Ellipsometry study of the adsorption of molecules at electrolyte interfaces with gold and stainless steel

We describe an ellipticity study of the adsorption onto gold and Type 304 stainless steel of the molecules bovine serum albumin (BSA), sodium dodecyl sulfate (SDS), and polyethylenimine (PEI) in sodium perchlorate electrolyte solutions. The ellipsometric response is made of two effects, one due to surface-charging of the metal electrodes, and the other due to adsorption. The surface-charging effect is much larger for gold than for stainless steel. Misleading estimates for the adsorption are obtained if this charging response is not taken into account. We find that for the most part the adsorbed layers are not sensitive to the applied potential, and only on one occasion did we observe reversible desorption with varying potential. The surface-charging effect is not altered by BSA (the largest molecule), but SDS and PEI show evidence for changes in the surface-charging effect (and thus in the Stern layer) and changes in conformation. (C) 1998 Academic Press.