25th Anniversary Article: “Cooking Carbon with Salt”: Carbon Materials and Carbonaceous Frameworks from Ionic Liquids and Poly(ionic liquid)s

This review surveys recent work on the use of ionic liquids (ILs) and polymerized ionic liquids (PILs) as precursors to synthesize functional carbon materials. As solvents or educts with negligible vapour pressure, these systems enable simple processing, composition, and structural control of the resulting carbons under rather simple and green synthesis conditions. Recent applications of the resulting nanocarbons across a multitude of fields, such as fuel cells, energy storage in batteries and supercapacitors, catalysis, separation, and sorption materials are highlighted.     

Extractive solubilization, structural change, and functional conversion of cytochrome c in ionic liquids via crown ether complexation

This article reports on the extraction behavior of heme proteins from an aqueous phase into ionic liquids (ILs) with dicyclohexano-18-crown-6 (DCH18C6), and the structure-function relationship of cytochrome c (Cyt-c) dissolved in ILs. We have found that DCH18C6 enables transfer of Lys-rich proteins into ILs via supramolecular complexation. The hydrophobicity and functional groups of ILs have a great influence on protein partitioning, and a hydroxyl group-containing IL with DCH18C6 is capable of the quantitative partitioning of Cyt-c. On the other hand, protein transfer using conventional organic solvents is negligibly small. UV-visible, CD, and resonance Raman spectroscopic characterizations indicate that the sixth ligand Met 80 in the heme group of the Cyt-c-DCH18C6 complex in IL is replaced by other amino acid residues of the peptide chain and that a non-natural, six-coordinate, low-spin ferric heme structure is induced in IL. Solubilization of Cyt-c in IL causes the environmental change of the heme vicinity of Cyt-c, which triggers the functional conversion of Cyt-c from an electron-transfer protein to peroxidase. The Cyt-c-DCH18C6 complex in IL provides remarkably high peroxidase activity compared with native Cyt-c, because of enhancement of the affinity for H2O2.     

A fundamental study on electrowetting by traditional and multifunctional ionic liquids: possible use in electrowetting on dielectric-based microfluidic applications

Water or aqueous electrolytes are the dominant components in electrowetting on dielectric (EWOD)-based microfluidic devices. Low thermal stability, evaporation, and a propensity to facilitate corrosion of the metal parts of integrated circuits or electronics are drawbacks of aqueous solutions. The alternative use of ionic liquids (ILs) as electrowetting agents in EWOD-based applications or devices could overcome these limitations. Efficient EWOD devices could be developed using task-specific ILs. In this regard, a fundamental study on the electrowetting properties of ILs is essential. Therefore electrowetting properties of 19 different ionic liquids, including mono-, di-, and tricationic, plus mono- and dianionic ILs were examined. All tested ILs showed electrowetting of various magnitudes on an amorphous flouropolymer layer. The effects of IL structure, functionality, and charge density on the electrowetting properties were studied. The enhanced stability of ILs in electrowetting on dielectric at higher voltages was studied in comparison with water. Deviations from classical electrowetting theory were confirmed. The physical properties of ILs and their electrowetting properties were tabulated. These data can be used as references to engineer task-specific electrowetting agents (ILs) for future electrowetting-based applications.     

Design Principles of Ionic Liquids for Transdermal Drug Delivery

Ionic liquids (ILs) and deep eutectic solvents have shown great promise in drug delivery applications. Choline‐based ILs, in particular choline and geranic acid (CAGE), have been used to enhance the transdermal delivery of several small and large molecules. However, detailed studies outlining the design principles of ILs for transdermal drug delivery are still lacking. Using two model drugs of differing hydrophilicities, acarbose and ruxolitinib and 16 ILs, the dependence of skin penetration on the chemical properties of ILs is examined. First, the impact of ion stoichiometry on skin penetration of drugs is assessed using CAGE, which evidences that a molar ratio of 1:2 of choline to geranic acid yields the highest delivery. Subsequently, variants of CAGE are prepared using anions with structural similarity to geranic acid and cations with structural similarity to choline at a ratio of 1:2. Mechanistic studies reveal that the potency of ILs in enhancing transdermal drug delivery correlates inversely with the inter‐ionic interactions as determined by 2D NMR spectroscopy. Using this understanding, a new IL is designed, and it provides the highest delivery of ruxolitinib of all ILs tested here. Overall, these studies provide a generalized framework for optimizing ILs for enhancing skin permeability.     

