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Water is considered as an inert environment for the dispersion of many chemical systems. However, by simply spraying bulk water into microsized droplets, the water microdroplets have been shown to possess a large plethora of unique properties, including the ability to accelerate chemical reactions by several orders of magnitude compared to the same reactions in bulk water, and/or to trigger spontaneous reactions that cannot occur in bulk water. A high electric field (∼10 9 V/m) at the air-water interface of microdroplets has been postulated to be the probable cause of the unique chemistries. This high field can even oxidize electrons out of hydroxide ions or other closed-shell molecules dissolved in water, forming radicals and electrons. Subsequently, the electrons can trigger further reduction processes. In this Perspective, by showing a large number of such electron-mediated redox reactions, and by studying the kinetics of these reactions, we opine that the redox reactions on sprayed water microdroplets are essentially processes using electrons as the charge carriers. The potential impacts of the redox capability of microdroplets are also discussed in a larger context of synthetic chemistry and atmospheric chemistry., Competing Interests: The authors declare no competing financial interest., (© 2023 The Authors. Published by American Chemical Society.)

Due to the adverse health effects and the role in the formation of secondary organic aerosols, hydroxyl radical (OH) generation by atmospheric fine particulate matter (PM) has been of particular research interest in both bulk solutions and the gas phase. However, OH generation by PM at the air-water interface of atmospheric water droplets, a unique environment where reactions can be accelerated by orders of magnitude, has long been overlooked. Using the field-induced droplet ionization mass spectrometry methodology that selectively samples molecules at the air-water interface, here, we show significant oxidation of amphiphilic lipids and isoprene mediated by water-soluble PM 2.5 at the air-water interface under ultraviolet A irradiation, with the OH generation rate estimated to be 1.5 × 10 16 molecule·s -1 ·m -2 . Atomistic molecular dynamics simulations support the counter-intuitive affinity for the air-water interface of isoprene. We opine that it is the carboxylic chelators of the surface-active molecules in PM that enrich photocatalytic metals such as iron at the air-water interface and greatly enhance the OH generation therein. This work provides a potential new heterogeneous OH generation channel in the atmosphere.

The multiphase oxidation of SO 2 to sulfate in aerosol particles is a key process in atmospheric chemistry. However, there is a large gap between the observed and simulated sulfate concentrations during severe haze events. To fill in the gaps in understanding SO 2 oxidation chemistry, a combination of experiments and theoretical calculations provided evidence for the direct, spin-forbidden excitation of SO 2 to its triplet states using UVA photons at an air-water interface, followed by reactions with water and O 2 that facilitate the rapid formation of sulfate. The estimated reaction energy for the whole process, 3 SO 2 + H 2 O + 1/2O 2 → HSO 4 - + H + (298 K, 1 M), was Δ G r = -107.8 kcal·mol -1 . Moreover, calculations revealed that this was a multistep reaction involving submerged, small energy barriers (∼10 kcal·mol -1 ). These results indicate that photochemical oxidation of SO 2 at the air-water interface with solar actinic light may be an important unaccounted source of sulfate aerosols under polluted haze conditions.

Water serves as an inert environment for the dispersion and application of many kinds of herbicides. Viologen compounds, a type of widely used but highly toxic herbicide, are stable in bulk water, whose half-life can be up to 23 weeks in natural water, imposing a severe health risk to mammals. In this study, we present the striking results of the spontaneous and ultrafast reduction-induced degradation of three viologen compounds in water microdroplets and provide the concentration, time, temperature dependence, mechanism, and scale-up of the reactions. We postulate that the electrons existing at the air-water interface of the microdroplets due to the unique redox potential therein initiate the reduction, from which further degradation occurs. The host-guest complexation between cucurbit[7]uril and viologens only slightly changes the redox potential of viologens in the bulk but completely inhibits the reactions in microdroplets, adding to the uniqueness of the redox potentials at the air-water interfaces of microdroplets. Taken together, microdroplets might have been functioning as naturally occurring ubiquitous tiny electrochemical cells for a plethora of unique redox reactions that were thought to be impossible in the bulk water.

The cyclohexanehexone (C 6 O 6 ) octahydrate molecule was claimed to be synthesized as early as 1862. However, the chemical in the 1862 study and the chemicals used in most of the existing studies and sold by most chemical vendors are actually dodecahydroxycyclohexane dihydrate (C 6 (OH) 12 ·2H 2 O). Here we revisit our bulk synthesis method of C 6 O 6 by the dehydration of the C 6 (OH) 12 ·2H 2 O material, and report the mass spectrum of C 6 O 6 that has been highly challenging to obtain owing to its high sensitivity toward ambient conditions. A new home-built electrospray ionization mass spectrometry setup in a glovebox is utilized to detect C 6 O 6 in the form of C 6 O 6 H - . Tandem mass spectrometry MS n ( n = 2-4) presents consecutive losses of CO molecules, further confirming the structure of C 6 O 6 . Theoretical calculations are performed to recover the chemical bonding of C 6 O 6 and to rationalize the synthetic method. This work provides a benchmark understanding of the historically elusive C 6 O 6 .

The antioxidizing capability of membrane antioxidants is strongly affected by the submolecular regions of the membrane that they locate. However, the concurrent determination of their location in the membranes and the consequent antioxidizing effect remains difficult. Using our field-induced droplet ionization mass spectrometry methodology, here we show the rapid determination of the antioxidation effect and the spatial distribution of melatonin in POPC membranes. Melatonin effectively protects the membrane lipids against hydroxyl radicals originating from the Fenton reactions in the water phase but cannot protect the lipids against singlet oxygen generated by a lipophilic photosensitizer in the lipid tail region (oil phase). These varied antioxidizing behaviors indicate that melatonin dwells at the headgroup subregion of the membranes. We anticipate that the methodology in this study can be widely utilized in the screening of antioxidants' spatial distribution and antioxidizing efficiency, and eventually in designing novel antioxidants that could deliver specific functions.