Influence of Na-humate on electric surface properties and stability of γ-Al2O3 dispersions

Electro-optical properties and electrophoretic mobility of γ-Al2O3 particles in the presence of different Na-humate (NaHS) concentrations at pH 4–6 have been studied. The adsorption layer thickness was determined by measuring the rotation diffusion coefficient. The NaHS concentration corresponding to the charge reversal (isoelectric point, IEP of the γ-Al2O3/NaHS complex) decreases with pH increase. In the IEP, the dispersion system is fully precipitated and above the IEP it is restabilized. The increase of the electrophoretic mobility above the IEP supposes that some specific (non-electrostatic) forces operate after the charge reversal, leading to further adsorption of NaHS on alumina. The adsorption layer thickness under the IEP is ≈10 nm and above the IEP it is ≈30 nm. This considerable thickness value is an indication for some electrosteric stabilization. The frequency dependence of the electro-optic effect above the IEP is shifted to lower frequency, showing a considerably reduced mobility of counterions, probably as result of their condensation on the humate backbone. The comparison between the concentration dependencies of the electrophoretic mobility, the surface electric polarizability (determined electrooptically) and the particles’ stability shows that under the IEP the stability correlates better with the electric polarizability than with the electrokinetic potential.