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1 Bulletin of the Polish Academy of Sciences. Chemistry

1 2000

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EPR spectroscopic and rheological investigation during sol-gel transition in the systems TEOS/ethanol/water and sodium water-glass

Breyer T., Breitbarth F. W., Vogelsberger W.

Bulletin of the Polish Academy of Sciences. Chemistry

2000

Vol. 48, No. 4

325-336

Paramagnetic ions (V4+ , Mn2+ ) are used to detect mobility changes during the sol-gel transition of sols from sodium water glass as well as from TEOS hydrolysis. The viscosity of these samples is measured in the rotational and oscillating mode to determine the steady state viscosity and the storage and loss moduli. The overall kinetic rate constant for the formation of siloxane bonds has been determined in dependence on pH of the reaction mixture. In spite of the rapid increase of the viscosity during the solgel transition by several orders of magnitude, EPR spectra of solvated VO2+ and Mn2+ probe ions do not show significant changes of the mobility. These ions move freely in the space between the agglomerating sol particles until the gels are dried. Paramagnetic V(IV) species derived from reduction of VO(iPr)3, however, are adsorbed on the growing sol particles. They, therefore, show a powder-like EPR spectrum of immobilized ions even in the region of very low sol viscosity well before the sol-gel transition.