Dynamic Properties Of Ionic Liquids Research Articles

Dynamics of ionic liquids by means of nuclear magnetic resonance relaxation - overview of theoretical approaches.

This paper presents a comprehensive overview of the spin relaxation theory needed for exploring nuclear magnetic resonance (NMR) relaxometry to study the dynamical properties of ionic liquids. The term NMR relaxometry refers to relaxation experiments performed over a wide range of magnetic fields (resonance frequencies). In this way, dynamical processes occurring on timescales from milliseconds to nanoseconds can be studied, including translational and rotational dynamics of both types of ions (cations and anions). In order to take advantage of the remarkable experimental possibilities, appropriate theoretical models linking relaxation properties with ionic motion are needed. With the aim of providing such theoretical tools, 1H and 19F relaxation models for ionic liquids have been reviewed and their applications have been illustrated by several examples. The presented models are valid for an arbitrary magnetic field, include all relevant relaxation pathways and allow to extract detailed information about the translational and rotational dynamics of the ions. On the basis of the theoretical models, formulas allowing a straightforward determination of the translational diffusion coefficients of cations and anions from combined 1H and 19F relaxation studies have been derived and discussed in detail.

Ag, Au, Pt, and Au-Pt nanoclusters in [N1114][C1SO3] ionic liquid: A molecular dynamics study

A comprehensive study of the solvation of nanoclusters in ionic liquids (ILs) can provide essential understanding needed for the development of various applications of materials based on such systems, such as solar cells, lithium batteries, and nanocluster synthesis. In this work, the solvation of (Au)147, (Pt)147, (Ag)147, (Au-Pt)147, and (Au-Pt)309 clusters in the ionic liquid 1-butyl-1,1,1-trimethylammonium methane sulfonate [N1114][C1SO3] at 300, 500, and 700 K at atmospheric pressure was studied using molecular dynamics simulations. In addition, some thermodynamic, structural, and dynamic properties of ILs and different metal clusters at different temperatures were investigated. Our results showed that the Pt and Au-Pt systems have higher solvation energies than the rest of the systems. The solvation energy of different clusters decreased by increasing the temperature and increased by increasing the size of nanocluster. There was not significant change in the structure of the nanoclusters in the IL at different temperatures. However, the structures become more spherical at highest temperature. The core–shell structure of (Au-Pt)309 nanocluster did not change in IL even at higher temperatures. Our results also indicated that the anions are in closer distances to the nanocluster surfaces than the cations which is in an agreement with the DLVO theory. The charge separation is not complete in the IL because we cannot observe the individual anions and cations shells around the nanoclusters. Our dynamics results also showed that the silver atoms show higher self-diffusion values and the Pt atoms show lower self-diffusion values than the other metals.

Rheological Characteristics of Ionic Liquids under Nanoconfinement.

While the dynamic properties of ionic liquids (ILs) in nanoconfinement play a crucial role in the performance of IL-based electrochemical and mechanical devices, experimental work mostly falls short at reporting "solid-like" versus "liquid-like" behavior of confined ILs. The present work is the first to conduct frequency-sweep oscillatory-shear rheology on IL nanofilms, reconciling the solid-versus-liquid debate and revealing the importance of shear rate in the behavior. We disentangle and analyze the viscoelasticity of nanoconfined ILs and shed light on their relaxation mechanisms. Furthermore, a master curve describes the scaling of the dynamic behavior of four (non-hydrogen-bonding) ILs under nanoconfinement and reveals the role of the compressibility of the flow units.

Thermal, structural and dynamic properties of ionic liquids and organic ionic plastic crystals with a small ether-functionalised cation

A series of new salts with a small ether-functionalised trimethylammonium cation are synthesised and characterised to probe their unique structure–property relationships.

Polarizable molecular dynamics simulations of ionic liquids: Influence of temperature control.

Ionic liquids are an interesting class of soft matter with viscosities of one or two orders of magnitude higher than that of water. Unfortunately, classical, non-polarizable molecular dynamics (MD) simulations of ionic liquids result in too slow dynamics and demonstrate the need for explicit inclusion of polarizability. The inclusion of polarizability, here via the Drude oscillator model, requires amendments to the employed thermostat, where we consider a dual Nosé-Hoover thermostat, as well as a dual Langevin thermostat. We investigate the effects of the choice of a thermostat and the underlying parameters such as the masses and force constants of the Drude particles on static and dynamic properties of ionic liquids. Here, we show that Langevin thermostats are not suitable for investigating the dynamics of ionic liquids. Since polarizable MD simulations are associated with high computational costs, we employed a self-developed graphics processing unit enhanced code within the MD program CHARMM to keep the overall computational effort reasonable.

