Sonochemical variables

Control of sonochemical reactions is subject to the same limitation that any thermal process has the Boltzmann energy distribution means that the energy per individual molecule wiU vary widely. One does have easy control, however, over the energetics of cavitation through the parameters of acoustic intensity, temperature, ambient gas, and solvent choice. The thermal conductivity of the ambient gas (eg, a variable He/Ar atmosphere) and the overaU solvent vapor pressure provide easy methods for the experimental control of the peak temperatures generated during the cavitational coUapse. 

Design of sonochemical reactors is a very important parameter in deciding the net cavitational effects. Use of multiple transducers and multiple frequencies with possibility of variable power dissipation is recommended. Theoretical analysis for predicting the cavitational activity distribution is recommended for optimization of the geometry of the reactor including the transducer locations in the case of multiple transducer reactors. Use of process intensifying parameters at zones with minimum cavitational intensity should help in enhancing the net cavitational effects. 

Wayment DG, Casadonte DJ (2002) Design and calibration of a single-transducer variable-frequency sonication system. Ultrason Sonochem 9 189-195... 

The electrochemical synthesis developed by Reetz and co-workers offers at present the most rational method for control of particle size. Researchers have obtained at will almost monodisperse samples of colloidal Pd and Ni between 1 and 6nm using variable-current densities and suitable adjustment of further essential parameters [12]. For thermal decomposition methods the resulting particle size has been found to depend on the heat source [44f]. Size control has also been reported for the sonochemical decomposition method [45e] and y-radiolysis [48]. 

Torok, B., Balazsik, K., Torok, M., Szollosi, G., Bartok, M. (2000) Asymmetric sonochemical reactions. Enantioselective hydrogenation of a-ketoesters over platinum catalysts. Ultrasonics Sonochem. 7, 151-155. Torok, B., Balazsik, K., Torok, M., Felfoldi, K., Bartok, M. (2002) Heterogeneous asymmetric reactions 20. Effect of ultrasonic variables on the... 

For example, with regard to the hot-spot theory outlined above, it would clearly be useful to understand the effects of changing the solvent, or the ambient temperature and at which the reaction was carried out. Furthermore, the design of ultrasonic probe systems allows for ready variation of the power input and occasionally variation of the frequency of the output. Hence, there are a number of factors that must be bom in mind when setting up a viable system. For this reason, the following section is devoted to a discussion of the effects of extrinsic variables on the sonochemical process. 

The credibility of the hot spot theory is reinforced by its ability to account for the effects of extrinsic variables on the sonochemical process. Nevertheless, the frequency of ultrasound applied is surprisingly irrelevant to the course of the reaction. Cleaning baths produce a range of frequencies which often vary from day to day, or even during the course of a reaction, and yet this has no discemable effect on the sonochemistry observed. 

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