Ion Diffusion\nReveals Heterogeneous Viscosity in Nanostructured\nIonic Liquids

Many ionic liquids (ILs) are composed of interpenetrating\npolar\nand apolar networks. These nanoscale networks are sustained by different\nlocal intermolecular and electrostatic interactions and are predicted\nto differ in their physical properties by orders of magnitude. Nonetheless,\nit is commonplace for the physical properties of ILs to be described\nby bulk parameters, such as the bulk dynamic viscosity. This study\naddresses the limitations of using bulk parameter descriptions in\nnanostructured ILs by applying the Saffman-Delbrück model to\ninterpret the self-diffusion coefficient of ions within the homologous\nseries of [C<sub>n</sub>mim][NTf<sub>2</sub>] ILs. We demonstrate\nthat pulsed field gradient NMR spectroscopy can effectively probe\nthe relative viscosities of polar/charged and apolar networks within\nthese pure ILs. Our calculated polar viscosities show good agreement\nwith literature simulations. Our approach provides valuable insights\ninto the local viscoelastic environments within nanostructured media.\nThis work not only contributes to the understanding of mass and charge\ntransport in ILs but also offers a new experimental perspective for\nstudying structured fluids more broadly.