Structure and stability of concentrated boehmite sols

Abstract

The viscoelastic properties of dispersions of microcrystalline boehmite (AlOOH) particles, covering a range of high concentrations (>10% w/w) and containing different electrolytes, have been measured under oscillatory shear with a Weissenberg rheogoniometer. Stable dispersions were highly elastic and often thixotropic—properties which are ascribed to short range (<10 nm) interparticle repulsion forces. When destabilised by addition of certain counterions (IO₃^–, BrO₃^–, F^–, SO₄^2–) this elasticity was lost and plastic properties developed. Interparticle repulsion is attributed to extensively solvated polynuclear aluminium cations, formed at the boehmite surface during acid peptisation, whose presence was consistent with quasielastic neutron scattering and other evidence.

Light scattering measurements on dilute dispersions (<2% w/w) showed that the latter contain large and very open aggregates of primary sol particles–the number of primary particles per aggregate being dependent on the electrolyte concentration. In more concentrated dispersions (with volume fractions, φ > 0.1) a stable and coherent structure analogous to the individual aggregates is proposed.