Specific Ionic Liquid Orientational Analysis

Cation-Specific Ionic Liquid Orientational Analysis 

One important use of SFG vibrational spectroscopy is the orientational analysis of ionic liquids at gas-liquid interfaces. For example, the study of the structural orientation ofionic liquids using common cation types, that is, [BMIM], combined with different anions, gives information on the effects of both cation and anion types [3, 22, 26-28]. Additional surface analytical work includes SFG studies under vacuum conditions for probing the second-order susceptibility tensor that depends on the polar orientation of the molecule and can be correlated to the measured SFG signal intensities. Supporting information is frequently obtained by complementary bulk spectroscopic techniques, such as Raman and Fourier transform infrared (FTIR) analysis, for the analysis of the pure ionic liquids. 

The final cation orientation may be influenced by the anion that is also present at the surface, depending on its type and size. These findings from SFG analytical work are generally in agreement with the results from other surface-sensitive analytical techniques that are presented in more detail in the following section. 

Once the alkyl ligands reach a certain length, which appears to be around a carbon number of 9 or 10, the ionic Hquids with long alkyl chains behave similarly to self-assembled monolayers (SAMs) where interaction of long alkyl chains results 

The ratio of the strength of the d-F mode to the number of CHj—CHj bonds in the alkyl chain. The broken line is a guide to highlight the dependence on the alkyl chain length. 

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Cation-Specific Ionic Liquid Orientational Analysis... 

The combination of cation and anion can be varied and tailored to specific chemical requirements, which makes them attractive for many chemical and industrial appHcations. The mechanisms of ionic liquid-surface interactions have been investigated by analysis of cation and anion orientation and its impact on the overall ionic Hquid behavior using surface-sensitive spectroscopic techniques [1, 7, 10, 11],... 

These surface-specific techniques provide information on the ionic liquid structure and composition at the surface. They probe the interfacial region at various technique-specific probing depths for analysis of surface concentration and structural identification. Vibrational spectroscopy, SFG in particular, is a widely used tool for orientational analysis of ionic Uquids at interfaces [1]. 

Ionic liquids at the gas-liquid and solid-liquid interface have been extensively studied by a variety of surface analytical techniques. The most prominent technique for surface orientational analysis proves to be SFG. Other vibrational spectroscopic and surface-sensitive techniques such as surface-enhanced Raman spectroscopy (SERS) and total internal reflection Raman spectroscopy (TIR Raman) have been employed for studying surface processes these techniques, however, have not been applied yet specifically for the study of ionic hquids. 

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