Recent Study of Counterion-mediated Attraction between Colloidal Particles and between Particles and Interface Studied by the Ultra-small-angle X-ray Scattering

The application of the 1Dand 2D-ultra-small-angle X-ray scattering (USAXS) methods to the structural study of dilute colloidal silica particle dispersions is reviewed. A single colloidal crystal showed 22 sharp diffraction peaks, which allowed us to uniquely determine the lattice symmetry (body-centered-cubic), lattice constant (around 0.3–0.4 m) and crystal orientation. 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 profile had six-fold and four-fold symmetries with 111 and 001 directions being parallel to the axis of the capillary (dispersion container), 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 interaction between negatively charged interface and particles. After the capillary 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) planes (and hence greater stabilization of the system) than for the four-fold symmetry. CROATICA CHEMICA ACTA CCACAA 71 (4) 1005¿1017 (1998)