Surface structures of equimolar mixtures of imidazolium-based ionic liquids using high-resolution Rutherford backscattering spectroscopy

Surface structures of equimolar mixtures of imidazolium-based ionic liquids (ILs) having a common cation (1-butyl-3-methylimidazolium (C₄MIM]) or 1-hexyl-3-methylimidazolium (C₆MIM])) and different anions (bis(trifluoromethanesulfonyl)imide (TFSI]), hexafluorophosphate (PF₆]) or chlorine) are studied using high-resolution Rutherford backscattering spectroscopy (HRBS). Both cations and anions have the same preferential orientations at the surface as in the pure ILs. In the mixture, the larger anion is located shallower than the smaller anion. The TFSI] anion is slightly enriched at the surface relative to PF₆] with coverage of ~ 60% for the equimolar mixtures of C₄₍₆₎MIM] TFSI] and C₄₍₆₎MIM] PF₆]. No surface segregation is observed for C₆MIM] TFSI]_0.5Cl]_0.5 and C₆MIM] PF₆]_0.5Cl]_0.5. These results are different from the recent TOF-SIMS measurement where very strong surface segregation of TFSI] was concluded for the mixture of C₄MIM] TFSI] and C₄MIM] PF₆].