Yield stress and zeta potential of nanoparticulate silica dispersions under the influence of adsorbed hydrolysis products of metal ions—Cu(II), Al(III) and Th(IV)

Abstract

The effects of hydrolysable Cu 2+, Al 3+ and Th 4+ ions on the zeta potential and yield stress behaviour of silica dispersions were evaluated as a function of pH and metal ions concentration. Silica dispersion remained dispersed at its point of zero charge (pzc) of pH ∼2.0 (CR1). Adsorbed hydrolysis products of Cu 2+ and Al 3+ caused the dispersion to display two further points of charge reversal (CR2 and CR3) at moderate ions concentration. CR2 occurred near the pH for the formation of the first hydrolysis product. This pH is about 2.8 for Al 3+ and 5.0 for Cu 2+. For all three metal ions, CR3 approached the pzc of the metal hydroxides at complete surface coverage. At CR3, the dispersions displayed a maximum yield stress. As many as three type of attractive forces; bridging, charged patch and van der Waals, may account for the maximum yield stress at low surface coverage. At complete coverage, only the van der Waals force is in play—the adsorbed hydrolysis products must have increased significantly the effective Hamaker constant of silica. With Al 3+ the yield stress was absent at CR2 because particle bridging and charged patch attraction are unimportant as the silica surface charge is near zero. Adsorption of strongly hydrolysed Th 4+ ions at pH < 2.0 caused the dispersion to display only one pzc (CR3).