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History and Development of Ionic Liquids

Sumana Brahma and Ramesh L. Gardas

Indian Institute of Technology Madras, Department of Chemistry, IIT P.O., Chennai 600036, India

1.1 Introduction

For the past two decades, the term ionic liquid (IL) has been familiar to a very small number of research groups. However, ILs have attracted significant attention as innovative fluids in a wide range of research fields during this period [8, 60]. Generally, ILs are liquids that exist only in ionic form [79]. ILs can be defined as liquids consisting of ions with a melting point =100?°C. In another way, ILs, which exist as liquids at or near room temperature, are frequently termed room temperature ionic liquids (RTILs) [54]. In 1914, Paul Walden reported ethylammonium nitrate as the first IL [13]. According to Walden, the liquid, i.e. ethylammonium nitrate, composed of cations and anions and a minimal amount of molecular species, is an IL. Since the nineteenth century, several synonyms and abbreviations have been given to ILs by different research groups. Among the scientific community, the most frequent synonyms of ILs are molten salt, molten organic salt, low-melting salt, fused organic salt, ambient temperature ILs, neoteric solvent, and many more [40].

ILs are associated with unique features such as high ionic conductivity, high viscosity, low volatility, nonflammability, negligible vapor pressure, tunable solubility, and a wide electrochemical potential window [82]. All the mentioned IL properties can be altered by tuning the combination of the cations and anions of the ILs. Hence, ILs can also be termed "designer solvents" [55]. Due to their unique properties, ILs are used in various research applications. A multidisciplinary research on ILs is developing, including materials science, biotechnology, chemical engineering, chemistry, energy field, and atmospheric chemistry. Due to the low-volatile, nonflammable nature of ILs, they are highly preferred over any conventional organic volatile solvents or catalysts in various physical and chemical processes [73].

Furthermore, recently, green technology has been the greatest challenge for researchers concerning environmental hazards. The linkage between ILs and green chemistry is associated with the solvent properties of ILs [17]. ILs are also entitled to green solvents as they possess negligible vapor pressure and high thermal stability, resulting in advantages such as product recovery, desulfurization of liquid fuel, ease of containment, and recycling capability [42, 51]. ILs never possess the explorer risk compared with volatile organic solvents. In terms of volatility, molecular solvents could not (except molten polymers) reach even near the ILs.

ILs can exhibit high polarity. Based on the normalized polarity scale, the polarities of tetramethylsilane and water are 0.0 and 1.0, respectively, whereas the polarity of ILs is usually in the range of 0.6-0.7 [85]. Due to their high polarity, ILs are used as catalysts in various chemical and biochemical reactions. ILs easily dissolve in different solvents, including organic, inorganic, polar and nonpolar, and polymeric compounds. From the chemical engineering perspective, the most critical disadvantage, i.e. gas/liquid-solid mass transfer limitations during catalytic reactions, is resolved using efficient IL catalysts, as reported in detail by Tan et al. [75].

In view of the growing field of renewable energy, it is necessary to replace the conventional volatile electrolytes with green electrolytes in energy storage devices such as batteries, supercapacitors, fuel cells, and dye-sensitized solar cells. [44, 84]. ILs are appropriate in energy storage devices because of their high conductivity, low volatility, nonflammability, and high electrochemical and thermal stability. Imidazole- and pyrrolidinium-based electrolytes have exhibited promising outcomes as electrolytes in lithium-ion batteries and capacitors [14]. However, the investigation and deep learning of ILs as electrolytes for new devices such as hybrid batteries and Al oxygen/ion batteries and for CO2 reduction are in the early stages [53].

