common to all gel types is the mobility reduction of network The formation of thermoreversible gels from aqueous alumielements, as compared to that in a solution or dispersion. num polyphosphate solutions was previously assigned to hydra-On the other hand, a large volume fraction of a gel may be tion-dependent interionic associations, but these were not elucimade of plain interstitial solution, in which ion and small dated. In the present work, aluminum-phosphate interactions molecule diffusion coefficients are in the same order of magwere examined by 31 P-and 27 Al-NMR spectroscopy of aqueous nitude as in a liquid solution.
aluminum polyphosphate solutions and gels, at various P / Al ratios and temperatures. The fraction of aluminum and phos-In a previous work ( 5 ) , we found that an admixture of phate groups actually involved in the sol -gel transition is small, sodium polyphosphate and aluminum salt aqueous soluas evidenced by the comparison of spectra of solutions at various tions leads to the formation of thermoreversible gels, P / Al ratios and temperatures, in which only minor spectral which are formed above room temperature and change changes are actually associated with gel formation. Chemical back to clear solutions upon cooling. Within 3 § P / Al § groups responsible for network formation are mainly Al-2 molar ratios, gels are stable at room temperature, but in ( H 2 O ) 4 (PO 3 ) 2 groups, in which the phosphate ligands are polyboth sides of this range ( 4.3 § P / Al ú 3 and 2 ú P / Al § phosphate chain-ends. Within the gels, a large fraction of total 1.3 ) solutions form thermoreversible gels at temperatures aluminum is found as hexaquoaluminum ions, as evidenced by which depend on the actual P / Al ratio and on the other 27 Al-NMR spectra; on the other hand, 31 P-NMR spectra show that most phosphate ions are associated. These data indicate anions within the solution. Gel formation was observed that bonding and bridging involve both inner-and outer-coordiusing various polyphosphate fractions; it is also dependent nation sphere interactions. The former depend on ligand exon polyphosphate chain size and is more pronounced when change reactions with half-life on the order of magnitude of a oligomers ( e.g., pentamer to decamer ) are used. The rate few seconds, which explains the time dependence of thermoreof syneresis was determined and was found to be depenversible sol -gel transformations, in this system. ᭧ 1995 Academic dent on the inertial field. Transmission electron micros-Press, Inc.