Kinetics of formation and dissolution of weak polyelectrolyte multilayers: Role of salt and free polyions

The layer-by-layer deposition method to prepare multilayers of polyelectrolytes of alternating charge has been followed in situ by means of optical reflectometry experiments. It turns out that in solutions containing both polyelectrolyte and appropriate salts up to a certain concentration, the regular build up of multilayers is modified and becomes an adsorption/redissolution process. We explain this by taking into account (i) that during the regular multilayer formation process the macromolecules cannot equilibrate, (ii) that the added salt plasticizes the multilayer to a state where the molecules are sufficiently mobile to enable them to equilibrate between the layer and the surrounding solution, and (iii) that the presence of excess polyelectrolyte brings the system to a one-phase region of the polyelectrolyte complex phase diagram, implying that polyelectrolyte complexes must dissolve under these conditions.