Interaction and structure of surfaces coated by poly(vinyl amines) of different line charge densities

Interactions between preadsorbed films of poly(vinyl amine) (PVA) of two different line charge densities on silica substrates were studied with the colloidal probe technique based on the atomic force microscope (AFM). The preadsorbed films were prepared by adsorption of PVA from a pH 4 solution without any added salt. The highly charged PVA adsorbs in a flat configuration and in laterally heterogeneous layers, while the more weakly charged PVA analog adsorbs in thicker and more homogeneous films. As revealed by reflectivity measurements, such preadsorbed PVA films are stable in polyelectrolyte-free solutions. However, force measurements with the colloidal probe reveal that their interactions depend strongly on the ionic strength. Upon approach, interactions are dominated by electrostatic diffuse layer overlap forces. Both PVA films have very similar diffuse layer charge densities of about 1.5 mC/m2. Since these values are substantially lower than what would be expected from the total charge of the adsorbed polyelectrolytes measured by reflectivity, we infer that coadsorption of anions represents the principal mechanism in charge neutralization. Upon retraction, the adhesion between the films is dominated by bridging forces due to single polymer chains. Such bridging adhesion becomes progressively important with increasing ionic strength, whereby their range and frequency increase. The work of adhesion due to bridging is about 0.3 mN/m. At low ionic strengths, the films behave differently. While the highly charged PVA shows unspecific adhesion at small distances, the more weakly charged PVA analog shows few adhesion events occurring at long distances.