POLYELECTROLYTE EFFECTS ON COUNTERION SELF-DIFFUSION

The self-diffusion coefficient of tetramethylammonium (TMA(+)) counter ion has been measured in aqueous solutions of polymethacrylic acid (PM A) of 100 and 1000 monomers as well its in persistence-length DNA frag ment solutions. The concentration ranged from 5 x 10(-4) M to 1 M (mon omolar) and no low-molecular-weight salt was added. The counterion sel f-diffusion coefficient was found to be independent of the kind of pol yion and of molecular weight over a large concentration range. In the low-concentration region the TMA(+) self-diffusion coefficient is a fu nction of polyion length and species. In this concentration region the TMA(+) self-diffusion coefficient is the smallest in solutions of the polyion with the largest persistence length (TMADNA). The TMA(+) self -diffusion coefficients in solutions of the two PMAs start to deviate upon dilution at the crossover concentration from semidilute to dilute for the shorter PMA (100 monomers). In the dilute solutions of the sh ort PMA (100 monomers) the counterion diffusion coefficient is lower t han that in semidilute solutions of the higher-molecular-weight polyel ectrolyte at the same monomer concentration.