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), sodiu m dodecyl sulfate (SDS), and polyethylenimine (PEI) in sodium perchlor ate electrolyte solutions. The ellipsometric response is made of two e ffects, one due to surface-charging of the metal electrodes, and the o ther due to adsorption. The surface-charging effect is much larger for gold than for stainless steel. Misleading estimates for the adsorptio n 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 reversi ble desorption with varying potential. The surface-charging effect is not altered by BSA (the largest molecule), but SDS and PEI show eviden ce for changes in the surface-charging effect (and thus in the Stern l ayer) and changes in conformation. (C) 1998 Academic Press.