Sonolysis

Sonoelectrochemistry has been employed in a number of fields such as in electroplating for the achievement of deposits and films of higher density and superior quality, in the deposition of conducting polymers, in the generation of highly active metal particles and in electroanalysis. Furtlienuore, the sonolysis of water to produce hydroxyl radicals can be exploited to initiate radical reactions in aqueous solutions coupled to electrode reactions. 

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). 

In contrast, the ultrasonic irradiation of organic Hquids has been less studied. SusHck and co-workers estabHshed that virtually all organic Hquids wiU generate free radicals upon ultrasonic irradiation, as long as the total vapor pressure is low enough to allow effective bubble coUapse (49). The sonolysis of simple hydrocarbons (for example, alkanes) creates the same kinds of products associated with very high temperature pyrolysis (50). Most of these products (H2, CH4, and the smaller 1-alkenes) derive from a weU-understood radical chain mechanism. 

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. 

The most intensive development of the nanoparticle area concerns the synthesis of metal particles for applications in physics or in micro/nano-electronics generally. Besides the use of physical techniques such as atom evaporation, synthetic techniques based on salt reduction or compound precipitation (oxides, sulfides, selenides, etc.) have been developed, and associated, in general, to a kinetic control of the reaction using high temperatures, slow addition of reactants, or use of micelles as nanoreactors [15-20]. Organometallic compounds have also previously been used as material precursors in high temperature decomposition processes, for example in chemical vapor deposition [21]. Metal carbonyls have been widely used as precursors of metals either in the gas phase (OMCVD for the deposition of films or nanoparticles) or in solution for the synthesis after thermal treatment [22], UV irradiation or sonolysis [23,24] of fine powders or metal nanoparticles. 

Weavers LK, Ling FH, Hoffmann MR (1998) Aromatic compound degradation in water using a combination of sonolysis and ozonolysis. Environ Sci Tech 32 2727-2733... 

To confirm the relation between the Pd(II) reduction and the change in the concentration of H+ ions, the sonolysis of 1-propanol-water solution without Pd(II) was carried out and the change in the pH value was investigated. The pH value without Pd(H) decreased from 5.7 to 5.2 in the 10 min irradiation, while that with Pd(II) decreased from 3.5 to 2.7 in the 10 min irradiation. This result suggests that the change... 

Based on the above results, ultrasonic irradiation to ion-exchanged [Pd(NH3)4]2+-zeolite powders was performed in an aqueous solution containing 2-propanol. The reduction of [Pd(NH3)4]2+-zeolite to Pd°-zeolite was confirmed by XPS analyses. However, from XPS depth analyses of the prepared samples, it was suggested that the [Pd(NH3)4]2+ complexes in the zeolite pore were not sufficiently reduced even in the presence of 2-propanol. Presumably, the reductants formed from 2-propanol sonolysis could not easily diffuse into the zeolite nano-pore (size 1.2 nm) and/or reductants undergo recombination reactions and quenching reactions with the walls. In addition, the results of XPS spectral analyses of the sonochemically prepared Pd-zeolite powders indicated that the average size of the Pd clusters on the zeolite surface is roughly estimated to be less than 1 nm and composed of several dozen Pd atoms. 

Gutierrez MS, Henglein A, Dohrmann JK (1987) H atom reactions in the sonolysis of aqueous solutions. J Phys Chem 91 6687-6690... 

Some of the reports are as follows. Mizukoshi et al. [31] reported ultrasound assisted reduction processes of Pt(IV) ions in the presence of anionic, cationic and non-ionic surfactant. They found that radicals formed from the reaction of the surfactants with primary radicals sonolysis of water and direct thermal decomposition of surfactants during collapsing of cavities contribute to reduction of metal ions. Fujimoto et al. [32] reported metal and alloy nanoparticles of Au, Pd and ft, and Mn02 prepared by reduction method in presence of surfactant and sonication environment. They found that surfactant shows stabilization of metal particles and has impact on narrow particle size distribution during sonication process. Abbas et al. [33] carried out the effects of different operational parameters in sodium chloride sonocrystallisation, namely temperature, ultrasonic power and concentration sodium. They found that the sonocrystallization is effective method for preparation of small NaCl crystals for pharmaceutical aerosol preparation. The crystal growth then occurs in supersaturated solution. Mersmann et al. (2001) [21] and Guo et al. [34] reported that the relative supersaturation in reactive crystallization is decisive for the crystal size and depends on the following factors. 

