Radiation-induced darkening of ionic liquid [C 4mim][NTf 2] and its decoloration

Abstract

The radiation effect on a hydrophobic room-temperature ionic liquid (RTIL), 1-butyl-3-methyl-imidazolium bis[(trifluoromethyl)sulfonyl]imide ([C 4mim][NTf 2]), was studied by γ-irradiation under nitrogen atmosphere. Accompanied by color darkening and increase of light absorbance in a wide wavelength range, a distinct absorption peak at around 290 nm for irradiated [C 4mim][NTf 2] appeared when acetonitrile was used as solvent, and the intensity of the peak enhanced with increasing dose. The spectrophotometric study on the irradiated RTILs containing 1,3-dialkylimidazolium cations associated with different inorganic anions revealed that the peak is ascribed to the radiolysis products of the [C 4mim] +. And the wavelength of the peak was affected by alkyl chain length on imidazolium cation, while the intensity of the peak was influenced by anions. With incorporating a little amounts of oxidants, such as KMnO 4 and HNO 3 into irradiated [C 4mim][NTf 2], the intensity of the peak at 290 nm decreased obviously and the decoloration of [C 4mim][NTf 2] occurred, suggesting that the peak at 290 nm is assigned to the colored species and the species can be oxidized.