Ionization-Controlled Stability of Polyelectrolyte Multilayers in Salt Solutions

In situ FTIR measurements of functional group ionization are reported for polyelectrolyte multilayer thin films, composed of a weak polycarboxylic acid and one of several polycations each having different charge densities. The stability of the multilayers in salt solutions increased dramatically when the pH of the solution was lowered below a critical point. This critical pH was shown to depend on both the charge density of the polycation and the nature of the polyacid. Stabilization of the multilayer was controlled exclusively by polyacid ionization within the film and occurred when the same fraction of carboxylic groups was ionized regardless of the nature of the polyacid or the polycation charge density.