Radiation chemistry liquid-phase

Evidence indicates [28,29] that in most cases, for organic materials, the predominant intermediate in radiation chemistry is the free radical. It is only the highly localized concentrations of radicals formed by radiation, compared to those formed by other means, that can make recombination more favored compared with other possible radical reactions involving other species present in the polymer [30]. Also, the mobility of the radicals in solid polymers is much less than that of radicals in the liquid or gas phase with the result that the radical lifetimes in polymers can be very long (i.e., minutes, days, weeks, or longer at room temperature). The fate of long-lived radicals in irradiated polymers has been extensively studied by electron-spin resonance and UV spectroscopy, especially in the case of allyl or polyene radicals [30-32]. 

This definition of electrochemistry disregards systems in which nonequilibrium charged species are produced by external action in insulators for example, by electric discharge in the gas phase (electrochemistry of gases) or upon irradiation of liquid and sohd dielectrics (radiation chemistry). At the same time, electrochemistry deals with certain problems often associated with other fields of science, such as the structure and properties of sohd electrolytes and the kinetics of ioific reactions in solutions. 

Radiation chemistry highlights the importance of the role of the solvent in chemical reactions. When one radiolyzes water in the gas phase, the primary products are H atoms and OH radicals, whereas in solution, the primary species are eaq , OH, and H" [1]. One can vary the temperature and pressure of water so that it is possible to go continuously from the liquid to the gas phase (with supercritical water as a bridge). In such experiments, it was found that the ratio of the yield of the H atom to the hydrated electron (H/eaq ) does indeed go from that in the liquid phase to the gas phase [2]. Similarly, when one photoionizes water, the threshold energy for the ejection of an electron is much lower in the liquid phase than it is in the gas phase. One might suspect that a major difference is that the electron can be transferred to a trap in the solution so that the full ionization energy is not required to transfer the electron from the molecule to the solvent. 

Horvath, Zs. Ausloos, P Foldiak, G. Comparison between liquid phase photolysis and radiolysis of C3—C4 hydrocarbons mixtures. In Proceedings Fouth Tihany S5unposium on Radiation Chemistry Hedvig, P. Schiller, R., Eds. Akademiai Kiado Budapest, 1977 57 pp. Antonova, E.A. Pichuzhkin, V.I. High Energy Chem. 1977, 11, 201. 

In general, however, the effect of phase is much less marked than for ionic species and results for different phases will not be considered separately in this section. Since, in fact, more experiments have been carried out on the radiation chemistry of liquids than of gases or solids, most of the results discussed in this section refer to the liquid state. 

During this paper, many questions will be raised for which only partial answers can be given. In an emerging field such as this, that is to be expected. In this way we hope to arrive at a fairly complete understanding of not only radiation-induced polymerization but the broader field of liquid-phase organic radiation chemistry. 

Early experiments in liquids were quite variable for many reasons. The conductivity technique, which was used in the gas phase to measure dose, was not applicable to the liquid phase. Reactions were measured using dissolved radium salts or radon gas as the ionization source. Some thought the chemistry was due to the reactions with radium however, it was soon recognized that it was the emitted rays that caused the decomposition. Both radium and radon could cause radiation damage. Because the radon would be partitioned between the gas and liquid phase, the amount of energy that was deposited in the liquid depended critically on the experimental conditions such as the pressure and amount of headspace above the liquid. In addition, because the sources were weak, long irradiation times were necessary and products, such as hydrogen peroxide, could decompose. 

In summary, in this first era of radiation chemistry it was discovered that the medium absorbs the energy and the result of this energy absorption leads to the initiation of the chemical reactions. The role of radium in these systems was not as a reactant or as a catalyst, but instead as a source of radiation. Most quantitative work was done with gases. It was learned that there was a close correspondence between the amount of ionization measured in a gas and the yield of chemical products. Solid and liquid-phase radiolysis studies were primarily qualitative. 

