Excellent catalytic oxidation inhibition and lubrication performance of carboxylic acid based ionic liquids as ester oil additives

In this study, a series of sulfur-free and phosphorus-free ionic liquids derived from fatty acids and tertiary ammonium as multifunctional additives for ester oils were synthesized and evaluated to assess the catalytic oxidation inhibition and tribological properties. Notably, all multifunctional ionic liquids effectively suppressed the catalytic effect from ferric ion on the oxidative degradation of bis (2-ethylhexyl) sebacate. Among them, trioctylammonium stearate ([N888H][SA]) demonstrated exceptional performance, extending the oxidation induction period base oil by 2.7 times and significantly reducing friction coefficient and wear scar diameter. This superior phenomenon of the oxidation induction period enhancement was due to a unique synergistic mechanism: the anion potently chelated the ferric ions to inhibit the catalysis, while the tertiary ammonium cation acted as an efficient radical scavenger. Tribological results confirmed the formation of a protective tribofilm by [N888H][SA], preventing the direct contact between the friction pairs. This work establishes a novel and sustainable paradigm for developing next-generation, multifunctional lubricant additives with high performance.