Ionic liquid

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Structure of BMIIm TFSI as an example of an ionic liquid

Ionic liquids ( English Ionic Liquids , also Room Temperature Ionic Liquids (RTIL) ) are molten salts with a melting temperature of less than 100 ° C. The formation of a stable crystal lattice is hindered by charge delocalization and steric effects. Even a small amount of thermal energy is therefore sufficient to overcome the lattice energy and break up the solid crystal structure. These are salts that are liquid at temperatures below 100 ° C without the salt being dissolved in a solvent such as water.

Examples of cations used , which can in particular be alkylated : imidazolium , pyridinium , pyrrolidinium , guanidinium , uronium , thiouronium , piperidinium , morpholinium , ammonium and phosphonium . As anions are halides and more complex ions such as tetrafluoroborates , trifluoroacetates , triflates , hexafluorophosphate , phosphinates and tosylates in question. Organic ions, such as imides and amides , can also be anions.

By varying the substituents of a given cation and by varying the anion, the physico-chemical properties of an ionic liquid can be varied within wide limits and optimized for technical requirements. In addition to the melting point, the solubility of, for example, homogeneous catalysts , products or starting materials in the ionic liquid can be influenced.


1914 described Paul Walden with ethylammonium , which has a melting point of 12 ° C, the first ionic liquid in the literature. Science paid no particular attention to the potential of this class of substances. In the following years mainly electrochemical publications dealt with the properties of the novel substance. It was not until 1983, with the synthesis of chloroaluminate melts as non-aqueous and polar solvents for transition metal complexes , that the broad field of application of ionic liquids was recognized. The first publications on their use as catalysts and solvents for organic reactions were made in the late 1980s.

The synthesis of hydrolysis-stable ionic liquids was achieved in 1992 by the research group led by John Wilkes and the development progressed rapidly. Current publications and numerous patents deal with the synthesis of new liquids. The work continues to deal with the application as solvents and catalysts, with the systematic investigation of their chemical and physical properties, with toxicological investigations and their application in the field of analytical separation processes.


Ionic liquids are characterized by a number of interesting properties. They are comparatively thermally stable, difficult to ignite, have a very low, hardly measurable vapor pressure and have very good dissolving properties for numerous substances. Due to their purely ionic structure, they also have interesting electrochemical properties, such as B. electrical conductivity, which is often accompanied by a high electrochemical stability against oxidation and reduction. The electrostatic interactions between the ions can be described with the Poisson-Boltzmann equation .

By varying the side chains of the cation and selecting suitable anions, for example, the solubility in water or organic solvents can be largely freely determined. The same applies to the melting point and the viscosity. They can be synthesized as acids , bases or ligands by means of appropriate functional groups .


In principle, the molecular diversity of ionic liquids enables them to be used in a large number of technical areas of application:


Dye solar cells

By means of dye solar cells, a wide range of the light spectrum can be used compared to crystalline silicon solar cells , which also enables use with diffuse light or lower light intensity. The ionic liquids can be used as a novel electrolyte material. The benefit here lies in their conductivity, which ensures the necessary charge transport. In combination with the properties of a low melting point and a very low vapor pressure, it can be used at temperatures from −20 to +80 ° C. They also have a high level of electrochemical stability. One application aims at using these cells e.g. B. as a charger for mobile phones in regions without nationwide power supply.


In many commercial lithium-ion batteries , organic solvents , e.g. B. carbonates are used as electrolytes . However, these have a number of disadvantages. Compared to ionic liquids, organic carbonate solvents have a limiting stability against oxidation at high positive potentials (approx. 4.3-4.9 V vs. Li + / Li) depending on the electrode used. This leads to an increasing decomposition of the electrolyte with progressive cycling at these potentials. Due to the associated loss of efficiency, organic solvents are only suitable for a small potential window. Another advantage is the thermal stability and low volatility of ionic liquids. Unwanted exothermic reactions can occur between the electrolyte and other battery components due to local overheating or a short circuit. The rapidly increasing cell temperature can lead to ignition of the organic solvent. A major disadvantage for the use of ionic liquids as electrolytes is their low ionic conductivity, since they have a low charge density and high steric hindrance .

Cellulose finishing

With an occurrence of around 700 billion tons, cellulose is the largest natural organic chemical on earth in terms of quantity and is of great importance as a renewable raw material . Of the 40 billion tons reproduced annually by nature, only about 0.2 billion tons are used as raw material for further processing. An extended use of cellulose as a raw material stands in the way of the lack of a suitable solvent. Robin Rogers and colleagues from the University of Alabama have found that cellulose solutions can be made available in technically usable concentrations by using ionic liquids. For example, when manufacturing synthetic cellulose fibers (e.g. viscose ) from so-called chemical pulp, various auxiliary chemicals, especially carbon disulfide (CS 2 ), currently have to be used in large quantities and then recycled or disposed of. In addition, due to the nature of the process, considerable amounts of wastewater have to be processed. These processes can possibly be simplified by the use of ionic liquids, since they can be used as solvents and almost completely recycled. The Institute for Textile Chemistry and Man-Made Fibers (ITCF) in Denkendorf and BASF are jointly investigating the properties of fibers that are spun in a pilot plant from cellulose dissolved with the help of ionic liquids.

The good solubility of cellulose in ionic liquids opens up a wide range of possibilities for the production of materials based on renewable raw materials. In a one-step synthesis, cellulose can be converted to 5-hydroxymethylfurfural (HMF) in the presence of coupled copper (II) chloride / chromium (II) chloride catalysts in the solvent 1-ethyl-3-methylimidazolium chloride at 80-120 ° C. a possible building block for plastics based on biomass applies.


A large number of commercially available pharmaceuticals are in the form of salts. Pharmaceutically active ionic liquids are also already known. One combines a pharmaceutically active cation with an equally active anion. The aim is to obtain a pharmaceutically active salt with the properties of an ionic liquid. In the ideal case, the pharmaceutical effect of the new ionic liquid has a synergetic effect.

Environmental balance

The long-term environmental effects of ionic liquids are still being investigated. Compared to common organic solvents, ionic liquids are somewhat more toxic. Within the ionic liquids there is an understandable tendency that the more hydrophobic they are, the more toxic they are, e.g. B. by long alkyl chains, as they then accumulate more easily in the adipose tissue of living things and more difficult by, for. B. urine (aqueous solution) can be excreted. Halogen-containing ILs are also harmful to the environment, just like halogenated organic substances, because they exist for a long time in the environment and through only a small amount of energy (light, heat, enzymes, ...) form reactive intermediate stages such as radicals, which are in the atmosphere as well as in the body are equally harmful. Even if there is no risk of poisoning by inhalation due to the non-volatility of the compounds, wastewater can be problematic. Due to the large number of possible combinations, one expects to achieve the desired physicochemical properties with the lowest possible toxicity in the medium term.

See also

Web links


  • Thomas Waldmann, Hsin-Hui Huang, Harry E. Hoster, Oliver Höfft, Frank Endres, R. Jürgen Behm: Imaging an Ionic Liquid Adlayer by Scanning Tunneling Microscopy at the Solid | Vacuum Interface. In: ChemPhysChem. 12, 2011, pp. 2565-2567, doi: 10.1002 / cphc.201100413 . (Adsorbed monolayers of an ionic liquid imaged with a scanning tunneling microscope)
  • Liquid / Solid Interface of Ultrathin Ionic Liquid Films. In: Langmuir . 27, 2011, pp. 3662-3671, doi: 10.1021 / la105007c .

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