Characterization of polyelectrolyte multilayers on mica and oxidized titanium by streaming potential and wetting angle measurements

Multilayer adsorption of polyelectrolytes on mica and electrochemically oxidized titanium was studied by the streaming potential method using a parallel-plate channel arrangement. Streaming potential data were complemented by wetting angle measurements. Two types of polyelectrolytes (PE) were used: polyallylamine hydrochloride (PAH), of a cationic type, and polysodium 4-styrenesulfonate (PSS) of an anionic type (both having molecular weight of ∼70,000). Layer by layer adsorption of polyelectrolytes from NaCl and TRIS solutions of various concentrations was studied in situ. It was demonstrated that after completing a bilayer, periodic variations in the apparent zeta potential between positive and negative values appeared for multilayers terminated by PAH and PSS, respectively. This was in accordance with zeta potential of the polymers in the bulk measured by electrophoresis. It also was observed that streaming potential data correlated well with the wetting angle characteristics of dried polyelectrolyte films. Variations in the contact angle between 70° for PAH and 40° for PSS were exactly in phase with the variation in the zeta potential. The stability of polyelectrolyte films against prolong washing (reaching 24 h) was determined using the streaming potential method. It was revealed that both for mica and titanium, the PSS layer was considerably more resistant to washing, compared to the PAH layer. The experimental data obtained were consistent with a model postulating particle-like adsorption of polyelectrolytes.