The imidazole ring, shown in Figure 1, is a versatile ionic liquid scaffold. Imidazole-based ionic liquids have been used in a variety of applications including potential water treatment agents because they have the ability to coordinate with metal atoms and are not volatile. They can also be used as green organic solvents. In addition, the unique combination of various alkyl substituents allows adjustment of the properties of the imidazolium ionic liquid to meet the requirements of different kinds of applications. Imidazolium ionic liquids have drawn the attention for a variety of reasons, particularly because of their tunable structure, thermal stability, relatively high ionic conductivity, wide electrochemical window, and amphoteric behavior in solution.
Figure 1. The structure of imidazole ring
Imidazolium ionic liquids are generally divided into four categories, namely monosubstituted, disubstituted, trisubstituted imidazolium ionic liquids, and benzimidazole ionic liquids.
Monosubstituted imidazolium ionic liquids: The structure of the imidazole monosubstituted ionic liquid is usually that the N atom on the imidazole ring is replaced by the R group. N-alkylimidazolium chloride is one of the representative substances that can be used in biochemistry. The N-alkyl and N-hydroxyalkyl-N-methylimidazolium chlorides have an effect on the renaturation of the two model proteins - egg white lysozyme and antibody fragment ScFvOx. Imidazolium ionic liquids can be used as refolding enhancers with different efficacy and efficiency.
Figure 2. The structure of monosubstituted imidazolium ionic liquids
Disubstituted imidazolium ionic liquids: When both nitrogen atoms in the imidazole ring are replaced by an alkyl group, the resulting molecule is typically an ionic liquid. Therefore, ionic liquids of disubstituted imidazoles have been extensively studied. Not all ionic imidazole molecules are liquids, and the formation of ionic liquids depends on the length or type of substituents on the molecule.
Figure 3. The structure of disubstituted imidazolium ionic liquids
The imidazolium ionic liquids can act as solvents to dissolve the less soluble polysaccharide as well as carbohydrates. These imidazolium cations, regardless of their structure, only interact weakly with the carbohydrate hydroxy groups, and the solvation mechanism is strongly dependent on the anion selected. Imidazolium ionic liquids containing chloride, acetate, phosphonate and dicyanamide also can be used as solvents. The midazolium ionic liquids can also be applied to produce cellulose beads, fibers or films when cellulose dissolved in an ionic liquid is embedded in magnetite particles to produce magnetic cellulose fibers. The absorption of 1-hexyl-3-methylimidazolium or 1-butyl-3-methylimidazolium in the polymer film affects the thermal stability, mechanical properties and ionic conductivity of the polymer film.
Trisubstituted imidazolium ionic liquids: Trisubstituted imidazolium ionic liquids are generally those molecules in which two nitrogen atoms and one carbon atom in the imidazole ring are replaced by alkyl group.
Figure 4. The structure of trisubstituted imidazolium ionic liquids
Trialkyl-substituted imidazolium-based ionic liquids are considered to be potential electrolytes for electroplating and other electrochemical applications, and trialkyl-substituted imidazolium-based ionic liquid are better than disubstituted ionic liquid in some electrochemical applications. For example, the basic properties density of trialkyl-substituted imidazolium-based ionic liquids such as 1-butyl-2,3-dimethylimidazolium tetrafluoroborate (BMMImBF4), 1-butyl-2,3-dimethylimidazolium azide (BMMImN3) and viscosity and conductivity of their mixture have a good performance. The thermal stability of BMMImN3 allows it to be used under temperature up to 423.15 K, while the upper temperature limit for BMMImBF4 extends to 623.15 K. Both ionic liquids have the same electrochemical stability in cathodic reduction. The values of density, molar volume and other physical properties indicate that the mixture of these two salts behaves as an ideal solution.
Benzimidazolium ionic liquids: Benzimidazolium ionic liquid are the compounds containing benzimidazol, which could give some electronic properties for the ionic liquids.
Figure 5. The structure of benzimidazolium ionic liquids
Ionic liquids and ionic melts consisting of imidazole or other bases and bis(trifluoromethanesulfonyl)imide are proton conductors to be used as fuel cell electrolytes. The combination of bis(trifluoromethanesulfonyl)imide and benzimidazole produces a novel proton ionic liquid and ionic melt. Proton ionic liquids and ionic melts composed of bis(trifluoromethanesulfonyl)imide and benzimidazole have high thermal stability and hydrophobicity. H2 oxidation and O2 reduction can be performed at the interface between the Pt electrode and the bis(trifluoromethanesulfonyl)imide-benzimidazole melt. The H2/O2 fuel cell polarization curve shows that by using this melt as an electrolyte, it can be operated under non-humidity conditions of 150 °C. The addition of water to bis(trifluoromethanesulfonyl)imide-benzimidazole has little effect on the electrochemical H2 oxidation and O2 reduction reactions, indicating the compatibility of the melt with the water produced by the fuel cell cathode.
1. Green, M. D.; Long, T. E. (2009). “Designing Imidazole-Based Ionic Liquids and Ionic Liquid Monomers for Emerging Technologies.” Polymer Reviews 49(4), 291–314.
2. Andriyko, Y. O.; et al. (2009). “Trialkyl-Substituted Imidazolium-Based Ionic Liquids for Electrochemical Applications: Basic Physicochemical Properties.” Journal of Chemical & Engineering Data 54(3), 855–860.