How ionic are room-temperature ionic liquids? An indicator of the physicochemical properties

Room-temperature ionic liquids (RTILs) are liquids consisting entirely of ions, and their important properties, e.g., negligible vapor pressure, are considered to result from the ionic nature. However, we do not know how ionic the RTILs are. The ionic nature of the RTILs is defined in this study as the molar conductivity ratio (Lambda(imp)/Lambda(NMR)), calculated from the molar conductivity measured by the electrochemical impedance method (Lambda(imp)) and that estimated by use of pulse-field-gradient spin-echo NMR ionic self-diffusion coefficients and the Nernst-Einstein relation (Lambda(NMR)). This ratio is compared with solvatochromic polarity scales: anionic donor ability (Lewis basicity), E(T)(30), hydrogen bond donor acidity (alpha), and dipolarity/polarizability (pi), as well as NMR chemical shifts. The Lambda(imp)/Lambda(NMR) well illustrates the degree of cation-anion aggregation in the RTILs at equilibrium, which can be explained by the effects of anionic donor and cationic acceptor abilities for the RTILs having different anionic and cationic backbone structures with fixed counterparts, and by the inductive and dispersive forces for the various alkyl chain lengths in the cations. As a measure of the electrostatic interaction of the RTILs, the effective ionic concentration (C(eff)), which is a dominant parameter for the electrostatic forces of the RTILs, was introduced as the product of Lambda(imp)/Lambda(NMR) and the molar concentration and was compared with some physical properties, such as reported normal boiling points and distillation rates, glass transition temperature, and viscosity. A decrease in C(eff) of the RTILs is well correlated with the normal boiling point and distillation rate, whereas the liquid-state dynamics is controlled by a subtle balance between the electrostatic and other intermolecular forces.     

Applications of functionalized ionic liquids

Recent developments of the synthesis and applications of functionalized ionic liquids (including dual-functionalized ionic liquids) have been highlighted in this review. Ionic liquids are at-tracting attention as alternative solvents in green chemistry, but as more functionalized ILs are pre-pared, a greater number of applications in increasingly diverse fields are found.     

Acidic functionalized ionic liquids as catalyst for the isomerization of α-pinene to camphene

An acidic functionalized ionic liquids (ILs) [HSO₃-(CH₂)₃-NEt₃]Cl-ZnCl₂ was synthesized and used to catalyze the isomerization of α-pinene in a homogeneous system. The optimum conditions for isomerization were obtained as follows: n(α-pinene):n(ILs) = 9:1, reaction temperature 140 °C, and reaction time 4 h, α-pinene 0.04 mol. Under the optimal conditions, the conversion of α-pinene was 97.6 % and the selectivity for camphene could reach 64.8 %. In addition, the catalyst could be easily separated by centrifugation after the isomerization completely finished. When the ILs were repeatedly used for four times, the conversion of α-pinene and the selectivity for camphene were still excellent, indicating the superb recycle ability of the acidic functionalized ILs catalyst.     

Do anions influence the polarity of protic ionic liquids?

Polarity studies in two classes of imidazolium-based protic ionic liquids (PILs) possessing [HSO(4)](-), [HCOO](-), [CH(3)COO](-) and [CH(3)CH(2)COO](-) anions were carried out using a solvatochromic method from 298.15 to 353.15 K. For 1-methylimidazolium class of PILs, E(T)(30) was found to be independent over the entire range of temperature, while E(T)(30) was noted to decrease with a rise in temperature in the case of 1-butylimidazolium class of PILs containing [CH(3)COO](-) and [CH(3)CH(2)COO](-) anions. The E(T)(30) value decreases in both the classes upon varying the anions ([HSO(4)](-), [HCOO](-), [CH(3)COO](-) and [CH(3)CH(2)COO](-)). The E(T)(30) value is controlled by hydrogen bond acceptor basicity, β, and dipolarity/polarizability, π*. The E(T)(30) value for PILs varies inversely to the strength of the coulombic interaction between ions in PILs. Strong interactions between ions lead to lower E(T)(30) values. Unlike the poor thermal effect on E(T)(30), the Kamlet-Taft parameters i.e. α, β and π* have pronounced thermal effect in the imidazolium-based PILs. Variation in the Kamlet-Taft parameters is controlled by the stabilization of ions and the degree of proton transfer from Br?nsted acid to Br?nsted base.     

