Surface structure of an ionic liquid with high-resolution Rutherford backscattering spectroscopy

The surface of an ionic liquid, trimethylpropylammonium bis(trifluoromethanesulfonyl)imide ([TMPA] [TFSI]), is observed by high-resolution Rutherford backscattering spectroscopy (HRBS). The composition depth profiles are derived from the observed HRBS spectra through spectrum simulation. The observed composition is in good agreement with the stoichiometric composition at depths larger than ∼1 nm. The observed composition profiles, however, show pronounced structures at the surface. Fluorine profile has a sharp peak at ∼0.1 nm and a broad peak at ∼1.0 nm. The sulfur profile also has a peak at ∼0.35 nm. These results indicate that the molecules show preferred orientations at the surface. From the observed profiles, it was concluded that the C1 conformer of the [TFSI] anion is dominant over the C2 conformer at the surface in contrast to bulk, where the C2 conformer is known to be dominant. It was also found that C1 conformers are oriented with their CF3 groups pointing toward the vacuum in the outermost molecular layer.