Effect of divalent ions on the structure of polyelectrolyte gels

Abstract

We perform an exploratory study of the influence of adding divalent metal ion salts such as CaCl₂ to the structure of model monovalent metal counterion polyelectrolyte gels to test the hypothesis that such counterions lead to a tendency of polyelectrolyte chains to form fiber-like structures. Our experimental methodology is based on osmotic pressure Π measurements and small angle neutron scattering measurements on typical synthetic [poly(acrylic acid) and polystyrene sulfonate] and biopolymer (DNA and hyaluronic acid) polyelectrolyte gels over large polymer and CaCl₂ concentration ranges. The results of the present combined small-angle neutron scattering (SANS) and Π measurements strongly suggest that the Ca²⁺ counterions are causing a tendency of the polyelectrolyte chains to associate all along the chain axis to form semi-flexible fiber structures rather than as randomly positioned physical cross-links between the chains. This form of association is consistent with the added divalent salt giving rise to attractive interactions between the chains that reduce phase stability of such polymers and with the observed scaling properties of the osmotic pressure of the gels and their structure as inferred from small angle neutron scattering. Further studies are required to confirm this structural assignment to this class of hydrogels given the uncertainty in assigning a unique structure by neutron scattering.