Towards an understanding of the mutual solubilities of water and hydrophobic ionic liquids in the presence of salts: the anion effect

Phase Behavior of Ionic Liquid-Based Aqueous Two-Phase Systems

As an environmentally friendly separation medium, the ionic liquid (IL)-based aqueous two-phase system (ATPS) is attracting long-term attention from a growing number of scientists and engineers. Phase equilibrium data of IL-based ATPSs are an important basis for the design and optimization of chemical reactions and separation processes involving ILs. This article provides the recent significant progress that has been made in the field and highlights the possible directions of future developments. The effects of each component (such as salting-out agents and ILs) on the phase behavior of IL-based ATPSs are summarized and discussed in detail. We mainly focus on the phase behavior of ATPSs by using ILs, expecting to provide meaningful and valuable information that may promote further research and application.

Development and Challenges of Biphasic Membrane-Less Redox Batteries

Ion exchange membranes (IEMs) play important roles in energy generation and storage field, such as fuel cell, flow battery, however, a major barrier in the way of large-scale application is the high cost of membranes (e.g., Nafion membranes price generally exceeds USD$ 200 m-2 ). The membrane-less technology is one of the promising approaches to solve the problem and thus has attracted much attention and been explored in a variety of research paths. This review introduces one of the representative membrane-less battery types, Biphasic membrane-less redox batteries that eliminate the IEMs according to the principle of solvent immiscibility and realizes the phase splitting in a thermodynamically stable state. It is systematically classified and summarizes their performances as well as the problems they are suffering from, and then several effective solutions are proposed based on the modification of electrodes and electrolytes. Finally, special attention is given to the challenges and prospects of Biphasic membrane-less redox batteries, which could contribute to the development of membrane-less batteries.

Chiral Ionic Liquids: Structural Diversity, Properties and Applications in Selected Separation Techniques

Ionic liquids (ILs) are chemical compounds composed of ions with melting points below 100 °C exhibiting a design feature. ILs are commonly used as the so-called green solvents, reagents or highly efficient catalysts in varied chemical processes. The huge application potential of ionic liquids (IL) justifies the growing interest in these compounds. In the last decade, increasing attention has been devoted to the development of new methods in the synthesis of stable chiral ionic liquids (CILs) and their application in various separation techniques. The beginnings of the successful use of CILs to separate enantiomers date back to the 1990 s. Most chiral ILs are based on chiral cations or chiral anions. There is also a limited number of CILs possessing both a chiral cation and a chiral anion. Due to the high molecular diversity of both ions, of which at least one has a chiral center, we have the possibility to design a large variety of optically active structures, thus expanding the range of CIL applications. Research utilizing chiral ionic liquids only recently has become more popular. However, it is the area that still has great potential for future development. This review aimed to describe the diversity of structures, properties and examples of applications of chiral ionic liquids as new chiral solid materials and chiral components of the anisotropic environment, providing chiral recognition of enantiomeric analytes, which is useful in liquid chromatography, countercurrent chromatography and other various CIL-based extraction techniques including aqueous biphasic (ABS) extraction systems, solid-liquid two-phase systems, liquid-liquid extraction systems with hydrophilic CILs, liquid-liquid extraction systems with hydrophobic CILs, solid-phase extraction and induced-precipitation techniques developed in the recent years. The growing demand for pure enantiomers in the pharmaceutical and food industries sparks further development in the field of extraction and separation systems modified with CILs highlighting them as affordable and environmentally friendly both chiral selectors and solvents.

