“Solvent-in-salt” systems for design of new materials in chemistry, biology and energy research

Vladimir A. Azov a, Ksenia S. Egorova a, Marina M. Seitkalieva a, Alexey S. Kashin a and Valentine P. Ananikov *ab
aN.D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, Leninsky Prospect 47, Moscow 119991, Russia. E-mail: val@ioc.ac.ru; Web: http://AnanikovLab.ru
bDepartment of Chemistry, Saint Petersburg State University, Stary Petergof, 198504, Russia

Received 22nd September 2017

First published on 7th February 2018

Inorganic and organic “solvent-in-salt” (SIS) systems have been known for decades but have attracted significant attention only recently. Molten salt hydrates/solvates have been successfully employed as non-flammable, benign electrolytes in rechargeable lithium-ion batteries leading to a revolution in battery development and design. SIS with organic components (for example, ionic liquids containing small amounts of water) demonstrate remarkable thermal stability and tunability, and present a class of admittedly safer electrolytes, in comparison with traditional organic solvents. Water molecules tend to form nano- and microstructures (droplets and channel networks) in ionic media impacting their heterogeneity. Such microscale domains can be employed as microreactors for chemical and enzymatic synthesis. In this review, we address known SIS systems and discuss their composition, structure, properties and dynamics. Special attention is paid to the current and potential applications of inorganic and organic SIS systems in energy research, chemistry and biochemistry. A separate section of this review is dedicated to experimental methods of SIS investigation, which is crucial for the development of this field.

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Vladimir V. Azov

Vladimir Azov is a Professor of Organic Chemistry at the University of the Free State, Bloemfontein, South Africa. He graduated from the Higher Chemical College of the Russian Academy of Sciences with a MSc degree in Chemistry. He obtained his PhD in Organic Chemistry from Emory University, Atlanta, USA in 2001. After two postdoctoral fellowships at ETH Zürich, Switzerland, and at Ludwig Maximilian University, Munich, Germany, Vladimir moved to the University of Bremen in Germany, where he completed his Habilitation in 2011. In 2016–2017, he spent several months as a visiting professor at the Zelinsky Institute of Organic Chemistry in Moscow. His research interests cover molecular self-organization, supramolecular chemistry, redox- and light-controllable molecular receptors and devices and mechanistic studies.

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Ksenia S. Egorova

Ksenia Egorova graduated from Lomonosov Moscow State University with a MSc in Biochemistry in 2006. Between 2006 and 2011, she worked at the Institute of Molecular Genetics of Russian Academy of Sciences and obtained PhD in Molecular Biology in 2010. In 2012–2017, she worked as a researcher at the Zelinsky Institute of Organic Chemistry. Since June 2017, she has been a senior researcher at the same institution. Her scientific interests include biological activity, natural products, cancer proteomics, ionic liquids, and carbohydrate research.

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Marina M. Seitkalieva

Marina Seitkalieva graduated from Volgograd State Technical University in 2011. In 2011–2014, she was a PhD student at the Zelinsky Institute of Organic Chemistry, and obtained PhD in Organic Chemistry in 2014. In 2014–2017, she worked as a researcher at the same institution. Her research interests cover dynamic ionic systems based on ionic liquids.

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Alexey S. Kashin

Alexey Kashin graduated from Higher Chemical College of the Russian Academy of Sciences with a MSc degree (chemistry) in 2011. Since 2009, he has been working at the Zelinsky Institute of Organic Chemistry, where he received his PhD degree (organic chemistry) in 2014 and got a senior researcher position in 2017. His research interests are focused on the study of soft and dynamic chemical systems, electron microscopy and X-ray spectroscopy.

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Valentine P. Ananikov

Valentine Ananikov received his MSc degree in 1996 (biochemistry), PhD degree in 1999 (organic chemistry and catalysis), Habilitation in 2003, and in 2005 he was appointed Professor and Laboratory Head of the Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences. In 2008 he was elected as a Member of the Russian Academy of Sciences. In 2012 he became Professor of the Chemistry Department of Moscow State University. He was a recipient of the Russian State Prize for Outstanding Achievements in Science and Technology (2004), an Award of the Science Support Foundation (2005), a Medal of the Russian Academy of Sciences (2000), Liebig Lecturer by German Chemical Society (2010), and the Balandin Prize for outstanding achievements in the field of catalysis (2010), Organometallics Distinguished Author Award Lectureship by American Chemical Society (2016), Hitachi High-Technologies Award In Appreciation for Novel Approach and Outstanding Contribution to Setting New Standards for Electron Microscopy Applications in Chemistry (2016). His research interests are focused on mechanistic studies, catalysis, ionic liquids and molecular complexity.

1. Introduction

Ionic interactions in the liquid phase are ubiquitously involved in a number of key areas dealing, among many others, with highly selective enzymatic biochemical processes, fine chemical syntheses, pharmaceutical industry, electrochemical processes, operation of batteries and fuel cells, and energy research.1–18 The approach of varying concentrations of salts in a single pure solvent or in binary, ternary or higher-order solvent–solvent or solvent–salt mixtures is developing rapidly. The IOLIOMICS research discipline has emerged dealing with the studies on ions surrounded by liquid phases and exploring the fundamental nature of ionic interactions.14,19

Recently, it has been shown that highly concentrated salt solutions possess special properties making them indispensable for the development of high-voltage lithium batteries with non-flammable aqueous electrolytes. By using a 21 mol kg−1 solution of lithium bis(trifluoromethylsulfonyl)imide (Li[(CF3SO2)2N], or LiNTf2) in water, it was possible to increase the working window for an aqueous electrolyte from the thermodynamic voltage limit of 1.23 V to 3.0 V with the formation of a stable electrode–electrolyte interphase,20 making a revolutionary step in the design of rechargeable lithium batteries. Such systems have adapted the term “solvent-in-salt” (SIS) to denote highly concentrated low-melting electrolytes with salt/solvent ratios >1 by volume or by weight (Fig. 1).21,22 For aqueous-based electrolytes, the term “water-in-salt” (WIS) can also be applied.

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Fig. 1 Distribution map of salt/solvent mixtures with weight and volume ratios of salt-to-solvent. Regions (A, B, and D) represent solvent-in-salt mixtures with the salt/solvent ratio over 1.0 by volume or weight; for region (C), salt/solvent is below 1.0 by weight and volume. Reprinted with permission from Suo et al.21 Copyright 2013 Nature Publishing Group.

Although the concept of “solvent-in-salt” has emerged only recently, the phenomenon of low-melting inorganic salt hydrates and their mixtures resulting in water-depleted liquid phases at low temperatures has long been known.23 Some of these salt hydrates, such as LiClO3·3H2O, are liquid even at ambient temperatures which makes the definition “solvent-in-salt” also relevant in relation to these well-known old compositions. This type of low-melting hydrated salt was studied, for example, in heat storage, but was never defined as SIS.23 It should be noted that such examples of low-melting inorganic salt hydrates are relatively rare, since most of the hydrates melt at temperatures above or far above the ambient conditions.

On the other hand, the number of organic low-melting salts, commonly known under the name of “ionic liquids” (ILs), is vast and is expanding continuously.14,24,25 Ionic liquids (although this term can be employed for both organic and inorganic low-melting salts, usually it implies salts with organic molecular cations) are fascinating high-performance solvents with a number of unique properties. In recent years, ionic liquids have been employed in diverse fields of chemistry ranging from their use as solvents for simple chemical transformations26–28 and more complex enzymatic processes14,29–32 to tailor solvents for special solutes such as cellulose,33,34 to electrolytes in batteries,35,36 and to pre-treatment reagents for electron microscopy.37,38 Given the wide variety of available cation head groups that have potential to form ionic liquids at ambient temperature (e.g. quaternary ammonium salts, imidazolium, pyridinium, phosphonium derivatives, etc.), the even larger scope of organic substituents (such as alkyl chains of various length or their poly-fluorinated derivatives), and, finally, the broad range of inorganic and organic anions, which can be employed as counter-ions, it is possible to synthesize and screen the properties of a plethora of organic salts in a systematic manner. Such screening can be employed to prepare ILs with desired properties and functions that are classified as “task-specific ionic liquids”.14,39,40 It was approximated that about 1018 of different ionic liquids were in principle accessible for task-specific optimizations.24 Apparently, experimental screening of ionic liquid characteristics remains the major reliable approach to achieve the desired properties.

An efficient strategy for “tuning” the properties of ionic liquids for a specific need is combining them with salts, other solvents or water. The addition of a solvent can dramatically change the properties of ionic liquids, whereas water is the most studied co-solvent employed to “tune” the ionic liquids.41 The major part of the research done to date concerns water-rich mixtures of ionic liquids in which the ILs typically play the role of surface-active compounds forming micellar-type aggregates. Water-depleted mixtures of ionic liquids, which belong to SIS systems (in relation to ionic liquids, they can be generally called “solvent-in-ionic liquid” or, more precisely, “water-in-ionic liquid”), have attracted significant attention only recently. Indeed, these water-depleted systems represent very interesting types of binary IL–water mixtures. Due to the low water content, water molecules cannot form large interconnected pools or domains. In contrast, trace amounts of water remain tightly connected to the molecules of ionic liquids by specific interactions (H-bonds and electrostatic interactions) or form separate microdroplets or interconnected networks penetrating through the bulk of the ionic liquids. Several recent experimental studies and theoretical modeling of “water-in-ionic liquid” systems have provided evidence for this type of water aggregation.42–46 Recent literature affirms the enormous potential of the practical applications of SIS in synthetic chemistry and battery research.20,21,47–52

The goal of this review is to give an overview of “solvent-in-salt” systems and to discuss the composition, properties, structure and dynamics of inorganic and organic SIS systems in view of their rapidly developing fascinating applications in chemistry, biology and energy research.

2. Types of “solvent-in-salt” systems, their properties and applications

As we have pointed out above, “solvent-in-salt” systems are generally defined as mixtures with a salt/solvent ratio of over 1.0 by volume or weight. Both salts and solvents can contain organic and inorganic components. Such mixtures can be either homogeneous or can enclose various microstructures such as channels or microdroplets (microheterogeneous components). The following classification can be used to describe different types of the phase behavior of “solvent-in-salt” systems: complete mixing with formation of a single phase, macroscopic phase separation and microscopic phase separation caused by solvent or salt/IL aggregation (Fig. 2).
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Fig. 2 Possible phases of “solvent-in-salt” systems: (A) homogeneous mixing of two components; (B) separation into two phases; (C) formation of interconnected channel networks containing the minor component of the system and of non-interconnected aggregates (reverse micelles, vesicles); (D) formation of ionic aggregates in water (micelles, vesicles). Adapted from Kashin et al.42 Copyright 2016 Wiley-VCH Verlag GmbH & Co. KGaA.

Most of the low-melting inorganic salt hydrates, solutions of inorganic salts in organic solvents and aqueous mixtures of hydrophilic ionic liquids are supposed to form homogeneous solutions, in which components are distributed uniformly (Fig. 2A). In contrast, ionic liquids with cations or anions containing large lipophilic domains (alkyl chains and aromatic groups) tend not to mix with water uniformly and often form phases with distinct separation between their components (Fig. 2B). When two components do not undergo bulk phase separation, the morphology of the phases formed by two immiscible liquids varies depending on the ionic liquid-to-solvent ratio. At very high ratios (low water content), water molecules display very low mobility and are virtually attached to the polar moieties (of both anions and cations) of ionic liquids due to electrostatic interactions and H-bridges. A gradual increase of the water content can lead to the formation of local pools of water or microchannels penetrating the bulk of the ionic system (Fig. 2C). Upon a further increase of the water concentration, buildup of aggregates pertinent to solutions of common amphiphiles in water (micelles and vesicles) can be observed (Fig. 2D).

In the present review, we give a brief description of inorganic and organic SIS systems (Section 2.1) followed by discussion of their structural studies (Section 2.2) and practical applications in most rapidly developing areas of chemical reactions, biochemical transformations and energy research (Sections 2.3–2.5). Finally, experimental methods relevant to studies on various properties of SIS systems are summarized (Section 2.6).

2.1 Overview of general types of inorganic and organic “solvent-in-salt” systems

Here we briefly discuss the representative examples and properties of inorganic and organic SIS systems. The former can be defined as those consisting only of inorganic components, whereas the latter contain one or several organic components.

Low-melting salt hydrates CpAq·nH2O (C = cation, A = anion) and their mixtures were the first ionic compositions complying with the definition of “solvent-in-salt” systems.23 Among illustrative examples of simple salt hydrates liquid at ambient temperature or slightly above, there are lithium salts LiClO3·3H2O (melting point Tm = 281 K) and LiNO3·3H2O (Tm = 303 K), as well as zinc salts ZnCl2·4H2O (Tm < 298 K) and Zn(NO3)2·6H2O (Tm = 310 K).23,53,54 In hydrate melts, a metal salt is mixed with an extremely small amount of water. In such very concentrated solutions, all water molecules are coordinated with the metal cations, still retaining their fluidity. The temperature of the melts can be further decreased by the use of ternary mixtures salt 1–salt 2–H2O. The first prominent example was the lowering of the melting point of Ca(NO3)2·4H2O (Tm = 315 K) by the addition of KNO3, leading to the formation of eutectic mixtures with the melting temperature well below the ambient temperature.55 These low-melting hydrates were classified as solvate (or chelate) ILs (SILs) in the review of Angell et al.,56 denoting low-melting molecular systems, in which ligand molecules (water in the case of hydrates) were strongly coordinated with the cations and/or anions of salts in solid and liquid phases.

Solvates of polyethers can be called a new class of SIS with adjustable properties. When mixing a lithium salt, such as LiNTf2, with the equimolar amount of ethylene glycol oligoether (glyme, designated as Gn, where “n” denotes the number of ethylene glycol units) or a cyclic polyether (crown ether, designated as “m-crown-n”, where “m” is the total number of atoms in the cycle and “n” is the number of oxygen atoms), the formation of non-covalent, but stable complex cations [Li(glyme)]+ or [Li(crown)]+ is observed.57,58 If a suitable weakly coordinating anion is available, a low-melting phase is formed, making these systems to comply with the classification of solvate ILs as well as SIS systems. Among examples of such SIS are [Li(G3 or G4)]X (X = [NTf2], [BETI] (bis((perfluoroethyl)sulfonyl)imide), and ClO4), and [Li(18-crown-6)][NTf2]. Interestingly, Li complexes with 12-crown-4, possessing a cavity size complementary to Li+ ions, show high melting points implying the formation of tight complexes with low entropy leading to stronger intermolecular interactions in the crystal phase. On the other hand, weekly coordinating ligands (short polyethers or other coordinating solvents) cannot stabilize Li+ sufficiently well, leading to the formation of concentrated solutions that do not satisfy the criteria neither of SIS nor of SILs (Fig. 3). Salts of other alkali and of alkaline earth metals should potentially form solvate ILs with glymes, which allows expanding the scope of these remarkable SIS systems.

