A model of how protons pass through water has been vindicated using X-ray spectroscopy. Surprisingly, the model it replaces dates back to 1806, despite the fact that protons were not discovered until more than 100 years later.
At the beginning of the 19thu century, the discovery of the electric cell made possible many experiments in electricity, but observation trumped theory. Having discovered electrolysis, scientists tried to explain how electricity could pass through water. Remarkably, Theodor Grotthuss provided a solution that has proven remarkably robust, despite the vast gaps in his knowledge.
However, physical chemists have long known that Grotthuss’s explanation requires modification. In 2005, Professor Ehud Pines of the Ben-Gurion University of the Negev presented his progress on the Grotthuss mechanism, but struggled to gain acceptance for his proposal. Now, however, a paper published in Angewandte Chemie supports Pines’ theory and hopes it will finally win over the doubters.
According to Grotthuss, charge is transferred from one water molecule to another like buckets between people seeking to put out a fire before the tube grows. Not only did Grothus not know about protons, but (like his contemporaries) he believed that the chemical formula of water was HO, not H2O. However, what became known as “proton hopping” remained the basis for scientific thinking about why protons diffuse so easily in an electric field in the face of the random motions of water molecules.
Pines has a more sophisticated version, in which each proton creates a train of three water molecules. Like Gromit inside The wrong pantsPine suggests that trains “build the tracks” in front of them, allowing them to move through the water as if on rails and dismantle the track behind them to rebuild.
Pine acknowledges that he is not the first to suggest something along these lines, but he claims to have identified the true molecular structure formed by the proton train.
“Discussions about the Grotthuss mechanism and the nature of proton solvation in water have increased,” Pines said in a statement, “as this is one of the most fundamental challenges in chemistry. Understanding this mechanism is pure science, which pushes the boundaries of our knowledge and changes one of our fundamental understandings of one of nature’s most important mass and charge transport mechanisms.”
In the face of such doubt, Pines assembled a team that developed custom-built flatjet X-ray equipment to observe the passage of protons through water. In recognition of the importance with which the subject is being treated among chemists, the equipment alone cost millions of dollars, but the European Research Council deemed it worth the investment.
The experiment revealed that each proton changes the electron structure of the oxygen atoms in three water molecules, but to a different extent, creating a train of three molecules, with much weaker effects on more distant molecules.
“Everyone has been thinking about this problem for over 200 years, so it was quite a challenge for me to decide to tackle it,” Pines said. “Seventeen years later, I’m satisfied that I probably found and demonstrated the solution.”
University chemistry textbooks still use the Grotthuss Mechanism to explain the progress of a proton in water, and a copy may be required before they move to the “Pine Mechanism”. However, the paper raises the possibility that fundamental progress has been made.
With electrolysis carrying many of the world’s hopes for a low-carbon future, anything that contributes to a better understanding could be valuable. The mechanism could also lead to improvements in hydrogen fuel cells and membrane design.
The work is published in Angewandte Chemie.