Hydrophobic deep eutectic solvents with anti-wear properties for MEMS/ NEMS

Most known deep eutectic solvents (DESs) are hydrophilic but, recently, hydrophobic DESs have been developed with some interesting features that make them potential alternative lubricants. The advantage of this type of compounds is their stability relative to the presence of water vapor in humid environments during industrial application. In this work, we report the use of hydrophobic DESs based on combinations of salts and acids or natural components (NADES) to lubricate Si surfaces. Two sets of previously reported DESs were prepared and studied: ionic DESs containing ammonium salts, and non-ionic DESs based on menthol. The friction coefficients were measured using steel and Si spheres against Si surfaces and their values were compared to those obtained with hexadecane, the reference lubricant. Non-ionic hydrophobic DESs presented poor lubrication for Si surfaces when compared to the reference lubricant, while all ionic hydrophobic DESs outperformed hexadecane. Among the most promissory hydrophobic DESs, [Aliquat]Cl:menthol (1:2) stood up showing CoF < 0.1 and the best surface protection against wear, independently of its water content, under stresses up to 1.3 GPa. The resistance of the adsorbed boundary layer might be justified by the strong interactions between its HBD and HBA components.