The formation of H202 was commonly observed when ultrasonically irradiated water contained oxygen. Relatively low amount of H2O2, seen on sonolysis in the absence of air, was due to the fact that H and OH radicals readily recombined [30]. Thus, in the presence of O2 the recombination of OH and H is slowed down by the following reaction ... 

The behavior of Cu(II)(aq) is relatively more understood than other metal ions. Haas and Gedanken [74] found only a partial reduction of Cu2+ ions to Cu+ (95%) instead of metallic copper (5.1%) in the presence of cetyltrimethylammonium bromide in an ultrasonic field and thus obtained CuBr particles instead of Cu. Nevertheless, when polymers such as poly(N-vinyl 2-pyrrolidone) or poly(vinyl alcohol) were used, the end product was metallic copper particles, as expected. They have proposed the reduction of Cu2+ ions to copper as the first stage, however, in the second stage Cu reacted with OH radicals or H2O2, formed by sonolysis of water to produce Cu+ and OH- ions as under ... 

Hart Edwin J, Henglein Amim (1986) Sonolysis of ozone in aqueous solution. J Phys Chem 90 3061-3062... 

Rosentel IK, Mosobba MM, Riesz P (1981) Sonolysis of perhalomethane as studied by EPR and spin trapping. J Magn Reson 45 359-361... 

Buttner J, Gutierrez M, Henglein A (1991) Sonolysis of water - methanol mixtures. J Phys Chem 95 1528-1530... 

Misik V, Riesz P (1997) Effect of Cd2+ on the H atom yield in the sonolysis of water. Evidence against the formation of hydrated electrons. J Phys Chem A 101(8) 1441-1444... 

Sonochemical reduction of permanganate to manganese dioxide the effect of H202 formed in the sonolysis of water on the rates of reduction Kenji O, Masaki I, Ben Nishimura Rokura N, Yasuaki M (2009) Ultrason Sonochem 16(3) 387—391... 

In the experiment involving oxidative enzyme HRP (EC 1.11.1.7, RZ 1.9, 240 purpuro gallin (units/mg)) [89] for the enzymatic treatment and ultrasonic waves of 423 kHz and 5.5 W, the phenol degradation rate was found to increase. The ultrasound assisted biodegradation method has been found to be more efficient method than the sonolysis and enzyme treatment when operated individually. 

Tauber A, MarkG, Schuchmann H-P, Von Sonntag C (1999) Sonolysis oftert-butyl alcohol in aqueous solution. J Chem Soc, Perkin Trans 2 1129-1136... 

Misik V, Miyoshi N, Riesz P (1995) EPR spin-trapping studies of the sonolysis of H2O/D2O mixtures probing the temperatures of cavitation regions. J Phys Chem 99 3605-3611... 

Hart EJ, Fischer CH, Henglein A (1990) Sonolysis of hydrocarbons in aqueous solution. Radiat Phys Chem 36 511-516... 

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 ... 

It is now clearly demonstrated through the use of free radical traps that all organic liquids will undergo cavitation and generate bond homolysis, if the ambient temperature is sufficiently low (i.e., in order to reduce the solvent system s vapor pressure) (89,90,161,162). The sonolysis of alkanes is quite similar to very high temperature pyrolysis, yielding the products expected (H2, CH4, 1-alkenes, and acetylene) from the well-understood Rice radical chain mechanism (89). Other recent reports compare the sonolysis and pyrolysis of biacetyl (which gives primarily acetone) (163) and the sonolysis and radiolysis of menthone (164). Nonaqueous chemistry can be complex, however, as in the tarry polymerization of several substituted benzenes (165). 

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