Adsorption of polyacrylic acid on α-Al2O3 colloid particles as studied by electro-optics

The adsorption of polyacrylic acid (PAA) on partially soluble α-Al2O3 particles is studied by electric light scattering and electrophoresis at pH 5 and constant ionic strength in the presence of aluminium ions. The stability and the thickness of adsorbed polymer layer were determined from the changes of relaxation times of particle disorientation obtained from transient electric light scattering. The electrophoretic mobility changes sharply from positive to negative in a narrow concentration range the isoelectric point (iep) being at an adsorption density of ΓPAA=2.5 mg m−2. The changes of electro-optical effect, proportional to particles surface electric polarizability, with PAA concentration follow the changes of the electrophoretic mobility. A minimum of polarizability is observed at the iep. The frequency dependence of electro-optical effect giving information on the different polarization mechanisms is shifted to lower frequencies above the iep indicating a decreased mobility of counterions whose movement is responsible for polarizability. The thickness of the adsorption layer increases above the iep. This increase and the decreased counterions mobility can be an indication to conformational changes in the adsorbed polymer layer.