In this study, three sterically-hindered quaternary ammonium phosphate ionic liquids (N11116-D.P, N111616-D.P, N88816-D.P) were synthesised, and the effect of the alkyl chain structure of oil soluble ILs on their lubricating and anti-corrosion abilities were investigated. In contrast to conventional ILs, oil soluble ILs exhibit prominent van der Waals interactions owing to the complex structure of the nonpolar alkyl chain, leading to special interfacial adsorption. It was found that when lower steric hindrance was present, the amount of ILs adsorbed increased, resulting in a compacted ad sorption layer with more mass, which facilitated the formation of more antiwear compounds in the tribochemical reaction and a hydrophobic layer that isolated the corrosive environment.