Picosecond Fickian Proton Diffusion in a Deep Eutectic Solvent Revealed by Backscattering QENS

We report anomalous continuous Fickian proton diffusion in the 40 mol % imidazole/levulinic acid deep eutectic solvent, a transport signature lacking a resolvable jump length or residence time and marking the first observation in a liquid proton conductor. In every hydrogen-bonded proton conductor previously characterized by quasi-elastic neutron scattering (QENS), Grotthuss-type transport manifests as jump diffusion with a finite step length set by the hydrogen-bond spacing. Using isotopic contrast QENS difference-spectrum analysis to isolate the acidic proton dynamics from molecular background, we find that molecular species undergo conventional jump diffusion. In contrast, the acidic proton exhibits purely quadratic q dependence (Γ ∝ q2) across the entire picosecond time scale measurement window.