Electric properties and conformation of polyethylenimine at the hematite-aqueous solution interface

The variations of electro-optical behavior and pH of ellipsoidal hematite (alpha-Fe2O3) particle suspension are investigated in the presence of polyethylenimine (PEI). Suspension stability and particle electrophoretic mobility are also followed. Considerable shift of pH and the particle isoelectric point are found at extremely low PEI concentrations, indicating that nearly all amino groups of unfolded PEI molecules contact with the oxide surface. The small polymer layer thickness in stabilized suspension (from electro-optical data) shows "flattening" of the PEI molecules even at higher polymer adsorption. Similar electro-optical behavior of suspensions with either a positive or a negative oxide particle, saturated with PEI, indicates that the colloid-polymer complex assumes charge characteristics of the polymer. Reduced counterion mobility, found in both systems, is attributed to counterion condensation on the polyion surface and, possibly, to conformational transition in adsorbed low-charged polymer molecules. (C) 1997 Academic Press.