Abstract
The adsorption of a humic substance — Na-humate — on aluminium oxide (γ-Al 2O 3) and aerosil (SiO 2) particles was investigated using electric light scattering and electrophoresis. The electro-optic effect in γ-Al 2O 3 dispersion is changed when varying the concentration of the humate polyanion and reduces to zero together with the electrophoretic mobility. A decrease of the rotation diffusion coefficient was registered above the isoelectric point (IEP), which indicates formation of a thick adsorption layer, stabilizing the dispersed system. The electro-optic effect relaxation frequency of the stabilized dispersed particles is greatly decreased, which is evidence of decreased counterion mobility. In the aerosil dispersed system the electro-optic effect increases with the addition of Na-humate. The frequency dependence of the electro-optic effect is also influenced by the presence of negative polyions. These changes in the electro-optic behaviour of the aerosil suspension are an indication of the fact that the polyion is adsorbed on a negatively charged surface, probably because of a strong specific interaction.
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