Microrheological study of polyelectrolyte collapse and reexpansion in the presence of multivalent counterions

We studied salt-induced changes in the viscosity of dilute solutions of a flexible polyelectrolyte, sodium poly(styrenesulfonate) (PSS), using particle tracking microrheology. It was found that PSS responds distinctively different to the presence of various trivalent cations. In addition to the well-known polyelectrolyte precipitation and redissolution, we also observed more subtle changes in solution viscosity, which can be attributed to chain collapse and reexpansion. In particular, the addition of indium chloride (InCl₃) to PSS solutions leads to polyelectrolyte chain collapse at low salt concentrations, followed by chain reexpansion upon further increasing the salt concentration, as evidenced by an increase in relative viscosity of the polyelectrolyte solution; at even higher salt concentrations, the relative viscosity decreases and chains exhibit a secondary collapse The salt concentration at which the onset of reexpansion is observed is a function of the polyelectrolyte molecular weight and concentration; rescaling of the data suggests that the key physical quantity is the number of indium ions per polyelectrolyte repeat unit. Upon the addition of chloride salts with other trivalent cations (AlCl₃ and GaCl₃), PSS undergoes precipitation and redissolution, which makes it difficult to draw conclusions about chain expansion at high salt concentrations. A determining factor for the polyelectrolyte response seems to be the size of the trivalent ions. The study provides the first clear experimental evidence in support of recent claims based on numerical simulations [Hsiao and Luijten, Phys. Rev. Lett. 2006, 97, 148301; Hsiao, J. Chem. Phys. 2006, 124, 044904; Hsiao, Macromolecules 2006, 39, 7125]. The hypothesis is that the association of multivalent ions with the monovalent charges on the polyelectrolyte chain leads to charge inversion and that intramolecular repulsion between associated trivalent ions results in chain reexpansion.