# A Presentation of Ionic Liquids as Lubricants: Some Critical Comments
## 2 Why Should ILs Be Used as Lubricants? Ionic liquids (ILs) may be broadly defined as molten (i.e., in liquid state) organic salts at room temperature [5]. They are composed by an organic cation and an organic (or inorganic) anion to produce an electrically neutral ion pair. Generally, the cations are large, with long alkyl chains or a polysubstituted heteroaromatic _core_, while the anionic portion is relatively small. The typical IL cationic and anionic composition is reported in Figure 1.  
The different size of cations and anions may be determinant for the liquid state of ionic liquids at a temperature lower than 100 \({}^{\circ}\)C [6; 7] due to the difficulty in the building up of a regular crystal lattice. In addition, this peculiar asymmetry between the cation and anion size may favour the molecules in the fluid to self-assemble in various ways (charge-by-charge assemblies) [8], similar to liquid crystals [9] but less ordered than a typical solid crystal. This is shown in Figure 1. However, the complex pattern of anion-cation piling is also influenced by other interactions of different strengths and at different extents, such as polar and apolar interactions, steric hinderance, eventual H-bonds, and \(\pi\)-\(\pi\) interactions, etc. Thanks to this peculiar chemical property, other unique properties generally arise, such as high viscosity, negligible volatility, high polarity, nonflammability, high thermal stability, and miscibility with water and other organic solvents. These properties make ILs good lubricants as promising alternatives to the conventional lubricants [10; 11], overcoming the typical tribological problems in chemical engineering fields, such as the friction between metals or other materials and their wear [12; 13; 14]. In particular, the structure of ILs determines their high viscosity, and for this reason, they have also shown very good boundary lubricating capacity (see mechanisms of lubrication in Section 3) [15]. Another valid advantage is surely represented by the wide variety of cations and anions that can be combined to construct different ILs with tuneable properties and more flexible applications. More clearly, specific ILs can be designed and modulated to act only on the surface of sliding materials or to react with their surface or as additives to be dissolved in typical lubricants (i.e., mineral or synthetic oils). Figure 1: Common cations and anions of ILs. In any case, the key factor for the expectance of a good lubrication effect by an IL is that the electrostatic forces between anions and cations and their eventual capability to establish strong interactions with a solid surface can give the IL a better capacity to resist squeezing out from the sliding surfaces under high pressures. In brief, they can form a stable film on the surfaces held up by intermolecular interactions that are higher than the usual intermolecular interactions in conventional lubricants [16]. However, the high pressure may inhibit the aggregation of ionic liquids in macroclusters, favouring clusters of smaller sizes. This phenomenon reduces the viscosity of IL by decreasing its lubricating capacity [17; 18; 19]. However, other value-added properties arise thanks to their peculiar characteristics: the great miscibility with (or solubility in) a wide range of materials makes these important additives in lubricants. This is justified by the fact that, even if dissolved in another solvent, the strong electrostatic interactions between anions and cations of the IL allow their self-assembly within the solvent, so that the above considerations about the possibility to form a film on solid surfaces, hold up by electrostatic interactions, still holds. It is not the case that ILs can retain their lubrication performances even if dissolved up to 1:10-100 [20]. If smaller amounts of IL can be used, benefits in terms of costs are achieved [21]. Their low volatility allows them to be considered as green lubricants, avoiding the typical complications of common lubricants (i.e., in high-vacuum systems).