Acoustic wavelength in liquids range from approx. 110 to 0.15mm for frequencies between 18kHz and 10Mhz, much greater than molecular dimensions.  Therefore, there is no direct coupling of the acoustic field with molecules.  The effects of ultrasonication are to a large degree a result of the ultrasonic cavitation in liquids.  

Ultrasonication contributes to heterogeneous and homogeneous catalysisin many ways. Individual effects can be promoted or reduced adapting the ultrasonic amplitude and liquid pressure.

Chemical reactions involving reagents and a catalyst of more than one phase (heterogeneous catalysis) are limited to the phase boundary as this is the only place, where the reagent as well as the catalyst are present. Exposure of the reagents and of the catalyst to each other is a key factor for many multi-phase chemical reactions. For this reason, the specific surface area of the phase boundary becomes influential for the chemical rate of reaction

When reagents react at a phase boundary, the products of the chemical reaction accumulate at the contact surface. This blocks other reagent molecules from interacting at this phase boundary. Mechanical shear forces caused by cavitational jet streams and acoustical streaming result in turbulent flow and material transport from and to particle or droplet surfaces. In the case of droplets, the high shear can lead to the coalescence and subsequent formation of new droplets. As the chemical reaction progresses over time, a repeated sonication, e.g. two-stage or recirculation, may be required to maximize the exposure of the reagents.

Ultrasonic cavitation is a unique way to put energy into chemical reactions. A combination of high speed liquid jets, high pressure (>1000atm) and high temperatures (>5000K), enormous heating and cooling rates (>109Ks-1) occur locally concentrated during the implosive compression of cavitational bubbles. Kenneth Suslick says: "Cavitation is an extraordinary method of concentrating the diffuse energy of sound into a chemically usable form."

Cavitational erosion on particle surfaces generates unpassivated, highly reactive surfaces. Short-lived high temperatures and pressures contribute to molecular decomposition and increase the reactivityofmany chemical species. Ultrasonic irradiation can be used in the preparation of catalysts, e.g. to produce aggregates of fine-size particles. This produces amorphous catalysts particles of high specific surface area. Due to this aggregate structure, such catalysts can be separated from the reaction products (i.e. by filtration).

(Hielscher, Sonocatalysis, 6/16/2011)

Acoustic Cavitation andIts Chemical Consequences
Phil. Trans. Roy. Soc. A, 1999, 357, 335-353.

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