Ionic Liquid Surfaces by Spectroscopic Techniques

Characterization of ionic Liquid Surfaces by Spectroscopic Techniques 

Types of interfaciai Systems Involving Ionic Liquids 

Ionic liquids are involved in chemical processes at surfaces that may be sohd-hquid, solid-gas, and liquid-liquid interfaces. They are found in homogeneous or biphasic catalytic processes where they serve as process catalytic enhancement, immobilization medium, cocatalyst, or electrolytes [7, 9-12]. They may also find applications in nanotechnology, surface coatings, adsorbent materials, and solar energy storage cells [13, 14]. 

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], 

Overview of Surface Analytical Techniques for Characterization of Ionic Liquids 

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Characterization of Ionic Liquid Su faces by Spectroscopic Techniques 155 Surface normal... 

Conventional approaches based on electrochemical techniques, surface tension, and extraction methods have allowed the estabhshment of thermodynamic and kinetic information concerning partition equilibrium, rate of charge transfer, and adsorption of surfactant and ionic species at the hquid/Uquid interface [4—6]. In particular, electrochemical methods are tremendously sensitive to charge transfer processes at this interface. For instance, conventional instm-mentation allowed the monitoring of ion transfer across a hquid/hquid interface supported on a single micron-sized hole [7, 8]. On the other hand, the concentration profile of species reacting at the interface can be accurately monitored by scanning electrochemical microscopy [9, 10]. However, a detailed picture of the chemical environment at the junction between the two immiscible liquids caimot be directly accessed by purely electrochemical means. The implementation of in-situ spectroscopic techniques has allowed access to key information such as ... 

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. 

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. 

Structure of ionic liquids at interfaces, another fascinating and important subject, was studied by Baldelli and co-workers [63] and by Ouchi [64] using surface spectroscopic techniques. Non-polar alkyl chains are segregated towards vacuum at the free surface of ionic liquids, as a result of the balance between coulombic and van der Waals forces [63, 64]. 

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