Sonochemistry: The Production of Chemical Changes with Sound Waves

Abstract

An extensive survey is made of the chemical effects of ultrasonic waves. Increased understanding of the mechanism of these effects is sought through a consideration of the significance of certain experimental variables such as frequency, intensity, pressure, and temperature. Also considered is the role of cavitation. NTENSE ultrasonic waves produce unusual chemical, physical, and biological phenomena. Although a fully satisfying explanation of the mechanisms involved is not yet available (even after 25 years of study by laboratories in many countries), much relevant information has been obtained. CAVITATION Practically all of the observed chemical effects of ultrasonics in liquid systems have been attributed to "cavitation," which is the formation and violent collapse of small bubbles or cavities in the liquid as a result of pressure changes2 -7 In the usual case of water saturated with air, the negative pressure portion of the sound wave causes some of the air to come out of solution as minute bubbles, which act as weak spots for the further tearing apart of the liquid to form larger cavities. Then when the pressure increases, as in the other half of the sound wave cycle, the cavities collapse with a violent hammering action which may generate local pressures of thousands of atmospheres and local temperatures of several hundred degrees. 8 Electrical discharges also occur, as the result of the electric potential built up between opposite walls of the cavity. 9-•4 The ultrasonic intensity re suired to produce cavitation in.ordinary distilled water is about 0.03 watt/cm 2, equivalent to a pressure variation of =t=0.3 atmosphere25 * This article is based on a paper presented at

Sonoluminescence and acoustic emission spectra at different stages of cavitation zone development

“…It is now widely accepted that ultrasound can accelerate physical and chemical processes [1][2][3], as well as generate specific ultrasonic phenomena, such as the inverse ultrasonic capillary effect [4], and the direct electromotive force [5].…”

Effects and Mechanisms

“…However, the nature of the gas also affects the free radicals produced, and therefore the sonoluminescence, through the resulting chemical reactions (the noble gases are generally chemically inactive). The sonoluminescence from a variety of gas-saturated liquids has been investigated experimentally, workers including Weissler [252], Parke and Taylor [132], Günther et al [137], Prudhomme and Guilmart [138], Srinivasan and Holroyd [128], Young [173] and Vaughan et al [253].…”

Palynological Techniques used in New Zealand

“…These effects appear to be at a minimum before oxidation has taken place (McIntyre and Norris, 1964) and ultrasonic irradiation is used to great advantage on many samples at that stage with negligible effect on the microfossils. It should be noted that several factors influence the intensity of cavitation with individual machines (Weissler, 1953;Weissler and Hine, 1962). Details of the machine used are given below (see Equipment).…”