Reload Index (ZRiChK UMCS)

 

 

INFLUENCE OF THE MOLECULAR WEIGHT OF SOME NONIONIC POLYMERS ON THE ADSORPTION AND ELECTROCHEMICAL PROPERTIES OF THE TITANIA-SOLUTION INTERFACE

 

ST. CHIBOWSKI, M. PASZKIEWICZ

DEPARTMENT OF RADIOCHEMISTRY AND COLLOID CHEMISTRY

 

 

Because of their wide application in cosmetics, dye and pharmaceutical industry, polyethylene glycols (PEG) and polyethylene oxides (PEO), found its place in model studies of the adsorption on the surface of the dispersed solids. The general formula of the polyethylene glycol segment and polyethylene oxide segment is presented as (-CH2-CH2-O-). The difference in the chain structure of these polymers is connected with different form of ending groups.

Pole tekstowe: Fig. 1. Adsorption isotherms of nonfractioned samples of PEG and PEO on the surface of TiO2 in NaCl solution (cNaCl 10-3 mol/dm3).Presented studies aimed on the determination and comparison of the kinetic and adsorption properties of nonionic polymers (PEG) and (PEO) on the surface of titania. Respective molecular weights of applied polymers were 2,000 and 35,000 (PEG) and 100,000 and 300,000 (PEO). The specific surface of TiO2 (anatase), found by BET method, was 8.3 m2/g. From the obtained adsorption isotherms of PEG and PEO on the surface of titania, (Fig. 1) the influence of the molecular weight on the adsorption is well visible. The magnitude of the adsorption depends on the absolute number of big particles, as they, adsorbing on the surface remove the smaller ones [1].

The more detailed characteristics of the adsorption processes and mechanism, which is responsible for them, give electrochemical measurements.

Zeta potential measurements and potentiometric titrations of the systems  TiO2-NaCl, TiO2-NaCl-PEG and TiO2-NaCl-PEO confirm the influence of the molecular weight on the surface charge and zeta potential.

Although the addition of the polymer to the titania suspension changes the surface charge density and zeta potential it does not have the influence on the position of pHpzc (about 6.4) and pHiep (about 6.1). Probably, the mechanism that leads to these behavior is based on the hydrogen bridge type interactions between -TiOH groups and hydroxyl groups, presented in the polymer chain. The influence of the polymer adsorption on the surface charge may be connected with a blocking of the active sites on the TiO2 surface, whereas zeta potential changes with changes of the molecular weight are the summed up effect results from the blockade of the active sites as well as the shift of the shear plane. A comparison of some differences in the diffuse layer charge Dsd { Dsd = sd (no polymer) - sd (with polymer)} and difference of surface charge Dso { Dso = so (no polymer) - so (with polymer)} calculated from the following equation was useful to confirm these supposition (Fig 2 and 3).

                                               

where: c - concentration of background electrolyte, z - charge of counter ion, yd  - potential of diffuse layer

Fig. 2. Changes of the difference of TiO2 surface charge as a function of the pH of the solution.

Fig. 3. The changes of the difference of the diffuse charge as a function of the pH of the solution.

Consideration of the obtained relations and the model of the electrical double layer in the presence of the polymer, allowed us to confirm foreseen mechanism, leading to the changes of zeta potential as a function of pH and the polymer molecular weight. The calculation of the thickness of the PEG and PEO adsorption layers on the surface of TiO2 from the following equation:

                                           

where: e - 1.6021 x10-19C, elementary charge, k - 1.3805 x10-23J/K (Boltzman constant), T - 298K

Pole tekstowe: Table 1. Thickness of PEG and PEO adsorption layers on the surface of TiO2. cPolymer = 10-3g/ml, cNaCl =10-3 mol/dm3
Molecular weight of the polymer	dE [nm]
PEG 2 000	16,3
PEG 35 000	17,3
PEO 100 000	17,4
PEO 300 000	22,8

z - 1 (valency of the counterions in the electric double layer), Y » zeta potential of polymer covered particles Yd» zeta potential of bare particles, D - Stern layer thickness, estimated to be 0.4 nm, C-1 - 10 nm (reciprocal thickness of the electric double layer), d = thickness of the adsorption layer

confirm the influence of the molecular weight on the conformation of the polymer chain. The lack straight proportionality of the thickness of the adsorption layer with molecular weight of the studied polymers may be connected with the type and adsorption affinity of  the ending groups and number of hydrogen bonds per one macromolecule (Tab 1).

 

[1]. B. Vincent and S. G. Whittington, in E. Matijević (Ed.), Surface and Colloid Science, Plenum Press, New York, Vol.12, 1-116, (1981)