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Fig. 3 Schematic classification of Li-based organic solvated ionic liquids. Kcomplex, complex formation constant; ELi+X, Li–X ion pair dissociation energy. Reprinted with permission from Mandai et al.57 Copyright 2014 Royal Society of Chemistry.

Ionic liquids are organic salts with low melting points and currently they are the most diverse tool for designing organic “solvent-in-salt” systems. Within the last few decades, ILs have been extensively studied due to their unique properties. The possibility to combine various building blocks in both cations and anions has allowed synthesizing and studying a plethora of different ILs (Fig. 4). All of them contain organic cations with delocalized (imidazolium and pyridinium) or localized (ammonium, pyrrolidinium, phosphonium, and sulfonium) positive charges. Commonly, inorganic anions (such as halogenides, nitrates, and hexafluorophosphates) are taken as counterions, although organic anions (for example, carboxylates, amino acids, and phosphonates) are also frequently employed. Depending on the type of the cationic head group, ionic liquids can be divided into two types. More commonly used aprotic ILs contain cationic moieties, which cannot be easily deprotonated, for example, imidazolium, pyridinium, or quaternary ammonium derivatives. Protic ionic liquids (PILs) contain cationic head groups, which can be easily deprotonated, for example, the ammonium cation.

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Fig. 4 Common components of ionic liquids.

Initially, only pure ILs were the preferred objects of the study, whereas low molecular weight impurities, including water, were regarded as harmful contaminants. Due to strong intermolecular interactions between water molecules and ions of ILs, it usually costs much effort to remove the traces of water in order to obtain an anhydrous IL. However, using only structural variations of the cationic and anionic components of ILs, it is not always possible to achieve the desired properties. Mixing of two or more different ILs with each other or with other compounds has established itself as another method of fast and straightforward tuning of ionic liquid properties.59 When considering the potential components that can be directly mixed with ILs, the first choice should be given to accessible and low molecular weight liquid substances, such as water, alcohols, or other organic solvents. When the concentration of the second component (water or another solvent) is less than 50% by volume or by weight, such mixtures belong to the class of “solvent-in salt” systems that can be described as either “water-in-ionic liquid” (WIIL) or “solvent-in-ionic liquid” (SIIL), respectively. Depending on the hydrophilic/hydrophobic character of ILs and the added components, they can either mix with each other affording a homogenous solution, or, if non-miscible, form a biphase system or more complex microheterogeneous phases (Fig. 2). As water represents the most obvious and abundant solvent, most of the studies performed on IL–solvent mixtures were conducted with H2O as an additive.

Cationic components in the majority of ionic liquids demonstrate the amphiphilic nature due to the presence of positively charged cationic domains and lipophilic non-charged groups (usually alkyl chains). Intricate mutual interactions between amphiphilic cationic and anionic components of ionic liquids lead to complex molecular arrangements even in pure ILs, making the prediction of fundamental IL properties based on their chemical structures rather challenging. Due to the fact that the physical and chemical properties of ILs strongly influence the outcome of the reactions performed in them, understanding the structure of ILs is of tremendous importance for their broad practical use as media for chemical processes. This aspect is well recognized by the scientific community. In recent years, much effort, both experimentally and theoretically, has been made into the investigation of the molecular arrangement of ILs and its dependence on chemical structural variations, as it is clearly indicated by the large number of reviews discussing the structure of ionic liquids on the basis of experimental and theoretical studies, or combination of both (see e.g.ref. 25 and 60–62). A recent review by Hayes et al.62 represents an excellent comprehensive summary of the available literature on the studies of the structure and nanostructure of ILs to the year of 2015.

Investigating the mixtures of ILs with added components, such as water or other solvents, is even more challenging. Although the significance of co-solvents as property-altering impurities in ILs has been recognized on the eve of the IL era,63 only relatively recently such additives have gained recognition as useful modifiers, not as hostile admixtures in ILs, and have become a common subject of systematic investigation. In recent years, the number of papers related to the systematic study of IL–solvent mixtures has been constantly increasing.

2.2 Structural studies in “solvent-in-salt” systems

In the discussion of chemical systems, two major aspects should be considered. The first one is related to their physical and structural properties and the second one concerns their function.
2.2.1 Phase behavior. In the discussion below, we will focus on IL–water mixtures, since water is the most studied solvent component and the one that can alter the physicochemical properties of ILs most significantly. Although the addition of alcohols can also change the properties of ILs, their influence is much less pronounced.

The miscibility of an IL with water depends on the properties of both its cations and anions. In general, ILs containing cations with shorter alkyl chains possess higher water solubility than those with longer alkyl chains. ILs with highly fluorinated and charge-delocalized anions, e.g. hexafluorophosphate PF6 and bis(trifluoromethylsulfonyl)imide [NTf2], are typically much less water-soluble than the ILs with the same cations and more hydrophilic anions like halides (Cl or Br), tetrafluoroborate BF4, hydrogen sulphate HSO4, or acetate [OAc]. The phase partition coefficient of octanol–water systems (Kow) can be employed to estimate the hydrophobicity of ILs, with larger Kow coefficients denoting more hydrophobic substances.64 For ILs with the 1-alkyl-3-methylimidazolium cation [Cnmim]+ (“n” denotes the length of the alkyl chain), values of Kow increase upon introduction of longer alkyl chains into the cation, whereas for the same [Cnmim]+ cation with different anions, Kow increases in the row BF4 < Br < NO3 < Cl < PF6 < [NTf2]. Thus, at room temperature [Cnmim]+-based ILs are miscible with water when in combination with hydrophilic anions like BF4, Cl, Br, or NO3, but afford split phases if combined with the PF6, [NTf2], [C(CN)3], or 2-(2-methoxyethoxy)-ethylsulfate [CH3(OC2H4)2OSO3] anions.65 Many ionic liquids with lipophilic anions are very hygroscopic and easily absorb water from air, gaining up to 7% water (w/w) within several hours of air exposure.63

IL–solvent mixtures often show a complex behavior with a strong temperature dependence. As an example, [C4mim][BF4], which is fully miscible with water at room temperature, splits into two phases after cooling below 278 K,66 the upper critical solution temperature (UCST, Tc, temperature above which the components of a mixture are miscible in all proportions) for this mixture. Thus, for the 1[thin space (1/6-em)]:[thin space (1/6-em)]1 (w/w) component mixture, a homogeneous solution is formed above 5 °C; below this temperature, it separates into two phases. This effect can be employed for product separation when IL–water mixtures are used as reaction media. A precise thermodynamic analysis of [C4mim][BF4]/water mixtures was performed later by Rebelo et al.;67 it showed that UCST was at 277.71 K and 0.4585 mass fraction (wIL) or 0.0628 mole fraction (xIL) of the ionic liquid, but was about 5 K higher for the IL–D2O mixtures (Fig. 5).

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Fig. 5 Temperature–IL mass fraction (A) and temperature–IL mole fraction (B) phase diagrams of [C4mim][BF4]/H2O (○) and [C4mim][BF4]/D2O (●) solutions. The two phase region is located inside the envelope. Reprinted with permission from Rebelo et al.67 Copyright 2004 Royal Society of Chemistry.

Such a liquid–liquid equilibrium (LLE) phase behavior featuring the upper critical solution temperature is typical for mixtures of different ILs with water and alcohols.68–70 For example, [Cxmim][A] (x = 8, 10 and [A] = Cl, Br) mixtures with water demonstrated the UCST behavior71 with the critical point Tc increasing with the alkyl chain length and decreasing with the increase of the anion size for x = 8. The critical temperature was at 278 K and 306 K for x = 8 ([A] = Cl and Br, resp.) and at 360 K and 362 K for x = 10 ([A] = Cl and Br, resp.). Since ILs even with relatively small hydrophobic domains (short alkyl chains) often do not form homogeneous mixtures with water even at elevated temperatures, such a LLE phase behavior has been better explored for IL–alcohol mixtures. For example, the miscibility data for the [Cxmim][NTf2] (x = 2, 4, 6, 8, 10, 12) ionic liquids with CnH2n+1OH (3 ≤ n ≤ 20) were summarized and systematically analyzed.72 For all x, n combinations, the phase behavior with UCST (similar to the one shown in Fig. 5) was observed. The UCST increased upon the increase of the chain length of the alcohols (Cn), but decreased upon the extension of the alkyl substituent attached to the imidazolium ring (Cx, Fig. 6). The authors developed a simple empirical relationship based on the consideration of the ratio of molar volumes of the alcohols and the IL cation, which allowed the prediction of UCST with the accuracy of ca. 10 K.

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Fig. 6 Dependence of UCST for [Cxmim][NTf2] (x = 2, 4, 6, 8, 10, 12)/CnH2n+1OH (3 ≤ n ≤ 20) mixtures on alcohol chain length. Lines are shown as eye guides to connect points for the same ILs. Alcohol melting points are shown for comparison. Reprinted with permission from Vale et al.72 Copyright 2012 Wiley-VCH Verlag GmbH & Co. KGaA.

Nockemann et al. discovered the same behavior for a choline-based IL with the [NTf2] counteranion. This ionic liquid was not miscible with water at room temperature, but above 345 K, a one-phase system was formed.73 When the Eosin dye was added to the high-temperature monophasic IL–water mixture, it concentrated exclusively in the aqueous layer after phase separation (Fig. 7). This phenomenon illustrates the enormous potential of such temperature-dependent IL–solvent systems as reaction media for chemical processes, affording the possibility to accumulate products in one of the phases after the reaction by cooling the homogeneous reaction mixture. Thermo-reversible split-phase behavior with UCST was also observed for mixtures of ILs with other organic solvents, both with upper (UCST)74,75 and, much less often, lower critical solution temperatures (LCST).76,77 Very complex and bizarre thermodynamic liquid–liquid equilibria, such as a closed-loop type of demixing in [C4mim][NTf2]–CHCl3 systems, are observed sometimes for multicomponent IL–solvent mixtures, but they are relatively rare and not well explored.78

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Fig. 7 Temperature-dependent phase behavior of the binary mixture [choline][NTf2]–water. The dye Eosin Y accumulates in the IL layer after phase separation. Reprinted with permission from Nockemann et al.73 Copyright 2009 American Chemical Society.

Ionic liquids with sufficiently long hydrophobic domains usually display amphiphilic behavior, forming lyotropic phases common for surface-active compounds. At high dilutions, hydrophobic ILs typically form simple micelles that can transform into more complex structures (cylindrical micelles, vesicles, lamellar phases, microemulsions, etc.) upon concentration increase, addition of a third component, or temperature variation. Below, we will give an overview of colloidal behavior of IL–water and IL–solvent mixtures from historical and structural perspectives.

Bowers et al. were one of the first who performed a comprehensive study on IL aggregation in water at low IL concentrations by means of surface tensiometry, conductometry, and small-angle neutron scattering (SANS), using [C4mim][BF4], [C8mim][Cl], and [C8mim][I] as the objects.79 From the surface tension and conductivity measurement data (Fig. 8), as well as from the consideration based on the calculation of Israelachvili's packing parameter,80 it was concluded that spherical micelles should form at concentrations exceeding the critical aggregation concentration (CAC). Structural information obtained from the SANS data was used to model the shape and sizes of aggregates. It was suggested that the short-chained [C4mim][BF4] was likely to give polydisperse spherical aggregates, colloids of [C8mim][I] – a system of regularly sized near-spherical charged micelles, whereas [C8mim][Cl] displayed weak long-range ordering at higher concentrations and possibly formed lyotropic mesophases above the CAC, although this assumption was not supported by proper investigation.

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Fig. 8 Conductivity isotherms for aqueous solutions of (A) [C8mim][Cl] (●) and [C8mim][I] (□) and (B) [C4mim][BF4] (○); the insets show the regions near the break points. (C) Surface tension σ vs. IL concentration isotherms c for aqueous solutions of [C8mim][Cl] (●), [C8mim][I] (□) and [C4mim][BF4] (○). All measurements were performed at 298 K. Lines are used as eye guides to highlight the break points associated with CAC. Reprinted with permission from Bowers et al.79 Copyright 2004 American Chemical Society.

The aggregation behavior of the same ILs was reexamined by Singh and Kumar using 1H NMR, steady-state fluorescence spectroscopy and refractometry.81 CACs determined in this study were in good agreement with the results obtained by Bowers using surface tension and conductivity measurements and the SANS technique. Changes in chemical shifts and peak shapes were used to make assumptions about the structural changes upon aggregation and about the sizes and shapes of molecular aggregates. In the study of Wang et al.,82 densities, conductivities, and polarity indexes of pyrene for aqueous solutions of several [Cnmim][X] (X = BF4, Br) were determined as a function of the concentration of ILs in water. Analysis of the data led to the interpretation of the aggregation behavior similar to the results of the previous studies performed using different physical methods.79,81 Later, the same group performed a detailed 1H NMR study on water colloids, affording similar evidence for their aggregation properties.83

Since imidazolium-based ILs [Cnmim][X] represent one of the most common IL classes, their colloidal properties in dilute water solutions have been examined for different numbers of chain methylene groups “n”, as well as for different counteranions X, using various physicochemical methods commonly employed for the determination of the CAC.84–88 All these methods have produced evidence for the behavior of ILs in dilute water solutions to be similar to that of typical surfactants, which display plot breakpoints of several physical characteristics around their CACs. The results have been summarized in the review of Łuczak et al. with the general conclusion that imidazolium ionic liquids with the alkyl chains longer than C4 usually display the behavior of typical surfactants: they undergo self-aggregation at certain concentrations, and the CAC point decreases upon the extension of the amphiphilic chain.89

Self-aggregation behavior of [C4mim][BF4] was re-investigated by Almásy et al. using SANS in water solutions at room temperature,90 in the region of the full water–IL miscibility on the phase diagram (Fig. 5). Although the previous data established by surface tension, conductivity, and NMR measurements79,81 implied some sort of possible structural transition between the mixing schemes, they could not afford any reliable information on the nature of the structure. Data analysis performed by Almásy et al. led to the conclusion that the mixture could be well described by the Ornstein-Zernike form of statistical concentration fluctuations. These statistical fluctuations implied that the system was close to phase separation, which occurred less than 20 K below the investigated temperature. Strong pre-critical scattering masks the nature of the underlying microheterogeneity, which manifests itself from other physical methods in a manner similar to micelle formation. Therefore, indirect physicochemical methods cannot provide the final evidence on the formation of nanoscale aggregates and do not reliably describe their physical nature. Only direct methods, such as electron microscopy (EM), can be employed for obtaining the ultimate proof of their formation.

