Hydrogen Bond-Driven Tribological Properties of Natural Deep Eutectic Solvents for Green Lubrication

Natural deep eutectic solvents (NADESs) have recently attracted considerable attention as sustainable lubricants due to their excellent tribological performance, biocompatibility, tunable composition, and low toxicity. Nevertheless, the relationship between their molecular structures and lubrication mechanisms remains insufficiently understood. In this work, five NADESs with distinct hydrogen-bonding strengths and hydrogen bond acceptor (HBA) structures were prepared using choline chloride, betaine, or L-carnitine combined with glycerol, urea, or malic acid. Their physicochemical and tribological properties were systematically evaluated. The results indicate that stronger hydrogen-bonding interactions restrict molecular mobility, thereby improving lubrication stability, reducing wear track dimensions, and generating smoother surface morphologies. Moreover, functional groups such as carboxyl moieties in HBAs can coordinate with metal surfaces, further enhancing anti-wear effects. Compared with the commercial lubricant PAO40, the optimized NADES achieved a 32.6% reduction in the friction coefficient and an 89.1% reduction in wear volume. These findings highlight the synergistic role of hydrogen bonding and HBA molecular design in determining lubrication behavior, and provide a theoretical basis for the rational design of high-performance, environmentally friendly lubricants based on NADESs.