Electrostatic characterisation of Al-modified, nanosized silica particles

Al-modified, nanosized silica particles have been studied with an electroacoustic technique. Also slightly aggregated silica particles have been investigated with this method. zeta-potentials calculated from the measured dynamic mobilities were found to be in good agreement with previous reported zeta-potentials obtained from streaming potential measurements. This indicates that the magnitude of the dynamic mobility is very similar to the electrophoretic mobility for moderately charged nanosized particles. The degree of Al-substitution was varied so that between 0 and 16% of the surface silicon atoms were replaced by aluminium. It was found that the magnitude of the titratable surface charge density increased with increasing degree of Al-substitution. This was especially pronounced in the neutral pH region. The behaviour of the zeta-potential as a function of Al-substitution was more complex than the behaviour of the surface charge density: increasing the degree of Al-substitution stepwise up to 7%, raised the magnitude of the zeta-potential both in 2 and 10 mM NaCl. A further increase of the degree of Al-substitution lead to a reduction of the magnitude of the zeta-potential. However, in 50 mM NaCl, it was favourable to include as much as 16% aluminium in the surface structure of the particle. The behaviour of the zeta-potential as a function of pH for the Al-modified samples can be explained in terms of adsorbed and non-adsorbed aluminium.