2.2.2 Micro- and nanostructuring. Using electron microscopy, it is possible to capture images of nanoscale aggregates formed from ILs in dilute water solutions. Since spherical micelles are nanometer-size objects, it is extremely difficult to visualize them even using the methods of transmission (TEM) or scanning electron microscopy (SEM); only larger rod-like micelles can be reliably detected by EM. Using cryo-TEM (TEM performed on deeply frozen samples), it was possible to see the formation of wormlike micelles from the dilute solution of [C4mim][Br] or N-alkyl-N-methylpyrrolidinium ILs upon addition of sodium p-toluenesulfonate to the mixture.91,92 Large aggregates were discovered in TEM images of [C8mim][X] (X = Br, Cl, NO3, OAc, TFA), although their possible nature was not discussed by the authors.93 Another group reported the detection of formation of colloidal aggregates in water solutions of [C12mim][Br] using negative stain TEM and cryo-TEM techniques.94 The aggregates were tentatively classified as spherical micelles, rod-like micelles, and vesicles (Fig. 9).
image file: c7cs00547d-f9.tif
Fig. 9 TEM images of aggregates formed in [C12mim][Br] in aqueous solution at (A) 0.06; (B) 0.56; and (C) 0.93 mol L−1 concentrations of IL. (A) Negative stain TEM; (B and C) cryo-TEM. Reprinted with permission from Wang et al.94 Copyright 2013 Royal Society of Chemistry.

Microscale behavior of diphase IL–water mixtures at concentrations far above the CAC in the solvent-depleted “solvent-in-salt” regime is less investigated, although it is likely to present much higher diversity of possible phases and microstructures. Pioneering NMR studies gave evidence for a significant change of the ionic liquid structure due to the participation of water in C(sp2)–H mediated hydrogen bonds in [C4mim][BF4],95 disrupting the network of C(sp2)–H···F interactions that governed the short-range structure in this IL. Thus, increasing amounts of water progressively replaced the C(sp2)–H···F interactions with hydrogen bonds, leading to ion pair dissociation. Such disruption of the attractive cation–anion interactions can easily lead to layer segregation and nanostructuring of ILs, since another attractive interaction, namely lipophilic segregation of alkyl chains, becomes stronger in the presence of water. The assumption that water mostly remained exclusively tightly bound to the ionic domain within the bulk of IL medium was later confirmed by several other similar NMR studies.96,97 It should be noted that, according to an MD study on a mixture of [C8mim][BF4] with 0–50 mol% water, molecules of the latter formed a diffuse coordination envelope around the imidazolium ring.98 In such a case, the presence of water should also be considered to describe the coordination of the anions to the cation rings.

The first evidence implying the presence of nanoscale features in ILs with small amounts of added water came from electrochemical investigation of ion mobility in water-depleted IL–water mixtures.99 In a study on diffusion of redox-active ferricyanide and methylviologen ions using voltammetry, it was found that even the addition of traces of water induced a much more significant acceleration effect on the diffusion of the ionic species in comparison to the neutral ones. As a possible explanation for this behavior, the authors suggested segregation of ionic liquids into more and less polar domains in the manner similar to liquid crystalline or concentrated surfactant media. Ion diffusion through the bulk of the mixture should occur along the water-rich channels penetrating through the medium. Based on the data obtained from ATR and transmission IR spectroscopy, Cammarata et al. also proposed the water aggregation within the ionic liquid accompanied by the formation of a H-bonding network between the water and anions.100

The first direct observation of the microstructure formation came from Firestone et al., who observed a lyotropic liquid crystalline phase in the water-depleted solutions of [C12mim][Br]. Addition of 5–40% of water to the viscous liquid [C12mim][Br] led to its fast conversion to a stable homogenous flow-resistant gel, named “ionogel” due to the high content of ionic species.101 In contrast to featureless polarized optical microscopy (POM) images of the pure IL, the gel phase showed the emergence of optical birefringence, which revealed its liquid crystalline nature (Fig. 10). Upon a modest temperature increase, the gel could be reversibly converted into an optically isotropic free-flowing liquid. Additional analysis of the samples performed by SAXS (small-angle X-ray scattering) displayed an isotropic scattering pattern for the pure IL liquid sample, indicating that it consisted of non-oriented domains with the average domain size of ca. 25 nm. On the other hand, the gel sample displayed a strong anisotropic 2D-pattern with two diffraction rings (at Q = 0.22 and 0.44 Å) demonstrating the existence of highly ordered mesogens with the average domain size of 164 nm. The presence of the second diffraction ring indicated a lamellar structure with a lattice spacing of 2.9 nm which implied the layered structure of the sample with water channels between the lamellae of 3.6 Å or more.

image file: c7cs00547d-f10.tif
Fig. 10 Two-dimensional small-angle X-ray scattering patterns, azimuthally averaged intensity as a function of scattering vector, and polarized optical micrographs for fluid (panels A–C, respectively) and lyotropic liquid crystalline (16% w/w H2O; panels D–F, respectively) phases of [C12mim][Br] (T = 296 K). Reprinted with permission from Firestone et al.101 Copyright 2002 American Chemical Society.

The next report of Firestone et al. expanded the previous study by comparing bromide and nitrate of the same IL and using 1H NMR and IR spectroscopies as additional characterization means.102 The spectroscopic methods gave evidence for the disruption of the H-bonds between the imidazolium ring and the IL anion, which were partially replaced by H-bonds between the anion and water upon gelation. The particularly important discovery was that the alteration of the ionogel composition could be performed by anion selection or adjustment of the water content. Thus, it was possible to create near-monodomain materials with either lamellar, 2D hexagonal or 3D cubic structures with tunable lattice dimensions (Fig. 11), clearly demonstrating that the water-depleted ionogel phases could adopt a rich variety of structural motifs. Later, the same research group exploited unique ordering properties of ionogels for the photochemical preparation of anisotropic gold nanoparticles.103 Previously unattainable trigonal prismatic nanorods, as well as nanoparticles with other morphologies, could be prepared within the constrained aqueous domains of the nanostructured ionogel template. Although the exact mechanism of templating could not be elucidated, it was clear that the liquid crystalline phase played a significant role in the process of nanoparticle formation.

image file: c7cs00547d-f11.tif
Fig. 11 Schematic representation of structural phases that can be prepared from [C10mim]+-containing IL. [C10mim][Br] with 11–16% (w/w) water: lamellar structure (LAM); [C10mim][Br] and [C10mim][NO3] with 17% (w/w) water: hexagonal perforated layers (HPL) and gyroid structure (G); [C10mim][Br] and [C10mim][NO3] with 40% (w/w) water: hexagonal structure (HEX). Reprinted with permission from Firestone et al.102 Copyright 2004 Elsevier.

Another proof of the organized microstructuring of water-depleted IL–water mixtures came from Inoe et al., who studied [C12mim][Br]/H2O mixtures using differential scanning calorimetry (DSC) and POM.104 The formation of two types of lyotropic liquid-crystalline gel phases, the lamellar phase Lα and the hexagonal phase H1, was discovered in this study. DSC was used to construct the T-X phase diagram, defining the regions of various phases in the mixture (Fig. 12). With the help of POM it was possible to visualize the morphology of these phases at different temperatures and mixture compositions (Fig. 13), giving evidence for a rich variety of phase morphologies in the IL–water systems.

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Fig. 12 T-X phase diagram of the [C12mim][Br]/H2O mixture. SIL: solid phase of [C12mim][Br], Lα: lamellar phase, H1: hexagonal phase, L1: aqueous solution of [C12mim][Br]. Reprinted with permission from Inoue et al.104 Copyright 2007 Elsevier.

image file: c7cs00547d-f13.tif
Fig. 13 Morphologies of [C12mim][Br]/H2O mixtures obtained using POM at different compositions (wt% of H2O) and temperatures: (A) 0 and 318.2 K, (B) 0 and 393 K, (C) 5.01 and 318.2 K, (D) 5.01 and 338 K, (E) 24.7 and 293.7 K, (F) 24.7 and 343 K, (G) 50.0 and 293.1 K, and (H) 59.9 and 276 K. Reprinted with permission from Inoue et al.104 Copyright 2007 Elsevier.

A similar phase behavior with the formation of ionogel phases was observed for a number of molecular systems comprising ILs, water, and also other solvents such as alcohols. Zhang et al. comprehensively investigated IL–water–alcohol ternary systems and discovered the formation of lyotropic lamellar phases upon variation of their component ratios.105 Long-chain alcohols induced a prominent effect on the behavior of these systems. The tested IL, [C8mim][Cl], an imidazolium IL with an alkyl chain of intermediate length, is liquid at room temperature, is fully miscible with water, and does not form any lyotropic phases in water mixtures because of its relatively poor amphiphilic nature. Its ternary mixtures with H2O and four alcohols CnH2n+1OH (n = 6, 8, 10, 12) were investigated at 298 K using POM and SAXS and gave evidence for the formation of an anisotropic lamellar liquid crystalline phase (Lα) for the ternary mixture (Fig. 14A), whereas isotropic (L1 and L2) solutions were observed upon mixing the IL with one or both of the co-solvents in the pure form. The liquid crystalline phase Lα was separated from the L1 and L2 phases by the biphase regions Lα + L1 and Lα + L2, respectively (Fig. 14B). The formation of the lamellar phase Lα was clearly observed in a polarizing microscope and was confirmed by the SAXS spectra, which displayed two Bragg peaks with the relative positions 1[thin space (1/6-em)]:[thin space (1/6-em)]2, typical for lamellar structures. The values of lattice spacing of 2.76, 3.22, 3.36, and 4.55 nm for n = 6, 8, 10, 12 (as calculated from the scattering factor of the first Bragg peak) were similar in order to those calculated by Firestone for his IL-based ionogel.101

image file: c7cs00547d-f14.tif
Fig. 14 (A) POM microphotograph of an LLC system in the [C8mim][Cl]–C6H13OH–H2O ternary mixture; the sample composition is 60–25–15 (w/w%). (B) Phase diagram of the [C8mim][Cl]–C6H13OH–H2O ternary mixture. Reprinted with permission from Zhang et al.105 Copyright 2007 American Chemical Society.

Interestingly, the counterion plays an important role in the shaping of phase diagrams: the formation of lamellar phases was not observed for the BF4 or PF6 salts. In the [C8mim][PF6]–H2O–C10H21OH system, only the formation of a triphase was found due to the immiscibility of all three components with each other. According to the analysis of the available data concerning the formation of ionogels with IL–chlorides and inability of the BF4 and PF6 salts to form the lamellar phase, the authors suggested a model for its formation and microstructuring. The self-ordering of the phase is based on two sorts of interactions: the formation of an H-bond network between the head groups of IL cations, H2O molecules and Cl, and the self-sorting of the alkyl chains into lipophilic bilayers (Fig. 15).

image file: c7cs00547d-f15.tif
Fig. 15 Scheme of the possible structure of the Lα phase in the [C8mim][Cl]–H2O–C10H21OH ternary system. Inset shows the H-bonded network interconnecting the imidazolium cation, Cl, H2O, and C10H21OH. Reprinted with permission from Zhang et al.105 Copyright 2007 American Chemical Society.

The formation of ternary systems with imidazolium-based ILs was also observed for the cases when the third component was a lipophilic, fully water insoluble compound, such as p-xylene.106 When mixing [Cnmim][Br] (n = 12, 14, 16) with p-xylene and water, ternary phase diagrams showed several different types of liquid crystalline phases, namely the lamellar phase Lα, cubic phase C, and hexagonal phase H1, which could exist in the pure as well as in the mutually mixed forms (Fig. 16). Such a behavior was explained by the aromatic nature of xylene, which could form stacks with positively charged imidazolium cycles due to π–π and π–cation interactions. These systems were characterized using POM and SAXS, which allowed the unambiguous assignment of the nature of liquid crystalline phases.

image file: c7cs00547d-f16.tif
Fig. 16 Above: ternary phase diagrams of [C12mim][Br]/p-xylene/H2O, 298 K. C, cubic liquid crystalline phase; L1, isotropic solution; H1, hexagonal liquid crystalline phase; Lα, lamellar liquid crystalline phase. Below: POM images of hexagonal and lamellar liquid crystalline phases for samples with a [C12mim][Br]/p-xylene/H2O ratio of 45.0[thin space (1/6-em)]:[thin space (1/6-em)]11.0[thin space (1/6-em)]:[thin space (1/6-em)]44.0. Reprinted with permission from Li et al.106 Copyright 2009 Elsevier.

Not only aprotic ILs display the ability to form organized IL–water phases with separated lipophilic and amphiphilic domains. It has been shown that PILs, such as ethyl- or butyl-ammonium nitrate, can also display nanoordering upon dilution with water and other solvents in a similar manner to imidazolium-based ILs.107,108 With the help of small- and wide-angle X-ray scattering (SWAXS), it was discovered that the aggregate structure of neat PILs was retained upon dilution with water within the broad concentration range.108 In this case, the nanoscale arrangement, consisting of micellar-type aggregates surrounded by nitrate ions, was simply expanded by the intercalation of water molecules in between them (Fig. 17). Later the formation of structured mesophases in PIL–water mixtures was also discovered for ethylammonium-based PILs with polyfluorinated carboxylate counterions,109 as well as for PIL–alkanol mixtures.110

image file: c7cs00547d-f17.tif
Fig. 17 Schematic diagram of the microstructure of (A) neat butylammonium nitrate and (B) the butylammonium nitrate–water mixture. Reprinted with permission from Greaves et al.108 Copyright 2011 American Chemical Society.

