Surface Structural Study on Ionic Liquids Using Metastable Atom Electron Spectroscopy

The surface structures of ionic liquids 1-C n H 2 n +1 -3-methylimidazolium tetrafluoroborate (BF 4 − ), trifluoromethanesulfonate (OTf) and bis(trifluoromethylsulfonyl)amide (TFSA) with n = 4, 8, and 10 were investigated by ultraviolet photoemission spectroscopy (UPS) and metastable atom electron spectroscopy (MAES). The MAES spectra reveal that the anions and cations share the surface of the room-temperature ionic liquids (RTILs) with a shorter alkyl chain at n = 4 although the alkyl chain layer tends to cover the outermost surface in the case of RTILs with longer alkyl chains of n = 8 and 10. Comparison of UPS and MAES spectra demonstrates that the nonpolar groups such as the alkyl chain and the CF 3 group point toward the vacuum side, while the polar groups such as the SO 3 and SO 2 groups face toward the bulk side. A double-layered structure at the surface, which consists of an alkyl chain layer and a polar layer containing anions and imidazolium rings, is proposed from these observations.