No abstract

Surface structure of equimolar mixture of 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([CCIm][TfN]) and 1-ethyl-3-methylimidazolium tetrafluoroborate ([CCIm][BF]) is studied using high-resolution Rutherford backscattering spectroscopy (HRBS) and molecular dynamics (MD) simulations. Both HRBS and MD simulations show enrichment of [TfN] in the first molecular layer although the degree of enrichment observed by HRBS is more pronounced than that predicted by the MD simulation. In the subsurface region, MD simulation shows a small depletion of [TfN] while HRBS shows a small enrichment here. This discrepancy is partially attributed to the artifact of the MD simulations. Since the number of each ion is fixed in a finite-size simulation box, surface enrichment of particular ion results in its artificial depletion in the subsurface region.

Elemental depth profiles of 1-alkyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([CnMIM][TFSI], n = 4, 6, 8) are measured using high-resolution Rutherford backscattering spectroscopy (HRBS). The profiles are compared with the results of molecular dynamics (MD) simulations. Both MD simulations and HRBS measurements show that the depth profiles deviate from the uniform stoichiometric composition in the surface region, showing preferential orientations of ions at the surface. The MD simulations qualitatively reproduce the observed HRBS profiles but the agreement is not satisfactory. The observed discrepancy is ascribed to the capillary waves. By taking account of the surface roughness induced by the capillary waves, the agreement becomes almost perfect.