An age-old physical chemistry conundrum is finally solved

How does a proton move through water? The theory that chemist Theodor Grotthuss proposed over 200 years ago may have satisfied other scientists, but not Prof. Ehud Pines of Ben-Gurion University of the Negev (BGU), Israel. Now, after 17 years of research, he has found vindication through another group of researchers who – with support from the EU-funded projects XRayProton and SMART-X – replicated his experiment, revealing the solution he has put forth all along. The study, of which Prof. Pines is a co-author, was published in the international edition of ‘Angewandte Chemie’.As described in a news item posted on BGU’s website, what Prof. Pines had previously suggested was that the proton moves through water in chains, or “trains”, of three water molecules: “The proton train ‘builds the tracks’ underneath them for their movement and then disassembles the tracks and rebuilds them in front of them to keep going. It’s a loop of disappearing and reappearing tracks that continues endlessly.” Although similar theories had been proposed by different scientists in the past, the news item reports that the water molecules “were not assigned to the correct molecular structure of the hydrated proton which by its unique trimeric structural properties leads to promoting the Grotthuss mechanism.”

Prof. Pines explains: “The debates on the Grotthuss mechanism and the nature of proton solvation in water have grown heated, as this is one of the most basic challenges in chemistry. Understanding this mechanism is pure science, pushing the boundaries of our knowledge and changing one of our fundamental understandings of one of Nature''s most important mass and charge transport mechanisms.”

To tackle the global scientific community’s reluctance to accept the solution of a hydrated proton accommodated by a chain of three water molecules, Prof. Pines joined forces with a team of researchers from Germany and Sweden. Led by study senior author Dr Erik Nibbering of the Max Born Institute for Nonlinear Optics and Short Pulse Spectroscopy, Germany, the team replicated the experiment. They X-rayed the chemical system using specially designed equipment financed by the European Research Council, confirming Prof. Pines’ findings: the presence of the proton has the greatest effect on three water molecules – affecting each to a different extent – and, together with the proton, these molecules form protonated three-water molecule trains.

“Everyone thought about this problem for over 200 years, so that was a sufficient challenge to me to decide to take it up. Seventeen years later, I am gratified to most likely have found and demonstrated the solution,” states Prof. Pines in the news item.

The study supported in part by XRayProton (Ultrafast Structural Dynamics of Elementary Water-Mediated Proton Transport Processes) and SMART-X (Study of carrier transport in MAterials by time-Resolved specTroscopy with ultrashort soft X-ray light) demonstrates a new approach that focuses on a gradual transition from strong orbital interactions of the hydrated proton with the closest water molecules to orbital-energy shifts induced in more distant water molecules. As reported in the study, these findings “will be of much use in steady-state investigations of hydrated protons and time-resolved studies of the underlying mechanisms of aqueous proton transport.”

For more information, please see:

SMART-X project website

XRayProton project