Organophosphate anion based low viscosity ionic liquids as oil-miscible additives for lubrication enhancement

A single-step approach is addressed to synthesize the bis(2-ethylhexyl)phosphate (BEHP) anion-based ammonium and phosphonium ionic liquids. These ionic liquids exhibit significantly low viscosity, attributed to tsteric hindrance of the isoalkyl chain in the ammonium and phosphonium cations. A linear correlation between the shear rate and shear stress of BEHP ionic liquids signified their Newtonian behavior. The quaternary structure of cation, the presence of alkyl chains in both cation and anion, and +I effect of alkyl chains to subsidize the charge on the cationic center, are believed to facilitate the compatibility of these ionic liquids with mineral lube base oil. The BEHP ionic liquids as additives to fully formulated engine oil significantly reduced the friction (22-32%) and wear volume (56-72%) of steel balls. Elemental mapping of worn surfaces revealed the formation of BEHP ionic liquids-derived tribo-chemical thin film, which is accounted for lowering of friction and protection of tribo-interfaces against the undesirable events of wear. A facile approach to prepare the BEHP ionic liquids, low viscosity, and high miscibility of BEHP ionic liquids with hydrocarbon lube oils and organophosphate-driven tribo-chemistry to form a tribo thin film promises the potential of BEHP ionic liquids for lubricant applications. (C) 2018 Elsevier B.V. All rights reserved.