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 colla pse of acoustic cavitation bubbles produced by ultrasound in liquids. The semiclassical model of the temperature dependence of the kinetic i sotope 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 mix tures (1:1) exposed to 50 kHz ultrasound and were detected by spin tra pping with the nitrone spin traps N-tert-butyl-alpha-phenylnitrone (PE N), alpha-(4-pyridyl-1 -oxy)-N-tert-butylnitrone (POBN), alpha-(4-pyri dyl-1-methyl)-N-tert-butylnitrone (PYBN), and 5,5-dimethyl-1-pyrroline N-oxide (DMPO). The resulting spin adducts were identified and quanti fied 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 D MPO for the measurement of H-. and D-. atoms (in our experimental syst em the half-lifes of PBN/H-. and DMPO/H-. were similar to 600 and simi lar 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 th e decay rate of DMPO/H-. was 2.4 times higher than that of DMPO/D-.. T he effective temperatures of O-H bond pyrolysis determined from the se miclassical treatment are in the region of similar to 2000-4000 K usin g the PEN-type spin traps. This estimate may be compared to the temper atures (similar to 5000 K) determined by Suslick ct al. for the gas ph ase of the cavitation bubbles in alkanes and (approximate to 1900 K) f or the interfacial region of the cavitation bubbles [Suslick, K. S., c t al. J. Am. Chem. Sec. 1986, 108, 5641].