Picolinium-Based Hydrophobic Ionic Liquids as Additives to PEG200 to Lubricate Steel-Silicon Contacts

Three new ionic liquids (ILs) containing the picolinium cation were synthesized using a more sustainable method based on microwave (MW) irradiation: 1-hexyl-2, 3 or 4-picolinium trifluoromethanesulfonate ([C₆-x-pic][TfO] where x=2, 3 and 4. The objective was, besides testing MW reactions with higher yields and lower reaction times comparing to conventional heating process, to assess the tribological behavior of these ILs as additives to base oil polyethylene glycol (PEG200). The friction coefficients (CoFs) for the pair silicon surface/steel sphere were compared to those obtained with other ILs based on the same anion. The ILs with cations containing one hexyl or butyl side chain led to lower CoFs, and [C₆-4-pic][TfO] was the most efficient additive, which may be attributed to the cation symmetry. The roughness of the sliding surfaces was reduced after the tribological test using neat PEG200, but, when the additive [C₆-4-pic][TfO] was added, the roughness was practically not affected. It seems that [C₆-4-pic][TfO] adsorbs forming ordered layers that support the load together with the liquid film retained among the asperities of the opposing surfaces.