ИСТИНА

Due to numerous important industrial applications of polyelectrolytes (for example, for waste-water treatment, paper processing, production of functional polymeric coatings, etc.), considerable attention of scientists during the last years has been focused on the adsorption of such polymers at a solid-liquid interface. A deep understanding of this phenomenon is required to control technological processes dealing with polyelectrolytes. The Langmuir-Blodgett (LB) technique provides a unique opportunity to construct well-defined surfaces with desired characteristics. The use of such model surfaces as substrates makes it possible to obtain fundamental information on the adsorption of polyelectrolytes at a solid-liquid interface; the effects of key factors governing the process can be studied in a systematic way. In this work, we deliver our results on the adsorption of a cationic polyelectrolyte, poly(N-ethyl-4-vinylpyridinium bromide) (PVPEtBr, Pw = 3000, 500, 300), onto LB films formed by amphiphilic derivatives of alternating copolymers of maleic acid and alkenes. The amphiphilic copolymers bearing carboxylic groups, poly(maleic acid hexadecyl monoamide - alt - propene) (copolymer 1, Pw = 200) and poly(maleic acid octyl / pyrenyl monoamide - alt - ethene) (copolymer 2, Pw = 850) with the molar content of (maleic acid pyrenyl monoamide - ethene) units of 5%, were used to build up the LB films composed of 4 monolayers: 1st, 2nd, and 4th monolayers were formed by the copolymer 1 whereas 3rd monolayer was formed by the copolymer 2 containing pyrenyl groups as fluorescent labels. The tranformations of absorbance spectra and quenching of fluorescence of the LB films were detected after their contact with a dilute aqueous solutions of PVPEtBr. These changes provide clear evidence that the adsorption of cationic macromolecules onto the LB films takes place, this process was found to be irreversible. The adsorption of PVPEtBr onto the LB films was found to be rather sensitive to the variations in pH of the surrounding medium. The amount of the cationic macromolecules adsorbed at different pH was characterized by the difference in optical densities ΔOD at 257 nm and the relative fluorescence intensity I/I0 at 396 nm. The pH-dependencies of both ΔOD and I/I0 indicate that the adsorption of PVPEtBr onto the LB films is pronounced in basic media (when surface charge density of the LB films is high) but rather weak in acidic media (when surface charge density of the LB films is low), the drastic changes in the amount of the cationic macromolecules adsorbed being clearly detected for pH varying from 6 up to 8. The effect of pH on the adsorption of PVPEtBr onto the LB films suggests this process to be mainly of electrostatic nature.