Interactions of PEI (polyethylenimine)–silica particles with citric acid in dispersions

Adsorbed polyethylenimine (PEI) of M _w 1,800 and 70,000 on silica (SiO₂) dispersions produced flocculated slurry in the pH range of 5 to 12. Adsorbed citrate widens this flocculated pH regime. It also increases the strength of the interparticle attractive force or the yield stress over the pH range of between 3 and 8. The stronger attractive force is due to particle bridging by the citrate anions bonding with positively charge sites of the adsorbed PEI layer of the interacting particles at the closest point of interaction. The higher M _w PEI being more strongly attached to the silica particle produced a stronger attractive interparticle force with adsorbed citrate anions. Via charge balance calculation using contributions from SiO₂, PEI, and citrate, the pH of zero charge was found to correspond to the pH of zero zeta potential for PEI of M _w 70,000. This suggests 100% adsorption of PEI and citrate on SiO₂. The bridging interaction was confirmed by a linear relationship between yield stress and the square of the limiting citrate charge content. Adsorbed citrate was found for the first time to play the role of a bridging agent, a result of the positive charges being located on a more flexible adsorbed layer rather than being fixed to a rigid surface.