Application of deep eutectic solvents in lubrication: A review

• Hydrophobic type V deep eutectic solvents (HDESs) offer a good choice to lubricate. • A methodology for the use of HDESs in the lubrication field is proposed. • Research on corrosion and long-term thermal stability of HDESs should be addressed. • The study of toxicity and biodegradability of HDESs highlights a research gap. Deep eutectic solvents (DESs) have emerged as a promising alternative for lubricant formulation due to their low cost, ease of synthesis, hydrophobic nature, and wide liquid window, which is evident in a significant number of DESs. However, the majority of review papers have focused on DES applications other than lubrication, and the majority of DESs used in lubrication studies were selected without a clearly defined rationale. Consequently, research endeavors in this field exhibit a paucity of direction, and it is imperative to adopt a more systematic and methodologically robust approach. The diversity of hydrogen bond acceptors (HBA) and hydrogen bond donors (HBD) available for DES synthesis, along with the impact of these precursors and their molar ratio on DES properties, renders a trial-and-error approach to obtaining a DES for a specific application impractical. This review paper aims to address the synthesis and types of DESs, their main lubrication-related properties, the application of DESs in the lubrication field, and proposes a methodology for DES selection for this purpose. The conclusions drawn from this review underscore the importance of using molecular modeling and machine learning methodologies to determine quantitative structure–property relationships and predict the physicochemical properties and tribological behavior of DESs. Additionally, the review emphasizes the necessity of studying the corrosion, long-term thermal stability, toxicity, and biodegradability properties of DESs prior to their utilization in lubrication applications.