Abstract
The various stages of the TEOS-based sol-gel process were studied by EPR spectroscopy using a VO(II) spin-probe. A detailed analysis of the multicomponent VO(II) EPR spectra recorded at 77 K showed that three main types of individual signals appeared superimposed at the various stages of the sol-gel process, which corresponds to different environments of the paramagnetic VO(II) cations. The spin Hamiltonian parameters of each individual subspectrum remain unchanged throughout the various stages of the sol-gel process. However, the relative contribution of individual signals changed significantly as the sol-gel process proceeded. The results show that the VO(II) cations are not directly incorporated into the siloxane network, but are trapped relatively freely as [VO(H2O)(5)](2+) complexes in a solution-like environment during the evolution of the sol-gel process. It was amply demonstrated that the sol-to-gel transformation from the viscous, colloid suspension to the viscoelastic gel can be successfully monitored by EPR spectroscopy with VO(II) as a spin-probe.
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