Equilibrium Overcompensation in Polyelectrolyte Complexes

Association between positive, Pol(+), and negative, Poi(-), units on polyelectrolytes drives spontaneous formation of a range of morphologies, some with fuzzy structure but most essentially amorphous. An excess of one type of charge over the other, known as overcompensation or overcharging, is essential for certain types of processing, such as the formation of polyelectrolyte multilayers on substrates or polyplex nanoparticles in solution. In this work, uniform, stoichiometric, smooth thin films of polyelectrolyte complex, PEC, from poly(diallyldimethylammonium), PDADMA, and poly(styrenesulfonate), PSS, were prepared starting from rough, nonstoichiometric multilayers of these materials. A narrow concentration range of added salt was found which promoted steady-state bulk overcompensation of PEC films in the presence of a large excess of polycation or polyanion without loss of PEC to solution. The extent of overcompensation, about 35% for PDADMA in 1.0 M NaCl and about 40% for PSS in 1.4 M NaCI, was independent of solution polymer concentration and only weakly dependent on salt concentration. A virtual dependence of overcompensation on molecular weight was also determined. Erosion/instability of films for [NaCl] > 1.4 M was observed, with more prominent or faster erosion for higher molecular weight PSS. The mechanism for overcompensation in this entropically driven system was attributed to the formation of a Donnan ion equilibrium between the PEC and solution phases.