Optimization of cation structure of imidazolium-based ionic liquids as ionic solvents for rechargeable magnesium batteries

A series of imidazolium cation-based ionic liquids (ILs) have been synthesized and examined as ionic solvents for rechargeable magnesium batteries. The electrolyte solutions consist of these ILs dissolving methylmagnesium bromide with tetrahydrofuran (MeMgBr/THF). The chemical structure of imidazolium cation much influenced the ionic conductivity and the electrochemical window of the system. A reversible process of cathodic deposition and anodic dissolution of magnesium has been successfully achieved at room temperature. The highest value of anodic peak current for magnesium dissolution was obtained in an optimized-structure IL with allyl and methoxyethyl groups as the substituents.     

Surface characterization of functionalized imidazolium-based ionic liquids

The surface composition of oligo(ethylene glycol) ether functionalized bis(trifluoromethylsulfonyl)imide ionic liquids has been studied by means of X-ray photoelectron spectroscopy (XPS). For [Me(EG)MIM][Tf 2N], [Et(EG) 2MIM][Tf 2N], and [Me(EG) 3MIM][Tf 2N], which vary by the number of ethylene glycol (EG) units (from 1 to 3), we have shown that the stoichiometry of the surface near region is in excellent agreement with the bulk stoichiometry, which confirms the high purity of the ionic liquid samples investigated and rules out pronounced surface orientation effects. This has been deduced from the experimental observation that the angle-resolved XP spectra of all elements present in the IL anions and cations (C, N, O, F, S) show identical signals in the bulk and surfaces sensitive geometry, i.e., at 0 degrees and 70 degrees emission angle, respectively. The relative intensity ratios of all elements were found to be in nearly perfect agreement with the nominal values for the individual ILs. In contrast to these findings, we identified surface-active impurities in [Me(EG)MIM]I, which is the starting material for the final anion exchange step to synthesize [Me(EG)MIM][Tf 2N]. Sputtering of the surface led to a depletion of this layer, which however recovered with time. The buildup of this contamination is attributed to a surface enrichment of a minor bulk contamination that shows surface activity in the iodide melt.     

Dielectric response of imidazolium-based room-temperature ionic liquids

We have used microwave dielectric relaxation spectroscopy to study the picosecond dynamics of five low-viscosity, highly conductive room temperature ionic liquids based on 1-alkyl-3-methylimidazolium cations paired with the bis((trifluoromethyl)sulfonyl)imide anion. Up to 20 GHz the dielectric response is bimodal. The longest relaxation component at the time scale of several 100 ps reveals strongly nonexponential dynamics and correlates with the viscosity in a manner consistent with hydrodynamic predictions for the diffusive reorientation of dipolar ions. Methyl substitution at the C2 position destroys this correlation. The time constants of the weak second process at the 20 ps time scale are practically the same for each salt. This intermediate process seems to correlate with similar modes in optical Kerr effect spectra, but its physical origin is unclear. The missing high-frequency portion of the spectra indicates relaxation beyond the upper cutoff frequency of 20 GHz, presumably due to subpicosecond translational and librational displacements of ions in the cage of their counterions. There is no evidence for orientational relaxation of long-lived ion pairs.     

X-ray photoelectron spectroscopy of pyrrolidinium-based ionic liquids: cation-anion interactions and a comparison to imidazolium-based analogues

We investigate seven 1-alkyl-1-methylpyrrolidinium-based ionic liquids, [C(n)C(1)Pyrr][X], using X-ray photoelectron spectroscopy (XPS). The electronic environment for each element is analysed and a robust fitting model is developed for the C 1s region that applies to each of the ionic liquids studied. This model allows accurate charge correction and the determination of reliable and reproducible binding energies for each ionic liquid studied. The electronic interaction between the cation and anion is investigated for ionic liquids with one and also two anions. i.e., mixtures. Comparisons are made to imidazolium-based ionic liquids; in particular, a detailed comparison is made between [C(8)C(1)Pyrr][X] and [C(8)C(1)Im][X](-), where X(?) is common to both ionic liquids.     