Non‐Newtonian Thermosensitive Nanofluid Based on Carbon Dots Functionalized with Ionic Liquids

Non‐Newtonian nanofluids present outstanding features in terms of energy transfer and conductivity with high application in numerous areas. In this work, non‐Newtonian nanofluids based on carbon dots (Cdots) functionalized with ionic liquids (ILs) are developed. The nanofluids are produced using a simple, single‐step method where the raw materials for the Cdots synthesis are glucose and waste biomass (chitin from crab shells). The use of ILs as both reaction media and functionalization molecules allows for the development of a new class of nanofluids, where the ILs on the Cdots surface represent the base‐fluid. Here, the well‐known benign IL 1‐butyl‐3‐methylimidazolium chloride ([Bmim]Cl) and a novel home‐made IL (1‐tosylate‐3‐methyl‐imidazolium triflate) [Tmi][Trif] are used. The nanofluids obtained from both substrates show, apart from high conductivity and viscosity, light absorption, and good wettability, an appealing thermal sensitivity behavior. This thermal sensitivity is preserved even when applied as thin films on glass slides and can be boosted using the surface plasmon resonance effect. The results reported demonstrate that the new Cdots/IL‐based nanofluids constitute a versatile and cost‐effective route for achieving high‐performance thermosensitive non‐Newtonian sustainable nanofluids with tremendous potential for the energy coatings sector and heat transfer film systems.     

New Frontiers in Materials Science Opened by Ionic Liquids

Ionic liquids (ILs) including ambient‐temperature molten salts, which exist in the liquid state even at room temperature, have a long research history. However, their applications were once limited because ILs were considered as highly moisture‐sensitive solvents that should be handled in a glove box. After the first synthesis of moisture‐stable ILs in 1992, their unique physicochemical properties became known in all scientific fields. ILs are composed solely of ions and exhibit several specific liquid‐like properties, e.g., some ILs enable dissolution of insoluble bio‐related materials and the use as tailor‐made lubricants in industrial applications under extreme physicochemical conditions. Hybridization of ILs and other materials provides quasi‐solid materials, which can be used to fabricate highly functional devices. ILs are also used as reaction media for electrochemical and chemical synthesis of nanomaterials. In addition, the negligible vapor pressure of ILs allows the fabrication of electrochemical devices that are operated under ambient conditions, and many liquid‐vacuum technologies, such as X‐ray photoelectron spectroscopy (XPS) analysis of liquids, electron microscopy of liquids, and sputtering and physical vapor deposition onto liquids. In this article, we review recent studies on ILs that are employed as functional advanced materials, advanced mediums for materials production, and components for preparing highly functional materials.     

Ionic liquid high-temperature gas sensor array

A novel sensor array using seven room-temperature ionic liquids (ILs) as sensing materials and a quartz crystal microbalance (QCM) as a transducer was developed for the detection of organic vapors at ambient and elevated temperatures. Ethanol, dichloromethane, benzene, and heptane were selected as representative gas analytes for various kinds of environmental pollutants and common industrial solvents. The QCM/IL sensors responded proportionately and reversibly to the organic vapor concentrations (i.e., ethanol, heptane, and benzene) in the gas phase from 0 to 100% saturation at room and elevated temperatures (e.g., 120 degrees C) but deviated from this linear relationship at high concentrations for dichloromethane, a highly volatile compound. Linear discriminant analysis was used to analyze the sensing patterns. Excellent classifications were obtained for both known and unknown concentrations of vapor samples. The correct classifications were 100% for known concentration samples and 96% for samples with unknown concentrations. Thermodynamics and ATR-FT-IR studies were conducted to understand specific molecular interactions, the strength of the interaction between ILs and organic vapors, and the degree of ordering that takes place upon dissolution of the vapors in ILs. The different response intensity of the QCM/IL sensors to the organic vapors depends on the different solubilities of organic vapors in ILs and varying molecular/ion interactions between each organic vapor and IL. The diverse set of IL studied showed selective responses due to structural differences. Therefore, a sensor array of ILs would be able to effectively differentiate different vapors in pattern recognitions, facilitating discrimination by their distinctive patterns in response to organic vapors in both room and high temperatures.     

离子液体/金属-有机骨架复合材料制备方法、理论计算及应用研究进展

金属-有机骨架(MOF)是一种多孔、高比表面积的新型纳米材料.离子液体(IL)具有稳定性好、功能可设计的特点,将IL负载到MOF的孔中,实现离子液体和MOF材料的有效组合,开发新型功能化复合多孔材料,有利于充分发挥两种材料的优势.本文主要介绍IL/MOF复合材料合成方法、分子模拟及其应用的最新研究.总结目前IL/MOF...     

