Structure of polyelectrolyte layers on colloidal particles at different ionic strengths

The structure of polystyrenesulfonate (PSS) layer adsorbed onto ellipsoidal β-ferric hydrous oxide (β-FeOOH) particles is examined using electro-optics. Changes in the thickness are investigated by varying the ionic strength of the solution for rinsing the adsorbed PSS layer. An increase in the layer thickness is found after rinsing with water, which turns to a decrease when salt (NaCl) is added to the rinsing solution. No changes are registered if formation and rinsing of the layer is carried out at constant ionic strength. Compression of the PSS layer is also found after adsorption of an oppositely charged polyethylenimine (PEI) on the layer rinsed with water or in the presence of 10−4 M NaCl. Complex-formation between the two oppositely charged polyions on the particle surface is supposed to explain the behavior of the polyelectrolyte bilayer.