Radiation induced diffusion controlled reactions

Rate constants for the reaction of the hydrated electron, e-sub(aq) with certain solutes, which are diffusion controlled in normal aqueous solution, have been investigated using pulse radiolysis techniques in binary solvent systems of different viscosities. The same reactions were studied in various polysaccharide matrices, such as viscous polyanionic sols of kappa (kappa) carrageenan and sodium carboxymethylcellulose (CMC), and also polyanionic and neutral gels of kappa-carrageenan and agarose. The vastly enhanced viscosities do not reduce the reaction rates, and in certain instances, significant enhancement of rates is observed. A similar behaviour was encountered with the unimolecular decomposition of the cystamine (RSSR)-, anion radical, formed by reaction of the hydrated electron with cystamine, and also for the electron transfer reaction between the cystamine anion radical and oxygen to form the superoxide anion radical. The results can best be understood in terms of facilitated solute movement rather than Donnan equilibria. Facilitated solute movement is a phenomenon noted in certain types of polyanionic polysaccharide matrix, where enhanced diffusion of both ionic and neutral small molecular species is observed relative to that in a matrix of agarose. (author)