The technique of layer-by-layer (lbl) deposition of polyelectrolytes allows to control the total thickness of multilayer film and its structure on a molecular level. Due to versatility of this technique, such self-assembled polyelectrolyte films have a great application potential. That is, they can be used for chemical and biochemical sensing, as separation membranes for gases or dissolved species and for modification of surface properties, which is of special importance in biomaterial area. It is often assumed that the properties of the polyelectrolyte multilayer films are independent of the underlying substrates. However, there should exist a minimal number of layers above which the properties of the system are governed by the choice of polycation/polyanion pair. The structure of the multilayer films is also strongly dependent on the deposition conditions and the most frequently used control parameters are ionic strength and pH of the solution. Also the long-time exposure to the conditions different than those encountered during deposition usually leads to the change of the polyelectrolyte multilayer structure.
We developed efficient method for characterizing wettability of heterogeneous surfaces produced by 'layer-by-layer' adsorption of polyelectrolytes. Three types of polyelectrolytes were used in our studies: polyallylamine hydrochloride (PAH) of a cationic type, polysodium 4-styrenesulfonate (PSS) of an anionic type, both having molecular weight of about 70 000 and polyethyleneimine (PEI) of about 75 000. As the support materials for deposition a natural ruby mica, glass, titanium foils and unpolished silicon wafers were used. Adsorption of polyelectrolytes was performed from NaCl solutions of various concentrations under natural pH and from Tris buffer at pH=7.4. The contact angle of substrates covered by polyelectrotyle multilayers was determined by using direct image analyzing technique of sessile drops. Periodic oscillations in contact angle values were observed for multilayers terminated by PAH and PSS, respectively, PAH being more hydrophobic. However, amplitude of the contact angle variations depended on the electrolyte used for deposition and was the smallest for low electrolyte concentration suggesting significant interpenetration of oppositely charged layers of polyelectrolytes. It was also observed that starting from the third PAH/PSS layer deposited from 0.15 M NaCl, the contact angle of water on consecutive polyelectrolyte layers was practically independent of the substrate. When examining the effect of long time conditioning in solution of various pH on wettability of films, we found that the film becomes more hydrophilic after conditioning in pH=3 and pH=11 but in neutral pH the wetting of the film does not change. However, while after acidic treatment of the multilayer film the periodic oscillations in contact angle values were still observed, there were practically no oscillations between consecutive layers when they were treated with basic solution. This behavior suggests either the partial removal of the film or some major structural changes within the film together with OH- incorporation.