Monitoring the Tetraethyl Orthosilicate (TEOS)-Based Sol–Gel Process with Cu(II) Ions as a Spin Probe

The various stages of the tetraethyl orthosilicate-based sol–gel process were investigated using electron paramagnetic resonance (EPR) spectroscopy with Cu(II) cations as a spin probe. The latter were introduced to the starting reaction mixture in the form of various copper(II) salts containing anions of different basicity (CuCl 2 , Cu(ac) 2 , or CuSO 4 ). At the various defined stages of the sol–gel process, the experimental EPR spectra, recorded at both ambient and liquid nitrogen temperature, were found to be a superimposition of three main types of individual subspectra ( Γ 1 , Γ 2 , and Φ ), which reflect the different local environment in which the Cu(II) ions were located. The spin Hamiltonian parameters of each individual subspectrum remained identical, within experimental error, throughout the various stages of the sol–gel process. In contrast, the relative proportion of the individual subspectra varied significantly as the sol–gel process proceeded, from which the liquid-state to solid-state transition could be monitored as it occurred in the sol–gel reaction medium. Identical results were obtained, irrespective of the nature of the copper(II) salt employed. The results demonstrate that the EPR method provides an effective means with which to monitor the sol-to-gel transition from the viscous, colloid suspension to the final viscoelastic gel.