Liquid phase behavior of ionic liquids with alcohols: experimental studies and modeling

Ionic liquids (ILs) have been suggested as potential "green" solvents to replace volatile organic solvents in reaction and separation processes due to their negligible vapor pressure. To develop ILs for these applications, it is important to gain a fundamental understanding of the factors that control the phase behavior of ionic liquids with other liquids. In this work, we continue our study of the effect of chemical and structural factors on the phase behavior of ionic liquids with alcohols, focusing on pyridinium ILs for comparison to imidazolium ILs from our previous studies. The impact of different alcohol and IL characteristics, including alcohol chain length, cation alkyl chain length, anion, different substituent groups on the pyridinium cation, and type of cation (pyridinium vs imidazolium) will be discussed. In general, the same type of behavior is observed for pyridinium and imidazolium ILs, with all systems studied exhibiting upper critical solution temperature behavior. The impacts of alcohol chain length, cation chain length, and anion, are the same for pyridinium ILs as those observed previously for imidazolium ILs. However, the effect of cation type on the phase behavior is dependent on the strength of the cation-anion interaction. Additionally, all systems from this study and our previous work for imidazolium ILs were modeled using the nonrandom two-liquid (NRTL) equation using two different approaches for determining the adjustable parameters. For all systems, the NRTL equation with binary interaction parameters with a linear temperature dependence provided a good fit of the experimental data.     

Retention characteristics of organic compounds on molten salt and ionic liquid-based gas chromatography stationary phases

The interest of using ionic liquids (ILs) as stationary phases in gas chromatography (GC) has increased in recent years. This is largely due to the fact that new classes of ILs are being developed that are capable of satisfying many of the requirements of GC stationary phases. This review highlights the major requirements of GC stationary phases and describes how molten salts/ILs can be designed to largely meet these needs. The retention characteristics of organic solutes will be discussed for ammonium, pyridinium, and phosphonium-based molten salts followed by imidazolium, pyridinium, pyrollidinium, and phosphonium-based IL stationary phases. The versatility of ILs allows for the development of stationary phases based on dicationic ILs, polymeric ILs, and IL mixtures. To aid in choosing the appropriate IL stationary phase for a particular separation, the reader is guided through the different types of stationary phases available to identify those capable of providing the desired separation selectivity of organic solutes while allowing for flexibility in ranges of temperature used throughout the separation.     

Application of 1-Alkyl-3-methylimidazolium-Based Ionic Liquids as Background Electrolytes in Nonaqueous Capillary Electrophoresis for the Analysis of Coptidis Alkaloids

Interest in ionic liquids (ILs) for their potential in analytical chemistry is increasing because they are environmentally benign and are good separation solvents. The aim of the presented investigation was to verify whether ILs would be a suitable background electrolyte (BGE) in nonaqueous capillary electrophoresis (NACE) for organic cations analysis of the closely related analogues. In this study, a novel and very simple NACE method has been established for analyzing seven quaternary alkaloids in Coptis rhizome using 1-alkyl-3-methylimidazolium tetrafluoroborate-based ionic liquid as BGE. The effects of the alkyl group, imidazolium counterion (anionic part), along with the concentration of IL, are investigated and discussed. Baseline separation, high efficiencies, and symmetrical peaks of the seven alkaloids were obtained. The separation mechanism could be hydrophobic and hydrogen-bonding interactions between the alkaloids and the imidazolium cations. The optimum conditions were 70 mM 1-decyl-3-methylimidazolium tetrafluoroborate (1D-3MI-TFB) methanol solution (apparent pH 2.66) and 30 kV applied voltage. The detection was performed at 254 nm. Seven quaternary alkaloids in Coptis rhizome were separated within 14 min. The proposed NACE separation procedure is highly reproducible and can be applied in the qualitative and quantitative analysis of Coptidis alkaloids.     

Characterization of hexacationic imidazolium ionic liquids as effective and highly stable gas chromatography stationary phases

Polycationic ionic liquids (ILs) are an attractive class of ILs with great potential applicability as gas chromatography stationary phases. A family of hexacationic imidazolium ILs derived from the cycloalkanol family was chemically first prepared in a straightforward manner and then applied for analytical separation purposes. Four tuneable engineering vectors, namely cation ring size structure, anion nature, spatial disposition of cycloalkanol substituents and O‐substitution, were considered as experimental parameters for the design of the desired ionic liquids. A total number of five new phases based on a common benzene core respectively exhibited column efficiencies around to 2500 plates/m, broad operating temperature ranges and also, even more importantly, good thermal stabilities (bleeding temperature between 260 and 365°C), finding variations in the selectivity and analytes elution orders depending on the IL structures. Their solvation characteristics were evaluated using the Abraham solvation parameter model, establishing clear correlations between their cation structure and retention capability with respect to certain analytes. The study of relationships between the ILs structure and solvation parameters gives us an idea of the IL stationary phase to be used for specific separations.     