Structural and dynamical properties of ionic liquids: a molecular dynamics study employing DL_POLY 4

ABSTRACT Molecular dynamics simulations are often used to study the structures and dynamics of ionic liquids. Here, we have simulated three ionic liquids, trihexyl(tetradecyl)-phosphonium chloride [P66614][Cl], 1-butyl-3-methylimidazolium acecate [BMIm][Oac] and 1-ethyl-3-methylimidazolium dicyanamide, [EMIm][DCA] in a comparison of two force fields, GAFF and CL&PFF. In most cases, the resulting theoretical densities agree with experimental values within a 2% error. Diffusive properties were characterised by mean squared displacements to show the significant effect of the alkyl chain on the movement of the [P66614] cation. Activation energies of diffusion were calculated from linear Arrhenius plots which agree with previous studies. Simulations of the dynamical behaviour show retention of short and medium-range structure of the ionic liquids with temperature. However, although with increasing temperature more high energy local configurations become accessible, they are observed less frequently as energy barriers are more easily overcome, resulting in more ordered time-averaged structures.

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 (ρc, Tc), 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.

Dynamic Heterogeneity and Flexibility of the Alkyl Chain in Pyridinium-Based Ionic Liquids.

Changing the number of carbon atoms in the substituents of ionic liquids (ILs) is a way to shift the balance between Coulomb and van der Waals forces and, thus, to tune physicochemical properties. Here we address this topic on the microscopic level by employing quasielastic neutron scattering (QENS) and provide information about the stochastic ionic motions in the N-alkylpyridinium based ILs in a relatively expanded time range, from short time (subpicosecond) particle rattling to long time diffusive regime (hundreds of picoseconds). We have systematically investigated the effect of the alkyl chain length on the picosecond dynamics by employing partial deuteration of the samples and varying the number of carbon atoms in the alkyl substituent. The localized dynamics of the side groups have appeared to be enhanced for bulkier cations, which is opposite to the trend observed for the translational motion. This result highlights the role of the conformational flexibility of the alkyl group on the dynamical properties of ILs.

The effect of various quantum mechanically derived partial atomic charges on the bulk properties of chloride-based ionic liquids

Partial atomic charges using various quantum mechanical calculations for [Cnmim]Cl (n=1, 4) ionic liquids (ILs) are obtained and used for development of molecular dynamics simulation (MD) force fields. The isolated ion pairs are optimized using HF, B3LYP, and MP2 methods for electronic structure with 6-311++G(d,p) basis set. Partial atomic charges are assigned to the atomic center with CHELPG and NBO methods. The effect of these sets of partial charges on the static and dynamic properties of ILs is evaluated by performing a series of MD simulations and comparing the essential thermodynamic properties with the available experimental data and available molecular dynamics simulation results. In contrast to the general trends reported for ionic liquids with BF4, PF6, and iodide anions (in which restrained electrostatic potential (RESP) charges are preferred), partial charges derived by B3LYP-NBO method are relatively good in prediction of the structural, dynamical, and thermodynamic energetic properties of the chloride based ILs.

Molecular Simulations of Anion and Temperature Dependence on Structure and Dynamics of 1-Hexyl-3-methylimidazolium Ionic Liquids

In this study, we examine the effect of various anions and temperature on structure and dynamics of 1-hexyl-3-methylimidazolium ionic liquids (ILs) from molecular dynamics simulations. The structural properties show that ILs containing smaller anions like Cl(-) and Br(-) are relatively higher cation-anion interactions, compared to ILs containing larger anions like OTf(-) and NTf2(-). In all ILs, the spatial distribution of anions is closer to the acidic hydrogen atom of the cation compared to the two nonacidic hydrogen atoms of the cation. The diffusion coefficients of cations and anions (ionic conductivity) increase with anionic size. At each temperature, the cationic and anionic diffusions and ionic conductivity are lowest in ILs containing anions like Cl(-) and Br(-) and highest in ILs containing anions like BF4(-), OTf(-), and NTf2(-). Consistent with experiments, simulations predict that ILs with an intermediate size BF4(-) anion show the highest cationic and anionic diffusion (and ionic conductivity). At each temperature, the interactions between ion pairs of each IL show that a decrease in ion-pair lifetimes is directly related to the increase in diffusion coefficients and conductivity in ILs, suggesting that characterization of ion-pair lifetimes is sufficient to validate the trends seen in dynamical properties of ILs.

