Polyelectrolyte effects on counterion self-diffusion

Abstract

The self-diffusion coefficient of tetramethylammonium counterion has been measured in aqueous solutions of polymethacrylic acid (PMA) of 100 and 1000 monomers as well as in persistence-length DNA fragment solutions. The concentration ranged from to 1 M (monomolar) and no low-molecular-weight salt was added. The counterion self-diffusion coefficient was found to be independent of the kind of polyion and of molecular weight over a large concentration range. In the low-concentration region the self-diffusion coefficient is a function of polyion length and species. In this concentration region the self-diffusion coefficient is the smallest in solutions of the polyion with the largest persistence length (TMADNA). The 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 short PMA (100 monomers) the counterion diffusion coefficient is lower than that in semidilute solutions of the higher-molecular-weight polyelectrolyte at the same monomer concentration.