IL Mixtures Research Articles

Synergism in the solvation of solvatochromic probes in binary mixtures with ionic liquids

The preferential solvation of two perichromic dyes, 4-amino-N-methylphthalimide (4-AMP) and Nile Red (NR) in binary mixtures of an ionic liquid ([bmim][X], where X = BF4, PF6, Tf2N and DCA) with a protic or non-protic solvent, was investigated, in search of examples of a synergetic or “hyperpolar” behavior. Synergism was generally observed in those mixtures where the two pure components had similar polarity values, according to the employed probe. A parallel was drawn between the observed synergism in these IL mixtures and the phenomenon of hydrotropism, also observed with organic solutes in aqueous IL mixtures. More generally, in binary mixtures with organic solvents, ionic liquids can act as solvatotropes, promoting the dye solvation by formation of solute-specific aggregates with all species present in the mixture. By stabilizing the dipolar excited state of 4-AMP or NR through hydrogen-bonds and electrostatic interactions in these aggregates, a “hyperpolar” milieu is generated, which surpasses the polarities of the two pure components. Solvatotropism seems to offer a more realistic picture than the solvent-exchange model, originally developed for simpler systems, to the phenomenon of preferential solvation in binary mixtures with ionic liquids.

Investigation of Nonidealities through an Extended Flory–Huggins Model of Ionic Liquid Mixtures with Ammonia

The phase behavior of ionic liquids and ammonia is critical due to their application in various fields such as sorption-based heating/cooling cycles and direct ammonia capture. To date, various ionic liquids have been developed and investigated to enhance the solubility of ammonia without compromising the reversibility due to recycling concerns and various thermodynamic models have been used to correlate the experimental vapor–liquid equilibria data. Surely, high ammonia sorption capacities are required and understanding the interaction between ammonia and ionic liquids can lead to synthesizing targeted ionic liquids for ammonia. In this study, we developed an extended Flory–Huggins model on ammonia and imidazolium-based ionic liquid systems, where both combinatorial and residual terms are considered, to understand the interaction between ammonia and ionic liquids. The results showed that the nonidealities in ammonia and imidazolium-based IL mixtures are due to both entropic and enthalpic contributions. Therefore, increasing the size of the IL would not be enough to achieve higher solubilities. A deeper understanding of the molecular-level interaction between ionic liquids and ammonia can be investigated via molecular simulations.

Size-Polydisperse Model Ionic Liquid in Bulk.

The static and the dynamic properties of a size-polydisperse model ionic liquid is studied using molecular dynamics simulations. Here, the size of the anions is derived from a Gaussian distribution while keeping cation size fixed, resulting in a system that closely corresponds to IL mixtures with a common cation. We systematically explore the behavior of thermodynamic transition temperatures, spatial ordering of ions, and the resulting screening behavior as a function of polydispersity index, δ. We observe a nonmonotonic dependence of transition temperatures on δ, and this nonmonotonic behavior is also reflected in other properties such as screening length. Furthermore, from the radial distribution function analysis it is found that, upon varying δ, the spatial ordering of cations is affected, while no such changes is seen for anion. On the other hand, the analysis of ion motion through mean-square displacement show that for all δ values considered both inertial and diffusive regimes are observed (as expected in the liquid state). However, in contrast to the neutral counterpart, the overall relaxation time of the polydisperse IL system increases (and hence decreasing diffusion coefficient) with increasing δ.

Dey-Biswas equation is highly effective for viscosity prediction of binary and multicomponent liquid mixtures including Ionic liquid mixtures

The recently developed Dey - Biswas has been put to test to demonstrate its consistency and efficacy on a wide range of organic liquid and Ionic liquid mixtures. It has also been tested over a broad range of temperatures to check its robustness since viscosity as a property is highly sensitive to temperature variations. This equation has been tested on 200 binaries, 30 ternaries, and 7 quaternary mixtures at varying temperatures. The results obtained have then been compared with eight well-established and some recently developed approaches. Furthermore, 2 recently developed approaches in 2021 and 2022 have also been included in the comparative study. The findings have been discussed by comparing the Average Absolute Percentage Deviations (AAPD) from the experimental values as a benchmark of predictive efficacy and consistency. The Grand AAPDs indicate that the Dey-Biswas approach is clearly superior being way ahead of its competing counterparts.It is seen to be performing extremely well even in the case of the IL mixtures.

