THE INFLUENCE OF SCREENING OF THE POLYION ELECTROSTATIC POTENTIAL ON THE COUNTERION DYNAMICS IN POLYELECTROLYTE SOLUTIONS

The self-diffusion coefficient of tetra-methylammonium counterion in s olutions of polymethacrylic acid in D2O has been measured over a broad polyion concentration range at a constant degree of neutralization an d at different ratios of added monovalent or bivalent salt to polyions . A maximum counterion self-diffusion coefficient was observed as a fu nction of polyion concentration. The value of the self-diffusion coeff icient at the maximum did not depend on the valency of the added salt. The maximum was found at lower polymer concentrations and with a high er value, when the ratio of added salt to polyions was increased, as p redicted by the Poisson-Boltzmann-Smoluchowski equation in the cylindr ical cell model for polyelectrolytes. At higher polyion concentrations a maximum counterion self-diffusion coefficient against the ratio of added salt and polyions was observed, which has not been reported befo re. Upon increasing this ratio the electrostatic potential of the poly electrolyte gets screened, leading to an increase of the counterion se lf-diffusion coefficient. Concentration effects of the added salt on t he other hand ultimately lead to a decrease of the counterion self-dif fusion coefficient, which explains the occurrence of a maximum.