Liquid gold: there are a possible million simple ionic liquids and over a trillion complex ionic liquids. Researchers are looking at how we can use these strange salts in new cars, houses, clothing and even drugs.

The rash of research into ionic liquids in the 1990s has opened the door to many new applications. Previously these chemicals, which are essentially liquid salts, were touted as 'green' solvents that would eliminate the use of volatile, toxic solvents and speed up reactions or make them cheaper. But as work has continued apace, scientists have found new and novel applications for these strange salts. These range from air-conditioning and heating systems, to drugs and even transparent solar panels in windows. If there is a problem that needs fixing, it looks like there could be an ionic liquid to fill the hole.

Ken Seddon, at Queen's University Ionic Liquid Laboratories (QUILL) in Belfast, is one of the pioneers of ionic liquids research. He has calculated that there are a possible million simple ionic liquids and over a trillion complex ionic liquids, giving researchers a huge toolbox for every eventuality.

'When you look at the explosion in the scientific and patent literature on ionic liquids, a whole lot of novel applications are being tried, including unexpected things like ion propulsion for space travel and embalming,' says Neil Winterton, a visiting professor in the department of chemistry at the University of Liverpool. But he says: 'The difficulty is telling which are wholly speculative or close to breaking into commercial applications.'

Blowing hot and cold

One firm looking to exploit this new-found interest in ionic liquids is Germany's lolitec, lolitec has a portfolio of over 200 ionic liquids and is helping universities and businesses find the right one. And lolitec is more than just a supplier, it is also involved in helping its clients move from the R&D stage to pilot applications.

One of the most exciting projects is a novel air conditioning system. 'We have a way to cool things down that doesn't need electricity - just heat,' says Thomas Schubert, managing director and founder of lolitec. Sorption cooling relies on the hygroscopic nature, or ability to attract water, of certain ionic liquids. Just as sweating cools your body, the evaporation of water, as it is attracted to the ionic liquid, can be used to cool buildings. Passive solar panels on a building's roof could be used to drive the water out of the ionic liquid, recycle it and keep the cooling cycle going.

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'We believe that sorption cooling will have a major market in the future; 80% of US public buildings have air conditioning and 100% are cooled in Japan,' Schubert adds. A 7kW pilot plant is already up and running at the University of Karlsruhe and Iolitec hopes to have a megawatt device in a hospital in Magdeburg, Germany by 2009.

Ionic liquids could also be used to heat buildings. Iolitec is working to develop phase-changing ionic liquids for passive heating systems. 'The goal is to integrate them into buildings to store energy between day and night,' Schubert says. These materials could quickly become big business and the market is predicted to reach 150-200 m [euro] by 2012.

Phase-changing ionic liquids move from liquid to a crystalline solid, and vice-versa, at close to room temperature. So during the day when it is warmer the ionic liquid soaks up the heat. But as night approaches and the temperature begins to drop the ionic liquid crystallises, releasing the stored heat just like a pocket hand warmer. And scientists at the TITK Institute in Germany are hoping to create suits of intelligent textiles with pockets of these phase-changing ionic liquids to warm or cool the wearer.

Drug Crystals

Ionic liquids are even being considered as drugs. Robin Rogers, a co-director of QUILL, who first came up with the idea of ionic liquid drugs, explains that the active form of many medicines, like ibuprofen, are ions. 'The pharmaceutical companies have looked at ions for a long time,' Rogers notes. 'However, they are still 100% reliant on crystalline salts and we feel that liquid salts have not been foremost in people's minds.'

The use of ionic liquids as the active pharmaceutical ingredient offers several potential benefits. One of the biggest advantages is that there is no need to worry about the drug existing in several different crystalline forms, a condition known as polymorphism. At the moment pharma companies spend considerable time and money ensuring that drugs are produced in only one form. Indeed this is often a requirement that regulatory bodies insist on, unless the polymorphic version has been shown to have a therapeutic effect beyond that of the single drug form.

If the drug is formulated as a liquid there is no need to worry about getting the right kind of crystal, simplifying the manufacturing process. Winterton says, '[Ionic liquids as drugs] is a neat idea and avoids the complication of polymorphism.' However he adds, 'This would only work if the drug is a salt and you also have to find a compatible anion or cation to go with the counter-ion to ensure the drug is a stable liquid at ambient temperatures.'

