Abstract
The various stages of the tetraethyl orthosilicate-based sol-gel process were studied using Gd(III) cations as a spin probe, using EPR spectroscopy at ambient and liquid nitrogen temperatures. The Gd(III) EPR spectra (being of the 'U-spectrum' type) were very similar, as recorded at 77 K, at the various defined stages of the sol-gel process. The analysis of the experimental EPR spectra showed that as the sol-gel process proceeded, the spin Hamiltonian parameters remained unchanged, but the relative amplitude of the alpha-line (at g similar to 2.8) and beta-line (at g similar to 2.0) changed significantly, such that the alpha/beta-ratio decreased progressively. This is the first report that the silica sol-to-gel transition from the low viscosity, starting reaction mixture, to the high viscosity, colloid suspension, up to the viscoelastic gel and finally to the dry gel, can be successfully monitored using Gd(III) EPR spectroscopy.