(483a) Pressure-Induced Reduction of Shielding for Improving Sonochemical Activity

Irradiation of a liquid with high intensity ultrasound is known to enhance or alter a wide variety of chemical reactions. It has been reported that ultrasound successfully increases conversion, changes reaction pathways, initiates reactions, and accelerates mixing. These physical and chemical effects of ultrasound predominantly arise from acoustic cavitation. Acoustic cavitation is the growth and subsequent contraction of a cavity in a liquid induced by pressure variations from a sound wave. During the contraction of the cavity, its contents can be almost adiabatically heated, leading to hot-spots in the liquid in which temperatures of ?î5000 K and pressures of several hundreds of bars can be reached. These conditions enable demanding (sono)chemistry, while the macroscopically observable properties of the system remain unchanged.

Frequently, single-bubble dynamics models are employed to gain more insight into acoustic cavitation. From a modeling point of view, an increase in hydrostatic pressure would result in a decrease in ultrasound-induced reactivity. Our measurements have demonstrated, however, that an increase in hydrostatic pressure results in an increase in yield for the oxidation of potassium iodide in aqueous solution.1 This increase can be attributed to the more intense sound field at elevated pressure, which is confirmed by visualization studies as well as sound attenuation measurements. For intense sound fields, a large cloud of bubbles is formed close to the emitter. The high gas content below the emitter leads to a strong attenuation of the sound field. Light scattering has been used to visualize this bubble cloud and to determine the effect of hydrostatic pressure on the volume of the bubble cloud. These studies have demonstrated that the volume of the bubble cloud is reduced at elevated pressure. This reduction implies that shielding of the sound wave is impeded and that the sound field is more efficient. This notion has been confirmed by the measurements performed with the microphone. The intensity level of the applied frequency increases with increasing pressure. At static pressures above 5 bar, a further increase in pressure does not suppress shielding effects any further. Accordingly, the effect predicted from the single-bubble model prevails and the sonochemical reaction rate decreases substantially.2

This counter-intuitive effect of hydrostatic pressure on sonochemical reactivity has illustrated the importance of encountering a variety of phenomena while performing sonochemistry. In addition to the commonly anticipated effects, which are derived from single-bubble models, several effects acting on a macroscopic scale determine the outcome to a large extent.

[1] van Iersel, M. M.; Van den Manacker, J-P. A. J.; Benes, N. E.; Keurentjes, J. T. F. J. Phys. Chem B 2007, 111, 3081 [2] van Iersel, M. M.; Cornel, J.; Benes, N. E.; Keurentjes, J. T. F. J. Chem. Phys. 2007, 126, 064508/1