Thermostimulated Sol-Gel Transition in Suspensions of Sulfate-Zirconium Oxychloride

The sol-gel transition of an aqueous solution of zirconium oxychloride modified by sulfuric acid (Zr:S 3:1) has been studied by small-angle X-ray scattering (SAXS) performed in situ during one cycle of heating and cooling between 298 and 358 K. The experimental SAXS curves exhibit three regions, at small, medium and high q values, characteristic of the Guinier, 'fractal' and Porod regimes, respectively. The value of 5.5 Å for the radii of the primary particles, obtained from the cross over of the Porod and fractal regimes, is consistent with the size of the inner core of the polynuclear Zr₁₈O₄(OH)_38.8(SO₄)_12.6.33H₂O molecule. These molecules aggregate as small clusters (31 Å) of fractal structure, with a dimensionality D = 2.16 in the sol. The value of the fractal dimensionality, which is characteristic of ideal branched polymers, decreases during heating, attaining a value of 2.0, characteristic of swollen polymers. Above the critical temperature (329 K), the average size of aggregates increases abruptly to 200 Å and the fractal dimensionality decreases to D = 1.75, this value being characteristic of a cluster-cluster aggregation process.