## 2 Topical Research Areas Related to Ionic Liquids and the Contribution of Spiroammonium Compounds
### Green Chemistry and the IL Concepts
#### 2.2.1 The State of the Art
ILs have a set of attractive properties, such as chemical and thermal stability, making them nearly ideal solvents in terms of extraction techniques, and they also have high ionic conductivity and a wide electrochemical potential window. ILs are a new class of purely ionic salt-like materials that are in a liquid state below 100 \({}^{\circ}\)C. This is the existing predominant meaningful definition, leaving significant opportunities for flexibility in perceiving ILs as \"designer solvents\". Therefore, the representative of IL systems is 1-ethyl-3-methylimidazolium chloride aluminum chloride, [emim]Cl\({}^{-}\)AlCl\({}_{3}\). It is a liquid and retains thermal stability from almost \(-\)100 \({}^{\circ}\)C to around 200 \({}^{\circ}\)C, depending on the molar ratio of [emim]Cl\({}^{-}\) and A1 Cl\({}_{3}\) added to the system. For example, when the molar proportions are equal, the system is a neutral stoichiometric compound, [emim]\({}^{+}\)[A1 Cl\({}_{4}\)]\({}^{-}\), which melts at a melting point (m.p.) of about 6 \({}^{\circ}\)C. The lowest melting point, \(-\)96 \({}^{\circ}\)C, is achieved when the molar percentages of the system are 35% [emim]Cl\({}^{-}\) and 65% A1 Cl\({}_{3}\). Chloroaluminate ILs are sensitive to water and air, and harsh conditions are required for their synthesis and utilization [26]. Tetrafluoroborates and hexafluorophosphates in aqueous environments are subject to hydrolysis with the release of toxic gas HF [27]. The choice of cation has a strong impact on the properties of an ionic liquid and will often define its stability. The chemistry and functionality of ILs are, in general, controlled by the choice of anion. The combination of a wide variety of cations and anions leads to a theoretically possible number of 1018 ILs. However, a realistic number will be magnitudes lower. Today, about 1000 of these compounds are described in the literature, and approximately 300 are commercially available. The typical structures combine organic cations with inorganic or organic anions (Figure 3) [28]. ILs with organic cations such as [emim]+ or [bmim]+ generally have high thermal and chemical stability. ILs are good solvents for a wide range of inorganic, organic, and polymeric materials; moreover, even stones, coal, and almost all organics are highly soluble in ionic liquids. In organic synthesis, ILs could have a variety of applications in various fields like pharmaceuticals, fine chemicals, biotechnology, medical sciences, nanotechnology, bioremediation, and environmental and nuclear sciences [29]. ILs have played multiple roles such as catalyst, solvent, and catalyst support. In some cases, it was observed that IL enables efficient catalytic reactions in comparison with conventional molecular solvents [30]. Ionic  
Figure 3: The common ILs and their structures. Reproduced from [28]. liquids can serve as solvents in reactions using biocatalysis to improve the efficiency and cost-effectiveness and, hence, the sustainability of biocatalytic reactions [31]. Furthermore, ILs serve as extractants of metal ions, organic compounds, and biomolecules [32]. Due to their asymmetry, there are reduced electrostatic forces between the anion and cation in these salts, and the formation of a regular crystalline structure is difficult; therefore, they can remain in a liquid state from temperatures of \(-20\)\({}^{\circ}\)C (cholinium and imidazolium ILs) [33] up to elevated temperatures of around 80 \({}^{\circ}\)C for bis(fluorosulfonyl)imide (FSI) ILs lithium ion batteries [34]. ILs, which are molten salts at r.t., can meet the latest requirements for lubricants in terms of improving maintenance with reduced emissions and have good properties that prevent the wear of rubbing materials and energy loss.