Reload Index (ZRiChK UMCS)

 

 

STUDIES ON THE ADSORPTION MECHANISM AND THE STRUCTURE OF ADSORBED LAYERS OF THE POLEYELECTROLYTE AT THE METAL OXIDE – POLYMER SOLUTION INTERFACE.

 

Stanisław CHIBOWSKI

DEPARTMENT OF RADIOCHEMISTRY AND COLLOID CHEMISTRY

 

 

The main task of the research project was determination of the adsorption mechanism, structure and thickness of the adsorption layers of the polyacrylic acid (PAA) and polyacrylamide (PAM) on the surface of metal oxides (ZrO2, Fe2O3, MnO2).

Adsorption measurements as well as surface charge, zeta potential and thickness of the adsorption layer measurements with application of viscosity and SAXS methods proved that polymer adsorption is a complex process that depends on many parameters. These ones which influence the polymer conformation at the interface are: polymer molecular weight, type of functional groups of the polyelectrolite, pH and ionic strength of the solution, type of the electrolyte, purity of the solid surface and temperature [1-3].

Obtained data allowed determine free energy of polymer adsorption on the surface of the solid from Pradip’s equation [4]:

where:

‑ difference of pHIEP of the oxide without polymer and with adsorbed polymer,

‑ free adsorption energy of the polymer on the solid,

c0‑ initial concentration of the polymer,

R‑ gas constant,

K‑  absolute temperature.

            Free energy of the adsorption of PAA and PAM segments, on the surface of MnO2 and Fe2O3 is presented in Table 1.

            As can be seen from data listed in Table 1, free energy of the adsorption increases with the increase of the polymer molecular weight for all studied systems. However, it is not proportional to the increase of the number of the segments in the polymer chain. Lack of proportionality results from conformation changes of the polymer chains of various lengths. Moreover polyacrylic acid is stronger than polyacrylamide adsorbed on the surface of the solid. The bonding of the macromolecule with surface of the solid goes through carboxylic groups, which are more numerous in the PAA chain.

 

Table 1. Free energy of the adsorption of PAA and PAM on the surface of MnO2 and Fe2O3 CNaCl=1´10‑2 mol/dm3.

 

System

C0 [ppm]

C0 [mol/l]

DG0sp [kJ/mol]

Metal oxide

Polymer

Fe2O3

PAA 2 000

0.01

5´10-9

-44.67

PAA 170 000

0.01

5.88´10-11

-57.88

PAA 240 000

0.01

4.167´10-11

-59.37

PAM 1 500

1

6.67´10-7

-36.30

PAM 10 000

1

1´10-7

-39.72

MnO2

PAA 2 000

0.01

5´10-9

-48.7

PAA 170 000

0.01

5.88´10-11

-60.92

PAA 240 000

0.01

4.167´10-11

-61.14

PAM 1 500

1

6.67´10-7

-36.46

PAM 10 000

1

1´10-7

-40.68

 

Computer calculations based on Scheutjensa-Fleera theory [5‑6] allowed characterize the structure of adsorption layers of the polymer. Obtained data, concerning amount and length of „train”, „loops” and „tails” structures in adsorbed polymer layer prove that increase of the adsorption layer thickness, connected with increase of the polymer molecular weight for systems with PAA is determined by “tails” whereas for systems with PAM by “loops” and “tails”.

 

References:

[1] S. Chibowski, M. Wiśniewska, Adsorp. Sci. Technol., 19 (2001) 409.

[2] S. Chibowski, M. Wiśniewska, Colloids Surf., 208 (2002) 131.

[3] S. Chibowski, M. Wiśniewska, M. Paszkiewicz, Adsorp. Sci. Technol., 20 (2002) No 6.

[4] Pradip, Trans. Indian Inst. Metals, 41 (1988) 15.

[5] J. M. H. M. Scheutjens and G. J.Fleer, J. Phys. Chem., 83 (1979) 1619.

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

 

Grant 7T09B 010 21