Studies Of The Mechanism Of The Polyelectrolyte Adsorption At Metal Oxide/electrolyte Interface
STUDIES OF THE MECHANISM OF THE POLYELECTROLYTE ADSORPTION AT METAL OXIDE/ELECTROLYTE INTERFACE
S. Chibowski, M. Krupa
Department of Radiochemistry and Colloid Chemistry,Faculty of Chemistry, Maria Curie-Skłodowska University,
Pl. M. Curie-Skłodowskiej 3, 20-031 Lublin, Poland
An influence of the molecular weights of polyacrylic acid (PAA) and polyacrylamide (PAM) as well as inorganic contamination of the surface of ZrO2 on the magnitude of these polymer adsorption and electrokinetic properties of the metal oxide/polymer solution interface was determined. From obtained dependencies, the mean factors, responsible for the observed changes of the surface charge and zeta potential of ZrO2 and Fe2O3 were determined. It was revealed that carboxyl groups of PAA and PAM macromolecules are responsible for the conformation of polymer chains at the interface. These groups also directly influence the magnitude of the both polymer adsorption.
Calculated concentrations of various surface groups, and observed changes of zeta potential and surface charge of ZrO2 and Fe2O3 allowed determine the probable mechanism of bonding both polymers with the surface of examined oxides. The analysis of these values showed that bonding of PAA and PAM macromolecules, with the surface of the solid, runs by SOH groups of both oxides. Polyacrylic acid and polyacrylamide may interact with SOH groups by hydrogen bridge. Calculated concentration of surface groups is higher for ZrO2 so more polymer segments may join to its surface. From this reason, obtained adsorption of examined polymers was higher for zirconia.
It was showed that inorganic ions, present on the surface of ZrO2, are blocking part of the active sites, making them inaccessible for adsorbing polymer chains. This effect explains lower adsorption of polyelectrolyte on the contaminated zirconia, in comparison to the pure ZrO2.
From the analysise of the determined values of diffuse layer charge and surface charge, the main factors were determined, having influence on the changes of the zeta potential of the examined oxides resulting from pH of the solution, molecular weight and polymer concentration.
A parameter, which gives some additional information, concerning the mechanism of the adsorption of examined polyelectrolytes, is free energy of adsorption. Using zeta potential measurements the free energies of adsorption for PAA and PAM were calculated for both oxides. It was noticed that generally the free energy of the polymer adsorption increases with the increase of its molecular weight although it is not proportional to the length of the chain. Moreover it was proved that polyacrylic acid is tied with the surface stronger than polyacrylamide.
Also, the thickness of adsorption layer of polyacrylic acid and polyacrylamide on the surface of Fe2O3 was determined ,as a function of pH of the solution, molecular weight and concentration of PAA and PAM. For this purpose an experiment was applied, based on the measurements of the viscosity changes of the polymer solution and suspension of the solid with adsorbed polymer. It was showed that increase of the pH of the solution, molecular weight and polymer concentration caused in all cases increase of the thickness of the adsorbed layer of the polymer. Responsible for this effect are loops and tails of the polymer chains that form during adsorption of the polymer.
Experimental measurement of the number and length of the polymer train, loops and tails is impossible. For the determination of the participation of these forms in PAA and PAM chain segments on Fe2O3, some computer calculations, based on Schutjens-Fleer model of the polymer adsorption, were applied. From these calculations it was shown that increase of the adsorption layer thickness of PAA, connected with increase of the molecular weight of the polymer, results from the increase of the length of tails, creating at the interface. In the case of PAM for the same increase both loops and tails, occurring at the interface, are responsible.
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