Parameters Influencing the Sonochemical Reactivity

Since sonochemistry takes its origin in cavitation, the reactivity depends on the characteristics of the bubbles. Their size and lifetime, and the content of the gaseous phase, depend on the physical properties of the medium and the parameters (amplitude and frequency) of the wave. Conducting a sonochemical reaction implies that a multiparameter problem is examined. 

The role of frequency is discussed with respect to its physical implications in Ch. 1 (p. 3) and topological consequences for the ultrasonic field in Ch. 8. (pp. 302, 316, 323). In contrast to many applications of diagnostic ultrasounds, which make use of frequencies above 3-5 MHz, sonochemistry employs the lower range of the spectrum. Frequencies below 50 kHz are preferred for heterogeneous systems due to the more intense mechanical effects. The problem is less clear for solutions. The frequency is chosen, more or less arbitrarily, among a few values considered as important, 20, 30, or 50 kHz for the lower, 500 or 800 kHz in the medium range (recently 200 and 300 kHz), 7,8 and 1,1.5, or 2 MHz for the highest values. 

Some bonds are cleaved more efficiently in the medium range. A general explanation for this observation is still missing. Tentatively, the difference in the nature of the products formed at different frequencies was assigned to the relative lifetimes of the bubbles and the radicals (Fig. 3).  

Radical lifetime bubble lifetime Radical lifetime bubble lifetime 

If the radicals first formed in the gas phase exist for a longer time than the bubble, they can undergo chemical changes before reaching the interface or the bulk solution where normal conditions exist. The reactive species reaching this solution are not the primarily generated species. On the other hand, if the bubble lifetime is shorter than that of the radicals, the latter are injected into the solution before any evolution, and a different chemistry can occur. 

---