Influence of the Alkyl Chain Length on the Low Temperature Phase Transitions of Imidazolium Based Ionic Liquids

Journal of Solution Chemistry - The effects induced by alkyl chain length on the thermal phase behavior for a series of imidazolium based ionic liquids (ILs) containing the anion...     

Tunable Lower Critical Solution Temperature of Poly(butyl acrylate) in Ionic Liquid Blends

We describe the lower critical solution temperature(LCST)-type phase behavior of poly(butyl acrylate)(PBA) dissolved in hydrophobic 1-alkyl-3-methylimidazolium bis{(trifluoromethyl) sulfonyl}amide ionic liquids(ILs). The temperature-composition phase diagrams of these PBA/ILs systems are strongly asymmetric with the critical composition shifted to low concentrations of PBA. As the molecular weight increases from 5.0×10~3 to 2.0×10~4, the critical temperature decreases by about 67 °C, and the critical composition shifts to a lower concentration.Furthermore, the LCST of PBA/ILs system increases as increasing the alkyl side chain length in the imidazolium cation. Using IL blends as solvents,the LCST of PBA can be tuned almost linearly over a wide range by varying the mixing ratio of two ionic liquids without modifying the chemical structure of the polymers.     

The dielectric response of room-temperature ionic liquids: effect of cation variation

In continuation of recent work on the dielectric response of imidazolium-based ionic liquids (ILs) (J. Phys. Chem. B, 2006, 110, 12682), we report on the effect of cation variation on the frequency-dependent dielectric permittivity up to 20 GHz of ionic liquids. The salts are comprised of pyrrolidinium, pyridinium, tetraalkylammonium, and triethylsulfonium cations combined with the bis-((trifluoromethyl)sulfonyl)imide anion. The dielectric spectra resemble those observed for imidazolium salts with the same anion. In all cases, the major contribution results from a diffusive low-frequency response on the time scale of several 100 ps, which shows a broadly distributed kinetics similar to that of spatially heterogeneous states in supercooled and glassy systems rather than that observed in fluid systems. There is evidence for a weak secondary process near 10-20 ps. Perhaps the most interesting difference to imidazolium salts is founded in the missing portions of the spectra due to processes beyond the upper cutoff frequency of 20 GHz. These are lower than that observed for imidazolium-based salts and seem to vanish for tetraalkylammonium and triethylsulfonium salts. As for imidazolium salts, the extrapolated static dielectric constants are on the order of epsilon(S) congruent with 10-13, classifying these ILs as solvents of moderate polarity.     

N-羧基吡啶功能化离子液体的表征

合成了系列新型N-羧基吡啶功能化离子液体, 利用1H NMR、13C NMR、IR、DSC对其进行表征并研究了其与常规溶剂的相溶性, 采用酸碱滴定法测量了系列离子液体的酸离解常数pKa值. N-羧基取代吡啶功能化离子液体的pKa值在2.5~4.0之间, 并随阳离子取代羧基碳链的增长而增大; 离子之间形成氢键及阴、阳离子的大小是影响离子液体熔点的主要因素. 阴离子越小, 熔点越高. 所合成的N-羧基吡啶功能化离子液体具有相同的相溶性且由取代羧基所决定, 与常见烷基咪唑离子液体相比, N-羧基吡啶功能化离子液体与丙酮、二氯甲烷并不相溶. 功能化离子液体的阳离子取代基是影响其物化性能的主要因素, 通过改变功能化基团碳链的长短及与不同阴离子进行组合, 可以对功能化离子液体物理、化学性能进行调节.     

Water sorption in ionic liquids: kinetics, mechanisms and hydrophilicity

Most of the ionic liquids (ILs) are hygroscopic in air. The effects of structural factors of ILs (cation, anion, alkyl chain length at cation, and C2 methylation at cation) and external factors (temperature, relative humidity, and impurity) on the kinetics of water sorption by 18 ILs were investigated. A modified two-step sorption mechanism was proposed to correlate the water sorption data in the ILs. Three type of parameters (sorption capacity, sorption rate and degree of difficulty to reach sorption equilibrium) based on the modified two-step mechanism were derived to comprehensively characterize the water sorption processes. These parameters have similar tendencies, providing an efficient way to evaluate them by one parameter that can be easily obtained. The hydrophilicity of the ILs was classified to four levels (super-high, high, medium, low) according to the water sorption capacity. The results show that cation of the ILs also plays an important role in water sorption, and the impurities affect the water sorption enormously. Acetate and halogen-based ILs have the highest hydrophilicity when combined with the imidazolium or pyridinium cation.     

