Changes in the interaction characteristics of polyelectrolyte complex covered silica surfaces

The adsorption of polyelectrolyte complexes, PEC, made from the cationic poly (diallyldimethylammonium) chloride (PDADMAC) and the anionic maleic acid-co-propene copolymer (MA-P) on a Si-wafer surface has been studied. The application of highly diluted colloidally dispersed PEC solutions led to the deposition of single PEC particles onto the surface of the Si-wafer. The interaction forces of the heterogeneously covered surface were monitored by direct force measurements with an atomic force microscope (AFM) in the force volume mode. On the surface of a single PEC particle drastic changes in the interaction forces were found in comparison with the unmodified Si-wafer: in all force vs. distance curves a strong increase of the adhesion was measured that can be attributed to the formation of electrostatic bonds between the negatively charged Si3N4-tip and the cationic excess charge of the PEC. Additionally, the behavior during approach of both surfaces has been distinct: at pH 6.1 we see a long range electrostatic attraction between the tip and the PEC particle. The attraction becomes even stronger at pH 4.1, because of an increased positive net charge. Generally, a heterogeneous surface with a wide variety of interaction features can be created by the adsorption of PEC particles.