Water influence on ionics

New research on ionic liquids — molten salts with applications as ‘green’ solvents — has revealed that they have an unusual property in the presence of water.

According to Jason Widegren and co-workers at the US National Institute of Standards, the liquids react to small amounts of water with a dramatic fall in viscosity.

Researchers in the ionic liquids field have always found it difficult to obtain reliable readings of viscosity, with published results differing by 30% or more.

The NIST research reveals why this might be so. For one of the solvents the team studied, a 0.01% increase in water dissolved in the sample led to a 1% drop in flow resistance — a hundred-fold effect.

Even the slightest contamination with water vapour absorbed from the air can lead to dramatic changes in viscosity, Widegren says. The team had to dry their instruments and samples carefully, and measure water content before and after each viscosity measurement to obtain their results.

Ionic liquids, which are liquid at room temperature, are not flammable and do not evaporate, have great potential in industrial processes, but the difficulty in measuring their viscosity has delayed their uptake.

Viscosity is one of the parameters of a solvent which must be known precisely before processes can be designed. Now that their unusual behaviour is better understood, their variable viscosity might even become useful in designing processes such as chemical separations.