Interplay between cationic head-groups and oxygen-containing side-chains and the effects on the physicochemical properties of ionic liquids

I&EC 6

Will Pitner, william-robert.pitner@merck.de, Anja Amann, Jens Eichhorn, and Ingrid Skupch-Martinez. IOLI R&D Laboratories, LSA, Merck KGaA, Frankfurter Straße 250, Darmstadt, D-64271, Germany
The effects of n-butyl, 3-hydroxy-n-propyl, 2-methoxyethyl and ethoxymethyl side-chains and pyridinium, N-methylimidazolium, N-methylpyrrolidinium, N-methylpiperidinium, N-methylmorpholinium and N,N-dimethylethylammonium head-groups on the conductivity, viscosity, density, phase transitions, thermal and electrochemical stability and relative hydrophobicity of ionic liquids containing the bis(trifluoromethylsulfonyl)imide anion have been investigated. Clear trends can be seen moving from different cation head-groups: for example, the N-methylimidazolium head-group leads to lower viscosities and higher conductivities, whereas the N-methylmorpholinium head-group increases viscosities and lowers conductivities. Some trends also become immediately obvious in changes in the side-chains: 3-hydroxy-n-propyl-substituted ionic liquids are denser than ethereal-substituted, which in turn are denser than n-butyl-substituted ionic liquids. However, the effect of the side-chains on viscosity and conductivity is subtler, and, especially for ethereal side-chains, is strongly dependent upon the head-group. The dependence of electrochemical and thermal stability upon the choice of head-group is well established, but side-chains also play a role in determining ionic liquid stability.