Structural investigation of hydrous titanium dioxide precipitates and their formation by small-angle X-ray scattering

The mechanisms of formation and structure of precipitates hydrolyzed from a queous solutions of titanium tetrachloride and titanium sulfate were studie d by small-angle X-ray scattering (SAXS). SAXS was found a powerful method for that purpose. The small colloidal primary particles that were present i n the solutions at room temperature started to aggregate when thermal preci pitation started. The aggregated particles coalesced until their radius was approximately 1 nm depending of the precipitation conditions to some exten t. After that, the aggregation continued producing either mass fractal or s urface fractal structures. The structure was mass fractal when primary part icle concentration was high enough and surface fractal when concentrations were closer to the equilibrium state. With a delay ionic titanium started t o precipitate. Because of its rather slow precipitation rate in the conditi ons of this study, only surface fractal aggregates were formed. The mass fr actal structure was found X-ray amorphous and the surface fractal aggregate s nano crystalline. The mass and surface fractal dimensions of these aggreg ates were 2.2 (1 s +/- 0.1) and 2.7 (1 s +/- 0.2), respectively. The "titan ic acids" were found to be aggregates of small titanium dioxide particles t hat have mass fractal structure in ortotitanic acid and surface fractal str ucture in metatitanic acid. The loose structure of the mass fractal aggrega tes causes the relatively easy solubility and the X-ray amorphous state fou nd in ortotitanic acid. On the contrary, the more compact structure of meta titanic acid explains its nano crystallinity and insolubility. The changing of ortotitanic acid to metatitanic acid upon aging is obviously a conseque nce of the restructuring of the primary titanium dioxide particles toward t he close-packed porous structure.