Flash thermolysis, subsonic normal beams

44 Brown R.F.C. Pyrolytic Methods in Organic Chemistry Academic Press, New York, 1980. 

The last point which can be evoked here is conceptually linked to the hot-spot theory. If the limit liquid layers around a bubble are in direct contact with the heated and pressurized bubble content, far above the critical point of the liquid, these layers should be in a supercritical state. 23 This attractive hypothesis (see p. 61) was used by Hoffmann et al to rationalize the sonolysis of nitrophenyl derivatives. Supercritical fluids are characterized by a very high flexibility of important parameters (density, dielectric constant, solubilizing power) as a function of pressure. Experts in the field distinguish gas-like and liquid-like media, in which the kinetics of a reaction can vary over a broad range. For instance, the conjugate addition of piperidine to methyl propiolate was studied in supercritical ethane or fluoroform (Fig. 5).  

In the latter solvent, the dielectric constant undergoes a sharp increase ( 3 times) with pressure, which explains why the polar transition state of the reaction is highly favored under pressure. In a number of examples, supercritical media were shown to 

With these data in mind, one can assume that the supercritical shell aroimd a cavitation bubble is able to concentrate solutes and bring them to temperature, pressure, and concentration conditions in an environment in which diffusion is much easier, with a modified dielectric constant. Under such conditions, even polar processes, not only radical ones, should imdergo sonochemical effects. A reaction similar to the one mentioned in Fig. 5 was indeed reported to be improved by sonication.59 

The obstacles encoxmtered by this attractive interpretation are of a different nature. A calculation has shown recently that the total volume of the shells around all the bubbles in a given solution represents only a very small fraction.60 Consequently, even if the sonochemical rate increase in this zone is high, the overall effect observed will remain weak. A second point is that the temperature distribution inside the bubble is by itself controversial, and some authors reject even the idea of the existence of such supercritical phases. 

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