Irradiated heterogeneous systems

II. Experimental Studies of Some Irradiated Heterogeneous Systems. 56... 

Table 1 Coefficients AT(10), lf(15), andlt(25) for y-irradiated heterogeneous system(/retention time, X and Af mean characteristic and molecular, respectively).

The existence of the so-called microwave effect has not been proved. It does, however, seem to have been demonstrated that overheating of polar liquids [17] occurs and that hot spots are present in heterogeneous systems, especially at the interface [38]. Similarly, microwave irradiation results in an increase in the molecular mobility in solids [5 b]. 

Rule 2 applies to heterogeneous systems where a more complex situation occurs and here reactions proceeding via ionic intermediates can be stimulated by the mechanical effects of cavitational agitation. This has been termed false sonochemistry although many industrialists would argue that the term false may not be correct because if the result of ultrasonic irradiation assists a reaction it should still be considered to be assisted by sonication and thus sonochemical . In fact the true test for false sonochemistry is that similar results should, in principle, be obtained using an efficient mixing system in place of sonication. Such a comparison is not always possible. 

Figure 15. Photo-oxidation of LCV in the heterogeneous system. Sensitizer 3 (o), 4 ( ), 5 (o). Solution [LCV] = 4 x 10-3M in acetonitrile, irradiation at 375 nm, OD3-/5 of the films > 1.5 nm. "Reproduced with permission from Ref. 15. Copyright 1983, John Wiley < Sons, Inc...

It was originally thought that the sonication approach would promote miscibility at the interface of the heterogeneous system and also would raise the electronic states of a small portion of the reactants above their ground state, which would hopefully enhance the addition reaction. We later found, that only the simultaneous photoexcitation and ultrasonic irradiation plus the addition of an appropriate gas (e.g., hexafluoropropane), which Increased the pressure in the reaction vessel, formed the fully saturated products. The added pressure above the liquid phase is another parameter required to form the desired saturated product and should have affected the temperature and pressure at the localized hot spots created by sonication. 

In the case of simultaneous irradiation of the catalyst and the reactants, we are most interested in the phenomenon related to the imperfections w ith transient character, i.e., essentially to the electronic imperfections. However, this does not mean that we do not take account of the quasi-permanent imperfections. Three groups of phenomena may be distinguished here, depending upon the heterogeneous system under consideration. 

It should be noted again that the present paper deals essentially with heterogeneous systems under irradiation, which is the reason why in the experimental part no examples of catalyst activation through preliminary irradiation are mentioned. But the latter mode of activation is narrowly related to activation under irradiation, so that we have preferred to discuss simultaneously these two points in the present chapter. 

Ultrasound irradiation has a beneficial influence on sample digestion, physical dispersion, the development of chemical reactions, analyte separation/concentration and precipitation reactions carried out in flow systems [144,145]. In general, sensitivity is improved, especially for heterogeneous systems, reactions with slow kinetics and low analyte concentrations. 

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