Ion chromatography for monitoring [NTf2]- anion contaminants in pure and saline water

Hydrophobic ionic liquids containing bis(trifluoromethanesulfonyl)imide, [NTf2]-, anions partially dissolve in aqueous phases. The potential ecotoxicity of [NTf2]- means wastewater streams must be closely monitored to avoid environmental release. A new ion chromatography method is presented, which improves on existing techniques and methods by significantly decreasing analysis time and improving chromatographic peak properties. Consequently, the limit of detection is lowered to 5 μM (1.4 ppm) and limit of quantification lowered to 30 μM (8.5 ppm). The method is reproducible and has a high precision for short and medium chain length ionic liquids (RSD = 0.95%); however, microemulsion effects increase measurement errors for long chain ionic liquids (RSD = 2.32%), albeit by a relatively small amount. Hence, the method is a highly accurate analytical method for highly polarizable [NTf2]- anions, and quantification in the presence of high salinity samples, such as seawater, is readily achieved. Importantly, this method is a significant improvement on existing techniques (chromatography, NMR, UV-Vis) for many reasons, making it ideal for environmental monitoring or process design of [NTf2]- ionic liquid-based applications.

Development and Application of Supported Ionic Liquid Membranes in Microbial Fuel Cell Technology: A Concise Overview

Membrane separators are key elements of microbial fuel cells (MFCs), especially of those constructed in a dual-chamber configuration. Until now, membranes made of Nafion have been applied the most widely to set-up MFCs. However, there is a broader agreement in the literature that Nafion is expensive and in many cases, does not meet the actual (mainly mass transfer-specific) requirements demanded by the process and users. Driven by these issues, there has been notable progress in the development of alternative materials for membrane fabrication, among which those relying on the deployment of ionic liquids are emerging. In this review, the background of and recent advances in ionic liquid-containing separators, particularly supported ionic liquid membranes (SILMs), designed for MFC applications are addressed and evaluated. After an assessment of the basic criteria to be fulfilled by membranes in MFCs, experiences with SILMs will be outlined, along with important aspects of transport processes. Finally, a comparison with the literature is presented to elaborate on how MFCs installed with SILM perform relative to similar systems assembled with other, e.g., Nafion, membranes.

Binary Mixtures of Ionic Liquids in Aqueous Solution: Towards an Understanding of their Salting-In/Salting-Out Phenomena

The order of the salting-in or salting-out inducing ability of ions on the aqueous solubility of macromolecules in aqueous solutions is known as the Hofmeister series. Taking into account that ionic liquids (ILs) are constituted by ions, they can exert similar effects on the solubility of other ILs in aqueous media. In order to expand the knowledge on the salting-in/-out ability of ILs, experimental studies on the solubility of 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonylimide) in water in presence of other IL/salts were conducted at 298.15 K at atmospheric pressure. Both the impact of the anion and cation of the IL were evaluated with the following ILs/salts: 1-butyl-3-methylimidazolium chloride, 1-butyl-3-methylimidazolium hydrogensulfate, cholinium bis(trifluoromethylsulfonyl)imide, 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide and lithium bis(trifluoromethylsulfonyl)imide, in a wide composition range. As happens with common salts, both salting-in and salting-out effects exerted by ILs were observed, with a higher impact exerted by the IL anion on the salting-out phenomenon. These data allow to better understand the ILs impact when designing liquid-liquid separation processes.

Salting-in species induced self-assembly of stable MOFs

A general strategy based on the Hoffmeister effect is proposed for the aqueous-phase and mild synthesis of stable metal–organic frameworks.

Metal–organic frameworks (MOFs) are attracting immense research interest despite the fact that their synthesis usually proceeds in organic media or under harsh conditions depending on specific cases. Herein, Hofmeister effect was firstly introduced for the construction of MOFs and thereafter a general salting-in species (SS) induced self-assembly strategy was proposed for the aqueous-phase and mild synthesis of stable MOFs based on a unique “solubilization-mediating” mechanism. The SS not only improved the solubility of organic ligands, but also effectively mediated the mutual proximity of the organic linkers and the inorganic nodes, thus facilitating the crystallization of MOFs under mild conditions. Several typical and highly useful stable MOFs were exemplified owing to the availability of various SS. This strategy could set a framework for the development of more stable MOFs in aqueous phase and drive the large-scale and economic production of MOFs.

Mechanisms of phase separation in temperature-responsive acidic aqueous biphasic systems

The thermal and acid responsive behaviour of bulky phosphonium-based ILs is elucidated using a mixed experimental and computational approach.

