Combining in-situ quadrupole mass spectrometer analyses: Study on the structure–activity relationship and lubrication mechanisms of ammonium phosphate ionic liquids

Since the lubrication performances of ionic liquids are closely linked to their unique structures, four protic ionic liquids (PILs), [N 44 HH][DEHP], [N 444 H][DEHP], [N 448 H][DEHP] and [N 4412 H][DEHP], are designed to investigate the effect of cationic structures on the lubrication behaviors and lubrication mechanisms. Vacuum tribological tests were carried out to minimize the impact of the atmospheric environment. The structure–activity relationships and lubrication mechanisms of PILs were emphatically examined by in-situ quadrupole mass spectrometer (Q-MS) and XPS analyses. Results demonstrated that all four PILs exhibited outstanding lubrication performance under a vacuum condition. Unusually, extended alkyl chain cationic structures resulted in a changed lubrication mechanism, leading to a slight decrease in performance. In-situ Q-MS analyses and XPS further revealed that the length of the alkyl chain in cations influenced the strength of intermolecular forces in PILs, leading to changes in their lubrication performance and mechanisms.