Sonochemical degradation rates of volatile solutes

We report degradation rates of chlorinated methanes, ethanes, and ethenes-spanning the range of Henry's law constants 0.9 less than or equal to H/(atm M-1) less than or equal to 24.5-in water solutions sonicated at f = 205, 358, 618, and 1078 kHz. First-order degradation rate constants, k(-X), vary as k(-X) similar to H-X(0.30+/-0.03) at all frequencies, change with f by less than a factor of 2 in this range, and peak at about 600 kHz for all species. We show that experimental rates are consistent with (1) complete decomposition of the solute contained in collapsing bubbles, (2) about 15% ultrasound power efficiency for transient cavitation, and (3) a rather flat N(R-o) proportional to R-o(n), n similar to 0, initial radius bubble distribution under continuous sonication. The solute content of collapsing bubbles is composed of the equilibrated vapor at R-o, plus the amount incorporated by diffusion froth the surrounding solution during the acoustically driven expansion from R-o to R-max, the maximum radius attained prior to collapse. The finding that k(-X)'s decline above 600 kHz is ascribed to the fact that increasingly smaller bubbles collapse at rates reaching a limiting value at sufficiently high frequencies.