Surface Structure of 1-Hexyl-3-methyl-imidazolium Bis(trifluoromethanesulfonyl)- imide Observed by High-resolution Rutherford Backscattering Spectroscopy

Abstract

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.