Uniform polystyrene particles in aqueous suspensions have been subject to ionizing from Co-60. Particles of 0.234-μm diameter have been most extensively studied. The suspensions are stable with regard to coagulation and the particles undergo no detectable configurational change up to doses of at least <tex-math>$5\times 10^{20}\ {\rm eV}\ {\rm ml}^{-1}$</tex-math> of suspension. When irradiated in the presence of iodide, a chemical bonding of iodine to the surface occurs, presumably through the formation of C-I bonds. At a population of <tex-math>$1.4\times 10^{12}$</tex-math> particles of 0.234-μm diameter <tex-math>${\rm cc}^{-1}$</tex-math> the yield of surface-bonded halogen in <tex-math>$5\times 10^{-4}\ M$</tex-math> iodide corresponds to 0.02 iodine atoms bonded to the particle surface per 100 eV of total energy absorbed. In <tex-math>$5\times 10^{-4}\ M$</tex-math> iodide, the reaction rate is proportional to the surface area and depends upon the first power of the radiation intensity. The experimental results are consistent with a reaction mechanism in which radical sites are produced on the particle surface by reaction of OH and other radicals from the bulk phase. A surface bonding of iodine results from a reaction of these sites with halogen intermediates formed in the aqueous phase, predominantly by reaction of OH radicals with iodide. Energy absorbed in the particle apparently plays no major role in the reaction, and a complex dependence of the reaction upon the iodide concentration suggests that a surface adsorption of halogen intermediates plays an important role in the reaction.

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