Oil-miscible, halogen-free, and surface-active lauryl sulphate-derived ionic liquids for enhancement of tribological properties

Abstract

Halogen-free ionic liquids, comprising low-cost surface-active precursors, having good miscibility with synthetic and mineral lube base oils, and enhance the lubrication properties by reducing the friction and wear, are gaining increasing attention for tribological applications. The present work addresses lauryl sulphate-derived tetraalkylammonium/phosphonium ionic liquids. Total four ionic liquids comprising tetrabutyl- and tetraoctyl-based quaternary structure are synthesized and then demonstrated them as additives to mineral and synthetic lube base oil for enhancement of lubrication properties. The tetraalkylphosphonium cation-based ionic liquids exhibited higher thermal stability, whereas the tetraalkylammonium cations-based analogue showed superior viscosity. These ionic liquids as an additive to PEG 200 and SN 150 lube base oils increased the viscosity index. The 1.5 w/v% of lauryl sulphate anion-based ionic liquids as an optimized dose to PEG 200 oil significantly improved the tribological properties by decreasing the friction (28%) and wear volume (91%). The enhancement of tribological properties by these ionic liquids is attributed to the formation of tribochemical thin film on the contact interfaces as deduced from their spectroscopic measurements. Furthermore, the tribo results revealed that the alkyl chains in cationic moieties and the viscosity of ionic liquids govern the frictional characteristics.