Equation of state modeling of the phase equilibria of ionic liquid mixtures at low and high pressure

Accurate design of processes based on ionic liquids (ILs) requires knowledge of the phase behavior of the systems involved. In this work, the truncated perturbed chain polar statistical associating fluid theory (tPC-PSAFT) is used to correlate the phase behavior of binary and ternary IL mixtures. Both non-polar and polar solvents are examined, while methyl imidazolium ILs are used in all cases. tPC-PSAFT accounts explicitly for weak dispersion interactions, highly directive polar interactions between permanent dipolar and quadrupolar molecules and association between hydrogen bonding molecules. For mixtures of non-polar solvents, tPC-PSAFT predicts accurately the binary mixture data. For the case of polar solvents, a binary interaction parameter is fitted to the experimental data and the agreement between experiment and correlation is very good in all cases.     

Thermal Analysis of Binary Mixtures of Imidazolium,Pyridinium, Pyrrolidinium,and Piperidinium Ionic Liquids

Ionic liquids (ILs) are being widely studied due to their unique properties, which make them potential candidates for conventional solvents. To study whether binary mixtures of pure ionic liquids provide a viable alternative to pure ionic liquids for different applications, in this work, the thermal analysis and molar heat capacities of five equimolar binary mixtures of ionic liquids based on imidazolium, pyridinium, pyrrolidinium, and piperidinium cations with dicyanamide, trifluoromethanesulfonate, and bis(trifluoromethylsulfonyl)imide anions have been performed. Furthermore, two pure ionic liquids based on piperidinium cation have been thermally characterized and the heat capacity of one of them has been measured. The determination and evaluation of both the transition temperatures and the molar heat capacities was carried out using differential scanning calorimetry (DSC). It was observed that the thermal behavior of the mixtures was completely different than the thermal behavior of the pure ionic liquids present, while the molar heat capacities of the binary mixtures were very similar to the value of the average of molar heat capacities of the two pure ionic liquids.     

Effects of Cationic Structure on Cellulose Dissolution in Ionic Liquids: A Molecular Dynamics Study

In recent years, great progress has been made in the dissolution of cellulose with ionic liquids (ILs). However, the mechanism of cellulose dissolution, especially the role the IL cation played in the dissolution process, has not been clearly understood. Herein, the mixtures of cellulose with a series of imidazolium‐based chloride ionic liquids and 1‐butyl‐3‐methyl pyridinium chloride ([C₄mpy]Cl) were simulated to study the effect that varying the heterocyclic structure and alkyl chain length of the IL cation has on the dissolution of cellulose. It was shown that the dissolution of cellulose in [C₄mpy]Cl is better than that in [C₄mim]Cl. For imidazolium‐based ILs, the shorter the alkyl chain is, the higher the solubility will be. In addition, an all‐atom force field for 1‐allyl‐3‐methyl imidazolium cation ([Amim]⁺) was developed, for the first time, to investigate the effect the electron‐withdrawing group within the alkyl chain of the IL cation has on the dissolution of cellulose. It was found that the interaction energy between [Amim]⁺ and cellulose was greater than that between [C₃mim]⁺ and cellulose, indicating that the presence of electron‐withdrawing group in alkyl chain of the cation enhanced the interaction between the cation and cellulose due to the increase of electronegativity of the cations. These findings are used to assess the cationic effect on the dissolution of cellulose in ILs. They are also expected to be important for rational design of novel ILs for efficient dissolution of cellulose.     

Novel One-step Preparation of Functional Ionic Liquids Using Epoxides as Alkylation Reagents

Owing to their unique chemical and physical properties, ionic liquids (ILs) have received focus attention for application as solvent alternatives. ILs can be used in place of organic solvents in synthesis, catalysis, electrochem istry and liquid/liquid extractions. The commonly reported ILs have relied on pyridinium or imidazolium cations bearing simple alkyl groups.     

