Small-angle X-ray scattering study of the smart thermo-optical behavior ofzirconyl aqueous colloids

The smart thermo-optical systems studied here are based on the unusual ther moreversible sol-gel transition of zirconyl chloride aqueous solution modif ied by sulfuric acid in the molar ratio Zr/SO4:3/1. The transparency to the visible light changes during heating due to light scattering. This feature is related to the aggregates growth that occurs during gelation. These rev ersible changes can be controlled by the amount of chloride ions in solutio n. The thermoreversible sol-gel transition temperature increases from 323 t o 343 K by decreasing the molar ratio Cl/Zr from 7.0 to 1.3. In this work t he effect of the concentration of chloride ions on the structural character istics of the system has been analyzed by in situ SAXS measurements during the sol-gel transition carried out at 323 and 333 K. The experimental SAXS curves of sols exhibit three regions at small, medium and high scattering v ectors characteristics of Guinier, fractal and Porod regimes, respectively. The radius of primary particles, obtained from the crossover between the f ractal and Porod regimes, remains almost invariable with the chloride conce ntration, and the value (4 Angstrom) is consistent with the size of the mol ecular precursor. During the sol-gel transition the aggregates grow with a fractal structure and the fractal dimensionality decreases from 2.4 to 1.8. This last value is characteristic of a cluster-cluster aggregation control led by a diffusion process. Furthermore, the time exponent of aggregate gro wth presents values of 0.33 and 1, typical of diffusional and hydrodynamic motions. A crossover between these two regimes is observed.