Effect of ionic strength on the adsorption of cationic polyelectrolytes onto alumina studied using electroacoustic measurements

The adsorption of numerous cationic polyelectrolytes onto alumina in aqueous suspensions was investigated using electroacoustic measurements. Initially, the isoelectric point (IEP) of the Sumitomo AA05 alumina was identified at several different electrolyte (KCl) strengths. The IEP was found to depend on the presence of the electrolyte. With no electrolyte present, the IEP was 9.4; but with electrolyte present, there was a slight shift to higher pH, possibly due to adsorption of potassium ions on the alumina surface. The natural pH of the aqueous suspensions was 4.9; at this pH value, the particles were highly positively charged and none of the polyelectrolytes adsorbed. The pH of the suspensions was then increased to 11.4; at this value, the particles were highly negatively charged and all the polyelectrolytes adsorbed, causing charge reversal. The amounts of polyelectrolyte then required to obtain both zero zeta potential and the maximum plateau zeta potential were found to depend on the electrolyte strength of the suspension. This was attributed to the different configurations taken up by the polyelectrolyte on adsorption.