Time evolution of a sol-gel process monitored by Mn2+ EPR spectroscopy

The time evolution of the tetraethoxysilane (TEOS)-based sol-gel process was investigated by EPR spectroscopy of a Mn2+ spin probe. Analysis of the EPR spectra recorded at room temperature (RT) and liquid nitrogen temperature (77 K) showed that the values of the g-factor (for the central Mn2+ sextet) were found to be, within experimental error, identical during the sol-gel process evolution. However, the peak-to-peak half width, ΔBpp, of the spectral lines recorded at RT, the splitting of the forbidden hyperfine lines (and thus the zero-field splitting parameter, D) recorded at 77 K, and the averaged hyperfine splitting constant, Aav, at both temperatures, were found to change significantly during the various stages of the sol-gel process. These EPR spectral parameters allow us to monitor changes in the TEOS-based sol-gel reaction mixture during the liquid-to-solid-state transition. It was clearly demonstrated that the sol-to-gel transition as well as the hydration and dehydration process of gels could be successfully studied by Mn2+ EPR spectroscopy.