CHEMICAL BUBBLE DYNAMICS AND QUANTITATIVE SONOCHEMISTRY

We model the collapse of a bubble by taking into account all the energ y forms involved (i.e., mechanical, thermal, chemical, and radiative) and compare the calculated radical yields with sonochemical data in H2 O. Water decomposition plays a critical role in the energy balance, bu t trails equilibrium even in bubbles collapsing at subsonic speeds. In tegration of the equation of bubble motion coupled with a full chemica l mechanism reveals that (1) terminal gas temperatures and Mach number s M-L increase in cooler water, (2) Gamma(OH), the number of OH-radica ls produced per unit applied work at maximum M-L-when bubbles become u nstable and disperse into the liquid-decreases at small and very large sound intensities. We show that available data on the sonochemical de composition of volatile solutes-such as CCl4, which is pyrolyzed withi n collapsing bubbles-are compatible with the efficient conversion of u ltrasonic energy into transient cavitation. On this basis we calculate Gamma(OH) = (1 +/- 0.5) x 10(17) molecules/J for R-0 = 2 mu m bubbles optimally sonicated at 300 kHz and 2.3 W/cm(2) by assuming mass and e nergy accommodation coefficients of alpha less than or equal to 7 x 10 (-3) and epsilon less than or equal to 0.04, respectively, in gas-liqu id collisions, and values about 3-fold smaller after averaging over th e nuclei size distribution. Since there is negligible radical recombin ation during dispersal, these Gamma(OH) values represent available oxi dant yields, that agree with experimental data on iodide sonochemical oxidation. Bubbles emit little radiation, suggesting that only radial shock waves may heat small regions to the 10(4)-10(5) K range required by some sonoluminescence experiments. The contribution of this sonolu minescent core to sonochemical action is, however, insignificant. We s how that much larger accommodation coefficients would lead to higher t emperatures, but also to O atoms rather than OH radicals and ultimatel y to excess O-2, at variance with experimental evidence.