Structure and rheology of polyelectrolytes with anisotropic counterions

We study the scattering and solution rheology of polyelectrolytes with counterions of varying anisotropy and valence. Increasing counterion valence reduces the effective polymer charge, leading to smaller chain dimensions, reflected in a higher overlap concentration and a weakened scattering peak, consistent with diminished electrostatic repulsion. Higher counterion valence decreases the solubility of the polyelectrolyte in aqueous and organic solvents. A comparison of anisotropic divalent counterions such as hexamethonium, with two individual charges separated by 0.7 nm, to monocentric divalent ions such as Mg²⁺ or Ca²⁺, shows that anisotropy promotes counterion dissociation, as demonstrated by conductivity and SAXS. Multivalent counterions composed of spatially separated charges also maintain higher solubility than their monocentric counterparts. The higher effective charge density of polyelectrolytes with anisotropic counterions becomes increasingly pronounced at high concentrations. In excess salt, divalent counterions increase solution viscosity, suggesting the formation of temporary inter-chain cross-links via neutralization of groups on different chains. Monovalent surfactant counterions largely preserve the polyelectrolyte solution structure but impart interfacial activity.