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

The influence of the effective particle surface charge density, sigma(e), o n the order-disorder phase transition was examined for aqueous dispersions of charged colloidal silica and polymer latex particles (diameters = 0.11-0 .13 mu m). The sigma(e) value of the silica particle was continuously tuned by changi ng the concentration of added NaOH. The three-dimensional phase diagram of the order-disorder transition far the silica system was determin ed as a function of sigma(e), particle volume fraction phi, and salt concen tration C-s, by observing iridescence due to Bragg diffraction from the ord ered structure, and further by applying an ultra-small-angle X-ray scatteri ng method. With increasing sigma(e), the disordered dispersion became order ed and thereafter reentered into the disordered state. The presence of the reentrant disordered phase at high sigma(e) conditions was observed for ion ic polymer latex systems. The reentrant phase transition was not explainabl e in terms of the Yukawa potential and charge renormalization model.