Adsorption of ionic liquids onto silver studied by XPS

X-ray photoelectron spectroscopy has been used to study the adsorption of two related imidazolium ionic liquids: 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide and 1-hexyl-3-methylimidazolium chloride (HMIM NTf2 and HMIM Cl) onto silver. In both cases, adsorption occurred from ethanol solutions. Conventional XPS allowed us to detect the adsorbed layers of ionic liquids, but the signal-to-noise was too low, particularly after rinsing. However, synchrotron XPS, which is more surface sensitive thanks to the high X-ray flux and the ability to vary the energy of the incident beam, was able to detect the elements of the different components of the ILs with better signal-to-noise ratio. There was no evidence of chemical interaction between HMIM NTf2 and silver, in direct contrast to previous research from this group on IL adsorption onto gold (PCCP, 2015, vol. 17 (6), pp 4199–4209). HMIM Cl was seen to induce a significant surface chemical reaction of silver upon immersion in an ethanol solution of the IL, but as with HMIM NTf2, the exposure did not result in the adsorption of a significant film of IL. Synchrotron XPS did allow for the acquisition of IL spectra for C, N, and Cl, and as with HMIM NTf2, there was no evidence that the cation interacted chemically with the underlying silver substrate, although there was visual and spectral evidence for a reaction of the anion to form AgCl.