CONTROL OF CRYSTALLIZATION OF IONIC SILICA PARTICLES IN AQUEOUS DISPERSIONS BY SODIUM-HYDROXIDE

Crystallization behavior of colloidal silica particles in aqueous disp ersion (particle diameter = 0.12x10(-6) m) is examined at a silica vol ume fraction of 3x10(-2) and various sodium hydroxide and salt concent rations. We report that the crystallization could be controlled by add ing 10(-5) to 10(-4) M sodium hydroxide, which increased the surface c harge density, sigma, of the silica particle. Conditions for crystalli zation were determined as a function of sigma and salt concentration. The grain size was large near the phase boundary between crystal and L iquidlike regions. The structure of the crystals was studied using an ultra-small-angle x-ray scattering method. Several orders of Bragg ref lection having a sixfold symmetry was observed for large grains. For s mall grains, the scattering profile was powderlike. In both cases, the crystals had body-centered-cubic lattice structures. The observed clo sest interparticle distances were smaller than average interparticle d istance, suggesting the non-space-filling nature of the crystal.