Response of polyelectrolyte complexes to subsequent addition of salts with different cations

The field of applications of polyelectrolyte complexes ranges from large-scale industrial products up to special uses in biotechnology and medicine. An important problem is their stability against environmental conditions, particularly the presence of different salts, that is, the colloidal stability as well as the stability of ionic bindings. Most of the previous work has been focused on the effect of sodium chloride. This paper deals with the effect of salts of different cations, stressing both the influences of the ion radius of monovalent cations and the valency of the cations. Polyelectrolyte complexes were prepared between sodium poly(styrene sulfonate) and poly(methacrylate) as polyanions and poly(diallyldimethylammonium chloride) and a copolymer of this polycation with 53 mol % of acrylamide in pure water. The changes in the structural parameters of these complexes during addition of LiCl, NaCl, KCl, CaCl2, MgCl2, AlCl3, and FeCl3, respectively, were followed by light scattering measurements. The ion radius of monovalent cations has practically no effects on the response of the complex particles. In the case of polyanions being in excess in the complexes, higher valency of the cation led to rapid flocculation in agreement with theoretical expectations. The polyelectrolyte complexes with polycation excess were strongly stabilized by addition of salts with multivalent cations, against both coagulation and dissolution. Especially for the complexes with sodium poly(methacrylate), the multivalent cations take part in complex formation and partly displace the polycations.