pH-dependent aggregation state of highly dispersed alumina, titania and silica particles in aqueous medium

Abstract

Highly dispersed commercial products (Aluminium oxide C, Titanium dioxide P25 and Aerosil 200) of Degussa were investigated. Their pH-dependent surface charge state was determined from acid-base titrations in the presence of KNO₃. The p.z.c. values were measured at pHs 8.0 for Al₂O₃, 5.9 for TiO₂ and ∼4 for SiO₂. The calculated intrinsic equilibrium constants for surface charge (σ ₀) formation are log K ^int_a1 =5.8, log K ^int_a2 =10.2 for Al₂O₃, log K ^int_a1 =3.6, log K ^int_a2 =8.1 for TiO₂ and log K ^int_a2 =8.02 for SiO. The experimental σ ₀ vs. pH curves were fitted by using the DDL model of MICROQL-UCR program. The aggregation state in dense suspensions was investigated by means of rheology and SAXS method. The rheological character of flow curves changed from Newtonian to pseudoplastic or to highly thixotropic, depending on the pH of suspensions. The experimental yield values (initial or Bingham) showed maximum at the p.z.c. Scattering curves of aggregated (at p.z.c.) and well-stabilized (at pH far from p.z.c.) oxide suspensions were determined. The correlation length values were calculated on the basis of theoretical approach of Debye-Bueche which were independent of the solid/liquid ratio of suspensions at p.z.c., indicating that aggregation was controlled by attractive forces between particles, while those increased with increasing dilution in well stabilized systems.