Purification or contamination? The effect of sorbents on ionic liquids

Commonly used for purification, alumina and silica are found to contaminate ionic liquids with particles; these particles cannot be removed easily and can have a non-negligible impact on the electrochemical, spectroscopic and physical properties of an ionic liquid, including its nucleation and crystallisation kinetics.     

Thermal characterization of the effect of fillers and ionic liquids on the vulcanization and properties of acrylonitrile–butadiene elastomer

Different thermal analysis techniques were used to study the effect of fillers and ionic liquids (ILs) on the vulcanization process, thermal and dynamic mechanical properties of acrylonitrile–butadiene elastomer (NBR). The products of the studies were composites of NBR filled with hydrotalcite, nanosized silica or carbon black. ILs such as 1-butyl-1-methylpyrrolidinium (BMpyrrolBF₄), 1-butyl-4-methylpyridinium (BMpyrBF₄) or 1-butyl-1-methylpiperidinium (BMpipBF₄) tetrafluoroborates were applied to improve the dispersion degree of the curatives and filler particles in the elastomer and to increase the efficiency of vulcanization. The differential scanning calorimetry results indicated that ILs reduced the vulcanization temperature of NBR compounds and increased the homogeneity of cross-link distribution in the elastomer network. NBRs filled with carbon black or silica exhibited similar thermal stabilities, whereas hydrotalcite reduced the temperature of thermal decomposition. The lowest mechanical loss factors were determined for vulcanizates filled with nanosized silica.

     

Syntheses of CuO nanostructures in ionic liquids

A simple and efficient approach is developed to fabricate single-crystalline CuO nanostructures through an ionic liquid assisted one-step low-temperature solid-state route.Both nanoparticles(5 nm in size)and nanorods(5-10 nm in diameter and 50-100 nm in length)of monoclinic CuO were obtained. These synthesized CuO nanostructures were characterized by X-ray diffraction(XRD),transmission electron microscopy(TEM),selected area electron diffraction(SAED),X-ray photoelectron spectros- copy(XPS),energy dispersive spectroscopy(EDS)and nitrogen adsorption analysis.The morpholo- gies of the nanostructures can be controlled by tuning the amount of NaOH and ionic liquids.The growth mechanism of CuO nanostructures is investigated.     

Coordination of 1,10-phenanthroline and 2,2'-bipyridine to Li+ in different ionic liquids. How innocent are ionic liquids?

On the basis of (7)Li NMR measurements, we have made detailed studies on the influence of the ionic liquids [emim][NTf(2)], [emim][ClO(4)], and [emim][EtSO(4)] on the complexation of Li(+) by the bidentate N-donor ligands 2,2'-bipyridine (bipy) and 1,10-phenanthroline (phen). For each of the employed ionic liquids the NMR data implicate the formation of [Li(bipy)(2)](+) and [Li(phen)(2)](+), respectively. X-ray diffraction studies were performed to determine the coordination pattern in the solid state. In the case of [emim][ClO(4)] and [emim][EtSO(4)], crystal structures confirmed the NMR data, resulting in the complexes [Li(bipy)(2)ClO(4)] and [Li(phen)(2)EtSO(4)], respectively. On the contrary, the ionic liquid [emim][NTf(2)] generated the C(i) symmetric, dinuclear, supramolecular cluster [Li(bipy)(NTf(2))](2), where the individual Li(+) centers were found to be bridged by two [NTf(2)] anions. Density functional theory (DFT)-calculations lead to further information on the effect of stacking on the coordination geometry of the Li(+) centers.     

Alkylsulfate-based ionic liquids in the liquid–liquid extraction of aromatic hydrocarbons

The (liquid + liquid) equilibrium data (LLE) for the extraction of toluene from heptane with different ionic liquids (ILs) based on the alkylsulfate anion (R-SO₄) was determined at T = 313.2 K and atmospheric pressure. The effect of more complex R-SO₄ anions on capacity of extraction and selectivity in the liquid–liquid extraction of toluene from heptane was studied. The ternary systems were formed by {heptane + toluene + 1,3-dimethylimidazolium methylsulfate ([mmim][CH₃SO₄]), 1-ethyl-3-methylimidazolium hydrogensulfate ([emim][HSO₄]), 1-ethyl-3-methylimidazolium methylsulfate ([emim][CH₃SO₄]), or 1-ethyl-3-methylimidazolium ethylsulfate ([emim][C₂H₅SO₄])}. The degree of quality of the experimental LLE data was ascertained by applying the Othmer–Tobias correlation. The phase diagrams for the ternary systems were plotted, and the tie lines correlated with the NRTL model compare satisfactorily with the experimental data.     

