Cavitation heterogeneous systems

Alex CCT, Goh NN, Chia LS (1995) Effects of particle size morphology on ultrasound induced cavitational mechanism in heterogeneous systems. J Chem Soc Chem Commun 2 201-201... 

Rule 2 applies to heterogeneous systems where a more complex situation occurs and here reactions proceeding via ionic intermediates can be stimulated by the mechanical effects of cavitational agitation. This has been termed false sonochemistry although many industrialists would argue that the term false may not be correct because if the result of ultrasonic irradiation assists a reaction it should still be considered to be assisted by sonication and thus sonochemical . In fact the true test for false sonochemistry is that similar results should, in principle, be obtained using an efficient mixing system in place of sonication. Such a comparison is not always possible. 

It is worth noting that the use of a very high-speed stirrer e. g. an Ultra-Turax can produce similar effects to sonication in heterogeneous systems. This may well be a case of chemistry induced by hydrodynamic rather than acoustic cavitation [32, 33]. 

However, this commonly accepted theory is incomplete and applies with much difficulty to systems involving nonvolatile substances. The most relevant example is metals. For a heterogeneous system, only the mechanical effects of sonic waves govern the sonochemical processes. Such an effect as agitation, or cleaning of a solid surface, has a mechanical nature. Thus, ultrasound transforms potassium into its dispersed form. This transformation accelerates electron transfer from the metal to the organic acceptor see Chapter 2. Of course, ultrasonic waves interact with the metal by their cavitational effects. 

In any heterogeneous system cavitation which occurs in the bulk liquid phase will be subject to... 

A number of reactirais, especially those in heterogeneous systems, such as alkoxide/ water mixtures, have been shown to give enhanced reaction rates and yields under sonication. The process is driven by acoustic cavitation [1—4]. 

For flow systems, the strength and size of the cavitating zone should be optimized so that it matches the flow rate into the system. Care should be taken to find the best frequency for both homogeneous and heterogeneous reactions. A particular problem with heterogeneous systems is that it is necessary to ensure... 

Effects of Sonication. In a subsequent study [92] on the accelerating effect of ultrasonic waves on the Barbier reaction of benzaldehyde, 1-bromo-heptane and lithium, Luche and co-workers could not state that cavitation was the only important phenomenon. No information had been obtained on the effects of non-cavitational shock waves or the frequency. The authors came to the conclusion that the work published had to be considered as an approach for a better knowledge of the interaction of ultrasound with a heterogeneous system, but the problem in its generality could not be considered as fully understood. 

Sonochemistry in heterogeneous systems is the result of a combination of chemical and mechanical effects of cavitation, and it is very difficult to ascribe sonochemistry to any single global origin, other than the overriding source of activity, namely, cavitation. 

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. 

In heterogeneous liquid/liquid reactions, cavitational collapse at or near the interface will cause disruption and mixing, resulting in the formation of very fine emulsions. When very fine emulsions are formed, the surface area available for the reaction between the two phases is significantly increased, thus increasing the rates of reaction. The emulsions formed using cavitation, are usually smaller in size and more stable, than those obtained using conventional techniques and often require little or no surfactant to maintain the stability [8]. This is very beneficial particularly in the case of phase-transfer catalyzed reactions or biphasic systems. 

As we have mentioned before, acoustic streaming, cavitation and other effects derived from them, microjetting and shock waves take also relevance when the ultrasound field interacts with solid walls. On the other hand, an electrochemical process is a heterogeneous electron transfer which takes place in the interphase electrode-solution, it means, in a very located zone of the electrochemical system. Therefore, a carefully and comprehensive read reveals that all these phenomena can provide opposite effects in an electrochemical process. For example, shock waves can avoid the passivation of the electrode or damage the electrode surface depending on the electrode process and/or strength of the electrode materials [29]. 

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