To make hydrogen peroxide, all you need is water

Water in tight spaces sometimes doesn’t behave like bulk water. Microdroplets can, for example, accelerate the synthesis of gold nanostructures . Discovering new droplet properties could have far-reaching research implications, including in industrial food and drug processing, atmosphere–ocean interactions, and immune responses in cells.

Now Hong Gil Nam from the Institute for Basic Science in South Korea, Richard Zare of Stanford University, and their colleagues have found that under certain environmental conditions, water spontaneously forms hydrogen peroxide. What’s more, the process doesn’t require a catalyst, external electric potential, radiation, or other means of generating a reaction.

To track hydrogen peroxide production, the researchers sprayed a solution of peroxyfluor-1 in water onto a hydrophobic glass surface that molds the water into microdroplets. When it interacts with hydrogen peroxide, the peroxyfluor-1 releases fluorescein, which can be imaged with microscopy. The left picture shows fluorescing microdroplets (green); the right picture is of bulk water with no hydrogen peroxide yield.

The authors suggest that the intrinsic electric field at the air–water interface of a microdroplet can instigate a series of reactions that produce hydroxyl radicals and, in turn, hydrogen peroxide. As the curvature of the droplet increases, more ions can adhere to the droplet’s surface, which effectively increases the electric field strength and hydrogen peroxide production. An analysis of the images revealed that the fluorescence intensity, and thus the yield of hydrogen peroxide, increased significantly for microdroplets with a diameter of less than 20 µm. With additional experiments, the new observations may help to explain why hydrogen peroxide forms in raindrops at night when UV radiation isn’t present to catalyze a peroxide-forming photochemical reaction. (J. K. Lee et al., Proc. Natl. Acad. Sci. USA, 116, 19294, 2019 .)

Thumbnail image credit: wikimedia/pt6120190926a_Fir0002, CC-BY-SA 3.0