Gelificação termorreversivel em soluções aquosas de polifosfato de aluminio

AUTOR(ES)
DATA DE PUBLICAÇÃO

1995

RESUMO

Admixture of aqueous sodium polyphosphate solutions and aluminum salts (nitrate, sulfate and chloride) lead to the formation of thermoreversible gels. Gels formed above roam temperature change back to clear solutions, upon cooling. For some compositions, gel formation happens at roam temperature. This behavior was observed in polymeric systems, but it has not been previously described for a purely inorganic system. Gel formation was observed using polidisperse sodium polyphosphate as well as various polyphosphate fractions obtained by fractional precipitation and characterized by P- NMR spectra. The average degree of polymerization of the fractions ranged from 5.9 to 19.8. The temperature of gel formation depends on the actual P/AI ratio, on polyphosphate chain size and on the other anions within the solution. The aluminum-polyphosphate ínteractions were examined by P-NMR and AI- NMR spectroscopy of aluminum polyphosphate gel-forming solutions, at various P/AI ratios and temperatures. The fraction of aluminum and phosphate groups actually involved in the sol -gel transformation is small, as evidenced by the comparison of spectra of solutions of various P/AI ratios and temperatures, which shows that only minar spectral changes are associated with gel formation. This is confirmed by conductivity measurements. Chemical complexes responsible for network formation are mainly AI (H2O)4 (PO3)2 groups, in which the phosphate ligands are chain terminais. Within the gels, there is always a large fraction of Al(H2O)6 groups, as evidenced by AI- NMR spectra; on the other hand, P-NMR spectra show that most phosphate ions are bound, Times required for gel liquefaction, which is obtained by cooling, increase as the gel formation temperature and the time during which lhe gel was kept at higher temperatures increases. Gels formed at roam temperature undergo syneresis, yielding viscous fluids that can be molded and beco me glassy by drying at roam temperature. Dried gels and glasses were examined by chemical analysis, infrared and Raman spectroscopy. Energy-loss spectroscopic imaging of glass fragments revealed the existence of aluminum-richer domains, disperse in a phosphate-rich matrix. A model is proposed for gel formation, which main, features are: i) gelation is due to the formation of a supramalecular ionic network, by assaciation of the high-cannectivity aluminum and polyphosphate ions; ii) temperature-dependent gelation is related to the large heats of hydration af both anions and cations, with cause a sharp dependence af the extent of ion association with the temperature.

ASSUNTO(S)

coloides solução (quimica)

ACESSO AO ARTIGO

Documentos Relacionados