After 200 Years, a Key Physical Chemistry Mystery May Have Been Solved

There's more evidence for how protons move through water. (Newannyart/iStock/Getty Images Plus)

The question of how precisely protons move through water in an electric field has fascinated scientists for centuries. Now, more than 200 years after the last major insight into the phenomenon, scientists have some clarity. 

In 1806, Theodor Grotthuss put forward a hypothesis, which came to be known as the Grotthuss mechanism for 'proton jumping', about how a charge might flow through a solution of water. 

While Grotthuss's hypothesis was very forward-thinking for its time – coming before protons, or even the actual structure of water, were even known about – modern-day researchers have long known that it didn't provide a complete understanding of what happened at a molecular level. 

The latest findings on the topic may have figured out the mystery by resolving the electronic structures of hydrated protons that have for so long remained elusive. 

The findings suggest that protons move through water in 'trains' of three water molecules, with 'tracks' built in front of the train as it goes and pulled up once it's passed. 

This loop can carry on indefinitely to transport protons through water. While the idea has been suggested before, the new study assigns a different molecular structure that fits better into the solution proposed by Grotthuss, according to the study authors. 

"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," says chemist Ehud Pines from the Ben-Gurion University of the Negev in Israel. 

The new study is compelling because it combines a theoretical approach with physical experimentation made possible by recent technological advances. The researchers used an X-ray absorption spectroscopy (XAS) experiment to monitor how proton charges affected electrons in the single oxygen atoms of water. 

As predicted, the impact was greatest on three water molecules, though to different extents on each individual molecule within the trimeric cluster. Researchers found the groups of three molecules forming chains with the proton. 

The researchers also incorporated chemical simulations and calculations at the quantum level to determine the interactions between protons and neighboring water molecules as protons move through liquid. 

"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," says Pines. 

The discovery plays into many other chemical processes, including photosynthesis, cell respiration, and energy transport in hydrogen fuel cells. 

It's not just the solution that's notable but also how the researchers were able to get to it – testing and validating theoretical predictions against experimental results, and vice versa, in a long, winding process that has taken almost two decades from beginning to end. 

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

The research has been published in Angewandte Chemie International Edition.