The superior lubricating performance and unique mechanism of oil-soluble protic ionic liquids with short alkyl chains

Hypothesis: Ionic liquids (ILs), as lubricant additives, can greatly improve the lubricating behavior of the frictional interfaces. However, it is urgent to explore ILs with good oil solubility in nonpolar oils, and it is necessary to further study and verify the lubrication mechanism of ILs from the perspective of alkyl chain length. Experiments: Five protic ILs (PILs) with varying alkyl chain lengths were synthesized by proton transfer method. As additives in PAO oil, their tribological properties were investigated on SRV-V tester. Through molecular dynamics simulation, the adsorption behavior of PILs at the frictional interface was illustrated. Findings: The tribological properties of base oil could be significantly improved by adding PIL additives, but interestingly, PILs with short-chain anions showed better lubricating performance, which contradicted most of the early findings. Further analyses revealed that PILs achieved effective lubrication by the tribochemical interaction between anions and frictional interface, and the formation of cationic protective layer. However, PILs with shorter-chain anions form a denser protective layer that can better support the interfacial anions to participate in tribochemical reactions and thus abnormally exhibit superior lubricating performance than those with longer-chain anions. (C) 2022 Elsevier Inc. All rights reserved.

Automated summary

This study investigates the lubrication behavior of ionic liquids (ILs) as lubricant additives through experiments and molecular dynamics simulation. The results show that ILs with short-chain anions exhibit better lubricating performance, which is attributed to the formation of a denser protective layer that promotes tribochemical reactions at the frictional interface.