engleski

PAH/PAA polyelectrolyte multilayers on silica surface

Polyelectrolytes are macromolecules composed of substantial portion of the constitutional units which contain ionic or/and ionisable groups. By alternating adsorption of positively and negatively charged polyelectrolytes on a solid surface, polyelectrolyte multilayers are formed. These nanocomposites have interesting properties and are applied in various fields, mainly in biomedicine and pharmacology. Many factors including pH, salt concentration and temperature influence the composition and multilayer structure and in that way designate the properties of polyelectrolyte multilayers. Using the layer-by-layer (LbL) technique PAH/PAA multilayers were formed at silicon dioxide surface (Figure 1). Poly(allylamine hydrochloride), PAH, and poly(acrylic acid), PAA, are weak polyelectrolytes which means that the degree of ionization of these macromolecules depends on pH of solution. That is why the influence of pH on the thickness of each polyelectrolyte layer during the formation of multilayer was monitored by means of ellipsometry. It was found that PAH layers are thinner in the acidic pH region than in the alkaline pH region. The reasons therefore are the repulsive interactions between PAH functional groups and the formation of elongated chain conformation of molecules. At the contrary, PAA layers are thicker in the acidic pH region due to the low degree of ionization resulting in “loopy” conformations of PAA molecules. Furthermore, the effects of ionic strength and salt type on the thickness of polyelectrolyte layers were also examined by means of ellipsometry. It seems that the PAH chains prefer binding of weakly hydrated oxyanions. In contrast, the binding of small cations of high charge density appears to be favoured by PAA. This study has shown a way to design a PAH/PAA multilayers of desirable properties and the obtained results could be related to the behaviour of the same polyelectrolyte pair in solution.

polyelectrolyte multilayers ; silica ; ellipsometry

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