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.

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The surface structure of trimethylpropylammonium bis(trifluoromethanesulfonyl)imide ([TMPA] [TFSI]) is studied by high-resolution Rutherford backscattering spectroscopy at room temperature. The results provide direct evidence of the molecular ordering at the surface. The C1 conformer of the [TFSI] anion is dominant among two stable conformers, and the anions are oriented with their CF3 groups pointing toward the vacuum in the outermost molecular layer. The anions in the second molecular layer also show preferred orientation although it is rather weak.

The surface structure of 1-butyl-3-methylimidazolium hexafluorophosphate is studied by high-resolution Rutherford backscattering spectroscopy (HRBS) at room temperature. Elemental depth profiles are derived from the observed HRBS spectrum through spectrum simulation. While the obtained carbon profile has a sharp peak at the surface, the nitrogen profile shows a broader peak at a depth approximately 0.3 nm. These observations indicate that the butyl chain protrudes from the bulk liquid to the vacuum at the surface. The profiles of phosphorous and fluorine also have a broad peak at almost the same depth as the nitrogen profile, indicating that the anions are located near the imidazolium rings. These results are in good agreement with recent molecular dynamics simulations.

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The surface of an ionic liquid, 1-hexyl-3-methyl-imidazolium bis(trifluoromethanesulfonyl) imide ([HMIM] [TFSI]), is observed by high-resolution Rutherford backscattering spectroscopy (HRBS). The composition depth profiles are derived from the observed HRBS spectrum through spectrum simulation. Although the overall composition is in good agreement with the stoichiometric composition the profiles have some structures at the surface. The concentrations of fluorine and carbon are enhanced while those of oxygen and nitrogen are reduced at the surface. These results indicate that both [HMIM] cation and [TFSI] anion show preferred orientations at the surface. The hexyl group of [HMIM] and CF3 groups of [TFSI] point toward the vacuum in the outermost molecular layer.