How homogeneous are "homogeneous dispersions"? Counterion-mediated attraction between like-charged species

In solutions or dispersions, solute distributions are considered to be more or less homogeneous and space-filling, particularly for concentrated ones. This is not experimentally the case, however, at low concentrations. Trave rse photographs of a homogeneous dispersion of ionic latex particles (volum e fraction phi = 0.05) taken by a Lang camera show the coexistence of order ed domains of particles (as studied by Kossel line analysis) and disordered regions. The video imagery study indicates the presence of at least two di ffusion modes for particles in a "homogeneous" dispersion (phi = 0.02). The confocal laser scanning microscope (CLSM) study shows that negatively char ged latex particles are positively adsorbed near likewise negatively charge d glass interface. The ultra-small-angle X-ray scattering (USAXS) patterns of 4- or 6-fold symmetry are observed with five or four orders of Bragg dif fraction from colloidal silica dispersions (phi = 0.0376), suggesting the f ormation of a bcc single crystal with a lattice constant of 0.3 mu m. The t wo-dimensional USAXS study of the colloidal silica dispersion (phi = about 0.025) gives 22 scattering peaks below a scattering angle of 203 ", which u niquely prove that a single bcc crystal is formed, allowing us to accuratel y determine the lattice constant and direction of the crystal. The USAXS in vestigations again confirm the previously found fact that the closest inter particle distance was systematically smaller than the average distance expe cted from the overall particle concentration. For latices of poly(chlorosty rene-styrenesulfonate) copolymers, which allow concurrent scattering and mi croscopic studies, the inequality relation of the interparticle spacing is observed and the presence of void structures is visually confirmed at phi = 0.03 or below. The re-entrant phase transition is found when the net charg e density of particles is increased. The bcc-fcc transition, void formation , and the re-entrant behavior can be accounted for by the Monte Carlo simul ation with the Sogami potential containing a short-range repulsion and a lo ng-range attraction.