Triple Screening and Plasmon Mode in Polyelectrolyte Solutions

Understanding relation between structure and dynamics of polyelectrolyte chains in aqueous solutions is of fundamental and technological interest. Almost thirty years ago, theories for the structure (J. Chem. Phys. 105, 5183 (1996)) and dynamics (J. Chem. Phys. 107, 2619 (1997)) of polyelectrolyte solutions were developed by Muthukumar laying a theoretical basis for the relation. The theories were developed using the concept of triple screening in polyelectrolyte solutions, which postulated that counterions, co-ions, and chains screen topological, electrostatic, and hydrodynamic interactions. In this work, we test the concept by probing concentration fluctuations in aqueous solutions containing a well-studied polyelectrolyte, sodium poly(styrene sulphonate) (NaPSS) with neutron scattering, theory, and molecular dynamics simulations. Neutron spin-echo (NSE) and small-angle neutron scattering (SANS) data from semi-dilute solutions of NaPSS will be presented at different polymer and salt (NaCl) concentrations revealing a plasmon mode. The theories provide a quantitative description of the decay rate obtained from the NSE measurements while capturing shape and concentration dependencies of the polyelectrolyte peak observed in the SANS spectra. These results show that the dynamics of concentration fluctuations in semi-dilute solutions of polyelectrolytes is dictated by the screening of hydrodynamics, topological, and electrostatic interactions.