This narrative echoes the themes addressed in our recent review on the properties of uncommon solvent anions. We do not pretend to be comprehensive or inclusive, as the literature on electron solvation is vast and rapidly expanding. This increase is cnrrently driven by ultrafast laser spectroscopy studies of electron injection and relaxation dynamics (see Chap. 2), and by gas phase studies of anion clusters by photoelectron and IR spectroscopy. Despite the great importance of the solvated/ hydrated electron for radiation chemistry (as this species is a common reducing agent in radiolysis of liquids and solids), pulse radiolysis studies of solvated electrons are becoming less frequent perhaps due to the insufficient time resolution of the method (picoseconds) as compared to state-of-the-art laser studies (time resolution to 5 fs ). The welcome exceptions are the recent spectroscopic and kinetic studies of hydrated electrons in supercriticaF and supercooled water. As the theoretical models for high-temperature hydrated electrons and the reaction mechanisms for these species are still rmder debate, we will exclude such extreme conditions from this review. 

Ion-molecule reactions in interstellar clouds Radiation chemistry in interstellar grain mantles Condensation in stellar outflows Equilibrium reactions in the solar nebula Surface catalysis (Fischer-Tropsch) in the solar nebula Kinetically controlled reactions in the solar nebula Radiation chemistry (Miller-Urey) in the nebula Photochemistry in nebular surface regions Liquid-phase reactions on parent asteroid Surface catalysis (Fischer-Tropsch) on asteroid Radiation chemistry (Miller-Urey) in asteroid atmosphere... 

The radiation chemistry of cyclic oligoenes was studied, and the radiolytic yields of the final products are summarized in Table 3, which shows that l, 4-cyclohexadiene differs from all others in its high yield of hydrogen, both in the gas phase and in the liquid phase. Cserep and Foldiak attributed it to the presence of two doubly allylic CH2 groups. In addition, the geometric orientation of the allylic hydrogens is favourable for hydrogen... 

T he aggregation state is known to exert a great influence on radiation chemistry processes. In the gas phase, active species formed by decomposition of a parent molecule can easily escape recombination. In the liquid phase, diffusion competes with recombination in the cage formed by the surrounding molecules. In the solid phase, diffusion is very limited, and the most probable fate of fragments issued from a parent molecule is recombination. 

Radiation Chemistry of Solvents Water. The successful design of a radiation chemistry experiment depends upon complete knowledge of the radiation chemistry of the solvent. It is the solvent that will determine the radicals initially present in an irradiated sample, and the fate of all these species needs to assessed. Among the first systems whose radiation chemistry was studied was water, both as liquid and vapor phase, as discussed by Gus Allen in The Story of the Radiation Chemistry of Water , contained in Early Developments in Radiation Chemistry (8), Water is the most thoroughly characterized solvent vis-a-vis radiation chemistry. So to illustrate the power of radiation chemical methods in the study of free radical reactions and electron-transfer reactions, I will focus on aqueous systems and hence the radiation chemistry of liquid water. Other solvents can be used when the radiation chemistry of the solvent is carefully considered as noted previously, Miller et al. (I) used pulse radiolysis of solutions in organic solvents for their landmark study showing the Marcus inversion in rate constants. 

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. 

While most studies of radiation chemistry track effects have been carried out in condensed phase, and particularly in liquids, evidence of Pl due to interaction with track electrons or other paramagnetic species (e.g. N2 has also been seen in gases at high pressures and as well near the critical point in ethane [19]. 

Due to the intensive investigations more is known about radiation chemistry of water than any other liquid (Buxton 1987). Many of the principles of radiation chemistry as applied to the liquid phase have been developed in the course of studies on the radiation chemistry of water and aqueous solutions. Water and aqueous solutions have been studied because of the part they play in chemistry in general and in radiochemistry in particular, because they are readily available and not too difficult to work with, and because water is a polar Uquid that responds in characteristic ways to radiation. A practical motivation for the studies has been the desire to understand the effect of radiation on biological systems. Irradiation of water and several aqueous systems is an important consideration in various aspects of nuclear technology (Swallow 1973). 

---