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

Abstract

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 H 2O/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 H 2O 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 H 2O are immobilized in the silica network. The bulk structure of the dried gel skeleton resembles the surface layer of colloidal silica particles.