Effect of thiocyanate counterion condensation on poly(allylamine hydrochloride) chains on the buildup and permeability of polystyrenesulfonate/polyallylamine polyelectrolyte multilayers

In this study, we investigate the buildup of PEI-(PSS-PAH)(n) polyelectrolyte multilayers at pH 7.4 in the presence of either NaCl or NaSCN as a supporting electrolyte. It appears that in the presence of increasing thiocyanate concentrations (from 0.1 to 0.5 M), the thickness increment, obtained from optical waveguide lightmode spectroscopy experiments, increases whereas it stays practically constant for increasing sodium chloride concentrations (between 0.1 and 0.5 M). The hydration of the films differs also markedly between both electrolyte solutions. The differences in the construction of the polyelectrolyte multilayers in the presence of both supporting electrolytes are rationalized in terms of strong SCN(-) condensation on the PAH chains. The occurrence of this ion condensation is indirectly demonstrated by means of zeta potential measurements and directly demonstrated by means of attenuated total internal reflection infrared spectroscopy on the multilayer films. Moreover when the films are built up in the presence of SCN(-), these ions are only slowly exchanged by the Cl(-) ions introduced in the bulk. Conversely the thick films obtained from 0.5 M NaSCN solutions do not deswell when the buffer solution is replaced by a 0.5 M NaCl containing buffer. The permeability of the films constructed in the presence of both sodium salts is also studied by means of cyclic voltametry and is found to be markedly different in the case of films made from five bilayers at 0.5 M salt concentration. This difference is due to the different morphology and porosity of the films constructed in the presence of 0.5 M NaCl and 0.5 M NaSCN.