Sonochemistry solvent choice

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). 

Sonochemistry is strongly affected by a variety of external parameters, including acoustic frequency, acoustic intensity, bulk temperature, static pressure, choice of ambient gas, and choice of solvent. These are important considerations in the effective use of ultrasound to influence chemical reactivity, and are also easily understandable in terms of the cavitational hot-spot mechanism. A summary of these effects is given in Table II. 

By the proper choice of solvent and experimental conditions (i.e., low volatility, highly stable liquids at low temperature e.g., decane, -10° C), the rates of degradation of nonaqueous liquids can be made quite slow, well below those of water. This is of considerable advantage, since one may then observe the primary sonochemistry of dissolved substrates rather than secondary reactions with solvent fragments. In general, the examination of sonochemical reactions in aqueous solutions has produced results difficult to interpret due to the complexity of the secondary reactions which so readily occur. One may hope to see the increased use of low-volatility organic liquids in future sonochemical studies. 

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