Study of structural evolution of silica gel using ¹H and ²⁹Si NMR

Silica sols and gels were prepared by hydrolysis and condensation of tetraethyl orthosilicate (TEOS) under acid conditions. By use of ²⁹Si and ¹H NMR the chemical evolution of the system was followed through gelation, aging, and drying at temperatures ≤ 50 °C. The effects of higher H₂O/TEOS ratios, refluxing, and stripping on the promotion of short- and long-term condensation and gelation were characterized. The extent of crosslinking (i.e., the number of siloxane bonds) was found to increase very slowly after gelation. The driest gels still retained about one SiOH per two Si atoms. The proton spectra showed that the dynamic state of the protons in dried gels correlates well with the residual water content, but not with the details of the drying treatment: when the moisture level exceeds 0.5 H₂O per Si atom, the water is uniformly distributed throughout the drying gel network, where it provides a mechanism for high diffusional mobility of all the silanol and water protons. When less than 0.2 free-water molecules per Si remain in the gel, the hydroxyls of both SiOH and residual H₂O are immobilized in the silica network. The bulk structure of the dried gel skeleton resembles the surface layer of colloidal silica particles.