Collapsing cavitation bubbles, chemical

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 collapsed 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 applications of ultrasound to enhancing chemical reactivity has been explored, with important applications in mixed-phase synthesis, materials chemistry, and biomedical uses. 

Hua et al. (1995) proposed a supercritical water region in addition to two reaction regions such as the gas phase in the center of a collapsing cavitation bubble and a thin shell of superheated liquid surrounding the vapor phase. Chemical transformations are initiated predominantly by pyrolysis at the bubble interface or in the gas phase and attack by hydroxyl radicals generated from the decomposition of water. Depending on its physical properties, a molecule can simultaneously or sequentially react in both the gas and interfacial liquid regions. 

Flint and Suslick (1991) and Seghal and Wang (1989) clearly demonstrated that temperature and pressure within a collapsing cavitation bubble exceed the critical point of water, on the basis of previously estimated temperatures within a collapsed bubble and a smaller layer of surrounding liquid. However, no experimental data are available for the density of nuclei or actual cavitation bubbles in water during ultrasonic irradiation or SCW accelerated chemical reactions. 

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. 

TTHE ACTION OF ULTRASONIC WAVES IN LIQUIDS can induce or accelerate a wide variety of chemical reactions (1, 2) The chemical effects of ultrasound have been explained in terms of reactions occurring inside, at the interface, or at some distance away from cavitating gas bubbles. In the interior of a collapsing cavitation bubble, extreme but transient conditions exist. Temper-... 

Much of the work done in recent years on polymer mechanochemistry has made use of the high elongational strain rates observed around collapsing cavitation bubbles in sonicated solutions, as outlined in the section on mechanoluminescence [27]. In addition to the distinctive features of sonochemically-induced mechanical reactivity described above, further attention needs to be paid to the sonication conditions in the case of mechanochemical catalysis, because catalyst lifetime and turnover number are reduced by sonochemical byproducts. Implosion of cavitation bubbles is essentially an adiabatic process which leads to formation of local hotspots within the bubble in which temperature and pressure increases drastically. The content of cavitation bubbles pyrolyses under these extreme conditions and results in formation of reactive species, such as radicals and persistent secondary byproducts acidic byproducts may also form from the degradation of the substrates [75]. Chemical impurities deactivate the reactive catalyst partially if not completely. Recent studies in our group have shown that heat capacity of gas... 

Kotronarou, A. Hoffinann, M. R. The Chemical Effects of Collapsing Cavitation Bubbles Mathematical Modeling. In Aquatic Chemistry Intetfacial and Interspecies Processes Huang, C. P., O Melia, C. R., Morgan, J. J., Eds. Advances in Chemistry Series 244 American Chemical Society Washington, DC, 1995 pp 233-251. 

Ultrasonic irradiation has also been employed for chemical remediation of water but the mode of sonochemical degradation of organic compounds in aqueous solution depends upon their physical and chemical properties. This is because there are two ways in which the cavitation bubble can function. In the case of volatile chemicals which enter the bubble, destruction occurs through the extreme conditions generated on collapse. In the case of chemicals remaining in the aqueous phase the bubble acts as a source of radicals (H, HO and HOO ) which enter the bulk solution and react with pollutants. 

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... 

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