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 ·CH 2 CH 2 OH produced by H abstraction from CH 3 group of the ethanol and ·CH 3 radicals were detected. The latter was due to the decomposition of unstable CH 3 ·CHOH to form the ·CH 3 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 H 2 O 2 –UV photolysis of the reaction mixtures. The evidence of ·CH 3 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 ·CH 3 radicals were not found in both plasma and H 2 O 2 –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.