The question of how protons move through water in an electric field has fascinated scientists for centuries. Now, more than 200 years after the last major glimpse of the phenomenon, scientists have some clarity.
In 1806, Theodor Grotthuss proposed a hypothesis, which became known as the Grotthuss mechanism for “proton hopping”, about how a charge could flow through a water solution.
Although Grotthuss’s hypothesis was very cutting edge for its time, before protons were even known, or even the actual structure of water, modern researchers have long known that it did not provide a complete understanding of what happened in a moment. molecular level.
The latest findings on the subject may have solved the mystery by solving the long-elusive electronic structures of hydrated protons.
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 passes.
This loop can continue indefinitely to transport protons through the water. Although the idea has been suggested before, the new study assigns a different molecular structure that better fits the solution proposed by Grotthuss, according to the study authors.
“Debates about the Grotthuss mechanism and the nature of proton solvation in water have intensified, as this is one of the most basic challenges in chemistry,” says chemist Ehud Pines of Ben-Gurion University in 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 the proton charges affected the electrons of the individual oxygen atoms in water.
As predicted, the impact was greatest in three water molecules, although to varying degrees in each individual molecule within the trimeric cluster. The researchers found groups of three molecules forming chains with the proton.
The researchers also incorporated quantum-level chemical simulations and calculations to determine the interactions between the protons and neighboring water molecules as the protons move through the 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 charge and mass transport mechanisms,” says Pines.
The discovery has implications for many other chemical processes, including photosynthesis, cellular respiration and energy transport in hydrogen fuel cells.
Not only is the solution remarkable, but so is how the researchers were able to arrive at it: testing and validating theoretical predictions with experimental results, and vice versa, in a long and winding process that has taken nearly two decades from start to finish.
“Everyone thought about this problem for over 200 years, so it was enough of a challenge for me to decide to take it on,” says Pines. “Seventeen years later, I am pleased to have found and demonstrated the solution.”
The research has been published in Angewandte Chemie International Edition.