Surface Structural Study on Ionic Liquids Using Metastable Atom Electron Spectroscopy

초록

The surface structures of ionic liquids 1-CnH2n+1-3-methylimidazolium tetrafluoroborate (BF4-), 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 CF3 group Point toward the vacuum side, while the polar groups such as the SO3 and SO2 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.

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