On-Surface Metathesis of an Ionic Liquid on Ag(111).

We investigated the adsorption, surface enrichment, ion exchange, and on‐surface metathesis of ultrathin mixed IL films on Ag(111). We stepwise deposited 0.5 ML of the protic IL diethylmethylammonium trifluoromethanesulfonate ([dema][TfO]) and 1.0 ML of the aprotic IL 1‐methyl‐3‐octylimidazolium hexafluorophosphate ([C8C1Im][PF6]) at around 90 K. Thereafter, the resulting layered frozen film was heated to 550 K, and the thermally induced phenomena were monitored in situ by angle‐resolved X‐ray photoelectron spectroscopy. Between 135 and 200 K, [TfO]− anions at the Ag(111) surface are exchanged by [PF6]− anions and enriched together with [C8C1Im]+ cations at the IL/vacuum interface. Upon further heating, [dema][PF6] and [OMIm][PF6] desorb selectively at ∼235 and ∼380 K, respectively. Hereby, a wetting layer of pure [C8C1Im][TfO] is formed by on‐surface metathesis at the IL/metal interface, which completely desorbs at ∼480 K. For comparison, ion enrichment at the vacuum/IL interface was also studied in macroscopic IL mixtures, where no influence of the solid support is expected.

Anomalously high solubility behavior of methanethiol in alkylimidazolium–based ionic liquids

In this work, the absorption capacity of four 1-butyl-3-methylimidazolium ([C4mim]+) ionic liquids containing tetrafluoroborate ([BF4]–), hexafluorophosphate ([PF6]–), trifluoromethanesulfonate ([OTf]–), and bis(trifluoromethyl)sulfonylimide ([Tf2N]–) anions for methanethiol has been experimentally and theoretically studied. The experimental measurements were conducted below 100 kPa and temperatures ranging from (303.15 to 343.15) K. Henry’s law constants, as well as the thermodynamic functions for dissolution of methanethiol in the ionic liquids were evaluated using the experimental solubility data and compared with the corresponding data for the solubility of hydrogen sulfide reported in the literature. The solubility of CH3SH was found to be unexpectedly higher (1.3 to 2.9 times) than that of H2S in the ionic liquids studied in this work. The comparison of Henry’s constants showed that the solubility of CH3SH in the ionic liquids follows the order: [C4mim][Tf2N] > [C4mim][PF6] ≈ [C4mim][BF4] > [C4mim][OTf]. The measured data for the solubility of CH3SH in the ionic liquids were correlated using both the γ – φ approach, based on the Pitzer’s activity coefficient model, and the φ – φ approach, on the basis of the generalized Redlich-Kwong cubic equation of state (GRK EoS). Surprisingly, the GRK EoS indicated a better correlative accuracy compared to the Pitzer’s model for the four CH3SH + IL mixtures studied in this work. Quantum chemical density functional theory (DFT) as well as atoms-in-molecules (AIM) calculations, based on M06-2X level of theory, using the 6–311++G(2d,2p) basis set, was employed to explore the irregularly high solubility behavior of CH3SH compared to H2S in the ionic liquids studied in this work. According to DFT calculations CH3SH establishes a stronger interaction with the ionic liquids compared to H2S. In addition, the interaction of the cation with the anions diminishes in the presence of CH3SH compared with H2S, which results in greater void volume available to CH3SH guest molecules compared to H2S.

