A new affinity for electrons and water
US scientists find a key value used in molecular engineering is wrong. Andrew Masterson reports.
Models predicting the behaviour of electrons interacting with water are wide of the mark, new research shows.
The finding, made by a team led by molecular engineer Alex Gaiduk of the University of Chicago in the US, means that standard calculations used to assess a wide range of real-world physical phenomena, from digestion to bridge corrosion, are inaccurate.
At the basis of the faulty calculations lies a problem that until now has proven impossible to solve. It concerns a property known as the “electron affinity of water”.
In a wide set of chemical reactions, electrons strike water. When this occurs, the electron is “captured” by the water, which gains energy as a result. It is not a trivial effect: electron affinity affects the speed and magnitude of many processes, including food breakdown in the gut, rust on iron exposed to water, and the splitting of water into oxygen and hydrogen by photoelectrochemical cells.
Although the nature of the reaction is well known, the precise value for it has never been determined.
“Most of the results quoted in the literature as experimental numbers are actually values obtained by combining some measured quantities with crude theoretical estimates,” explains co-author Giulia Galli.
The reason for the uncertainty is simple: simulating the interactions of electrons with water is extremely complex and requires enormous amounts of costly computational grunt.
To do so, the researchers united refined theoretical frameworks, derived by Gaiduk and others at the University of Chicago, liquid water models created by University of California biochemist Francesco Paesani, and a set of results obtained from the Argonne National Laboratory in Chicago and the Lawrence Livermore national Laboratory in California.
In particular, the scientists sought a better understanding of the precise mechanism by which electrons and water interact – and the energy gain involved.
The results were surprising.
“We found large differences between the affinity at the surface and in the bulk liquid,” says co-author Tuan Anh Pham. “We also found values rather different from those accepted in the literature, which prompted us to revisit the full energy diagram of an electron in water.”
The team found that although the ionisation potential of surface and bulk water are almost identical, the electron affinity differs markedly. Importantly, the energy gain from electron-binding is much lower than previously assumed.
The findings will enable researchers and engineers in other fields to refine their own models, resulting in more accurate predictions for oxidisation rates in aqueous environments – with potential boons for safety and expenditure.