EFFECT OF SOLUBLE ALUMINUM IONS ON POLYELECTROLYTE-ALUMINA INTERACTION - KINETICS OF POLYMER ADSORPTION AND COLLOID STABILIZATION

We investigated the adsorption of high-molecular-weight polyacrylamide, hydrolyzed polyacrylamide and polyacrylic acid onto partially soluble colloidal alumina in aqueous suspensions containing 10(-3) N KCl and 3 x 10(-4) N A1Cl(3). The polymer solution and the colloidal suspension were mixed at pH 5.0, and the electrophoretic mobility and pH were recorded as a function of time. The polymer-aluminium ion interaction resulted in polymer ionization and complexation between carboxylic acid groups and aluminium ions characterized by a maximal degree of complexation close to 0.6. When the polymer was added to the oxide suspension, we also determined the concentration of the different species by potentiometric titration, and characterized the polyelectrolyte adsorption kinetics on the colloidal alumina by determining the variation with time of the amount of free and complexed polymer segments in the supernatant liquid phase. Two different situations were investigated depending on the ratio of carboxylic acid to dissolved aluminium ions. For a low value of the ratio, the polymer was quickly adsorbed in a form which was highly complexed by aluminium ions pre-existing in the solution. For a high value of the ratio, the adsorption was very slow. Before adsorption, the polyacid also underwent an aluminium-hydrogen ion-exchange, the extent of which depended on the rate of oxide dissolution: it appeared that adsorption increased very strongly with complexation. From an investigation of the colloidal stability of the system and the zeta-potential of the colloid-polymer complex as a function of the amount of polymer added to the solution, the domain of electrosteric stabilization was determined and the instability near the point of zero charge was attributed to the electrostatic attraction between the positively and negatively charged groups.