# A Presentation of Ionic Liquids as Lubricants: Some Critical Comments
## 5 Some Applications
### Aluminium Surfaces  
Aluminium is widely used in the automobile and aerospace industries [107]. In these applications, aluminium is often subjected to problems due to the sliding of moving components because of its poor tribological properties that make the system difficult to lubricate. For this reason, different types of lubricants are employed to reduce friction and wear, and in particular, protic ionic liquids (PILs) were found to be very promising [108]. More recently, PILs based on 2-hydroxyethylamine (2 HEA) and a carboxylic acid (formic  
Figure 11: Coefficient of friction and wear rate of Fomblin Z25 compared to IL2.  
Figure 12: Magnetic ionic liquid with 1-hexyl-3-methylimidazolium as cation and a dysprosium complex as anion.  
and pentanoic acids) (Figure 13) were synthesize by [PERSON] et al. to study, among other aspects, the effect of anion chain length on the lubrication of aluminium parts [109].  
The PILs were compared to a commercial lubricant, and the results of the coefficients of friction are grouped into Table 4.  
The coefficients of friction measured for an aluminium-steel system showed that the 2 HEAPe gave value similar to that of commercial lubricant, highlighting the better ability of the protic IL with a longer chain to support a major load. This example confirms the important role exerted by the length of the alkyl chain, offering a way to effectively control the coefficient of friction by simply tuning the number of C-atoms involved in the chain. However, the performance of 2 HEAPe suffers from the distance of sliding surfaces because the lubricant regime is based on the boundary lubrication; therefore, the instability and oscillation of the COF were observed when the distance of surfaces was increased. An elongation of the alkyl chain of the anionic portion might amplify the interactions between chains or with the surface metal, permitting, for example, a wider stacking or micelle-like arrangement that may reflect a wider adjustment to the several lubrication mechanisms.  
In confirmation of the effect of alkyl chain length in changing the tribological properties of IL lubricants, a study conducted by [PERSON] et al. on imidazolium ILs with variable alkyl chains (Figure 14) highlighted that (1) the derivatives with the longest chain of cations (hexyl and octyl imidazolium ILs) showed a better-performing lubricating activity; (2) different anions did not alter the results; (3) in the sliding process the more active ILs provided an excellent boundary film, which is very important for antiwear performance; and (4) the temperature did not influence the tribological action of the examined lubricants [110].  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Lubricant** & **Cefficient of Friction** \\ \hline Commercial & 0.08 \(\pm\) 0.011 \\
2 HEAPe & 0.14 \(\pm\) 0.026 \\
2 HEAF & 0.35 \(\pm\) 0.12 \\ Dry & 0.64 \(\pm\) 0.10 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Coefficient of friction value of two synthetized PILs.  
Figure 13: Structure of synthesized PILs and their precursors.  
In 2018, the lubricating ability of PILs on aluminium-steel contact was studied by the use of tri-[bis(2-hydroxyethylammonium)]citrate (DCi) (Figure 15) as an additive to mineral oil (MO) [111].  
Three sliding velocities (0.10, 0.15 and 0.20 m/s) were used to compare the lubricating performance of MO with respect to MO mixed with 1 wt% or 2 wt% DCi. The friction coefficient and wear volume were reduced for each sliding velocity, prevalently at 0.15 m/s and a MO + 2% DCi solution with which friction and wear decreases of 16% and 40%, respectively, were observed (Figure 16).  
Figure 16: Fiction coefficient and wear volume for different concentrations at sliding velocity of 0.15 m/s.  
Figure 14: Structure of imidazolium ILs with variable alkyl chain.  
Figure 15: Structure of tri-[bis(2-hydroxyethylammonium)]citrate (DCi).