Surface Characterization of Functionalized Imidazolium-Based Ionic Liquids

Abstract

The surface composition of oligo(ethylene glycol) ether functionalized bis(trifluoromethylsulfonyl)imide ionic liquids has been studied by means of X-ray photoelectron spectroscopy (XPS). For [Me(EG)MIM][Tf 2N], [Et(EG) 2MIM][Tf 2N], and [Me(EG) 3MIM][Tf 2N], which vary by the number of ethylene glycol (EG) units (from 1 to 3), we have shown that the stoichiometry of the surface near region is in excellent agreement with the bulk stoichiometry, which confirms the high purity of the ionic liquid samples investigated and rules out pronounced surface orientation effects. This has been deduced from the experimental observation that the angle-resolved XP spectra of all elements present in the IL anions and cations (C, N, O, F, S) show identical signals in the bulk and surfaces sensitive geometry, i.e., at 0 degrees and 70 degrees emission angle, respectively. The relative intensity ratios of all elements were found to be in nearly perfect agreement with the nominal values for the individual ILs. In contrast to these findings, we identified surface-active impurities in [Me(EG)MIM]I, which is the starting material for the final anion exchange step to synthesize [Me(EG)MIM][Tf 2N]. Sputtering of the surface led to a depletion of this layer, which however recovered with time. The buildup of this contamination is attributed to a surface enrichment of a minor bulk contamination that shows surface activity in the iodide melt.

Surface Enrichment in Equimolar Mixtures of Non‐Functionalized and Functionalized Imidazolium‐Based Ionic Liquids

“…For IL4, only minor changes with emission angle are seen (Figure S2), indicating the absence of major enrichment/orientation effects at the outer surface. This is in line with our earlier47 observations for other PEG ILs. For the signals of the [Tf 2 N] − anion, we find a minor surface enrichment of the CF 3 groups and a depletion of the corresponding SO 2 groups 21,22,47,52…”

“…This analysis, along with the charge on atoms calculations, will allow for a comparison of the electronic structure of these methoxy and polyethylene‐glycol (PEG) ILs. Notably, for none of the ILs Si 2p signals, indicative of surface‐active polysiloxane contaminations,47,48 are observed in the presented data, confirming the high purity of our ILs.…”

“…[(MeO) 2 Im][PF 6 ] was synthesized with a purity of >99 % 37,38. [Me(EG) 2 C 1 Im][Tf 2 N] and [C 8 C 1 Im][Tf 2 N] were synthesized as was published earlier;45,47 the PEG‐functionalized IL [Me(EG) 2 C 1 Im] [Tf 2 N] was used for comparison of binding energy positions. [(MeO) 2 Im][PF 6 ] shows a minor contamination in its solid state at room temperature, which almost disappears by heating the sample to 90 °C.…”

Ionic Liquid Surface Composition Controls the Size of Gold Nanoparticles Prepared by Sputtering Deposition

“…Indeed, both experimental and theoretical investigations have recently found that the IL surface composition may be more or less populated with alkyl and/or fluorinated moieties depending on the type of anion and cation substituents. [27][28][29][30][31][32][33][34][35] However, the experimental conditions (distance between the metal target and liquid surface, gas pressure, current and time, and voltage change) used thus far have varied from one study to another, making it almost impossible to compare and correlate the influence of these parameters. 11,[22][23][24][25][26] Nonetheless, it is apparent that the size of NPs depends on the type of IL and on the discharge current.…”

Changes of the near-surface chemical composition of the 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide room temperature ionic liquid under the influence of irradiation

“…9 In plasma processing as well as in material characterisation, the ionic liquids are exposed to high energy photons and electrons that can have an impact on the chemical reactions taking place or in worst case can be the cause of degradation effects in the material. Especially in some analytical methods such as photoelectron spectroscopy, [10][11][12][13][14] infrared spectroscopy, 15 Raman spectroscopy 16 or Auger electron spectroscopy, the ILs are exposed to different kinds of radiation by photons or electrons. A commonly used analytical method for the surface characterisation of different ionic liquids is XPS where monochromated or non-monochromated X-ray radiation is used for excitation.…”