Spontaneous Formation of Hydrogen Peroxide in Water Microdroplets

There is accumulating evidence that many chemical reactions are accelerated by several orders of magnitude in micrometer-sized aqueous or organic liquid droplets compared to their corresponding bulk liquid phase. However, the molecular origin of the enhanced rates remains unclear as in the case of spontaneous appearance of 1 mu M hydrogen peroxide in water microdroplets. In this Letter, we consider the range of ionization energies and whether interfacial electric fields of a microdroplet can feasibly overcome the high energy step from hydroxide ions (OH-) to hydroxyl radicals (OH center dot ) in a primary H2O2 mechanism. We find that the vertical ionization energies (VIEs) of partially solvated OH- ions are greatly lowered relative to the average VIE in the bulk liquid, unlike the case of the Cl- anion which shows no reduction in the VIEs regardless of solvation environment. Overall reduced hydrogen-bonding and undercoordination of OH- are structural features that are more readily present at the air-water interface, where the energy scale for ionization can be matched by statistically probable electric field values.

Automated summary

There is growing evidence that chemical reactions in micrometer-sized liquid droplets can occur at significantly accelerated rates. However, the molecular mechanism behind this enhancement is still unclear. In this study, the authors investigate the energy step of hydroxide ions converting to hydroxyl radicals in microdroplets, and find that the ionization energy of partially solvated hydroxide ions is greatly reduced at the droplet's interface.