Colloidal properties of synthetic hectorite clay dispersions. II: Light and small angle neutron scattering

Abstract The structure and interactions in aqueous dispersions of a synthetic hectorite-type clay, Laponite RD, have been investigated at low ionic strength ([Na] ∼ 10 −3 mole dm −3 ) by static (SLS) and dynamic light scattering (DLS) and small angle neutron scattering (SANS). At low concentrations (≲0.01 g ml −1 ) both SLS and DLS show that the dispersions contain discrete particles, which do not interact strongly. The molecular weight and hydrodynamic properties of these particles are similar to those of circular disks of diameter ∼30 nm and thickness ∼ 1 nm. At higher concentrations (≳0.03 g ml −1 ), where gelation and the development of elasticity occurs, there are clear indications of interparticle interactions from changes in the light scattering and SANS behavior. These interactions are ascribed to an overlap of the electrical double layers surrounding the clay particles. It is proposed that the resultant interparticle repulsion gives rise to an equilibrium structure, having elastic properties, in which the translational mobility of the particles is restricted. From the scattering behavior there is no evidence of an association of the particles in an edge-to-face type structure which is now accepted to occur extensively at much higher ionic strengths in other clay dispersions, such as kaolinites and bentonites. The proposed gelation mechanism of Laponite dispersions at low ionic strength is, however, likely to be similar to that which has been suggested previously from rheological studies to occur with montmorillonite, which is a clay which can also undergo osmotic swelling.