Synergistic Effect of Ionic Liquid (Il) Cation and Anion Inhibits Negative Difference Effect on Mg in Water - Il Mixtures

Synergistic Effect of Ionic Liquid (Il) Cation and Anion Inhibits Negative Difference Effect on Mg in Water - Il Mixtures

Binary mixtures of ionic liquids: Ideal, non-ideal, or quasi-ideal?

The mixing of ILs provides an opportunity for fine tuning the physiochemical properties of ILs for various applications. However, a suitable mixture having desired properties can only be designed when the physiochemical properties of the mixtures of ILs along with their spectroscopic properties are well understood. With an aim to achieve this objective, three different mixtures with a common anion, namely, [C2C1im][C4C1im][NTf2], [C3C1pyr][C4C1pyr][NTf2], and [C3C1im][C3C1pyr][NTf2], have been investigated in the current study. Investigations have been carried out at the macroscopic level by observing the thermophysical properties, such as molar volume and thermal expansion coefficient, and at the microscopic level with time-resolved fluorescence measurements and the pulse field gradient nuclear magnetic resonance (NMR) technique. The results obtained from the thermophysical study have indicated that excess molar volume for imidazolium-based IL-IL mixtures may be linked to the free volume created by the alkyl chain of the imidazolium cation whereas for the mixture of pyrrolidinium ILs, lowering of density can give rise to free volume. Analysis of time-resolved fluorescence anisotropy data has provided clear evidence in favor of the presence of free volume in the binary mixture of ILs. NMR studies have also supported the fluorescence anisotropy data. The outcome of the present investigation reveals that the mixtures show appreciable deviation from ideal behavior and the deviation from the ideal behavior is caused due to the generation of free volume in the resultant mixture, describing these IL mixtures as quasi-ideal rather than ideal or non-ideal.

Enrichment effects of ionic liquid mixtures at polarized electrode interfaces monitored by potential screening

The behavior of ionic liquids (ILs) at charged interfaces is pivotal for their application in supercapacitors and electrochemical cells. Recently, we demonstrated for neat ILs that potential screening at polarized electrode interfaces shows a characteristic voltage dependence, as determined in situ by X-ray photoelectron spectroscopy. Herein, we use this fingerprint-type behavior to characterize the nature of the IL/electrode interfaces for IL mixtures of [C8C1Im][Tf2N] and [C8C1Im]Cl on Au and Pt electrodes. For Au, the IL/electrode interfaces are dominated by the Cl− anions, even down to a 0.1 mol% [C8C1Im]Cl content. In contrast, [Tf2N]− anions enrich at the IL/Pt electrode interfaces down to 10 mol% [C8C1Im][Tf2N]; only at lower concentrations does a transition to Cl− enrichment occur. These mixture studies demonstrate that even small concentrations of another IL or contamination, e.g. remaining from synthesis, can strongly influence the situation at charged IL interfaces.

Current Status of AMOEBA-IL: A Multipolar/Polarizable Force Field for Ionic Liquids.

Computational simulations of ionic liquid solutions have become a useful tool to investigate various physical, chemical and catalytic properties of systems involving these solvents. Classical molecular dynamics and hybrid quantum mechanical/molecular mechanical (QM/MM) calculations of IL systems have provided significant insights at the atomic level. Here, we present a review of the development and application of the multipolar and polarizable force field AMOEBA for ionic liquid systems, termed AMOEBA–IL. The parametrization approach for AMOEBA–IL relies on the reproduction of total quantum mechanical (QM) intermolecular interaction energies and QM energy decomposition analysis. This approach has been used to develop parameters for imidazolium– and pyrrolidinium–based ILs coupled with various inorganic anions. AMOEBA–IL has been used to investigate and predict the properties of a variety of systems including neat ILs and IL mixtures, water exchange reactions on lanthanide ions in IL mixtures, IL–based liquid–liquid extraction, and effects of ILs on an aniline protection reaction.

