Radiation chemistry of aqueous solutions

While many of the important reactions in radiation and photochemistry are fast, not all are diffusion-limited. The random flight simulation methodology has been extended to include systems where reaction is only partially diffusion-controlled or is spin-controlled [54,55]. The technique for calculating the positions of the particles following a reflecting encounter has been described in detail, but (thus far) this improvement has not been incorporated in realistic diffusion kinetic simulations. Random flight techniques have been successfully used to model the radiation chemistry of aqueous solutions [50] and to investigate ion kinetics in hydrocarbons [48,50,56-58]. 

The irradiation of water is immediately followed by a period of fast chemistry, whose short-time kinetics reflects the competition between the relaxation of the nonhomogeneous spatial distributions of the radiation-induced reactants and their reactions. A variety of gamma and energetic electron experiments are available in the literature. Stochastic simulation methods have been used to model the observed short-time radiation chemical kinetics of water and the radiation chemistry of aqueous solutions of scavengers for the hydrated electron and the hydroxyl radical to provide fundamental information for use in the elucidation of more complex, complicated chemical, and biological systems found in real-world scenarios. 

In comparison with the abundant information on the reactivity of different compounds toward e aq, there is a lack of information on the products of these reactions. It has been shown throughout this review that some of these products are extremely reactive oxidizing or reducing agents. The formation of such products may occasionally reverse the radiolytic damage or sometimes enhance it. The characterization of the reactive intermediates formed from solutes is the major task of radiation chemistry of aqueous solutions in the coming decade. Pulse radiolysis is the most appropriate tool for this type of investigation. 

Seddon WA, Allen AO (1967) Radiation chemistry of aqueous solutions of ethanol. J Phys Chem 71 1914-1918... 

It has already been pointed out (p. 73) that the postulate of free radical chain reactions provided a reasonable explanation for early results of the radiolyses of gases. It was later suggested10 that the radiation chemistry of aqueous solutions could best be explained by the production of H atoms and OH radicals. Subsequently, the results of a large variety of radiolyses were explained in terms of radical reactions occurring therein. Although such experiments did not provide conclusive evidence for the existence of free radicals in the systems, the results obtained, e.g. product analysis, rate coefficients, were not inconsistent with the occurrence of free radical processes. 

Radiation chemistry of aqueous solutions has also been applied to the study of micellar systems. Considerable micellar effects on the yield of radiolytic products and on rates of radical reactions have been observed by several authors (Gebicki and Allen, 1969 Fendler and Patterson, 1970 Bansal et al., 1971 Patterson et al., 1971, 1972 Fendler et al., 1972 Wallace and Thomas, 1974 Gratzel et al., 1974). These observations led to conclusions on the permeability of micelles to various radicals and on the location of substrates in micelles. Recent experiments have also demonstrated a very efficient trapping of e4q by positively charged micelles even when chemical reaction between them did not take place (L. K. Patterson, personal communication). 

Kuppermarm A. (1967) Diffusion model of the radiation chemistry of aqueous solutions. In Silini G. (ed.). Radiation Research North-Holland Publishing Company, Amsterdam, pp. 212-234. 

Schwarz HA. (1969). Applications of the spur diffusion model to the radiation chemistry of aqueous solutions. J Phys Chem 73 1928-1937. 

The radiation chemistry of aqueous solutions may be considered from two points of view. The first, called the Target Theory, considers the direct effect of ionizing radiations on the solute molecules. The second approach regards transformations in the solute molecules to be attributed to interactions with the reactive intermediates formed by the radiolysis of water. Because most aqueous systems are relatively dilute, the latter approach seems statistically more reasonable. Kinetic studies of dilute aqueous systems have indeed borne out this supposition. The radiation chemistry of aqueous solutions then becomes the free radical and redox chemistry of H-, OH-, and e q. 

