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.