Anion Configuration at the Air/Liquid Interface of Ionic Liquid [bmim]OTf Studied by Sum-Frequency Generation Spectroscopy

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• 作者：Takashi Iwahashi ; Takayuki Miyamae ; Kaname Kanai ; Kazuhiko Seki ; Doseok Kim ; Yukio Ouchi
• 刊名：Journal of Physical Chemistry B
• 出版年：2008
• 出版时间：September 25, 2008
• 年：2008
• 卷：112
• 期：38
• 页码：11936-11941
• 全文大小：209K
• 年卷期：v.112,no.38(September 25, 2008)
• ISSN：1520-5207

文摘

The air/liquid interface of a room temperature ionic liquid, 1-butyl-3-methylimidazolium trifluoromethanesulfonate ([bmim]OTf), is investigated using infrared−visible sum frequency generation (SFG) spectroscopy. The SFG spectra clearly show low-frequency modes [CF₃-symmetric stretching (ss) mode and SO₃-symmetric stretching (ss) mode] of the OTf anion, demonstrating the existence of anions polar oriented at the interface. The amplitude of the CF₃-ss peak of the OTf anion has the opposite sign with respect to that of the SO₃-ss peak, indicating that OTf anions at the surface have polar ordering where the nonpolar CF₃ group points away from the bulk into the air, whereas the SO₃ group points toward the bulk liquid. The line width of the SFG peak from the submerged SO₃ group is appreciably narrower than that from IR absorption, suggesting the environment of the surface OTf anions is much more homogeneous than that of the bulk. The vibrational calculations also suggest that the anions and the cations form a more specific aggregated configuration at the surface as compared to the bulk.