Abstract
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.
Highlights
We start with analyzing the ARXP spectra collected at 0° and 80° emission angles after the successive deposition of [PFBMIm][PF6] and [C8C1Im][PF6] on Ag(111) at room temperature (RT)
We studied the ion distribution within mixed thin ionic liquid (IL) films after the deposition of [C8C1Im][PF6] on top of a wetting layer (WL) of [PFBMIm][PF6] on Ag(111)
At RT, we observe the preferential enrichment of the [PFBMIm]+ cations at the IL/vacuum interface because of an immediate ion exchange at the IL/Ag interface after the deposition of [C8C1Im][PF6]
Summary
Thin films of ionic liquids (ILs) have been the focus of ultrahigh vacuum (UHV) surface science in the past 10 years as they provide powerful ways for molecular level studies of liquid/solid interfaces in general.[1−10] The detailed knowledge of the structure and the formation of the IL/solid interface, in particular, enables a more complete description and control of the interface properties and the system’s overall stability and performance in applications where ILs are in contact with solid surfaces, such as catalysis,[11,12] sensors,[13] lubrication,[14,15] separation,[16,17] and electrochemistry.[18]. In a reference heating experiment (Figures S6 and S7 in the Supporting Information), after the deposition of a pure 1.5 ML [C8C1Im][PF6] film onto Ag(111) at 82 K, we find the Calkyl peak to increase up to 220 K during heating This indicates that the decrease of the Calkyl peak in the mixed film above 180 K is due to the competing enrichment of the PFB side chains at the vacuum interface, following the cation-exchange process at the IL/Ag. Article show the quantitative analysis of the respective F 1s signals; the corresponding sets of F 1s spectra acquired during heating can be found in Figures S8−S10 in the Supporting Information. The remaining pure [C8C1Im][PF6] WL desorbs like a pure [C8C1Im][PF6] film until 455 K
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