Interactions between multivalent ions and exponentially growing multilayers:: Dissolution and exchange processes

The interactions between multivalent ions (small ions or polyelectrolytes) and two exponentially growing polyelectrolyte multilayers, namely, (HA-PLL)(n) and (HA-PAH)(n) films, are investigated (HA = hyaluronic acid, PLL = poly-L-lysine, PAH = poly(allylamine)). Ferrocyanide and ferricyanide ions are used as small ion probes. The most striking finding is that, even though these two ions differ only by one charge unit, the ferrocyanide ions induce a partial dissolution of both multilayers whereas these films remain stable in the presence of ferricyanide ions. The dissolution process of (HA-PLL)(n) films is more rapid than that of (HA-PAH)(n) films, indicating a stronger interaction between HA and PAH compared to HA and PLL. This is confirmed by polyelectrolyte exchange experiments: when an (HA-PLL)(n) multilayer film is put into contact with a PAH solution, PLL is quantitatively exchanged with the PAH chains and transformed into an HA-PAH film, whereas an (HA-PAH)(n) multilayer remains stable in the presence of a PLL solution.