Computational analysis of conductivity contributions in an ionic liquid mixture of 1-ethyl-3-methylimidazolium dicyanamide and tetrafluoroborate

Ionic liquid mixtures not only possess the well-known advantages of pure ionic liquids, but allow for a continuous and very exact fine-tuning of physico-chemical properties. Molecular dynamics simulations are a valuable tool for understanding the molecular processes that govern the behavior of IL mixtures. Considering electrochemical applications, especially the accurate prediction of dynamics is of great interest. In this work, we investigate mass and charge transport properties of binary mixtures of 1-ethyl-3-methylimidazolium tetrafluoroborate [C2mim][BF4] and 1-ethyl-3-methylimidazolium dicyanamide [C2mim][N(CN)2]. Due to the high statistical accuracy required for the reliable calculation of transport properties, we employ a coarse-grained model of the ionic liquids which is validated by comparison to experimental data. Via calculation of the non-Gaussian parameter, we find that the dynamics in this ionic liquid mixture are heterogeneous for all ion species. To elucidate how the constituents of the mixture influence overall dynamics, separate contributions of the ion species to the total conductivity are analyzed.

Densities and viscosities for ionic liquids [BMIM][BF4] and [BMIM][Cl] and their binary mixtures at various temperatures and atmospheric pressure

Abstract The density and viscosity of 1-butyl-3-methylimidazolium tetrafluoroborate [BMIM][BF4] and 1-butyl-3-methylimidazolium chloride [BMIM][Cl] and their binary mixtures within the temperatures from 303.15 K to 323.15 K and at ambient pressure were determined in this work. The temperature dependences of density and viscosity were satisfactorily described with the linear model and the Vogel-Tammann-Fulcher type equation, respectively. The molar volume and viscosity of binary IL mixtures were predicted through ideal mixing rules showing that almost null deviations for IL mixtures were observed and their mixing was remarkably close to linear ideal behavior in the molar volumes, while comparatively large errors in viscosity occurred. Additionally, the molar volume of the investigated pure ILs and their mixtures could well be predicted by a predictive model presented by Valderrama et al. (Fluid Phase Equilib., 275 (2009) 145).

Neat ionic liquids versus ionic liquid mixtures: a combination of experimental data and molecular simulation.

Simple mixtures of ionic liquids (IL-IL mixtures) can become a promising approach for the substitution of task-specific ILs. Such a concept was explored in this article by comparison of the thermophysical properties of neat 1-ethyl-3-methylimidazolium 2,2,2-trifluoromethylsulfonyl-N-cyanoamide, [C2mim][TFSAM], and equimolar mixtures of two structurally similar ILs having more common ions. Molecular dynamics (MD) simulations were additionally used to further highlight structural aspects of these systems at a molecular level.

Understanding the Microscopic Behavior of Binary Mixtures of Ionic Liquids through Various Spectroscopic Techniques.

In recent times, it has been shown that certain binary mixtures of pure ionic liquids having appropriate chemical composition can behave like a new chemical entity. However, current knowledge about the microscopic behavior of these interesting systems is rather limited. The present study is undertaken with an objective to understand the microscopic behavior in terms of intermolecular interaction, structure, and dynamics of these systems. In the present study, few (IL + IL) mixtures are chosen with a common cation and a variation of anion. The investigations are also carried out by taking individual pure ILs so that the difference in the behavior of pure IL and (IL + IL) mixtures is understood. Initially, the systems have been investigated by studying the thermophysical properties of the concerned mixtures. The synergistic effect between combining pure ILs through photochromism has also been studied. These mixtures have been investigated further through steady-state and time-resolved fluorescence spectroscopy, electron paramagnetic resonance (EPR), nuclear magnetic resonance (NMR), and fluorescence correlation spectroscopy (FCS). Interestingly, time-resolved fluorescence data also pointed out that (IL + IL) mixtures are not only spatially heterogeneous but also dynamically heterogeneous. EPR measurements have suggested that the micropolarity ( ET(30)) of the (IL + IL) mixture is close to that of aliphatic polyalcohols. Measurements of translational diffusion coefficients of the diffusing species through NMR and FCS studies have provided an idea about the nanostructural organization within (IL + IL) binary mixtures. The analysis of data essentially reveals that the mixtures of ILs that are used in the current study do not behave like a nonideal solution. The behavior of the IL mixtures is observed to be more like quasi-ideal type.

