Effect of sodium carboxymethylcellulose on the reactivity of hydrated electrons and cystamine anion radicals in aqueous solutions

Abstract

Rate constants for several reactions of the hydrated electron with different solutes, shown to be diffusion controlled in normal aqueous solution, were carried out in an aqueous solution of sodium carboxymethylcellulose with a relative viscosity of 250 cP, using the pulse radiolysis technique. In carboxymethylcellulose solutions, hydrated electron rate constants are not dependent on macroscopic viscosity : rate constants are ≈ 10¹⁰ dm³ mol^–1 s^–1 for solutions with 250 cP viscosity. For comparison the same reactions were observed in water + glycerol solutions with comparable solution viscosities. The results are rationalised on the basis that a small molecule binary solvent system such as water + glycerol has a higher microscopic and macroscopic viscosity, whereas a carboxy-methylcellulose solution has a high macroscopic viscosity but the microscopic viscosity is little or no different to the pure solvent.

The first order decomposition of the cystamine anion radical, formed in solutions of cystamine to which carboxymethylcellulose has been added, was also studied. The rate of this reaction increased in carboxymethylcellulose solutions. A similar increase in reactivity was found also for electron transfer from the cystamine anion radical to oxygen. The ordered structure of carboxymethylcellulose in aqueous solutions, and the electrostatic potential at the surface of the macroion, would appear to enhance the rate.