Ionic Liquids in Gas Sensors
Room-temperature ionic liquids (RTILs) are a unique class of compounds containing organic cations and anions, which melt at or close to room temperature, and thus they are called as room-temperature molten salts. Present chapter describes these compounds
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Ionic Liquids in Gas Sensors
No commercial gas sensors use ionic liquids (ILs), but the research into their use is gaining momentum rapidly as of this writing (Welton 1999; Buzzeo et al. 2004d; Silvester and Compton 2006; Ahmad 2009; Sun and Armstrong 2010; Silvester 2011; Singh et al. 2012). Room-temperature ionic liquids (RTILs) are a unique class of compounds containing organic cations and anions, which melt at or close to room temperature, and thus are known as room-temperature molten salts (Seddon 1997; Demus et al. 1998). The simplest explanation that can be given for this circumstance focuses on the difficulty of finding efficient packing modes for the more complex and size-mismatched ions characteristic of IL and inorganic salt hydrates. The big difference in the size of a bulky organic cation and a small organic or inorganic anion does not allow packing of lattice, which happens in many inorganic salts; instead, the ions are disorganized. The result of this is that some of these salts remain liquid at the room temperature. The solvent properties of ILs such as melting point, dielectric constant, viscosity, polarity, and water miscibility can be tailored by combining different cations with suitable anions (Demus et al. 1998; Wasserschied and Welton 2003). The influence of chloride, water, and organic solvents on the physical properties of ILs has been investigated by Seddon et al. (2000). The cations in RTILs are generally organic compounds with asymmetrically substituted N-containing cation that are bulky in nature with varying heteroatom functionality paired with charged diffused anion. The class of cations explored till now includes 1-allyl-3-methylimidazolium, N-alkylpyridinium, N-methylalkyl pyrrolidinium, pyrazolium, and tetraalkyl ammonium types; more importantly, phosphonium salts are also finding greater utility. Regarding anions acceptable for synthesis of ILs, it was found that a wide range of organic and inorganic species can be used for these purposes. In particular, anions ranging from simple halides, which generally inflect high melting points, to inorganic anions such as tetrafluoroborate, [BF4]−, and hexafluorophosphate, [PF6]−, and to large organic anions like bistriflimide, [(CF3SO2)2N]−, triflate, [CF3SO3]−, or tosylate may be employed (Ahmad 2009). Commonly used cations and anions are shown in Fig. 7.1. Typical abbreviations used for ILs are listed in Table 7.1. It was established that the change of anion dramatically affects the chemical behavior and stability of the ionic liquid; the change of cation has a profound effect on the physical properties, such as melting point, viscosity, and density (Bowlas et al. 1996). There are also many interesting uses of ionic liquids with simple non-halogenated organic anions such as formate, alkylsulfate, alkylphosphate, or glycolate. The melting point of 1-butyl-3-methylimidazolium tetrafluoroborate with an imidazole skeleton is about −80 °C, and it is a colorless liquid with high viscosity at room temperature (Earle and Seddon 2000; Ahmad 2009).
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