Abstract
Admixture of sodium polyphosphate and aluminum salt aqueous solutions leads to the formation of thermoreversible gels. Gels formed above room temperature change back to clear solutions upon cooling. Gel formation was observed using various polyphosphate fractions obtained by van Wazer's method of fractional precipitation and characterized by 31P NMR spectra. Gel formation is dependent on polyphosphate chain size and is more pronounced when oligomers (e.g., pentamer to decamer) are used. The rate of syneresis was determined, and was found to be higher under centrifugation. Glassy, dried gels were examined by energy-loss spectroscopic imaging; this revealed the existence of aluminum-richer domains dispersed in a phosphate-rich matrix. Thermoreversible gel formation is rationalized by considering the thermodynamics of ion hydration, together with the large connectivity of both cations and anions, in aluminum-polyphosphate systems.
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