High-performance protic ionic liquids as additives in lubricating oils with varying polarity

Although ionic liquids (ILs) have proven to be multifunctional and universal lubricant additives, their solubility in different polar lubricating oils varies greatly. This variability restrains their widespread use as general-purpose lubricating additives. To address this issue, three protic ionic liquids (PILs) with high solubility in lubricating oils with differing polarities were selected as additives, and their tribological performances in polyethylene glycol (PEG) and synthetic pentaerythritol carboxylic ester oil (5750) were systematically investigated. These findings were interesting, as PILs effectively improved the lubrication properties of PEG but impaired those of 5750. Additionally, a variety of surface analysis techniques and MD simulation were employed to further explore the lubrication mechanisms of PILs, and results indicated that they exhibited distinct tribological performance.