Rheology of the Electric Double Layer In Ionic Liquid Solutions: Effects of Ion Concentration and Anion Structures

Ionic liquids (IL) are molten salts with melting temperatures below 100°C. Combined with other unique properties such as high thermal and electrochemical stability, non-volatility and high ionic conductivity, they have been used as electrolytes in batteries and lubricants, where the viscoelasticity of the electric double layer (EDL) plays an important role. Due to the small length scale of EDL thickness, it has been challenging to characterize their viscoelastic properties at nanoscale. Herein, by using a quartz crystal microbalance (QCM), we measure the changes in the resonant frequency and energy dissipation of a gold-coated quartz crystal on which IL solutions are deposited. Since the gold surface of the quartz crystal is negatively charged at an open circuit potential, we can estimate the loss modulus of the EDL near the charged surface through a wave propagation model. Using this approach, we investigate the viscoelastic properties of the EDL formed on the quartz crystal from three ionic liquids with different anions: 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (Bmim-TFSI); 1-butyl-3-methylimidazolium trifluoromethanesulfonate (Bmim-TfO); 1-butyl-3-methylimidazolium tetrafluoroborate (Bmim-BF4). We find that the loss modulus of the EDL increases rapidly with increasing ionic liquid concentrations in the low concentration regime, but reaches about 3 orders of magnitude larger than the loss modulus of the bulk solution in the higher concentration regime. This behavior is independent of the type of anions. Our results can provide useful information on the rheological properties of the EDL in IL solutions, optimizing IL-based material design with improved system performances.