Water Can Spontaneously Form Hydrogen Peroxide, and We Finally Understand Why

Since we’ve been plunged into a global pandemic, we’re probably much more aware of being surrounded by micro-droplets of water. In moist air or expelled by coughing, sneezing or talking, these droplets can carry tiny particles with them, from pollution to viruses like the flu.

In 2019, the basic chemistry of these environmental water particles created a surprise: researchers found that ordinary, otherwise benign water droplets could somehow spontaneously form small but significant amounts of hydrogen peroxide. (H2O2).

Yes, the same weakly acidic substance we use to bleach hair or disinfect wounds.

Now, a team of researchers has discovered that the spontaneous reaction occurs upon contact with solid surfaces and may play a role in flu seasonality.

“We think we know so much about water, one of the most commonly encountered substances, but we’re humbled,” says Stanford University chemist Richard Zare, who was involved in both discoveries.

In bulk, water is fairly stable, but breaking it up into tiny droplets seems to drastically change its behavior in comparison.

As the size of objects decreases, their relative volume decreases faster than their area. This means that a tiny droplet of water will have a far greater proportion of its molecules exposed to the surrounding environment than those in a glass, bucket, or lake.

It is on this exposed surface that hydrogen peroxide forms, Jianghan University chemist Bolei Chen, Zare and colleagues confirmed.

Use of a dye that glows in the presence of H2O2 the team mapped its presence in droplets in contact with a glass surface and found that it was most concentrated at the interface between the two types of matter.

The researchers demonstrated H2O2 was also produced when the droplets hit nine other solids, including soil or finer dust floating in our air.

In a previous study, they also showed that it happens naturally, simply when water condenses from the air onto cold surfaces. Also, the amount of H2O2 increased with humidity.

“It appears that contact electrification producing hydrogen peroxide is a universal phenomenon at water-solid interfaces,” says Zare.

To determine where the droplets are stealing the extra oxygen atom, the researchers treated the glass surface with a heavy oxygen isotope, 18O.

Sure enough, the droplets formed hydrogen peroxide adopting the glass surface 18O atoms, confirming that excitable hydrogen and oxygen elements called hydroxyl radicals attached to the surface material were the source.

As the acid formed, Chen and the team could also measure an electric current flowing from the solid to the ground in time with the increasing glow of the dye.

This confirmed the results of previous studies which suggested that an exchange of electrons took place in a process called contact electrification, creating the hydroxyl radicals.

Of course, this does not exclude that some of the raw materials may be provided by other sources in the environment such as ozone (O3), writes the team in its article. But it confirms that water droplets and a solid surface alone are enough.

“Contact electrification provides a chemical basis to partially explain why there is seasonality in viral respiratory illnesses,” says Zare.

With increased humidity in the hot summer air carrying small amounts of hydrogen peroxide, this could be one more barrier for circulating pathogens.

By contrast, the cool, dry winter air might just give viruses that little advantage by jumping from one sniffling nose to another.

This research was published in PNAS.

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