Tuning Binary Ionic Liquid Mixtures: Linking Alkyl Chain Length to Phase Behavior and Ionic Conductivity

The use of mixed salts to generate new composite ionic liquids (ILs) provides a facile means of readily tuning or tailoring the desired properties of ionic media. Despite this, very little information is available about how the structure of the selected ions and composition impacts the properties of salt mixtures. To explore this, six binary IL1-IL2 mixtures based on N-alkyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide salts have been characterized. The physicochemical properties (density, viscosity, and ionic conductivity) and phase behavior of these mixtures are reported. The variation of the alkyl chains lengths on the cations plays a significant role in determining both the phase behavior and the physicochemical properties of the mixtures. Notably, the "tunability" of the properties of the IL mixtures is much easier to control than is found by simply making small structural changes to the ions in a given salt.