Formation of Aqueous Biphasic Systems with an Ionic Liquid Induced by Metallic Salts: Nanoscopic Views from Molecular Dynamics Simulations

The formation of aqueous biphasic systems (ABSs) based on aqueous ionic liquid (IL)/salt mixtures has been investigated via molecular dynamics simulations (with IL butyl-methyl-imidazolium triflate; salts NaCl, CsCl, SrCl₂, and EuCl₃). The analysis of ion distributions, solvation, and mutual interactions during the dynamics reveals the heterogeneity of all solutions due to ion segregation into mutually exclusive IL and salt domains, even in monophasic solutions ("ionic sociology"). Ion segregation and ABS formation are found to increase with (i) the salt content and (ii) the IL content, (iii) in the order Na⁺ < Sr²⁺ < Eu³⁺, and (iv) when the IL ion "polarity" is diminished, following experimental trends. The structuration of the solution is rationalized as a synergistic water transfer from the best donating ion pair (first hydration shell of hydrophobic moieties of IL ions) to the best accepting pair (M ⁿ⁺ and Cl^- ions, beyond their first shell). In ABSs, the IL- and salt-containing phases are linked by a well-defined "interface" that decreases in width when MCl _n becomes more hydrophilic and/or more concentrated. In the IL-rich phase of ABSs, the hydration of IL ions and their mutual interactions are shown to be similar to those displayed at aqueous interfaces.

Extraction in cholinium-based magnetic ionic liquid aqueous two-phase system for the determination of berberine hydrochloride in Rhizoma coptidis

A magnetic ionic liquids (MILs)-based aqueous two-phase system (MIL-ATPs) obtained by mixing with a series of inorganic salts, which involves five cholinium MILs with the piperidinyloxy radical anion is reported for the first time. Phase diagrams for the new ATPs were experimentally determined at different temperatures (298.15–318.15 K) and the liquid–liquid equilibrium data for two-phase systems were correlated according to the empirical nonlinear expression. The effects of the types of MILs, temperature and inorganic salts on the binodal curve are discussed in detail. The MIL-ATPs coupled with HPLC-UV analysis was developed in the quantitation of berberine hydrochloride in Rhizoma coptidis. Under optimal conditions, the partition coefficient of berberine hydrochloride was 127.68 with the precision values (RSD%) of 1.40% and 2.83% for intra-day (n = 6) and inter-day (n = 3), respectively. The limit of detection (LOD) and limit of quantification (LOQ) for berberine hydrochloride were 0.023 mg L⁻¹ and 0.077 mg L⁻¹, respectively. The recoveries were obtained in the acceptable range of 97.4–101.2%. Moreover, the content of berberine hydrochloride in the raw material of Rhizoma coptidis was measured as 123.95 mg g⁻¹ with this method. Finally, 99.8% MIL was recovered for cycle application after the removal of berberine hydrochloride by using D101 resin. This study provides a meaningful reference for the application of MIL-ATPs with great prospects.

Five cholinium type organic magnetic ionic liquids have been applied in ionic liquid-based aqueous two-phase systems by mixing with a series of inorganic salts, which is reported to extract berberine in quantitative analysis for the first time.

Mutual solubility of water and hydrophobic ionic liquids in the presence of hydrochloric acid

Possible mechanism for ionic liquid dissolution in HCl solution.