Although X-ray diffractometry cannot be used in liquid samples, X-ray structures of hydrated ILs can shed some light on their aggregation in the liquid state. Only recently it has become possible to investigate the structural behavior of long-chain imidazolium-based ionic liquid [C10mim][Cl]–water systems employing low-temperature and high-pressure crystallization methods.111 One of the crystallized forms was a trihydrate that was obtained under high-pressure conditions above 0.55 GPa from solutions containing >80% (w/w) water; so far, it represents the only highly hydrated long-chain imidazolium-based ionic liquid characterized in the solid state. Examination of the crystal packing (Fig. 18) clearly shows that water molecules are segregated between the layers of the IL molecules forming a complex H-bonding network with each other as well as with Cl anions located near the cationic imidazolium cycles. Thus, a mesoscopic structure with IL lamellae segregated by water nanolayers is being formed. Since this IL forms liquid crystalline phases upon mixing with small amounts of water, it is very likely that it maintains many features of its solid state structure in the liquid crystalline state.

image file: c7cs00547d-f18.tif
Fig. 18 Crystal packing of [C10mim][Cl]·H2O as seen along the b-axis. (1) a hydrophobic layer with interdigitated cation side chains; (2) a hydrophilic layer; (3) O–H···Cl H-bonded chain along the a-axis; (4) C(4)–H, C(5)–H, [Cl]2–H-bonded ring; (5) water–imidazolium contact via C(2)–H. Chloride anions are shown in green, water molecules are shown in red. H atoms are omitted for clarity. Reprinted with permission from Saouane et al.111 Copyright 2015 American Chemical Society.

Indirect evidence for the formation of nanostructures in IL–water systems comes from other physical methods, such as NMR. In a 1H and 19F NMR study of Rollet et al., the self-diffusion properties of cations, anions and water molecules were investigated in water-depleted (0.3–30 mol% of water) solutions of [C4mim][NTf2], an IL only weakly miscible with water at room temperature.112 Although the measured self-diffusion coefficients increased with the water content, their values raised much higher for the water molecules than for the cations and anions. Such an anomalous behavior was contributed to the phase separation on the microscopic scale, which was represented as the formation of a network of water-rich microchannels (Fig. 19) disrupting the homogeneous bulk of the IL. The authors estimated the ratio of linked and bulk water as 10 and the characteristic size of water channels as ∼5 μm at 30 mol% of water.

image file: c7cs00547d-f19.tif
Fig. 19 Artistic representation of the microstructure of [C4mim][NTf2]/water systems for increasing amounts of water (from left to right). RTIL is represented in dark color and water in light color. Reprinted with permission from Rollet et al.112 Copyright 2007 American Chemical Society.

[C10mim][Br] was investigated using 1H and 81Br NMR spectroscopy in the pure form as well as in water mixtures.113 Due to the extended lipophilic domain, aqueous compositions of this IL form liquid-crystalline ionogels at room temperature. Upon solidification to the ionogel structure, drastic changes were observed for the water signal due to the rise of new water peaks between 3.6 and 4.4 ppm. The emergence of the additional peaks was attributed either to the placement of H2O molecules in inhomogeneous regions of the sample or to the appearance of non-equivalent water sites in ionogels with slow exchange rates. 81Br NMR evidenced strong broadening and a very complex shape of the Br signal in the neat IL, and even more so in the ionogel phase formed upon water addition. This complexity disappeared in the diluted water solution, indicating a complex microstructural arrangement of Br ions in the ionogel phase.

Investigation of phosphonium halide ILs using methods of 1H and 2H NMR spectroscopy showed that the addition of a small amount of water and other solvents (CD3OD, CDCl3, DMSO-d6, CD3CN) induced critical changes in the liquid crystalline structure of these ILs.114,115 The effect was due to the attenuation of electrostatic interactions between the anions and cations by the solvent molecules leading to the induction of structural changes.

Optically-heterodyne-detected optical Kerr effect (OHD-OKE) spectroscopy was employed by Fayer's group to study the microstructure of 1-alkyl-3-methylimidazolium-based ILs and their mixtures with water116–120 by means of tracking the molecular orientational relaxation dynamics over the time scale ranging from hundreds of femtoseconds to hundreds of nanoseconds. A biexponential decay (indicative of the presence of fast and slow relaxing components in the mixture) of orientational anisotropy was clearly observed by OHD-OKE for the water mixtures of ILs with hexyl or longer chains upon approaching the gelling concentration range from either high or low water concentrations.117 Such a behavior was interpreted to be due to the local stiffening of the cation alkyl tail–tail associations upon water addition. Thus, the fast decay component was attributed to “wobbling-in-a-cone” motions of the imidazolium head groups, whereas the slow component was assigned to sluggish general cation reorientation determined by the aggregation of the alkyl chain. The second component of the biexponential decay was much more pronounced for the longer-tailed (decyl) than for the shorter-tailed (hexyl) ILs, as could be obviously expected due to stronger mutual interactions between the longer alkyl chains. It was also possible to trace the role of water molecules in IL–water mixtures.119 At very low H2O concentrations, all hydroxyl groups interact only with the anions. As the water concentration increases to ca. 1[thin space (1/6-em)]:[thin space (1/6-em)]1 IL ion pairs to water (mol mol−1), a second population of water hydroxyl groups is established, which is consistent with the observation of several water signals in the NMR spectra. Upon further increase of the water concentration, the water cluster ordering starts to dominate the structural modification. Based on the available data, a schematic model of the gel structure was proposed (Fig. 20). It comprised tubular- or lamellar-type IL aggregates with lipophilic cores separated from each other by thin polar layers containing interacting cationic head groups, anions, and water molecules.

image file: c7cs00547d-f20.tif
Fig. 20 Schematic representation of the formation of the [C8mim][Cl]/water gel. An inverted vial with gel is shown on the right. Reprinted with permission from Sturlaugson et al.117 Copyright 2013 American Chemical Society.

In a recent study, Gao and Wagner employed small-angle neutron scattering (SANS) to investigate the microstructure of [C4mim][BF4] mixtures with D2O.121 In the salt-rich regime of the [C4mim][BF4]/D2O mixtures, a microphase transition to water nanoclusters, resembling in their structure an inverse microemulsion, was observed at room temperature for the D2O concentrations exceeding ∼70 mol%. Based on the results of the study combined with the analysis of literature data, it was possible to draw a coherent picture of the behavior of the [C4mim][BF4]/water mixture for the whole concentration range (Fig. 21). Upon addition of small amounts of water to IL, water molecules remain isolated within the anion–cation polar network and interact with it via H-bonds, whereas the mixture retains its homogeneous nature. In this state, water traces in imidazolium-based ILs can be regarded as flexible clathrate complexes with the water molecules trapped inside the ionic lattice of ILs through the H-bond network. In this regime, water is soluble in the ionic liquid until the limit of ∼2 water molecules per ionic liquid ion pair is reached; then the microphase separation of water is observed with the transition to water nanoclusters similar to an inverse microemulsion. Upon further gradual addition of water, the water nanoclusters grow in size, while the mole ratio of the water dissolved in the ionic liquid nanostructure increases to ∼4[thin space (1/6-em)]:[thin space (1/6-em)]1 and the water percolation occurs.122 At higher concentrations, the inversion of the colloid system with the formation of IL micelles in the water solution starts due to the amphiphilic character of the [C4mim] cations.

image file: c7cs00547d-f21.tif
Fig. 21 Schematic illustration of the evolution of a nanostructure in [C4mim][BF4]/D2O mixtures upon increase of the D2O content. Reprinted with permission from Gao and Wagner.121 Copyright 2016 American Chemical Society.
2.2.3 Molecular modeling. Molecular modeling has been employed for the elucidation of IL microstructures from the eve of their research.123,124 Later, it has become possible to gain insights into the microstructuring of ILs for different IL/water ratios using computational methods.125–127 For mixtures with low IL concentrations, multiple molecular dynamics have predicted the formation of micellar-type aggregates surrounded by the bulk of water. This result is fully consistent with experimental observations not only for dilute solutions of ionic liquids, but also for other charged amphiphilic molecules in general. The studies were typically focused on the determination of ion pairing between counterions, aggregation numbers and the shape of micelles, as well as dynamical evolvement of the mixtures from a monodispersed solution to colloid systems.

In this review, we would like to focus on the studies of IL–water mixtures related to the water-depleted regime, when gel-type liquid crystalline structures are often being formed. These phases are more complex in their arrangement than micelles, which usually contain less than a hundred molecules in a micellar aggregate. For molecular dynamics simulations of such water-depleted systems, it is desirable to take several hundreds of IL ion pairs and thousands of water molecules, making these calculations extremely demanding of computer power.

Self-aggregation of [C10mim][Br] in water in the broad range of concentrations was studied by Bhargava and Klein using coarse-grade molecular dynamics.128 In dilute solutions, spontaneous formation of quasi-spherical micelles with aggregation numbers in the range of 40–60 was observed. The increase of the concentration to 37% of water (w/w, corresponding to 2.2 M solution of [C10mim][Br]) led to the formation of the hexagonal columnar phase, in which disordered columns formed by the IL cations were separated from each other by polar regions comprising water, the anions and the charged head groups (Fig. 22). The spacing between the layers organized by columns was in good agreement with the experimental data,101 affording supportive evidence for the proposed microstructuring of concentrated IL–water mixtures.

image file: c7cs00547d-f22.tif
Fig. 22 Snapshots of MD simulations of the [C10mim][Br]–water system containing 37% (w/w) of water. (A) View from side of hexagonal columns. (B) View from top of hexagonal columns. Reprinted with permission from Bhargava and Klein.128 Copyright 2009 Taylor & Francis Ltd.

A study of the imidazolium-based IL [C6mim][NTf2] with the relatively hydrophobic [NTf2] anion showed a slightly different solvation behavior for this IL.129 At low H2O concentrations, small water pools were surrounded by several cation–anion pairs (Fig. 23). Upon the increase of the water amount, phase separation started; it was accompanied by more tight segregation of the cationic alkyl chains. Finally, in a dilute water solution of the IL, tail aggregation led to the formation of quasi-spherical micelles. Due to hydrophobicity of the anion, it did not participate in H-bonding with water molecules, but formed H-bonds with the acidic hydrogen of the positively charged imidazolium cation via its nitrogen and oxygen atoms. These results were in agreement with poor water miscibility of imidazolium-based ILs with the [NTf2] anion.

image file: c7cs00547d-f23.tif
Fig. 23 Snapshots from annealing of IL–water mixtures at different ratios between the number of water molecules and the number of ion pairs (λ). Color scheme: (green) [C6mim]+ cation; (red) [NTf2] anion, and (blue) water. Reprinted with permission from Ramya et al.129 Copyright 2014 American Chemical Society.

A molecular dynamics study on water mixtures of [C2mim]+-based ILs with three different anions (namely, acetate [OAc], trifluoroacetate [TFA], and [BF4]) gave evidence for dramatic changes in H-bonding networks depending on the counterion.130 In the [C2mim][OAc]–water binary mixtures, water molecules were incorporated into the anion layers and led to the formation of anion–water–anion chains, which propagated across the mixture and caused significant nanostructuring of the medium (Fig. 24). On the other hand, anion–water wires in the [C2mim][TFA]–water and [C2mim][BF4]–water binary mixtures were interrupted by water clusters hindering the anion structuring. The [C2mim][BF4]–water binary systems featured the weakest anion–water H-bonded units for all three ILs. In terms of molecular mobility, the more structured [C2mim][OAc]–water mixtures exhibited retarded H-bond dynamics in comparison with the two other IL–water binary systems. Thus, the microstructure of the water layers separating domains formed by the IL cations was significantly influenced by the nature of the anion and its ability to form H-bonds with water molecules and cationic head groups of IL cations.

image file: c7cs00547d-f24.tif
Fig. 24 (A) Average number of hydrogen bonds formed by each water molecule with IL anions at different water concentrations. (B–D) Snapshots of the equilibrated mixture at Xw = 0.5 for (B) [C2mim][OAc]–water, (C) [C2mim][TFA]–water and (D) [C2mim][BF4]–water mixtures depicting anion–water (blue) and water–water (green) hydrogen bond networks. Representative anion–water wires are highlighted with CPK representation. Different nature of H-bonds in the three IL–water mixtures is evident from the prevalence of blue dashes in [C2mim][OAc], green dashes in [C2mim][BF4] and a mix of both in [C2mim][TFA]. Color scheme: grey, anions; red, water molecules. Hydrogens on anions are omitted for clarity. Reprinted with permission from Ghoshdastidar and Senapati.130 Copyright 2016 Royal Society of Chemistry.

Molecular dynamics simulations also were used to study the absorption of water molecules on electrode surfaces in a bulk imidazolium IL. The nature of the ions was shown to influence the water accumulation because of variations in the volume of the ions and their contacts with water molecules. In general, water molecules accumulated near the charged electrodes, predominantly positive ones (Fig. 25). In the case of [C4mim][PF6], this effect was explained by stronger association of water molecules with the hexafluorophosphate anions, which were denser near the positive electrode.48 Of note, a different behavior was observed for a lithium bis(trifluoromethylsulfonyl)imide–lithium trifluoromethanesulfonate–water WIS electrolyte, where water was excluded from the positive electrode, thus increasing the electrochemical stability window (see Section 2.5.2 below).131

image file: c7cs00547d-f25.tif
Fig. 25 Accumulation of water molecules near the charged positive electrode in [C4mim][PF6]. Reprinted with permission from Feng et al.48 Copyright 2014 American Chemical Society.

An interesting observation was made in a combined simulation-experimental study of the solvent influence on the ion aggregation in mixtures of N-methyl-N-pentylpyrrolidinium bis(trifluoromethylsulfonyl)imide with aprotic solvents (propylene carbonate, dimethyl carbonate (DMC), and acetonitrile).132 Conductivity, viscosity, coordinated NMR, and MD simulation studies of RTILs in three solvents (acetonitrile, DMC and propylene carbonate) demonstrated rather different aggregation behavior in these solvents. Unexpectedly, an increase in ionic aggregation and correlation in very dilute solutions was observed upon increasing the DMC solvent fraction. Therefore, the strongest correlation between ions and large aggregate formation was observed in the most dilute electrolyte considered.

