Sonochemical degradation rates of volatile solutes

We report degradation rates of chlorinated methanes, ethanes, and ethenes-s panning 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, 3 58, 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 l ess than a factor of 2 in this range, and peak at about 600 kHz for all spe cies. We show that experimental rates are consistent with (1) complete deco mposition of the solute contained in collapsing bubbles, (2) about 15% ultr asound 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 distribut ion under continuous sonication. The solute content of collapsing bubbles i s composed of the equilibrated vapor at R-o, plus the amount incorporated b y diffusion froth the surrounding solution during the acoustically driven e xpansion 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 a t sufficiently high frequencies.