Physical Chemistry of Water Microdroplets.

Orders-of-magnitude reaction accelerations and the formation of HO in aerosolized water microdroplets are in search of explanation. These processes are not overall spontaneous as generally assumed because they are preceded by the forced aerosolization of water. Nor are they caused by internal electric fields because the electrical activation of chemical reactions requires external static fields vectorially aligned with transition state dipoles. The formation of HO is enabled by the partial dehydration of HO and H ions in low-density interfacial water, a process that raises their free energies of formation and lifts the endoergicity of the prerequisite electron transfer HO + H = HO + H in bulk water. The lack of unimolecular reaction accelerations suggests the accelerations of bimolecular reactions are due to the higher frequencies of molecular encounters, fewer transition state recrossings, and smaller entropy losses for expanding solvent cavities to hold their transition states in interfacial vs bulk water.