Ultrasound, chemical effects

The chemical effects of ultrasound do not arise from a direct interaction with molecular species. Ultrasound spans the frequencies of roughly 15 kH2 to 1 GH2. With sound velocities in Hquids typically about 1500 m/s, acoustic wavelengths range from roughly 10 to lO " cm. These are not molecular dimensions. Consequently, no direct coupling of the acoustic field with chemical species on a molecular level can account for sonochemistry or sonoluminescence. 

Homogeneous Sonochemistry Bond Breaking and Radical Formation. The chemical effect of ultrasound on aqueous solutions have been studied for many years. The primary products are H2O2 there is strong evidence for various high-energy intermediates, including HO2,... 

Makino K, Mosobba MM, Riesz P (1982) Chemical effects of ultrasound in aqueous solutions. Evidence for OH and H by spin-trapping. J Am Chem Soc 104 3537-3539... 

Henglein A (1995) Chemical effects of continuous and pulsed ultrasound in aqueous solutions. Ultason Sonochem 2 S 115—S121... 

The effect of ultrasound on liquid-liquid interfaces between immiscible fluids is emulsification. This is one of the major industrial uses of ultrasound (74-76) and a variety of apparatus have been devised which will generate micrometer-sized emulsions (9). The mechanism of ultrasonic emulsification lies in the shearing stresses and deformations created by the sound field of larger droplets. When these stresses become greater than the interfacial surface tension, the droplet will burst (77,78). The chemical effects of emulsification lie principally in the greatly increased surface area of contact between the two immiscible liquids. Results not unlike phase transfer catalysis may be expected. 

The early studies of the chemical effects of ultrasound have been thoroughly reviewed (5-7). Only the most important and most recent research is mentioned here as needed to provide a perspective on sonochemical reactivity patterns. The sonolysis of water is the earliest and most exhaustively studied (3,93,96,98-105). The first observations on the experimental parameters which influence sonochemistry come from these reports. The primary products are H202 and H2, and various data supported their formation from the intermediacy of hydroxyl radicals and hydrogen radicals ... 

The purpose of this chapter will be to serve as a critical introduction to the nature and origin of the chemical effects of ultrasound. We will focus on organo-transition metal sonochemistry as a case study. There will be no attempt to be comprehensive, since recent, exhaustive reviews on both organometallic sonochemistry Q) and the synthetic applications of ultrasound (2) have been published, and a full monograph on the chemical, physical and biological effects of ultrasound is in press (3). 

Since this type of result shows that sonication is definitely not just another method of providing agitation of a medium, but exhibits its own peculiarities, it stands to reason that it should obey some rules of its own. An examination and classification of published material led to an empirical systemisation of sonochemistry [28, 30). This classification concentrates on the chemical effects in sonochemistry but it should also be recognised that in some cases ultrasound does act in a mechanical sense achieving remarkable results through super agitation. Sometimes the mechanical and chemical effects occur together. 

Recently, liquid water was decomposed to hydrogen and oxygen stoichiometrically and continuously by irradiations of ultrasound and light with particulate photocatalyst.n) This reaction system is thought to be a joint one for sonolysis and photocatalysis. Furthermore, this system also is a hybrid of mechanical effects and chemical effects. In this chapter, the effect of ultrasound on photocatalytic reaction is considered. The joint system of sonochemical and photocatalytic reactions, in particular s explained. 

Sonochemistry is defined as the chemical effects produced by ultrasonic waves. Ultrasound, with frequencies roughly between 15 kHz and 10 MHz, has a drastic effect on chemical reactions. It is the most important... 

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