Search for a Grotthuss mechanism through the observation of proton transfer

The transport of protons is critical in a variety of bio- and electro-chemical processes and technologies. The Grotthuss mechanism is considered to be the most efficient proton transport mechanism, generally implying a transfer of protons between 'chains' of host molecules via elementary reactions within the hydrogen bonds. Although Grotthuss proposed this concept more than 200 years ago, only indirect experimental evidence of the mechanism has been observed. Here we report the first experimental observation of proton transfer between the molecules in pure and 85% aqueous phosphoric acid. Employing dielectric spectroscopy, quasielastic neutron, and light scattering, and ab initio molecular dynamic simulations we determined that protons move by surprisingly short jumps of only similar to 0.5-0.7 angstrom, much smaller than the typical ion jump length in ionic liquids. Our analysis confirms the existence of correlations in these proton jumps. However, these correlations actually reduce the conductivity, in contrast to a desirable enhancement, as is usually assumed by a Grotthuss mechanism. Furthermore, our analysis suggests that the expected Grotthuss-like enhancement of conductivity cannot be realized in bulk liquids where ionic correlations always decrease conductivity.

Automated summary

The transport of protons is critical in various biochemical and electrochemical processes. The Grotthuss mechanism, proposed more than 200 years ago, is considered as the most efficient proton transport mechanism. However, direct experimental evidence has been lacking until now. In this study, we provide the first experimental observation of proton transfer between molecules in phosphoric acid solutions. Our analysis reveals that protons move by short jumps of approximately 0.5-0.7 angstroms, contrary to previous assumptions. Moreover, these proton jumps show correlations that actually reduce conductivity, challenging the expected enhancement predicted by the Grotthuss mechanism.