Sol-gel transition of colloidal suspensions of anisotropic particles of laponite

The thermodynamical properties, structure and rheology of laponite dispersions have been studied as a function of solid fraction and ionic strength. The dispersions undergo a transition from Newtonian liquid to viscoelastic solid without phase separation. The increase of the ionic strength shifts the transition to lower solid fraction and a phase diagram has been constructed for the onset of viscoelastic solid like behaviour. This sol-gel transition line coincides with the appearance of a singularity in the equation of state as determined by osmotic pressure measurements. Analysis of the dispersions by transmission electron microscopy (TEM) and small-angle X-ray scattering (SAXS) shows correlated but well separated particles. In order to elucidate the nature of the observed sol-gel transition and the particle organization, we discuss the role of particle anisotropy coupled with diffuse-layer repulsion.