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 (