Radical surface chemistry: Augmentation of reactivity by radicals at aqueous interfaces

Cooperativity and non-additive interactions play central roles in the unusual and surprising behavior of water. A host of reactive oxygen species (ROS) including the hydroxyl radical ^•OH, superoxide radical anion (O₂^−•), hydroperoxide radical (HO₂^•), singlet oxygen (¹O₂), and also the more recently discussed water radical cation/anion pair (H₂O^+•/H₂O^−•) all add to the more familiar acid/base chemical pathways tread by hydronium (H₃O⁺) and hydroxide (OH⁻). This is amplified in surface science because interfacial water – whether found at the gas/liquid, gas/solid, or liquid/solid interface – poses yet more unique behavior. This review explores the unexpected chemistry associated with ambient temperature aqueous interfaces much of which is mediated not only by ions and neutrals as expected, but also radical species. Water microdroplets catalyze numerous reactions and can also support simultaneous oxidation and reduction reactions through the production of reactive intermediates that owe their existence to the unique influence of the air/water or oil/water interface. Interfacial water influences and is influenced by the ubiquitous phenomenon of contact electrification, a manifestation of spontaneous symmetry breaking. The mechanisms of chemistry not only on and in microdroplets but also at the gas/solid and liquid/solid interfaces rely on a broad set of chemical transformations mediated by radicals. Furthermore, because aqueous macro- and micro-interfaces are ubiquitous on Earth, we find that water radical-mediated chemistry has applications to atmospheric chemistry, geochemistry, mineral weathering, pre-biotic chemistry, enhanced enzyme performance, wastewater remediation, public health, mechanochemistry, and potentially novel routes to pharmaceuticals.