Effect of Dialysis on the Osmotic Pressure, Conductivity, and Rheology of Aqueous Polyelectrolyte Solutions

Dialysis, as a purification step, removes residual salts present in as-received commercial polyelectrolytes. Poly-(diallyldimethylammonium chloride) (PDADMAC) samples, for example, contain substantial amounts of salts from the manufacturing process. PDADMAC is a water-soluble polycation with high charge density, a stable quaternary ammonium structure, and broad pH tolerance. The presence of residual salts increases the ionic strength and can significantly influence measured solution properties. In this work, we compare the viscosity, osmotic pressure measured by freezing point depression, and conductivity of aqueous solutions of dialyzed PDADMAC and its copolymer with acrylamide, poly-(acrylamide-co-diallyldimethylammonium chloride), with those of their as-received, not dialyzed counterparts to assess the influence of residual salts and impurities on these polyelectrolytes. The residual salts dominate the measured osmolality, so osmotic pressure cannot be used to determine the fraction of dissociated counterions for samples that have not been properly dialyzed to remove residual salts. The presence of residual salts also significantly lowers the solution viscosity of the as-received samples compared with that of the dialyzed samples at the same polymer concentration. The dialyzed samples do not show the expected concentration dependences of correlation length, suggesting that PDADMAC and its copolymers are likely branched polyelectrolytes.