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

Area of Science:

• Materials Science
• Surface Science
• Analytical Chemistry

Background:

• Ionic liquids (ILs) are salts that are liquid at ambient temperatures, with tunable properties.
• Understanding the surface structure of ILs is crucial for their application in various fields.
• Previous studies have debated the preferential orientation of cations at the IL surface.

Purpose of the Study:

• To determine the near-surface elemental depth distributions of ionic liquids.
• To investigate the surface structure of 1-alkyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([CnC1Im][Tf2N]) for n = 2, 6, 10.
• To resolve the controversy regarding the preferential orientation of the [C2C1Im] cation at the IL surface.

Main Methods:

• High-resolution Rutherford backscattering spectroscopy (HRBS).
• High-resolution elastic recoil detection analysis (HR-ERDA).
• Combined HR-ERDA/HRBS measurements to derive elemental depth profiles, including hydrogen.

Main Results:

• Elemental depth profiles for all constituent elements were successfully derived.
• The derived profiles accurately reproduced both HR-ERDA and HRBS spectra.
• The results confirmed a preferential orientation of the [C2C1Im] cation with the ethyl chain towards the vacuum in the topmost molecular layer.
• The findings align with state-of-the-art molecular dynamics simulations.

Conclusions:

• The combined HR-ERDA/HRBS technique is a feasible method for analyzing near-surface elemental distributions in ionic liquids.
• The study definitively resolved the orientation of the [C2C1Im] cation at the IL surface.
• This provides valuable insights into the surface structure of ionic liquids, aiding in their design and application.