Introduction to ionic liquids

Since the early 1980s a new class of room temperature ionic liquids has been available that has attracted great interest due to the potential commercial and environmental advantages they offer over existing liquids.

Conventional liquids are molecular, being constituted of molecules, regardless of whether they are polar or non-polar.

Ambient temperature ionic liquids are comprised of ions that allow them to potentially behave in a very different manner to conventional molecular liquids when they are used as solvents.

The simple physical properties of ionic liquids yield great promise, and by combining different anions and cations it is possible to synthesise a huge library of different ionic liquids each with specific properties.

As low temperature ionic liquids represent a recent development, they remain a mystery to even the most experienced organic chemist.

There are roughly 300 organic solvents that are used widely throughout the chemical industry.

However there are over a trillion possible ionic liquids and for the most part little data exists about even the most basic physical properties, such as the density or melting point.

To try and remedy this situation and to make ionic liquids a realistic alternative to organic solvents, a consortium comprised of academics and industrial representatives known as the Queen's University Ionic Liquids Laboratories (Quill) was set up.

Quill shares commercial secrets with its members collaborating and distributing any patented technology, primarily because a fundamental knowledge of ionic liquids does not currently exist.

Conventional organic solvents are used in a variety of industrial applications that include the production of pharmaceuticals, the manufacturing of electronic components, the processing of polymers, refrigeration systems and the synthesis of chemicals.

As a result of their unstable nature, organic solvents readily evaporate into the environment and are known collectively as volatile organic compounds (VOCs).

The use of VOCs poses a risk to those people working in or living close to such processing facilities.

In addition, VOCs have been heavily implicated in causing changes to the global climate, the formulation of smog as well as being identified as a source of ozone depletion.

It is estimated the current usage of VOCs worldwide is worth some £4,000,000,000 per annum, representing a massive global market.

The Montreal Protocol has forced many industries and organisations to re-evaluate their chemical operations, due to the adverse environmental impact caused by the use of VOCs, by investing in clean technology that reduces waste and by-products from an industrial process to a minimum.

It is from this background that the pioneering work on ionic liquids was commenced.

Ionic liquids have the potential to make ideal green solvents as they have negligible vapour pressure and do not evaporate into the atmosphere, making them a more environmentally responsible material than traditional organic solvents.

They can be recyclable, and different combinations of ions make solutions that can dissolve a large range of substances that include coal, plastics, metals, and rocks.

Ionic liquids are relatively undemanding and inexpensive to manufacture.

Ionic liquids - a brief overview.

Ionic liquids are usually associated with high temperature processes, as ionic substances are often comprised of two very small ions with opposite electrical charges.

The similar size and shape of these ions ensures that the electrical attraction between them is so strong that it requires an enormous amount of energy to make the ionic bond.

A common salt melts at around 800C.

A high temperature molten salt would be an unsuitable solvent for heat sensitive organic molecules, but the melting point can be lowered by producing ionic liquids from bulky, asymmetrical ions that can only loosely fit together.

As less of the ions attractive forces are utilised - due to their ill fitting nature - the rest can attract and so dissolve other compounds.

Ambient temperature ionic liquids typically consist of a heterocyclic cation based on substituted imidazole or pyridine and an inorganic anion such as [PF6]-.

By changing the anion or alkyl chain of the cation, one can vary the hydrophobicity, viscosity, density, and solvation of the ionic liquid system.

The composition of ionic liquids may be adjusted to control their acidity or basicity.

Ionic liquids are often composed of poorly co-ordinating ions meaning they have the potential to be highly polar co-ordinating solvents that can be useful when using transition metal based catalysts.

They are good solvents for a range of inorganic and organic materials and unusual combination of reagents can be solved into a single phase.

Ionic liquids appear to have a high capacity for organic solutes with up to 50% v/v solutions of benzene having been reported.

Ionic liquids can allow easy separation of organic molecules by direct distillation without losing any of the ionic liquid.

The constituents of ionic liquids are constrained by coulombic forces and exert practically no vapour pressure above the surface of the liquid.

This may allow the development of novel recovery schemes for certain organic species in relation to normal liquid/liquid extraction in which the product recovery can be affected by distillation.

The liquid range can be as large as 300C which is higher than that of water and offers the potential for considerable kinetic control of extractive processes.

The major disadvantage that currently exists with ionic liquids is that there is little information on their properties, how they work, or how effective they are in various applications - hence the importance of work performed by Quill and others.

Chromatography is a method for separating mixtures of substances using two phases, one of which is stationary and the other mobile moving in a particular direction. Chromatography techniques are divided up according to the physical states of the two participating phases.

