Green hydrophobic deep eutectic solvents as low-viscosity and efficient lubricants

Development of novel green lubricants that meet low viscosity and effective anti-wear requirements is a tireless pursuit to improve fuel economy and reduce exhaust emissions, as well as enhance reliability. Herein a series of hydrophobic deep eutectic solvents (HDESs) with ultra-low viscosity were successfully synthesized by combining eco-friendly economical DL-menthol (DL) with fatty acids of various alkyl chain lengths. According to quantum chemistry method, weak hydrogen bond interaction between DL and fatty acid was calculated. The as-prepared DL-based HDESs lubricants present prominent physicochemical properties and lubrication functions. Thereinto, DL+Dodecanoic acid HDES with a low viscosity of 8.73 mm2/s (40 °C) achieves a 23.05 % reduction in friction and a 44.22 % reduction in wear compared to the PEG 200 base oil at 491N. According to experimental characterizations and first-principles calculations to reveal their anti-wear mechanism, the excellent wetting and ultra-low viscosity of the HDESs fluid facilitates rapid physisorption and chemisorption. Meanwhile, the absorbed film contributes to the formation of the long alkyl chains flow layer and the tribo-chemical film composed of hydrocarbon (CxHy), oxy hydrocarbon (CxHyOz) fragments and iron oxide/hydroxide-rich layer, which endows HDESs with excellent lubrication properties. This founding furnishes a new strategy to design green low-viscosity and high-performance anti-wear HDESs lubricants in sustainable lubrication applications.