Ionic Liquids

Green Chemistry Jemma Vickery jv2237@bris.ac.uk

Introduction

What are Ionic Liquids?

Ionic liquids are highly solvating, non-coordinating medium in which a variety of organic and inorganic solutes are able to dissolve. They are outstanding good solvents for a variety of compounds, and their lack of a measurable vapour pressure makes them a desirable substitute for VOCs. Ionic liquids are attractive solvents as they are non-volatile, non-flammable, have a high thermal stability and are relatively inexpensive to manufacture. They usually exist as liquids well below room temperature up to a temperature as high as 200^oC.

The key point about ionic liquids is that they are liquid salts, which means they consist of a salt that exists in the liquid phase and have to be manufactured, they are not simply salts dissolved in liquid. Usually one or both of the ions is particularly large and the cation has a low degree of symmetry results, these factors result in ionic liquids having a reduced lattice energy and hence lower melting points.

The first room temperature ionic liquid [EtNH₃] [NO₃], was discovered in 1914 but this did not initiate a huge amount of interest in ionic liquids until the development of binary ionic liquids from mixtures of aluminium (III) chloride and N-alkylpyridinium or 1,3-dialkylimidazolium chloride.

Examples of an alkylpyridinium cation is shown below:

Examples of dialkylimidazolium cations are shown below:

Problems with Ionic Liquids:

Extracting the chemical product from the ionic liquid in pure form can pose a problem. Water soluble compounds can easily be extracted with water and distillation can be used to separate compounds with high vapour pressure, however higher temperatures would be required to extract chemical products with low vapour pressures which will most likely result in the decomposition of the chemical product.

See references (8, 9)