Reactivity of anionic nucleophiles in ionic liquids and molecular solvents

The nucleophilic reactivity of a representative series of anions has been measured in [hmim][ClO₄] 3i, [hm₂im][ClO₄] 3′i, and [hmim][PF₆] 3l ILs in the reaction with n-alkyl methanesulfonates and compared with that found in common molecular solvents (MeOH, DMSO, PhCl). The reactivity is found to depend on both the imidazolium cation-anion interaction and the specific solvation by water present in the IL, the water playing the main effect, in particular with hydrophilic anions. Removal of the largest quantity of water remarkably increases the ion pair reactivity in the IL up to rate constant value k comparable with those obtained in DMSO and in low polarity media (PhCl).     

Bimolecular photoinduced electron transfer in imidazolium-based room-temperature ionic liquids is not faster than in conventional solvents

The fluorescence quenching of 3-cyanoperylene upon electron transfer from N,N-dimethylaniline in three room-temperature ionic liquids (RTILs) and in binary solvent mixtures of identical viscosity has been investigated using steady-state and time-resolved fluorescence spectroscopy. This study was stimulated by previous reports of bimolecular electron transfer reactions faster by one or several orders of magnitude in RTILs than in conventional polar solvents. These conclusions were usually based on a comparison with data obtained in low-viscous organic solvents and extrapolated to higher viscosities and not by performing experiments at similar viscosities as those of the RTILs, which we show to be essential. Our results reveal that (i) the diffusive motion of solutes in both types of solvents is comparable, (ii) the intrinsic electron transfer step is controlled by the solvent dynamics in both cases, being slower in the RTILs than in the conventional organic solvent of similar viscosity, and (iii) the previously reported reaction rates much larger than the diffusion limit at low quencher concentration in RTILs originate from a neglect of the static and transient stages of the quenching, which are dominant in solvents as viscous as RTILs.     

Self-aggregation of nonionic surfactants in imidazolium-based ionic liquids with trifluoromethanesulfonate anion

Self-aggregation of polyoxyethylene (POE)-type nonionic surfactants in ionic liquids, 1-butyl- and 1-ethyl-3-methylimidazolium trifluoromethanesulfonate (bmimCF₃SO₃ and emimCF₃SO₃), was investigated by means of ¹H-NMR chemical shift, dynamic light-scattering (DLS), and surface tension measurements. The surfactants showed no definite aggregate formation in bmimCF₃SO₃. This shows a remarkable contrast to the previous observation in bmimBF₄ and bmimPF₆, and demonstrates an importance of anion species to determine the property of ionic liquids as a solvent to support the self-assembly of amphiphilic compounds. On the other hand, the surfactants formed micelles in emimCF₃SO₃, which shows an importance of alkyl chain attached to imidazolium ring to determine the solvophobic interaction between surfactant hydrocarbon chains in imidazolium-based ionic liquids. The low solvophobicity of the surfactants to the ionic liquid composed of imidazolium cation with long alkyl chain is attributed to an affinity of the surfactant hydrocarbon chain to the imidazolium alkyl chain. The values of micellization parameters and surface adsorption parameters obtained for the surfactant solutions in emimCF₃SO₃ are reported.     

Ambient temperature imidazolium-based ionic liquids with tetrachloronickelate(II) anions

Deep blue tetrachloronickelate(II) salts of 1-alkyl-3-methylimidazolium cations [C_nmim]⁺ with intermediate chain lengths (n = 5, 7, 8 and an n = 4/6 mixture) have been prepared from imidazolium chlorides and NiCl₂. They are liquid at ambient temperatures (ca. 20 °C) and above, and display viscosities and thermal stability comparable to related [BF₄]⁻ compounds. The similarities may reflect the compensating effects of the dinegative charge of the [NiCl₄]²⁻ ion and its larger size.     

