Ultrasound radicals formation

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,... 

Riesz P, Kondo T (1992) Free radical formation induced by ultrasound and its biological implications. Free Radic Biol Med 13 247-270... 

Unlike conventional chemical reactions, the altered reactivity of chemical reactions undergoing ultrasonic irradiation is principally due to acoustic cavitation which essentially involves the free radical formation. The ultrasound produces highly reactive free radical species like H and OH radicals from the homolytic cleavage of water. Further they may react with any of other free radicals present or with neutral molecules like 02 and O3 to produce peroxy species, superoxide, hydrogen peroxide or hydrogen. When the aqueous solution is saturated with 02, extra... 

For the sonochemical mineralization of reactive dye Cl Reactive Black 5 with 20, 279 and 817 kHz irradiation, the discoloration and radical formation both are directly dependent upon ultrasonic frequency, acoustic power and irradiation time and indirectly on the number of free radicals thus generated, as their suppression decreased the discoloration rate due to radical scavenging effect. Although ultrasound alone is capable of decolorizing Reactive Black 5 but inefficient in mineralization as only 50% degradation was observed after 6 h of ultrasonic irradiation [121]. The sonochemical... 

Mark G, Schuchmann MN, Schuchmann H-P, von Sonntag C (1990) The photolysis of potassium peroxodisulphate in aqueous solution in the presence of tert-butanol a simple actinometer for 254 nm radiation. J Photochem Photobiol A Chem 55 157-168 Mark G, Korth H-G, Schuchmann H-P, von Sonntag C (1996) The photochemistry of aqueous nitrate revisited. J Photochem Photobiol A Chem 101 89-103 Mark G, Tauber A, Laupert R, Schuchmann H-P, Schulz D, Mues A, von Sonntag C (1998) OH-radical formation by ultrasound in aqueous solution, part II. Terephthalate and Fricke dosimetry and the influence of various conditions on the sonolytic yield. Ultrason Sonochem 5 41-52 MarkG, Schuchmann H-P, von Sonntag C (2000) Formation of peroxynitrite by sonication of aerated water. J Am Chem Soc 122 3781-3782... 

Fang X, Mark G, von Sonntag C (1996) OH-Radical formation by ultrasound in aqueous solutions, part I. The chemistry underlying the terephthalate dosimeter. Ultrason Sonochem 3 57-63 Fang X, Schuchmann H-P, von Sonntag C (2000) The reaction of the OH radical with pentafluoro-, pentachloro-, pentabromo- and 2,4,6-triiodophenol in water electron transfer vs. addition to the ring. J Chem Soc Perkin Trans 2 1391-1398... 

It is probably now widely accepted that the high temperatures and pressures generated by cavitation are sufficiently extreme to initiate radical formation and reaction. Indeed, spin trap electron spin resonance techniques have been employed to confirm the presence of radicals in some sonochemical experiments. Hydrogen and hydroxyl radical formation due to ultrasound has been positively identified by ESR measurements, and in the relevant reactions are now believed to be formed through thermal dissociation of water molecules at the temperatures generated within the cavitating bubble [31]. 

Viscosity is an important factor during ultrasound-induced bulk polymerizations as the long polymer chains formed upon reaction cause a drastic increase in the viscosity of the reaction mixture, thereby hindering cavitation and consequently reducing the production rate of radicals. Precipitation polymerization forms a potential solution to this problem, because a constant viscosity and hence a constant radical formation rate can be maintained. In this perspective, high-pressure carbon dioxide is an interesting medium as most monomers have a high solubility in CO2, whereas it exhibits an anti-solvent effect for most polymers. ... 

In ultrasound-induced polymerization reactions, the viscosity has a large influence on the radical formation rate. Therefore, it is important to monitor the viscosity during these reactions. By coupling the overall heat transfer coefficient U to the viscosity of the reaction mixture, the influence of the C02-concentration on the viscosity of polymer solutions has been deter-... 

In the following sections, the physical backgroimd of ultrasound-induced cavitation and radical formation is described. Subsequently, an overview of the various types of ultrasoimd-induced polymerizations is given, including hulk, precipitation, and emulsion pol5unerization. Finally, the breakage of polymer chains by cavitation is discussed. 

Temperature. When the reaction temperature is altered, the liquid properties will change. Although all these properties (viscosity, surface tension, soimd velocity, vapor pressure, etc) have an influence on the chemical effect of cavitation, the change in vapor pressure dominates the other liquid properties. As the temperature is raised, the vapor pressure in the bubble is increased, which cushions the implosion of the cavity. This results in a lower local temperature inside the cavity at higher overall temperatures. Consequently, fewer radicals are formed per cavitation bubble. On the other hand, a higher vapor pressure can lead to easier bubble formation because of the decrease of the cavitation threshold. In most cases, however, an increase in reaction temperature will result in an overall decrease in the radical formation rate. Therefore, ultrasound-induced reactions exhibit opposite behavior as compared to common radical reactions (20). 

