From the Cover: Condensing water vapor to droplets generates hydrogen peroxide

It was previously shown [J. K. Lee et al., Proc. Natl. Acad. Sci. U.S.A., 116, 19294–19298 (2019)] that hydrogen peroxide (H₂O₂) is spontaneously produced in micrometer-sized water droplets (microdroplets), which are generated by atomizing bulk water using nebulization without the application of an external electric field. Here we report that H₂O₂ is spontaneously produced in water microdroplets formed by dropwise condensation of water vapor on low-temperature substrates. Because peroxide formation is induced by a strong electric field formed at the water–air interface of microdroplets, no catalysts or external electrical bias, as well as precursor chemicals, are necessary. Time-course observations of the H₂O₂ production in condensate microdroplets showed that H₂O₂ was generated from microdroplets with sizes typically less than ∼10 µm. The spontaneous production of H₂O₂ was commonly observed on various different substrates, including silicon, plastic, glass, and metal. Studies with substrates with different surface conditions showed that the nucleation and the growth processes of condensate water microdroplets govern H₂O₂ generation. We also found that the H₂O₂ production yield strongly depends on environmental conditions, including relative humidity and substrate temperature. These results show that the production of H₂O₂ occurs in water microdroplets formed by not only atomizing bulk water but also condensing water vapor, suggesting that spontaneous water oxidation to form H₂O₂ from water microdroplets is a general phenomenon. These findings provide innovative opportunities for green chemistry at heterogeneous interfaces, self-cleaning of surfaces, and safe and effective disinfection. They also may have important implications for prebiotic chemistry.

Water molecules in liquid water are considered stable and inert. We and other investigators have reported that water molecules become electrochemically active and catalytic for various reactions when bulk water is formed into micrometer-sized droplets (microdroplets). Reaction rates for various chemical reactions are accelerated in microdroplets by factors of 10² or more compared to bulk solution (1). The microdroplet environment provides conditions for a lowered entropic barrier, which allows thermodynamically unfavorable reactions to proceed in microdroplets at room temperature (2, 3). We also have shown that water microdroplets induce spontaneous charge exchanges between solutes and water molecules to induce the spontaneous reduction of organic molecules and metal ions as well as the formation of nanostructures without any added reducing agent or template (4, 5). Moreover, we have reported that water molecules undergo spontaneous oxidation to form reactive oxygen species, including hydroxyl radicals (OH) and hydrogen peroxide (H₂O₂) (6–8). Recent investigations attributed the origin of these unique physicochemical properties observed in microdroplets to the enrichment of reactants at the interface (9–11), restricted molecular rotations (12), partial solvation at the water surface (1, 13), and a strong interfacial electric field at the surface of the water microdroplet (14).Microdroplets can be formed either by atomizing bulk water (top down) with various methods such as high-pressure gas nebulization (15), ultrasonic nebulization (16), vibrating micromesh nebulization (17), and piezoelectric nebulization (18), or by condensing vapor-phase molecules (bottom up) (19). A question may be asked whether those unique properties of microdroplets arise only in microdroplets formed by atomization of bulk water. In addition, it may be wondered whether the spontaneous oxidation of water to form H₂O₂ in microdroplets (6) was caused by the atomizing process involving friction or vibration. These questions motivated us to investigate whether H₂O₂ becomes spontaneously generated in water microdroplets formed by the condensation of water vapor in air on cold surfaces, and how universal might this process be. We have paid special attention to the influence of different surface properties, including hydrophilicity and surface roughness, as well as environmental factors, including relative humidity and surface temperature.