The Properties of Ionic Liquids

Posted by Tylor Keller on July 25th, 2019

Melting point

In general, the lower the structural symmetry of the ionic liquid, the lower the intermolecular force and the lower the melting point of the ionic liquid. The melting point of the ionic liquid is closely related to the size of the anion constituting the ionic liquid, and the larger the size of the anion constituting the ionic liquid, the smaller the melting point of the ionic liquid. The cations constituting the ionic liquid are generally determined by the size of the group of atoms constituting the ionic liquid. The larger the group of atoms, the lower the structural symmetry, and the lower the melting point of the ionic liquid composed of the cation.

Viscosity

The viscosity of the ionic liquid is several tens or even hundreds of times greater than the viscosity of water. The viscosity of ionic liquids is mainly related to van der waals forces, coulomb forces between ions, and hydrogen bonding. The structure of the anion and cation also has a great influence on the viscosity of the ionic liquid, wherein the anion has a large influence, and the viscosity of the ionic liquid mainly increases as the volume of the anion increases.

Thermal stability

Ionic liquids have high thermal stability and their thermal decomposition temperature is above 300°C. The stability of the ionic liquid is affected by the interaction between the heteroatom-carbon atoms and the interaction between the heteroatom-hydrogen bonds. The thermal stability of ionic liquids is related to the structure and properties of anions and cations. For example, the longer the carbon chain of the cation of the ionic liquid, the worse the stability of the ionic liquid.

Density

Density is also one of the important parameters of ionic liquids. Generally, the density of the ionic liquid is greater than that of water. A comparison of the densities of chloroaluminates containing different substituent imidazolium cations revealed that the density has a linear relation with the length of the N-alkyl chain on the imidazole cation, and the density of the ionic liquid becomes smaller as the organic cation becomes larger. Therefore, the density of the ionic liquid can be adjusted by slightly adjusting the structure of the cation. The effect of anions on density is more pronounced, and usually the greater the anion, the greater the density of the ionic liquid. Therefore, if you want to obtain ionic liquids of different densities, you should first select the corresponding anions to determine the approximate density range of the ionic liquid, and then slightly adjust the structure of the cation to fine-tune the density.

Solubility

Ionic liquids are capable of dissolving different substances such as organic matter, inorganic substances and polymers, and are good solvents for many chemical reactions. The solubility of ionic liquids is closely related to the properties of their cations and anions. The effect of cations on the solubility of ionic liquids can be seen by the solubility of n-octene in the ionic liquid containing the same tosylate anion quaternary ammonium salt. As the quaternary ammonium cation side chain of the ionic liquid becomes larger, the non-polar characteristics increase, and the solubility of n-octene becomes larger. Therefore, the solubility of the ionic liquid can be adjusted by changing the alkyl group of the cation.