The term in exchange chromatography or ion chromatography (IC) is a subdivision of high performance liquid chromatography (HPLC).

A general definition of ion chromatography can be applied as follows: "ion chromatography includes all rapid liquid chromatography separations of ions in columns coupled online with detection and quantification in a flow-through detector".

A stoichiometric chemical reaction occurs between ions in a solution and a solid substance carrying functional groups that can fix ions as a result of electrostatic forces.

For anion chromatography these are quaternary ammonium groups.

In theory ions with the same charge can be exchanged completely reversibly between the two phases.

The process of ion exchange leads to a condition of equilibrium, the side to which the equilibrium lies depends on the affinity of the participating ions to the functional groups of the stationary phases.

Ion chromatography can be used to successfully determine the quantity of chloride present in an ionic liquid.

In many cases, chloride acts as a catalyst poison so it is thus important to measure the amount of chloride and if possible try to minimise its presence by taking appropriate control steps during synthesis.

Many of the reactions carried out in ionic liquids are catalytic, using a variety of heterogeneous and homogeneous catalysts.

One of the easiest methods for producing ionic liquids is to make the chloride salt such as the reaction of methylimidazole with 1-ethylchloride to give 1-ethyl-3-methylimidazolium chloride.

The salt can then be transformed into other salts through metathesis but one is left with a small amount of residual chloride which can be quantified using ion chromatography.

It is possible to make ionic liquids without producing the chloride salt.

For example the reaction of diethylsulphate with methylimidazole produces 1-ethyl-3-methylimidazolium ethylsulphate [C2mim][EtSO4], which is chloride free.

This can be combined with the organic salt, 1-butyl-3-methylimidazolium chloride [C4mim]Cl, where the chloride component can be determined using ion chromatography. Method for determination of chloride in sample of [C2mim][EtSO4] combined with [C4mim]Cl.

1g of the ionic liquid system was solved into 10ml of acetonitrile before being diluted up to 50ml with deionised water.

20ml of the diluted ionic liquid was injected directly into the Metrohm 761 Compact IC.

The response for the peaks was recorded using a mobile phase eluent of sodium carbonate/sodium bicarbonate with the Cetac AN2 analytical column.

The calculation was carried out automatically using integration software 761 Compact IC 1.1 against a previously prepared calibration plot There are no external displays or switches on the instrument, all the hardware is fully controlled via a single RS232 connection between the IC and the PC.

All the instrument parameters can be called upon with a click of the mouse.

80 system configurations with more than 300 applications are stored within the software and it is also possible to download further systems from the Metrohm homepage for direct use with the software.

The 761 Compact IC comprises a low-pulsation dual-piston pump, pulsation dampner, electromagnetic injection valve, two channel peristaltic pump, conductivity detector, eluent organiser as well as a data recording and processing module.

All the components that come into contact with the eluent and sample are metal-free.

The detector is the heart of every ion chromatograph.

The Metrohm detector's temperature varies by less than 0.01C and can be optimally adapted to the ambient conditions.

This outstanding temperature stability reduces interference and allows exact conductivity measurements.

Two hardware configurations are available; either with direct conductivity detection or additionally with chemical suppression using the Metrohm Suppressor Module (MSM).

When a large sample throughput is required, it is recommended to automate the system with the Metrohm autosamplers; the 766 IC Sample Processor or the 788 IC Filtration Sample Processor.

Conclusion.

Ion chromatography as an analytical technique has seen an enormous surge in popularity, due partly to the simplicity of many of the methods as well as other factors such as market forces driving down the expenditure costs of the instrumentation and an improved power.

Different ionic liquid systems can be investigated using ion chromatography, for example [C2mim][EtSO4] contains no chloride whereas [C2mim][EtSO4] combined with [C4mim]Cl does contain chloride which can be verified and quantified.

Ion chromatography can be used as a quantitative tool during the development and synthesis of novel ionic liquid systems to determine whether or not certain species are present.

The low running costs of ion chromatography with Metrohm instruments are surprisingly low requiring only the acquisition of chemicals required for the eluent and suppressor module as well as a clean, reliable source of deionised water.

A major advantage of ion chromatography is that often little sample preparation is required as long as the sample is in an ionic, homogenous form. Only a very small amount of sample is required for the analysis and the quantified results obtained, often, within a matter of minutes.

Ion chromatography is a clean technique as all the reagents are enclosed, its robustness and reliability are assured demonstrating precisely the reason why it is rapidly becoming the method of choice for many analysts.

References.

The following websites were used extensively as a references and can be used to obtain further information:- www.chem.orst.edu www.ch.qub.ac.uk www.isis.rl.ac.uk www.metrohm.com www.nature.com www.metrohm.co.uk