Ammonium-based ionic liquid crystals

Ionic liquid crystals are liquid crystals containing ionic moieties. Ionic hquid crystals having ammonium and pyridinium salts were reported to show stable thermotropic liquid crystalline behavior [35 0]. Seddon, Bruce, and coworkers showed that some imidazolium and pyridinium salts exhibit low melting points and liquid crystalline behavior [41]. Ionic liquid crystals with lower temperature mesophase ranges are called hquid crystalline ionic liquids. A large variety of thermotropic liquid crystaUine ionic liquids exhibiting anisotropic fluid states were prepared. In particular, the ionic liquid crystals based on imidazolium salts 1 [42-46], 2... 

As in the case of the salts discussed above, appropriate pairs of cations and anions yield ionic liquids that are partially ordered but essentially nondimensional (OD) states [13]. In ionic liquids, the bulky geometries of both the cationic and the anionic species effectively prevent crystallization due to weak ionic interactions. Such ionic liquids can be converted to gelated materials. As an example, Guerrero-Sanchez and Schubert et al. investigated the gelation behavior of a quaternary ammonium ohgo(propylene oxide)-based ionic liquid 4 (Fig. 4.3a(i)) upon mixing with water... 

Halide salts of quaternary ammonium and phosphonium cations, and those derived from heterocyclic bases (both aromatic and alicyclic), have long been known (Table 11.14). Most are relatively high melting (and would not be considered ionic liquids), though the structural characteristics of quaternary ammonium halides have been studied by those with interests in plastic crystals and so-called rotator phases [7]. Many deconpose at or near their melting temperatures, a characteristic that would limit interest in them as reaction media [667]. Indeed, it is probably as a result of the view, widely held in the past [668], that such materials may be intrinsically unstable that the onset of the recent massive interest in ionic liquids was delayed. 

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