Transport Properties Of Ionic Liquids Research Articles

Viscosity of imidazolium ionic liquids and mixtures of ILs from entropy scaling using the PC-SAFT and ePC-SAFT equations of state

Entropy scaling is a promising method for the development of engineering models for transport properties of ionic liquids (ILs). Here we present two entropy scaling models for the viscosity of ionic liquids and their mixtures at various temperatures and pressures: 1) a molecular-based approach treating each IL as one pure species and using the PC-SAFT equation of state (EoS); and 2) treating each ion as a separate species and using the ePC-SAFT EoS. Both approaches were correlated to viscosity data of 12 ionic liquids from the imidazolium cation class and three different anions comprising 4233 data points with overall average absolute deviations (%AARDs) of less than 8 % over a wide range of temperature and pressures. The ion-based approach was better able to correlate IL viscosity at low temperatures and/or high pressure (to 4011 bar) than the molecular approach. The functional entropy-scaling coefficients for both models were regressed as linear functions of molecular weight, which allowed good prediction of another nine ionic liquids not in the training set. Interestingly, both approaches well predict the viscosity of binary mixtures of imidazolium ionic liquids. In addition, the effect of various reference viscosities and comparison with the Peng-Robinson EoS was performed.

Impact of fluorination on Li+ solvation and dynamics in ionic liquid-hydrofluoroether locally concentrated electrolytes

The thermal, physical, structural, and transport properties of ionic liquid (IL) electrolytes based on n-methyl-n-butylpyrrolidinium bis(trifluoromethanesulfonyl)imide [PYR14][TFSI] and Li-salts of lithium (nonafluorobutane)(trifluoromethanesulfonyl)imide [Li][IM14] and [Li][TFSI] (0 ≤ xLi ≤ 0.3), with addition of 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether (TTE) (0 ≤ xTTE ≤ 0.45) were studied. While [IM14] increases glass transition, TTE improves conductivity over a wider liquidus range. Li+ is solvated primarily by [TFSI] with some contribution by [IM14]. However, both 7Li- 19F HOESY NMR and spatial distribution functions (SDFs) derived from molecular dynamics (MD) indicate short contacts between the fluorines of TTE and Li+. This is a result of tighter anion-solvated Li+ without aggregation of the Li+ solvates, consistent with Raman measurements, thus confirming the existence of fluorous domains in the bulk which leads to improved fluidity and a Li+ diffusivity of 1.26 × 10−12 m2/s at 0 °C (xLi = 0.20) with a Li+ transference of 0.16. Improved transport properties translated to a higher capacity of the TTE/IL electrolyte in a Li||LiFePO4 half-cell with 95mAh/g at 0.1C, compared to the IL electrolyte without TTE (60 mAh/g). This study demonstrates the tunability of solvation and transport properties in IL electrolytes by asymmetric fluorinated anions and hydrofluoroether co-solvents for low temperature Li-ion batteries.

Molecular dynamic study on the transport properties of ionic liquids in ZTC porous carbon materials

The adsorption and diffusion behavior of ions within electrodes have a significant impact on the energy density and charging/discharging speed of supercapacitors. Zeolite-Templated Carbon (ZTC) material, a type of porous carbon material characterized by well-defined pore shape and spatial structure, is recognized as a promising electrode material owing to its numerous advantages. This study employs molecular dynamics simulations to investigate the diffusion and adsorption profiles of ionic liquids [EMI][BF4] in different types of ZTC materials as electrodes. Additionally, the effect of ion size, pore dimension, pore shape, and the properties of the adsorbed ions are discussed. The degree of confinement (DOC) and ion diffusivity are thoroughly analyzed to elucidate the transport behavior of ions within the ZTC pores. The results demonstrate that the diffusion coefficient and degree of confinement of ionic liquids predominantly depend on the anion size and the pore size of the porous carbon. Surprisingly, in certain cases, larger anions exhibit enhanced diffusion ability and adsorption effect. These findings unravel the underlying transport mechanism of ions in ZTC materials and provide valuable theoretical insights for the application of ZTC materials as electrodes in the field of supercapacitors.

Liquid Structures and Ion Dynamics of Ionic Liquids Viewed from Intermolecular Interactions.