Molecular Insights into the Regulatable Interfacial Property and Flow Behavior of Confined Ionic Liquids in Graphene Nanochannels

The understanding of confined structure and flow property of ionic liquid (IL) in a nanochannel are essential for the efficient application of ILs in the green chemical processes. In this work, the ionic structure and various flow behaviors of ILs inside graphene nanochannels via molecular dynamics simulations are shown. The effect of the nanochannel structure on confined flow is explored, showing that the width mainly heightens the viscosity while the oxidation degree primarily enhances the interfacial friction coefficient. Tuning the width and oxidation degree of nanochannel, three different flow behaviors including Poiseuille, partial plunger and full plunger flow can be achieved, where the second one does not occur in water or other organic solvents. To describe the special flow behavior, an effective influence extent of the nanochannel (w _EIE) is defined, whose value can distinguish the above flows effectively. Based on w _EIE, the phase diagrams of flow behavior for the nanochannel structure and pressure gradient are obtained, showing that the critical pressure gradient decreases with width and increases with the oxidation degree. Based on the quantitative relations between confined structures, viscosity, friction coefficient, flow behavior, and nanochannel structure, the intrinsic mechanism of regulating the flow behavior and rational design of nanochannel are finally discussed.     

Facile Ionic Liquid‐Assisted Strategy for Direct Precipitation of Eu2+‐Activated Nanophosphors under Ambient Conditions

This work describes a novel ionic liquid (IL)‐assisted synthesis strategy for a direct and easy production of Eu²⁺‐doped nanoparticles (NPs), where ILs are also used as fluoride sources to avoid the use of elemental fluorine or toxic hydrofluoric acid. Up to now, the direct synthesis of Eu²⁺‐doped nanophosphors consisted of an enormous challenge, due to the oxidation to Eu³⁺ observed in hydrous solution, which is commonly used for the preparation of NPs, generating lattice defects and undesired particle growth or agglomeration by additional reducing steps at high temperatures. In contrast, ILs, unless containing ClO₄ ^– or NO₃ ^– anions, do not present an oxidizing character, allowing the direct precipitation of NPs, e.g., using Eu²⁺ containing starting materials. Here, the undoped and Eu²⁺‐doped BaFCl NPs have been prepared under atmospheric conditions for the first time using ILs as solvents and also as fluoride source, applying sonochemical and microwave‐assisted approaches. In general, this method bears an enormous potential for an easy synthesis of fluoride materials compared to inconvenient solid‐state methods. In addition, the IL plays the role of a strongly attached protecting shell which represents ≈7–8% of the total NPs weight.     

Dynamics of redox processes in ionic liquids and their interplay for discriminative electrochemical sensing

Motivated by the use of ionic liquids (ILs) as green replacers of traditional electrolytes, a mechanistic study has been systematically conducted to comprehend various design principles responsible for electrochemical profiling of redox-active species in ILs. The full spectrum of properties associated with ILs is exploited to assess the viability of this platform, thus revealing the correlation between the redox properties and the physiochemical parameters of the species involved. This includes the evaluation of (1) the variation of redox responses toward analytes with similar molecular structures or functionalities of ILs, (2) the influence in terms of physical criteria of the system such as viscosity and conductivity as well as chemical structure of ILs, and (3) the sustainability in harsh conditions (high temperature or humidity) and interferences. The principle is exemplified via trinitrotoluene (TNT) and dinitrotoluene (DNT) with inherent redox activity as analytes and IL membranes as solvents and electrolytes using glassy carbon (GC) electrodes. A discrete response pattern is generated that is analyzed through linear discriminant analysis (LDA) leading to 100% classification accuracy even for the mixture of analytes. Quantitative analysis through square wave voltammetry (SWV) gave rise to the detection limits in liquid phase of 190 and 230 nM for TNT and DNT, respectively, with a linear range up to 100 μM. Gas-phase analysis shows strong redox signals for the estimated concentrations of 0.27 and 2.05 ppm in the gas phase for TNT and DNT, respectively, highlighting that ILs adopt a role as a preconcentrator to add on sensitivity with enhanced selectivity coming from their physiochemical diversity, thus addressing the major concerns usually referred to most sensor systems.     

