Iionic liquids refers to a liquid composed entirely of ions, such as KCI at a high temperature, and KOH is in a liquid state, in which case they are ionic liquids. A substance composed of ions at a temperature near room temperature or near room temperature, called room temperature ionic liquid, room temperature molten salt (room temperature ionic liquid is often accompanied by the presence of hydrogen bonds, defined as room temperature molten salt is somewhat reluctant), organic ionic liquid, etc. There is currently no uniform name, but it tends to be referred to as ionic liquid. In ionic compounds, the force between anions and cations is Coulomb force, the size of which is related to the number and radius of charge of anion and cation. The larger the ionic radius, the smaller the force between them, the more the melting point of this ionic compound low. Some ionic compounds have large anions and cations and are loose in structure, resulting in a low force between them, so that the melting point is close to room temperature.

History

The history of ionic liquids dates back to 1914, when Walden reported the synthesis of (EtNH2) + HNO3- (melting point 12 ° C). This substance is produced by the reaction of concentrated nitric acid and ethylamine, but its discovery is highly prone to explosion due to its instability in the air. Its discovery was not of interest at the time, and it was the earliest ionic liquid. In general, the melting of an ionic compound into a liquid requires a high temperature to overcome the ionic bond, and the state at this time is called "molten salt".The ionic bond in the ionic compound becomes weaker as the radius of the cation increases, and the melting point also decreases. For most substances, the melting point of the mixture is lower than the melting point of the pure substance. For example, the melting point of NaCl is 803 ° C, and the melting system composed of 50 % LICI-50 %AICl3 (molar fraction) has a melting point of only 144 °C. If the volume and structure asymmetry of the cation or anion are further increased to weaken the force between the anion and the cation, the liquid ionic compound at room temperature can be obtained. According to this principle, in 1951 F.H.Hurley and T.P. Wiler first synthesized an ionic liquid that was in a liquid state at ambient temperature. The cation they chose was N-ethylpyridine, and the synthesized ionic liquid was a mixture of n-ethyl bromide and aluminum chloride (the molar ratio of aluminum chloride to ethyl bromide was 1:2). However, the liquid temperature range of the ionic liquid is relatively narrow, and the aluminum chloride ionic liquid will release hydrogen chloride when exposed to water, which is irritating to the skin.Until 1976, Robert of the Colorado State University in the United States used AICl3/[N-EtPy]Cl as an electrolyte for organic electrochemical research. It was found that this room temperature ionic liquid is a good electrolyte and can be miscible with organic matter. Contains protons and a wide electrochemical window. In 1992, Wilkes synthesized 1-methyl-3-ethylimidazolium chloride with 1-methyl-3-ethylimidazole as a cation. Its melting point reached 8 °C in the presence of 50% mole fraction of AIC1. After this, the application research of ionic liquids has really been widely carried out.

Classification

An ionic liquid refers to a salt which is liquid at room temperature or near room temperature and consists entirely of anions and cations, also known as a low temperature molten salt. The main reason for the lower melting point of ionic liquids as ionic compounds is that the ions cannot be regularly deposited into crystals due to the asymmetry of certain substituents in their structure. It is generally composed of organic cations and inorganic or organic anions. Common cations are quaternary ammonium salt, quaternary phosphonium ion, imidazolium salt and pyrrole salt ion (as shown in the figure below). Anion has halogen ion and tetrafluoroborate. Ions, hexafluorophosphate ions, and the like.

Preparation

A wide variety of ionic liquids, changing the different combinations of cations and anions, can be designed to synthesize different ionic liquids. There are basically two basic methods for the synthesis of ionic liquids: direct synthesis and two-step synthesis.

Direct synthesis

The ionic liquid is synthesized in one step by an acid-base neutralization reaction or a quaternization reaction, and the operation is economical and simple, and there is no by-product, and the product is easy to be purified. Hlrao and other acid-base neutralization methods have synthesized a series of tetrafluoroborate ionic liquids with different cations. Further, a plurality of ionic liquids such as a halogenated 1-alkyl 3-methylimidazolium salt, a halogenated pyridinium salt and the like can be produced in one step by a quaternization reaction.

two-step synthesis

It is difficult to obtain the target ionic liquid by the direct method, and a two-step synthesis method must be used. There are many applications for preparing ionic liquids in a two-step process. The preparation of commonly used tetrafluoroborate and hexafluorophosphate ionic liquids is usually carried out in a two-step process. First, a halogen salt containing a target cation is prepared by a quaternization reaction; then a halogen ion is replaced with a target anion or a Lewis acid is added to obtain a target ionic liquid. In the second step reaction, when the metal salt MY (usually AgY), HY or NH4Y is used, the Ag salt precipitate or the amine salt and the HX gas are easily removed, and the strong protonic acid HY is added, and the reaction is required to be stirred under low temperature. The mixture is then washed with water several times to neutrality, the ionic liquid is extracted with an organic solvent, and finally the organic solvent is removed by vacuum to obtain a pure ionic liquid. It is important to note that in the exchange of the X-(halogen) anion with the target anion Y, it is necessary to complete the reaction as much as possible, ensuring that no x. anions remain in the target ionic liquid because the purity of the ionic liquid is The characterization of applications and physicochemical properties is critical. The synthesis of high purity binary ionic liquids is typically carried out by anion exchange using an ion exchange resin in an ion exchanger. In addition, the Lewis acid (MY) can be directly combined with a halogen salt to prepare a [cationic] [MnXny+l] type ionic liquid, such as a chloroaluminate ionic liquid prepared by utilizing this method, as described in the properties of ionic liquids. The acidity of the ionic liquid can be adjusted as needed.

Due to the designability of ionic liquids, the design of functionalized ionic liquids as needed is the direction of our experimental research.

Author's Bio: 

As a global Contract Research Organization (CRO), Alfa Chemistry has served the pharmaceutical and biotechnology industries for eight years. We offers an extensive catalog of m-MTDATA, Pyridinium ionic liquids, Ammonium ionic liquids and Functionized ionic liquids in a wide range of applications.