硅烷基咪唑双三氟甲烷磺酰亚胺离子液体气相色谱固定相的性能评价

以双三氟甲烷磺酰亚胺离子([NTf₂]^-)为阴离子,合成阳离子烷基取代不同(C1、C2和C4)的硅烷基咪唑离子液体,以其为固定相制备气相色谱填充柱。 硅烷基咪唑离子液体为强极性固定相;阳离子结构影响固定相的热稳定性、极性和分离性能。 在这些离子液体固定相中,1-丁基-3-[(3-三甲氧基硅基)-丙基]咪唑双三氟甲烷磺酰亚胺([PBIM]NTf₂)对Grob试剂分离性能较好。 利用溶剂化作用参数模型,评价[PBIM]NTf₂固定相特性,研究固定相-组分分子之间相互作用机制;同时考察[PBIM]NTf₂色谱柱对不同类型化合物的分离性能。 结果表明,[PBIM]NTf₂固定相主要作用力是氢键碱性和偶极作用,对烷烃、醇、酯和胺等不同类型的样品组分表现出良好的分离能力。     

On the stabilisation and surface properties of soluble transition-metal nanoparticles in non-functionalised imidazolium-based ionic liquids

Doubtless ionic liquids (ILs), particularly those based on the 1,3-dialkyl imidazolium cation, provide a flexible liquid platform to prepare, soluble and stable transition metal nanoparticles (TMNPs). ILs can act as a “solvent”, stabiliser, ligand and support for TMNPs. Soluble and stable TMNPs for specific applications can be easily prepared in ILs using a bottom-up or top-down approach. The stability of TMNPs in non-functionalised ILs is mainly related to the surface electronic stabilisation provided by protective layers of discrete supramolecular imidazolium aggregates, non-polar imidazolium alkyl side chains and NHC carbene species as well as surface hydrogen species, together with an oxide layer when present on the metal surface. The IL provides a template-like effect and does not exist as a pure double layer, rather, the IL interacts directly with the TMNP surface through both cationic and anionic species, and the non-polar groups are preferentially directed away from the surface, forming a protective layer at the interface of at least one layer thick. The main aspects involving the stabilisation, in particular the interface of TMNP surface with the ionic liquids and other species present in the media, will be presented and discussed in light of the recent experimental and theoretical results reported.     

Simultaneous determination of fangchinoline and tetrandrine in Stephania tetrandra S. Moore by using 1-alkyl-3-methylimidazolium-based ionic liquids as the RP-HPLC mobile phase additives

A reversed phase high performance liquid chromatography (RP-HPLC) method for simultaneous determination of fangchinoline (FAN) and tetrandrine (TET) in Stephania tetrandra S. Moore was established by using 1-hexyl-3-methylimidazolium tetrafluoroborate as the mobile phase additives in this paper. Four types of 1-alkyl-3-methylimidazolium-based ionic liquids (ILs) were used as additives of the mobile phase to separate FAN and TET by RP-HPLC. The effects of the length of the alkyl group on the imidazolium ring and its counterion, the concentrations of IL and the pH of the mobile phase, which influenced the chromatographic behaviors of FAN and TET, were investigated in detail. The linearity, sensitivity, accuracy and repeatability of the proposed method were also investigated. The probable mechanism of the separation with ILs as the mobile phase additives was explored and discussed.     

Syntheses and characterization of few bio-ionic liquids comprising of cholinium cation and plant derived carboxylic acids as anions

Due to the increasing trend to use ionic liquids (ILs) for number of applications, it is of utmost importance to ensure non toxicity of the solvent systems which may contaminate the processed products. The reported toxicity of several imidazolium based ionic liquids posed a need to develop bio based ILs for various applications which are due to their bio-origin are bio compatible, nontoxic and biodegradable. Herein eleven bio-based ionic liquids were prepared using acid moieties available in various plants and characterized. Although some of the ILs were used to separate antibodies such as IgG from rat serum in the form of aqueous biphasic systems but to find their direct application for material preparation and food applications, herein rheological behavior of the ILs were investigated. The choline based IL containing coumarine-3-carboxylate was found to have highest zero shear viscosity while the IL with D-(−)-quinate was found to have the lowest. The viscoelastic behavior of the ionic liquids established anion dependent viscous and liquid like behavior of the ionic liquids. Interestingly the ILs showed viscosity independent ion conductivity. Due to the high conductivity, stable physical state and bio-origin such ILs have the potential for applications in electrochemistry, food and material science.     

