Water in contact with low temperature plasma (LTP) is of interest for chemical, biomedical, and agricultural applications. The LTP is typically an impinging jet. The electrochemical potential of the aqueous phase in contact with LTP has not been described, frustrating comprehension of reaction outcomes. In this work, the spatial profiles of electrochemical potential near the plasma-water interface are elucidated. Floating surfaces in contact with LTPs acquire a net negative charge. Water in direct contact with the plasma is expected to have a more negative electrochemical potential with respect to the potential far away from the plasma (Einf). The overall charge of the solution is expected to be neutral. To balance the negative charge of the plasma-water interface, a region is expected in the liquid that has a more positive electrochemical potential with respect to Einf. Measurements of the electrochemical potential were made as a function of (i) radial distance from the centerline of a 3.5 mm radius helium plasma jet and (ii) depth below the surface of the water. Water directly contacting the plasma has a negative electrochemical potential, in some cases more than -1 V with respect to Einf. With radial distance away from the jet centerline, the electrochemical potential increases and crosses zero, peaking at 5 mm with values as large as +0.2 V with respect to Einf, before decaying back to 0 V at approximately 20 mm. As a function of depth from the plasma-water interface, electrochemical potential decays to zero over a distance of 3.5 mm. The aforementioned profiles vanish quickly in less than 1 second when the plasma is turned off. Evidently, the affected zone in the aqueous phase is much larger than previously thought. Experiments to determine the contributions of electrostatic and chemical to the electrochemical potential will be described, and experiments focused on understanding the effect of plasma parameters and solution conductivity.

Electrochemical structure of water in contact with low temperature plasma

2019 Spring National Meeting

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