Reload Index (ZRiChK UMCS)

 

 

STUDY OF THE INFLUENCE OF THE POLYACRYLIC ACID AND POLYACRYLAMIDE ON THE ELEKTROCHEMICAL PROPERTIES OF THE ZrO2/SOLUTION INTERFACE.

 

STANIS£AW CHIBOWSKI, MA£GORZATA KRUPA

DEPARTAMENT OF RADIOCHEMISTRY AND COLLOID CHEMISTRY

 

 

Presented studies aimed on the determination of the influence of the molecular weight of a polyacrylic acid and polyacrylamide on the sorption and electrokinetic properties of ZrO2/polyelectrolyte solution interface. The adsorption isoterms of PAA and PAM on the surface of ZrO2 , surface charge and zeta potential of zirconia in the presence of these polymers were determined. The adsorption isoterm for polacrylic acid is presented on Fig.1. Similar plots were obtained for polyacrylamide.

 

Fig.1. Adsorption isotherms of PAA on the surface of ZrO2.

 

Fig.2. Zeta potential of ZrO2 particles as a function of the pH of the solution without and with PAM.

 

 

From the analysis of the adsorption isotherms, one can see that the amount of adsorbed polymer increases with the increase of its molecular weight. The increase of the polymer molecular weight causes greater participation of segments as a loops and tails in the interface region. Also, the pH of the solution has an influence on the magnitude of the adsorption. For both studied polymers, the amount of the adsorbed polymer substance decreases with increase of pH although for PAA this effect is more visible. That is because carboxylic groups from the polymer chains, that may dissociate at some pH values, are responsible for the adsorption process of PAA and PAM on the ZrO2. The adsorption of PAA and PAM has an influence on the surface charge density, as well as the distribution of the charge in the diffuse part of the electrical double layer.

The main factor, responsible for the changes of the surface charge of examined oxide, in the presence of the polymers, is existence in their macromolecules some functional groups (-COO-), that being at the interface of the solution lowers the ZrO2 charge. This lowering depends on the type of the polymer used and pH of the solution.

The change of zeta potential values of the oxide with adsorbed polyelectrolyte (Fig.2) is caused by two fact: shift of the shear plane from the solid phase [1] and blocking of active sites of the oxide by adsorbing polymer chains. In the case of PAA, the first effect plays a dominant role whereas for PAM the shift of the shear plane is responsible for the changes of the zeta potential at higher concentrations and higher molecular weights of the polyacrylamide. At lower concentrations of PAM the blockade of active sites is a dominant effect.

Using zeta potential measurements, the thickness of the adsorption layers [2] and free energy of the  adsorption for polyacrylic acid and polyacrylamide on the surface of ZrO2 were calculated [3]. Obtained results are in agreement with potentiometric titrations and the adsorption measurements. Free energies of the adsorption of both polymers are presented in Table 1.

 

Table 1. Free energy of the adsorption of PAA and PAM on the surface of ZrO2.

 

System

C0

[ppm]

C0

[mol/dm3]

 

[RT]

[kcal/mol]

ZrO2+PAA 2 000

0.1

5´10-8

2.15

-17.54

-10.38

ZrO2+PAA 170 000

0.1

5.88´10-10

2.79

-20.19

-11.95

ZrO2+PAA 240 000

0.1

4.16´10-10

3.15

-22.71

-13.44

ZrO2+PAM 1500

1

6.67´10-7

0.44

-13.36

-7.9

ZrO2+PAM 10 000

1

1´10-7

0.12

-13.96

-8.26

 

Free energy of adsorption increases with the increase of the molecular weights of both polymers. It is caused by the fact that for longer polymer chains more segments interact with the surface of the solid and macromolecules form conformation with different number and length of loops and trains configurations. Moreover, it results from the comparison of the free energies of the PAA and PAM adsorption that polyacrylic acid is adsorbed stronger on the surface of ZrO2. The reason for this is greater participation of carboxylic groups, that bonds polymer with the surface of the solid.

 

References:

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

[2] M.J.Garvey, Th.F.Tadros and B.Vincent, J. Colloid Interface Sci., 55 (1976) 440.

[3] R.S.Pradip, Trans. Indian Inst.Met., 41 (1988) 15.