and should be more broadly evaluated.

DFT and ab initio molecular dynamics simulation study of the infrared spectrum of the protic ionic liquid 2-hydroxyethylammonium formate

Protic ionic liquids (PILs) typically show a complex band shape in their infrared (IR) spectra in the high-frequency range due to the hydrogen stretching vibrations of functional groups forming rather strong hydrogen bonds (H-bonds). In the low-frequency range, the intermolecular stretching mode of the H-bond leaves a mark in the far-IR spectrum of PILs. In this study, the IR spectrum of the PIL 2-hydroxyethylammonium formate, [HOCH2CH2NH3][HCOO], is investigated in order to identify the different modes that contribute to the high-frequency band shape, i.e. the cation ν(NH), ν(OH), and ν(CH) modes, and the anion ν(CH) mode, as well as the intermolecular mode of the strongest H-bond in the far-IR spectrum. The assignment is validated by quantum chemistry calculations of clusters at the density functional theory (DFT) level for four ionic pairs and by ab initio molecular dynamics (AIMD) simulations of ten ionic pairs. There is good agreement between the vibrational frequencies obtained from DFT and AIMD simulations for both the high- and low-frequency ranges. Based on the calculations, the strong H-bond interaction between the cation -NH3 group and [HCOO]- gives a broad band envelope associated with the ν(NH) mode in the high-frequency range of the IR spectrum on which there are narrower peaks corresponding to the ν(OH) and ν(CH) modes. In the far-IR (FIR) spectrum, the anions' rattling motion gives a broad feature with a maximum at 160 cm-1, while the H-bond's intermolecular NH⋯O stretching mode appears as a peak at 255 cm-1.

Long-Lived Weak Ion Pairs in Ionic Liquids: An Insight from All-Atom Molecular Dynamics Simulations

The microstructure and local dynamics of ions in room-temperature ionic liquids (RTILs) have drawn a lot of attention due to their extensive potential applications in numerous fields. It is well-known that the widely used definitions of ion pairs (IPs) cannot reflect the full picture of RTILs. In this study, we find a universal residence time (τ_MR), which is regardless of the number of counterions in the first solvation shell in RTILs. Inspired by this, we propose a weak IP (WIP) model from a spatiotemporal perspective and demonstrate that the WIPs are long-lived and that their lifetimes obey a log-normal distribution, which is different from the literature. In addition, the electrostatic interactions are the main factors in the formation of WIPs, and the reorientations of ions are vital to the ruptures of WIPs. This research provides a new perspective for understanding the microstructural and dynamical properties of RTILs.

Molecular Simulation-Guided Spectroscopy of Imidazolium-Based Ionic Liquids and Effects of Methylation on Ion-Cage and -Pair Dynamics

Classical molecular dynamics simulations were performed to assess an atomistic interpretation of the ion-probe structural interactions in two typical ionic liquids (ILs), 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [BMIm][NTf₂] and 1-butyl-2,3-dimethylimidazolium bis(trifluoromethylsulfonyl)imide [BDimIm][NTf₂] through computational ultrafast spectroscopy. The nitrile stretching vibrations of the thiocyanate anion, [SCN]^-, serve as the local mode of the ultrafast system dynamics within the imidazolium-based ionic liquid environment. The wavelet transform of classical trajectories determines the time-varying fluctuating frequencies and the stretch spectral signatures of SCN^- in the normalized distribution. However, computational modeling of the two-dimensional (2D) spectra from the wavelet-derived vibrational frequencies yields time evolution of the local molecular structure along with the varied time-dependent dynamics of the spectral diffusion process. We calculated the frequency-frequency correlation functions (FFCFs), time correlations associated with the ion-pair and -cage dynamics, and mean square displacements as a function of time, depicting diffusive dynamics. The calculated results based on the pair correlation functions and the distribution of atomic density suggest that the hydrogen and methylated carbon at the two-position of the imidazolium ring of [BMIm] and [BDimIm] cations, respectively, strongly interact with the probe through the N of the thiocyanate anion rather than the S atom. The center-of-mass center-of-mass (COM-COM) cation-probe radial distribution functions (RDFs) in conjunction with the site-specific structural analysis further reveal well-structured interactions of the thiocyanate ion and [BMIm]⁺ cation rather than the [BDimIm] cation. In contrast, the anion-probe COM-COM RDFs depict weak interactive associations within the vibrational probe [SCN]^- and [NTf₂]^- ions. Methylation at the two-position of the imidazolium ring predicts slower structural reorganization and breaking and reformation dynamics of the ion pairs and cages within the ionic liquid framework.

