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 (O₂, N₂, Ar, Ne, He, and H₂) at 25°C of water bulk temperature, free radical generation pattern differed dependently on the dissolved gas molecules. Only ^?OH was detected in the O₂-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 O₂-dissolved water is that the ^?H reacts with O₂ 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 H₂-dissolved water, only ^?H was detected, suggesting that H₂ reduces or neutralizes ^?OH. In the N₂-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.