Electro-optics of colloid-polyelectrolyte complexes: counterion condensation on free and adsorbed sodium carboxymethyl cellulose.

Electrical properties of sodium carboxymethyl cellulose (NaCMC) in aqueous solution and after addition to a dilute suspension of beta-ferric hydrous oxide particles (beta-FeOOH) are studied by means of electric birefringence method. We established extended conformation of the NaCMC chains in a solution at concentration 10(-2) g dm(-3), which is found high enough to assure overcharging of the particle surface. The frequency behavior of the electric birefringence of NaCMC solution with concentration 10(-2) gdm(-3) is found similar to the behavior of the suspension of beta-FeOOH particles containing same amount of polyelectrolyte. The observed decrease in the relaxation frequency of the electro-optical effect in both systems is attributed to polarization of condensed counterions near to the polyion surface. These results are in line with our previous estimation, showing that the condensed counterions are not released from NaCMC because of its adsorption onto weakly charged particle surface. They also reveal that, at overcompensation of the particle charge, the electrical properties of the adsorbed polyion dominate the electro-optical behavior of the weakly charged particle.