Spontaneous Oxidation in Aqueous Microdroplets: Water Radical Cation as Primary Oxidizing Agent

Abstract

Exploration of the unique chemical properties of interfaces can unlock new understanding. A striking example is the finding of accelerated reactions, particularly spontaneous oxidation reactions, that occur without assistance of catalysts or external oxidants at the air interface of both aqueous and organic solutions (provided they contain some water). This finding opened a new area of interfacial chemistry but also caused heated debate regarding the primary chemical species responsible for the observed oxidation. An overview of the literature covering oxidation in microdroplets with air interfaces is provided, together with a critical examination of previous findings and hypotheses. The water radical cation/radical anion pair, formed spontaneously and responsible for the electric field at or near the droplet/air interface, is suggested to constitute the primary redox species. Mechanisms of accelerated microdroplet reactions are critically discussed and it is shown that hydroxyl radical/hydrogen peroxide formation in microdroplets does not require that these species be the primary oxidant. Instead, we suggest that hydroxyl radical and hydrogen peroxide are the products of water radical cation decay in water. The importance of microdroplet chemistry in the prebiotic environment is sketched briefly and the role of partial solvation in reaction acceleration is noted.

Water Microdroplets in Air: A Hitherto Unnoticed Natural Source of Nitrogen Oxides

“…It can also explain the spontaneous formation of H 2 O 2 from the sprayed water microdroplets, 9 where H 2 O •+ serves as the precursor of the • OH radical. 45 Nevertheless, the activation of the inert nitrogen gas and its subsequent oxidation into nitrogen oxides under a high-voltage electrical discharge has been known for more than 100 years, 49,50 but the same phenomenon from water microdroplets seems to be without precedent. It should also be noted that there could be an interplay of other unknown factors contributing to the mechanism of the nitrogen oxide production from the sprayed water microdroplets, providing an incentive for more work in the future.…”

Spontaneous Degradation of the “Forever Chemicals” Perfluoroalkyl and Polyfluoroalkyl Substances (PFASs) on Water Droplet Surfaces

“…A decrease in the C 7 F 15 − peak was observed when we added NaNO 3 as scavengers of e − and added both NaNO 3 and DMPO, 30 indicating synergy between e − and •OH in the PFOA degradation (Figure S6). AIMD simulations (Figure S8) show that the water radical cations (H 2 O• + ) 31 in the microdroplets can rapidly react with H 2 O to produce •OH and H + via eq 2 within only 2.5 ps. Although H 2 O• + may react with PFOA via eq 3, the free-energy barrier for the eq 3 is much higher than that for the barrierless eq 2 by ca.…”

Mild Diarylmethylation of Indoles through Regioselective 1,6-Hydroarylation of para-Quinone Methides in Microdroplets/Thin Film

“…As shown in Entries 8−9 of Table 1 and Table S3, the microdroplet/thin film reaction can proceed smoothly in the three polar solvents like methanol, ethanol, and tetrahydrofuran similar to acetonitrile whereas almost no conversions are observed in DMF, DMSO, and DCM. In addition, we also screened three binary aqueous solvents (a mixture of acetonitrile and water at three ratios of 9:1, 7:3, and 1:1, v/ v) considering the extraordinary power of water/aqueous microdroplets, 16 where the microdroplet/thin film yield was sharply reduced with the increase of the water proportion (Entries 10−11 of Table 1 and Table S3). For comparison, the microdroplet/thin film reaction in anhydrous acetonitrile afforded a slightly improved yield of 91%.…”

“…), oxidation/reduction, ,,,,,− biphasic reactions (gas–liquid, ,,, liquid–liquid, − and liquid–solid , reactions), multicomponent reactions (Hantzsch, Biginelli, Betti, Petasis and Passerini reaction, N-alkylation, five-component spiro-pyrrolidine construction, etc. ), and others (Pinacol and Beckmann rearrangement, metal catalysis, etc.). In addition, a lot of efforts have been made for the scale-up of microdroplet reactions including high-flux strategy ,, (by mesh-integrated and ultrasonic nebulization), parallel array sprayers, and closed solvent-recycle strategy .…”