Hydrodynamic size and electrophoretic mobility of poly(styrene sulfonate) versus molecular weight

The effective charge of a macromolecule in solution is significantly smaller than the nominal charge, derived form the number of charged groups and the degree of dissociation. It is reduced by counterion condensation, shielding a considerable fraction of the macromolecules charge. As an example the influence of the molecular weight on hydrodynamic size and electrophoretic mobility of poly(styrene sulfonate) has been investigated. While from diffusion the hydrodynamic size is inferred, in a combination with electrophoresis NMR the effective charge is calculated. At low molecular weight the effective charge is equal to the nominal charge, in an intermediate range the effective charge is that predicted from counterion condensation theory, while at higher molecular weights E it becomes considerably smaller.