The next generation of ionic liquid drugs, meanwhile, would combine two ions that are both active in the body Rogers says. As the ionic liquid mixes with body fluids, the ions are delivered in doses proportionate to the liquid's ion composition. This could be useful for controlling doses in drug combination therapies.

Formulating drugs as ionic liquids could even breathe new life into failed drugs. Many drugs fail early on because they cannot get where they need to be or have poor solubility and this could potentially be fixed by formulating them as an ionic liquid.

Rogers' group has now taken this idea into the lab, formulating ionic liquid drugs with off-patent medicines. His group paired lidocaine with docusate and ibuprofen with didecyldimethylammonium to create two of the first ionic liquid drugs (New J Chem 2007, 31, 1429). Each of the ions used had a therapeutic action: lidocaine is a local anaesthetic and docusate an emollient. Ibuprofen an antiinflammatory and didecyldimethylammonium an antibacterial. Rogers says that tests on mice with lidocaine docusate suggest 'there's tantalising clues and hints that there may be a synergistic effect.' But he cautions that it is too early to predict whether these will always be positive effects.

Trains, planes ...

Another company, Nisshinbo in Japan, has found that ionic liquids can make excellent electrolytes for storing energy. Nisshinbo has developed an electric double layer capacitor (EDLC) that uses the ionic liquid [DEME-BF.sub.4] as the electrolyte. '[DEME-BF.sub.4] is a better electrolyte compared to [typical] solid ones, especially in long term use,' says Gen Masuda, senior research associate at Nisshibo. 'Where other solid electrolytes often solidify out of the electrolyte, lowering performance, [DEME-BF.sub.4] does not.'

These capacitors have a very high energy density and Masuda's group has run a small buggy for 10km on only a 1 minute charge (Electrochim Acta 2004, 49, 3603). Nisshinbo has now commercialised this technology and the capacitors are finding their way into hybrid cars and trains where the EDLCs are continually recharged by braking which turns kinetic energy into electrical energy.

Soak up the sun

And ionic liquids are being exploited for other power applications. The ColorSol consortium is combining several European companies' manufacturing skills with expertise from the Fraunhofer Institute for Solar Energy Systems to produce dye-sensitised solar cells (DSCs). These cells use a light-harvesting dye, immobilised on nanocrystalline titanium dioxide. But to allow the dye to transfer its energy, an electrolyte is needed and ionic liquids once again fit the bill. Whereas other electrolytes tend to evaporate as they heat up in the sun, ionic liquids do not. Andreas Hinsch, project leader at Fraunhofer Institute for Solar Energy Systems, says that this is very important for sealed systems like DSCs where electrolyte evaporation can break the seal, damaging the solar panel. The market for ionic liquids in DSCs is expected to reach 13-26m [euro] by 2012.

DSCs have several advantages over photovoltaic cells as they work better in low light and are transparent, so can be incorporated in windows or even beach parasols. Hinsch says that large buildings have potentially a huge area to install these cells. 'These solar cells combine aesthetics with energy production,' he adds.

However, problems may still remain for ionic liquids. Ionic liquids' high viscosity, compared with classical molecular solvents, means that pumping large volumes of them about is difficult. 'To get around this problem you need to find clever new ways to do things--new approaches, new materials and that adds cost,' Winterton points out. The other issue is what to do with the ionic liquid once it's served its purpose as recycling them is still problematic. But Seddon remains upbeat about ionic liquids' potential. 'Wherever a liquid is being used an ionic liquid could be doing the job better,' he says.

Ionic liquids explained

* Ionic compounds like table salt melt at over 800[degrees]C, whereas ionic liquids have melting points below 100[degress]C, And many of the ionic liquids that are piquing chemists' interest are liquid at room temperature.

* The reason they are liquid is the size and shape of their ions. The sodium and chloride ions in table salt are very similar in size and pack together tightly to create a solid crystal. Ionic liquids, on the other hand, are made up of asymmetric ions of varying shapes and sizes and it is this disorganised structure that makes them liquid.