Revised state diagram of Laponite dispersions

We propose a state diagram of charged disk-like mineral particle (Laponite) dispersions as a function of the Laponite concentration (C) and the concentration of added salt (Cs), based on simple observation and light-scattering measurements. At low C or high Cs the dispersions separate into two domains due to sedimentation of Laponite aggregates, while at high C and low Cs they form homogeneous gels that do not flow upon tube reversal. The aggregation rate and the structure factor of the Laponite dispersions is determined with light scattering as a function of C and Cs. We discuss in detail the controversy on the origin of gelation of Laponite dispersions in the absence of added salt. We argue that aggregation rather than glass formation causes gelation.