How Homogeneous Are “Homogeneous Dispersions”? Counterion-Mediated Attraction between Like-Charged Species

Abstract

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. Traverse photographs of a homogeneous dispersion of ionic latex particles (volume fraction Φ=0.05) taken by a Lang camera show the coexistence of ordered domains of particles (as studied by Kossel line analysis) and disordered regions. The video imagery study indicates the presence of at least two diffusion modes for particles in a homogeneous dispersion (Φ = 0.02). The confocal laser scanning microscope (CLSM) study shows that negatively charged latex particles are positively adsorbed near likewise negatively charged 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 diffraction from colloidal silica dispersions (Φ = 0.0376), suggesting the formation of a bcc single crystal with a lattice constant of 0.3 μm. The two-dimensional USAXS study of the colloidal silica dispersion (Φ = about 0.025) gives 22 scattering peaks below a scattering angle of 203, which uniquely prove that a single bcc crystal is formed, allowing us to accurately determine the lattice constant and direction of the crystal. The USAXS investigations again confirm the previously found fact that the closest interparticle distance was systematically smaller than the average distance expected from the overall particle concentration. For latices of poly(chlorostyrene-styrenesulfonate) copolymers, which allow concurrent scattering and microscopic studies, the inequality relation of the interparticle spacing is observed and the presence of void structures is visually confirmed at Φ = 0.03 or below. The re-entrant phase transition is found when the net charge density of particles is increased. The bcc-fcc transition, void formation, and the re-entrant behavior can be accounted for by the Monte Carlo simulation with the Sogami potential containing a short-range repulsion and a long-range attraction.