Structure and dynamics of polyelectrolyte solutions with multivalent salts

Aqueous solutions of highly charged polyelectrolyte solutions phase separate in the presence of multivalent salts. Phase diagrams of sodium polystyrene sulfonate (Na+PSS-) with LaCl3, Th(NO3)(4) were studied over a wide range of polyelectrolyte and salt concentrations. The concentration of salt at the transition C-s* was found to be proportional to the monomer concentration C-m and nearly molecular weight independent: C-s*/Cm congruent to 0.23 for La3+ and C-s*/C-m congruent to 0.12 for Th4+. With further addition of salt a new transition from a two phase system back to a single phase was observed at constant salt concentration independent of NaPSS concentration but sensitive to molecular weight. A theoretical model which explains this behavior in termes of electrostatic attractive interaction between monomers due to the high valency of condensated counterions has been developped recently. The form of the phase diagram results from a competition between the classical long range Coulombic interaction, the short range bridging attraction via the condensed counterion and the electrostatic screening. Dynamic light scattering measurements were performed in the lower monophase solution as a function of NaPSS concentration and over a wide LaCl3 concentration range. Two relaxation times were observed at very low salt concentration. The dynamic studies reveal some interesting behaviours of apparent diffusion coefficient as a function of polyelectrolyte and salt concentrations (C-m, C-s): i) a q(2) dependence of the fast mode and a q(3) dependence of the slow mode; ii) the power law D-fast varies approximately as C-m(1/2) at constant low salt concentration. This mode can be explained assuming cooperative diffusion in a transient polyelectrolyte network. The slow mode of the bimodal decay disappears increasing the added salt (LaCl3) concentration at constant C-m. It is concluded that the slow mode is related to labile clusters or temporal domains of polyelectrolyte chains which are formed in solutions al low concentration of added salt. Both the disappearance of the slow mode on a very long time scale and the non reappearance of this mode when cycling al constant C-s/C-m between high and low salt concentration indicate that low salt or salt free polyelectrolyte solutions tend very slowly to thermodynamic equilibrium.