Reload Index (ZRiChK UMCS)

 

 

STUDIES OF ADSORPTION MECHANISM AND THE STRUCTURE OF ADSORBED POLYELECTROLYTE LAYERS ON METAL OXIDE-SOLUTION INTERFACE

 

STANISŁAW CHIBOWSKI, MAŁGORZATA WIŚNIEWSKA

DEPARTMENT OF RADIOCHEMISTRY AND COLLOID CHEMISTRY

 

 

 

Fig. 2. Surface charge of ZrO2 versus pH of the solution, with and without of PAA.

 

 

 

 

 


Presented studies aimed to the determination of an influence of the molecular weight of polyacrylic acid (PAA) and polyacrylamide (PAM) on their adsorption and electrical properties of metal oxide-polymer solution interface. ZrO2 and Fe2O3 were used as adsorbents. Figure 1 presents adsorption isotherms of PAM on the surface of hematite. Similar dependencies were obtained for PAA on both oxides.

Fig. 1. Adsorption isotherms of PAM on the surface of Fe2O3.

 


A possible mechanism of the adsorption of both polyelectrolytes on the surface of studied oxides was proposed on the base of their adsorption isotherms and computed concentrations of various surface groups of ZrO2 and Fe2O3 as a function of the pH of the solution. It was stated that bonding of PAA and PAM chains with the surface of the solid goes through -SOH groups of both adsorbents, which may interact with carboxyl groups of studied polyelectrolytes through the hydrogen bridge.

Adsorption of PAA and PAM on ZrO2 and Fe2O3 influences the surface charge density and its distribution in the diffuse layer of electrical double layer (edl).

The -COO- groups along the PAA and PAM macromolecule chain, are main factor responsible for lowering the surface charge of the oxide (Fig.2). Change of surface charge is proportional to the concentration of these groups in the polymer chain.

Observed changes of zeta potential in the studied systems results from the shift of the slipping plane [1] and blockade of active sites on the surface of the oxide.

Share of these effects depends on pH of the solution, molecular weight and concentration of the polymer.

A thickness of adsorption layer (d) of PAA and PAM on the surface of ZrO2 and Fe2O3 was determined from the measurements of viscosity changes of the pure polymer solution and the suspension without and with adsorbed polymer [1].

Obtained data are listed in table 1.

Table 1. Thickness of PAA and PAM adsorption layers on the surface of ZrO2 and Fe2O3.

System

Cp [ppm]

d [nm]

Polymer

Mw

Oxide

pH=3

pH=9

PAA

2000

240000

ZrO2

100

1.19

4.14

2.18

6.06

PAA

2000

240000

ZrO2

500

2.77

5.97

4.14

7.95

PAM

1500

10000

Fe2O3

100

1.22

1.84

2.74

3.65

PAM

1500

10000

Fe2O3

500

3.13

4.03

5.75

6.68

A thickness of PAA and PAM adsorption layer on the surface of ZrO2 and Fe2O3 increases with the increase of pH of the solution, molecular weight and concentration of the polymer. The increase of these last two factors favors more compact ordering of the polymer chains on the surface of the solid. Obtained conformations are more abound in loops and tails structures.

Observed increase of d for higher pH of the solution results from mutual interaction of negatively charged polyelectrolyte chains and their interaction with negatively charged surface. It leads to developing of the polymer coils, which form on the surface of both adsorbents thicker and more compact adsorption layers.

 

References:

[1] A. M’Pandou and B. Siffert, Colloids and Surfaces, 24 (1987) 159.