Small-angle neutron scattering investigations of the structure of thixotropic dispersions of smectite clay colloids

The structure and interparticle interactions in colloidal dispersions of montmorillonite and Laponite RD (a model hectorite-type clay) have been investigated by small-angle neutron scattering (SANS). Measurements have been made under static conditions and during shear, using a Couette-type cell, for concentrations in the range (5 × 10–3)–(6.5 × 10–2) g cm–3, where time-dependent gelation and thixotropic behaviour occur. It is shown that in dispersions of low ionic strength ( < 10–3 mol dm–3) spatial and orientational correlations between the plate-like particles can develop. Such self-organised structures, which have a limited range, as is evident from the anisotropic scattering behaviour, depend on the dispersion concentration and are also influenced by the particle size and shape. Thus orientational correlation is more extensive for the larger, or more anisotropic montmorillonite particles; here it appears that some preferential alignment may persist at distances 103A under equilibrium conditions. The effect of shear on such structures has been investigated by anisotropic SANS. Thus under low shear, montmorillonite dispersions show preferential alignment of particles in the direction of flow. This orientation occurs over long range, giving rise to anisotropic scattering; furthermore, spatial correlations persist. Under high shear (ca. 104 s–1) a breakdown in structure occurs and only preferential alignment is observed. These microstructural changes are discussed in the context of the bulk rheological and thixotropic properties of the clay dispersions.