Illuminating the “wet” electron

One of the strangest-sounding puzzles of chemistry is that of the “wet electron”: when an excess electron is added into liquid water, some molecules re-organize themselves to form a cavity around it. When this happens, the electron is referred to as “hydrated”.

Even though the hydrated electron is well studied, we know little about how it is actually formed. Recent experiments have suggested that the excess electron goes through two changes before it finally becomes hydrated: the “quasi-free” electron stage, where the excess electron is fully de-localized in the water, and the “wet” electron stage, which is still a mystery.

It’s not just a fundamental question; the hydrated electron is involved in reduction reactions in aqueous chemistry or even in mutagenic lesions in the DNA of cells. Which means that understanding the response of liquid water to an excess charge is important for a number of applications.

Now, scientists at EPFL’s Institute of Physics have performed advanced computer simulations to clarify the mechanism leading to the formation of the hydrated electron and to deliver a well-rounded picture of the wet electron’s properties. The research, now published in Chemical Science, was led by Professor Alfredo Pasquarello and carried out in the context of his Master’s course at EPFL.

“We found that, before the formation of the hydrated electron, the excess electron repeatedly switches between the quasi-free electron and wet electron precursor states,” says Michele Pizzochero, the first author of the study. The study also revealed that the wet electron occurs when the excess electron partially breaks the hydrogen-bond network of liquid water. All this happens before the entire system (water plus excess electron) reaches the thermodynamic equilibrium.

The work is the first to determine the atomic structure and the energy level of the wet electron. “Overall, our findings shed new light on the elusive wet electron, and offer an appealing theoretical picture to the interpretation of experimental observations,” says Pizzochero.