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 similar to 1 rim. The observed composition profiles, however, show pronounced structures at the surface. Fluorine profile has a sharp peak at similar to 0.1 nm and a broad peak at similar to 1.0 nm. The sulfur profile also has a peak at similar to 0.35 nm. These results indicate that the molecules show preferred orientations at the surface. From the observed profiles, it was concluded that the C, conformer of the [TFSI] anion is dominant over the C-2 conformer at the surface in contrast to bulk, where the C-2 conformer is known to be dominant. It was also found that C-1 conformers are oriented with their CF3 groups pointing toward the vacuum in the outermost molecular layer. (C) 2008 Elsevier B.V. All rights reserved.