Density functional theory study on the –SO3H functionalized acidic ionic liquids

In this work, the structures of the –SO₃H functionalized acidic ionic liquid 1-(3-sulfonic acid) propyl-3-methylimidazolium hydrogen sulfate ([C₃SO₃Hmim]HSO₄), including its precursor compound (zwitterion), cation, and cation–anion ion-pairs, were optimized systematically by the DFT theory at B3LYP/6-311++G** level, and their most stable geometries were obtained. The calculation results indicated that a great tendency to form strong intramolecular hydrogen bonds was present in the zwitterion, and this tendency was weakened in the cation that was the protonation product of zwitterion. The intramolecular hydrogen bonds and intermolecular hydrogen bonds coexisted in the ionic liquid, and they played an important role in the stability of the systems. The strongest interaction in the ionic liquid was found between the anion and the functional group. The transition state research and the intrinsic reaction coordinate analysis of the hydrogen transfer reaction showed that, when the cation and the anion interacted near the functional group by double O–H···O hydrogen bonds, the ionic liquid was inclined to exist in a form of the zwitterion and H₂SO₄.     

Effects of protic ionic liquids on the oxygen reduction reaction – a key issue in the development of intermediate-temperature polymer-electrolyte fuel cells

Intermediate-temperature polymer-electrolyte fuel cells (IT-PEFCs), operated at an elevated temperature of ≈120 °C, would enable simplified system design and a potential increase in fuel cell performance compared to state-of-the-art low-temperature (LT-)PEFCs. As LT-PEFC membranes rely on the presence of water and high-temperature (HT-)PEFCs suffer from sluggish oxygen reduction reaction (ORR) kinetics, alternative materials must be developed. Promising candidates are protic ionic liquids (PILs) immobilized in, e.g., a host polymer. PILs’ properties, such as weak ion adsorption, high acidity of the proton-carrying ion, an excess of the anion precursor, and a high oxygen diffusivity and solubility, are favorable for achieving high ORR rates. Concepts proposed in the literature for incorporating PILs into MEA components are presented herein, and their utility for future IT-PEFCs is discussed.     

Periodicity and map for discovery of new ionic liquids

1 Introduction In undertaking the researches on ionic liquids, we wished to establish periodicity and draw a “map” of ionic liquids for providing definite guidance to dis-cover, design, and choose the proper ionic liquids to meet the specific applicatio…     

What causes extended layering of ionic liquids on the mica surface?

Extended layering of ionic liquids (ILs) on the mica surface has been reported by several groups previously and it is generally accepted that the electrostatic interaction at the IL/mica interface is critical to the observed extended layering. Here we report that, indeed, water adsorption on the mica surface is the key to the extended layering of ionic liquids. The atomic force microscopy (AFM), attenuated total reflectance-fourier transform infrared spectroscopy (ATR-FTIR) and contact angle (CA) results show that ionic liquids form extended layering on a mica surface under ambient conditions when water is adsorbed on the mica surface under such conditions. However, when airborne hydrocarbon contaminants replace the water on the mica surface at the elevated temperatures, instead of layering, ionic liquids exhibit droplet structure, i.e., dewetting. Based on the experimental results, we propose that water enables ion exchange between K+ and the cations of ILs on the mica surface and thus triggers the ordered packing of cations/anions in ILs, resulting in extended layering.     

Preparation and characterization of amino or carboxyl-functionalized ionic liquids

New functionalized ionic liquids,1-carboxylmethyl-3-methylimimidazolium hexafluorophosphate or fluoborate and 1-ami- noethyl-3-methylimimidazolium hexafluorophosphate or fluoborate have been synthesized and investigated.The obtained amino or carboxyl-functionalized ionic liquids were all characterized by FT-IR,~1H NMR and MS(ESI)and their properties such as freezing point,viscosity,solubility,specific gravity,surface tension,and interracial tension were also determined.     