Physico-chemical properties and phase behaviour of piperidinium-based ionic liquids

The sufficient review of the existing literature of the 1-alkyl-1-methylppiperidinium-based ionic liquids has been presented. The phase diagrams for the binary systems of {1-butyl-1-methylpiperidinium thiocyanate [BMPIP][SCN] + an alcohol (1-hexanol, 1-heptanol, 1-octanol, 1-nonanol, 1-decanol, 1-dodecanol), or + water, or + aliphatic hydrocarbons (n-hexane, n-heptane, n-octane), or + cyclohexane, or, + cycloheptane, or + aromatic hydrocarbons (benzene, toluene, ethylbenzene)} and for the binary systems of {1-ethyl-1-methylpiperidinium bis{(trifluoromethyl)sulfonyl}imide [EMPIP][NTf₂] + an alcohol (ethanol, 1-propanol, 1-butanol, 1-hexanol, 1-heptanol, 1-octanol, 1-nonanol), or + water} have been determined at atmospheric pressure using a dynamic method. The influence of an alcohol chain length was discussed for these ionic liquids. A systematic decrease in the solubility was observed with an increase of the alkyl chain length of an alcohol. (Solid + liquid) phase equilibria with complete miscibility in the liquid phase region were observed for the systems involving water and the alcohols for the thiocyanate-based ionic liquid. Opposite, the bis{(trifluoromethyl)sulfonyl}imide-based ionic liquid reveal the immiscibility gap in the liquid phase. The correlation of the experimental data has been carried out using the NRTL equation. The phase diagrams reported here have been compared to the systems published earlier with the 1-alkyl-1-methylpiperidinium-based ionic liquids. The influence of the cation and anion on the phase behaviour has been discussed. The basic thermal properties of pure ILs, i.e. melting temperature and the enthalpy of fusion, the solid-solid phase transition temperature and enthalpy have been measured using a differential scanning microcalorimetry technique.     

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.     

Effect of Cation Structure in Quinolinium-Based Ionic Liquids on the Solubility in Aromatic Sulfur Compounds or Heptane: Thermodynamic Study on Phase Diagrams

Experimental and theoretical studies on thermodynamic properties of quinolinium-based ionic liquids (ILs) based on bis(trifluoromethylsulfonyl)imide anion (namely N-butyl-quinoloinium bis(trifluoromethylsulfonyl)imide, [BQuin][NTf₂], N-hexylquinoloinium bis(trifluoromethyl-sulfonyl)imide, [HQuin][NTf₂], and N-octylquinoloinium bis(trifluoromethyl-sulfonyl)imide, [OQuin][NTf₂]) with aromatic sulfur compounds and heptane, as a model compound of fuel were examined in order to assess the applicability of the studied ionic liquids for desulfurization of fuels. With this aim, the temperature-composition phase diagrams of 13 binary mixtures composed of organic sulfur compounds (thiophene, benzothiophene, or 2-methylthiophene) or heptane and ionic liquid (IL) were investigated at ambient pressure. A dynamic method was used to determine the (solid–liquid) equilibrium phase diagrams in binary systems over a wide composition range and temperature range from T = 255.15 to 365.15 K up to the fusion temperature of ILs. The immiscibility gap with an upper critical solution temperature (UCST) was observed for each binary system under study. The influence of the alkane chain length of the substituent on the IL cation and of the sulfur compounds (the aromaticity of the solvent) was described. The experimental (solid + liquid) phase equilibrium dataset were successfully correlated using the well-known NRTL equation.     

Imidazolium Salt Ion Pairs in Solution

The formation, stabilisation and reactivity of contact ion pairs of non‐protic imidazolium ionic liquids (ILs) in solution are conceptualized in light of selected experimental evidence as well theoretical calculations reported mainly in the last ten years. Electric conductivity, NMR, ESI‐MS and IR data as well as theoretical calculations support not only the formation of contact ion pairs in solution, but also the presence of larger ionic and neutral aggregates even when dissolved in solvents with relatively high dielectric constants, such as acetonitrile and DMSO. The presence of larger imidazolium supramolecular aggregates is favoured at higher salt concentrations in solvents of low dielectric constant for ILs that contain shorter N‐alkyl side chains associated with anions of low coordination ability. The stability and reactivity of neutral contact species are also dependent on the nature of the anion, imidazolium substituents, and are more abundant in ILs containing strong coordinating anions, in particular those that can form charge transfer complexes with the imidazolium cation. Finally, some ILs display reactivities as contact ion pairs rather than solvent‐separated ions.     

