Transitions between ordered and disordered phases and their coexistence indilute 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 tr eating the particle density, the effective surface charge number, and the i onic strength as variables. On the basis of the phase diagram, the structur es of the dispersions were studied by means of ultrasmall-angle X-ray scatt ering and confocal laser scanning microscopy. The results clearly revealed the presence of a reentrant order- disorder transition vith increasing char ge number, and a wide biphasic region where ordered and disordered phases c oexist. The nearest interparticle distance 2D(exp) was almost equal to that calculated assuming a homogeneous particle distribution, 2D(0), in the ord ered single-phase region, while 2D(exp) was smaller than 2D(0) under the bi phasic condition. Further studies concerning the crystallization process of the dispersion in the biphasic region demonstrated the development of diso rdered 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 interpa rticle interactions:are considered, cannot explain the phase behavior of th e dilute ionic colloids.