Measurement and correlation of CO2 solubility in 1-butyl-3-methylimidazolium ([BMIM]) cation-based ionic liquids: [BMIM][Ac], [BMIM][Cl], [BMIM][MeSO4

The solubility of CO2 in three different 1-butyl-3-methylimidazolium ([BMIM]) cation-based ionic liquids: 1-butyl-3-methylimidazolium acetate ([BMIM][Ac]), 1-butyl-3-methylimidazolium chloride ([BMIM][Cl]), and 1-butyl-3-methylimidazolium methyl sulfate ([BMIM][MeSO4]) was investigated. The CO2 solubility was determined by the measurement of the bubble-point or cloud-point pressure of the CO2 + IL mixtures. The temperature and pressure ranges were 303.15–403.15 K and 0.39–34.55 MPa, respectively.The results of this study show that the CO2 solubility in ILs were strongly affected by the different type of anions and also increased with pressure, and decreased with temperature. The sequence of magnitude of the CO2 solubility was [BMIM][Ac] >> [BMIM][MeSO4] > [BMIM][Cl]. For correlating the experimental data, we used the PR-EoS with one fluid mixing rule and the modified Lydersen-Joback-Reid method. The average absolute deviations of pressure (AAD-P) were 0.0335 for CO2 + [BMIM][Ac], 0.0128 for CO2 + [BMIM][Cl], and 0.0213 for CO2 + [BMIM][MeSO4] systems.

Influence of the anion on the properties of ionic liquid mixtures: a molecular dynamics study.

Mixing of ionic liquids provides new opportunities for their tuning, enabling the applications of ionic liquid mixtures to expand. At the same time, the genesis of the fundamental properties of ionic liquid mixtures is still poorly understood. In this study we carried out a molecular dynamics simulation of binary mixtures of 1-buthyl-3-methylimidazolium hexafluorophosphate, 1-buthyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, and 1-buthyl-3-methylimidazolium tris(perfluoroethyl)trifluorophosphate ([C4mim][PF6] + [C4mim][NTf2], [C4mim][PF6] + [C4mim][FAP], [C4mim][FAP] + [C4mim][NTf2]) in a wide concentration range at 303.15 K and complemented it with quantum mechanical calculations. Three pure ionic liquids underwent the same kind of analysis for comparison purposes. We found that the addition of the [FAP]--anion to a mixture enhances the segregation of non-polar domains and weakens the hydrogen-bond network. The H-bonds in the studied mixtures are rather weak, as follows from QTAIM analysis, with the rarest occurrence for the [FAP]--anion. The competition of two anions in the mixtures for the most acidic hydrogen of the 1-butyl-3-methylimidazolium cation is reported. In most of the cases, the smaller anion ([PF6]- or [NTf2]-) with stronger charge concentration displaces the bigger one ([NTf2]- or [FAP]-) from the preferred coordination site. The existing nano-segregation in some mixtures notably slows down ion diffusion. Our results show that the differences in anion size, shape and nature are the main reasons for nano-segregation and the non-ideal behavior of ionic liquid mixtures.

Sorbent Properties of Halide-Free Ionic Liquids for Water and CO2 Perfusion

Aqueous solutions of lithium halides are state-of-the-art sorbents in sorption-based heat pump and heat storage devices, with the drawbacks of high corrosivity and limited natural resources of lithium. Halide-free ILs could be well-suited alternative sorbents and have been explored with isothermal titration calorimetry showing superior thermochemical performance. Here, we have explored the performance of selected halide-free ILs in perfusion measurements monitoring the uptake of water vapor and CO2 from an air stream with well-defined flow and humidity as a model for open sorption processes. We have determined the saturation vapor pressure of water in the corresponding water–IL mixtures and moreover determined the diffusion coefficients of molecular species in binary [EMIM][Ac]–water mixtures via pulsed-field-gradient nuclear magnetic resonance (PFG NMR) measurements. These, together with variation of the absorption rates with calorimetric monitoring, indicate that diffusion is approximately 3 orders of m...

