Ionic liquids are generally composed of specific organic cations with relatively large volume and asymmetric structure and inorganic anions with relatively small volume. Ionic liquids have a low melting point and they are liquid at or near room temperature. Compared with other solid or liquid materials, ionic liquids often exhibit special physical and chemical properties and special functions. Ionic liquids have good chemical and thermal stability and wide electrochemical stable potential window, which is a kind of new functional materials worthy of research and development. According to the type of cations, ionic liquids can be classified as quaternary ammonium salts-based ionic liquids, imidazole-based ionic liquids, pyridinium-based ionic liquids and others. Among them, pyridinium-based ionic liquids have been more and more widely studied and applied because of their excellent properties. In the structure of pyridinium-based ionic liquids, pyridinyl groups can be replaced at multiple sites. The pyridinium-based ionic liquids with only one active site replaced by other functional groups is called monosubstituted pyridinium-based ionic liquids. The general structural formula of the monosubstituted pyridinium-based ionic liquid is shown in Figure 1. R is usually an alkyl chain. Y- can be PF6-, Br-, BF4- and others.
Figure 1 The general structural formula of monosubstituted pyridinium-based ionic liquids.
Applications:
- Extraction agents: Monosubstituted pyridinium-based ionic liquids are new green organic solvents with excellent solubility and significant ionic properties, which can be used as extraction agents to purify various elements. For example, using N-octyl pyridine bromide salt as extraction agent and n-amyl alcohol as diluent, the recovery and utilization of vanadium in aqueous solution of vanadium slag is realized, and it has the advantages of high efficiency, no pollution and low cost.
- Electrochemical biosensors: Monosubstituted pyridinium-based ionic liquids are widely used in the preparation of electrochemical sensors, especially as modifiers for electrodes. For example, taking the advantages of simple preparation method, low cost, easy renewal of electrode surface, small background current and wide potential window, carbon paste electrode has become one of the most popular electrode materials for the design and preparation of electrochemical sensors. However, the sensitivity of carbon paste electrode is reduced due to the addition of non-conductive adhesive in the preparation process. Therefore, monosubstituted pyridinium-based ionic liquids, such as N-hexylpyridinium hexafluorophosphate and N-butylpyridinium hexafluorophosphate, are often added as modifiers to improve the electrochemical performances of carbon paste electrodes.
- Others: In addition to above mentioned applications, monosubstituted pyridinium-based ionic liquids can be used in other fields. For example, monosubstituted pyridinium-based ionic liquids can be used as catalysts for chemical reactions, increasing the reaction rate. Monosubstituted pyridinium-based ionic liquids are noncombustible and nonvolatile, which can be used as a safe heat medium. In addition, monosubstituted pyridinium-based ionic liquids can be used as lubricants for bearings. Moreover, monosubstituted pyridinium-based ionic liquids can be used as electrolytes for secondary batteries by virtue of their high ionic conductivity.
Classification:
Commonly used monosubstituted pyridinium-based ionic liquids include N-octyl pyridinium bromide, N-hexyl pyridinium bis(trifluoromethyl sulfonyl)imide, N-hexyl pyridinium hexafluorophosphate, N-hexyl pyridinium tetrafluoroborate and others. The chemical formulas of these monosubstituted pyridinium-based ionic liquids are shown in Figure 2.
Figure 2. The chemical formulas of the typical monosubstituted pyridinium-based ionic liquids.