Effect of salts on the solubility of ionic liquids in water: experimental and electrolyte Perturbed-Chain Statistical Associating Fluid Theory

Due to scarce available experimental data, as well as due to the absence of predictive models, the influence of salts on the solubility of ionic liquids (ILs) in water is still poorly understood. To this end, this work addresses the solubility of the IL 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C4C1im][NTf2]), at 298.15 K and 0.1 MPa, in aqueous salt solutions (from 0.1 to 1.5 mol kg(-1)). At salt molalities higher than 0.2 mol kg(-1), all salts caused salting-out of [C4C1im][NTf2] from aqueous solution with their strength decreasing in the following order: Al2(SO4)3 > ZnSO4 > K3C6H5O7 > KNaC4H4O6 > K3PO4 > Mg(CH3CO2)2 > K2HPO4 > MgSO4 > KH2PO4 > KCH3CO2. Some of these salts lead however to the salting-in of [C4C1im][NTf2] in aqueous medium at salt molalities lower than 0.2 mol kg(-1). To attempt the development of a model able to describe the salt effects, comprising both the salting-in and salting-out phenomena observed, the electrolyte Perturbed-Chain Statistical Associating Fluid Theory (ePC-SAFT) was applied using ion-specific parameters. The gathered experimental data was modelled using ePC-SAFT parameters complemented by fitting a single binary parameter between K(+) and the IL-ions to the IL solubility in K3PO4 aqueous solutions. Based on this approach, the description of anion-specific salting-out effects of the remaining potassium salts was found to be in good agreement with experimental data. Remarkably, ePC-SAFT is even able to predict the salting-in effect induced by K2HPO4, based on the single K(+)/IL-ions binary parameter which was fitted to an exclusively salting-out effect promoted by K3PO4. Finally, ePC-SAFT was applied to predict the influence of other sodium salts on the [C4C1im][NTf2] solubility in water, with experimental data taken from literature, leading to an excellent description of the liquid-liquid phase behaviour.

Aqueous Solutions of Ionic Liquids: Microscopic Assembly

Aqueous solutions of ionic liquids are of special interest, due to the distinctive properties of ionic liquids, in particular, their amphiphilic character. A better understanding of the structure-property relationships of such systems is hence desirable. One of the crucial molecular-level interactions that influences the macroscopic behavior is hydrogen bonding. In this work, we conduct molecular dynamics simulations to investigate the effects of ionic liquids on the hydrogen-bond network of water in dilute aqueous solutions of ionic liquids with various combinations of cations and anions. Calculations are performed for imidazolium-based cations with alkyl chains of different lengths and for a variety of anions, namely, [Br](-), [NO3](-), [SCN](-) [BF4](-), [PF6](-), and [Tf2N](-). The structure of water and the water-ionic liquid interactions involved in the formation of a heterogeneous network are analyzed by using radial distribution functions and hydrogen-bond statistics. To this end, we employ the geometric criterion of the hydrogen-bond definition and it is shown that the structure of water is sensitive to the amount of ionic liquid and to the anion type. In particular, [SCN](-) and [Tf2N](-) were found to be the most hydrophilic and hydrophobic anions, respectively. Conversely, the cation chain length did not influence the results.

Good's buffers as novel phase-forming components of ionic-liquid-based aqueous biphasic systems

Aiming at the development of self-buffering and benign extraction/separation processes, this work reports a novel class of aqueous biphasic systems (ABS) composed of ionic liquids (ILs) and organic biological buffers (Good's buffers, GBs). A large array of ILs and GBs was investigated, revealing than only the more hydrophobic and fluorinated ILs are able to form ABS. For these systems, the phase diagrams, tie-lines, tie-line lengths, and critical points were determined at 25 °C. The ABS were then evaluated as alternative liquid-liquid extraction strategies for two amino acids (L-phenylalanine and L-tryptophan). The single-step extraction efficiencies for the GB-rich phase range between 22.4 and 100.0 % (complete extraction). Contrarily to the most conventional IL-salt ABS, in most of the systems investigated, the amino acids preferentially migrate for the most biocompatible and hydrophilic GB-rich phase. Remarkably, in two of the studied ABS, L-phenylalanine completely partitions to the GB-rich phase while L-tryptophan shows a preferential affinity for the opposite phase. These results show that the extraction efficiencies of similar amino acids can be tailored by the design of the chemical structures of the phase-forming components, creating thus new possibilities for the use of IL-based ABS in biotechnological separations.

