Ion Diffusion Reveals Heterogeneous Viscosity in Nanostructured Ionic Liquids

Abstract

Many ionic liquids (ILs) are composed of interpenetrating polar and apolar networks. These nanoscale networks are sustained by different local intermolecular and electrostatic interactions and are predicted to differ in their physical properties by orders of magnitude. Nonetheless, it is commonplace for the physical properties of ILs to be described by bulk parameters, such as the bulk dynamic viscosity. This study addresses the limitations of using bulk parameter descriptions in nanostructured ILs by applying the Saffman-Delbrück model to interpret the self-diffusion coefficient of ions within the homologous series of [C_nmim][NTf₂] ILs. We demonstrate that pulsed field gradient NMR spectroscopy can effectively probe the relative viscosities of polar/charged and apolar networks within these pure ILs. Our calculated polar viscosities show good agreement with literature simulations. Our approach provides valuable insights into the local viscoelastic environments within nanostructured media. This work not only contributes to the understanding of mass and charge transport in ILs but also offers a new experimental perspective for studying structured fluids more broadly.