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Orientation and Structure of Ionic Liquid Cation at Air/[bmim][BF4] Aqueous Solution Interface
Gang-hua Deng,Xia Li,You-qi Guo,Shi-lin Liu,Zhou Lu*,Yuan Guo*
Author NameAffiliationE-mail
Gang-hua Deng Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Tech-nology of China, Hefei 230026, China  
Xia Li Beijing National Laboratory for Molecular Sciences, State Key Laboratory of Molecular Reaction Dynamics, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China  
You-qi Guo Nanjing University of Aeronautics and Astronautics, Nanjing 211106, China  
Shi-lin Liu Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Tech-nology of China, Hefei 230026, China  
Zhou Lu* Beijing National Laboratory for Molecular Sciences, State Key Laboratory of Molecular Reaction Dynamics, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China zhoulu@iccas.ac.cn 
Yuan Guo* Beijing National Laboratory for Molecular Sciences, State Key Laboratory of Molecular Reaction Dynamics, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China guoyuan@iccas.ac.cn 
Abstract:
The water-miscible room temperature ionic liquid 1-butyl-3-methylimidazolium tetrafluorob-orate ([bmim][BF4]) is a model system for studying the interactions between ionic liquid and water molecules. In this work the orientational structure of the low concentrated aqueous solution of [bmim][BF4] at the air/liquid interface was investigated by sum frequency gener-ation vibrational spectroscopy. It has been found that at very low concentrations, the butyl chain exhibited a significant gauche defect, indicating a disordered conformation; and the cation ring oriented with a fairly small tilting angle at the surface. When the concentration increased, the cation ring tended to lie flat at the surface, and the gauche defects of the butyl chain decreased due to the intermolecular chain-chain interactions and the consequent more ordered interfacial molecular arrangement. Additionally, the anti-symmetric stretching mode in the PPP and SPS spectra exhibited a peak shift, showing that there exists more than one kind of orientation or chemical environment for the butyl CH3 group. These results may shed new light on understanding the surface behavior of water-miscible ionic liquids as well as the imidazolium based surfactants.
Key words:  Sum frequency generation vibratinal spectroscopy, Ionic liquid, Aqueous solu-tion, Interface, orientation
FundProject:
离子液体阳离子在空气/[bmim][BF4]水溶液界面的取向和结构
邓罡华,李霞,郭有祺,刘世林,陆洲*,郭源*
摘要:
采用和频振动光谱研究了空气/[bmim][BF4]低浓度水溶液界面的取向结构. 研究发现,在体相浓度非常低时,丁基链具有较大的旁式扭曲,表明此时的取向比较无序;阳离子咪唑环则采取一个较小的取向角. 随着浓度的升高,阳离子咪唑环趋向平躺在界面. 由于链-链相互作用,此时丁基链的旁式扭曲也减小,说明界面分子的取向变得有序. 进一步研究发现,PPP和SPS光谱上甲基反对称的峰存在位移,表明界面丁基链上的甲基存在不同取向或具有不同的化学环境.结果有助于从微观层次理解水溶性离子液体和基于咪唑的表面活性剂在界面上的物理化学行为.
关键词:  和频振动光谱,离子液体,水溶液,界面,取向
DOI:10.1063/1674-0068/26/05/569-575
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