Potential Screening at Electrode/Ionic Liquid Interfaces from In Situ X-ray Photoelectron Spectroscopy

A new approach to investigate potential screening at the interface of ionic liquids (ILs) and charged electrodes in a two-electrode electrochemical cell by in situ X-ray photoelectron spectroscopy has been introduced. Using identical electrodes, we deduce the potential screening at the working and the counter electrodes as a function of applied voltage from the potential change of the bulk IL, as derived from corresponding core level binding energy shifts for different IL/electrode combinations. For imidazolium-based ILs and Pt electrodes, we find a significantly larger potential screening at the anode than at the cathode, which we attribute to strong attractive interactions between the imidazolium cation and Pt. In the absence of specific ion/electrode interactions, asymmetric potential screening only occurs for ILs with different cation and anion sizes as demonstrated for an imidazolium chloride IL and Au electrodes, which we assign to the different thicknesses of the electrical double layers. Our results imply that potential screening in ILs is mainly established by a single layer of counterions at the electrode.     

Extraction separation of rare-earth ions via competitive ligand complexations between aqueous and ionic-liquid phases

The extraction separation of rare earth elements is one of the most challenging separation processes in hydrometallurgy and advanced nuclear fuel cycles. The TALSPEAK process (trivalent actinide lanthanide separations by phosphorus-reagent extraction from aqueous komplexes) is a prime example of these separation processes. The objective of this paper is to explore the use of ionic liquids (ILs) for the TALSPEAK-like process, to further enhance its extraction efficiencies for lanthanides, and to investigate the potential of using this modified TALSPEAK process for separation of lanthanides among themselves. Eight imidazolium ILs ([C(n)mim][NTf(2)] and [C(n)mim][BETI], n = 4,6,8,10) and one pyrrolidinium IL ([C(4)mPy][NTf(2)]) were investigated as diluents using di(2-ethylhexyl)phosphoric acid (HDEHP) as an extractant for the separation of lanthanide ions from aqueous solutions of 50 mM glycolic acid or citric acid and 5 mM diethylenetriamine pentaacetic acid (DTPA). The extraction efficiencies were studied in comparison with diisopropylbenzene (DIPB), an organic solvent used as a diluent for the conventional TALSPEAK extraction system. Excellent extraction efficiencies and selectivities were found for a number of lanthanide ions using HDEHP as an extractant in these ILs. The effects of different alkyl chain lengths in the cations of ILs and of different anions on extraction efficiencies and selectivities of lanthanide ions are also presented in this paper.     

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.     

Aqueous interfaces with hydrophobic room-temperature ionic liquids: a molecular dynamics study

We report a molecular dynamics study of the interface between water and (macroscopically) water-immiscible room-temperature ionic liquids "ILs", composed of PF6(-) anions and butyl- versus octyl-substituted methylimidazolium+ cations (noted BMI+ and OMI+). Because the parameters used to simulate the pure ILs were found to exaggerate the water/IL mixing, they have been modified by scaling down the atomic charges, leading to better agreement with the experiment. The comparison of [OMI][PF6] versus [BMI][PF6] ILs demonstrates the importance of the N-alkyl substituent on the extent of solvent mixing and on the nature of the interface. With the most hydrophobic [OMI][PF6] liquid, the "bulk" IL phase is dryer than with the [BMI][PF6] liquid. At the interface, the OMI+ cations retain direct contacts with the bulk IL, whereas the more hydrophilic PF6(-) anions gradually dilute in the local water micro-environment and are thus isolated from the "bulk" IL. The interfacial OMI+ cations are ordered with their imidazolium moiety pointing toward the aqueous side and their octyl chains toward the IL side of the interface. With the [BMI][PF6] liquid, the system gradually evolves from an IL-rich to a water-rich medium, leading to an ill-defined interfacial domain with high intersolvent mixing. As a result, the BMI+ cations are isotropically oriented "at the interface". Because the imidazolium cations are more hydrophobic than the PF6(-) anions, the charge distribution at the interface is heterogeneous, leading to a positive electrostatic potential at the interface with the two studied ILs. Mixing-demixing simulations on [BMI][PF6]/water mixtures are also reported, comparing Ewald versus reaction field treatments of electrostatics. Phase separation is very slow (at least 30 ns), in marked contrast with mixtures involving classical organic liquids, which separate in less than 0.5 ns at the microscopic level. The results allow us to better understand the specificity of the aqueous interfaces with hydrophobic ionic liquids, compared with classical organic solvents, which has important implications as far as the mechanism of liquid-liquid ion extraction is concerned.     

