Linear alkyl-benzenesulfonate-based protic ionic liquids: Physicochemical properties and tribological performance as lubricant additives to a non-polar base oil

Abstract

• Two new families of protic ionic liquids were synthesized, characterized, and studied as additives to a non-polar oil. • The presence of any protic ionic liquid in the non-polar oil reduced friction and wear of steel disk. • The improved lubrication performance is attributed to an oxygen-rich tribolayer on steel surfaces. Advanced additives are crucial to improve the tribological performance of the base lubricants. A series of protic ionic liquids (PILs) derived from linear alkyl-benzenesulfonic acids with different alkyl chain length, and alkanol amines with different basicity were easily synthesized in the laboratory. The molecular structures of the PILs were established through proton magnetic resonance and Fourier transform infrared spectroscopy. Their thermal behavior was evaluated by differential scanning calorimetry and thermogravimetric analysis. The sliding wear and frictional performance of the PILs used as additives to mineral oil were studied by using a reciprocating ball-on-flat tribometer in a steel-steel contact. The solubility and stability of the PILs were also examined and related to their frictional behaviors. The worn steel surfaces were characterized by a non-contact profilometer, scanning electron microscope, energy-dispersive X-ray spectroscope and Raman microscope. The results showed that all PILs improved the lubricity of mineral oil with the concentration of only 1 wt%. The friction-reducing and anti-wear capacities of these PILs as additives were analyzed and discussed from their structures (different cations associated with anion moieties in different alkyl chain length) and the formed tribo-films.