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Ionic Liquids Overview

Ionic liquids (ILs) are frequently defined as salts whose melting point lies below the temperature of 100 °C. A more specific and highly interesting variety are the room temperature ionic liquids (RTILs), which have melting points at or below room temperature, as the name suggests. The prototypical outline of an ionic liquid consists of a bulky, floppy, usually asymmetric organic cation paired with a weakly coordinating anion, which can be organic or inorganic. This combination of structural and interaction properties serves to frustrate crystallization and lower the melting point significantly as compared to inorganic salts, such as the ubiquitous sodium chloride (melting point: 801 °C). Ionic liquids inherit properties from both the organic molecular solvents, on the one hand, and simple inorganic salts, on the other. They are, at the simplest level, still liquids characterized by long range disorder, with no specific repeating pattern of intermolecular arrangements. But conversely, cation-anion charge alternation dominates the overall liquid structure. Each cation is surrounded on average by a shell of anions, which is further surrounded by shell of cations, and so on. However, ionic liquids are not limited by their heritage; the liquid state presents a whole new set of features and phenomena unique to these materials.

Figure 1. Sample aprotic ionic liquid, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EmimNTf₂, structure, R=CH₃) and a protic ionic liquid, 1-ethylimidazolium bis(trifluoromethylsulfonyl)imide (EhimNTf₂, structure, R=H)

ILs have attracted a great deal of interest for applications in energy storage and generation, chemical synthesis, materials processing, and carbon (CO₂) capture, to name a few. The particular excitement regarding these solvents stems from several of their characteristic features. These can be divided into three general areas which guide and organize IL/RTIL research in the Fayer group. First, ionic liquids are necessarily very highly concentrated electrolytes. They are generally not volatile due to the strong coulombic interactions between charged constituents, leading to typically low flammability. Thus, ILs are potentially a much safer alternative to the organic solvent systems currently used in lithium ion batteries, for example. An additional consequence of the low volatility is their freedom from evaporative losses, which can lead to significant environmental and economic impacts from large scale chemical processes in the case of normal organic solvents.

Second, ionic liquids are modular in nature. They are assembled from a cation and anion that can have completely separate synthetic pathways prior to combination into the final salt. Furthermore, many of the cations (and in some cases, the anions as well) have tunability in their structures, such as varying alkyl chain length, the inclusion of hydrogen bond donating or accepting groups, π-π stacking interactions, and so on. The structural variety of ionic liquids is enormous – many millions of simple ionic liquids (with only a single cation and anion) are possible. Beyond this, IL mixtures and the introduction of molecular cosolvents broadens the range of IL-based solvent systems to an immense scale. ILs have been discussed as “designer” solvents, whose properties can be modified easily for a desired outcome. Realizing this potential, of course, requires a great deal of further study to understand the interplay between IL composition, interactions, and the performance aspects relevant to a chosen task.

The third, and final, key feature characterizing ionic liquids is the rich and fascinating space of nanoscale or mesoscopic ordering which is accessible through many IL structural families. Mesoscopic ordering is often denoted “intermediate” range, because it exists on length scales longer than the sizes of cations and anions or their local charge alternation patterns, but shorter than macroscopic length scales of, e.g., micrometers, which are accessible in day-to-day life. A great deal of the ionic liquid research to date has focused on clarifying the size and structural motifs of nanoscopic domains, as well as how changing the IL composition affects these structures. The presence of alkyl chains longer than 2 or so carbon atoms placed asymmetrically on the cation, typically, results in self-assembled structures which add a new degree of structural organization to the ionic liquid.