Radiolytic limitations

In dibenzothiophene-S,S-dioxide the S atom is in a ring, and hence more constrained. The yield of SOz in the radiolysis is linear with the dose to about 13 Mrad after which it levels off as in p,p -ditolyI sulfone. However, the yield of S02 in this case is much lower (a factor of 25) than in the case of p,p -ditolyl sulfone (G = 0.002 compared to G = 0.05). This stability of the dibenzothiophene sulfone could be partially due to back reaction to reform the parent sulfone and partially due to more efficient energy delocalization. The expected biphenylene product was not detected due to limitations of the analytical method. Bowmer and O Donnell70 studied the volatile products in y-radiolysis of dialkyl, alkyl aryl and diaryl sulfones. Table 2 gives the radiolytic yields of S02 and of the hydrocarbon products of the alkyl or aryl radicals. The hydrocarbon products are those obtained either by H atom abstraction or by radical combination. The authors69 suggested the mechanism... 

In experiments where ion-radicals are generated by radiolytic reduction or oxidation in solid matrices, the concentration of solute molecules must be at least 10 M to ensure efficient scavenging of the initially generated electrons or holes. At the same time, the upper limit of the solute concentration should not exceed 10 -10 M. It is necessary that the direct effect of radiation on the solute molecule should be ignored. This is one of the major requirements for successful use of radiolytic methods for generating ion-radicals. 

Finally, solute radical ions can be generated by light-induced, one-photon or multiphoton ionization of their parent compounds (Chaps. 5 and 16). This approach is particularly useful in the ultrafast studies of short-lived, unstable radical ions that aim to unravel their solvation, recombination, reaction, and vibrational relaxation dynamics of the primary charges (see, e.g., Chap. 10). Whereas the time scale of radiolytic production of secondary ions is always limited by the rate with which the primary species reacts with the dispersed parent molecules, light-induced charge separation can occur in <100 fsec. There are many studies on photoionization of solute molecules in liquid solutions we do not intend to review these works. 

Figure 8 Nucleation and growth of clusters generated by radiolytic radicals at various dose rates, with or without electron donor D. The stabilizing effect of the polymer prevents exclusively coalescence beyond a certain limit of nuclearity, but does not prevent successive ion and electron transfers at low dose rate from the radicals. The donor allows the cluster to become much larger at any dose rate. (From Ref. 19.)...

Schmidt KH, Flan P, Bartels DM (1995) Radiolytic yields of the hydrated electron from transient conductivity improved calculation of the hydrated electron diffusion coefficient and analysis of some diffusion-limited (e )aq reaction rates. J Phys Chem 99 10530-10539 Schoneich C, Aced A, Asmus K-D (1991) Halogenated peroxyl radicals as two-electron-transfer agents. Oxidation of organic sulfides to sulfoxides. J Am Chem Soc 113 375-376 Schuchmann Fl-P, von Sonntag C (1981) Photolysis at 185 nm of dimethyl ether in aqueous solution Involvement of the hydroxymethyl radical. J Photochem 16 289-295 Schuchmann Fl-P, von Sonntag C (1984) Methylperoxyl radicals a study ofthey-radiolysis of methane in oxygenated aqueous solutions. Z Naturforsch 39b 217-221 Schuchmann Fl-P, von Sonntag C (1997) Heteroatom peroxyl radicals. In Alfassi ZB (ed) Peroxyl radicals. Wiley, Chichester, pp 439-455... 

Collect intrinsic data pertaining to the emerging process liquid-liquid distribution, kinetics, hydrolytic and radiolytic stability, and the maximum metal loading in an organic phase without TPF, referred to as the limiting organic concentration (LOC). Consequently, build a database. 

To limit the radiolytic degradation of extractants, the influences of free-radical inhibitors have been measured. The addition of dimethoxybenzaldehydes (DMBA), particularly 3,5- and 3,4-DMBA, to the PUREX solvent could improve its stability and decrease its contamination (307). DMBA has a double effect, including a protective effect for the excited molecules of TBP (because of its low ionization potential), and the aldehyde radiolysis products could react with the HDBP present and therefore inhibit its complexing properties. 

The selection of a suitable diluent is important to limit radiolytic degradation. Diluents currently used in nuclear applications are hydrocarbons, despite their well-known sensitization effect on radiolysis, as mentioned for alkylphosphates or amide extractants (90, 182, 183, 199), and as discussed in Section 8.4.2. To avoid this negative effect or to enhance the solubility of ligands and metallic complexes, other diluents have been selected and their influence on degradation investigated. 

