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: HO_S^- + H_S⁺ = HO_S^• + H_S^•, 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: ΔH_hydration(H⁺ + OH^-) = -1670 kJ mol^-1, relative to neutral radical products: ΔH_hydration(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.