Ether-functionalized Imidazolium Ionic Liquids
Laura LAIMINA1, Eduards BAKIS1
1University of Latvia Faculty of Chemistry, Riga, Latvia
Ionic liquids (ILs) are ambient-temperature liquid organic salts composed entirely of ions. Combination of organic cations and inorganic anions allows for the creation of ILs with desirable physical properties such as low melting points, relatively low viscosities, electrochemical stability, and relatively high ionic conductivity. These properties are important for ILs to be used as electrolytes in batteries, fuel cells, and supercapacitors.
It is challenging to choose the most favorable cation-anion combination to achieve desired properties. Fluorinated anions produce low IL melting points, relatively low viscosities, and electrochemical stability. Studies of PEG ethers have revealed that these can form complexes with metal ions such as Na+. This is one of the reasons why ether-functionalized ILs have potential use as fillers for sulfonated poly(ether ether ketone) (SPEEK) membranes to improve the Na+ ion conduction. Additionally, ether functionalization has demonstrated to significantly reduce pure IL viscosities and densities.
In this work, we have developed multigram synthetic routes to novel PEG-monomeric and PEG-dimeric imidazolium ILs based on bis-(trifluoromethanesulfonyl)imide [NTf2]- anion. IL structures and purity have been validated using 1H and 13C NMR spectroscopy and high-resolution mass spectrometry. Density and viscosity analysis has been carried out using rolling ball viscometry and vibrating tube densimetry.
To understand the role of connectivity between the PEG chain and the ionic core, the linker length in cations has been varied from one (3, 4, 5, 6,7) to two (1, 2) carbon atoms. To facilitate cooperative interactions of the two chains with plausible solute ions, the chains have been placed in a parallel (1,2-substituted) fashion (4). Controls (1,3-substituted) have also been prepared (5, 6,7). As a longer ether chain may better coordinate plausible solute ions, the ether sidechain length has been systematically extended from one to three oxygen atoms.
The results of this study reveal that even minor structural variations can lead to significant changes in IL physical properties. Further work on using these ILs for crafting and characterization of IL-functionalized SPEEK membranes for Na+ batteries is currently underway.
References
[1] F. Philippi, D. Rauber, B. Kuttich, T. Kraus, C. Kay, R. Hempelmann, P. Hunt, T. Welton, J.Chem. Phys. Chem. Chem. Phys. 2020, 22, 23038-23056.
Acknowledgements
This work was supported by the Latvian Council of Science (Grant No. lzp-2020/1-0391)