In situ determination of the structural properties of initially deposited polyelectrolyte multilayers

The buildup of the first layers of polystyrenesulfonate (PSS)/polyallylamin e (PAH) multilayer is studied in situ by means of streaming potential measu rements (SPM) and by scanning angle reflectometry (SAR). The results are di scussed in the framework of a schematic representation of the multilayer in three zones: a precursor zone (I), a core zone (II), and an outer zone (II I). This view seems to be supported by our experimental findings. The zeta potential of the multilayer determined by the SPM shows a symmetrical and c onstant charge inversion during the multilayer buildup. This seems to indic ate an exact charge compensation in zone II and an excess charge that is en tirely located in the outer zone III. It is also shown by SAR that a regula r buildup regime, in which the thickness increment per layer is constant, i s reached after the deposition of the first six polyelectrolyte layers, whi ch gives an indication of the extension of zone I. The influence of the sal t concentration C-NaCl present in the polyelectrolyte solutions during mult ilayer buildup is also investigated. It is found that an increase of the sa lt concentration in the polyelectrolyte solutions leads to larger amounts o f deposited polyelectrolytes and to thicker multilayers. The amount deposit ed per polyelectrolyte layer delta Q (PSS or PAH) is correctly predicted by the law delta Q = a.C-Nacl(a) + b where a lies between 0.05 and 0.15. In a ddition, when a multilayer built up in salty solutions is brought in contac t with pure water, it expands, indicating that the rinsing step mainly affe cts zone III of the multilayer, which appears thus to behave like a polyion layer. The structural changes of the multilayer consecutive to the replace ment of the salt solution by pure water occur with characteristic times ran ging from a few tens of minutes to several hours depending on the initial s alt concentration. Finally, it is also found that the structural modificati ons of the film are fully reversible so that the initial multilayer structu re is recovered when water is replaced again by the initial salt solution.