Phase Diagram of Colloidal Dispersions of Anisotropic Charged Particles: Equilibrium Properties, Structure, and Rheology of Laponite Suspensions

Abstract

We discuss the phase diagram of aqueous dispersions of colloidal platelike charged particles (300 A x 10 A). Particle concentration and ionic strength are the two parameters controlling the system. The suspensions undergo a sol/gel transition without macroscopic phase separation. Shear rheology is used to monitor this transition and to locate the appearance of the mechanical gel phase. Increasing the ionic strength shifts the sol/gel transition to lower volume fraction. Direct inspection of this gel phase by cryofracture, TEM and SAXS shows correlated but well-separated particle populations. In order to check the reversibility and the equilibrium properties of this transition, the equation of state was determined by osmotic stress. At fixed ionic strength, the osmotic pressure first increases at low particle concentration, then reaches a pseudoplateau, and increases again for higher concentrations. The location of such a singularity in the equation of state of the suspension defines a thermodynamical transition coinciding with the mechanical phase transition. In order to analyze the origin of this gel or glassy phase, the role of particle anisotropy, coupled with diffuse layer repulsion, is discussed.