Influence of the Counterions on the Behaviour of Polyelectrolytes

The influence of the counterions on the hydrodynamic size and the effective charge of poly(styrene sulfonate) has been investigated applying a combination of pulsed-field gradient (PFG) NMR and electrophoresis NMR. From the diffusion coefficient, determined by PFG NMR the hydrodynamic radius as a measure of the size is inferred. Electrophresis NMR yields the electrophoretic mobility and thus the effective charge of the macromolecule. An increased ionic strength of the solution results in a more coiled conformation of the polyelectrolyte, an effect which is more pronounced for bivalent ions. If the dielectric constant of the solution is lowered, the effective charge is reduced as well. The effect on the overall conformation strongly depends on the kind of the counterion. While a small effect is observed for the acid form, the most drastic effect is found for sodium as a counterion.