Influence of divalent counterions on the solution rheology and supramolecular aggregation of carboxymethyl cellulose

Abstract: Divalent counterions promote attractive forces between polyelectrolyte chains via electrostatic bridging, which play an important role in the conformation of ionic biopolymers. Further, counterion valence is known to affect the flexibility and aggregation properties of polyelectrolytes in solution. The present study seeks to resolve the effect of counterion valence and type on the structure and flow properties of a model semiflexible polyelectrolyte. We report rheology and light scattering data for the Na ⁺ , Mg ^{2 +} , Ca ^{2 +} , Mn ^{2 +} , Co ^{2 +} , Ba ^{2 +} salts of carboxymethyl cellulose in aqueous solutions. The Na ⁺ and Mg ^{2 +} counterions do not interact specifically with the carboxylate groups, and their CMC salts form clear solutions in the concentration (c) range studied (0.001 M < c < 0.3 M), which spans from the dilute to the entangled regimes. The other salts form clear solutions at low concentrations and become turbid at higher ones. The specific viscosity as a function of molar polymer concentration falls into a single curve for all divalent salts, with small differences occurring only for c > 0.2 M. Compared NaCMC, divalent salts display a lower viscosities at low concentrations (in the non-entangled regime), suggesting less expanded chains, in agreement with earlier experimental results on flexible polyelectrolytes. Above the entanglement crossover (c≃ 0.07 M), solutions with divalent counterions display viscosities up to an order of magnitude larger than NaCMC, possibly because interchain crosslinks form by electrostatic bridging. Dynamic light scattering measurements on semidilute non-entangled solutions reveal a bimodal decay function, where the relative amplitudes of the two modes vary with counterion valence, size as well as with the filter size employed and the time after filtration. These variables (except for counterion valency) do not strongly affect the solution viscosity, indicating that polyelectrolyte clusters only contain a small fraction of the total number of chains in solution. Graphical abstract: [Figure not available: see fulltext.]