Pulsed Electrical Discharges in Water: Can Non-volatile Compounds Diffuse into the Plasma Channel?

The objective of this research effort is to develop a more comprehensive understanding of how molecules get degraded in plasma during an electrical discharge in water. The study correlates the intensity of hydroxyl (OH) radicals in the plasma and physicochemical properties of aqueous solutions of methanol, ethanol, acetonitrile, acetone, dimethyl sulfoxide (DMSO), dimethyl formamide (DMF), phenol, hydroquinone, caffeine, and bisphenol A (BPA). To determine the tendency of the used compounds to penetrate the plasma, their vapor pressures, Henry’s constants, aqueous solubilities, reaction rate constants with OH radicals, and octanol–water partition coefficients are compared and correlated with plasma spectroscopic and hydrogen peroxide (H₂O₂) measurements. OH radicals are precursors to the formation of hydrogen peroxide and any compound that diffuses into the plasma will react with and lower the intensity of OH radicals and therefore the concentration of hydrogen peroxide in the bulk liquid. Optical emission spectroscopy (OES) reveals that all the used compounds diffuse inside the plasma channel regardless of their vapor pressure where they get oxidized (primarily by OH radicals) and thermally degraded. Results also indicate that hydrophobicity (i.e., octanol–water partition coefficient) is the most important property that determines a compound’s tendency to diffuse inside the plasma channel; hydrophobic compounds readily penetrate the plasma whereas hydrophilic compounds tend to stay in the bulk liquid. The rate of formation of hydrogen peroxide is independent of the type of the compound present in the bulk liquid which confirms that this molecule is formed at the plasma interface.