Modeling of complexation of oppositely charged polyelectrolytes in aqueous solutions

The association of oppositely charged polyelectrolytes in aqueous solutions can lead to formation of polyelectrolyte complexes (PECs) whose properties are governed by many physiochemical parameters. PECs can be divided into overcharged complexes, where one type of polyelectrolyte charge is present in excess, and neutral ones, which contain equimolar ratios of polyanion and polycation. Using a theory, which captures the effects of chain connectivity and ion-specificy of charged species, here we rationalize why in non-stoichiometric mixtures, excess polyelectrolyte appears in the PEC and we pinpoint theoretical underpinnings of the overcharging phenomenon. Further, using the same theory, we investigate “doping” of stoichiometric PECs, in which salt concentration is increased, leading to the breaking of ion-pairs between oppositely charged groups of polyelectrolytes and their replacement with counterions. We find that the predictions of the level of counterion replacement during doping follow a similar trend to data from doping experiments. The development of such theories to could lead to methods of rational design of practical applications of PECs.