EPR SPIN-TRAPPING STUDY OF THE SONOLYSIS OF H2O/D2O MIXTURES - PROBING THE TEMPERATURES OF CAVITATION REGIONS

High temperatures and pressures are generated during the violent collapse of acoustic cavitation bubbles produced by ultrasound in liquids. The semiclassical model of the temperature dependence of the kinetic isotope effect for H-. and D-. atom formation was used to estimate the effective temperature of the hot cavitation regions in which H-. and D-. atoms are formed by ultrasound-induced pyrolysis of water molecules. The H-. and D-. atoms were formed in argon-saturated H2O and D2O mixtures (1:1) exposed to 50 kHz ultrasound and were detected by spin trapping with the nitrone spin traps N-tert-butyl-alpha-phenylnitrone (PEN), alpha-(4-pyridyl-1 -oxy)-N-tert-butylnitrone (POBN), alpha-(4-pyridyl-1-methyl)-N-tert-butylnitrone (PYBN), and 5,5-dimethyl-1-pyrroline N-oxide (DMPO). The resulting spin adducts were identified and quantified by EPR spectroscopy. Because of the higher stability of H- and D-adducts, the PEN-type spin traps were found to be more suitable than DMPO for the measurement of H-. and D-. atoms (in our experimental system the half-lifes of PBN/H-. and DMPO/H-. were similar to 600 and similar to 40 s, respectively). An isotope effect on spin adduct stability was also observed: the decay rates of PEN-type H-adducts were similar to 1.2 times higher than those of the corresponding D-adducts, and the decay rate of DMPO/H-. was 2.4 times higher than that of DMPO/D-.. The effective temperatures of O-H bond pyrolysis determined from the semiclassical treatment are in the region of similar to 2000-4000 K using the PEN-type spin traps. This estimate may be compared to the temperatures (similar to 5000 K) determined by Suslick ct al. for the gas phase of the cavitation bubbles in alkanes and (approximate to 1900 K) for the interfacial region of the cavitation bubbles [Suslick, K. S., ct al. J. Am. Chem. Sec. 1986, 108, 5641].