Reversed phase liquid chromatography of alkyl-imidazolium ionic liquids

Eleven 1-alkyl-3-methyl imidazolium ionic liquid (IL) salts were analyzed in reversed phase mode with a Kromasil C18 column. The mobile phases were water-rich acetonitrile solutions (water content > or =70%, v/v) without any added salts. It is shown that it is possible to separate different ILs sharing the same cation and differing by the anion when salt-free mobile phases are used. When a buffer, acetate or phosphate salt, or any salt, such as sodium chloride or sodium tetrafluorobarate, is added to the mobile phase, the ILs differing only by their anions cannot be separated. ILs with different alkyl chains in the imidazolium cation are separated by mobile phases with or without added salts following a hydrophobic interaction behavior: log k is proportional to nC, the carbon number of the alkyl chain. Important differences in ion/stationary phase interactions are observed depending on the ionic content of the mobile phase. With salt-free mobile phases, the IL/C18 stationary phase interactions correspond to concave isotherms associated with fronting peaks for all ILs. With mobile phase containing 0.01 M of salt, tailing IL peaks correspond to convex adsorption isotherms. Also, the IL retention factor depends on the concentration and nature of the added salt. Hexafluorophosphate chaotropic anions can adsorb on the Kromasil C18 surface dramatically increasing the imidazolium cation retention factors.     

Xenon Diffusion in Ionic Liquids with Blurred Nanodomain Separation

The dynamics of xenon gas, loaded in a series of 1-alkyl-3-methylimidazolium based ionic liquids, probes the formation of increasingly blurred polar/apolar nanodomains as a function of the anion type and the cation chain length. Exploiting ¹²⁹Xe NMR spectroscopy techniques, like Pulse Gradient Spin Echo (PGSE) and inversion recovery (IR), the diffusion motion and relaxation times are determined for 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [C_nC₁im][TFSI]. A correlation between the ILs nano-structure and both xenon diffusivity and relaxation times, as well as chemical shifts, is outlined. Interestingly, comparison with previous results of the same properties in the homologous imidazolium chlorides and hexafluorophospate shows an opposite trend with the alkyl chain length. Classical molecular dynamics (MD) simulations are used to calculate the xenon and cation and anion diffusion coefficients in the same systems, including imidazolium cations with longer chains (n=4, 6, 8 … 20). An almost quantitative agreement with the experiments validates the MD simulations and, at the same time, provides the necessary structural and dynamic microscopic insights on the nano-segregation and diffusion of xenon in bistriflimide, chloride and hexafluorphosphate salts allowing to observe and rationalize the shaping effect of the cation in the nanostructure.     

Alkylimidazolium Protic Ionic Liquids: Structural Features and Physicochemical Properties

We focus on a series of protic ionic liquids (PILs) with imidazolium and alkylimidazolium (1R3HIm, R=methyl, ethyl, propyl, and butyl) cations. Using the literature data and our experimental results on the thermal and transport properties, we analyze the effects of the anion nature and the alkyl radical length in the cation structure on the above properties. DFT calculations in gas and solvent phase provide further microscopic insights into the structure and cation-anion binding in these PILs. We show that the higher thermodynamic stability of an ion pair raises the PIL decomposition temperature. The melting points of the salts with the same cation decrease as the hydrocarbon radical in the cation becomes longer, which correlates with the weaker ion-ion interaction inthe ion pairs. A comparative analysis of the protic ILs and corresponding ILs (1R3MeIm) with the same radical (R) in the cation structure and the same anion has been performed. The lower melting points of the ILs with 1R3MeIm cations are assumed to result from the weakening of both the ion-ion interaction and the hydrogen bond.     

