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

The buildup of the first layers of polystyrenesulfonate (PSS)/polyallylamine (PAH) multilayer is studied in situ by means of streaming potential measurements (SPM) and by scanning angle reflectometry (SAR). The results are discussed 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 (III). This view seems to be supported by our experimental findings. The zeta potential of the multilayer determined by the SPM shows a symmetrical and constant charge inversion during the multilayer buildup. This seems to indicate an exact charge compensation in zone II and an excess charge that is entirely located in the outer zone III. It is also shown by SAR that a regular buildup regime, in which the thickness increment per layer is constant, is reached after the deposition of the first six polyelectrolyte layers, which gives an indication of the extension of zone I. The influence of the salt concentration C-NaCl present in the polyelectrolyte solutions during multilayer buildup is also investigated. It is found that an increase of the salt concentration in the polyelectrolyte solutions leads to larger amounts of deposited polyelectrolytes and to thicker multilayers. The amount deposited 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 addition, when a multilayer built up in salty solutions is brought in contact with pure water, it expands, indicating that the rinsing step mainly affects zone III of the multilayer, which appears thus to behave like a polyion layer. The structural changes of the multilayer consecutive to the replacement of the salt solution by pure water occur with characteristic times ranging from a few tens of minutes to several hours depending on the initial salt concentration. Finally, it is also found that the structural modifications of the film are fully reversible so that the initial multilayer structure is recovered when water is replaced again by the initial salt solution.