As a summary of the discussion of physical properties of SIS systems, the phase behavior and the structural features of ionic liquid-based organic SIS have been studied using a number of techniques including polarized optical microscopy, electron microscopy, NMR spectroscopy, X-ray diffraction, scattering methods, conductrometry and others. The results of these studies have revealed the key structural properties of the ionic liquid mixtures with water and organic solvents under SIS conditions. Molecular modeling applied to the IL–water systems has given a picture of the water behavior in ionic liquid media and has visually demonstrated the formation of nano-phases. Undoubtedly, such a prolific phase behavior opens up multiple opportunities for the use of IL–solvent systems as media for chemistry processes, biochemical systems, and energy applications.

2.3 Application of the “solvent-in-salt” concept in chemical systems

Ionic liquids have always been positioned as solvents tailored for special applications, for the cases when “common” solvents fail to function, or when special physicochemical or miscibility properties of ILs facilitate the product separation. Both organic and inorganic ionic liquids were used as media for different processes and reactions.

Although molten inorganic salt hydrates were relatively rarely used as reaction media, they were successfully employed as solvents for dissolution and chemical transformations of cellulose.133 Surprisingly, many inorganic salt hydrates are capable of dissolving cellulose, an organic polymer. Some of the reported cellulose-dissolving molten salt hydrates are LiClO4·3H2O, LiI·2H2O, LiSCN·2H2O, ZnCl2·4H2O, and a eutectic mixture of NaSCN/KCN/LiSCN·3H2O.134–136 It was also demonstrated that melts of lithium salt hydrates not only could dissolve cellulose, but also could be used for its chemical modification. For example, LiClO4·3H2O (Tm = 368 K)134 was employed for carboxymethylation of cellulose using sodium monochloroacetate in the presence of NaOH.135,137 The reaction led to carboxymethylcellulose with the degree of substitution of 2 and the substituent distribution ratios of C-6 > C-2 ∼ C-3. Conversion of cellulose to isosorbide, a valuable compound for pharmaceutical industry, in concentrated aqueous ZnCl2 (molar ratio of ZnCl2[thin space (1/6-em)]:[thin space (1/6-em)]H2O = 1[thin space (1/6-em)]:[thin space (1/6-em)]3.2) in a single-pot reaction in the presence of HCl (Scheme 1) was reported by de Almeida et al.138 Regeneration of dissolved cellulose or the product of its conversion was done by addition of a precipitating solvent, most commonly water.

image file: c7cs00547d-s1.tif
Scheme 1 Hydrolysis of cellulose in the ZnCl2/H2O solvent-in-salt system followed by dehydration and formation of isosorbide. Reprinted with permission from Sen et al.133 Copyright 2013 American Chemical Society.

SIS systems based on organic ionic liquids have been widely applied for the synthesis of organic substances and structured inorganic materials. Reactions in ionic liquid media have attracted much attention in recent years, and a great number of publications on this topic have been devoted to the effect of water or another co-solvent on the process outcome.

In the synthesis of structured inorganic materials and metal organic frameworks (MOFs), the broad interest to ionic liquids has led to the creation of the new methodology called “ionothermal synthesis”, where ionic media are used both as a solvent and template. This approach has been successfully used in the preparation of porous materials, such as zeolite-like compounds and others.139–142

The key features of the ionothermal method including the role of added water have been studied in detail by Parnham and Morris.143 One of the vivid examples is the water effect on the preparation of aluminum phosphate zeolite analogues called SIZs, or St Andrews ionothermal zeolites.144 The synthesis was carried out in the [C2mim][Br] or choline chloride/urea media at 423–453 K with the use of aluminum isopropoxide, phosphoric acid and HF or water additives. X-ray diffraction studies of the obtained materials clearly showed that water had a significant impact on the structure of the aluminum phosphate framework (Fig. 26). The addition of water to the imidazolium ionic liquid led to the alteration of the SIZ structure. In the case of choline chloride/urea, non-zeolitic aluminophosphate was formed in the presence of water excess. The mechanism of the water action was suggested by the authors. As it was mentioned above, under the ionothermal conditions the ionic liquid served both as a solvent and template, so the force of interactions between the ionic media and substrates had a critical influence on the reaction progress. Strong shielding of the ionic liquid anions by water changed the templating properties of the media and led to the variation of the product geometry. Further statistical studies showed that the water-induced template restructuring occurred in a stochastic manner, and it was difficult to control the product structure by changing the water content in the reaction mixture.145

image file: c7cs00547d-f26.tif
Fig. 26 Aluminum phosphate zeolite analogue synthesis under ionothermal conditions in [C2mim][Br] (1–4) or choline chloride/urea (5 and 6) media. Reactions were carried out in the absence of additives (1 and 5) or in the presence of HF (2, 3 and 6) or water (4 and 6). For more details, see ref. 144. Reprinted with permission from Cooper et al.144 Copyright 2004 Nature Publishing Group.

Along with the template modification, water can affect the kinetics of crystalline product formation. On the example of aluminum phosphate molecular sieve synthesis under ionothermal conditions, it was shown that the addition of water into an ionic liquid media significantly enhanced the crystals growth rate and reduced the induction time.146

Another important area of application of ionic liquids in chemistry is organic synthesis as manifested by a plethora of publications that have appeared during the last few decades. Reaction media based on water and ionic liquids, including SIS systems, have attracted much attention due to their importance for the development of green chemical methods. Varieties of organic transformations in ionic liquid/water mixtures are described in detail in a recent review.147 The reactions, which can be successfully carried out in homogeneous or biphasic ionic liquid/water mixtures, include: hydrogenation,148,149 oxidation,150,151 catalytic cross-coupling reaction,152 Michael addition,153,154 enantioselective Diels–Alder155 and aldol156,157 reactions, epoxidation,158 dihydroxylation,159 Mannich reaction,160 and others.

For multi-component organic chemical transformations with the use of ionic liquids, water can be applied as a tool for tuning the solvent properties. The change of the water/ionic liquid ratio allows altering the solubility of a desired compound in the reaction mixture in order to promote the chemical transformation.

Recent studies have shown that scanning electron microscopy (SEM) can be used as a powerful method for the direct observation of morphology and dynamics of water-in-salt systems. A set of ionic liquids compatible with harsh conditions of the electron microscope specimen chamber was employed as a “salt” component, so the observations were directly carried out without any special sample protection. By using the developed technique, a variety of morphologies was detected in [C4mim][BF4]–water mixtures with different water content.42 It was found that the pure, dry ionic liquid was fully homogeneous and contained no microphase (Fig. 27A). The presence of even trace amounts of water in ionic liquid led to the formation of microdroplets within the liquid media (Fig. 27B). Further increase of the water content resulted in the growth and aggregation of droplets accompanied by the formation of dense inclusions (Fig. 27C–F). Dynamic behavior of the IL–water mixtures under electron beam was captured on the SEM-video. The observed processes included: movement of droplets and alteration of their shape, as well as reorganization of the aggregate structure.

image file: c7cs00547d-f27.tif
Fig. 27 SEM-images of [C4mim][BF4]–water mixtures with different water contents: (A) dry ionic liquid; (B) ionic liquid with traces of water; (C–F) ionic liquid with 5 (C), 10 (D) and 20 (E and F) vol% of added water. Reprinted with permission from Kashin et al.42 Copyright 2016 Wiley-VCH Verlag GmbH & Co. KGaA.

The results of the SEM studies indicate that very sensitive structures in liquid systems could be directly detected by means of electron microscopy. Moreover, complex multi-component solutions on the basis of water-in-salt systems were suitable for microscopy studies. For example, the role of water in the acid-catalyzed conversion of carbohydrates to the renewable chemical industry platform 5-hydroxymethylfurfural (5-HMF) was revealed on the basis of microscopy data.42 It was demonstrated that an excess of water led to aggregation of the carbohydrate and decrease in the product yield.

Another example of SEM observations of multi-component mixtures is the study of extraction of a dipeptide from a water-in-salt system at the microscale level.45 It was found that dialanine–water–IL mixtures had a complex microstructure with structural units such as liquid droplets and solid inclusions within the droplets or on the microphase boundaries (Fig. 28). The behavior of these microobjects during extraction was studied, and it was demonstrated that the retention ability of the system decreased in the following order: uniform phase > microdroplets > solid microinclusions.

image file: c7cs00547d-f28.tif
Fig. 28 SEM-image of dialanine–[C4mim][BF4]–water mixture (A) and schematic representation of the system morphology (B). Reprinted with permission from Seitkalieva et al.45 Copyright 2017 Elsevier.

An extensive study on the reactivity of organic compounds in a SIS system was reported by Weber et al.161 Nucleophilic substitution of [Ar3CPic][Cl] in the mixture of an ionic liquid with a nucleophile (water of alcohol) was chosen as a model reaction (Scheme 2). Classical imidazolium and 2-methylimidazolium ionic liquids and their mixtures were employed as reaction media. Kinetic measurements for the transformation carried out in the mixture of [C4mim][NTf2] and [C4C1mim][NTf2] (2-methyl-substituted [C4mim][NTf2]) revealed unique properties of water-containing solutions. It was found that the rate constant of the substitution reaction had high values for [C4C1mim][NTf2] and continuously decreased with an increase of the [C4mim][NTf2] molar fraction due to the changes in hydrogen-bond acidity. The most interesting observation was that the rate constant decreased almost linearly for water and non-linearly for alcohols (Fig. 29).

image file: c7cs00547d-s2.tif
Scheme 2 Nucleophilic substitution reaction in ionic liquids (R = H, Alk, Bn, Ph). Reprinted with permission from Weber et al.161 Copyright 2013 Royal Society of Chemistry.

image file: c7cs00547d-f29.tif
Fig. 29 Correlation between the observed nucleophilic substitution rate constant and the [C4mim][NTf2] content in the reaction mixture. Comparison of water and MeOH as nucleophiles is shown. Reprinted with permission from Weber et al.161 Copyright 2013 Royal Society of Chemistry.

On the basis of these observations, the authors suggested that in the case of alcohols, structural heterogeneity caused a non-linear behavior of the system. In its turn, water promoted partial dissociation of the ions accompanied by disruption of the ionic liquid structure.

The role of water in the microwave-assisted Suzuki cross-coupling in an ionic liquid was revealed.152 For the model Pd-catalyzed coupling between phenylboronic acid and 4-bromoacetophenone in 1-butyl-4-methylpyridinium tetrafluoroborate ([C4MPy][BF4]), it was found that the addition of excess water (from 1- to 3-fold excess) increased the product yield from 30% to more than 90%. Interestingly, the reaction in pure water gave only 68% product yield. As reported by the authors, the ionic liquid/water system had the most suitable heating profile and allowed to reach the desired reaction temperature faster than pure water; it also allowed to avoid overheating, which was observed for the reaction mixture in dry [C4MPy][BF4] under microwave conditions.

Among the organic transformations in ionic liquid media, the most studied process is the conversion of sugars and biomass into the top-value platform chemical 5-hydroxymethylfurfural (5-HMF). The examples of the water effect on the fructose, glucose and cellulose conversion in ionic liquids is discussed below.

The negative effect of water on the outcome of the fructose conversion to 5-HMF was demonstrated in a number of publications.42,162,163 It was clearly shown that an increase of the water content in a chloride ionic liquid-based medium resulted in the drop of the fructose conversion and 5-HMF yield. The same effect was observed in the case of glycerol addition to the reaction mixture and was explained by the lowering of the dielectric constant of the reaction media in the presence of the co-solvent.164 Although SIS systems may be inappropriate media for the fructose conversion to 5-HMF, they can be used to facilitate the transformation in the case of glucose.165 ZrO2-Catalysed glucose conversion in [C6mim][Cl] in the absence of water resulted only in 7% 5-HMF yield, but it could be increased to 32% by the addition of 10 wt% water and to 53% by the addition of 50 wt% water to the ionic liquid. The positive cooperative effect of water in the IL media is provided by the following factors: promotion of the glucose conversion to fructose, which is directly transformed into 5-HMF; lowering the viscosity of the reaction media which facilitates the mass transfer; and inhibition of the polymeric by-product formation. On the other hand, the above-mentioned negative effect of the co-solvent cannot be neglected. The particular case of the “water-in-ionic liquid system” usage for the 5-HMF synthesis is the cellulose conversion reaction. The formation of this product from plant biomass is a stepwise process including hydrolysis of cellulose to glucose with subsequent transformation of the monosaccharide into 5-HMF. Taking this into account, water can be obviously considered as a crucial player in the reaction. However, it should be noted that cellulose is insoluble in water, so a high water content in an ionic liquid can lead to the substrate precipitation from the reaction mixture. The question about the appropriate water content in the ionic liquid was addressed.166 It was found that the addition of only 5 wt% water resulted in partial precipitation of cellulose with gel formation, and at the 10 wt% water content, the system became completely heterogeneous. In order to overcome this problem, the gradual addition of water was suggested. This approach provided the aqueous conditions required for successful cellulose hydrolysis and glucose conversion; the stepwise water addition also allowed avoiding the complete system demixing. For the cellulose conversion in the [C2mim][Cl] media in the presence of HCl, the desired product was synthesized in the 30% yield with the use of the developed procedure.

Ionic liquids are widely employed as solvation media for other biomass products, such as lignin.34 Remarkably, the presence of small amounts of water increased the solubility of lignin in polar ILs such as 1-ethyl-3-methylimidazolium trifluoromethanesulfonate ([C2mim][TfO]) and tetrabutylphosphonium trifluoroacetate ([P4444][TFA]), which did not dissolve lignin without the addition of water. The water addition was supposed to decrease the hydrogen accepting ability and increase the hydrogen donating ability thus supporting the dissolution of lignin.167 In a simulation study on dissolution of cellulose compounds in hydrated 1-methyltriethoxy-3-ethylimidazolium acetate, 75% w/w IL was established as the critical IL content.168 Upon lowering the IL content below this point, the transition from IL-like to water-like nanostructuring of the solvent took place, since the hydrogen bonding between water molecules destroyed the structural characteristics of the pure IL, thus dramatically impacting its solvation ability.

As the conclusion of this part, several points should be emphasized. First, the tuning of the wide range of ionic liquid properties at the molecular level (dissolving ability, viscosity, conductivity, etc.) with the addition of a co-solvent can be used for the improvement of performance of chemical processes. Second, the unique properties of SIS systems at the supra-molecular level provide the possibilities for the use of such media in structured material synthesis. Lastly, multi-component ionic liquid-based systems can be employed as complex reagents for carrying out a variety of transformations.

