Some remarks on the dynamics of charged macromolecules in solution

With reference to experimental results obtained by dynamic light scattering and electric birefringence decay it is shown that, for aqueous solutions of a highly charged macromolecule with a relative flexible backbone, in the presence of an excess low molar mass salt monovalent cations and anions, the dynamics of the charged macromolecules in the absence of an external force are comparable to what is expected and found for conventional but uncharged macromolecules in good solvents. It is concluded that under these conditions it may be assumed that the charged macromolecules move, in a first approximation, in a continuum constituted by both the solvent and the low molar mass salt, and that the coupling between the motions of polyions and small ions is weak. A calculation based on a simple model, in which the fundamental asymmetry between polyions and small ions is explicitly taken into account, however, leads to the conclusion that such a coupling may in principle be expected although it is not excluded that it is rather weak. The calculation also justifies the experimentally found linear concentration dependence of the macromolecular cooperative diffusion coefficient in dilute solutions and the decrease of the corresponding slope with increasing salt concentration.