Proton transport is crucial in various biochemical and electrochemical processes. The Grotthuss mechanism, involving proton transfer between host molecules via hydrogen bonds, is considered the most efficient transport mechanism. While proposed over 200 years ago, direct experimental evidence has been lacking. This study provides the first experimental observation of proton transfer in pure and 85% aqueous phosphoric acid using dielectric spectroscopy, quasielastic neutron and light scattering, and ab initio molecular dynamics simulations. Protons move via surprisingly short jumps (0.5-0.7 Å), much smaller than in ionic liquids. Correlated proton jumps were observed, but these correlations surprisingly reduce conductivity, contradicting the expected Grotthuss enhancement. The analysis suggests that Grotthuss-like conductivity enhancement is not achievable in bulk liquids due to inherent ionic correlations.

Ivan Popov, Zhenghao Zhu, Amanda R. Young-Gonzales, Robert L. Sacci, Eugene Mamontov, Catalin Gainaru, Stephen J. Paddison, Alexei P. Sokolov