Cooperative Relaxation Governs the Viscosity of Ionic Liquids

Abstract

Ionic liquids are promising solvents for batteries, biomass processing and critical mineral recovery, but their viscosities can exceed those of conventional molecular solvents by several orders of magnitude, impeding mass and charge transport and complicating process design. Using complementary neutron spin echo spectroscopy and nuclear magnetic resonance diffusometry, we probe local and long-range dynamics of three protic ionic liquids over time scales from 10^–12 to 10^–3 seconds. We identify a fast relaxation with closely corresponding self and collective time scales, indicating cooperative rearrangement of the surrounding molecular network. Its characteristic time scale correlates with bulk viscosity, while its temperature dependence reveals distinct Arrhenius activation energies depending on whether extended networks are dominated by hydrogen bonding or electrostatic interactions. These results link cooperative network relaxation to macroscopic viscosity and show how molecular structure can tune the energetic cost of structural relaxation through network connectivity.