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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.
A. J. Colussi (Fri,) studied this question.