2.4 Biochemical systems and their applications

Concentrated saline solutions are traditionally thought to impose deleterious effects on biological molecules with their fine structures maintained by complex combinations of hydrogen and van der Waals bonding, as well as electrostatic interactions.169,170 In contrast to common inorganic salts, ionic liquids often have a pronounced positive influence on stability and activity of proteins and nucleic acids.14 Moreover, ionic interaction-based drug delivery systems have been attracting significant attention lately,171,172 opening another potential niche for the application of ILs.

Whereas there are numerous publications dedicated to the systems consisting of aqueous solutions of biomolecules (mostly proteins) with small admixtures of ionic liquids (see e.g. review by Zhao),173 those dealing with the behavior of biomolecules in ILs with small amounts of water are rather scarce. Nevertheless, some useful data on the properties of such systems have been accumulated so far.

As we have mentioned above, neat ILs are supposed to be non-homogenous solvents, or media with rather complex inner micro- and nanostructuring ensured by H-bonding.62 Introduction of other molecules can change this structure leading to the formation of various polar and non-polar domains in the bulk solution.174 Thus, traces of water have a significant impact on the electrochemical properties and diffusion in IL-rich media.99 Moreover, water forms liquid aggregates inside 1-alkyl-3-methylimidazolium ILs with anions with stronger basicity, such as [NO3] or [TFA], and the presence of these aqueous ‘microcapsules’ is suggested to impact the solubility of substances otherwise insoluble in ILs.100 Therefore, mixtures of water and ILs represent an excellent environment for ordered molecular assemblies.175

Such structural effects are especially pronounced in systems containing large biomolecules. Water is one of the major determinates of the structural integrity of proteins.176 Notably, small amounts of water improve the protein solubility in IL systems177–179 and are required for the manifestation of enzymatic activity, e.g. in the case of horseradish peroxidase.180 An addition of highly polar zwitterions (such as 3-(1-methylimidazol-3-io)-butane-1-carboxylate) and 35 wt% water allowed the renaturation of cytochrome c dissolved in the polar IL 1-ethyl-3-methylimidazolium methylphosphonate.181

Upon dispersing an aqueous solution of an enzyme in the IL, the protein is suggested to occupy the polar water-rich domains, which assist the maintenance of its active conformation (Fig. 30).173,174,182

image file: c7cs00547d-f30.tif
Fig. 30 Trapping of enzyme-containing aqueous drops within the IL network. Reprinted with permission from Feher et al.182 Copyright 2007 Portland Press Ltd.

Such assumptions are based on the experimental evidence obtained in the studies on stability and activity of various enzymes introduced as aqueous solutions into IL media. Thus, α-chymotrypsin (from bovine pancreas) and lipase B (Candida antarctica) were stabilized by 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C2mim][NTf2]), 1-butyl-3-methylimidazolium tetrafluoroborate ([C4mim][BF4]), 1-butyl-3-methylimidazolium hexafluorophosphate ([C4mim][PF6]) and methyltrioctylammonium bis(trifluoromethylsulfonyl)imide ([(C8)3C1N][NTf2]) in the presence of 2% (v/v) water.183–185 This stabilizing effect of the IL is thought to be attributed to the preservation of a water layer around the enzyme molecule. Therefore, the enzyme in the aqueous environment can be considered as inclusions into the bulk IL media.185 The necessity of water for enzymatic activity was proven by the following observation: when dry lyophilized powder of α-chymotrypsin was dissolved in neat ILs ([C2mim][BF4], [C2mim][NTf2], [C4mim][BF4]), ([C4mim][PF6], [(C8)3C1N][NTf2]), no activity was detected;183 the presence of small water contents was essential for the enzymatic activity of the protein.186 In the case of [C4mim][NTf2], [C4mim][BF4], [C4mim][PF6] and [C4mim][TfO], the systems with 10–20% (w/w protein) water corresponded to the protein structures closer to the native one. In contrast, solvation of α-chymotrypsin in 1-butyl-3-methylimidazolium chloride ([C4mim][Cl]) led to destabilization of the enzyme structure even at low water contents due to the penetration of the small chloride anion into the protein core.187 Similar effects were observed for Candida antarctica lipase B and Candida rugosa lipase.188 1.5–4% (v/v) water contents were found optimal for Pseudomonas capaci lipase transesterification activity in 1-isobutyl-3-methylimidazolium hexafluorophosphate ([i-C4mim][PF6]) and [C4mim][PF6].189

According to a molecular simulation study on cutinase (serine protease from Fusarium solani pisi), the enzyme was stabilized in [C4mim][PF6] containing 5–10% (w/w protein) water. The IL was shown to strip water molecules off the enzyme surface, whereas the remained water was organized in small clusters.190

Hydrophobic ILs containing cations with long alkyl side chains are suggested to behave like “sponges” capable of absorbing hydrophobic compounds. Afterwards, these “swollen” sponges can be “wrung out” by centrifugation (Fig. 31). The possible explanation of this phenomenon is related to smectic structures formed by the alkyl chains; these structures contain hydrophobic pockets, which can hold non-polar compounds.191

image file: c7cs00547d-f31.tif
Fig. 31 N-Octadecyl-N′,N′′,N′′′-trimethylammonium bis(trifluoromethylsulfonyl)imide as dry “sponge” (A), sponge soaked with methyl oleate (B), and “wet sponge” after wringing out (C). A non-polar solute (blue) can incorporate into particular areas (yellow), which expand accordingly. Upon applying external mechanical force (centrifuging), the solute is pushed out from the SLIL. Reprinted with permission from Lozano et al.192 Copyright 2013 Royal Society of Chemistry.

Such “spongy” ILs, or SLILs (sponge-like ionic liquids), have been proposed as an optimal environment for biocatalytic reactions for the synthesis of various flavor esters, as well as for biodiesel production. Usually, these systems contain 50–70% (w/w) SLILs, to which substrates and immobilized enzymes are added, and allow high yields of target products (Fig. 32).191,192

image file: c7cs00547d-f32.tif
Fig. 32 Biodiesel production in a SLIL system. In the case demonstrated, SLIL, methanol and triolein form a homogenous system (A), in which an immobilized lipase carries out methanolysis of triolein. Then the reaction mixture is supplemented with water, cooled down (a semisolid heterogeneous mixture is formed) (B) and centrifuged giving a three-phase system consisting of a biodiesel phase (top), glycerol-containing aqueous phase (middle) and solid SLIL phase (bottom) (C). The SLIL phase can be recovered for subsequent reuse (D). Reprinted with permission from Lozano et al.191 Copyright 2015 Royal Society of Chemistry.

Water is able to form reverse micelles in surfactant-supplemented ILs, and these micelles are found suitable for dissolving hydrophilic biomolecules.44,193 Such systems were shown to contain bulk water, which was present in the cores of surfactant aggregates.194 Such systems are called water-in-ionic liquid microemulsions and are proposed to be used in various biocatalytic applications.195–198 Water-in-IL-based organogels obtained by adding the corresponding biopolymers were suggested as matrices for protein immobilization and to be used as solid-phase catalysts in various organic solvents.196

A microinterface formed between ILs and water was suggested effective for preparing protein-containing aqueous microcapsules (Fig. 33).199,200 Upon addition of aqueous solutions of BSA (bovine serum albumin), HSA (human serum albumin), β-lactoglobulin or cytochrome c to [C4mim][NTf2], aqueous microdroplets were observed (Fig. 34 and 35). These microcapsules were cross-linked by using glutaraldehyde for subsequent extraction into the aqueous media. The suggested method was also successfully applied for encapsulating guest biopolymers, such as DNA, in the protein microdroplets.

image file: c7cs00547d-f33.tif
Fig. 33 Synthesis of protein microcapsules on the IL–water interface with their following extraction into the aqueous media. Reprinted with permission from Morikawa et al.199 Copyright 2012 American Chemical Society.

image file: c7cs00547d-f34.tif
Fig. 34 Aqueous BSA microdroplets in [C4mim][NTf2]. (A) The bright-field optical microscopic image of emulsion; (B) the confocal fluorescent image of microdroplet containing FITC-labeled BSA formed in [C4mim][NTf2]; and (C) the fluorescence intensity profile along the red line shown in (B). Reprinted with permission from Morikawa et al.199 Copyright 2012 American Chemical Society.

image file: c7cs00547d-f35.tif
Fig. 35 Bright-field optical microscopic images (A and B) and SEM images (C and D) of cytochrome c microcapsules prepared in [C4mim][NTf2]. (A and C), 5 mg mL−1 cytochrome c; (B and D), 50 mg mL−1 cytochrome c. Reprinted with permission from Morikawa et al.200 Copyright 2013 Chemical Society of Japan.

In the case of DNA, which demonstrated remarkable stability in IL–water systems, molecular dynamics simulations and circular dichroism experiments showed the disruption of the DNA-surrounding water shell, including that in the minor groove (Fig. 36). Such partial dehydration was suggested to prevent DNA hydrolysis. ILs interacted with the minor groove of DNA via van der Waals interactions, hydrogen bonding and electrostatic contacts of the IL cation, thus stabilizing the B-conformation of DNA.201 Water–IL systems with a low water content corresponded to DNA conformations closer to the native one. The strength of the water shells around the phosphate groups of DNA was supposed to correlate inversely with the IL stabilizing effect.202

image file: c7cs00547d-f36.tif
Fig. 36 Spatial distribution of water and IL cations around DNA in 80 wt% IL solution. Cyan, DNA; red, water; green, imidazolium moieties of IL cations. Reprinted with permission from Chandran et al.201 Copyright 2012 American Chemical Society.

These observations suggest the wide potential of water-in-IL systems in various biochemical applications, such as biocatalysis and biomolecule separation and storage.

2.5 Energy research applications

Ionic liquids in their pure form have been extensively tested in the area that can be broadly defined as “energy applications”. Initial interest was focused on applications of ionic liquids as high temperature electrolytes,203 but in the next years it extended to their use in other energy-storage and energy-generation-related areas. Most of the research efforts were aimed at the development of advanced electrolytes7,35,36,204–209 for Li-batteries, fuel cells, dye-sensitized solar cells, and supercapacitors, although other possible applications, such as carbon dioxide capture and separation and thermal energy storage, had also been explored. The use of ILs in energy-related research areas has been discussed in detail in several recent reviews.17,207,209–211 In contrast, potential of solvent-in-salt compositions remains mostly unexplored, and their possible chemical and technological advantages have started to draw broad attention only during the last five-six years in relation to their use in electrochemical devices. In this section, we discuss the examples of successful application of solvent-in-IL systems in thermal energy storage, batteries and supercapacitors. To the best of our knowledge, the efficient usage of such systems in solar cells has not been demonstrated yet.207,212–214
2.5.1 Thermal energy storage. Interestingly, inorganic salt hydrates, which can be regarded as SIS systems, have been employed for many years for the latent thermal energy storage. This relatively narrow niche application is based on the accumulation and release of thermal energy upon reversible phase transitions. Low-melting salt hydrates possess the required properties for reliable thermal energy storage: suitable melting points, high boiling points and very low vapor pressures at operational temperatures, low degradation and chemical stability over a very large number of regeneration cycles, and high thermal conductivity. Additionally, their molar enthalpies of melting are generally higher than those of similar anhydrous salts, which also typically melt at much higher temperatures that are not suitable for heat storage needs. Numerous salt hydrates and their mixtures have been screened with regard to these criteria, and a number of candidates, particularly suitable for low grade heat storage, have been identified. Low melting point mixtures that melt at about ambient temperature are especially desirable for sustainable air-conditioning systems operating at temperatures close to the ambient ones.

Several reviews54,215,216 discuss the potential of different salt hydrates and their eutectics as media for latent heat storage in the broad temperature range from 253 to 393 K (Fig. 37). Most of the data used for phase diagrams of salt–water mixtures was rather obsolete, being measured about 60–70 years ago, indicating relatively low interest to these systems in recent years. For successful practical applications on the industrial scale, the thermophysical properties of the most known salt hydrates and their mixtures should be re-determined using modern experimental equipment. Currently, a variety of phase change materials (PCMs) based on salt hydrates and their compositions are manufactured and available commercially for the use in latent heat storage systems, although the information provided by their producers usually does not disclose their composition.

image file: c7cs00547d-f37.tif
Fig. 37 Enthalpy of fusion given per unit volume for several salt hydrates and salt hydrate eutectics plotted against their melting points. For comparison, data for water, two paraffins and two clathrates are shown. Reprinted with permission from Gawron and Schröder.215 Copyright 1977 Wiley-VCH Verlag GmbH & Co. KGaA.
2.5.2 Batteries. The real advance paving the way for a broad practical use of inorganic solvent-in-salt systems is related to their application as electrolytes in rechargeable Li-ion batteries. The innovative concept of the “polymer-in-salt” solid electrolyte with a very high ratio of salt to solid polymer solvent was suggested by Angell et al. in 1993.217 In such polymers, glass transition is low enough to maintain the rubbery state and to demonstrate high conductivity and electrochemical stability at room temperature.218–220 However, the first example of a liquid “solvent-in-salt” (SIS) electrolyte was reported only two decades later in the groundbreaking paper of Suo et al., who employed the concentrated 7 M lithium bis(trifluoromethylsulfonyl)imide (LiNTf2) solution in 1,3-dioxolane (DOL)–dimethoxyethane (DME) mixtures (1[thin space (1/6-em)]:[thin space (1/6-em)]1 by volume) as an electrolyte in rechargeable metallic Li-S batteries.20 It was shown that the SIS electrolyte displayed a number of important advantages in comparison with traditional electrolytes (typical salt concentrations below 1.2 M), namely almost complete inhibition of dissolution of lithium polysulfide and suppression of lithium dendrite growth on the metallic lithium anode. It allowed achieving nearly 100% coulomb efficiency and long cycling stability. According to a recent study, in the superconcentrated aqueous electrolyte the liquid lithium polysulfide phase was thermodynamically phase-separated from the electrolyte. Upon coupling a sulfur anode with various Li-ion cathode materials, energy densities up to 200 W h kg−1 for >1000 cycles at nearly 100% coulomb efficiency were achieved.221 Another recent study confirmed that lithium metal anodes demonstrated excellent cycling (up to 6000 cycles) and coulomb efficiencies (up to 98.4%) in the concentrated 4 M lithium bis(fluorosulfonyl)imide (LiN(SO2F)2, LiFSI)222 in the 1,2-dimethoxyethane electrolyte.223

