A way to crystallise ionic liquids with very low melting points will make it easier to determine their structure, and could be used to purify them.

Neil Winterton and colleagues at the University of Liverpool, UK, grew crystals of two imidazolium-based ionic liquids, with melting points of around -26 °C, using a zone-melting method. The researchers sealed a small amount of the ionic liquid in a capillary tube and cooled it to below  -40 °C. A small section of the solid ionic liquid was heated with a laser, creating a molten zone that could be moved along the length of the tube. Further cycles of heating and cooling resulted in single crystals of the ionic liquid.

Janet Scott, deputy director of the Centre for Green Chemistry, Monash University, Melbourne, Australia, highlighted the importance of the technique for those interested in the structures and properties of the materials. 'This technique allows one to obtain good quality single crystals for diffraction experiments,' Scott said, 'which will make it possible to probe the structure of these fascinating materials, giving information about the structure and intermolecular interactions, some of which may help to explain liquid phase properties.'

Winterton agreed that the method could be used to grow crystals for the determination of the ionic liquids solid-state structures, but he also has other ideas for the technique. 'Zone-melting methods have been used industrially to purify metals at high temperature,' said Winterton. 'We are now interested in investigating the extent to which zone-melting can be used to remove impurities from ionic liquids.'

Susan Batten