Solvated electrons at the atmospheric pressure plasma-water anodic interface

We present results from a particle-in-cell/Monte Carlo model of a dc discharge in argon at atmospheric pressure coupled with a fluid model of an aqueous electrolyte acting as anode to the plasma. The coupled models reveal the structure of the plasma-electrolyte interface and near-surface region, with a special emphasis on solvated or hydrated electrons. Results from the coupled models are in generally good agreement with the experimental results of Rumbach et al (2016 Nat. Commun. 6 7248). Electrons injected from the plasma into the water are solvated, then lost by reaction with water within about 10-20 nm from the surface. The major reaction products are OH^- and H₂. The solvated electron density profile is controlled by the injected electron current density and subsequent reactions with water, and is relatively independent of the external plasma electric field and the salt concentration in the aqueous electrolyte. Simulations of the effects of added scavenger compounds (H₂O₂, \text{NO}₂^- , \text{NO}₂^- and H⁺) on near-surface solvated electron density generally match the experimental results. The generation of near-surface OH^- following electron-water decomposition in the presence of bulk acid creates a highly basic region (pH ~ 11) very near the surface. In the presence of bulk solution acidity, pH can vary from a very acidic pH 2 away from the surface to a very basic pH 11 over a distance of ~200 nm. High near-surface gradients in aqueous solution properties could strongly affect plasma-liquid applications and challenge theoretical understanding of this complex region.