Reload Index (ZRiChK UMCS)

 

 

STUDIES ON THE INFLUENCE OF FUNCTIONAL GROUPS OF SOME MACROMOLECULES ON THEIR CONFORMATION AT SOLID-WATER SOLUTION INTERFACE.

 

Stanisław CHIBOWSKI, Małgorzata PASZKIEWICZ

DEPARTMENT OF RADIOCHEMISTRY AND COLLOID CHEMISTRY

 

 

Results of studies on the polymer adsorption from solutions on the solids are of great importance not only for theoretical description of the process but also markedly contribute to their utilization. As practical application one can mention pharmacy, food and cosmetics industries, where polymers are used as emulsifiers or stabilizers of colloid dispersed systems.

Main task of the research was an attempt of the description of the adsorption mechanism and structure of the adsorption layers of the polymer substance forming at solid-solution interface, basing on experimental data and theoretical model by Scheutjens‑Fleer [1], describing adsorption of the polymers.

Experiments were made on model system, containing aluminum oxide (g‑Al2O3) and titanium oxide (anatase) as a solid. Macromolecules such as polyvinyl alcohol (PVA) and polyethylene glycol (PEG) were used as non-ionic polymers.

Adsorption measurements were done by static method. Concentration of the polymers, before and after the adsorption was determined by colorimetric and turbidimetric analysis. Viscosity measurements allowed calculate thickness of the adsorption layer of studied polymers. Obtained results had essential meaning at the determination of the structure of adsorbed polymer chain. For more detailed characteristics of the adsorption mechanism of the polymers some electrokinetic measurements of researched systems were taken; namely zeta potential and surface charge measurements. Above studies were complemented by microcalorimetric measurements, that allowed determine adsorption heat of the polymers on the surface of Al2O3. A comparison of the adsorption heat of the polymer with respective value of the heat for the monomer let us determine number of segments, which directly interacts with surface of the solid. It results from the obtained data that fraction of segments of the polymer chain, connected with surface of Al2O3 do not exceed 4% and decreases with the increase of molecular weight of PEG. That is because in the case of higher molecular weights of the polymer, number and length of loops and tails increases and decreases number of train type structures. Determined value of adsorption heat for single polymer segment on the surface of the oxide (cosegm equal 5,2 kT) was the same order as the mean value of hydrogen bond. Such result confirms that interaction between PEG and Al2O3 goes through hydrogen bridge. This interaction is strong enough for stable adsorption of the PEG macromolecule on the surface of the oxide.

 

A plot of obtained adsorption isotherms, for various molecular weights of PVA and PEG on the surface of TiO2 and Al2O3 reveals distinct increase of adsorbed polymer with the increase of its molecular weight. [2, 3] Experiments proved also marked influence of the number and type of the functional groups composing the macromolecule on the sorptive behavior of the polymer at metal oxide-solution interface. In macromolecule of polyvinyl alcohol, beside –OH functional groups, there are also acetate ones (CH3COO‑), as much as from 2% to 20%. They have considerable influence on the sorptive properties of PVA. At decrease of their number in macromolecule of PVA the increase of the polymer adsorption was observed (Fig.1) produced by the change of conformation of the macromolecule from train into loop and tail type.

 

Fig. 1.         Adsorption isotherms of fractionated samples of PVA of various amount of acetate groups in macromolecule, on the surface of TiO2 CNaCl 1x 10 – 3M.

Obtained dependencies well correspond with determined values of adsorption layer thickness (d). Analysis of obtained data proves that both molecular weight of the polymer as well as type of functional groups markedly influence (d) values [2,3,4].

 

References:

[1] J.M.H.M Scheutjens., G.J. Fleer, Phys. Chem., 84 (1980) 178.

[2] St. Chibowski, M. Paszkiewicz and M. Krupa, Powder Technology, 107 (2000) 251-255.

[3] St. Chibowski and M. Paszkiewicz,J. Dispersion Sci. Technol., 22(2001) 281‑289.

[4] St. Chibowski and M. Paszkiewicz, Adsorption Science and Technology, 19(2001) 397‑407.

 

Grant promotorski 7T09A12420