Nitrite ion is an undesirable by-product of radiolysis in waters containing high nitrate concentrations, and regulatory limits exist regarding its acceptable concentration. The presence of nitrate does not preclude radiolytic water treatment. One approach that compensates for nitrite generation in high nitrate waters is ozone addition, discussed later. 

Prior to the quoted investigations, Dorfman had reported rate constants for the reactions of radiolytically generated benzyl- and benzhydryl cations [126] with some alkenes and dienes [127]. Although Dorfman s data on benzhydryl cations agreed well with the results of the other methods, two independent groups came to the conclusion that the corresponding benzyl cation data must be erroneous [128,129], Because only a very limited number of compounds had been studied by the radiolytic method, details of this technique will not be presented here. 

In a series of experiments the kinetics of UOj (s) oxidation by different oxidants was studied. The results clearly showed that the logarithm of the rate constant for oxidation of U02(s) is linearly dependent on the one-electron reduction potential of the oxidant. The correlation was also in excellent agreement with previous kinetic studies on different radiolyric oxidants.On the basis of the correlation it was possible to estimate the rate constants for the more reactive (less stable) radiolytical oxidants of relevance (i.e. OH and CO f). The rate constants for these species were estimated to be limited by diffusion in particle suspensions of pm-sized particles. 

Figure I shows that the physicochemical stage is purely intrapore, the characteristic diffusion length of radiolytic species being in the nanometer range. Confinement in nanopores will impact this physicochemical step by limiting the diffusion of primary species out of their...

This decrease, observed in CPGs, is somewhat limited even for material domains as big as 100 nm. However, it must be noted that most studies on radiolytic hydrogen production at the surface of nanoporous material have been conducted on silica-based systems, and the exact role of substrate chemistry has still to be deciphered. 

An important observation is that there is a production of H2 even without HO scavengers (in bulk water only neglectable amounts of H2 are produced in the absence of such scavengers). This indicates that the chain reaction which controls the back reaction of H2 and H2O2 in pure bulk water does not occur in confined water. In confined water the diffusion of the radiolytic species is limited, thereby reducing the interpore encounter of HO and H2. Confinement, besides early radiolytic events, may also impact the long-term behavior of radiolytic species and their accumulation. 

When people consider confinement effects, they consider mainly an increase in the encounter probability inside a single pore and therefore, expect an acceleration of the reaction. Such in-pore acceleration has been quantified by Tachiya and co-workers for diffusion-limited reactions through the so-called confinement factor [see Eq. (11.58) in Ref. 40]. From this treatment, confinement effects are expected to disappear when the reaction radius is less than one tenth of the confinement radius. Considering the reaction radii of radiolytic species, no acceleration by confinement should be expected for pore diameter larger than 10 nm. For smaller pore size, acceleration of the recombination reactions within spurs would be critical in the determination of radiolytic yields in the nanosecond time range. However, the existence of such an acceleration of radiolytic reactions has not been suggested in the nanosecond pulse radiolysis of zeolites and has still to be assessed using picosecond pulse radiolysis. 

Experiments of implantation of reactive ions are not numerous and consequently their application has been little discussed. A noteworthy exception is the case of SO2. It has been observed on Europa and Callisto and it has been interpreted as formed by magnetospheric sulfur implanted in water ice. We have found only an upper limit to the formation yield of sulfur dioxide after S implantation that does not permit us to be conclusive about the effective relevance of the process at Europa. The finding of the formation of a sulfate, most probably hydrated sulfuric acid, could be of extreme relevance to support the suggested radiolytic sulfur cycle at Europa. 

The HPLC-UV-DAD was used for the analysis of final products results showed that many related compounds of cefotaxime were present prior to irradiation. Some of them had maxima around 320-380 nm and could be responsible for the change in color of the dissolved irradiated drug. The radiolytic products (25 kGy) were present in traces and were all below the qualification limit of0.1%.TheHPLC-UV-MS results showed that some radiolytic products detected were impurities already present in the non-irradiated cefotaxime and increased after irradiation. The other products were unique to radio-sterilization [14]. 

Irradiation of lipids, especially unsaturated ones, may lead to unwanted flavour, which is the main technological limit of the treatment. Even if lowering the irradiation temperature can always reduce the quantities of these undesirable radiolytic products, this generally increases the treatment cost. 

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