  In charged water microdroplets, which occur in nature or in the lab upon
ultrasonication or in electrospray processes, the thermodynamics for reactive
chemistry can be dramatically altered relative to the bulk phase. Here, we
provide a theoretical basis for the observation of accelerated chemistry by
simulating water droplets of increasing charge imbalance to create redox agents
such as hydroxyl and hydrogen radicals and solvated electrons. We compute the
hydration enthalpy of OH^- and H^+ that controls the electron transfer process,
and the corresponding changes in vertical ionization energy and vertical
electron affinity of the ions, to create OH* and H* reactive species. We find
that at ~20-50% of the Rayleigh limit of droplet charge the hydration enthalpy
of both OH^- and H^+ have decreased by >50 kcal/mol such that electron transfer
becomes thermodynamically favorable, in correspondence with the more favorable
vertical electron affinity of H^+ and the lowered vertical ionization energy of
OH^-. We provide scaling arguments that show that the nanoscale calculations
and conclusions extend to the experimental microdroplet length scale. The
relevance of the droplet charge for chemical reactivity is illustrated for the
formation of H2O2, and has clear implications for other redox reactions
observed to occur with enhanced rates in microdroplets.
