What are ionic liquids?
Ionic liquids are liquids composed entirely of ions. Molten salt is the term normally reserved for those systems that are liquid at high temperatures, for example NaCl (table salt is a liquid at ≈ 800ºC) Ionic liquid is normally reserved for those systems that are liquid at room temperature (or just above, up to approximately 100ºC.)
Ionic liquids are generally composed of a bulky organic cation, such 1-butyl-3-methylimidazolium (which you can see in the ion pair to the right). The other half of the ionic liquid is typically an inorganic anion such as a halide (the green atom of the pair to the right is a Cl- anion), BF4 or PF6. Below are the chemical structures of some common cations and anions used to make ionic liquids.
A large number of ion combinations is possible, which opens up a myriad of potential applications. However, characterising ionic liquids as a class is almost impossible, because each group of ionic liquids has highly individual characteristics.
Ionic liquids can be employed in a VERY wide range of applications.
The properties of an ionic liquid can be varied by changing the constituent ions. However, exactly how the ionic liquid behaves at a microscopic level is unknown. This means that ionic liquids for specific tasks cannot yet be "designed" but are obtained by laborious experimental trial and error. As there are vast numbers of cation and anion combinations possible this is an enormous task! It is important that we obtain a fundamental molecular level understanding of how ionic liquids behave. Once we understand ionic liquids at the molecular level we will be able to predict their physical and chemical properties without having to make them. This is where computational research comes in.
An ionic solid and ionic liquid at room temperature
Why are Ionic Liquids Interesting?
As a class, room temperature ionic liquids exhibit some unusual and highly desirable physical properties:
- vanishing vapour pressure
- large liquidous range
- high thermal stability
- good ionic conductivity
- high electrochemical stability
- favourable solvation behaviour
Ionic liquids are designer solvents. One or both of the constituent ions can be changed in order to control physical properties.
Ionic liquids are also task-specific materials. The chemical makeup of the cation and anion can be altered to achieve particular goals. This makes them ideal for developing new and innovative materials.
Ionic liquids are green. As replacements for volatile organic solvents ionic liquids have the potential to make a significant positive environmental impact.
An ion-pair from an IL
What is special about the properties of ILs?
- Vapour Pressure: Ionic liquids have a negligible vapour pressure. This means there is no "vapour" or gas sitting above the liquid. Think of the fumes that come off petrol, or the smell of coming out of an open bottle of vinegar. Petrol and vinegar have a positive vapour pressure, and you can smell the vapour that is sitting above the liquid.
How is this useful? Well, it means there is no toxic or potentially explosive gas sitting above the solvent, unlike for example, petrol! If industrial processes can be changed to use ionic liquids, instead of the dangerous and environmentally damaging organic solvents they use now, processes will be much safer for workers, greener for the environment and easier to handle for the industrialist. Ionic liquids are also known as green solvents.
- Good Conductivity: Ionic liquids can conduct, that is they can pass current, some even in their solid state. However, ionic liquids are more commonly used as a solvent or electrolyte for charge carrying ions which actually do the most of the "conduction".
How is this useful? Because ionic liquids are composed of ions, this makes solvating other ions very easy and carrier ions that can move freely in a solvent create a larger current. While ionic liquids have excellent solvating ability for ions, they also tend to have a higher viscosity compared to traditional solvents. So on the one hand while they solvate the ion very well thus increasing conductivity, on the other hand, due to the low viscosity the ion has a harder time moving through the ionic liquid, reducing conductivity. This is a big area of research at present, how to make low viscosity ionic liquids.
- High Electrochemical Stability: Ionic liquids remain chemically stable at high and low potentials. Electrochemical reactions work by applying a potential which rips electrons from one speices and forces them onto another. The potential is related to the energy required to remove or add an electron. Ionic liquids are very stable in this respect, they don't want to give up or recieve electrons, and this makes them very good solvents for electrochemical reactions. Ionic liquids also have a wide electrochemical window, this gives them advantages over aqueous based systems where the production of H2 and O2 limit the range of the electrochemical window.
How is this useful? There are some metals which cannot be electroplated from a normal solvent because of the potentials required, the advent of ionic liquids has changed this! Electroplating is important in the electronics industry and for forming anticorrosion and wear resistant coatings. These types of reactions typically also occur at very high temperatures, so thermal stability is important, in addition the metal has to dissolve in the electrolyte, thus ionic liquids are very good candidates, as they have high thermal stability and good solvation ability.
- Large Liquidous Range: Ionic liquids remain liquid over a large temperature range. This means that an ionic liquid remains liquid if you reduce the temperature (it doesn't freeze easily) and it remains liquid if you increase the temperature (it doesn't boil away).
How is this useful? Industrial machinery can run at very high temperatures, normal lubricants boil off, or chemically react with the steel components, an ionic liquid can remain stable and unreactive. It also means that devices that use ionic liquids can operate in a wider range of temperatures.
- High Thermal Stability: Ionic liquids remain chemically stable at high temperatures. Generally heating a reaction makes it go faster, and when exposed to very high temperatures molecules break-down (this is what happens in flames). Ionic liquids are made from very stable ions, they are unreactive to high temperatures, upto 400 degrees C. Actually not all ionic liquids are very stable, some anions can decompose at relatively low temperatures, such as N(CF3SO3)2-, this is a problem since ionic liquids with this ion are used in electrochemical applications, where we want high thermal stability. The search is on for anions which are stable, but also impart good electrochemical performance
How is this useful? We need solvents for reactions that occur at high temperatures, if the solvent decomposes during a reaction this is a serious problem. It is also an environmental problem as the product becomes contaminated, and if the decomosed fragments are volatile they become a pollutant. One use of ionic liquids is as a solvent in separation processes, organic molecules dissolved in the ionic liquid can be cleanly distilled off leaving the ionic liquid behind, this is a big area of research for ionic liquids. Having high thermal stability also means that devices which use ionic liquids can operate at a higher temperature.
- Solvation Properties Ionic liquids are excellent solvents because they are weakly-coordinating, and thus don't interfere explicitly with the chemical reaction, in this sense ionic liquids are "spectators". However, many reactions proceed through charged or highly polarised transition states or intermediates. Ionic liquids, because they are ions, are able to stabilise these states, lowering the energy of and facilitating the reaction. Ionic liquids can also be used to control product ratios, and even enantiomeric selectivity via preferential stabilisation of particular chemical species.
How is this useful? A significant problem in the industrial production of chemicals, is the separation of the products (for removal) and the catalyst (this precious metal is recovered for reuse). In a biphasic catalytic reaction, the cationic catalyst is carried in the ionic liquid (catalyst phase) and the organic substrate (or reactants) and eventually the products are carried in an organic phase (carrier phase). Once the reaction is complete the mixture is heated, and the product and organic phase evaporate off leaving the stable ionic liquid behind. Hence there is clean separation and recovery of both the product and solvent!