The Role of Charge in Microdroplet Redox Chemistry

TL;DR

This study provides a theoretical framework explaining how charge in microdroplets accelerates redox reactions by altering thermodynamics, with implications for understanding enhanced chemical reactivity in charged water droplets.

Contribution

The paper introduces a theoretical model linking droplet charge to redox chemistry acceleration, supported by nanoscale simulations and scaling arguments.

Findings

Charge increases reduce hydration enthalpy of ions.

Electron transfer becomes thermodynamically favorable at 20-50% of Rayleigh limit.

Charge effects extend from nanoscale to experimental microdroplet sizes.

Abstract

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.