On the Formation of Hydrogen Peroxide in Water Microdroplets

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Abstract
Recent reports on the formation of hydrogen peroxide (HO) in water microdroplets produced via capillary condensation or pneumatic spraying have garnered significant attention. How covalent bonds in water could break under such conditions challenges our textbook understanding of physical chemistry and the water substance. While there is no definitive answer, it has been speculated that ultrahigh electric fields at the air-water interface are responsible for this chemical transformation. This thesis documents the findings of our exploration of this mystery via a comprehensive experimental investigation of HO formation in (i) water microdroplets condensed on hydrophobic and hydrophilic substrates formed via hot water in the 50–70 ℃ range or ultrasonic humidifier under controlled air composition, and (ii) water microdroplets sprayed over a range of liquid flow-rates, the (shearing) air flow rates, and the air composition. Our glovebox experiments, with controlled gas composition, revealed that no HO(aq) was produced in water microdroplets condensed via heating water (detection limit ≥ 0.25 μM), regardless of the droplet size or the substrate wettability. In contrast, water droplets condensed via ultrasonic humidification contained significantly higher (~1 μM) HO concentration. We pinpointed that ultrasonic humidifiers induced cavitation of tiny bubbles in water, which is known to form HO(aq) and other reactive species. Next, in the case of sprayed water microdroplets, also, we did not detect HO(aq) unless O(g) was present in the ambient atmosphere. In contrast, water microdroplets (sprayed or condensed) exposed to O(g) concentration in the range 2–5000 ppb formed 2–100 μM HO(aq); increasing the gas–liquid surface area, mixing, and contact duration enhanced the HO(aq) concentration. Therefore, we submit that the original reports suffered from experimental artifacts due to the high regional O(g) giving rise to HO(aq), and the air–water interface does not spontaneously produce HO(aq). The water surface merely facilitates the O(g) mass transfer, which then undergoes chemical transformations in the water to form HO(aq). Taken together, these findings offer an alternative explanation to the mysterious production of HO in water microdroplets; These findings should also advance our understanding of the implications of this chemistry in natural and applied contexts.

Citation
Huve Musskopf, N. V. (2022). On the Formation of Hydrogen Peroxide in Water Microdroplets. KAUST Research Repository. https://doi.org/10.25781/KAUST-12621

DOI
10.25781/KAUST-12621

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