Revisiting structure of silica gels from water glass: An ¹H and ²⁹Si MAS and CP-MAS NMR study

The silica gels, derived from water glass solution with pH adjusted at 3.0 and 9.9, were revisited to investigate their constitution, although water glass has been studied for last tens of decades on gelation. Solid-state nuclear magnetic resonance spectroscopy was applied to the nuclei ¹H and ²⁹Si, by the use of magic angle spinning (MAS), ¹H → ²⁹Si CP-MAS (CP: cross-polarization), and modern techniques such as 2D HETCOR (two dimensional heteronuclear correlation), and variable-contact time CP techniques. Gelation time (t_gel) showed U-letter shape dependence on pH. All gels consisted of Qⁿ groups (n: 2, 3, and 4), where Qⁿ stands for a silicate unite [(O_1/2)_nSi (-O^-)_4-n] (n: 0-4). The analysis of the ¹H → ²⁹Si CP kinetics and ¹H-²⁹Si HETCOR spectra elucidated the presence of four kinds of ¹H nuclei, i.e., those giving a peak at 6.9 ppm in chemical shift δ: ¹H-OSi hydrogen bonded to H₂O molecules; one at 4.3 ppm: ¹H of adsorbed water molecules, hydrogen-bonded to the silanol groups; one at 1.7 ppm: ¹H-OSi confined in the gel lattice, including that forming aggregations like Si-O^H/_NaO-Si; and one at 4.2 ppm: ¹H of water molecules on the outermost hydration layer.