Studies on the phase behavior and solubilization of the microemulsion formed by surfactant-like ionic liquids with ɛ–β-fish-like phase diagram

The phase behavior and the solubilization of the microemulsion systems surfactant-like ionic liquids 1-hexadecyl-3-methylimidazolium bromide (C₁₆mimBr), 1-tetradecyl-3-methylimidazolium bromide (C₁₄mimBr), or 1-dodecyl-3-methylimidazolium bromide (C₁₂mimBr)/alcohol/alkane/brine have been studied with ɛ–β-fish-like phase diagram method at 40 °C and an oil-to-water mass ratio of 1:1. From the ɛ–β-fish-like phase diagram, the physicochemical parameters, such as the mass fraction of alcohol in the hydrophile–lipophile-balanced interfacial layer (A ^S), and the solubilities of ionic liquid (S ^O) and alcohol (A ^O) in alkane phase, were calculated. The solubilization of the microemulsion system has been discussed based on the ɛ–β-fish-like phase diagram. The smaller the oil molecule, the longer the alcohol chain length, and the larger the NaCl concentration in water, the larger the solubilization of the microemulsion system. In this paper, the solubilization of the microemulsion stabilized by both C₁₂mimBr and sodium dodecyl sulfonate (sodium dodecyl sulfate) was also investigated with the ɛ–β-fish-like phase diagram. The unequimolar composite of anionic and cationic surfactants can avoid the sedimentation aroused by the strong electrostatic attraction, and an obvious synergism effect in solubilization was obtained.     

On the mechanism of the unwanted acetylation of polysaccharides by 1,3-dialkylimidazolium acetate ionic liquids: part 2—the impact of lignin on the kinetics of cellulose acetylation

Cellulose acetylation has been reported as a side reaction of cellulose treatment with the ionic liquid 1-ethyl-3-methylimidazolium acetate ([EMIm][OAc]) (Karatzos et al. in Cellulose 19:307–312, 2012) and other 1,3-dialkylimidazolium acetate ionic liquids. 1-Acetylimidazole (AcIm), an [EMIm][OAc] impurity, has been found to be the actual acetylating agent (Zweckmair et al. in Cellulose 22:3583–3596, 2015), and the degree of acetylation was relatively low, below a DS of approx. 0.1%. Higher degrees of cellulose acetylation (DS > 10%) have been observed when the entire wood was mixed with [EMIm][OAc] instead of cellulosic pulp only (Abushammala et al. in Carbohydr Polym 134:609–616, 2015). In this paper, we explore the impact of wood constituents, mainly lignin, on cellulose acetylation using AcIm. The results demonstrate that lignin itself can be readily acetylated upon mixing with AcIm, and—noteworthy—that lignin presence significantly accelerates cellulose acetylation. The initial rate of cellulose acetylation by AcIm increased from 1.8 to 4.7%/h when only 1% of lignin, based on cellulose mass, was added. A mechanistic study employing cellulose and lignin model compounds showed lignin to be more susceptible to acetylation than cellulose and to act as an intermediate acetyl group source for further cellulose acetylation in a catalytic scenario.     

How polar are ionic liquids? Determination of the static dielectric constant of an imidazolium-based ionic liquid by microwave dielectric spectroscopy

In a pilot study of the dielectric constant of room-temperature ionic liquids, we use dielectric spectroscopy in the megahertz/gigahertz regime to determine the complex dielectric function of five 1-alkyl-3-methylimidazolium salts, from which the static dielectric constant epsilon is obtained by zero-frequency extrapolation. The results classify the salts as moderately polar solvents. The observed epsilon-values at 298.15 K fall between 15.2 and 8.8, and epsilon decreases with increasing chain length of the alkyl residue of the cation. The anion sequence is trifluoromethylsulfonate > tetrafluoroborate approximately tetrafluorophosphate. The results indicate markedly lower polarities than found by spectroscopy with polarity-sensitive solvatochromic dyes.     

Functional dependency of structures of ionic liquids: do substituents govern the selectivity of enzymatic glycerolysis?

The concept of regulating the preference of a reversible multi-step reaction by adjusting the substituents of ionic liquids (ILs) has been successfully exemplified with a group of tetraammonium-based ionic liquids as medium for the enzymatic glycerolysis. Simultaneous existence of long chain hydrophobic substituents and hydrophilic ethoxyl or hydroxyl moieties is found, respectively, to be essential for triglycerides (TG) dissolving and equilibrium shifting. The reactions in the ILs with cations consisting of long chain and free hydroxyl groups gave markedly higher conversion of TG and better preference to monoglyceride formation. Interestingly the predicted results from COSMO-RS (a quantum chemical model programme) achieved a good agreement with the experimental data, mapping out the specific solvation from the ILs as well as demonstrating the interaction between ILs, substrates and products being the intrinsic causes that govern reaction evolution and direct equilibrium shifting.