Fatty acids-derived protic ionic liquids as lubricant additive to synthetic lube base oil for enhancement of tribological properties

Rising environmental and energy concerns have propelled interests in the development of novel lubricants, which can effectively lubricate a diversified range of engineering systems and subsidize the environmental footprints. In this context, fatty acids-derived trioctylphosphonium cation-based protic ionic liquids having significantly low viscosity (15–48 mm2·s−1) compared to conventional ionic liquids are addressed as additives to synthetic lube oil. The variable chain length in the fatty acids anions governs the interactive forces; accordingly, the viscosity and rheological properties of these ionic liquids are changed. Protic ionic liquids as additive (1.5 w/v%) to synthetic oil decreased the friction (28–41%) between the steel balls and protected the contact surfaces by reducing the wear (15–32%) under the boundary lubrication. The structural changes in constituted fatty acids anions in the protic ionic liquids governed the friction and wear characteristics. The thorough distribution of phosphorus along with other characteristics elements of protic ionic liquids revealed the formation of tribochemical thin film on the contact interfaces. These protic ionic liquids as lubricants outperformed the synthetic oil and showed reduction in the coefficient of friction (60–65%) and wear (27–33%). A facile approach to prepare the fatty acids-derived protic ionic liquids, low viscosity, and formation of tribochemical thin film of low shear strength promise the potential of these ionic liquids for tribological applications.