Influence of cationic moieties on the tribolayer constitution shown for bis(trifluoromethylsulfonyl)imide based ionic liquids studied by X-ray photoelectron spectroscopy

The effect of varying the cationic structural moiety of bis(trifluoromethylsulfonyl)imide based ionic liquids (ILs) on the constitution of the tribolayer was studied by a X-ray photoelectron spectroscopic (XPS) approach. Tribolayers generated from five different ILs were characterised by means of XPS imaging of the top-most surface layers and by performing depth profiling. Differences in the concentration of tribochemical reaction products such as fluorides, detected on the topmost layers indicate that between the selected ILs an order of decreasing decomposition takes place phosphonium>imidazolium>pyrrolidinium>sulfonium>ammonium. Similar results could be shown in the depth profiles of the tribolayers. Hence, the influence of the cationic moiety cannot be neglected. [Display omitted] •Combination of XPS spot analysis, imaging and depth profiling for tribolayer characterisation of ionic liquids.•Cationic effect is not negligible.•Cations are not integral part of the tribolayers.