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 CH3CHOH 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.