Transitions between ordered and disordered phases and their coexistence in dilute ionic colloidal dispersions

Order-disorder transitions in dilute aqueous dispersions of ionic colloids were studied experimentally in colloidal silica and polymer latex systems. A three-dimensional phase diagram of the silica system was determined by treating the particle density, the effective surface charge number, and the ionic strength as variables. On the basis of the phase diagram, the structures of the dispersions were studied by means of ultrasmall-angle X-ray scattering and confocal laser scanning microscopy. The results clearly revealed the presence of a reentrant order-disorder transition with increasing charge number, and a wide biphasic region where ordered and disordered phases coexist. The nearest interparticle distance 2D_exp was almost equal to that calculated assuming a homogeneous particle distribution, 2D₀, in the ordered single-phase region, while 2D_exp was smaller than 2D₀ under the biphasic condition. Further studies concerning the crystallization process of the dispersion in the biphasic region demonstrated the development of disordered liquidlike regions inside the space-filling crystalline grains which was induced by lattice contraction. The results suggest that the commonly held view based on a Yukawa-type potential, in which only repulsive interparticle interactions are considered, cannot explain the phase behavior of the dilute ionic colloids.