Adsorption Kinetics of Polyelectrolyte onto Monolayers

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The adsorption kinetics of an electrolyte polymer onto a monolayer spread on the solution has been studied by means of the attenuated total reflection (ATR) method. The polymer was PSSK (potassium poly-(Stylenesulfonate) : anion) and the monolayer DOAB (dioctadecyl-dimethyl-ammonium-bromide : cation). A glass prism (SFS-1 glass) whose hypotenuse surface was coated with a thin (-500 A) Ag film was pressed onto a monolayer of DOAB spread on pure water. The ATR signal caused by the SPP (surface plasmon polariton) at the Ag/monolayer interface was monitored. The subphase water was then slowly replaced with the polymer solution of varing concentration. The shift of the ATR dip indicates the thickness of the polymer adsorbed onto the monolayer and the width signals the inhomogeneity of the polyemer layer. We found that the adsorption occurred in two steps ; first the adsorbed layer was monomolecular (thickness - 5A), then it grew into a thick and inhomogeneous film. The initial layer thickness was independent of the molecular weight of the polymer. The final thickness, however, depended on the molecular weight weakly (d=16A for n=3500 and d=33A for n=42000, where n is the degree of polymerization) suggesting loops and tails in the film at this stage. We also found that the adsorption was completely absent onto a non-cationic monolayer (stearic acid) within the resolution of our method. CONCLUSION : The polymer adsorption onto a spread monolayer is completely governed by the charge nutrality (at least in the early stage of the adsorption) and a 1 : 1 polyion complex is indeed formed at the solution/monolayer interface as has long been speculated.