A Surprising Failure Mechanism in Symmetric Supercapacitors at High Voltages

AbstractIonic liquids (ILs) are attractive candidates for high‐voltage electrochemical energy storage systems, owing to their high electrochemical stability. Recently, a unique eutectic mixture of ILs was reported to demonstrate outstanding performance in supercapacitor systems at low temperatures. Yet, many publications using this or similar IL mixtures reported only a limited voltage or cyclability when utilizing them with practical activated carbon electrodes. With supercapacitors consisting of symmetric electrodes, in which voltages higher than 3 V are applied, fast capacity fading and activity termination are observed. In order to exceed the limit of 3 V for supercapacitors that use electrolyte solutions possessing wide electrochemical windows, we thoroughly investigated the (unexpected) failure mechanism, using several analytical methods. This is the most important aspect of the paper. By this, we discovered a pronounced difference in the electrochemical behavior of the negative and the positive electrodes, which has significant implications on the operation of full symmetric cells at high voltages. Finally, we propose a solution that enables stable operation of cells up to 3.4 V. By balancing the mass of the electrodes, we prevent high‐voltage failure and control the voltage split to use the full electrochemical window of each electrode and obtain a higher cell voltage of 3.4 V and an energy density higher than 40 Wh/kg (of the electrode materials). The most important aspect of this work was a rigorous study of the failure mechanism.

Molecular Dynamics Investigation of Efficient SO2 Absorption by Anion-Functionalized Ionic Liquids

Ionic liquids are appropriate candidates for the absorption of acid gases such as SO2. Six anion-functionalized ionic liquids with different basicities have been studied for SO2 absorption capacity by employing quantum chemical calculations and molecular dynamics (MD) simulations. Gas phase quantum calculations unveil that the high uptake of SO2 in these ionic liquids originates from the basicity of the anions and the consequent enhanced anion-SO2 interactions. MD simulations of SO2–IL mixtures reveal the crucial role of both cations and anions in SO2 dissolution. Multiple-site interactions of SO2 with the anions have been identified. The calculated solvation free energy substantiates these observations. The order of computed Henry’s law constant values with change in the anion is in fair agreement with experimentally determined SO2 solubility order.

Tuning physical properties and mesomorphic structures in aqueous 1-ethyl-3-methylimidazolium octylsulfate rigid-gel with univalent salt doping

We report measurements of density, viscosity and electrical (ionic) conductivity of three aqueous mixtures of 1-ethyl-3-methyl imidazolium octyl-sulfate ([EMIm][OSO4] or C2C1Im-C8SO4) saturated with univalent sulfate salts in the temperature range 10°C–90°C. This ionic liquid presents a lyotropic mesophase about room temperature when mixed with about a 20wt% of water. The salts used are lithium, sodium and potassium sulfates, and the amount of water varies from 16wt% to 22wt% depending on the given mixture. As a reference, we have also prepared and characterized four aqueous IL mixtures (without salt addition) in the same water concentration range. The solubility of the different salts is scarce in the pure ILs (less than 20×10−6), while in the aqueous mixtures prepared it varies, going from 200×10−6 for Na up to 700×10−6 for Li. As expected, we find that with the addition of the salts, both density and viscosity increase while the ionic conductivity decreases. However, the extent of these effects depends very much on the nature of the salt and the amount of water (which also influence the solubility of the salt). The measured results are rationalized in terms of water content and salt solubility in the ternary mixtures. We observed that the transition from the liquid to the rigid-gel phases takes some temperature degrees, where the sample is in a stable quasi-gel state. Those transition temperatures, from the liquid to the quasi-gel states, and from the quasi-gel to rigid-gel states are related to the nature of the salt metal present in the mixture for samples with the same water content.