Free radical generation by non-equilibrium atmospheric pressure plasma in alcohol-water mixtures: An EPR-spin trapping study

Free radical species in aqueous solution - various alcohol-water reaction mixtures - by exposure to non-equilibrium cold atmospheric pressure Ar plasma (CAP), were monitored using electron paramagnetic resonance spin-trapping techniques with 3, 5-dibromo-4-nitrosobenzene sulfonate as a water soluble nitroso spin trap. The major radical species were formed by H-abstraction from alcohol molecules due to •OH radicals. In the ethanol-water mixture •CH₂CH₂OH produced by H abstraction from CH₃ group of the ethanol and •CH₃ radicals were detected. The latter was due to the decomposition of unstable CH₃•CHOH to form the •CH₃ radicals and the stable formaldehyde by C-C bond fission. These intermediates are similar to those observed by reaction with •OH radicals generation in the H₂O₂-UV photolysis of the reaction mixtures. The evidence of •CH₃ radical formation in the pyrolytic decomposition of the reaction mixtures by exposure to ultrasound or in methane irradiated with microwave plasma have been reported previously. However, the pyrolytic •CH₃ radicals were not found in both plasma and H₂O₂-UV photolysis condition. These results suggests that free radicals produced by Ar-CAP are most likely due to the reaction between abundant •OH radicals and alcohol molecules.