The 60Co γ-radiolysis of methyl iodide adsorbed on silica gel has been studied by examining the hydrocarbon products, which are mainly methane and ethane. These products are formed in large yields, indicating that a large fraction of the energy absorbed in the silica gel is able to cause decomposition of the methyl iodide. The “energy transfer” is thought to occur by electron or excitation transfer to the methyl iodide, leading to the production of methyl radicals.Evidence has been obtained that the silica gel takes part in the system as a chemical reactant as well as being an energy transfer medium, and that changing the nature of the surface changes the course of the reaction. This is most clearly shown in two ways. The ratio of methane to ethane decreases as the surface hydroxyl concentration decreases, and it is concluded that excited methyl radicals form methane by abstraction of hydrogen from surface hydroxyls. Experiments using methyl iodide-d3 adsorbed on protiated silica gel confirm the participation of hydrogen from the silica gel, as the methane is over 85% CD3H, while the ethane is over 95% C2D6.The effect of additives such as N2O and SF6, which are known to be electron scavengers, was also studied. It was shown that methyl iodide is a much better electron scavenger than N2O and is as good an electron scavenger as SF6 in this system.
