Effect of the Supporting Electrolyte Anion on the Thickness of PSS/PAH Multilayer Films and on Their Permeability to an Electroactive Probe

Quartz crystal microbalance and cyclic voltammetry are used to investigate the influence of the supporting salt of polyelectrolyte solutions on the buildup and the structure of PSS/PAH polyelectrolyte multilayers (PSS: poly(4-styrene sulfonate); PAH: poly(allylamine hydrochloride)). This film constitutes a model polyelectrolyte multilayer system. The supporting electrolytes were sodium salts where the nature of the anion was changed by following the Hofmeister series from cosmotropic to chaotropic anions (F−, Cl−, NO3−, ClO4−). For all the investigated anions, the film thickness increases linearly with the number of deposition steps. We find that chaotropic anions lead to larger thickness increments per bilayer during the film buildup than cosmotropic ones, confirming results found on PSS/PDADMA multilayers (PDADMA: poly(diallyldimethylammonium)). Films constituted by more than nine PSS/PAH bilayers are still permeable to hexacyanoferrate(II) ions, Fe(CN)64−, whatever the nature of the supporting salt anion. On the other hand, these films are impermeable to ruthenium(II) hexamine ions, Ru(NH3)62+, after the third PAH layer in the presence of NaF, NaCl, or NaNO3. These results are explained by the presence of an excess of positive charges in the film, which leads to a positive Donnan potential. We find that this potential is more positive when more chaotropic anions are used during the film buildup. We also find that a film constructed in the presence of chaotropic anions swells and becomes more permeable to Fe(CN)64− ions when the film is brought into contact with a solution containing more cosmotropic anions. All our experimental findings can be explained by a strong interaction between chaotropic anions with the NH3+ groups of PAH that is equivalent, as far as the multilayer buildup and electrochemical response is concerned, to a deprotonation of PAH as it is observed when the film is constructed at a higher pH. We thus arrive to a coherent explanation of the effect of the nature of the anions of the supporting electrolyte on the polyelectrolyte multilayer. We also find that great care must be taken when investigating polyelectrolyte multilayer films by electrochemical probing because electrochemical reactions involving the probes can appreciably modify the multilayer structure.