An unexpected reaction between 5-hydroxymethylfurfural and imidazolium-based ionic liquids at high temperatures

A new compound was detected during the production of 5-hydroxymethylfurfural (HMF) from glucose and cellulose in the ionic liquid 1-butyl-3-methylimidazolium chloride ([Bmim]Cl) at high temperatures. Further experiments found that it was derived from the reaction of HMF with [Bmim]Cl. The structure of new compound was established as 1-butyl-2-(5'-methyl-2'-furoyl)imidazole (BMI) based on nuclear magnetic resonance and mass spectrometry analysis, and a possible mechanism for its formation was proposed. Reactions of HMF with other imidazolium-based ionic liquids were performed to check the formation of BMI. Our results provided new insights in terms of side reactions between HMF and imidazolium-based ionic liquids, which should be valuable for designing better processes for the production of furans using biomass and related materials.     

Molecular dynamics simulation of dual amino-functionalized imidazolium-based ionic liquids

In this work, the all-atom (AA) force fields were set up for three kinds of dual amino-functionalized imidazolium-based ionic liquids (ILs), composed by cations with different alkyl chain length and amino acid anion [Gly]⁻. The force field was based on our previous work and the default parameters were developed in this study. Molecular dynamics simulations were performed. Validation was carried out by comparing simulation densities with experimental data, and good agreement was obtained. Molar volume and heat capacity at constant pressure were predicted. Mean square displacements for these ILs were computed and these ILs were proved to move very slowly. It may be caused by hydrogen-bonded network between ions and the terminal azyl. To depict the microscopic structures of the ILs, many types of radial distribution functions were investigated. It is interesting to find that not only the cation and anion, but also the anions themselves will form hydrogen bonds.     

Direct electrochemical reduction of hemin in imidazolium-based ionic liquids

The direct electrochemical reduction of hemin, protoporphyrin(IX) iron(III) chloride, ligated with strong or weak heterocyclic bases, was investigated in the ionic liquids (IL), 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF₆]) and 1-octyl-3-methylimidazolium hexafluorophosphate ([omim][PF₆]), using cyclic voltammetry and chronocoulometry. Hemin complexed with N-methylimidazole (NMI) or with pyridine had E_1/2 values slightly (4–59 mV) more positive in IL (without electrolyte) than in methanol (1.0 M electrolyte) using a gold electrode. NMI-ligated hemin had a lower E_1/2 than pyridine-ligated hemin in either IL, consistent with the stronger electron donor characteristic of NMI. [Bmim][PF₆] solutions consistently yielded E_1/2 values 30 mV more negative than [omim][PF₆] solutions. The diffusion coefficients D_o of hemin in the IL ranged between 1.50 and 2.80×10⁻⁷ cm² s⁻¹, while the heterogeneous electron-transfer rate constants k_s ranged between 3.7 and 14.3×10⁻³ cm s⁻¹. Cyclic voltammetry of hemin adsorbed to a gold surface through 4,4′-bispyridyl disulfide (AT4) linkages showed a large positive shift in the oxidation wave, indicating that adsorption stabilizes the reduced hemin state. The surface concentration Γ_o of the adsorbed hemin was determined to be 1.21×10⁻¹⁰ mol cm⁻², indicating the presence of one or more complete monolayers of hemin. These findings suggest that while hemin is electrochemically active in IL, its behavior is modified by the ligand field strength and surface adsorption phenomena.     

Eutectic-based ionic liquids with metal-containing anions and cations

Eutectic mixtures of zinc chloride and donor molecules such as urea and acetamide are described and it is proposed that these constitute a new class of ionic liquids. FAB-MS analysis shows that the liquids are made up of metal-containing anions and cations in which the donor is coordinated to the cation. Data on the viscosity, conductivity, density, phase behaviour and surface tension are presented and these are shown to be significantly different to other related ionic liquids that incorporate quaternary ammonium salts. The conductivity and viscosity are comparable with other ionic liquids and the data fit well to the Hole theory model recently proposed.     

