Picosecond Fickian Proton Diffusion in a Deep Eutectic Solvent Revealed by Backscattering QENS
Adam E. Imel, Thomas Simunovic, Shane Foister, Kevin J. Ledford, Brian Barth, Katherine D. Vogel, Mrinalini Ayilliath Kolaprath, Bhavya Sharma, Mark Dadmun, Eugene Mamontov, Thomas ZawodzinskiAbstract
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.