以离子液体为流动相添加剂高效液相色谱法分离钩藤药材中的钩藤碱和异钩藤碱

以常用流动相添加剂三乙胺作为对照,建立了以离子液体为流动相添加剂,分离钩藤药材中钩藤碱和异钩藤碱的高效液相色谱方法。以分离度及相关色谱参数为指标,选择了离子液体中咪唑阳离子烷基链长度及阴离子的种类。并分别考察了咪唑阳离子烷基链长度、离子液体浓度、流动相pH值和流动相比例对钩藤碱和异钩藤碱分离的影响,初步探讨了离子液体的分离机理。结果显示,咪唑阳离子的烷基链越长,阴离子的离子液体序列越高,分离效果越好,即[HMIM][BF_4]为最优的流动相添加剂。当[HMIM][BF_4]浓度为16 mmol/L,流动相pH值为3.0,甲醇比例为37%时,钩藤碱和异钩藤碱能够实现基线分离,满足样品分离测定的需求。     

Some Aspects of Ionic Liquids as Diverse and Versatile Sustainable Solvents

In this mini review several particular cases of ionic liquid solution behavior are discussed, namely solutions of ionic liquids with aliphatic alcohols, polyalcohols, arenes, chloromethanes and poly(ethyleneglycol) as well as salting-out and salting-in effects. The presented cases clearly expose ionic liquids as being diverse and versatile sustainable solvents that exhibit flexible phase behavior and, consequently, variable phase diagrams. These can be tuned either by changing the length of the alkyl chain(s) of the ionic liquid??s cation (and/or sometimes the anion), or by introducing a different aliphatic nature to the other solution constituent (e.g., by varying polymer chain length or the number of carbon atoms in chloromethanes). Sometimes, the evolution of the phase diagrams is fine and continuous, showing several consecutive stages, revealing both qualitative and quantitative changes. Finally, the diversity and versatility of ionic liquids are viewed as important features that contribute to their efficiency as tunable solvents or salting media.     

Identification of 1,3-dialkylimidazolium salt supramolecular aggregates in solution

The nature of the interactions between 1,3-dialkylimidazolium cations and noncoordinating anions such as tetrafluoroborate, hexafluorophosphate, and tetraphenylborate has been studied in the solid state by X-ray diffraction analysis and in solution by (1)H NMR spectroscopy, conductivity, and microcalorimetry. In the solid state, these compounds show an extended network of hydrogen-bonded cations and anions in which one cation is surrounded by at least three anions and one anion is surrounded by at least three imidazolium cations. In the pure form, imidazolium salts are better described as polymeric supramolecules of the type {[(DAI)(3)(X)](2+)[(DAI)(X)(3)](2-)}(n) (where DAI is the dialkylimidazolium cation and X is the anion) formed through hydrogen bonds of the imidazolium cation with the anion. In solution, this supramolecular structural organization is maintained to a great extent, at least in solvents of low dielectric constant, indicating that mixtures of imidazolium ionic liquids with other molecules can be considered as nanostructured materials. This model is very useful for the rationalization of the majority of the unusual behavior of the ionic liquids.     

Phase diagrams of ionic liquids-based aqueous biphasic systems as a platform for extraction processes

In the past few years, ionic liquid-based aqueous biphasic systems have become the subject of considerable interest as a promising technique for the extraction and purification of several macro/biomolecules. Aiming at developing guidelines for more benign and efficient extraction processes, phase diagrams for aqueous biphasic systems composed of ionic liquids and inorganic/organic salts are here reported. Several combinations of ionic liquid families (imidazolium, pyridinium, phosphonium, quaternary ammonium and cholinium) and salts [potassium phosphate buffer (KH₂PO₄/K₂HPO₄ at pH 7), potassium citrate buffer (C₆H₅K₃O₇/C₆H₈O₇ at pH 5, 6, 7 and 8) and potassium carbonate (K₂CO₃ at pH ∼13)] were evaluated to highlight the influence of the ionic liquid structure (cation core, anion and alkyl chain length), the pH and the salt nature on the formation of aqueous biphasic systems. The binodal curves and respective tie-lines reported for these systems were experimentally determined at (298 ± 1) K. In general, the ability to promote the aqueous biphasic systems formation increases with the pH and alkyl chain length. While the influence of the cation core and anion nature of the ionic liquids on their ability to form aqueous biphasic systems closely correlates with ionic liquids capacity to be hydrated by water, the effect of the different salts depends of the ionic liquid nature and salt valency.