Sonochemical phenomena

Somewhat later it was hypothesized that mechanochemical and sonochemical phenomena indnced by gas-bnbble transformations could be of decisive importance. Several serions scientihc gronps have np to now looked at this problem. The mechanism being tested by them is not able, of course, to satisfy the power levels claimed in nuclear reactions, bnt it is of high scientihc interest. It has already been found that it is not at all compnlsory to generate the gas electrolytically in order to utilize the energy of the bubbles, and that the gas itself need not at all contain deuterium. One of the model reactants nsed in cnrrent investigations in this area, more particularly, is deuterated acetone CjDgO. 

Chendke, P. K. Fogler, H. S. Second-Order Sonochemical Phenomena. Extensions of Previous Work and Applications in Industrial Process, Chem. Eng. J. 1974, 8, 165. 

A number of reactions have been developed in which the sonochemical mechanism is less clearly identified. In some cases, the exact nature of the transient species can be controversial also in terms of conventional organic chemistry. They were listed under this heading for the sake of convenience from the synthetic viewpoint, and many of them should be interesting topics for a deeper approach to the sonochemical phenomena. 

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... 

The chemical applications of ultrasound (Sonochemistry) have become an exciting new field of research during the past decade. Recently, Li and coworkers have found an efficient and convenient procedure for the preparation of oximes via the condensation of aldehydes and ketones in ethanol with hydroxylamine hydrochloride under ultrasound irradiation (Scheme 8). Compared with conventional methods, the main advantages of the sonochemical procedure are milder conditions, higher yields and shorter reaction periods. The reason may be the phenomenon of cavitations produced by ultrasound. 

If the yield of a silent reaction is n% after a specific period of time while the yield of the corresponding sonochemical reaction is m%, the ratio min higher than 1 is described as the effect of ultrasound. Since its beginning, ultrasound effects have been considered to originate in the general phenomenon of cavitation, which generates high temperatures, pressures, and shock waves. 

Solvent properties affect US-assisted digestion, as they impose a cavitation threshold above which sonochemical effects are perceived, as it were, by the medium. Therefore, any phenomenon altering the same solvent property can modify such a threshold. 

Recentiy this phenomenon was found to be induced at a reiatively low ultrasonic intensity, even with progressive waves by the second harmonic overlapping the fundamental one [10]. This finding is of paramount importance as regards the clinical applicability of US, and also in sonochemical uses involving systems sensitive to high ultrasound intensities. 

Sonochemistry is the research area in which molecules undergo chemical reaction due to the application of powerful ultrasound radiation (20 KHz-10 MHz) [4]. The physical phenomenon responsible for the sonochemical process is acoustic cavitation. Let us first address the question of how 20 kHz radiation can rupture chemical bonds (the question is also related to 1 MHz radiation), and try to explain the role of a few parameters in determining the yield of a sonochemical reaction, and then describe the unique products obtained when ultrasound radiation is used in materials science. 

Sonochemical enhancement of reaction rates is caused by a phenomenon called cavitation. Therefore, we largely confine the treatment in this chapter to the chemical and reaction engineering (scale-up) aspects of cavitation and its associated effects (see Shah et al., 1999, for a detailed treatment). An alternative means of achieving the same result is by mimicking the ultrasonic effect by inducing hydrodynamic cavitation. Because of the practical importance of this technique, we conclude the chapter by outlining its main features. 

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. - ... 

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