The elucidation of the factors determining liquid structures and transport properties of ionic liquids is important for the design and development of ionic liquid electrolytes. This personal account introduces the importance of computational methods for studying ionic liquids. Molecular dynamics simulations provide detailed information on liquid structures of ionic liquid such as the structures of solvated cation complexes in equimolar mixtures of glymes and Li[TFSA] and the effects of the charges of electrode on liquid structure near the electrode. Ab initio calculations reveal that the magnitude of the attraction between ions and conformational flexibility ions play important roles in determining transport properties of ionic liquids. First principle molecular dynamics simulations elucidate why solvated cation complex is stable in the equimolar mixtures, although the Li+ -[TFSA]- interaction is greater than Li+ -glyme interaction.

Study of Thermodynamic Properties of l-Serine and l-Threonine in Aqueous Solutions of 1-(2-Carboxyethyl)-3-methylimidazolium Chloride [HOOCEMIM][Cl

The stability of amino acids in solutions containing electrolytes and the effect of ambient pH on their biological activity is an important research area. In this research work, the volumetric and transport properties of ionic liquid (IL) aqueous solution with special performance of 1-(2-carboxyethyl)-3-methylimidazolium chloride ([HOOCEMIM][Cl]) were studied. The selected IL acts as an electrolyte and allows the study of systems containing amino acids at acidic pH. In this work, the density, speed of sound and viscosity of binary and ternary solutions of IL in aqueous solution of amino acids in the structure of proteins, namely l-serine and l-threonine within IL molality range of (0.05, 0.07 and 0.09 mol·kg−1) were measured at 298.15 K. From these quantities, apparent molar volume, limiting apparent molar volume, apparent molar isentropic compression, limiting apparent molar isentropic compression, transfer standard volumes for amino acids from water to the aqueous IL solutions and viscosity B-coefficients were calculated using the equations of Redlich–Meyer and Jones–Dole. Close examination of literature indicates that there is no data for water activity for solution of l-serine + H2O at 308.15 and 318.15 K. In this way water activity, osmotic coefficient and vapor pressure were measured and using the interaction parameters of Wilson, NRTL, NRF-NRTL and UNIQUAC models, activity coefficient values of l-serine calculated.

(Invited) Understanding Transport Properties of Ionic Liquids By Targeted Modifications of Ion Structures

Ionic liquids have been of great interest throughout this century and continue to find new applications in both academia and industry. This is largely due to their great versatility, with millions of potential combinations of cations and anions, which has led to them being described as 'designer solvents'. However, this leads to its own problems when it comes to choosing the optimum ionic liquid for any given application. To fully exploit this versatility, it is imperative to understand how the properties of ionic liquids arise from their constituent ions.To derive meaningful structure-property relationships for ionic liquids is a complex task. It can be particularly difficult to isolate the cause of any property change arising from even simple modifications of ions. For example, changing the length of the alkyl chain of a cation in an ionic liquid also changes the volume and mass of the ion, all of which can affect a property such as ion mobility and consequent properties such as the viscosity and conductivity of the ionic liquid. A ‘targeted modification’ is a deliberate change in the structure of an ionic liquid ion that is designed to minimise the introduction of such confounding factors. The change must isolate what is of interest, so that we can study only one concept at a time. Once these structure-property relationships have been clearly defined, it will be possible to use these as design elements for new ionic liquids.As well as experimental observations, it is possible to apply the targeted modification approach to a computer simulation. This provides both insight into experimental observations and predictions of potential targeted modifications before complex synthesis is attempted. Together, theory and experiment will enable the concept of ionic liquids as designer solvents to be realised.In this presentation we apply the targeted modifications approach to the transport properties of ionic liquids.

The refined determination of the ion pair lifetimes in ionic liquids

Assessing lifetimes of liquid’s local structures could bring impactful output. For example, the key transport properties of ionic liquids are governed by local structure dynamics to a large extent. Due to long-range Coulomb interactions, these systems show pronounced charge ordering. In this case, the local structure breaking rate could be described in terms of cation–anion pair lifetimes. Conventional pair lifetime analysis relies on geometrical criterion, defining the maximum distance from cation to anion in pair. In this study, we argue towards additional restriction to ensure pair stability. Based on vibrational properties, we introduce lower boundary for pair lifetime to exclude false pair recognition in the case of collision-like interaction. Also, we put forward a computational algorithm which allows obtaining both mean lifetime and lifetime distribution. The calculations of mean pair lifetimes in molten alkali halides support our reasoning.

