Solution Behavior of Semiflexible Polyelectrolyte Carboxymethylcellulose with Anisotropic Counterions That Have Spatially Separated Charges

The properties of polyelectrolytes depend on the counterions that neutralize their backbone. Although the effects of counterion valence and size have been studied, the influence of charge topology, i.e., the spatial distribution of charge within a multivalent counterion, has not been investigated experimentally. We address this question by comparing aqueous solutions of carboxymethyl cellulose (CMC) containing conventional divalent metal ions with solutions containing the organic ditopic counterions hexamethonium and suxamethonium. Conductivity measurements indicate comparable counterion dissociation at low polymer concentrations but substantially greater dissociation of the ditopic ions at higher concentrations. SAXS measurements reveal corresponding structural differences: CMC with ditopic counterions exhibits correlation peaks and smaller stretching parameters, consistent with a larger effective polymer charge and reduced local chain folding, whereas CMC with monotopic divalent metal counterions displays broad correlation shoulders, consistent with weaker electrostatic correlations. Partially “dangling” condensed ditopic ions provide a possible explanation for the observation that the monotopic and ditopic counterions show similar fractions of free counterions from conductivity at low concentrations, while the SAXS profiles indicate a higher effective charge for the ditopic salts. However, this mechanism cannot be established unambiguously from the present conductivity and scattering measurements. Overall, the results demonstrate that counterion charge topology provides a tuneable parameter for controlling polyelectrolyte conformation and charge transport.