Overview of the effect of salts on biphasic ionic liquid/water solvent extraction systems: anion exchange, mutual solubility, and thermomorphic properties

Hydrophobic (water-immiscible) ionic liquids (ILs) are frequently used as organic phase in solvent extraction studies. These biphasic IL/water extraction systems often also contain metal salts or mineral acids, which can significantly affect the IL trough (un)wanted anion exchange and changes in the solubility of IL in the aqueous phase. In the case of thermomorphic systems, variations in the cloud point temperature are also observed. All these effects have important repercussions on the choice of IL, suitable for a certain extraction system. In this paper, a complete overview of the implications of metal salts on biphasic IL/water systems is given. Using the Hofmeister series as a starting point, a range of intuitive prediction models are introduced, supported by experimental evidence for several hydrophobic ILs, relevant to solvent extraction. Particular emphasis is placed on the IL betainium bis(trifluoromethylsulfonyl)imide [Hbet][Tf2N]. The aim of this work is to provide a comprehensive interpretation of the observed effects of metal salts, so that it can be used to predict the effect on any given biphasic IL/water system instead of relying on case-by-case reports. These prediction tools for the impact of metal salts can be useful to optimize IL synthesis procedures, extraction systems and thermomorphic properties. Some new insights are also provided for the rational design of ILs with UCST or LCST behavior based on the choice of IL anion.

Influence of process variables on extraction of Cefalexin in a novel biocompatible ionic liquid based-aqueous two phase system

Despite the fact that ionic liquid-based aqueous two phase systems (ATPSs) have been widely studied for extraction purposes, the adequacy of biodegradable organic salts as salting out agents has been left unexploited.

Homogeneous liquid–liquid extraction of neodymium(iii) by choline hexafluoroacetylacetonate in the ionic liquid choline bis(trifluoromethylsulfonyl)imide

Choline hexafluoroacetylacetonate diluted in the ionic liquid choline bis(trifluoromethylsulphonyl)imide has been used as an extractant for the homogeneous liquid–liquid extraction of neodymium(iii) ions.

Salting-in with a salting-out agent: explaining the cation specific effects on the aqueous solubility of amino acids

Although the understanding of ion specific effects on the aqueous solubilities of biomolecules is crucial for the development of many areas of biochemistry and life sciences, a consensual and well-supported molecular picture of the phenomena has not yet been established. Mostly, the influence of cations and the nature of the molecular interactions responsible for the reversal of the Hofmeister trend in aqueous solutions of amino acids and proteins are still defectively understood. Aiming at contributing to the understanding of the molecular-level mechanisms governing the cation specific effects on the aqueous solubilities of biocompounds, experimental solubility measurements and classical molecular dynamics simulations were performed for aqueous solutions of three amino acids (alanine, valine, and isoleucine), in the presence of a series of inorganic salts. The evidence gathered suggests that the mechanism by which salting-in inducing cations operate in aqueous solutions of amino acids is different from that of anions, and allows for a novel and consistent molecular description of the effect of the cation on the solubility based on specific interactions of the cations with the negatively charged moieties of the biomolecules.

Role of the Hofmeister series in the formation of ionic-liquid-based aqueous biphasic systems

Among the numerous and interesting features of ionic liquids is their ability to form aqueous biphasic systems (ABSs) when combined with inorganic or organic salts in aqueous media. In this work, a wide range of salts was studied, aiming at gathering a detailed picture on the molecular mechanisms that govern the ability of the salt ions to induce the formation of ionic-liquid-based ABSs. For that purpose, 1-butyl-3-methylimidazolium trifluoromethanesulfonate was chosen due to its facility to undergo liquid-liquid demixing in aqueous media containing conventional salts. The corresponding ternary phase diagrams, tie-lines, and tie-line lengths were determined at 298 K and atmospheric pressure. With the large body of data measured in this work, it was possible to establish a scale on the salt cation and anion abilities to induce the formation of ionic-liquid-based ABSs, which follows the Hofmeister series, and to show that the molar entropy of hydration of the salt ions is the driving force for aqueous two-phase system formation.