On the green - Ionic liquids
Despite reports that some ionic liquids (ILs) - green solvents meant to cure all environmental ills - are toxic, this area continues to evolve, leading to hundreds of new patents each year.
Defined in the IL community as salts that melt below 100ºC, there have been over 7,500 papers (including more than 850 patents or applications) published since 1998 that use the term ‘ionic liquid’, even though salts meeting this definition have been known since early in the last century. Ionic liquids have been brought to the fore by chemists and engineers focusing on developing and implementing chemical processes and products that reduce or eliminate the use and generation of hazardous substances through so-called green chemistry. One of the main targets has been to replace volatile organic compounds (VOCs) with safer alternatives.
Negligible vapour pressure, a wide liquid range, and favourable solvating properties for diverse compounds make ILs great candidates to replace VOCs. This was emphasised at the first international IL meeting (NATO Advanced Research Workshop – Green Industrial Applications of Ionic Liquids), held in Crete, Greece, in April 2000. At the end of the meeting there were two significant outcomes:
• Ionic liquids are intrinsically interesting and worth studying for advancing science (ionic vs molecular solvents).
• Combined with green chemistry, ILs provide an opportunity for science, engineering and business to work together from the beginning of the field’s development.
The emphasis placed on the physical properties of ILs – the drive to discover more examples of low melting salts with negligible vapour pressure – has led to new classes of ILs and a plethora of solvent applications. Indeed ILs can be amazing solvents.
One example, 1-butyl-3-methylimidazolium chloride ([C4mim]Cl), will dissolve hydrophobic solutes such as carbamoylphosphine oxide (N,N-Diisobutyl-2-(octylphenylphosphinyl) acetamide, a hydrophobic extractant used in nuclear waste reprocessing), and hard to dissolve species such as cellulose. But it is completely miscible with water. When dissolved in water, ([C4mim]Cl) acts as a salt solution, and, when dry, behaves like an extremely versatile solvent.
Unfortunately, the green concept improperly attributed to many ILs has become a battlefield. The liquids were initially studied to solve a certain problem (often solvent replacement). This has led to the study of a limited number of ILs, and implicitly overgeneralisation of them as everything from nontoxic, nonflammable and nonvolatile to toxic, flammable and volatile. The above statements may be true for one particular IL, but not for the whole range of compounds.
Despite the controversy, there is still a growing interest in developing and using ILs in a range of applications. Though initially studied for their physical properties, ILs are now seen as tunable materials with applications in materials science, personal care, home products, chemistry, cell engineering and lunar mirrors. Combining ions with high energy content and oxygen balance could lead to novel energetic salts with enhanced efficacy, safety, or delivery options.
World of uses?
The composition of any IL can be tailored to the user’s needs. Desired physical, chemical, and biological properties can be realised in a single salt by selection of the component ions or in mixtures of component ions.
The combination of accessible physical and chemical properties represented by ILs has led to a boom in patent applications, where the unanticipated properties are used for what could be ‘transformational new technologies’. A quick review of these patents and their applications reveals the diversity in disciplines now working on IL technology.
Researchers have realised that, based on the dual functionality of ILs (having a cation and an anion), they can make customisable materials. Ionic liquids are used in or as lubricants, solvents, solar cells, solid-state photocells, batteries, magnetic and thermal fluids, propellants, optical fluids, extractants, hydraulic oils, and adsorbents, to name a few. The exploration of physical and chemical properties led to BASIL, the first industrial application in which the IL acts as a biphasic acid scavenger. Other product lines launched by chemical company BASF include BASIONIC (a range of ILs) and CELLIONIC (cellulose in IL solutions). Merck KGaA, in Germany, has developed hundreds of specialty products, and several small companies have been founded to synthesise ILs and uncover novel applications.
The debate over whether ILs are toxic is ongoing. Due to their tunability of ion combinations, ILs can be designed to be toxic, corrosive or flammable, just as easily as they can be designed to be the opposite. Recently, toxicity has become desirable, such as in the pharmaceutical sector. Even though the industry studied the use of ILs as VOC replacements in the synthesis of active pharmaceutical ingredients (APIs), they did not take this seriously, due to uncertainty over toxicity, purity, and regulatory approval for these ‘neoteric solvents’. Pharmaceutical companies, which rely mostly on crystalline APIs (facing problems such as polymorphism, poor dissolution, solubility and bioavailability), might now look at ILs as APIs themselves. This could solve these problems, or even introduce new treatment or delivery options which are not currently available with crystalline APIs.
The volume of IL literature demonstrates that they are fast becoming not just tunable solvents, but tunable materials. This bodes well for a strong continuation of the IL phenomenon, where pure liquid salt forms are considered a design strategy to overcome potential problems encountered from solids or molecular compounds.
Green chemistry should not define the field, but guide its development.
Drs D Robin Rogers and Gabriela Gurau, QUILL School of Chemistry and Chemical Engineering, Queen’s University of Belfast, Stranmillis Road, Belfast, BT9 5AG, Northern Ireland, UK. Tel: +44 (0)28 9097 4627. Email: R.Rogers@qub.ac.uk.
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