The Origin of Sonochemical Reactivity

The precise details of the way in which ultrasound acts to produce chemical reaction are not known. However, ultrasonic cavitation is well established as the originator of this phenomenon. This review does not attempt to cover all the literature that has been generated on this subject, but rather to present a qualitative overview in an attempt to clarify this subject which tends to be regarded as something of a black art For a more detailed report, interested readers are referred to Suslick s recently published review which deals comprehensively with this aspect of sonochemistry [3, 4]. 

The marked effects of ultrasound actually arise from the way in which sound is propagated through media. Longitudinal vibration of molecules in a liquid generates a series of compressions and rarefactions, that is areas of high and low local pressure. If the solvent molecules are tom apart with sufficient force during a rarefaction, cavities are formed at the points where the pressure in the liquid drops well below its vapour pressure. This creates 

Human hearing (16Hz to 16KHz) Power ultrasound (20KHzto100KHz) Q High frequency ultrasound (MO MHz) 

At this point it should be stressed that no reaction occurs in the absence of cavitation and a number of published examples contain parallel experiments which show that vigorous stirring or mechanical agitation fails to produce such marked increases in the rate of reaction [5, 6, 7]. That is, this is not primarily a mixing effect. 

Recent work by Lauterbom et al. [8] on high speed holography has produced pictures of the collapse of bubbles and the shock waves generated (see plates 1 and 2) by sonolysis of water. 

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However, the purpose of this review is to provide a critical introduction for those intrigued by the synthetic potential of ultrasound and this is reflected in the examples discussed. In addition, a brief overview of the physics of acoustic cavitation is included in order to explain the origin of sonochemical reactivity and the means by which ultrasound can be exploited to its full potential. 

Since sonochemistry takes its origin in cavitation, the reactivity depends on the characteristics of the bubbles. Their size and lifetime, and the content of the gaseous phase, depend on the physical properties of the medium and the parameters (amplitude and frequency) of the wave. Conducting a sonochemical reaction implies that a multiparameter problem is examined. 

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