Polyelectrolyte−Gelatin Complexation:  Light-Scattering Study

Abstract

Complex formation between negatively charged polyelectrolytes and a net negatively charged polyampholyte (gelatin) has been characterized by light scattering. The two polyanions studied are sodium poly(styrenesulfonate) (NaPSS) and sodium poly(2-acrylamido-2-methylpropanesulfonate) (NaPAMS). The molecular weights of the single polyelectrolyte chains NaPSS and NaPAMS increase 20- and 15-fold, respectively, upon saturation with gelatin. Despite such molecular weight increases, the radii of gyration of the complexes were found to be only slightly larger (∼15%) than those of the corresponding parent polyelectrolytes. Increasing negative values for the second virial coefficients were observed upon saturation of gelatin on the polyanion chains, indicating increasing interchain attraction. Both the stoichiometries and sizes of the complexes decrease monotonically with pH. At a constant pH, the complex stoichiometry peaks around 0.01 N NaAc. These results suggest that electrostatic interaction is the main driving force for complexation in these systems. Polyampholyte molecules are polarized in the electric field of a polyelectrolyte chain. This polarization-induced attraction is believed to be the main mechanism of complexation. Charge density of the polyanion is suggested to be an important factor for determining its extent of gelatin binding.