Adsorption of carboxymethyl cellulose on alumina particles.

The polyelectrolyte adsorption on colloid particles is often used for stabilization or flocculation of water suspensions. The aim of this work is to study the adsorption of carboxymethyl cellulose (CMC) on alumina (γ-Al2O3) colloid particles. The particles and polymer are chosen because of the capability of the metal-oxide ampholyte surface and the weak polyelectrolytes to alter their charge by pH. The measurements are done at pH 6.0 where the CMC carboxylic gropes are almost fully dissociated and the alumina surface is positively charged. The high linear charge density of the polyelectrolyte chain provides Na(+) counterions condensation on the COO(-) groups. The main employed method is the electric light scattering based on particle orientation in sinusoidal electric field. The electric polarizability and the relaxation time after field switching off (both depending on the particle charge and size) are used as criteria for polymer adsorption and particle aggregation. Micro-electrophoresis is applied as additional techniques indicating the sign and density of the surface charge. The results obtained give the conditions (time dependence, particle and polymer concentrations) where the CMC adsorption is complete and the suspension is stable.

Polymer concentration dependence of kilohertz electric polarizability of alumina colloid particles with adsorbed carboxymethyl cellulose.

Polyelectrolytes are soluble polymers composed of units having charged groups. Because of the high charge density, some of the counterions are adsorbed electrostatically (ion condensation) on the polyelectrolyte chain. It was shown that in direct electric field the condensed counterions and the chain move together as one whole but it is assumed that they are mobile in alternating field and participate in the polarization. Experimental evidence is obtained by electro-optical investigations of polyelectrolytes adsorbed on colloid particles-the observed low-frequency shift of the polarizability relaxation has been interpreted as condensed counterions' mobility. The present investigation aims to verify the reports for the condensed counterions' mobility in sinusoidal electric field. By means of electric light scattering we investigated a water suspension of γ-alumina particles with adsorbed carboxymethyl cellulose. Instead of the previously used frequency approach (dispersion dependence at saturated adsorption of the polyelectrolyte) we applied an amplitude approach-determination of the polarizability at frequency 1 kHz and increasing polyelectrolyte concentration (from zero to full adsorption saturation). The results indicate the absence of polarization owing to the condensed counterions. The main evidence was obtained by comparison of the concentration dependences of the polarizability (depending on all mobile counterions) and the electrophoretic mobility (determined only by the diffuse counterions). We concluded that the condensed counterions are immobile in sinusoidal field with intensity up to 0.5  kV cm (- 1) and frequency of 1 kHz and higher.