Sedimentation and diffusion of polyelectrolytes

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The sedimentation and diffusion behaviour of polyelectrolytes is discussed. The properties of polyelectrolyte solutions are profoundly influenced by intramolecular and intermolecular charge interactions. In the presence of a simple salt, the range of these interactions is reduced, but additional effects arise. For example, the Donnan effect needs to be taken into account in the quantitative analysis of sedimentation data. The Donnan parameter, Γ, may be estimated from the concentration dependence of sedimentation velocity. In the absence of a simple salt, sedimentation coefficients are rather low and diffusion coefficients, at moderate polymer concentrations, are rather high; this has been attributed to coupled motion of polyions and counterions. Results are presented from a synthetic boundary experiment on sodium poly(styrene sulphonate) in salt-free solution which show that the diffusion coefficient depends markedly on polymer concentration below 0.2 g dm−3. This diffusion process appears to be equivalent to the fast diffusion process observed by dynamic light scattering. Current theories of coupled-ion diffusion in salt-free polyelectrolyte solutions are inadequate; the effects of intermolecular interactions and the structure of polyelectrolyte solutions need to be considered.Key wordsPolyelectrolytesedimentation velocitydiffusionanalytical ultracentrifugedynamic light scatteringsodium poly(styrene sulphonate)