These preliminary results are very interesting and reveal that the amount of AgNPs in the thin films depends on the pH of the polyelectrolyte solutions. In general, the inhibitory activity observed in the thin films (4 41), (4 4 2) and (4 4 4) is less pronounced in relation to the thin films (8 41), (8 4 2) and (8 4 4).This difference indicates that there was a lower incorporation of AgNPs in the thin films where both polyelectrolytes have pH 4.0 because, at this pH, the NH3+ e COO– groups are fully interconnected electrostatically in the multilayers and there are no remaining free
NH3+ groups in the PAH that promote the interaction with the AgNPs. In addition, for this pH condition (4.0 and 4.0), the thin films are thinner and rougher than at the different pH condition (8.0 and 4.0), as the PAH chains are in a more linear conformation [61]. Thus, the surface area for the incorporation of AgNPs is reduced, which also explains the
presence of a lower ZOI when compared to thin films (8 41),(8 4 2) and (8 4 4). In the case of thin films (8 41), (8 4 2) and (8 4 4), the PAA chains have a low degree of ionization (7%) compared with the chains of
PAH (68%). For these pH values during formation of the
films, the
degree of ionization of PAA is increased from
7% to
63% due to
the interaction with PAH [62]. Thereby, the majority of –NH3
+
groups in PAH neutralize the charges of –COO– groups, with a small
fraction of –NH3
+ free groups remaining in the multilayers,
facilitating the incorporation of AgNPs. The degree of ionization
of PAA increases even more when the
film is placed in contact with
a solution of pH greater than 2.5 [61], as is the case of this study
where the AgNPs solution has a pH
6.0.