As the next breakthrough came the discovery that a >21 m (where “m” is molality, or molal concentration denoting the moles of solute per kilogram of the solvent) LiNTf2 water-in-salt (WIS) electrolyte could offer indispensable advantages for rechargeable Li-ion batteries.21,224 Such concentrated Li-salt solution remained a true liquid at ambient temperature and could be supercooled to 183 K. The use of the WIS system allowed overcoming the thermodynamic voltage limit of 1.23 V of aqueous electrolytes imposed by hydrogen (below 0 V) and oxygen (above 1.23 V) evolution that limited the performance of Li- and Na-based batteries with aqueous electrolytes.225–227 The battery, which used a common LiMn2O4/Mo6S8 electrochemical couple, demonstrated the open circuit voltage (OCV) of 2.3 V, 84 W h kg−1 energy density, and could be cycled up to 1000 times with nearly 100% coulomb efficiency. Electrochemical reduction of the [NTf2] anion at the anode surface led to the formation of a solid–electrolyte interphase, which suppressed the hydrogen evolution and allowed operation of the electrolyte at potentials far below its thermodynamic stability. On the cathodic side, the oxygen evolution was also suppressed, likely as a result of reduced water activity in a concentrated solution. Both factors increased the theoretical operation window for the aqueous electrolyte to 3.0 V. The very high concentration of Li+ ions also shifted the potentials of the LiMn2O4 cathode and the Mo6S8 anode towards positive values. This effect was attributed to the Li+ activity change according to the Nernst equation, making them compatible with the expanded operational window of the electrolyte (Fig. 38). The initial design of the SIS-based battery was revised by the use of LiFe2O4, which was known for its thermal stability, non-toxicity, safety and low cost, as a cathode material.228 This inexpensive aqueous Li-ion battery chemistry is aimed at large-scale energy storage applications. The batteries showed excellent cycling reversibility (>1000 cycles), a high coulomb efficiency and an energy density of 47 W h kg−1. Most recently, a TiS2 anode paired with a LiMn2O4 cathode in 21 m LiNTf2 in water demonstrated a discharge voltage of 1.7 V and an energy density of 78 W h kg−1.229 Protection of the surface of a LiCoO2 cathode paired with a Mo6S6 anode by an additive-formed interphase allowed an energy density of 120 W h kg−1 for 1000 cycles.230

One of the current goals of the rechargeable battery design is the construction of the next-generation 5V-class lithium-ion batteries. In connection with the discussion above, most likely, a viable electrolyte for such electrochemical devices will be created using SIS systems upon variation of already available organic electrolytes (acetonitrile, 1,3-dioxolane, dimethoxyethane, dimethyl carbonate, ethyl acetate, dimethyl sulfoxide) and lithium salts (LiPF6, LiNTf2, LiN(SO2F)2, LiTfO) in pure forms or as mixtures.231–234

image file: c7cs00547d-f38.tif
Fig. 38 Schematic illustration of the expanded electrochemical stability window for WIS electrolytes. In pure water, the thermodynamic stability window is 1.23 V, but it can be expanded in traditional electrolytes to ca. 2.0 V. In a WIS electrolyte, the window is expanded to ca. 3.0 V along with the modulation of redox couples of the LiMn2O4 cathode and Mo6S8 anode caused by the high salt concentration, allowing the construction of a 2.3 V aqueous battery. Reprinted with permission from Suo et al.20 Copyright 2015 American Association for the Advancement of Science.

This discovery paved the way for rapid development in the field of SIS electrolytes. Only one year later the same group further improved the performance of Li-ion batteries by using a 28 m water-in-bisalt electrolyte235 consisting of 21 m LiNTf2 and 7 m LiTfO. It afforded the liquid electrolyte of 28 m Li+ and the cation/water ratio of ca. 1[thin space (1/6-em)]:[thin space (1/6-em)]2, with even broader chemical stability window and lower water activity. A rechargeable Li-cell employing a LiMn2O4 and C-TiO2 couple in this electrolyte showed up to 100 W h kg−1 energy density and 2.1 V average discharge voltage. A molecular modeling study of this WIS system implied the formation of a special water-salt structure that was well permeable for lithium ions.236 According to the latest report, an “inhomogeneous additive”, that is, an electrolyte-immiscible hydrofluoroether, applied for coating the anode surface allows minimizing the water reduction upon the interphase formation.237 Such high-capacity anode materials can be coupled with various cathode materials producing 4.0 V aqueous Li-ion batteries. Moreover, it was shown that even highly concentrated aqueous solutions of simple inorganic lithium salts could be successfully used as electrolytes in lithium cobalt oxide (LCO) based batteries. Such electrolytes allowed employing safe water media and at the same time increasing the LCO cathode stability due to the low water content in the electrolyte. Less than 13% degradation of LCO was observed after 1500 cycles in the case of a Li2SO4 concentrated aqueous solution.238

Suppressed anodic dissolution in the aqueous medium was also observed for aluminum, opening the opportunity of roll-to-roll electrode fabrication on cost-effective light-weight aluminum current collectors for high-voltage aqueous lithium-ion batteries.50 Other recent reports describe the use of solvent-in-salt electrolytes in high-voltage Zn/LiMn0.8Fe0.2PO4, LiCoO2/Li4Ti5O12 and LiNi0.5Mn1.5O4/Li4Ti5O12 batteries,239,240 the use of less expensive and more affordable sodium for the anode in 1,2-dimethoxyethane-based241 or DMSO-based242 solvent-in-salt compositions, the successful testing of LiNTf2/DMSO compositions for their possible use in Li-air batteries,243 and even the creation of an all-organic rechargeable battery (cathode – polytriphenylamine, anode – 1,4,5,8-naphthalenetetracarboxylic dianhydride-derived polyimide) operating in a WIS electrolyte.244

The ability of SIS electrolytes to widen the electrochemical stability initiated the detailed studies of their electrochemical and structural properties. An electrochemical investigation of the behavior of a LiNTf2 WIS-electrolyte on platinum, gold and glassy carbon electrodes provided evidence that the increase of the electrochemical window was mainly due to the strong shift of the oxygen evolution onset potential.245 In a superconcentrated LiNTf2 solution, the fraction of free water is very low, and the free water molecules are shielded from the positively charged electrode surface by the [NTf2] anions (Fig. 39). These observations are in agreement with a recent MD simulation study of the electrolyte–electrode interface.131 Water is excluded from the interfacial layer at the positive electrode which lowers the fraction of free water and delays the onset of oxygen evolution (Fig. 39C). It should also be noted that the desolvation energy barrier for the lithium cation is lower in WIS electrolytes, as compared to conventional carbonate electrolytes, which leads to a lower interfacial resistance and a faster kinetics.131,246

image file: c7cs00547d-f39.tif
Fig. 39 Ionic environment of polarized electrode surfaces for diluted (A) and superconcentrated (B) electrolyte solutions. (C) Snapshots of interfacial layers for uncharged electrodes at the potential zero charge and for positively and negatively charged electrodes (WIS: 21 m lithium bis(trifluoromethylsulfonyl)imide–7 m lithium trifluoromethanesulfonate–water). Reprinted with permission from Coustan et al.,245 copyright 2017 Elsevier, and Vatamanu and Borodin,131 copyright 2017 American Chemical Society.

Another study employing the two-dimensional infrared vibrational echo and polarization-selective IR pump–probe technique showed the presence of two distinct populations of hydroxyl groups by differentiation of their vibrational lifetimes.247 One population of hydroxyl groups formed H-bonds with the [NTf2] anion whereas the second population – with other water molecules. It was found that the H-bonds to water in the SIS solution were significantly weaker than those in bulk water, meaning that the optimal quasi-tetrahedral arrangement of H-bonds was not possible. Dynamics of the hydroxyl groups was dominated by their hydrogen bonding with the [NTf2] anions.

Whereas the behavior of organic ionic liquids at electrified interfaces is well understood7,48,248,249 and they have been extensively tested as electrolytes in the neat form,7,35,36,204–206 SIS compositions based on them have not attracted much attention yet. The ‘ionic liquid-in-salt’ concept was suggested in 2014 in the context of creation of stable electrolytes for Li-ion batteries aimed for high temperature applications.250 The studied samples were based on the concentrated solution of LiNTf2 in the [C2mim][NTf2] ionic liquid with the composition of (1 − x)[C2mim][NTf2][thin space (1/6-em)]:[thin space (1/6-em)](x)LiNTf2, 0.66 ≤ x ≤ 0.97. The samples displayed a wide thermal stability window and high electric conductivities in the 0.7 ≤ x ≤ 0.9 composition range. With the help of NMR and Raman spectroscopy, [Li(NTf2)2] ions were identified as highly mobile charged species likely to be responsible for the high ionic conductivities observed in the study.

The energy density depends on the high Li+ transference number of an electrolyte (the part of the charge carried by electroactive lithium ions, tLi+), whereas low transference numbers lead to the electrolyte polarization and the corresponding internal resistance.251,252 Lithium salt-doped ILs are usually characterized by low tLi+, possibly due to the strong coordination of Li+ to IL anions.252–254 In contrast, an aqueous 21 m lithium bis(trifluoromethylsulfonyl)imide electrolyte is characterized by a high Li+ transference number, which can be explained by the formation of nanoheterogeneous domains in the liquid, where 40% of Li+ are available for fast ion transport even at high salt concentrations.255 Of note, such nanoheterogeneous liquid structures are characteristic of ionic liquids.

The usage of Li+–glyme solvates in IL systems has been proposed for facilitating the release of free mobile lithium solvates and increasing tLi+.256 Most extensive studies related to the use of IL-containing SIS systems as electrolytes were performed by Mandai et al. with glyme-based SILs.57 Although polyether solvates, i.e. glyme-based SILs, display favorable thermal and electrochemical properties, high viscosity and relatively low conductivity preclude their practical application as electrolytes in the pure form, without other additives. Upon dilution with polar solvents, such as water or propylene carbonate (PC), competitive solvation between the glyme and the solvent molecules leads to significant dissociation of the glyme–Li complex cations. The glyme–Li complexes are stable in the presence of less polar acetonitrile or acetone; however, the Li cations are partially solvated by the solvents. In contrast, dilution with non-polar solvents, such as toluene or hydrofluoroether (HFE), preserves the Li complexes, but increases the ionic conductivities and lowers the viscosities of SILs. Thus, addition of small fractions of non-polar organic solvents to SILs makes them well-suitable for the use as battery electrolytes,257 whereas polar organic additives lead to their instability because of oxidative decomposition due to the presence of the uncoordinated glyme.258 Recently, organic SIL-based electrolytes have been successfully tested for the construction of Li-ion,259 Li-S,259,260 and Li-air261,262 batteries, as well as Na-S batteries.263

The highly concentrated aqueous ionic liquid electrolyte [C4mim][Cl]–H2O has been successfully tested in a hybrid Zn/Ce aqueous flow battery operating in the broad electrochemical stability interval of 3 V.264 In another report, a solution of water in an ionic liquid (water-in-ionic liquid) was shown to perform as a practical electrolyte for Li-air batteries.51 One of the major problems of Li-air batteries, namely high lithium reactivity towards water, could be overcome by the use of an IL-based electrolyte; upon the addition of 1 vol% of water to the electrolyte system containing N-methyl-N-propylpiperidinium bis(trifluoromethylsulfonyl)imide and lithium bis(trifluoromethylsulfonyl)imide, only vanishing traces of chemical reaction between lithium and water were observed. The water was supposedly bound to the IL electrolyte and did not exist in the free form in the system.

Future efforts will be focused on identifying and optimizing the cathode and anode materials which will allow exploiting the full potential of the operational window of SIS electrolytes. Properties of supersaturated electrolytes for lithium-ion batteries are discussed in a recent review,265 although, due to the rapid development of the field, it does not include the most recent achievements. The discovery of the unique properties of SIS-based electrolytes is of paramount importance for the current endeavors to increase the performance and widen the operation window of rechargeable batteries.266 It will give a new impulse to the development of affordable high-energy batteries with lithium anodes.267

2.5.3 Supercapacitors. In recent years, the area of using SIS systems in supercapacitors has been developing actively, and some interesting results have been obtained.36,209 New ionic liquid-based electrolytes have been proposed to be beneficial in the development of high-energy supercapacitors.268–270 The main barrier against such IL applications is strong interactions of IL ions with each other, as well as with the electrode surface. This issue is suggested to be solved by using IL electrolytes with small amounts of additives.209 According to a simulation study, low concentrations of highly polar low molecular weight additives should increase the capacitance of electric double layer capacitors (EDLCs).271 The usage of mixtures of N-butyl-N-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide with butyronitrile allowed operating EDLCs at extreme temperatures (−20 to +80 °C).272 It should also be remembered that in conventional supercapacitors, the material porosity allows increasing the surface area but also establishes a higher area for possible contacts of IL electrolytes.209 Supplementing the IL electrolyte (N-propyl-N-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide or 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide) with acetonitrile improved the ion mobility in the micropores of a carbon electrode.273 These are just a few representative examples of the SIS employment in supercapacitors. We have no doubts that more results are to be expected in the nearest future.

