title: Structural change of ionic association in ionic liquid/water mixtures: a high-pressure infrared spectroscopic study.
abstract: High-pressure infrared measurements were carried out to observe the microscopic structures of two imidazolium-based ionic liquids, i.e., 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide [EMI(+)(CF(3)SO(2))(2)N(-), EMI(+)TFSA(-)] and 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)amide [EMI(+)(FSO(2))(2)N(-), EMI(+)FSA(-)]. The results obtained at ambient pressure indicate that the imidazolium C-H may exist in two different forms, i.e., isolated and network structures. As the sample of pure EMI(+)FSA(-) was compressed, the network configuration is favored with increasing pressure by debiting the isolated form. For EMI(+)TFSA(-)/H(2)O mixtures, the imidazolium C-H peaks split into four bands at high pressures. The new spectral features at approximately 3117 and 3190 cm(-1), being concentration sensitive, can be attributed to the interactions between the imidazolium C-H and water molecules. The alkyl C-H absorption exhibits a new band at approximately 3025 cm(-1) under high pressures. This observation suggests the formation of a certain water structure around the alkyl C-H groups. The O-H stretching absorption reveals two types of O-H species, i.e., free O-H and bonded O-H. For EMI(+)TFSA(-)/H(2)O mixtures, the compression leads to a loss of the free O-H band intensities, and pressure somehow stabilizes the bonded O-H configurations. The results also suggest the non-negligible roles of weak hydrogen bonds in the structure of ionic liquids.
authors.affiliation: Department of Chemistry, Faculty of Science, Kyushu University, Hakozaki, Higashi-ku, Fukuoko 812-8581, Japan.
authors.forename: Yasuhiro
authors.initials: Y
authors.lastname: Umebayashi
authors.forename: Jyh-Chiang
authors.initials: JC
authors.lastname: Jiang
authors.forename: Yu-Lun
authors.initials: YL
authors.lastname: Shan
authors.forename: Kuan-Hung
authors.initials: KH
authors.lastname: Lin
authors.forename: Kenta
authors.initials: K
authors.lastname: Fujii
authors.forename: Shiro
authors.initials: S
authors.lastname: Seki
authors.forename: Shin-ichi
authors.initials: S
authors.lastname: Ishiguro
authors.forename: Sheng Hsien
authors.initials: SH
authors.lastname: Lin
authors.forename: Hai-Chou
authors.initials: HC
authors.lastname: Chang