Polyphosphate Nanoparticles and Gels
Amorphous metal (Al, Ca, Fe, Zr) polyphosphate particles and monolyths may be obtained in a range of chemical compositions, by admixture of aqueous solutions of sodium polyphosphate, metal salts and ammonium hydroxide.
In the Al-polyphosphate system, phosphate-rich solids (P/A1>1.5) are soft glasses, which contain sodium ions, too. Aluminum-rich particles are refractory and have a low electrical conductivity up to 800–1000°C, which is assigned to a restrained ion mobility.
Two different, macroscopically opposite behaviors are also observed in the preparation of these materials: in one case, monolythic gels are obtained, and these are easily dried into glasses. In the other case, nanoparticles are formed.
Most reports on nanoparticles preparation in the literature use kinetic control to limit particle size. This is the case of the well-known Szigmondy colloidal gold particles, as well as the recent reports on nanoparticle synthesis using aerosols and templates. In the present case, the nanoparticles are obtained under dynamic quasi-equilibrium dissolution-precipitation conditions, in the presence of excess reagents in the aqueous medium. For this reason, we propose that their small size is due to a low interfacial tension, as e.g. in microemulsions.
This assumption is strengthened by the observation of thermoreversible gel formation, from aluminum polyphosphate solutions: a thermoreversible gel is the result of the reversible formation of a highly solvated supramolecular network, in which interfacial tension between different domains is necessarily low and does not contribute a significant driving force for further phase seggregation.
Aluminum polyphosphate powders containing nanoparticles can be cold-pressed into translucent, low-porosity and mechanically resistant macroscopic solid discs and tubes. This attractive property is assigned to two factors: the particle nanodimensions and the large water content of the powders. Together, these may open a new path towards low temperature materials processing.
KeywordsInterfacial Tension Nanoparticles Preparation Aluminum Phosphate Particle Coalescence Microscope Grid
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