Decoupling Interfacial Reactions between Plasmas and Liquids: Charge Transfer vs Plasma Neutral Reactions

Plasmas (gas discharges) formed at the surface of liquids can promote a complex mixture of reactions in solution. Here, we decouple two classes of reactions, those initiated by electrons (electrolysis) and those initiated by gaseous neutral species, by examining an atmospheric-pressure microplasma formed in different ambients at the surface of aqueous saline (NaCl) solutions. Electrolytic reactions between plasma electrons and aqueous ions yield an excess of hydroxide ions (OH-), making the solution more basic, while reactions between reactive neutral species formed in the plasma phase and the solution lead to nitrous acid (HNO2), nitric acid (HNO3), and hydrogen peroxide (H2O2), making the solution more acidic. The relative importance of either reaction path is quantified by pH measurements, and we find that it depends directly on the composition of the ambient background gas. With a background gas of oxygen or argon, electron transfer reactions yielding excess OH- dominate, while HNO2 and HNO3 formed in the plasma and by the dissolution of nitrogen oxide (NOx) species dominate in the case of air and nitrogen. For pure nitrogen (N-2) gas, we observe a unique coupling between both reactions, where oxygen (O-2) gas formed via water electrolysis reacts in the bulk of the plasma to form NOx, HNO2, and HNO3.