Sonochemical Degradation Rates of Volatile Solutes

Abstract

We report degradation rates of chlorinated methanes, ethanes, and ethenes -- spanning the range of Henry's law constants 0.9 ≤ H/(atm M^(-1)) ≤ 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) ∼ 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) ∝ R_o^n,n ∼ 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 from 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.