Recent study of counterion-mediated attraction between colloidal particlesand between particles and interface studied by the ultra-small-angle X-rayscattering

The application of the 1D- and 2D-ultra-small-angle X-ray scattering (USAXS ) methods to the structural study of dilute colloidal silica particle dispe rsions is reviewed. A single colloidal crystal showed 22 sharp diffraction peaks, which allowed us to uniquely determine the lattice symmetry (body-ce ntered-cubic), lattice constant (around 0.3-0.4 mu m) and crystal orientati on. In dilute dispersions, the closest interparticle spacing was confirmed to be smaller than the average spacing, pointing to the important role of a counterion-mediated attraction between the particles. The scattering profi le had six-fold and four-fold symmetries with [1 (1) over bar 1] and [001] directions being parallel to the axis of the capillary (dispersion containe r), respectively. In both cases, the (110) planes, the most densely packed planes of bcc, were in contact with and parallel to the capillary surface, confirming the presence of a possibly counterion-mediated attractive intera ction between negatively charged interface and particles. After the capilla ry was shaken, only the six-fold symmetry was found, which is reasonable in the light of larger contact between the capillary surface and the (110) pl anes (and hence greater stabilization of the system) than for the four-fold symmetry.