Capture of Hydroxyl Radicals by Hydronium Cations in Water Microdroplets

Abstract

Despite the high stability of bulk water, water microdroplets possess strikingly different properties, such as the presence of hydroxyl radicals (OH⋅) at the air–water interface. Previous studies exhibited the recombination of OH⋅ into H2O2 molecules and the capture of OH⋅ by oxidizing other molecules. By spraying pure water microdroplets into a mass spectrometer, we detected OH⋅ in the form of (H4O2)+ that is essentially OH⋅−H3O+, a hydroxyl radical combined with a hydronium cation through hydrogen bonding. We also successfully captured it with two OH⋅ scavengers, caffeine and melatonin, and key oxidation radical intermediates that bear important mechanistic information were seen. It is suggested that some previous reactions involving (H4O2)+ should be attributed to reactions with OH⋅−H3O+ rather than with the water dimer cation (H2O−OH2)+.

Spontaneous oxidation of I− in water microdroplets and its atmospheric implications

“…[21][22][23][24][25][26] More recently, our group reported the direct observation of OH by spraying pure water microdroplets into a mass spectrometer, where OH is in the form of a hydrogen-bonded HO -H 3 O + (H 4 O 2 + ) ion. 27 Based on the above observations, we anticipate that the spontaneously generated OH might promote the oxidation of I À in water microdroplets, providing a new channel for the formation of I and I 2 in atmospheric water. Details of the experimental conditions are described in the ESI.…”

“…It has been well established that sprayed water microdroplets can generate OH spontaneously. [19][20][21][22][23][24][25][26][27] The proposed mechanisms of forming OH include the ultrahigh electric field at the surface of microdroplets that splits OH À into an electron and an OH radical, 21 cavitation, 31 and reactions of O 3 at the microdroplet periphery. 32 Here we postulate that the OH generated by water microdroplets oxidizes I À into I (Fig.…”

Oxidation of Catechols at the Air–Water Interface by Nitrate Radicals

“…All experiments are performed in duplicate. The large aqueous microdroplets produced in this setup and the ultrafast contact time ( t c = 1 μs) discard any possible contribution from solvent evaporation in the surface reactions studied. − ,, Diffusion limitations have been discarded in this surface-sensitive setup for catechol reactions. , The conditions selected not only minimize any cluster formation but also are extremely soft to prevent any artifacts that could conduct to ionize molecules in the gas phase. − , Furthermore, the controls performed confirm that the products are not the result from redox chemistry in the probe but from the interfacial reactions examined. − ,,, In more detail, spontaneous reactions observed in other ESI probes with a fused silica capillary (e.g., to form HO • , H 2 O 2 , or facilitated electron transfer in water) are discarded by the controls performed and by previous studies with this OESI MS setup and complementary techniques. − ,,,,, For the quantification of nitroaromatic products, the method of standard addition was used as detailed in the Supporting Information with standards and pseudo-standards for commercially unavailable species.…”

Implementing reactive secondary electrospray ionization based on gas–droplet reactions for VOC analysis

“…9 In electrospray, as the generation and fission of the charged droplets take place in air, a large number of droplet/air interfaces are formed, creating sufficient contact sites for the components in the droplets and the surrounding gas. The reaction between the solution and air can occur at the gas-liquid interface, [13][14][15][16][17] which has already been used for reaction monitoring and to study interfacial chemistry. In contrast, to our knowledge, such interfacial reactions are rarely used for chemical analysis.…”