2.6 Characterization of “solvent-in-salt” systems using different analytical methods

Mechanistic studies and gaining deeper insight into the nature of physical and chemical processes occurring in the SIS systems are the key points required for further development of the field. Due to the high level of molecular complexity, different types of microstructures and various phase conditions (Fig. 2), these systems are extremely difficult to study. Therefore, we have briefly summarized the experimental methods employed for characterization of SIS (Table 1). The purpose of this section is not to provide a detailed description of each experimental method, since this information is available elsewhere in numerous dedicated reviews. Instead, a practical guide dealing with particular analytical studies of SIS systems is summarized, and the corresponding references are provided (Table 1).
Table 1 Experimental methods and modeling for structural characterization of “solvent-in-salt” systems
Method Description Potential for SIS Results/conclusions Ref.
Small angle X-ray scattering (SAXS) and wide angle X-ray scattering (WAXS) Determination of correlation lengths present in liquids using X-ray radiation. SAXS is a standard technique for delivering structural information on objects between 5 and 25 nm (macromolecules and small colloidal aggregates) and on repeat distances in partially ordered systems of up to 150 nm. WAXS is suited for the determination of finer structural features up to individual atoms Detection of colloidal aggregates and CAC in IL–water solutions, as well as of regular heterogeneities in the bulk of ILs and their mixtures; provides no detailed information about their structure Used for determination of CAC for different ILs in IL–water mixtures; provided evidence for formation of microheterogeneities in IL–water mixtures 20, 101–103, 105, 106 and 108–110
Small-angle neutron scattering (SANS) Structure investigation at mesoscopic scale of about 1–100 nm using thermal neutrons (λ ≈ 0.5 nm). Used to determine size, shape, and polydispersity of aggregates in solution. Similar to SAXS, but affords larger scale ranges and is sensitive to light elements Same as SAXS, but more sensitive and provides better resolution Same as SAXS 71, 79, 90, 107 and 121
X-ray diffraction (XRD) Determination of atomic and molecular structure of crystals and of packing of molecules in the crystalline state, also in hydrates, by using X-ray radiation Examination of molecular arrangement in salt hydrates and IL hydrates in crystalline states (below melting points) Showed formation of segregated layers of lipophilic and cationic moieties in crystals of ILs and segregation of water molecules between layers of cationic head groups and counteranions 73, 111 and 114
Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) Direct method for investigation of the morphology of materials with up to angstrom resolution based on sample interaction with electron beam Allows examination of ILs and their mixtures for formation of aggregates, nanostructures, and heterogeneities Detection and characterization of aggregates and microstructures in IL–water mixtures and microemulsions 42, 45, 91–94 and 103
Polarized optical microscopy (POM) Direct method for investigation of the surface structure and morphology of materials with resolution up to ca. 0.2 μm; the presence of crystalline and liquid crystalline phases. In combination with fluorescent dyes or NP allows enhanced visualization of microstructures and interfaces Affords detection of liquid crystalline phases and large colloidal aggregates in IL–water mixtures. Determination of phase transition temperatures Evidence for formation of liquid crystalline phases in pure ILs and IL–water, IL–solvent and ternary mixtures. Was employed for visualization of IL–water interfaces using colored nanoparticles 101, 103–106, 109, 111, 114, 115 and 117
NMR spectroscopy Spectroscopic technique for studying various systems at the molecular level based on interaction of radiofrequency radiation with samples in a strong magnetic field. Multiple capabilities depending on the type and setup of the experiment: 1H, 2H and heteronuclear spectra, relaxation and diffusion measurements, nuclear Overhauser effect experiments, two-dimensional NMR Can afford information on conformation, solvation, intermolecular interaction between system components and their dynamics, CAC, and other properties. Can be employed to follow chemical reactions Was used to determine CAC in dilute solutions; to study interactions between cations, anions, and H2O molecules; to determine self-diffusion coefficients for mixture components, giving evidence for segregation of IL–water domains 20, 63, 71, 73, 77, 81, 83, 84, 95–97, 102, 106, 112–115, 121, 243, 250, 257, 259, 260, 263, 274 and 283
IR (as well as polarization-selective IR pump probe and 2D IR vibrational echo spectroscopies) and Raman spectroscopy Spectroscopic technique used to measure frequencies of molecular vibrations and reveal their dependence on the local environment of molecules In particular, used to study H-bonding of components of ILs and IL–water mixtures Information about environment and interactions between cations, anions, and water or other solvents 20, 100, 102, 119, 231, 240–243, 247, 250, 257, 259, 263 and 284
Fluorescence spectroscopy Measurement of fluorescence spectra and their dependence on the molecular environment. Can be used for measurement of bulk weakly fluorescent substances or, much more often, for determination of fluorescence of minor amount of highly fluorescent probes (e.g. pyrene) added to the system In particular, used to determine critical aggregation concentrations or dynamic environment (e.g. dynamics of reorientation) of fluorescent probes Information about aggregation state of ILs as well as environment of individual molecules in the system 81, 82, 84, 85, 93, 94 and 118
Dynamic light scattering (DLS) Scattering technique used for measurement of size distribution and concentration of nm- and several μm-size aggregates in suspension or colloidal solution Can be employed to study IL-containing colloids, e.g. micellization of dilute IL solutions, or to determine “cloud points” in miscibility studies Determination of CAC (“cloud points”) for IL/water mixtures, as well as size of colloidal aggregates of ILs 67, 78 and 93
Optical Kerr effect spectroscopy Method for measurement of orientational relaxation dynamics of a liquid tracking collective motion of molecules over time scale ranging from hundreds of femtoseconds to microseconds and over six decades of signal amplitude Investigation of relaxation dynamics of ILs in pure form and in mixtures Evidence for structuring of alkyl chains and diminishing of orientational relaxation by small amounts of water 116 and 117
Calorimetry (DSC, ITC) Method for determination of thermal effect of different processes (e.g. phase transitions) used for thermodynamic characterization of studied systems Determination of enthalpy of phase transitions (e.g. melting of salt hydrates) or energy released upon mixing of two components Used to measure energy capacity of salt hydrates, as well as energy released by hydration of salts and ILs 51, 67, 104, 114 and 250
Surface tensiometry Measurement of surface tension Measurement of surface tension dependence on solute concentrations for determination of phase changes in two-component systems Determination of CAC of dilute solutions of ILs in water and its dependence on cation and anion properties 71, 79, 84, 87, 88 and 94
Ionic conductivity Measurement of ionic conductivity Determination of the degree of counterion dissociation and mobility Determination of the CAC based on different mobilities of individual ions and their aggregates 55, 71, 79, 82, 85–88, 93, 241, 243, 250, 257, 259, 260 and 263
Voltammetry Determination of electrochemical stability window and of redox potentials and diffusion coefficients of electroactive species Investigation of redox properties of electrolytes or redox-active solutes. Study of ion diffusion coefficients in pure ILs and their solvent mixtures Discovery of expanded redox windows for IL- and water-based electrolytes. Examination of ion diffusion coefficients in IL–water mixtures implied the formation of water channels in the bulk of IL 20, 50, 99, 223, 228, 231, 233, 235, 239–245, 259, 263 and 264
Rheological studies and densimetry Determination of rheological properties (e.g. viscosity) and density General information about the aggregation state, determination of apparent molar volumes Determination of critical aggregation concentration, indirect information about aggregates, data for plotting of phase diagrams 63, 77, 82, 83, 91–93, 106, 117, 243, 257, 259, 260 and 263
Visual miscibility studies Visual determination of “cloud point” Determination of phase separation or critical aggregation concentration Data can be used for plotting of phase diagrams for binary or ternary mixtures of ILs with co-solvents 54, 63–70, 72–76, 78, 85 and 110
Molecular modeling Different methods (molecular mechanics, ab initio, DFT) to calculate parameters of single ions, ion pairs and molecular clusters, to simulate behavior of pure ILs and their mixtures (molecular dynamics on different levels of theory) Theoretical modeling of structure and behavior of SIS systems, ILs and their mixtures; elucidation of observed behavior and phenomena Predicted and explained formation of separate phases in ILs and their mixtures with water due to segregation of lipophilic and hydrophilic domains, as well as association of water within hydrophilic domains 20, 76, 77, 107, 113, 123–130, 223, 231, 235, 236, 242, 257 and 263

The existing experimental methods for solvent-in-salt system characterization cover a wide range of SIS properties, ranging from molecular-level behavior to nano- and microscale characteristics and to bulk system features. Each method has its own object size range, accessible timescale, specific requirements for samples and other characteristic features. For the molecular-level characterization of SIS systems, a number of spectral methods, such as nuclear magnetic resonance (NMR), infrared (IR) and fluorescence spectroscopy, can be employed (Table 1). Spectral techniques provide indirect information about the molecular structure of the studied samples, intermolecular interactions and dynamic processes in the system of interest. Thorough data processing and, in some cases, supporting calculations are required for the collection and analysis of structural information.

Special attention should be paid to the application of NMR in the studies on ionic liquid systems. Until recently, NMR studies in IL media have been considered as complicated due to technical issues.25 The stability of IL microstructuring often requires special preparation procedures for obtaining good-quality NMR spectra (Fig. 40).274 Dedicated experimental procedures for studying IL-based systems have been developed, such as slice-selective NMR, which allows real-time visualization of solute transition between the system phases.275

image file: c7cs00547d-f40.tif
Fig. 40 Spectral evidence of the microheterogeneous nature of IL systems: The 1H NMR spectrum of glucose in [C4mim][Cl] (A) upon standard sample preparation, and (B) upon sample preparation wih additional external stirring. Magnified regions containing the glucose signals are shown in the insets. Reprinted with permission from Egorova et al.14 Copyright 2017 American Chemical Society.

NMR experiments have contributed to the studies of the impact of water on the IL structure.25 For instance, 1H NMR, 2H NMR, 19F NMR and other NMR techniques were employed for studying interactions between water and cations of [C4mim][BF4],95 for analyzing the dependence of water reactivity in ILs on their concentration,276 and for investigating the effect of the presence of small amounts of water on the reaction rate.277 When studying IL-based SIS, NMR is often supplemented by or is supplementary to other physicochemical methods of investigation. Thus, 1H NMR in combination with molecular dynamics simulations demonstrated the possibility of modulating solvation regimes by changing the water content in the [C4mim][BF4] system.2781H NMR in combination with infrared experiments showed that the presence of water influenced the formation of ionogels observed in [C10mim][Br] and [C10mim][NO3]; this effect was attributed to the disruption of H-bonds between the imidazolium ring and the anion and their replacement with H-bonds between the anion and water.10219F NMR together with SANS were used for deciphering the formation of water nanoclusters in [C4mim][BF4].121

In addition to spectral methods, X-ray diffraction provides the opportunity for the molecular-level characterization with atomic precision (Table 1). This technique can be used for the determination of the exact molecular structure and packing of molecules in crystals, but only solid crystalline samples can be used for the diffraction studies. Another group of methods is designed for the SIS characterization at nano- and microlevels. These methods are targeted to supramolecular architectures, nanostructures and their aggregates, and microsized objects. Various scattering methods, such as neutron scattering (SANS), X-ray scattering (SAXS) and light scattering (DLS), are employed for the determination of sizes and shapes of supramolecular structures, colloidal particles and other types of agglomerates (Table 1).

Important information for the characterization of nano- and microscale objects is provided by microscopy, particularly electron microscopy (scanning, SEM, and transmission, TEM). The key feature of these techniques is the possibility to observe the morphology of samples directly without any special data processing. Microscopic images reflect the morphology of the objects with sizes from about 1 nm (or less for modern TEM techniques) to hundreds of microns or even more. In spite of the great opportunity provided by electron microscopy for the direct observations of small particles, harsh conditions of the electron microscope chamber (i.e. volatile components may evaporate under vacuum) significantly confine the applicability of this method. For example, a study of liquids and solutions is currently a complicated task.279–281 Nevertheless, as mentioned above, SEM could be used in some cases as a convenient method for direct investigation of the morphology and dynamics of ionic liquid-based WIS. It allowed detecting various morphologies in [C4mim][BF4]–water mixtures depending on the water content,42 and was applied for studying microscale processes occurring in IL–water systems during extraction.45,282

A valuable group of the SIS characterization methods deals with the “bulk” properties of the solvent-in-salt systems, including conductivity, surface tension, viscosity, etc. (Table 1). The correlation between the “bulk” properties and the structure of samples not only allows characterizing SIS as a whole, but also makes it possible to propose their molecular-level or supramolecular-level structures. It should be mentioned that for almost all the methods listed above the studies of the SIS dynamics are accessible. This feature allows detecting structural changes in solvent-in-salt mixtures in real time, as well as observing chemical transformations in complex reaction mixtures.

The summary provided in Table 1 can by no means be considered as a complete and detailed description. Instead, illustrative examples of SIS systems and powerful analytic methods used for their characterization were summarized together with the corresponding representative references for practical guidance.

3. Conclusions and outlook

Several types of “solvent-in-salt” systems have been known in chemistry for decades, but have not been intensively studied. Initial scientific interest was due to the low temperature energetic solid–liquid phase transition with heat accumulation/release, but the attempts to employ SIS in commercial heat storage systems were not very fruitful. A recent revival of the scientific interest to very high-concentrated salt solutions was due to their successful application as electrolytes in rechargeable Li-ion batteries, which commonly used toxic and flammable organic or polymeric electrolytes. This achievement has triggered a new wave in the battery design. High importance of this research field due to its relevance to the acute problem of affordable energy storage will lead to an exponential rise of the number of reports in the coming years.

Organic “solvent-in-salt” systems are rather versatile, and their number grows rapidly. As a representative example, ionic liquids are non-volatile and thermally stable compounds, which allow employing them and their water mixtures as much safer electrolytes, as compared to the commonly used organic solvents. Due to the lipophilic nature of ILs, their “solvent-in-salt” compositions are highly promising special media for different types of synthetic transformations and biochemical applications. Water pools and channels in non-homogeneous systems can serve as soft reactors for hydrophilic substances, opening up new ways for tuning fine chemical processes.

Still, there is much to be done to understand the nature of organic non-homogeneous “solvent-in-salt” systems. This challenging task can be accomplished only by using a synergy of different physicochemical characterization methods, microscopy, and molecular modeling.

List of abbreviations

ATRAttenuated total reflection
BSABovine serum albumin
CACCritical aggregation concentration
DFTDensity functional theory
DLSDynamic light scattering
DMCDimethyl carbonate
DSCDifferential scanning calorimetry
EDLCElectric double layer capacitor
EMElectron microscopy
FITCFluorescein isothiocyanate
HSAHuman serum albumin
ILIonic liquid
ITCIsothermal titration calorimetry
LCOLithium cobalt oxide
LCSTLower critical solution temperature
LLELiquid–liquid equilibrium
MDMolecular dynamics
MOFMetal organic framework
NMRNuclear magnetic resonance
OCVOpen circuit voltage
OHD-OKEOptically-heterodyne-detected optical Kerr effect
PCMPhase change materials
PILProtic ionic liquid
POMPolarized optical microscopy
RTILRoom temperature ionic liquid
SANSSmall-angle neutron scattering
SAXSSmall-angle X-ray scattering
SEMScanning electron microscopy
SIILSolvent-in-ionic liquid
SILSolvate ionic liquid
SIZSt Andrews ionothermal zeolite
SLILSponge-like ionic liquid
SWAXSSmall- and wide-angle X-ray scattering
TEMTransmission electron microscopy
UCSTUpper critical solution temperature
WIILWater-in-ionic liquid
XRDX-ray diffraction





Conflicts of interest

There are no conflicts to declare.


This research was supported by the Russian Science Foundation (grant 14-50-00126).


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