Free radical formation from sonolysis of water in the presence of different gases

In the present study by applying electron spin resonance-spin trapping method, when a high frequency (1650 kHz) ultrasound was irradiated to water dissolved with different gas molecules (O(2), N(2), Ar, Ne, He, and H(2)) at 25°C of water bulk temperature, free radical generation pattern differed dependently on the dissolved gas molecules. Only (•)OH was detected in the O(2)-dissolved water sample, and the amount of the radical was much greater than that determined in any of other gas-dissolved water samples. One of the possible reasons to explain why the (•)H radical was not detected in the O(2)-dissolved water is that the (•)H reacts with O(2) to form (•)OOH. However, no electron spin resonance signals related to the adduct of not only 5,5-dimethyl-1-pyrroline-N-oxide but 5-(2,2-Dimethyl-1,3-propoxy cyclophosphoryl)-5-methyl-1-pyrroline N-oxide and (•)OOH were observed. In the H(2)-dissolved water, only (•)H was detected, suggesting that H(2) reduces or neutralizes (•)OH. In the N(2)-disolved water, both (•)OH and (•)H were detected at comparable level. In the water samples dissolved with rare gases (Ar, Ne, and He), the amount of (•)H was almost double as compared with that of (•)OH, and both (•)OH and (•)H yields increased in the order Ar > Ne > He.