Influence of Cation Size and Polarity on Charge Transport in Ionic Liquid Based Electrolytes

AbstractImidazolium‐based ionic liquids (ILs) with allyl and ether side chains were synthesized and characterized. Comprehensive structural and photoelectrochemical characterizations were performed, transport properties of ILs were also examined as electrolyte components in dye sensitized solar cells (DSSCs). The properties of synthesized materials and DSSC performances were compared with 1‐propyl‐3‐methyl imidazolium iodide (PMII) and 1‐allyl‐3‐ethyl imidazolium iodide (AEII) as reference ILs. Ionic conductivities, diffusion coefficients and charge transfer resistances of synthesized ionic liquids were investigated on DSSCs by Electrochemical Impedance Spectroscopy (EIS). The diffusion coefficient values of triiodide ions in different ionic liquid‐based electrolytes were measured by the means of diffusion limited current density method and found to be 1.75×10−7 cm2 s−1 and 2.05×10−7 cm2 s−1 with corresponding photocurrent densities of 10.38 mAcm−2 and 12.13 mAcm−2 for the reference AEII and PMII based electrolytes, respectively. However, for the electrolytes of 1‐(2‐methoxyethyl)‐3‐allyl imidazolium iodide and 1‐allyl‐3‐methyl imidazolium iodide ionic liquids, these values were found to be 0.86×10−7 cm2 s−1 and 0.57×10−7 cm2 s−1 with photocurrent densities of 9.53 mAcm−2 and 8.98 mAcm−2, respectively. Allyl and ether substituted imidazolium ILs exhibited promising results as potential alternative electrolyte materials for DSSCs.

Flexibility is the key to tuning the transport properties of fluorinated imide-based ionic liquids.

Ionic liquids are becoming increasingly popular for practical applications such as biomass processing and lithium-ion batteries. However, identifying ionic liquids with optimal properties for specific applications by trial and error is extremely inefficient since there are a vast number of potential candidate ions. Here we combine experimental and computational techniques to determine how the interplay of fluorination, flexibility and mass affects the transport properties of ionic liquids with the popular imide anion. We observe that fluorination and flexibility have a large impact on properties such as viscosity, whereas the influence of mass is negligible. Using targeted modifications, we show that conformational flexibility provides a significant contribution to the success of fluorination as a design element. Contrary to conventional wisdom, fluorination by itself is thus not a guarantor for beneficial properties such as low viscosity.

Effect of increasing the cation chain length on thermodynamic and transport properties of ionic liquids using molecular dynamics

In this study, Molecular Dynamics (MD) simulations are employed to observe the effect of different chain lengths on density, self-diffusion coefficient and viscosity of [Cnmim][TF2N] ionic liquids (ILs). Both density and self-diffusion coefficient decrease as the chain length increases. The rheological curves show a shear-thinning after Newtonian behavior. The results suggest that viscous thinning is related to the decrease in the number of hydrogen bonds (H-bonds) and the alignment of the alkyl chains in the flow direction. Viscosity values at zero-shear-rate increase as the chain size does. Moreover, the static structure factor indicates anisotropy in systems with cation alkyl chains of 6 and 8 carbon atoms. However, in a 10 carbon atom cation alkyl chain, a more ordering is observed.

Effect of water on microstructure and transport properties of ionic liquids

Effect of water on microstructure and transport properties of ionic liquids

(Invited) Relating Ion Structures and Transport Properties of Ionic Liquids

Whether used as electrolytes or solvents for synthesis both the bulk viscosity and the microscopic dynamics of ions in ionic liquids often determine the applicability of these. Hence, it is important to understand the connection between the structures of ionic liquid ions and these key properties. Features that contribute to these behaviours include ion mass, size, interactions, and flexibility and the ability of the ions to give rise to nanoscale segregation of charged and nonpolar domains.In this work, we combine experimental and theoretical studies of a wide range of ionic liquids composed of structurally similar ions in an attempt to separate the relative contributions of these features to the transport properties of ionic liquids. We target a number of design parameters, such as the ions' central atoms (e.g., ammonium or phosphonium cations), ion substitution (e.g., alkyl chain length and degree of substitution of cations and fluorination of the anion).In addition to the direct effects of structural changes to an individual ion, we show that the significance of these changes is often moderated by the nature of the counter ion of the ionic liquid. We also demonstrate that targeted design of functional groups based on structure-property relations can yield ionic liquids of exceptionally high fluidity.

Transport Properties of Ionic Liquid and Sodium Salt Mixtures for Sodium-Ion Battery Electrolytes from Molecular Dynamics Simulation with a Self-Consistent Atomic Charge Determination.

