Microdroplets as interfacial reactors: from bond breaking to atmospheric impacts

Microdroplets provide physicochemical environments that differ fundamentally from bulk liquids due to their large surface-to-volume ratios, partial solvation, and pronounced nonequilibrium dynamics. Over the past decade, a growing body of experimental and theoretical work has demonstrated that chemical reactions in microdroplets often exhibit markedly altered kinetics and mechanisms relative to bulk-phase systems. In particular, numerous studies have reported the activation and cleavage of relatively strong covalent bonds under mild conditions in aqueous microdroplets. Recent advances in this field have revealed that microdroplets can act as unique interfacial environments that facilitate bond-breaking reactions. The physicochemical characteristics of microdroplets can reshape reaction energy landscapes and influence reaction pathways. Representative examples of bond activation in microdroplets include C-F, C-H, C-S, C-X, S-O, and O-O bonds. These processes have been proposed to contribute to radical, ionic, or coupled radical-ionic reaction pathways and to alter the behavior of reactive intermediates under confined nonequilibrium conditions. At the same time, the mechanistic origins of many reported microdroplet effects remain actively debated, particularly regarding the roles of interfacial electric fields, spontaneous radical formation, dissolved oxygen, and potential instrumental artifacts. Despite these uncertainties, similar interfacial processes may also operate in natural aqueous microenvironments, particularly in atmospheric aerosols, cloud droplets, and sea spray particles. Interfacial bond cleavage and radical generation may influence sulfur cycling, organic aerosol aging, and the oxidative capacity of the marine boundary layer. Remaining challenges include the spectroscopic detection of transient intermediates, improved scaling between laboratory microdroplet systems and atmospheric particles, and the incorporation of interfacial chemistry into multiphase atmospheric models. Overall, these perspectives highlight the potential of aqueous interfaces as active sites of chemical reactivity linking molecular-scale processes to atmospheric chemistry.