Interactions of 1-butyl-3-methylimidazolium carboxylate ionic liquids with glucose in water: a study of volumetric properties, viscosity, conductivity and NMR

Extensive applications of ionic liquids (ILs) may result in their accumulation in the ecological environment and organisms. Although ILs are popularly called "green solvents", their toxicity, in fact, has been exhibited. Therefore the interaction of ILs with biomolecules is a cutting-edge research subject. Herein, the interactions of 1-butyl-3-methylimidazolium carboxylate ionic liquids ([C(4)mim][HCOO], [C(4)mim][CH(3)COO] and [C(4)mim][CH(3)CH(2)COO]) with glucose in water were studied for their volumetric properties, viscosity, conductivity and NMR spectra. Limiting apparent molar volumes (V(Φ, IL)(0)), viscosity B-coefficients, limiting molar conductivities (Λ(0)) and Walden products (Λ(0)η(0)) were evaluated for the ILs in glucose + water solutions. Volumetric interaction parameters were also obtained from the transfer volumes of the ionic liquids. The contributions of the solvent properties (B(1)) and the ionic liquid-solvent interactions (B(2)) to the B-coefficient were extracted, together with molar activation energies (Δμ(IL)(0≠)) of the ionic liquids for viscous flow of the aqueous glucose + IL solution. In addition, the (13)C and (1)H NMR spectra of methyl β-D-glucopyranoside and ILs in β-D-glucopyranoside + IL + D(2)O were studied. The NMR results show that no special and strong interactions were observed between glucopyranoside and the ILs. However, it was confirmed that the H2 on the imidazolium ring has more activity (acidity) than atoms H4 and H5. The macro-properties and their changes were also discussed in terms of the size, structure and solvation of the ILs and glucose.     

The preparation of supported ionic liquids (SILs) and their application in rare metals separation

This review summarizes the preparation methods of support ionic liquids (SILs) and their applications in rare metals separation. The rare metals separation includes the recovery of high value metal ions and the removal of heavy metal ions from wastewater. SILs can be used as a kind of highly efficient multifunctional separation materials. The preparation methods of SILs include chemical immobilization technique in which ILs moieties are supported on solid supports via covalent bonds and physical immobilization techniques in which ILs are immobilized on solid supports via physical method such as simple im- pregnation, sol-gel method. According to the difference of solid supports, this review summarizes the application of polymer supported ionic liquids (P-SILs), silica based material supported ionic liquids (SM-SILs) and membrane supported ionic liq- uids (M-SILs) in rare metals separation. P-SILs and SM-SILs prepared by chemical method with N-methylimidazolium group can be used as highly efficient anion exchangers with high thermal stability and good chemical stability for adsorption of Cr(VI), Re(VII), Ce(IV). P-SILs prepared via simple impregnation afforded IL functionalized solvent impregnated resins (SIRs) which showed high separation efficiency and selectivity in the separation of rare earths(III) (REs(III)). SM-SILs prepared via sol-gel method with IL doped in the support as porogens or extractant show high removal efficiencies and excellent stability for the separation of RE(III), Cr(III) and Cr(VI). M-SILs with IL as plasticizer or carrier show improved stability, high perme- ability coefficient and good selectivity for Cr(VI) transport. Different supports and different supporting methods were suffi- ciently compared. Based on the different practical application, different forms of SILs can be prepared for separation of rare metals with high separation efficiency and selectivity.     

Temperature-responsive ionic liquid/water interfaces: relation between hydrophilicity of ions and dynamic phase change

Phase separation between ionic liquids (ILs) and molecular liquids is of interest physico-chemically, and also has industrial relevance. IL/water mixtures are of great interest in many fields. Unlike static phase separation between IL and water, dynamic shifts of IL/water mixtures between a homogeneous mixture and separate phases have a wide variety of applications. The miscibility of ILs with water generally increases upon heating, and a few ILs undergo a lower critical solution temperature (LCST)-type phase transition with water in which the separated biphases become miscible upon cooling. As the phase transition is controlled by changing the temperature by a few degrees, the LCST-type phase response of IL/water mixtures makes it possible to use ILs as solvents in various energy-saving processes. Since many hydrophilic ILs do not undergo phase separation with water, we aim to determine the necessary conditions under which hydrophobic ILs undergo the phase transition. Based on physico-chemical analysis of many hydrophobic ILs that undergo a phase separation after mixing with water, we find there is a particular range of "hydrophilicity" of these hydrophobic ILs within which the LCST-type phase transition is possible. Accordingly, a hydrophilicity index (HI) of ILs is proposed, in terms of the number of water molecules in the separated IL phase. The HI value proves to be a good indicator of the phase behaviour of IL/water mixtures, as well as their phase transition temperature. Potential application of the LCST-type phase change to the selective extraction of water-soluble proteins is also summarised.     