Ionic liquid (IL) has been considered as a potential electrolyte for developing next-generation sodium-ion batteries. A highly concentrated ionic system such as IL is characterized by the significant influence of intramolecular polarization and intermolecular charge transfer that vary with the combination of cations and anions in the system. In this work, a self-consistent atomic charge determination using the combination of classical molecular dynamics (MD) simulation and density functional theory (DFT) calculation is employed to investigate the transport properties of three mixtures of ILs with sodium salt 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 self-consistent method is versatile to address the intramolecular polarization and intermolecular charge transfer in response to the cation-anion combination, as well as the variation in their compositions. The structure and dynamic properties of IL mixtures obtained from the method are in line with those from the experimental works. The comparison to the Nernst-Einstein estimates shows that the electrical conductivity is reduced due to correlated motions among the ions, and the contribution to the conductivity from each ion species is not necessarily ranked in the same order as the diffusion coefficient. It is further seen that the increase of the sodium-ion composition reduces the fluidity of the system. The results highlight the potential of the method and the microscopic description that it can provide to assist the investigation toward a sensible design of IL mixtures as an electrolyte for a high-performance sodium-ion battery.

Physical and transport properties of ionic liquids using geometric similitude and a cubic equation of state. Part 2: Thermal conductivity, and speed of sound of water + ionic liquid mixtures

The general equation of state based on the concept of geometric similarity presented in Part 1 of this series is extended to correlate and predict the thermal conductivity and the speed of sound of binary water + ionic liquid mixtures. The extension considers the classical mixing and combining rules commonly used in studies and applications of pressure-temperature-volume equations of state. Data on thermal conductivity and speed of sound of binary water + ionic liquids mixtures were gathered from the literature in the temperature range from 278 K to 343 K at atmospheric pressure. A total of 77 data for the thermal conductivity and 1560 data for the speed of sound for 28 binary mixtures are included in the study. The results of the extension to mixtures show that the empirical equation of state model provides adequate and consistent values using pure component data only, proving its good predictive capability.

Physical and transport properties of ionic liquids using the geometric similitude concept and a cubic equation of state. Part 1: Thermal conductivity and speed of sound of pure substances

A model based on the geometric similitude concept recently proposed by the authors is extended to calculate the thermal conductivity and the speed of sound of ionic liquids. Experimental data for these properties are employed to determine the constants and the parameters of the proposed general model. The experimental data are taken from ionic liquid database provided by the National Institute of Standards and Technology (NIST). In total, 320 thermal conductivity data (40 ionic liquids) and 2572 speed of sound data (61 ionic liquids), at the temperature range of 263.15 K to 390.15 K and pressure from 0.86 up to 2000 bar have been considered. The proposed empirical equation of state model contains only one adjustable parameter for each ionic liquid, parameter that can be determined using a single experimental data or can be generalized in terms of other available properties, as proposed in this work. The results show that the equation of state model provides acceptable results for the correlation and prediction of the thermal conductivity and the speed of sound for the studied pure ionic liquids.

How to detect possible pitfalls in ePC-SAFT modelling: Extension to ionic liquids

An ion-specific electrolyte perturbed-chain statistical associating fluid theory (ePC-SAFT) has been developed to describe the thermodynamic and transport properties of ionic liquids (ILs) for developing IL-based technologies to separate CO2 from gas mixtures. However, as it has been pointed out previously, SAFT-based models can lead to a pitfall, manifesting as an additional fictitious liquid-liquid critical line connected to an additional mechanically stable critical point when modeling pure substances. In this work, a method for detecting a pitfall in ILs was developed, where an expression with ionic term for calculating the critical point with ePC-SAFT was derived and an algorithm for detecting the additional fictitious phase equilibrium for ILs was proposed. The pitfall occurrence for an extended set ILs taken from our previous work was investigated. It shows that a pitfall occurs only for one single IL among all 96 ILs in the temperature and pressure range of interest. For [C8mpy][BF4], at 290.27–291.78 K, a pitfall may occur at the pressures of interest; at 273.15–290.20 K, it may occur at any given pressure. The ion-specific ePC-SAFT parameters for [C8mpy]+ may therefore need to be modified in the future when more reliable experimental results are available for parameter fitting.

