Cavitation bubbles, chemical effects

Further investigations of chemical kinetics and transformation products will be carried out during the final phase of the project. In order to truly understand sonochemical effects, the behavior of the individual bubbles and the bubble clouds must be more finely resolved. Physical characterization of cavitation bubble clouds will also be performed. Thus, a more fundamental link will be established between bulk, observable parameters and sonochemistry, via the physics and hydrodynamics of the cavitating cloud. 

Oxidation at the benzylic position of indane -with potassium permanganate (Eq. 3.30) gives indanone in good yields and no PTC is necessary [133]. In a two-phase system consisting of an aqueous solution of KMn04 and indane in benzene an 80 % yield can be obtained under a reduced pressure of ca. 450 Torr. The authors explain this effect by the size of the cavitation bubbles, which is dictated to some extent by the over pressure. An optimal energy transformation, from acoustic to chemical, can thus take place. 

The transient nature of the cavitation event precludes conventional measurement of the conditions generated during bubble collapse. Chemical reactions themselves, however, can be used to probe reaction conditions. The effective temperature realized by the collapse of clouds of cavitating bubbles can be determined by the use of competing unimolecular reactions whose rate dependencies on temperature have already been measured. The sonochemical ligand substitutions of volatile metal carbonyls were used as... 

The effects of ultrasonic irradiation on photochemical reactions have been also reported. In those papers, effects of cavitation were demonstrated. Cavitation means the process in which micro bubbles, which are formed within a liquid during the rarefaction cycle of the acoustic wave, undergo violent collapse during the compression cycle of the wave.5) The dissociation of water to radicals is an example of these effects. Since activated chemical species such as free radicals have high reactivity, chemical reactions proceed. In other words, this phenomenon is a chemical effect of ultrasonic waves. 

The concentration of volatile compounds in the cavitation bubbles increases with temperature thus, faster degradation rates are observed at higher temperatures for those compounds [23]. Conversely, in the case of nonvolatile substrates (that react through radicals reactions in solution), the effect of temperature is somehow opposed to the chemical common sense. In these cases, an increase in the ambient reaction temperature results in an overall decrease in the sonochemical reaction rates [24]. The major effect of temperature on the cavitation phenomenon is achieved through the vapor pressure of the solvent. The presence of water vapor inside the cavity, although essential to the sonochemical phenomenon, reduces the amount of energy... 

Thus, the presence of 03 as a background gas provides new sources for OH radical formation during sonolysis. The direct pyrolysis in the cavitation bubbles of volatile intermediates generated during ozonation also explains the improvement in the overall efficiency. In addition to these direct chemical effects, sonication also increases mass transfer coefficients of ozone from the bubbles to the solution, where the direct reaction of 03 with the substrates, or further radical formation, takes place. 

Chemat et al. [14] found the ]oint use of US and microwaves for the treatment of edible oils for the determination of copper to shorten the time taken by this step to about a half that was required in the classical procedure (heating in a Buchi digester) or with microwave assistance, nitric acid and hydrogen peroxide. However, they did not state the specific medium where the microwave-US-assisted method was implemented and assumed US to have mechanical effects only, even though they mentioned a cavitational effect. This is a very common mistake in working with US that is clarified in an extensive discussion by Chanon and Luche [15] of the division of sonochemistry applications into reactions which were the result of true and false effects. Essentially, these terms refer to real chemical effects induced by cavitation and those effects that can be ascribed to the mechanical impact of bubble collapse. The presence of one of these phenomena only has not been demonstrated in the work of Chemat et al. [14] — despite the illustrative figure in their article — so their ascribing the results to purely mechanical effects of US was unwarranted. 

Concerning chemical effects, US is known to increase the reactivity of some chemicals. The high temperature and pressure within a collapsing cavitation bubble produced by US irradiation causes the formation of free radicals and various other species. The primary chemical effects are therefore the promotion and acceleration of reactions involved in sample digestion. 

The effects of US on surviving cells may include structural changes and interactions with deoxyribonucleic acid (DNA) [83]. The biological effects observed in vitro include fragmentation of cell membranes caused by the collapse of cavitation bubbles, microstreaming near the boundary layer and formation of radicals, which promote chemical reactions leading to wall decomposition [84]. Carstensen et al. [85] found the extent of cell disruption to be inversely proportional to the cell concentration. 

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