Aminoguanidine-based deep eutectic solvents as environmentally-friendly and high-performance lubricant additives

Halogen-free, environmentally-friendly, and economically viable lubricant additives, which can furnish excellent miscibility with mineral lube base oils and enhance the tribological properties, are gaining increasing interest for the formulation of new generation lubricants. Herein, aminoguanidine salt-based deep eutectic solvents (DESs) having variable mole ratios of fatty acid (as a hydrogen bond donor; HBD) are synthesized through a single-step facile approach. The detailed chemical characterizations based on FTIR and NMR (1H and 13C) confirmed the synthesis of DESs via a hydrogen-bonding network. A linear correlation between the shear stress and shear rate implied Newtonian fluidic behaviour of these DESs. An increasing mole ratio of octanoic acid decreased the viscosity and thermal stability of resultant DESs. The long alkyl chain of HBD furnished good compatibility of these DESs with SN 150 mineral lube base oil. The DESs as additives to SN 150 mineral lube oil improved the viscosity index by 12% and enhanced the tribological properties for steel tribopair by decreasing the wear volume (94%) and coefficient of friction (30%). The detailed analyses of worn scars revealed the formation of DESs-derived tribochemical thin film. The low shear strength of DESs-derived lubricious and protective thin film between steel tribopair facilitated the sliding and avoided the direct contact between the steel tribopair, consequently, significant enhancement of tribological properties.