Reentrant Order−Disorder Transition in Ionic Colloidal Dispersions by Varying Particle Charge Density

Abstract

The influence of the effective particle surface charge density, σ e , on the order-disorder phase transition was examined for aqueous dispersions of charged colloidal silica and polymer latex particles (diameters = 0.11-0.13 μm). The σ e value of the silica particle was continuously tuned by changing the concentration of added NaOH. The three-dimensional phase diagram of the order-disorder transition for the silica system was determined as a function of σ e , particle volume fraction Φ, and salt concentration C s , by observing iridescence due to Bragg diffraction from the ordered structure, and further by applying an ultra-small-angle X-ray scattering method. With increasing σ e , the disordered dispersion became ordered and thereafter reentered into the disordered state. The presence of the reentrant disordered phase at high σ e conditions was observed for ionic polymer latex systems. The reentrant phase transition was not explainable in terms of the Yukawa potential and charge renormalization model.