Surface Chemistry of Hydrophobic Silica Aerogels

The properties of silica aerogels, and the ability to prepare them by ambient pressure drying, hinge on their chemical surface modification. Until now, quantitative analysis of the surface chemistry has not been possible by state-of-the-art analytical methods. Here, we determine the surface chemistry of six archetypal, ambient pressure dried, hydrophobic silica aerogels and open-porous foams with quantitative 1H, 13C, and 29Si and 1H–29Si heteronuclear solid-state NMR spectroscopy. The quality of the external calibration, the validation by elemental analysis, and the consistency among the 1H, 13C, and 29Si MAS NMR data enable us to robustly quantify the surface chemistry of these classical silica materials. Four aerogels were derived from tetraethoxysilane (TEOS), polyethoxydisiloxane (PEDS), or waterglass precursors and hydrophobized with trimethylsilyl (TMS) groups through immersion in hexamethyldisilazane (HMDZ) in heptane, trimethylchlorosilane (TMCS) in heptane, or hexamethyldisiloxane (HMDSO) in ethanol. The resulting aerogels display remarkably similar chemistries despite their very different precursors (alkoxides, waterglass), hydrophobization agents (HMDZ, TMCS, HMDSO), and gelation and hydrophobization solvents (water, ethanol, heptane). The TMS content is nearly the same (22–27 wt %), the ethoxy content is low (<4 wt %), and the silica speciation displays a narrow range for all four aerogels. The 1H–29Si heteronuclear correlation spectra provide unambiguous evidence for the grafting of the TMS groups on the silica surfaces. Remarkably, the NMR data enable the prediction of the silica particle size to within 1 to 2 nm of that derived by TEM analysis. Two additional, inherently hydrophobic, ambient pressure dried, open-porous silica foams were synthesized from methyltrimethoxysilane (MTMS) and from a MTMS–dimethyldimethoxysilane (DMDMS) mixture. The reduction in the number of bridging oxygens per Si atom, from 3.76 to 3.90 for TEOS, PEDS, or waterglass based aerogels, to 2.86 for the MTMS based open-porous foam, and to 2.48 for the MTMS–DMDMS derived open-porous foam, results in a brittle-to-superflexible transition. Our results highlight the importance of the chemical treatment of the wet gels (hydrophobization and solvent exchange) on the final surface chemistry of silica aerogels.