Polyelectrolyte adsorption onto planar surfaces: a study by streaming potential and ellipsometry measurements

Possible applications of polyelectrolyte monolayers and multilayers obtained via layer-by-layer deposition ask for well-defined growth as a function of the number of the adsorption cycles. Essential parameters for the regular adsorption of subsequent polymer layers by electrostatic interactions are the charge of the outermost surface region, the surface of the substrate, and the nature of the polyelectrolyte. Therefore such adsorbed polyelectrolyte layers were characterized by streaming potential measurements in combination with ellipsometric studies. After the adsorption of highly charged polycations such as poly(ethylene imine) on negatively charged surfaces, the surface charge changes: A marked increase of the zeta potential as well as a shift of the isoelectric point is observed for standard polyelectrolytes. These changes depend on the storage of the adsorbed layers under water, or on shear applied in the measuring cell. Only one polyelectrolyte disposing of a large hydrophobic fragments formed stable layers. In this case, no significant changes are observed in dependence on the storage conditions dr on shear. By varying the number of polyelectrolytes layers, the dependence of the zeta potential on the pH is found to be identical. Accordingly, the surface charge is nearly independent of the number of layers. The layer thickness in a dried state is determined by variable angle spectroscopic ellipsometry, demonstrating the step-by-step growth of the multilayers. (C) 1999 Elsevier Science B.V. All rights reserved.