It would be superfluous to review here the story of e aq in the radiation chemistry of aqueous solutions. High energy radiations cause ionizations and the free electrons so generated dissipate their excess energy and are eventually trapped in solvation shells. The discovery of hydrated electrons showed that electrons in water were chemical entities (as distinct from possessing purely physical characteristics) in having diffusion properties, size and sphere of influence, associated ion atmosphere, and reaction rate parameters all of which are comparable to normal chemical reagents. 

This general type of process has been discussed with respect to the radiation chemistry of aqueous solutions (1, 7, II, 20, 22, 24, 31) organic liquids (9), gas-phase mixtures (29), a model for radiobiological sensitization (I, 6), and with respect to some apparent conflicts between steady-state radiation chemistry and pulse radiolysis (22, 24). In this paper, some examples of electron transfer in pulse radiolysis have been chosen to illustrate various features of this phenomenon. 

Czapski G, Peled E (1968) On the pH-dependence of Greducing in the radiation chemistry of aqueous solutions. Isr J Chem 6 421 36... 

G. Scholes, The Radiation of Chemistry of Aqueous Solutions of Nucleic Acids and Nucleoproteins, in Progress in Biophysics, Vol. 12, Pergamon Press, London, 1963. 

P. Benes, V. Majer, Trace Chemistry of Aqueous Solutions, Elsevier, Amsterdam, 1980 R. Guillaumont, P. Chevallier, J. P. Adloff, Identification of Oxidation States of Ultra-trace Elements by Radiation Detection, Radiochim. Acta 40, 191 (1987)... 

The foundations for the edifice had been laid when I compiled my review on inorganic free radicals in solution ten years ago (23), in the sense that the basic concepts had been realized and kinetic studies based on sound energetics offered very great scope for further investigation in many fields polymerization and autoxidation reactions, photochemistry and radiation chemistry of aqueous systems, and even reactions in biological systems. 

The Fricke solution has been the most popular subject of ion beam radiation chemistry in aqueous solutions, and proton, helium ions, heavier ions including particle energy higher than GeV [52] and even uranium ions have been employed [53]. Some typical results are shown in Figure 4-1 [54]. The radical yields increase and the water decomposition decreases with increasing the LET of the particles. At a glance, it is clear that the value is strongly dependent on both LET and the kind of particles. 

Phillips, G.O. and Moody, G.J. (1960a). Radiation chemistry of carbohydrates. Part IV. The effect of gamma-radiation on aqueous solutions of sucrose. J. Chem. Soc. 155, 762-768. 

G. Czapski, Radiation chemistry of oxygenated aqueous solutions. Ann. Rev. Phys. Chem. 22, 171—208 (1971). 

Dajka, K Takacs, E Solpan, D Wojnarovits, L Guven, O. High-energy irradiation treatment of aqueous solutions of C.l. Reactive Black 5 azo dye pulse radiolysis experiments. Radiation Physics and Chemistry, 2003 67, 535-538. 

Allen, A.O. In The Radiation Chemistry of Water and Aqueous Solutions, D. Van Nostrand Princeton, 1961. 

There has been continued interest in the radiation chemistry of the purines since early reports on oriented DNA by Graslund et al. [35] which suggest that the main trapping site of one-electron oxidation in DNA is the guanine base. It is remarkable that in aqueous solution, the electron adducts of the purine nucleosides and nucleotides undergo irreversible protonation at carbon with a rate constant 2 orders of magnitude higher than that for carbon protonation of the electron adduct in thymidine [36]. It is therefore important to know the properties of the various purine reduction products and to ask why they have not been observed in irradiated DNA. 

Although the radiation-induced oxidation of ethanol has been fully in-- vestigated (2, 22, 23), little work has been published on the oxidation of other alcohols. In connection with a project concerned with the relative rates of hydroxyl radical reactions using 2-propanol as reference solute, it was thought desirable first to investigate the radiation chemistry of 2-propanol-oxygen solutions both in aqueous solution and pure 2-propa-nol. The results of this investigation are presented here. 

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