Natural Cholesterol-Derived Ionic Liquids Enhancing the Antiwear Property and Biodegradability of Mineral/Vegetable Oils

Using natural products as raw materials to prepare lubricating additives that can enhance both the tribological performance and biodegradability of base oils has huge application potential. Here, two natural cholesterol-derived ionic liquids (ILs) with long-chain alkyl amines or alkyl betaines as cations were successfully synthesized. The tribological properties of the two ILs as lubricating oil additives in white oil and castor oil were tested on the steel interface at room temperature and high temperature. The biodegradability of the ILs was evaluated according to the CECL-33-A-9 test method. The results showed that both ILs have an excellent ability to enhance the antiwear properties and to accelerate the biodegradation of base oils. Therein, the wear volume of white oil with 1.5 wt % alkyl amine IL decreases by 80%, the biodegradability at 21 days increases to 75%, the wear volume of castor oil with 1.5 wt % alkyl betaine IL decreases by 42%, and the biodegradability at 21 days increases to 85%. The improvement of the antiwear ability of base oils by the ILs was mainly ascribed to the formation of a more efficient boundary lubrication film due to the adsorption and tribochemical reactions of the ILs on the contact interfaces. The promotion of biodegradation is attributed to the nutrients needed for the early reproduction of microorganisms provided by cholesterol-based ILs and the increase in oil–water interfacial area owing to the surface activity of cations, thus causing microorganisms to better metabolize lubricants. Hence, it is feasible for some natural compounds to contribute to the development of green and sustainable products and processes.