Mechanism of Hydrogen Peroxide Formation on Sprayed Water Microdroplets

How is H₂O₂ formed in sprayed water is not well understood. It is believed to involve the association of HO• radicals spontaneously generated from HO– ions by internal electric fields on the surface of neutral microdroplets. Spraying water actually creates charged microdroplets carrying either excess OH– or H+ intrinsic ions that repel each other toward the very surface. The requisite electron transfer (ET) takes place between surface-bound ions: HOS– + HS+ = HOS• + HS•, during encounters between positive and negative microdroplets. The ET endothermicity in bulk water (ΔH = 448 kJ mol⁻¹) is reversed in low-density surface water by the destabilization of the strongly hydrated reactant ions: ΔHhydration(H+ + OH–) = −1670 kJ mol⁻¹, relative to neutral radical products: ΔHhydration(HO• + H•) = −58 kJ mol⁻¹. The formation of H₂O₂ is driven by the energy supplied for spraying water, and caused by restricted hydration on microdroplet surfaces.