Millions of ILs can be synthesized by tuning the combination of cations and anions with desired properties and applications. Based on their properties and applications, ILs can be classified as task-specific ILs, energetic ILs, magnetic ILs, polyionic liquids, and supported ILs. [52]. For a specific process, screening for appropriate ILs is a prerequisite. To identify the structure-performance relationships, it is required to determine the nature of the interactions between cations-cations, anions-anions, and cations-anions of IL species [12]. Therefore, experimental, theoretical, and computational methods are needed to summarize the proper nature of ILs. More profound knowledge of IL nature at the microscopic scale will support the interpretation of macroscopic fluid phenomena and therefore endorse the application of ILs in industry. The multiscale features of ILs extending from the molecular level to the industrial level have been described by Dong and his coworkers [38]. Because of a wide range of applications and prospects of the ILs in the industry, ILs were exclusively named as "solvents of the future" in industrial processes [65]. However, the toxicity of ILs is identified as an emerging limitation for practical applications of ILs. ILs containing high alkyl chain lengths or fluorine anions are more toxic [97]. The toxicity can be affected by changing the structure of ILs . Hence, a detailed toxicity analysis is recommended before real-life applications of ILs. The brief history, development, and future scope are further summarized in the next section.

1.2 Constituents of ILs

ILs are usually made up of organic cations and inorganic anions. Generally, nitrogen- (imidazolium, pyrrolidinium, pyridinium, ammonium, choline, etc.) or phosphorus-containing cation moieties with linear or branched alkyl chains are used to prepare ILs.

Figure 1.1 Widely studied cations and anions of ionic liquids.

The most commonly used anions are halides (Cl-, Br-, I-), nitrate [NO3-], chloroaluminates [AlCl4-, Al2Cl7-], hexafluorophosphates [PF6-], tetrafluoroborate [BF4-], alkyl carboxylate [RCOO-], acetate [CH3COO-], trifluoromethylsulfonate [CF3SO3-], triflate [OTf-], and bistriflamide [NTf2-]. Recently, amino acids are also used as anions. The most studied cations and anions are shown in Figure 1.1.

1.3 The Brief History

There are numerous inceptions to the story of ILs in which they were recognized independently. The reporter's opinion will essentially influence the history of ILs [88]. The background of the ILs started with the finding of molten liquid salt. In the early 1990s, Paul Walden was searching for liquid molten salt at a particular temperature at which he could have accomplished his experiment. In 1914, Walden discovered ethyl ammonium nitrate [EtNH3][NO3] with a melting point of 12?°C and termed it the first protic ionic liquid (PIL) [47]. Further, Walden and his coworkers formulated the "Walden rule", which correlates the equivalent conductivity (?) as well as viscosity (?) of the liquid (aqueous solution).

Later on, the Walden rule could not interpret the properties of low-melting silver salt. Further, the Walden rule was modified to the fractional Walden rule by a group of molten salt chemists from a German school [5]. The fractional Walden rule is as follows:

where ? is a constant 0?<???<?1. But after that, there was no potential progress for molten salt studies for a prolonged time. According to the partial Walden rule, the Arrhenius activation energy for conductivity was lower than that for viscosity in the case of a low-melting silver iodide salt. Therefore, the silver iodide salt is a good conductor even in its crystalline state near its melting point temperature. The Walden rule was unable to predict the "superionic" behavior of molten salt, which made Walden rule very useful for the classification of ILs.

Furthermore, in the mid-nineteenth century, chemists observed the so-called "red oil" during Friedel-Crafts reactions. The "red oil" was the first documented observation of ILs [88]. Using nuclear magnetic resonance (NMR) technique, chemists were able to identify the structure of "red oil," which was the stable intermediate in Friedel-Crafts reactions termed sigma complex, which was basically heptachlorodialuminate salt (Figure 1.2). Prof. Jerry Atwood from the University of Missouri termed the structure early IL. Afterwards, ILs started to be used as either catalysts or solvent systems for organic reactions. Their effects on the reaction rates and their antimicrobial activity and toxicity were further premediated.

Figure 1.2 Structure of heptachlorodialuminate salt.

Numerous literature surveys suggested that chloroaluminate...