The Site of Sonochemical Reactions

In order to probe the nature of the reactive site generated by acoustic cavitation, several new chemical dosimeters have been developed and the effects of physical conditions on their yields have been explored. Specifically, the effects of solution vapor pressure and dosimeter vapor pressure on sonochemical reaction rates have been examined. An excellent correlation between the log of the sonochemical rate and the solvent vapor pressure is found. This is the expected result if sonochemical reactions originate from an implosive cavitational collapse, generating a transient “hot-spot.” Thus as bulk temperature is increased (and solution vapor pressure increases), the intensity of cavitational collapse, the peak temperature reached during such collapse and, consequently, the rate of sonochemical reactions all decrease. In contrast, if the solution vapor pressure is held constant and the dosimeter's vapor pressure made variable, the rate of sonochemical reaction is linear with increasing dosimeter vapor pressure and exhibits a non-zero intercept. This demonstrates for the first time that primary sonochemical reactions occur both in the vapor phase and in the liquid surrounding the cavitation event. Thus sonochemistry occurs principally to chemical species with significant vapor pressure; nonetheless, involatiles will react in the heated liquid shell surrounding the collapsing cavity or with other diffusible high-energy intermediates in the bulk solution.