Evaluation of Binary and Ternary Deep Eutectic Solvents as Sustainable High-Performance Base Lubricants

The development of high-performance, environmentally friendly lubricants is crucial for sustainable industrial practices. This study investigates the synthesis and comprehensive characterization of binary and novel ternary deep eutectic solvents (DESs) made from choline chloride (ChCl) with various hydrogen bond donors (HBDs): urea, glycerol (Gly), ethylene glycol (EG), and oxalic acid (OA). Fourier-transform infrared FT-IR spectroscopy confirmed the formation of stable hydrogen-bonded networks in all DES formulations. Detailed analysis of their physicochemical, tribological, and electrochemical properties revealed the superior performance of ternary DESs. In particular, the ternary systems ChCl/Urea/EG (1:1:1) and ChCl/Gly/EG (1:1:1), show very high viscosity index (VI > 175), and excellent fluidity at low temperatures (pour points as low as −47°C). These are superior-performing compared to regular mineral base oils. Tribological tests demonstrated that several DESs, notably ChCl/Gly and ChCl/Urea/Gly, significantly reduced the coefficient of friction (μ≈0.170–0.186) through the formation of stable boundary films. Electrochemical analysis using Tafel plots unveiled a critical dichotomy in corrosion behavior towards iron. OA-based DESs caused active dissolution due to their acidic nature, while urea, glycerol, and EG-based DESs induced passivation. Glycerol and EG notably acted as corrosion inhibitors in the aggressive ChCl/OA system, although the ChCl/Urea/EG system exhibited pitting corrosion. This behavior was linked to mass transfer limitations imposed by DES viscosity, influencing both cathodic reactant supply and anodic passive layer formation. Overall, this study highlights that while ternary DESs offer a promising combination of physicochemical and tribological properties for lubrication, their interaction with metal surfaces is a critical design parameter. The choice of HBDs directly dictates the corrosion mechanism, underscoring the necessity for a holistic approach in designing next-generation DES-based lubricants.