Effect of Side-Chain Length on Structural and Dynamic Properties of Ionic Liquids with Hydroxyl Cationic Tails

The recent study has revealed that ionic liquids (ILs) with hydroxyl cationic tails are polar liquids without tightly aggregated nonpolar tail domains. Nevertheless, the influence of varying side-chain length on their microscopic structure and dynamics is still unclear. By performing all-atom molecular dynamics simulations for 1-(n-hydroxyalkyl)-3-methylimidazolium nitrate, where n varies from 2 to 12, we found that, with increasing side-chain length, both the nonpolar region and the flexibility of cationic tails increase. The larger nonpolar region pushes both the charged groups (heads and anions) and nonpolar groups (methylene groups on the side chains) to become more organized, while the increasing tail flexibility allows the hydroxyl terminals to retain a relatively uniform distribution. The increase of side-chain length does not apparently alter the polar nature of the ILs with hydroxyl tails, and has little effect on the total number of formed hydrogen bonds, but slows down the dynamics of ILs.

Ionic liquids in confined geometries

Over recent years the Surface Force Apparatus (SFA) has been used to carry out model experiments revealing structural and dynamic properties of ionic liquids confined to thin films. Understanding characteristics such as confinement induced ion layering and lubrication is of primary importance to many applications of ionic liquids, from energy devices to nanoparticle dispersion. This Perspective surveys and compares SFA results from several laboratories as well as simulations and other model experiments. A coherent picture is beginning to emerge of ionic liquids as nano-structured in pores and thin films, and possessing complex dynamic properties. The article covers structure, dynamics, and colloidal forces in confined ionic liquids; ionic liquids are revealed as a class of liquids with unique and useful confinement properties and pertinent future directions of research are highlighted.

Static and dynamic properties of ionic liquids

We performed molecular dynamics (MD) simulations of ionic liquids composed of 1-butyl-3-methylimidazolium ([bmim]) cation with PF6, NO3 and Cl anions to determine their static and dynamic properties. Large-scale simulation of 4096 ion pairs (131,072 particles in [bmim]PF6) was performed to estimate the system-size dependence of the static and dynamic properties. The diffusion constant, which is 100 times smaller than that of a normal liquid such as water, was estimated from long-time simulations. We also performed non-equilibrium MD simulations to determine the electrical conductivity. We obtained a nonlinear relationship between the electrical current and external electric field strength.

Structural and dynamical properties of ionic liquids: Competing influences of molecular properties

Room temperature ionic liquids differ from molten salts in many ways, our work concentrates on two distinguishing features. These are large cation-anion size disparities and at least one ionic species where the center of mass and the center of charge do not coincide. In earlier work, we examined the influences of these features in isolation on simple spherical models. This paper extends this work to ionic liquid models where both features are present, and where the characteristic distance sigma(+-) (') determining the strength of the Coulombic attractions is unconstrained. We consider the interplay among these molecular features and elucidate their relative importance to the behavior of ionic liquids. Particular attention is focused on the transport properties. We find that size disparity, charge location, and sigma(+-) (') can all have large (often competing) effects. In our models, size disparity and small charge displacements lead to weakly bound, directional ion pairs, and the resulting asymmetric ion-counterion distribution gives rise to increased diffusion coefficients, consequently lower viscosity, and increased conductivity. These observations are analogous to effects reported in the literature, and we see similarities between the directional ion pairs in our models and directional cation-anion pairing through weak hydrogen bonding in room temperature ionic liquids. In our models, large charge displacements lead to strongly bound, long-lived, directional ion pairs, and in this regime the trends noted above are reversed, increased viscosities, and decreased conductivities are observed. Recently, creating more strongly hydrogen bonded, directional ion pairs has been put forward as possible means of achieving larger viscosity reductions. The trend reversal that we observe suggests that this might not work in practice.

Influence of Polarization on Structural, Thermodynamic, and Dynamic Properties of Ionic Liquids Obtained from Molecular Dynamics Simulations

Utilizing the transferable, quantum-chemistry-based, Atomistic Polarizable Potential for Liquids, Electrolytes, & Polymers (APPLE&P) force field, we have systematically investigated the influence of polarization effects on the accuracy of properties predicted from molecular dynamics simulations of various room temperature ionic liquids (ILs). Simulations of ILs in which the atom-based polarizability was set to zero for all atoms (nonpolarizable APPLE&P potential) resulted in changes in thermodynamic and dynamic properties from those predicted by the polarizable APPLE&P potential that are qualitatively different from changes observed for nonionic liquids. Investigation of structural and dynamical correlations using both the polarizable and nonpolarizable versions of APPLE&P allowed us to obtain a mechanistic understanding of the influence of polarization on dynamics in the ILs investigated. Additionally, the Force Matching (FM) approach was employed to systematically obtain nonpolarizable two-body force fields for several ILs that reproduce as accurately as possible intermolecular forces predicted by the polarizable model. Unlike water, for which the FM approach was found to yield an accurate representation of the liquid phase structure predicted by a polarizable model, the FM approach does not result in a two-body potential that accurately reproduces either structure or dynamics predicted by the polarizable IL model.

