Sonochemistry

Sonochemistry can be roughly divided into categories based on the nature of the cavitation event homogeneous sonochemistry of liquids, heterogeneous sonochemistry of liquid—liquid or liquid—solid systems, and sonocatalysis (which ovedaps the first two) (12—15). In some cases, ultrasonic irradiation can increase reactivity by neady a million-fold (16). Because cavitation can only occur in liquids, chemical reactions are not generally seen in the ultrasonic irradiation of solids or solid-gas systems. 

The chemical effects of ultrasound do not arise from a direct interaction with molecular species. Ultrasound spans the frequencies of roughly 15 kHz to 1 GHz. With sound velocities in liquids typically about 1500 m/s, acoustic wavelengths range from roughly 10 to 10-4 cm. These are not molecular dimensions. Consequendy, no direct coupling of the acoustic field with chemical species on a molecular level can account for sonochemistry or sonoluminescence. 

The radius and velocity of the bubble wall are given by R and U respectively. The values for H, the enthalpy at the bubble wall, and C, the local sound speed, maybe expressed as follows, using the Tait equation of state for the liquid. 

Kirk-Othmer Encyclopedia of Chemical Technology (4th Edition) 

The linear speed of sound in the liquid is c. A, B, and n are constants that should be set to the appropriate values for water. Any acoustic forcing function is included in the pressure at infinity term, P1oo (t). The pressure at the bubble wall, P(R), is given by 

Sonochemistry and Other Novel Methods Developed for the Synthesis of Nanoparticles 

We are all aware of the use of ultrasound radiation in medicine, where it is being used mostly for diagnosis. More recently, however, focused ultrasound radiation is being used to burn cancer cells. Less is known regarding its application in chem- 

The Chemistry of Nanomaterials Synthesis, Properties and Applications, Volume 1. Edited by C. N. R. Rao, A. Miiller, A. K. Cheetham 

Copyright 2004 WILEY-VCH Verlag GmbH Co. KGaA, Weinheim ISBN 3-527-30686-2 

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. 

Most recently, sonochemistry was used as an efficient extraction technique. Extraction of carvone and limonene from caraway seeds has been successful. Sonochemistry helped increase the yield, lower the extraction temperature, and produce a purer extract than those obtained with conventional methods [10]. 

When a sound wave, propagated by a series of compression and rarefaction cycles, passes through a liquid medium it causes the molecules to oscillate aroimd their mean position. During the compression cycle the average distance between molecules is reduced and, conversely, it is 

Cleaning Laboratory glassware jewellery computer components large, delicate 

Engineering Welding and riveting of plastics, ceramic processing drilling aid for 

Biology Disruption of cell membrane to allow extraction of contents 

Ultrasonic irradiation of solutions containing volatile organometaUic compounds such as Fe(CO)j, Ni(CO), and Co(CO)3NO produced porous, coral-like aggregates of amorphous metal nanoparticles [25]. A classic example is the sonication of Fe(CO)j in decane at 0 C under Ar, which yielded a black powder. The material was 96% iron, with a small amount of residual carbon and oxygen present from the solvent and CO ligands. Bimetallic alloy particles have also been prepared in this way. Sonication of FeCCO) and Co(CO)3NO leads to Fe-Co alloy particles [26]. Nanostructured MoS can be synthesized by the sonication of Mo(CO)g with elemental sulphur in 1,2,3,5-tetramethylbenzene under Ar [27]. Metal nitrides are prepared by the sonication of metal carbonyls under a mixture ofNH3andH2atO°C [28]. 

FIGURE 8.3 (a) Schematic illustration of the sonochemical preparation of single-walled carbon nanotubes on silica powders, (b) Scanning electron microscope (SEM) image of carbon nanotube bundles on polycarbonate filter membrane, (c) High-resolution transmission electron microscopy (HRTEM) images of single-waUed carbon nanotubes within the bundles (From Ref. 29, J. Am. Chem. Soc., 126 (2004) 15982. 2004 American Chemical Society). 

Harris, G. Hyett, W. Jones, A. Krehs, J. Mack, L. Maini, A.G. Orpen, I.P. Parkin, W.C. 

---

Mason T J and Lorimer J P 1998 Sonochemistry Theory, Applications and Uses of Ultrasound In Chemistry (Chichester Ellis Florwood)... 

