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.

Deciphering the Microdroplet Acceleration Factors of Aza-Michael Addition Reactions

“…Microdroplet chemistry has been emerging as an exciting field in recent years because of its capabilities of accelerating reactions by several orders of magnitude compared to the same reactions in the bulk, , triggering reactions without external catalysts, − and yielding different products compared to the bulk reactions via different pathways. − Therefore, understanding how microdroplets promote reactions is very important in rationally utilizing them as green and efficient reactors for organic synthesis. Presently, the driving force of the unexpected acceleration in microdroplets is not fully known.…”