Aluminum Polyphosphate Thermoreversible Gels: A Study by31P and27Al NMR Spectroscopy

Abstract

The formation of thermoreversible gels from aqueous aluminum polyphosphate solutions was previously assigned to hydration-dependent interionic associations, but these were not elucidated. In the present work, aluminum-phosphate interactions were examined by31P- and27Al-NMR spectroscopy of aqueous aluminum polyphosphate solutions and gels, at various P/Al ratios and temperatures. The fraction of aluminum and phosphate groups actually involved in the sol–gel transition is small, as evidenced by the comparison of spectra of solutions at various P/Al ratios and temperatures, in which only minor spectral changes are actually associated with gel formation. Chemical groups responsible for network formation are mainly Al- (H2O)4(PO3)2groups, in which the phosphate ligands are polyphosphate chain-ends. Within the gels, a large fraction of total aluminum is found as hexaquoaluminum ions, as evidenced by27Al-NMR spectra; on the other hand,31P-NMR spectra show that most phosphate ions are associated. These data indicate that bonding and bridging involve both inner- and outer-coordination sphere interactions. The former depend on ligand exchange reactions with half-life on the order of magnitude of a few seconds, which explains the time dependence of thermoreversible sol–gel transformations, in this system.