Birkin P R and SilvaMartinez S 1997 A study on the effects of ultrasound on electrochemical phenomena Ultrasonics Sonochemistry 4 121... 

Fig. 1. Transient acoustic cavitation the origin of sonochemistry and sonoluminescence.

Homogeneous sonochemistry typically is not a very energy efficient process (although it can be mote efficient than photochemistry), whereas heterogeneous sonochemistry is several orders of magnitude better. Unlike photochemistry, whose energy inefficiency is inherent in the production of photons, ultrasound can be produced with neatly perfect efficiency from electric power. A primary limitation of sonochemistry remains the small fraction... 

Sonochemistry is strongly affected by a variety of external variables, including acoustic frequency, acoustic intensity, bulk temperature, static pressure, ambient gas, and solvent (47). These are the important parameters which need consideration in the effective appHcation of ultrasound to chemical reactions. The origin of these influences is easily understood in terms of the hot-spot mechanism of sonochemistry. 

Increases in the appHed static pressure increase the acoustic intensity necessary for cavitation, but if equal number of cavitation events occur, the coUapse should be more intense. In contrast, as the ambient pressure is reduced, eventuaUy the gas-fiUed crevices of particulate matter which serve as nucleation sites for the formation of cavitation in even "pure" Hquids, wiU be deactivated, and therefore the observed sonochemistry wiU be diminished. 

The choice of the solvent also has a profound influence on the observed sonochemistry. The effect of vapor pressure has already been mentioned. Other Hquid properties, such as surface tension and viscosity, wiU alter the threshold of cavitation, but this is generaUy a minor concern. The chemical reactivity of the solvent is often much more important. No solvent is inert under the high temperature conditions of cavitation (50). One may minimize this problem, however, by using robust solvents that have low vapor pressures so as to minimize their concentration in the vapor phase of the cavitation event. Alternatively, one may wish to take advantage of such secondary reactions, for example, by using halocarbons for sonochemical halogenations. With ultrasonic irradiations in water, the observed aqueous sonochemistry is dominated by secondary reactions of OH- and H- formed from the sonolysis of water vapor in the cavitation zone (51—53). 

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,... 

The sonochemistry of solutes dissolved in organic Hquids also remains largely unexplored. The sonochemistry of metal carbonyl compounds is an exception (57). Detailed studies of these systems led to important mechanistic understandings of the nature of sonochemistry. A variety of unusual reactivity patterns have been observed during ultrasonic irradiation, including multiple ligand dissociation, novel metal cluster formation, and the initiation of homogeneous catalysis at low ambient temperature (57). 

Sonochemistry is also proving to have important applications with polymeric materials. Substantial work has been accomplished in the sonochemical initiation of polymerisation and in the modification of polymers after synthesis (3,5). The use of sonolysis to create radicals which function as radical initiators has been well explored. Similarly the use of sonochemicaHy prepared radicals and other reactive species to modify the surface properties of polymers is being developed, particularly by G. Price. Other effects of ultrasound on long chain polymers tend to be mechanical cleavage, which produces relatively uniform size distributions of shorter chain lengths. 

Table 1. Some Representative Examples of Heterogeneous Sonochemistry...

The phenomenon of acoustic cavitation results in an enormous concentration of energy. If one considers the energy density in an acoustic field that produces cavitation and that in the coUapsed cavitation bubble, there is an amplification factor of over eleven orders of magnitude. The enormous local temperatures and pressures so created result in phenomena such as sonochemistry and sonoluminescence and provide a unique means for fundamental studies of chemistry and physics under extreme conditions. A diverse set of apphcations of ultrasound to enhancing chemical reactivity has been explored, with important apphcations in mixed-phase synthesis, materials chemistry, and biomedical uses. 

T. J. Mason and J. P. Lorimer, Sonochemistry Theory, Applications and Uses ofUltrasound in Chemisty, EUis Horword, Ltd., Chichester, U.K., 1988. 

K. S. SusHck, "Sonochemistry of Transition Metal Compounds," in R. B. King, ed., Tnyclopedia of Inorganic Chemisty,]ohxi Wiley Sons, Inc., New York, vol 7, pp. 3890—3905. 

---