Structural and dynamical properties of ionic liquids: The influence of charge location

The properties of ionic liquids depend on the chemical structure of the constituent ions. An important difference between molten inorganic salts and room temperature ionic liquids (RTILs) is that in RTILs the charge is frequently not located at the center of mass. This paper describes a molecular dynamics investigation of the influence of charge location on the structure and transport properties of ionic liquids. The model considered consists of univalent spherical ions with the cation charge moved away from its center of mass. It is shown that the charge location has an important influence on the liquid properties. As the charge is moved off center, the electrical conductivity initially increases, and the shear viscosity decreases. However, when the charge exceeds a certain displacement, this behavior is reversed. With further charge displacement, the conductivity decreases sharply and the viscosity increases rapidly. This behavior reversal can be traced to the formation of directional ion pairs that are present in sufficient numbers, and have lifetimes sufficiently long to strongly influence the liquid properties. We suggest that the influence of directional ion pairing can explain what appear to be anomalously low conductivities and high viscosities observed for some RTILs. The rotational and reorientational motions of the cations are examined, and shown to be strongly influenced by ion-pair formation when the charge is far off center. The temperature dependence of the transport properties is considered for selected systems, and deviations from Arrhenius behavior are found to be most important for the conductivity. Based on our results, this possibly indicates that directional ion pairs create an additional "barrier" to charge transport in some ionic liquids.

A comparative study of two classical force fields on statics and dynamics of [EMIM][BF4] investigated via molecular dynamics simulations

The influences of two different commonly employed force fields on statical and dynamical properties of ionic liquids are investigated for [EMIM][BF(4)]. The force fields compared in this work are the one of Canongia Lopes and Padua [J. Phys. Chem. B 110, 19586 (2006)] and that of Liu et al. [J. Phys. Chem. B 108, 12978 (2004)]. Differences in the strengths of hydrogen bonds are found, which are also reflected in the static ion distributions around the cation. Moreover, due to the stronger hydrogen bonding in the force field of Liu et al., the diffusive motions of cations and anions and the rotational behavior of the cations are slower compared with those obtained with the force field of Canongia Lopes and Padua. Both force fields underestimate the zero-field electrical conductivity, while the experimental dielectric constant can be reproduced within the expected statistical error boundaries.

Structural and dynamical properties of ionic liquids: The influence of ion size disparity

The influence of ion size disparity on structural and dynamical properties of ionic liquids is systematically investigated employing molecular dynamics simulations. Ion size ratios are varied over a realistic range (from 1:1 to 5:1) while holding other important molecular and system parameters fixed. In this way we isolate and identify effects that stem from size disparity alone. In strongly size disparate systems the larger species (cations in our model) tend to dominate the structure; the anion-anion distribution is largely determined by anion-cation correlations. The diffusion coefficients of both species increase, and the shear viscosity decreases with increasing size disparity. The influence of size disparity is strongest up to a size ratio of 3:1, then decreases, and by 5:1 both the diffusion coefficients and viscosity appear to be approaching limiting values. The conventional Stokes-Einstein expression for diffusion coefficients holds reasonably well for the cations but fails for the smaller anions as size disparity increases likely due to the neglect of strong anion-cation correlations. The electrical conductivity is not a simple monotonic function of size disparity; it first increases up to size ratios of 2:1, remains nearly constant until 3:1, then decreases such that the conductivities of the 1:1 and 5:1 systems are similar. This behavior is traced to the competing influences of ion diffusion (enhancing) and ion densities (reducing) on conductivities at constant packing fraction. The temperature dependence of the transport properties is examined for the 1:1 and 3:1 systems. In accord with experiment, the temperature dependence of all transport properties is well represented by the Vogel-Fulcher-Tammann equation. The dependence of the diffusion coefficients on the temperature/viscosity ratio is well described by the fractional Stokes-Einstein relation D proportional to (T/eta)(beta) with beta approximately = 0.8, consistent with the exponent observed for many molten inorganic salts.

On the use of memory functions in the study of the dynamical properties of ionic liquids

The molecular dynamics simulation results for a model of liquid LiBr are used to study the ionic motion in fused salts. It is shown that the particles exhibit a Brownian-like motion which is described by the generalized Langevin equation. The memory function introduced in this treatment is taken as a sum of two exponential terms M (t) = ω12 exp (−‖t‖/τ1)+ω22 exp (−‖t‖/τ2). The parameters are calculated from the velocity autocorrelation functions. The contributions of the two terms to the friction coefficient of the cation ξ =ω21τ1+ω22τ2 are respectively, 65% and 35%. The relaxation times are τ1=0.167 and τ2=2.19, both in units of 10−13s. An attempt is made to interpret the short time behavior in terms of relaxation in the force field of neighbouring particles. The longer decay is attributed to a collective mode of motion. This analysis is applied to the experimental data for physical LiBr with varying 6Li/7Li ratio.