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.