Magnetic ionic liquids are functionalized ionic liquids with magnetic property. Magnetic ionic liquids consist of an organic cation and an inorganic or organic anion. Magnetic ionic liquids are usually liquid state at room temperature. Unlike conventional ionic liquids, the cations or anions of magnetic ionic liquids contain magnetic centers. Because of the existence of magnetic center, the magnetic ionic liquid can be magnetized and can respond to the external magnetic field. Figure 1 shows that [bmim]FeCl₄, a typical magnetic ionic liquid, responds to a magnet.

Figure 1. The response of a magnetic ionic liquid ([bmim]FeCl₄) to a magnet.

Applications:

Magnetic ionic liquids can generate certain magnetization intensity under the action of external magnetic field, and can produce macro response to the external magnetic field. Due to the advantages of easy separation and reutilization, magnetic ionic liquids are widely used in the field of chemical, pharmaceutical and environmental protection.

• Catalyst: Magnetic ionic liquids can be catalyst in polymerization, Friedel-Crafts reaction and aryl Grignard cross-coupling reaction. For example, without dopant and oxidant, simply adding pyrrole into the [bmim]FeCl₄ solution can obtain polypyrrole(PPy) at room temperature. The particle size distribution of the obtained nano-spheres is narrow, and the nano-spheres are aligned with the applied magnetic field, which suggests that the applied magnetic field has an influence on the nano-spheres.

Figure 2. Molecular structure of [bmim]FeCl₄ (1-butyl-3-methylimidazolium tetrachloroferrate).

Figure 3. SEM images of PPy nano-particles obtained in magnetic ionic liquid at room temperature: (I) without external magnetic field, (II and III) with magnetic fields. IV is the particle size distribution graph.

• Magnetic carbon nanotubes: Magnetic ionic liquids react with carbon nanotubes to obtain magnetic carbon nanotubes, which can be applied in bio-nano engineering, biomedical and magnetic instruments. A group reported a magnetically responsive nanomaterial that was prepared by covalent attachment of a magnetic ionic liquid (MIL), specifically 1-butyl-3-methylimidazolium [FeCl₄], to single-walled carbon nanotubes (SWCNTs). The reported magnetic-SWCNTs (m-SWCNTs) have a strong magnetic response and can be easily dispersed in an organic solvent (N-methyl pyrrolidone).

Figure 4. Synthesis procedure of m-SWCNTs.

Classification:

According to the relationship between structure and magnetism, magnetic ionic liquids can be divided into two categories.

• Pure organic magnetic ionic liquid: Pure organic magnetic ionic liquid refers to the composition of magnetic ionic liquid does not contain metal elements, and its magnetism comes from its own structure.
• Metallic magnetic ionic liquid: The magnetism of metallic magnetic ionic liquid derives from the metal ion in the ionic liquid. Furthermore, the metallic magnetic ionic liquid can be divided into transition metal magnetic ionic liquid and Lanthanide metal magnetic ionic liquid.

References:

1. Hayashi S, Hamaguchi H O. Discovery of a Magnetic Ionic Liquid [bmim]FeCl₄[J]. ChemInform, 2005, 36(18):1590-1591.
2. Kim J, Song E, et al. Polypyrrole Nanostructures Self-Assembled in Magnetic Ionic Liquid as a Template[J]. Macromolecules, 2008, 41(8): 2886-2889.
3. Pei X, Yan Y H, Yan L, et al. A magnetically responsive material of single-walled carbon nanotubes functionalized with magnetic ionic liquid[J]. Carbon, 2010, 48(9):2501-2505.