Analytical Sciences

Perfect Composition Depth Profiling of Ionic Liquid Surfaces Using High-resolution RBS/ERDA

Kaoru NAKAJIMA,* Enkhbayar ZOLBOO,* Tomohiro OHASHI,* Martin LÍSAL,**,*** and Kenji KIMURA*

*Department of Micro Engineering, Kyoto University, Kyoto-daigaku-katsura, Nishikyo, Kyoto 615-8540, Japan
**Laboratory of Aerosols Chemistry and Physics, Institute of Chemical Process Fundamentals of the CAS, v. v. i., 165 02 Prague 6-Suchdol, Czech Republic
***Department of Physics, Faculty of Science, J. E. Purkinje University, 400 96 Ústí n. Lab., Czech Republic

In order to reveal the surface structures of large molecular ionic liquids (ILs), the near-surface elemental depth distributions of 1-alkyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([C_nC₁Im][Tf₂N], n = 2, 6, 10) were studied using high-resolution Rutherford backscattering spectroscopy (HRBS) in combination with high-resolution elastic recoil detection analysis (HR-ERDA). The elemental depth profiles of all constituent elements, including hydrogen, were derived from HR-ERDA/HRBS measurements, so that the profiles would reproduce both HR-ERDA and HRBS spectra simultaneously. The derived elemental depth profiles agree with state-of-the-art molecular dynamics simulations, indicating the feasibility of this method. A controversy concerning the preferential orientation of [C₂C₁Im] at the surface has been resolved by this new combination analysis; namely, the [C₂C₁Im] cation has a preferential orientation with the ethyl chain pointing towards the vacuum in the topmost molecular layer.

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