Mechanisms of adsorption of an azo-polyelectrolyte onto layer-by-layer films

The mechanisms which lead to the adsorption of the azopolyelectrolyte poly[1-[4-(3-carboxy-4-hydroxyphenylazo) benzene sulfonamido]-1,2-ethanediyl, sodium salt] (PAZO) layer adsorbed onto layer-by-layer films were investigated by monitoring the adsorbed amount per unit of area as a function of time. The adsorption kinetics curves were obtained as a function of adsorption variables as PAZO concentration and drying process. The PAZO adsorption kinetics data was shown to be described by two processes, one with smaller adsorption characteristic time, of the order of seconds, which was attributed to a nucleation mechanism and a second process, with adsorption characteristic times of the order of tenth of minutes, associated to diffusion. During the first process, PAZO molecules rapidly adsorb onto the substrate surface creating an electrostatic barrier that makes difficult the adsorption of more PAZO molecules. However, positive counterions are allowed to be adsorbed and as they are the potential barrier is lowered, allowing more PAZO molecules to be adsorbed. This last mechanism is accounting for the second adsorption process. The adsorption processes are not seen to be influenced by the layer drying and only contribute for the changes in the values of adsorption parameters, i.e., adsorption characteristic times and corresponding adsorbed amounts. These parameters were related with the available number of adsorption sites which are dependent of the presence of both water and counterions adsorbed on the last adsorbed layer.