Revealing intrinsic electric double layer viscoelasticity in ionic liquid solutions via quartz crystal microbalance

Ionic liquids (IL) are room-temperature molten salts that function as complex fluids with tunable interfacial and bulk properties, making them attractive for applications ranging from electrochemical energy storage to lubrication. In such systems, the viscoelasticity of the electric double layer (EDL) at charged interfaces can strongly influence performance, yet its characterization remains challenging due to the nanometric EDL thickness. Herein, we use quartz crystal microbalance (QCM) to measure changes in the resonant frequency and energy dissipation of a gold-coated quartz crystal upon deposition of IL solutions. Since the gold surface of the QCM 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 under non-confining conditions. Using 1-butyl-3-methylimidazolium (Bmim)-based ILs with three distinct anions-bis(trifluoromethanesulfonyl)imide (TFSI), trifluoromethanesulfonate (TfO), and tetrafluoroborate (BF₄), we find that the EDL loss modulus increases sharply with increasing IL concentrations in the low concentration regime, eventually reaching values up to three orders of magnitude higher than that of the bulk solution and saturating at high concentrations. Notably, this concentration-dependent scaling is consistent across the three anion types tested, in contrast to reports for nanoconfined ILs where ion identity markedly affects this behavior. Our results demonstrate that bulk viscoelastic properties can be used to infer the EDL loss modulus under non-confining conditions, providing a practical framework for engineering soft, ion-rich interfaces in electrochemical and tribological systems.