Effective charge of poly(styrenesulfonate) and ionic strength—an electrophoresis NMR investigation

The effective charge, the charge that is available for the interaction with other molecules or surfaces, for strong polyelectrolytes is determined by counterion condensation rather than the nominal charge density. Pulsed field gradient NMR is applied to investigate diffusion and electrophoretic flow of sodium poly(styrenesulfonate) in aqueous solution, in which the ionic strength has been adjusted by the addition of sodium chloride. From the electrophoretic mobility in conjunction with the friction determined from the diffusion experiment the effective charge of the polyelectrolyte is determined model free. An initial drastic drop of the effective charge with the ionic strength is observed, corresponding to the observed drop in the hydrodynamic radius.