Reentrant order-disorder transition in ionic colloidal dispersions by varying particle charge density

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 π, 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.