Oil-miscible and non-corrosive phosphonium-based ionic liquids as candidate lubricant additives

Ionic liquids (ILs) have been receiving considerable attention from the lubricants industry as potential friction and wear-reducing additives, but their solubility in oils is an issue. Unlike most ionic liquids that are insoluble in non-polar hydrocarbon oils, this study reports phosphonium-based ILs (PP-ILs) that are fully miscible with both mineral oil-based and synthetic lubricants. Both the cation and anion in quaternary structures, long alkyl chains, and capability of pairing the cation and the anion via a H–O bond are hypothesized to improve the compatibility between ions and neutral oil molecules. The measured viscosities of the oil-IL blends agree well with the Refutas equation that is for solutions containing multiple components. High thermal stability and non-corrosiveness were observed for the PP-ILs. Effective friction reduction and anti-wear functionality have been demonstrated in tribological tests when adding 5 wt% of a PP-IL into a base oil, suggesting potential applications for using the oil-miscible PP-ILs as lubricant additives.