Shape-controlled synthesis of silver crystals mediated by imidazolium-based ionic liquids

This work presents an approach toward the shape-controlled synthesis of Ag crystals with hierarchical structures by exploitation of ionic liquids (ILs) as a shape-regulating agent. The synthesis of Ag crystals involves the reduction of AgNO(3) by EG in the presence of ILs, specifically 1-butyl-3-methylimidazolium methylsulfate (bmim-MeSO(4)). In accordance with non-classical crystallization growth mechanism, the primary Ag nanoparticles were formed at the early stage of the reaction, and then self-organized into 1D or 3D Ag superstructures via an IL-mediated self-assembly process. Their final morphologies were strongly dependent on the reaction conditions such as the concentration of ILs in the reaction mixture and the reaction temperature, which suggests that ILs play an important role in controlling the shape of the Ag crystals.     

Ultrafast solvation dynamics of coumarin 153 in imidazolium-based ionic liquids

The dynamic Stokes shift of coumarin 153 has been measured in two room-temperature ionic liquids, 1-(3-cyanopropyl)-3-methylimidazolium bis(trifluoromethylsulfonyl)imide and 1-propyl-3-methylimidazolium tetrafluoroborate, using the fluorescence up-conversion technique with a 230 fs instrumental response function. A component of about 10-15% of the total solvation shift is found to take place on an ultrafast time scale < 10 ps. The amplitude of this component is substantially less than assumed previously by other authors. The origin of the difference in findings could be partly due to chromophore-internal conformational changes on the ultrafast time scale, superimposed to solvation-relaxation, or due to conformational changes of the chromophore ground state in polar and apolar environments. First three-pulse photon-echo peak-shift experiments on indocyanine green in room-temperature ionic liquids and in ethanol indicate a difference in the inertial component of the early solvent relaxation of <100 fs.     

Thermochemistry of imidazolium-based ionic liquids: experiment and first-principles calculations

In this work the molar enthalpy of formation of the ionic liquid 1-ethyl-3-methylimidazolium dicyanoamide in the gaseous phase [C(2)MIM][N(CN)(2)] was measured by means of combustion calorimetry and enthalpy of vaporization using transpiration. Available, but scarce, primary experimental results on enthalpies of formation of imidazolium based ionic liquids with the cation [C(n)MIM] (where n = 2 and 4) and anions [N(CN)(2)], [NO(3)] and [NTf(2)] were collected and checked for consistency using a group additivity procedure. First-principles calculations of the enthalpies of formation in the gaseous phase for the ionic liquids with the common cation [C(n)MIM] (where n = 2 and 4) and with the anions [N(CN)(2)], [NO(3)], [NTf(2)], [Cl], [BF(4)] and [PF(6)] have been performed using the G3MP2 theory. It has been established that the gaseous phase enthalpies of formation of these ionic liquids obey the group additivity rules.     

A comparison of ether- and alkyl-derivatized imidazolium-based room-temperature ionic liquids: a molecular dynamics simulation study

Molecular dynamics simulations of ether-derivatized imidazolium-based room-temperature ionic liquids (EDI-RTILs), [C(5)O(2)mim][TFSI] and [C(5)O(2)mim][BF(4)], have been performed and compared with simulations of alkyl-derivatized analogues (ADI-RTILs). Simulations yield RTIL densities, self-diffusion coefficients and viscosity in excellent agreement with experimental data. Simulations reveal that structure in the EDI-RTILs, quantified by the extent of nanoscale segregation of tails as well as cation-ion and cation-cation correlations, is reduced compared to that observed in the ADI-RTILs. Significant correlation between ether tail oxygen atoms and imidazolium ring hydrogen atoms was observed in the EDI-RTILs. This correlation is primarily intramolecular in origin but has a significant intermolecular component. Competition of ether oxygen atoms with oxygen atoms of TFSI(-) or fluorine atoms of BF(4)(-) for coordination of the ring hydrogen atoms was found to reduce the extent of cation-anion correlation in the EDI-RTILs compared to the ADI-RTILs. The reduction in intermolecular correlation, particularly tail-tail segregation, as well as weakening of cation-anion specific interactions due to the ether tail, may account for the faster dynamics observed in the EDI-RTILs compared to ADI-RTILs.     

