Ordinary–extraordinary transition in dynamics of solutions of charged macromolecules

Significance The experimentally observed phenomenon of the “ordinary–extraordinary” transition from dynamic light-scattering measurements of solutions of charged macromolecules has remained mysterious for the past four decades and has prohibited the use of the Stokes–Einstein law in characterizing the sizes of charged polymers, using light-scattering techniques. The diffusion coefficients of polymers with molecular weight of about 1 million can be merely one-fourth of those of metallic ions such as K+ , independent of polymer molecular weight and concentration. Similarly charged macromolecules can clump together to make aggregates when electrostatic repulsion between them is dominant and they break apart when the electrostatics are screened. A theory based on counterion dynamics and dipolar interactions is presented in resolving these mysteries.