Influence of polyelectrolyte adsorption on the electro-optical behavior of aerosil particles

The adsorption of polyelectrolyte, sodiumcarboxymethyl-cellulose (Na-CMC) on Aerosil (SiO2) particles is studied by means of electric light scattering and electrophoresis. The electro-optical effect alpha increases with the rise of polyelectrolyte concentration. Simultaneously the increase of particles' electrophoretic mobility reveals the adsorption of polyion on identically charged particles. The similarity of the frequency dependence of the electrooptic effect at different polyelectrolyte concentrations is an indication that the particles' induced dipole moment is mainly determined by polyelectrolyte' counterions behaviour. Two relaxations of the electro-optical effect are observed at frequencies ranging from 500 Hz to 100 kHz - the first relaxation is about 1 kHz, the second one - in the 10 kHz region. The increase of charge density parameter xi doesn't influence the first relaxation, but the second one becomes wider and more noticeable. According to Manning predictions the rising of xi (xi > 1) results in the increase of the fraction of diffuse counterions. So their contribution to the induced dipole moment is more clearly demonstrated in the frequeny dependence. Most probably the first relaxation of alpha is connected with the ''bound'' counterions whereas the second is due to the diffuse counterions. In the case of polyelectrolyte adsorption pn the particles of the same charge sign an analogy with Manning consideration for free polyelectrolytes in solution could be drawn.