Sonochemistry and cavitation

M.A. Margulis, Sonochemistry and Cavitation, Gordon and Breach Science Publishers, Luxembourg, 1995,... 

M. A. Margulis, Sonochemistry and Cavitation, translated by G. Leib (Taylor Francis, London, 1995) L. A. Crum, K. S. Suslick and J. L. Reisse (editors), Sonochemistry andSonoluminescence (Kluwer Acadmic, Dordrecht, The Netherlands, 1998). 

The observation that sonochemistry and luminescent activities are not necessarily coincident had never been reported and suggests that the conditions at the root of sonoluminescence and sonochemistry could be different in nature (or in intensity). Chemical species as tracers should serve to describe the liquid flows the knowledge of which is of prime importance in medical ultrasound (microstreaming, acoustic streaming and/or rectified acoustic streaming zy Even if synthetic exploitations are difficult to imagine, some fundamental aspects of sonochemistry and cavitation still to be explored are very probably involved, and in this respect deserve further development. 

Fig. 1. Transient acoustic cavitation the origin of sonochemistry and sonoluminescence.

The phenomenon of acoustic cavitation results in an enormous concentration of energy. If one considers the energy density in an acoustic field that produces cavitation and that in the coUapsed cavitation bubble, there is an amplification factor of over eleven orders of magnitude. The enormous local temperatures and pressures so created result in phenomena such as sonochemistry and sonoluminescence and provide a unique means for fundamental studies of chemistry and physics under extreme conditions. A diverse set of apphcations of ultrasound to enhancing chemical reactivity has been explored, with important apphcations in mixed-phase synthesis, materials chemistry, and biomedical uses. 

Abstract Acoustic cavitation is the formation and collapse of bubbles in liquid irradiated by intense ultrasound. The speed of the bubble collapse sometimes reaches the sound velocity in the liquid. Accordingly, the bubble collapse becomes a quasi-adiabatic process. The temperature and pressure inside a bubble increase to thousands of Kelvin and thousands of bars, respectively. As a result, water vapor and oxygen, if present, are dissociated inside a bubble and oxidants such as OH, O, and H2O2 are produced, which is called sonochemical reactions. The pulsation of active bubbles is intrinsically nonlinear. In the present review, fundamentals of acoustic cavitation, sonochemistry, and acoustic fields in sonochemical reactors have been discussed. 

The chemical effects of ultrasound do not arise from a direct interaction with molecular species. Ultrasound spans the frequencies of roughly 15 kHz to 1 GHz. With sound velocities in liquids typically about 1500 m/s, acoustic wavelengths range from roughly 10 to 10 4 cm. These are not molecular dimensions. Consequently, no direct coupling of the acoustic field with chemical species on a molecular level can account for sonochemistry or sonoluminescence. Instead, sonochemistry and sonoluminescence derive principally from acoustic cavitation, which serves as an effective means of concentrating the diffuse energy of sound. 

It should now be uimecessary to underline the mechanical connectimi between sonochemistry and other subfields of mechanochemistry. The former clearly possesses a series of inherent characteristics by virtue of various forces generated in a liquid under the action of pressure waves. Both chemical and physical activation, especially when cavitation is present, are able to drive numerous transformatimis and often provide a useful mechanistic rationale. 

The possibility of using sound energy in chemistry was established more than 70 years ago. By definition, sonochemistry is the application of powerful ultrasound radiation (10 kHz to 20 kHz) to cause chemical changes to molecules. The physical phenomenon behind this process is acoustic cavitation. Typical processes that occur in sonochemistry are the creation, growth and collapse of a bubble. A typical laboratory setup for sonochemical reactions is shown in Fig. 8.17. More details of sonochemistry and the theory behind it can be found elsewhere. - ... 

Ionic liquids have favorable intrinsic properties that make them of interest as solvents for various chemical reactions. The same properties that make the liquids effective solvents also make them interesting liquids for studies involving sonochemistry, acoustic cavitation, and sonoluminescence (Suslick et al., 1991, Suslick, 1988). Recent interest in using ultrasound to accelerate chemical reactions conducted in ionic liquids necessitates an understanding of the effects of acoustic cavitation on these solvents (Flannigan et al., 2005). 

Sonochemistry can be roughly divided into categories based on the nature of the cavitation event homogeneous sonochemistry of hquids, heterogeneous sonochemistry of hquid—hquid or hquid—sohd systems, and sonocatalysis (which overlaps the first two) (12—15). In some cases, ultrasonic irradiation can increase reactivity by nearly a million-fold (16). Because cavitation can only occur in hquids, chemical reactions are not generaUy seen in the ultrasonic irradiation of sohds or sohd-gas systems. 

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