Effect of Salt on the Ordinary-Extraordinary Transition in Solutions of Charged Macromolecules

Using dynamic light scattering technique, we address the role of added salt at higher concentrations on the ordinary-extraordinary transition in solutions of charged macromolecules. The ordinary behavior has previously been associated with a fast diffusion coefficient which is independent of salt concentration C-s, and polymer concentration C-p if the ratio C-p/C-s is above a threshold value. The extraordinary transition is associated with formation of aggregates, with a slow diffusion coefficient, formed from similarly charged macromolecules. By investigating aqueous solutions of sodium poly(styrenesulfonate) and sodium chloride with variations in C-p, C-s, and polymer molecular weight, M-w, we report the emergence of a new diffusive fast relaxation mode at higher values of C-p, C-s, and M-w, in addition to the previously known fast and slow relaxation modes. Furthermore, we find that M-w plays a crucial role on the collective dynamics of polyelectrolyte solutions with salt, instead of just the C-p/C-s ratio as previously postulated. As M-w is progressively decreased, the salty solution exhibits dynamical transitions from three modes to two modes and then to one mode of relaxation. The emergence of the new fast mode and the dynamical transitions are in marked departure from the general premise of the ordinary-extraordinary transition developed over several decades. In an effort to rationalize our experimental findings we present a theory for the collective dynamics of polyelectrolyte solutions with salt by addressing the coupling between the relaxations of polyelectrolyte chains, counterions from the polymer and added salt, and co-ions from the salt. The predictions are in qualitative agreement with experimental findings. The present combined work of experiments and theory forms the basis for accurately characterizing dynamics of charged macromolecules in salty solutions, which are ubiquitous in biological systems and polyelectrolyte-based technologies.