Observation of surface structure of 1-alkyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide using high-resolution Rutherford backscattering spectroscopy

Abstract

The surface structures of 1-alkyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([C(n)MIM][TFSI], n=2,4,6) are studied by high-resolution Rutherford backscattering spectroscopy. The average composition of the surface molecular layer is very close to the stoichiometric composition, showing that neither ion is enriched in the surface layer. A detailed analysis indicates that both cations and anions have preferential molecular orientations at the surface. The alkyl chains of the [C(n)MIM] cations protrude to the vacuum and the CF(3) groups of the [TFSI] anions are also pointing toward the vacuum. While the orientation of the [TFSI] anion becomes weaker with increasing alkyl-chain length, the protrusion of the alkyl chain occurs irrespective of the chain length. It was also found that the N(SO(2))(2) moiety is located nearly at the same depth as the imidazolium ring, suggesting that one of oxygen atoms in [TFSI] is bonded to the hydrogen of the C(2) carbon atom of the imidazolium ring.

Theoretical reconstruction and elementwise analysis of photoelectron spectra for imidazolium-based ionic liquids

“…However, in contrast to the UPS (He II) spectra the MIES difference spectrum reveals also a minor contribution from the F2p orbitals (Fig. 4 Notably, this interpretation, which is in our favor, is also in better agreement with the model of the surface structure given by K. Nakajima et al 36 obtained by their HRBS experiment. Moreover it is not in contradiction to the result that one or two trifluoro-group(s) of the anion point towards the vacuum.…”

“…Both ion pairs exhibit nearly the same relative geometry, where the nitrogen of the [Tf 2 N] À anion is used to form the connection to the mentioned most acidic hydrogen of the cation. A change in the preferred conformer of the [Tf 2 N] À anion at the surface depending on the alkylchain length of the cation was recently found by K. Nakajima et al 36 This fact was considered by the choice of an ion pair with a syn-conformer of the anion for [EMIm]Tf 2 N and one with the respective anti-conformer is chosen for [OMIm]Tf 2 N (see the displayed structure of the two utilized ion pairs in Fig. 1 52 have shown that the syn-conformer of the anion exhibits a slightly higher energy than the anti-conformer.…”

“…This has been investigated with various UHV techniques, e.g., X-ray photoelectron spectroscopy (XPS), 1,[25][26][27][28][29][30] soft X-ray emission spectroscopy, 31,32 secondary ion mass spectroscopy, 33 scanning atom probe, 34 high resolution Rutherford backscattering spectroscopy (HRBS). 35,36 Detailed information about the surface structure has also been obtained by other surface sensitive methods, e.g. sum frequency generation (SFG), which features an analysis of the orientation of components of the ions present at the surface.…”

“…4,6,37 A collection of ionic liquids, which consists of bis(trifluoromethylsulfonyl)imide anions ( , has been intensively studied by many methods, 38 inter alia angle-resolved XPS, 25,28 UPS, 8 SFG 37 and HRBS. 35,36 It has been shown that most probably the longer alkyl-chains (e.g. octyl for [OMIm]Tf 2 N) of the cation stick out of the ionic liquid surface.…”

Surface layering and melting in an ionic liquid studied by resonant soft X-ray reflectivity

“…This directly observed near-surface ordering of the molecular moieties is consistent with the ordering deduced from indirect studies by surface tension and spectroscopy of ILs comprising [C n≥4 mim] + cations. Those measurements were interpreted as showing that the first layer of imidazolium rings and anions is located at a depth close to that of a fully extended C 18 chain (20,21) and that the alkyl chains protrude into the vapor (6,(22)(23)(24). The molecular model presented in Fig.…”