Palladium catalyzed C-P cross-coupling reactions in ionic liquids

C-P cross-coupling reactions between phosphine-borane complexes and aryl iodides are successfully performed in imidazolium based ionic liquids (ILs). To improve the recycling of the catalyst, a monophosphine covalently attached to a pyridinium salt was prepared. Associated to palladium, this catalyst shows a high catalytic activity and stays immobilized in the IL. It was reused at least in six cycles.     

Specific phenomena in carboxylic acids extraction by selected types of hydrophobic ionic liquids

An overview on specific phenomena in extraction of carboxylic acids with hydrophobic ionic liquids (ILs) based on results of new measurements with selected phosphonium, ammonium and imidazolium ILs and published data is presented. Formation of IL – acid hydrated complexes with multiple molecules of organic acid per one IL ion pair was observed. The distribution coefficient of carboxylic acids and water content in ILs strongly decreases with the increasing acid concentration. Dependence of water content in the solvent passes through a minimum at loading of IL with butyric acid of about 3. Two extraction mechanisms are involved: competitive extraction of acid and water with the release of water from the solvent and co-extraction of water with acid depending on the IL concentration. A strong synergistic effect was observed between the cation and anion of ILs enhancing their extractive properties compared to IL precursors. A new extraction model suggests the formation of water bridges and polar nano-channels which is in agreement with the molecular modelling results. ILs are nano-segregated liquids with a structure sensitive to the content of molecular compounds. Water and carboxylic acids accumulate in polar domains and dodecane in non-polar domains modifying the IL structure and decreasing the solvent phase viscosity. The hypothesis of hopping mechanism in polar channels for acid molecules transport between acid chains at IL binding sites is suggested.     

Ionic liquid/water mixtures: from hostility to conciliation

Water was originally inimical to ionic liquids (ILs) especially in the analysis of their detailed properties. Various data on the properties of ILs indicate that there are two ways to design functions of ionic liquids. The first is to change the structure of component ions, to provide "task-specific ILs". The second is to mix ILs with other components, such as other ILs, organic solvents or water. Mixing makes it easy to control the properties of the solution. In this strategy, water is now a very important partner. Below, we summarise our recent results on the properties of IL/water mixtures. Stable phase separation is an effective method in some separation processes. Conversely, a dynamic phase change between a homogeneous mixture and separation of phases is important in many fields. Analysis of the relation between phase behaviour and the hydration state of the component ions indicates that the pattern of phase separation is governed by the hydrophilicity of the ions. Sufficiently hydrophilic ions yielded ILs that are miscible with water, and hydrophobic ions gave stable phase separation with water. ILs composed of hydrophobic but hydrated ions undergo a dynamic phase change between a homogeneous mixture and separate phases according to temperature. ILs having more than seven water molecules per ion pair undergo this phase transition. These dynamic phase changes are considered, with some examples, and application is made to the separation of water-soluble proteins.     

Influence of Substituents and Functional Groups on the Surface Composition of Ionic Liquids

We have performed a systematic study addressing the surface behavior of a variety of functionalized and non‐functionalized ionic liquids (ILs). From angle‐resolved X‐ray photoelectron spectroscopy, detailed conclusions on the surface enrichment of the functional groups and the molecular orientation of the cations and anions is derived. The systems include imidazolium‐based ILs methylated at the C2 position, a phenyl‐functionalized IL, an alkoxysilane‐functionalized IL, halo‐functionalized ILs, thioether‐functionalized ILs, and amine‐functionalized ILs. The results are compared with the results for corresponding non‐functionalized ILs where available. Generally, enrichment of the functional group at the surface is only observed for systems that have very weak interaction between the functional group and the ionic head groups.     

Surface tensions of imidazolium based ionic liquids: anion, cation, temperature and water effect

This work addresses the experimental measurements of the surface tension of eight imidazolium based ionic liquids (ILs) and their dependence with the temperature (288-353 K) and water content. The set of selected ionic liquids was chosen to provide a comprehensive study of the influence of the cation alkyl chain length, the number of cation substitutions and the anion on the properties under study. The influence of water content in the surface tension was studied for several ILs as a function of the temperature as well as a function of water mole fraction, for the most hydrophobic IL investigated, [omim][PF(6)], and one of the more hygroscopic IL, [bmim][PF(6)]. The surface thermodynamic functions such as surface entropy and enthalpy were derived from the temperature dependence of the surface tension values.