The Effect of pH on Adsorption of Poly(acrylic acid) and Poly(vinylpyrrolidone) on Alumina from Their Binary Mixtures

The effect of pH on adsorption of poly(acrylic acid) (PAA) and poly(vinylpyrrolidone) (PVP) on α-alumina from their binary mixtures has been investigated by measuring adsorbed amounts of polymers, ζ potential, and dispersion stability of alumina suspensions. ESR spectroscopy with the spin-labeled polymers has also been used to monitor the conformation of the polymers at the solid/liquid interface. The adsorption of PAA alone shows a strong affinity and the adsorbed amount decreases gradually with increasing pH from 5.2 to 10.2, whereas that of PVP alone is very small over a whole pH region studied. In the presence of PAA the adsorption of PVP is considerably enhanced due to the hydrogen bonding of PVP with PAA adsorbed on alumina at pHs of 5.2 and 7.2, but no enhancement of PVP adsorption at pH 10.2 is observed. It is found that at pHs of 5.2 and 7.2 PAA is mainly adsorbed as train segments while the conformation of PVP adsorbed on the PAA layer is taken as loops or tails. The alumina suspensions with PVP in the presence of PAA are flocculated at pHs of 5.2 and 7.2 but are stable at pH 10.2. These results can be explained on the basis of the conformation change of polymers adsorbed and the surface charge of alumina particles.