Cation Exchange at the Interfaces of Ultrathin Films of Fluorous Ionic Liquids on Ag(111)

In the context of applications with thin ionic liquid (IL) films on solid supports, we studied the ion distribution within mixed thin IL films by angle-resolved X-ray photoelectron spectroscopy. After the deposition of 1-methyl-3-octylimidazolium hexafluorophosphate, [C₈C₁Im][PF₆], on top of a wetting layer (WL) of 3-methyl-1-(3,3,4,4,4-pentafluorobutyl)imidazolium hexafluorophosphate, [PFBMIm][PF₆], on Ag(111) at room temperature (RT), we find a preferential enrichment of the [PFBMIm]⁺ cation at the IL/vacuum interface. In a similar deposition experiment at 82 K, this cation exchange at the IL/solid interface does not occur. Upon heating the film from 82 K to RT, we observe the replacement of [C₈C₁Im]⁺ by [PFBMIm]⁺ at the IL/vacuum interface between ∼160 and ∼220 K. No further changes in the surface composition were observed between 220 K and RT. Upon further heating the mixed IL film, we find the complete desorption of [PFBMIm][PF₆] from the mixed film below 410 K, leaving a WL of pure [C₈C₁Im][PF₆] on Ag(111), which desorbs until 455 K.