The thermochemistry of solvation of imidazolium-based ionic liquids in benzene

ABSTRACT

The present work is devoted to the thermochemical study of solvation of ionic liquids (IL) in benzene. The solution enthalpies of 1-ethyl-3-methylimidazolium tricyanomethanide [EMIM][C(CN)₃], 1-butyl-3-methylimidazolium tetrafluoroborate [BMIM][BF₄], 1-hexyl-3-methylimidazolium hexafluorophosphate [HMIM][PF₆], 1-octyl-3-methylimidazolium tetrafluoroborate [OMIM][BF₄], 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [EMIM][NTf₂], 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [BMIM][NTf₂] and 1-butyl-3-methylimidazolium trifluoromethanesulfonate [BMIM][TfO] in benzene were measured. The solvation enthalpies of imidazolium-based IL were calculated. Molar refractions of imidazolium-based IL form literature data on density and refractive indexes of IL were also calculated. The linear correlation between solvation enthalpy and molar refraction of IL was observed. This correlation can be used to calculate the vaporization enthalpy of imidazolium-based IL from solution calorimetry data.     

Green solvents from ionic liquids and deep eutectic solvents to natural deep eutectic solvents

Natural deep eutectic solvents (NADESs) are defined as mixtures of certain molar ratios of natural compounds such as sugars, organic acids, amino acids, and organic bases that are abundant in organisms. The melting points of these mixtures are considerably lower than those of their individual ingredients and far below ambient temperature. The first publications on the NADES concept in 2011 created a great expectation regarding their potential as green solvents that could replace conventional organic solvents in a wide range of applications. This was largely because many of the drawbacks of conventional synthetic ionic liquids (ILs) and deep eutectic solvents (DESs), particularly their toxicity and environmental hazards, could be solved using NADESs. Throughout the last 7 years, the interest in NADESs has increased enormously as reflected by the exponential growth of the number of related publications. The research on NADESs has rapidly expanded particularly into the evaluation of the feasibility of their application in diverse fields such as the extraction of (targeted) bioactive compounds from natural sources, as media for enzymatic or chemical reactions, preservatives of labile compounds, or as vehicles of non–water-soluble compounds for pharmaceutical purposes. Along with the exploration of these potential applications, there have been a large number of other studies related to their physicochemical features, the search for new NADESs, the research into the interactions between NADES components or with solutes, the recovery of solutes from NADES solutions, and the ways of circumventing inherent problems of NADESs such as their high viscosity and the consequent difficulties in handling them. This article contains a review of the applications of NADESs as extraction solvents, reaction media, and preservative, providing also a perspective of their future.     

Anion nucleophilicity in ionic liquids: a comparison with traditional molecular solvents of different polarity

The nucleophilic reactivity of a homogeneous series of anions (halides, pseudohalides and organic anions) in the ionic liquids [hexmim] [ClO₄] and [hexmim] [PF₆] has been measured in their reaction with n-alkyl methanesulfonates, and compared with that found in traditional molecular solvents of different polarity, that is, chlorobenzene, DMSO, and MeOH.     

Unexpected side reactions of imidazolium-based ionic liquids in the base-catalysed Baylis-Hillman reaction

Low yields are obtained when the Baylis-Hillman reaction is conducted in the presence of an imidazolium-based ionic liquid due to direct addition of the deprotonated imidazolium salt to the aldehyde. Ionic liquids are evidently not inert.