Designing ionic liquid solvents for carbon capture using property-based visual approach

Recently, ionic liquids (ILs) have been introduced as potential carbon dioxide (CO₂)-capturing solvents, as a substitute to conventional amine-based solvents. Conventional amine-based solvents that are used for CO₂ capture show some drawbacks, such as high solvent loss, high regeneration energy requirement, and solvent degradation. These shortcomings can be potentially overcome if IL-based solvents are considered. ILs have negligible vapour pressure, high thermal stability, and wide range of thermophysical properties. Nonetheless, using experimentation to identify suitable ILs as CO₂-capturing solvents is a tedious and costly task, as there are more than a million possible combinations of cations and anions that make up the ILs. Computer-aided tools have been previously developed for targeted IL design, which often involve non-linear programming. However, non-linear programming sometimes fails to converge, due to enlarged search space for optimal solution and its complex formulations. In this paper, the authors present a simple yet systematic visual approach to design IL solvents for carbon capture. Property integration framework is employed in this approach to systematically design IL, where the design problem can be mapped from the property domain into a cluster domain through clustering technique. The advantage of the visual approach is the ability to enumerate novel IL candidates. Group contribution (GC) method is included in the framework to estimate the properties of designed ILs. By combining property integration framework and GC method, the proposed approach is able to provide a property-based platform to visualise the performance of designed ILs on a ternary diagram. A case study is presented to illustrate the validity of the proposed approach.     

Immobilized ionic liquids as high-selectivity/high-temperature/high-stability gas chromatography stationary phases

Ionic liquids (ILs) are a class of nonmolecular solvents in which the cation/anion combination can be easily tuned to provide desired chemical and physical properties. When used as stationary phases in gas-liquid chromatography, ionic liquids exhibit dual nature retention selectivity. That is, they are able to separate polar molecules such as a polar stationary phase and nonpolar molecules such as a nonpolar stationary phase. However, issues such as optimization of the wetting ability of the ionic liquid on fused-silica capillaries, the maximum operating temperatures of the stationary phases, and nonuniform film thickness on the wall of the capillary at high temperatures have limited their use in gas chromatography. As described in this paper, these limitations are overcome by cross-linking a new class of ionic liquid monomers by free radical reactions to provide a more durable and robust stationary phase. By lightly cross-linking the ionic liquid stationary phase using a small amount of free radical initiator, high-efficiency capillary columns were produced that are able to endure high temperatures with little column bleed. Two types of cross-linked IL stationary phases are developed. A partially cross-linked stationary phase allows for high-efficiency separations up to temperatures of approximately 280 degrees C. However, by creating a more highly cross-linked stationary phase of geminal dicationic ILs, exclusively, an increase in efficiency is observed at high temperatures allowing for its use over 350 degrees C. In addition, through the use of solvation thermodynamics and interaction parameters, it was shown that the cross-linking/immobilization of the ionic liquid does not affect the selectivity of the stationary phase thereby preserving its dual nature retention behavior.     

Heteroatom‐Containing Porous Carbons Derived from Ionic Liquid‐Doped Alkali Organic Salts for Supercapacitors

A simple strategy for the synthesis of heteroatom‐doped porous carbon materials (CMs) via using ionic liquid (IL)‐doped alkali organic salts as small molecular precursors is developed. Doping of alkali organic salts (such as sodium glutamate, sodium tartrate, and sodium citrate) with heteroatoms containing ILs (including 1‐butyl‐3‐methylimidazolium chlorine and 3‐butyl‐4‐methythiazolebromination) not only incorporates the heteroatoms into the carbon frameworks but also highly improves the carbonization yield, as compared with that of either alkali organic salts or ILs as precursors. The porous structure of CMs can be tuned by adjusting the feed ratio of ILs. The porous CMs derived from 1‐butyl‐3‐methylimidazolium chlorine‐doped sodium glutamate exhibit high charge storage capacity with a specific capacitance of 287 F g^?1 and good stability over 5000 cycles in 6 m KOH at a current density of 1 A g^?1 for supercapacitors. This strategy opens a simple and efficient method for the synthesis of heteroatom‐doped porous CMs.     