Non-monotonic composition dependence of the breakdown of Stokes-Einstein relation for water in aqueous solutions of ethanol and 1-propanol: explanation using translational jump-diffusion approach

A series of experimental and simulation studies examined the validity of the Stokes-Einstein relationship (SER) of water in binary water/alcohol mixtures of different mixture compositions. These studies revealed a strong non-monotonic composition dependence of the SER with maxima at the specific alcohol mole fraction where the non-idealities of the thermodynamic and transport properties are observed. The translational jump-diffusion (TJD) approach elucidated the breakdown of the SER in pure supercooled water as caused by the jump translation of molecules. The breakdown of SER in the supercooled water/methanol binary mixture was successfully explained using the same TJD approach. To further generalize the picture, here we focus on the non-monotonic composition dependence of SER breakdown of water in two water/alcohol mixtures (water/ethanol and water/propanol) for a broad temperature range. In agreement with previous studies, maximum breakdown of SER is observed for the mixture with alcohol mole fraction x = 0.2. Diffusion of the water molecules at the maximum SER breakdown point is largely contributed by jump-diffusion. The residual-diffusion, obtained by subtracting the jump-diffusion from the total diffusion, approximately follows the SER for different compositions and temperatures. We also performed hydrogen (H-)bond dynamics and observed that the contribution of jump-diffusion is proportional to the total free energy of activation of breaking all H-bonds that exist around a molecule. This study, therefore, suggests that the more a molecule is trapped by H-bonding, the more likely it is to diffuse through the jump-diffusion mechanism, eventually leading to an increasing degree of SER breakdown.

Ion Transport in the EMITFSI/PVDF System at Different Temperatures: A Molecular Dynamics Simulation

We used all-atom molecular dynamics simulations to study the ion transport in the 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide/poly(vinylidene fluoride) (EMITFSI/PVDF) system with 40.05 wt % EMITFSI at different temperatures. The glass-transition temperature (T g = 204 K) of this system shows a good agreement with the experimental value (200 K). With the increase of temperature, the peaks of the pair correlation function show an increasing trend. Interestingly, the coordination numbers of ion pairs and the degree of independent ion motion are mainly affected by the binding energy between ion pairs as the temperature increases. In addition, the ion transport properties with increasing temperature can be studied by the ion-pair relaxation times, ion-pair lifetimes, and diffusion coefficients. The simulation results illustrate that the ion transport is intensified. Especially, the cations can always diffuse faster than the anions. The power law shows that mobilities of anions and cations are seen to exhibit a "superionic" behavior. With the increase of temperature, transference numbers of anions decrease first and then increase and transference numbers of cations show the opposite changes; ionic conductivity increases gradually; and viscosity decreases gradually, indicating that the diffusion resistance of ions decreases. In general, after adding PVDF into the EMITFSI system, the glass-transition temperature and viscosity increase, the ionic conductivity and degree of independent ion motion decrease, and diffusion coefficients of cations decrease faster than those of the anions.

Ultrafast dynamics and scattering of protic ionic liquids induced by XFEL pulses

X-rays are routinely used for structural studies through scattering, and femtosecond X-ray lasers can probe ultrafast dynamics. We aim to capture the femtosecond dynamics of liquid samples using simulations and deconstruct the interplay of ionization and atomic motion within the X-ray laser pulse. This deconstruction is resolution dependent, as ionization influences the low momentum transfers through changes in scattering form factors, while atomic motion has a greater effect at high momentum transfers through loss of coherence. Our methodology uses a combination of classical molecular dynamics and plasma simulation on a protic ionic liquid to quantify the contributions to the scattering signal and how these evolve with time during the X-ray laser pulse. Our method is relevant for studies of organic liquids, biomolecules in solution or any low-Z materials at liquid densities that quickly turn into a plasma while probed with X-rays.