Microdroplets can act as electrochemical cells

Abstract

A water microdroplet in air or oil typically possesses an electric double layer (EDL) from the preferential adsorption of surface-bound ions at the periphery. We present the calculations of the ion gradients within a microdroplet at equilibrium, including systems containing buffers and water autoionization. These ion gradients are used to calculate the potential energy stored within the microdroplet. We consider how this stored potential energy can be utilized to drive chemical reactions, much like an electrochemical cell. Effective voltages as high as 111 mV are found for microdroplets having a low surface charge density (0.01 ions per nm2). Two sources of potential energy are investigated: (1) the electrostatic energy of the EDL of the microdroplet and (2) shifts in other chemical equilibria coupled to the main reaction through the EDL. A particularly important example of the latter is water autoionization, wherein the reaction of interest causes a flattening of the [H+] gradient within the EDL, resulting in a net recombination of H+ and OH− throughout the microdroplet. Numerical calculations are performed using a continuum model consisting of a balance between the electromigration and diffusion of ions throughout the microdroplet. Our treatment accounts for the autoionization of water and any chemical equilibrium of buffers present. The results are presented for uncharged water microdroplets with low amounts of salts and simple buffers in them. However, the calculational method presented here can be applied to microdroplets of any net charge, composed of any solvent, containing ions of any valence, and containing complex mixtures of chemical equilibria.

Sprayed Water Microdroplets Are Able to Generate Hydrogen Peroxide Spontaneously

“…Contact electrification may account for all or part of the H 2 O 2 formed from condensed water vapor on a cold surface . The surface charges of microdroplets induce an electric double layer (EDL) − and cause microdroplets to act as electrochemical cells . In this sense, the anode is the negatively charged periphery of the water microdroplet, and the cathode is the positively charged species in the interior of the water microdroplet.…”

Simultaneous and Spontaneous Oxidation and Reduction in Microdroplets by the Water Radical Cation/Anion Pair

“…As illustrated in figure 4, the conversion from water molecules to the water radical cation/ anion pair and then to the highly reactive free cations and anions will be promoted by the strong electric field known to exist near the droplet surface. [19,[35][36][37] Due to their opposite polarities, these free ions will be accumulated in different layers and produce the double-layer structure. The outer layer is enriched in the water radical cation, which gives it strong oxidative abilities to convert phosphonate into phosphoric acid derivatives.…”

Sulfur radical formation from the tropospheric irradiation of aqueous sulfate aerosols

“…Lastly, it is possible that the unique interfacial region of particles plays a role in the reaction during the chamber experiments in this work and in tropospheric aerosols. Air-water interfaces have been demonstrated to promote redox chemistry spontaneously on AS particles ( 41 ) and water microdroplets ( 42 , 43 ). The reaction under study will require further characterization, but clearly can degrade organic compounds using low light energies available on a hazy day ( Fig.…”