离子液体及其在双电层电容器领域的新应用

离子液体作为一种新型绿色溶剂,在双电层电容器领域的应用受到极大关注。本文讨论了影响EDLC比电容的电极材料和电解液两大因素,着重对离子液体这一新兴的绿色溶剂进行探讨,并介绍了以碳纳米管为电极材料、离子液体为电解液制造EDLC的研究开发现状及发展趋势。     

Particulate composites based on ionic liquid-treated oil palm fiber and thermoplastic starch adhesive

Pretreatment of lignocellulosic materials is a highly essential and critical task for the manufacturing of engineered composite panels. Recently, ionic liquids (ILs) have emerged as a promising green solvent for lignocellulosic biomass disintegration. In this work, the impact of IL pretreatment on the flexural and thermal properties of the thermo-molded biocomposite panels made from oil palm biomass residue and thermoplastic starch biopolymer as binder was studied. Oil palm fiber was pretreated with IL [emim][dep] (1-ethyl-3-methylimidazolium diethyl phosphate) and IL [bmim][Cl] (1-butyl-3-methylimidazolium chloride) prior to mixing with plasticized starch. The compounded mixture was then hot-pressed into composite panels. To understand the effect of IL pretreatment, lignocellulosic characterization, morphology, and thermogravimetric analysis of the untreated and treated fibers were performed. It was found that thermal stability of the oil palm biomass and the biocomposites was improved after IL pretreatment due to partial removal of hemicellulose and lignin from raw fiber. Moreover, pretreated biocomposites exhibited superior strength and modulus as compared to that of untreated sample as evidenced from flexural testing. The study plainly demonstrates that IL-assisted pretreatment could be an extremely attractive and clean technology for the efficient use of agro-based industrial waste in biocomposite field.     

Toxicity and biodegradability of imidazolium ionic liquids

Several bioassays have been carried out to analyze the toxicity and biodegradability of several imidazolium ionic liquids (ILs) in aqueous phase. The synthetized compounds consist of an imidazolium cation with two alkyl substituents in positions 3 (R1) and 1 (R2) and a counter-ion. The alkyl substituent R1 has been fixed as a methyl group and the effect of the alkyl chain length (C1-C8) of the other substituent (R2) has been tested. Moreover, the influence of diverse counter-ions A- (Cl-, PF6, XSO4-) has been analyzed. Acute toxicity and EC50 values of each compound in the aqueous solution have been determined by using the Microtox standard procedure. Biodegradability of IL has been determined by measuring BOD5 of aqueous samples containing IL and/or D-glucose and the IL residual content and/or d-glucose concentration after this assay. The viability of the microorganisms used in the BOD5 has been related to the ATP in the samples, measured by a bioluminescence assay. All the ILs tested were not biodegradable in the considered conditions. Besides, it was found that the shorter the chain length of side chain R2, the lower the toxic effect is. On the contrary, the anion has a little effect on the IL toxicity.     

Nanometer‐Thick Ionic Liquids as Boundary Lubricants

Ionic liquids (ILs) are fascinating materials with unique combination of solid and liquid properties. Due to high thermal stability and promising tribological properties, there have been increasing research interests in applying ILs as boundary lubricants. In this review, the authors will discuss the recent progress on this topic with the emphasis on the relationship between the molecular arrangement of ILs confined to a solid and the tribological properties. First, the fundamentals on boundary lubrication and the state‐of‐the‐art lubricants will be reviewed briefly. Second, the progress on the molecular structure of ILs confined to a solid surface will be discussed. Afterward, the experimental and computational efforts on the ILs as boundary lubricants will be discussed in details with emphasis on the effect of IL structure, solid substrates, and IL‐soild interaction. Finally, the future research directions will be discussed.

     

Lithium-Ion Battery Separators for Ionic-Liquid Electrolytes: A Review

Ionic liquids (ILs) are widely studied as a safer alternative electrolyte for lithium-ion batteries. The properties of IL electrolytes compared to conventional electrolytes make them more thermally stable, but they also have poor wetting with commercial separators. In a lithium-ion battery, the electrolyte should completely wet out the separator and electrodes to reduce the cell internal resistance. Investigations of cell materials with IL electrolytes have shown that the wetting issues in IL–electrolyte cells are most likely due to poor separator compatibility, not electrode compatibility. A compatible separator must be developed before IL electrolytes can be used in commercial lithium-ion batteries. Herein, separators for IL electrolytes, including commercial and novel separators, are reviewed. Separators with different processing methods, polymers, additives, and different IL electrolytes are considered. Collated, the separator studies show a strong correlation between ionic conductivity and membrane porosity, even more than the electrolyte type. The challenge of a suitable separator for IL electrolytes is not solved yet. Herein, it is revealed that a separator for IL electrolytes will most likely require a combination of high thermal and mechanical stability polymer, ceramic additives, and an optimized manufacturing process.