Adsorption of polyelectrolyte-nanoparticle systems on silica: Influence oninteraction forces

In this work, we have studied the interfacial properties of cationic polyel ectrolyte (PE) and silica nanoparticle (NP) systems at macroscopic silica s urfaces by means of ellipsometry. The influence of adsorbed layers on the i nteractions between silica surfaces was also investigated using the bimorph surface force apparatus. Added nanoparticles were observed to strongly swe ll the interfacial polyelectrolyte layers, an effect partly related to neut ralization of charged polyelectrolyte groups. The effect was more pronounce d for low charged than for highly charged polyelectrolytes. Overall, the pr esence of nanoparticles seemed to increase the repulsive interaction measur ed between silica surfaces. The force measured on approach was long range a nd quite strongly repulsive. On separation, an attractive bridging interact ion was measured for polyelectrolyte-covered surfaces. For the low charged polyelectrolyte used in the study, the force turned repulsive on addition o f nanoparticles. For the highly charged polyelectrolyte used, a change from a very strong attraction (involving a jump of the surfaces out of contact) to a very long-range elastic attractive force was observed on adding nanop articles. The long-range elastic force indicates that polymer chains and na noparticles form a transient network in the gap between the surfaces. The o bserved difference in the outward force curves may explain why the addition of nanoparticles appears to improve, e,g., shear-resistance and refloccula tion characteristics of polymeric flocculants, (C) 2000 Academic Press.