Role of the Surface State and Structural Feature in the Thermoreversible Sol−Gel Transition of a Zirconyl Aqueous Precursor Modified by Sulfuric Acid

• Published in: Chemistry of materials Vol. 16; no. 21; pp. 3995 - 4004
• Main Authors: Chiavacci, L. A, Santilli, C. V, Pulcinelli, S. H, Bourgaux, C, Briois, V
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 19.10.2004
• Subjects: Applied sciences; Chemistry; Condensed matter: electronic structure, electrical, magnetic, and optical properties; Condensed matter: structure, mechanical and thermal properties; Exact sciences and technology; General and physical chemistry; Physicochemistry of polymers; Physics; Polymers; Inorganic compounds; Organic compounds
• ISSN: 0897-4756; 1520-5002
• DOI: 10.1021/cm035258d

The sols produced by admixture of ZrOCl2 acidified solutions to hot H2SO4 aqueous solutions were studied to clarify the effects of Cl- and SO4 2- ions on the kinetic stability of nanoparticles and to obtain some new evidence concerning the mechanism of a thermoreversible sol−gel transition observed in this system. The study of suspensions prepared with different molar ratios R S = [Zr]/[SO4 2-] and R Cl = [Zr]/[Cl-] revealed domains of composition of formation of thermoreversible gels, thermostable sols, and powder precipitation. The effects of R S and R Cl on the structural features of nanoparticles and on the particle−solution interface were systematically analyzed for samples of thermoreversible and thermostable sol domains. Small-angle X-ray scattering measurements revealed the presence of small fractal aggregates in all samples of thermoreversible domains, while compact packing aggregates of primary particles are present in the thermostable sol. Extended X-ray absorption fine structure and elemental chemical analysis revealed that irrespective of the nominal value of R S and R Cl all studied samples of the thermoreversible domain are constituted by a well-defined compound possessing an inner core made of hydroxyl and oxo groups bridging together zirconium atoms surrounded on the surface by complexing sulfate ligands. ζ potentials of powders extracted by freeze-drying from the thermoreversible gel revealed a point of surface charge inversion attributed to the specific adsorption of SO4 2- ion. Thermoreversible gel formation is rationalized by considering the effect of the specific adsorption on the electrical double-layer repulsion together with the temperature dependency of the physical chemical properties of ions in solution.