Department of Chemistry, University of Houston, 136 Fleming Building, Houston, Texas 77204-5003, USA.
Phys Chem Chem Phys. 2012 Apr 21;14(15):5122-31. doi: 10.1039/c2cp23920e. Epub 2012 Mar 9.
Advancement in the field of ionic liquid technology requires a comprehensive understanding of their surface properties, as a wide range of chemical reactions occur mainly at interfaces. As essential media currently used in several technological applications, their accurate molecular level description at the gas-liquid interface is of utmost importance. Due to the high degree of chemical information provided in the vibrational spectrum, vibrational spectroscopy gives the most detailed model for molecular structure. The inherently surface-sensitive technique, sum frequency generation (SFG) spectroscopy, in combination with bulk-sensitive vibrational spectroscopic techniques such as FTIR and Raman, has been used in this report to characterize the surface of cyano-containing ionic liquids, such as [BMIM][SCN], [BMIM][DCA], [BMIM][TCM] and [EMIM][TCB] at the gas-liquid interface. By structural variation of the anion while keeping the cation constant, emphasis on the molecular arrangement of the anion at the gas-liquid interface is reported, and its subsequent role (if any) in determining the surface molecular orientation of the cation. Vibrational modes seen in the C-H stretching region revealed the presence of the cation at the gas-liquid interface. The cation orientation is independent of the type of cyano-containing anion, however, a similar arrangement at the surface as reported in previous studies was found, with the imidazolium ring lying flat at the surface, and the alkyl chains pointing towards the gas phase. SFG results show that all three anions of varying symmetry, namely, DCA (C(2v)), TCM(D(3h)) and TCB (T(d)) in ionic liquids [BMIM]DCA], [BMIM][TCM] and [EMIM][TCB] are significantly tilted from the surface plane, while the linear SCN in [BMIM][SCN] exhibited poor ordering, as seen in the absence of its C-N stretching mode in the SFG vibrational spectra.
离子液体技术的发展需要全面了解其表面性质，因为许多化学反应主要发生在界面上。作为目前在多种技术应用中使用的重要介质，准确描述它们在气液界面上的分子水平特性至关重要。由于振动光谱提供了高度的化学信息，因此振动光谱为分子结构提供了最详细的模型。本报告结合了具有体相敏感性的振动光谱技术，如 FTIR 和拉曼光谱，使用固有表面敏感的和频产生（SFG）光谱技术，来表征氰基离子液体的表面，如[BMIM][SCN]、[BMIM][DCA]、[BMIM][TCM]和[EMIM][TCB]在气液界面上。通过保持阳离子不变而改变阴离子的结构，重点研究了阴离子在气液界面上的分子排列方式，以及其随后在确定阳离子表面分子取向方面的作用（如果有的话）。在 C-H 伸缩区域观察到的振动模式表明阳离子存在于气液界面上。阳离子的取向与含氰基阴离子的类型无关，然而，正如先前研究中报道的，在表面上发现了类似的排列，其中咪唑环平放在表面上，烷基链指向气相。SFG 结果表明，在具有不同对称性的三种阴离子中，即 DCA(-)(C(2v))、TCM(-)(D(3h))和 TCB(-)(T(d))，在离子液体[BMIM]DCA、[BMIM][TCM]和[EMIM][TCB]中，都明显从表面平面倾斜，而在[BMIM][SCN]中，线性SCN由于其 C-N 伸缩模式在 SFG 振动光谱中缺失，表现出较差的有序性。
