Aqueous Zirconium Complexes for Gelling Polymers. A Combined X-ray Absorption Spectroscopy and Quantum Mechanical Study

Abstract

The reinforcement of regulations concerning environment protection in most countries is a strong incentive to use “green” cross-linkers for sol−gel processes. Among them, water soluble zirconium complexes seem attractive for various applications in the surface coatings, oil production, sol stabilization, and agriculture industries. Moreover, recent works demonstrated that a zirconium lactate−polyacrylate solution leads to the formation of size controlled microgels under shear flow. The speciation of zirconium, both in aqueous solutions (zirconium lactate) and in polymer gels (a terpolymer of acrylamide), was determined by combining X-ray absorption spectroscopy (XAS) and quantum mechanical calculations using density functional theory. XAS experiments were performed at the Zr−K-edge at the European Synchrotron Radiation Facility by using fluorescence detection in order to get results down to very low concentrations (37 ppm). The X-ray absorption near edge structure (XANES) results show that Zr is mainly surrounded by eight oxygens in the first coordination sphere, forming a dodecahedron. The analysis of extended X-ray absorption fine structure (EXAFS) spectra combined with quantum mechanical calculations results indicates that, in the concentrated zirconium lactate solutions (53300 ppm), which was found to be stable over years, the zirconium complexes are dimers ((Zr) 2 (lactate) 6 ) surrounded by six bidentate lactate ligands. As zirconium lactate concentration is decreased, the dimers condense first in cyclic tetramers (Zr 4 (lactate) X ) and then in larger oligomers by tetramer association. In polyacrylamide microgels ([Zr] = 74 and 148 ppm) the Zr species remains dimers when gelation is completed at pH = 6, whereas tetramers are also observed at pH = 7. The XAS spectra were characterized by a very high signal-to-noise ratio, making possible to observe the gelation, i.e., the exchange of lactate by acrylate ligands.