Interfacial Restructuring of Ionic Liquids Determined by Sum-Frequency Generation Spectroscopy and X-Ray Reflectivity
Interfacial Restructuring of Ionic Liquids Determined by Sum-Frequency Generation Spectroscopy and X-Ray Reflectivity
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DOI：
10.1021/jp807873j
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发表时间：
2008-12-11
影响因子：
3.7
通讯作者：
Kim, Doseok
中科院分区：
文献类型：
--
作者：
Jeon, Yoonnam;Sung, Jaeho;Kim, Doseok
用表面和频光谱和X射线反射率研究了[BMIM]][X]离子液体（BMIM = 1-丁基-3-甲基咪唑阳离子，X = BF 4-，PF 6-，和I-来自表面处的阳离子的末端甲基的和频信号强度表明这些离子液体的最高表面被极性-取向的疏水丁基链具有完全填充的烷基链的面内密度的约1/3，如通过对于十六醇朗缪尔单层的相同方法所观察到的。X射线反射率数据揭示了一个层的密度大于散装。然而，反射率是不够敏感的确切位置，这层无论是在真空界面或夹在散装和低密度的烷基链，如观察到的和频测量。结合和频光谱对反射率数据的分析强烈表明，形成最顶层的分子平均而言是极性取向的，其（松散堆积的）丁基链朝向气/液界面，而（致密堆积的）咪唑鎓核心/阴离子与大量液体接触。
Surface sum-frequency generation spectroscopy and X-ray reflectivity were used to study the surface of [BMIM]][X] ionic liquids (BMIM = 1-butyl-3-methylimidazolium cation, X = BF4-, PF6-, and I- Sum-frequency signal strength from the terminal methyl groups of the cation at the surface indicates that the topmost surface of these ionic liquids is occupied by polar-oriented hydrophobic butyl chains having approximately 1/3 of the in-plane density of fully packed alkyl chains as observed by the same method for the hexadecanol Langmuir monolayer. X-ray reflectivity data reveal a layer with density larger than that of bulk. However, the reflectivity is not sufficiently sensitive to the exact location of this layer either at the vacuum interface or sandwiched between the bulk and-the low-density alkyl chain, as observed in the sum-frequency measurements. Analysis of the reflectivity data in conjunction with the sum-frequency spectra strongly suggests the molecules forming the topmost layer are on average polar-oriented with their (loosely packed) butyl chains toward the gas/liquid interface, while the (densely packed) imidazolium cores/anions are in contact with the bulk liquid.