Self-Consistent Scheme Combining MD and Order-N DFT Methods: An Improved Set of Nonpolarizable Force Fields for Ionic Liquids

The nonpolarizable force field of ionic liquids is tuned by using the self-consistent scheme of molecular dynamics (MD) simulation and first-principles calculation based on the order-N density functional theory (DFT). The atomic charges are determined by using the whole MD cell for DFT calculation and accounts effectively for the many-body effects of charge transfer and intramolecular polarization. The charges represent effective interactions in the condensed phase within the framework of the nonpolarizable force field and can be an alternative for an explicitly many-body model incorporating, for example, polarizability. Here we demonstrate the performance of nonpolarizable force field determined with the MD-DFT self-consistent scheme in imidazolium-, pyrrolidinium-, and ammonium-based ionic liquids. The variation ranges of molecular charges are much larger with the compositions of the ionic liquid than with the thermodynamic conditions, and the charge-ordering structures become systematically weaker with the effective charges. For energetic properties, while the calculated heat of vaporization depends on the atomic and molecular charges, the corresponding heat capacity is not strongly affected by the DFT-based variation. For transport properties, the self-diffusion coefficient, electrical conductivity, and viscosity vary much more in the self-consistent scheme. The effective DFT charge is observed to enhance the fluidity of ionic liquids and improve the accuracy of electrical conductivity and viscosity. This is due to the weakened interactions among the ions, and the too slow motions observed with a full-charge model are well corrected through the iteration of MD and DFT. We therefore conclude that the set of nonpolarizable force fields obtained with the MD-DFT self-consistent scheme leads to better description of transport properties of ionic liquids.

Effects of the Water Content on the Transport Properties of Ionic Liquids

Ionic liquids (ILs) have been applied in many fields in recent years, but the high viscosity of ILs has become an important factor hindering the further implementation of ILs. ILs/water binary mixtures often show a lower viscosity than pure ILs. However, the general effects of the water content on the viscosity remain unclear. In this work, the viscosities and self-diffusivity of a series of [Cnmim][Ac] (1-alkyl-3-methyl imidazole acetate) ILs were computed by molecular dynamics simulations with a water mass percentage of 0.02–10%. The results show a general rule that the viscosities of all [Cnmim][Ac] ILs (n = 1–6) first increase and then decrease by adding a certain amount of water, which correlate well with the corresponding reciprocals of self-diffusion coefficients. Interestingly, the inflection points of the viscosity gradually move forward in the direction of a lower water content when increasing the chain length, which is the result of increased hydrophobicity. Structural analysis shows that the chainlike structure of anion···water···anion is the key reason for the increase-to-decrease behavior of viscosity when water is added. Moreover, the sharp peaks of radial distribution functions between water and Ac– go up first and then move down, and the peak positions follow the same trend of inflection points.

Thermodynamic and transport properties of ionic liquids, 1-alkyl-3-methylimidazolium thiocyanate in the aqueous lithium halides solutions

Thiocyanate based ionic liquids, [RMIM][SCN], as fluorine free and low toxic were used effectively in different areas of resaerch. In this work, the thermodynamic and transport properties of aqueous solutions of the synthesized ionic liquids based on thiocyanate anion, [RMIM][SCN] (R = butyl, hexyl and octyl) have been investigated at T = (288.15–318.15) K. Also, effect of LiX (X = Cl−, Br−) salts on the investigated properties have been studied at T = 298.15 K. In this respect, density, speed of sound and viscosity of the solutions have been measured and these properties were used to calculate the apparent molar volume, Vφ, and apparent molar isentropic compressibility κφ. The properties were correlated with corresponding empirical relations to achieve standard partial molar volume, Vφ0, partial molar isentropic compressibility, κφ0, and viscosity B-coefficient. These parameters were used to analysis of the interactions between components. In order to separate Vφ0 contributions of the ILs in binary solutions, the scaled particle theory was used at T = (288.15–318.15) K. The results show that the hydrophobic hydration interactions are dominant interactions.

Assessment of thermal and transport properties of ionic liquids as suitable absorbent for absorption cooling applications

Ionic liquids, salts which have liquid phase at temperature below 100°C, have been widely introduced in engineering applications, including absorption cooling systems. The application of ionic liquids in absorption cooling systems is intended to remove the disadvantages of conventional working fluids such as corrosion and crystallization. In terms of thermodynamic performances, theoretical investigation based on solubility behavior of ionic liquids in natural refrigerants show a competitive performance in comparison with conventional working fluid. Nevertheless, heat transfer performance, which is also an important key in absorption cooling systems, particularly in terms of system design and size, needs to be deeply explored and investigated. This study aims to assess the thermal and transport properties of ionic liquids as absorbent in relation to the heat and mass transfer characteristics of these working fluids. The thermal and transport properties of ionic liquids proposed as absorbent for absorption cooling systems are collected, and heat and mass transfer characteristic of these ionic liquids based on their thermal and transport properties are investigated and analyzed. Finally, the most suitable ionic liquids for absorption machines, both in terms of thermodynamics and heat and mass transfer performances can be proposed.