The use of ultrasound has become an important synthetic tool in 'liquid-solid' reactions, and its ability to enhance chemical reactivity is a widely applicable and emerging laboratory technique. Sonochemistry is the application of ultrasound to chemical reactions and processes. Ultrasound is the part of the sonic spectrum which ranges from about 20 kHz to 10 MHz, and can be roughly subdivided into three main regions: low frequency, high-power ultrasound (20-100 kHz), high frequency, medium-power ultrasound (100 kHz-1 MHz), and high frequency, low-power ultrasound (1-10 MHz). The range from 20 kHz to around 1 MHz is used in sonochemistry, whereas frequencies far above 1 MHz are used as medical and diagnostic ultrasound. Ultrasound offers several potential advances compared to the more traditional synthetic routes. These include:

1. the use of mild ambient temperatures to protect reac-tants that might be thermally unstable,

2. enhanced mixing and transport properties,

3. the ability to generate unique, high-energy intermediates,

4. the production of materials normally associated with physical extremes, for example, molten metals and plasmas with relatively nominal energy input,

5. decrease of reaction time and/or increase of yield,

6. use of fewer forcing conditions, for example, lower reaction temperature,

7. possible switching of reaction pathway,

8. use of fewer or avoidance of phase transfer catalysts,

9. degassing force reactions with gaseous products,

10. use of crude or technical reagents,

11. activation of metals and solids,

12. reduction of any induction period.

The origin of sonochemical effects in liquids is the phenomenon of acoustic cavitation. Acoustical energy is a mechanical energy, that is, molecules do not absorb it. Ultrasound is transmitted through a medium via pressure waves by inducing vibrational motion of the molecules, which alternately compress and stretch the molecular structure of the medium due to a time-varying pressure. Therefore, the distance among the molecules varies as the molecules oscillate around their mean position. If the intensity of ultrasound in a liquid is increased, a point is reached at which the intramolecular forces are not able to hold the molecular structure intact. Consequently, it breaks down, resulting in the formation of a cavity. This cavity is called the cavitation bubble, as this process is called cavitation and the point where it starts the cavitation threshold. A bubble responds to the sound field in the liquid by expanding and contracting, that is, it is excited by a time-varying pressure. Two forms of cavitation are known: stable and transient. Stable cavita-tion means that the bubbles oscillate around their equilibrium position over several refraction/compression cycles. In transient cavitation, the bubbles grow over one (sometimes two or three) acoustic cycles to double their initial size, and finally collapse violently.

The size, lifetime, and fate of a cavitation bubble depend on the following parameters: frequency, intensity (acoustic pressure), solvent, bubbled gas, and external parameters (temperature, pressure). However, it should be noted that there is often no simple relationship. Since sonochemistry is the chemistry assisted/enhanced by ultrasound, this means that chemical reactions that take place under more conventional conditions are enhanced or even yield totally different products. The reason for this can be either physical or chemical effects of cavitation.

Copyright © 2004 by American Scientific Publishers All rights of reproduction in any form reserved.

Encyclopedia of Nanoscience and Nanotechnology Edited by H. S. Nalwa Volume 10: Pages (67-82)

0 0

Post a comment