Ultrasound Intensity. At first the radical formation rate will increase to a maximum with increasing ultrasound intensity (18). This is caused by the higher cavitation intensity per bubble and the larger number of cavitation bubbles. At too high ultrasound intensities, however, a cloud of cavitation bubbles is formed near the ultrasound source, because of which the pressure wave is no longer transmitted efficiently to the liquid. As a result, the cavitation intensity and, consequently, the radical formation rate decrease with a continued increase in intensity (18). An optimum radical formation rate with ultrasound intensity can thus be foimd. 

Ultrasound Frequency. The frequency of ultrasoimd has a significant effect on the cavitation process. At very high frequencies (>1 MHz), the cavitation effect is reduced as the inertia of a cavitation bubble becomes too high to react to fast changing pressures. Most ultrasoimd-induced reactions are therefore carried out at frequencies between 20 and 900 kHz. The optimum ultrasoimd effect as a function of frequency depends on the reaction system eg, water dissociation has an optimum frequency at approximately 500 kHz (21). For bulk pol5mierizations the maximum radical formation rate is obtained at 20 kHz. At this frequency the highest strain rates are produced, which results in a high radical formation rate by polymer scission (22). 

Generally, free-radical polymerization consists of four elementary steps initiation, propagation, chain transfer, and termination (see Radical Polymerization). When ultrasound is used to initiate polymerization, radicals can be formed both from monomer and from polymer molecules. This implies that because of radical formation by polymer scission, an additional elementary step is involved in ultrasound-induced polymerization, as indicated in Figure 4. 

Fig. 7. Radical formation rate constants from monomer and poljmier as a function of the wt% polymer dissolved at a temperature of 20°C and an ultrasound intensity of 62 W/cm (30).bMMA PMMA.

Precipitation Poiymerization. As described in the previous section, ultrasound-induced bulk polymerizations are limited to relatively low conversions, because a strong viscosity increase upon reaction hinders cavitation. To obtain higher conversions, precipitation polymerization forms a potential soln-tion. Because the produced polymer precipitates from the reaction medium, the viscosity and consequently the radical formation rate are expected to remain virtually constant. In this perspective, liquid carbon dioxide is a suitable reaction medium, because most monomers have a high solubility in CO2, whereas it exhibits an antisolvent effect for most polymers. Moreover, CO2 is regarded as an environmentally friendly compound, which is nontoxic, nonflammable, and naturally abundant. Since higher pressures are required for CO2 to act as an antisolvent (31-33), the possibility of ultrasound-induced cavitation in pressurized carbon dioxide systems has been studied (34). 

In contrast, contact as well as airborne ultrasound application avoids the occurrence of external cavitation and thus reduces the risk of related effects such as radical formation and oxidation. Thus, to the present author s knowledge, no important quality impairment due to such ultrasound-assisted processes has yet been reported. Soria et al. (2010) found only minor changes in the reducing sugar... 

For the development of sustainable polymer processes, ultrasound is an interesting technology, as it allows for polymerizations without the use of initiator. The radicals are generated in situ by cavitation events [116, 117], which make possible a dean and intrinsically safe polymerization reaction. As a result of the high strain rates outside the bubble, cavitation can also induce chain scission [118,119], which provides an additional means to control the molecular weight of the polymer produced. In Sections 21.3.1 and 21.3.2 the physical background of ultrasound-induced cavitation and radical formation will be described. Subsequently (see Section 21.3.3), an overview of the several types of ultrasound-induced polymerizations will be given, namely bulk, predpitation, and emulsion polymerization. 

Because of the extreme conditions during a cavitation event, radicals can be formed. Several parameters affect cavitation and thereby the polymerization reaction, since the radical formation rate is directly influenced by the cavitational collapse. The number of radicals formed due to sonification is a function of the number of cavities created and the number of radicals that are formed per cavitation bubble. The bubble wall velocity during collapse and the hot-spot temperature determine the rate at which radicals are formed, both inside and outside a single bubble. These two parameters depend on the physical properties of the liquid as well as on the physical and chemical processes occurring around the cavity. The most important properties and processes occurring in a cavitation bubble are depicted schematically in Figure 21.10. The number of cavities is determined, for instance, by the impurities in the liquid, the static pressure, the ultrasound intensity, and the vapor pressure. This emphasizes the complexity of the influences on the overall... 

Most ultrasound-induced bulk polymerizations are performed at room temperature [143]. This low temperature is chosen because radical formation induced by... 

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