Chemical effects of collapsing cavitation

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. 

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. 

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. 

Sonochemistry (chemical events induced by exposure to ultrasound) occupies an important place in organic chemistry. The chemical effects of high-intensity ultrasound were extensively smdied in aqueous solutions for many years, but is now applied to a variety of organic solvents. The origin of sonochemistry is acoustic cavitation the creation, growth, and implosive collapse of gas vacuoles in solution by the sound field. Acoustic cavitation is the phenomenon by which intense ultrasonic waves induce the formation, oscillation, and implosion of gas... 

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-... 

The chemical effects of ultrasound arise from cavitation, i.e. the collapse of microscopic bubbles in a liquid. Upon implosion of a cavity, locally extreme conditions in the bubble occur (5000 K and 200 bar) and high strain rates are generated outside the bubble (10 Monomer molecules are... 

The acoustic pressure amplitude determines the growth of a cavitation bubble and consequently the chemical effects upon collapse. The amplitude of the pressure wave can be measured with a hydrophone or can he calculated using a calorimetric method (9,10), in which it is possible to determine the ultrasoimd power (Qus) that is transferred to the liquid. With the ultrasound power and the surface area of the ultrasound source (Aus), the acoustic amplitude can he calculated according to equation (2), for which the ultrasoimd intensity is the power input divided by the surface area of the source (11). 

For decades, the accelerating effect of ultrasonic irradiation has been a useful reactivity paradigm most physical and chemical effects arise from cavitations without an alteration of the rotational or vibrational states of molecules. In contrast to classical chemistry, in sonochem-istry it is not necessary to go to higher temperatures in order to accelerate the chemical process. To drive the chemical transformations the released kinetic energy from the cavitational collapse is sufficient [177]. Such an effect was also observed in this esterification reaction, where at room temperature (Table 6.10, entry 5) both the reaction rate and the selectivity in the main product were enhanced in comparison to the values obtained at 80°C (Table 6.10, entry 4) the reaction rate increased 43 times when compared with thermal activation and around 6 times when compared with microwaves. Even more importantly the selectivity to DAG and TAG after 30 min was at almost the same level as that obtained by thermal heating at 100°C for 22 h. 

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