Phosphonium phosphate ionic liquids (PP-ILs) are promising lubricant additives for engine oils owing to their good miscibility in hydrocarbon fluids, high thermal stability, and excellent lubrication performance. The majority of the studies in the literature evaluating the lubricating properties of PP-ILs was carried out with high-purity ILs, whose high cost mainly derives from the difficult and time-consuming purification required to remove contaminants (<i>e.g.</i>, residual halide ions from the synthesis). Here, we evaluate the dependence of the lubricating properties of phosphonium-based ILs when used in steel/steel contacts on the relative concentration of phosphate and bromide anions. The results indicate no changes in friction and wear upon increasing the bromide-to-phosphate anion volume ratio up to 0.2:1, while a further increase in this ratio leads to a reduction of friction and wear. X-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectrometry analyses of steel surfaces at the end of tribological experiments highlight that the dependence of the lubricating properties of the ILs on the bromide-to-phosphate anion ratio originates from the balance between corrosivity and surface reactivity of this IL: while phosphate ions in the IL are critical to inhibit the corrosion of steel in the non-contact region, in the wear track the formation of species containing phosphorus-oxygen functional groups as a result of the hydrolysis of phosphonium ions enabled by the hydroxides formed during the pitting corrosion of iron occurring at high bromide concentration increases the surface coverage of phosphorus, which decreases friction by decreasing adhesion at steel/steel contacts and lowering the interfacial shear strength. These findings<sup>1</sup> demonstrate that PP-ILs can be effectively used in tribological applications to lubricate steel/steel contacts without the necessity of achieving the highest purity of this IL, thus opening the path towards the cost-effective and sustainable implementation of this IL in lubricant formulations.

Tuning the surface reactivity and tribological performance of phosphonium-based ionic liquid by mixing bromide/phosphate anions

ACS Spring 2022

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