PROTON CONCENTRATION PROFILE IN ULTRATHIN POLYELECTROLYTE FILMS

Embedding a pH-sensitive fluorescent dye at defined distances from the surface in a polyelectrolyte film, the pH dependence of fluorescence emission as a function of film thickness, surface charge, and ion conc entration is measured by total internal reflection fluorescence (TIRF) . From the analysis of the data, we demonstrate that the films are per meable for protons and derive the potential profile within the polyele ctrolyte film. The potential distribution within the polyelectrolyte f ilm near the charged outer surface is described by the Gouy-Chapman-St ern theory. The Debye lengths are a factor of 3 smaller than those in water at similar ionic conditions which may be ascribed to a lower die lectric constant or to a higher concentration of mobile ions in the fi lm. The local pH inside the film can be changed in a predictible way v ia the nature of the outer polyelectrolyte layer and the ionic milieu of the adjacent solution.