Paper PS+AS+BI+SE-MoM6
Deciphering Gas-Phase and Solution-Phase Reactions Initiated by Plasmas at the Surface of Aqueous Solutions

Monday, October 28, 2013, 10:00 am, Room 102 B

Recent advancements in atmospheric-pressure plasma technology have enabled applications in polymer processing, plasma medicine, and water treatment. Many of these applications rely heavily on physical and chemical interactions between plasmas and aqueous solutions. We have recently shown that plasma electrons are involved in electrolytic reactions such as the reduction of aqueous hydrogen ions (H+) to hydrogen gas[1]. In this work, we show that the reactions are more complex and involve a competition between plasma chemistry and solution chemistry.

To study interactions between a plasma and liquid, saline solutions were exposed to an argon (Ar) DC microplasma jet, and the effects of various reactions occurring in the plasma and solution phase were characterized. When the plasma jet was run in a background of argon or oxygen gas, traditional electrolytic reactions yielding sodium hydroxide (NaOH) were found to be dominant, making the solution more basic (pH ~ 8). Running the plasma jet in a background of atmospheric air produced significant amounts of nitric acid (HNO₃) and hydrogen peroxide (H₂O₂) in solution. Production of HNO₃ in air typically occurs at a rate two orders of magnitude higher than NaOH, making the solution more acidic (pH ~ 3). In a background of nitrogen gas, HNO₃ was also produced, but at a rate that is limited by oxygen gas evolution from water electrolysis. Overall, the chemical composition of the solution is affected by both electrolytic reactions at the plasma-liquid interface as well as reactions occurring in the bulk plasma.

[1] M. Witzke, P. Rumbach, D. B. Go, and R. M. Sankaran, J. Phys. D: Appl. Phys. 45, 442001 (2012).