Control of crystallization of ionic silica particles in aqueous dispersions by sodium hydroxide

Crystallization behavior of colloidal silica particles in aqueous dispersion (particle diameter = 0.12×[Formula Presented] m) is examined at a silica volume fraction of 3×[Formula Presented] and various sodium hydroxide and salt concentrations. We report that the crystallization could be controlled by adding [Formula Presented] to [Formula Presented] sodium hydroxide, which increased the surface charge density, [Formula Presented], of the silica particle. Conditions for crystallization were determined as a function of [Formula Presented] and salt concentration. The grain size was large near the phase boundary between crystal and liquidlike regions. The structure of the crystals was studied using an ultra-small-angle x-ray scattering method. Several orders of Bragg reflection having a sixfold symmetry was observed for large grains. For small grains, the scattering profile was powderlike. In both cases, the crystals had body-centered-cubic lattice structures. The observed closest interparticle distances were smaller than average interparticle distance, suggesting the non-space-filling nature of the crystal.