Hydrophobization of Silica Aerogels: Insights from Quantitative Solid-State NMR Spectroscopy

Silica aerogels have exceptional physical and chemical properties related to their nanoporous structure and high specific surface area. The hydrophobization of the silica surfaces, for example by modification with trimethylsilyl groups (TMS), is of central importance for silica aerogel production by ambient drying methods, particularly on an industrial scale. This study monitored the chemical modification of silica aerogels by quantitative and two-dimensional solid-state NMR spectroscopy. A series of two-step, acid/base-catalyzed silica alcogels were hydrophobized for different times in hexamethyldisiloxane (HMDSO) and subsequently dried at ambient pressure. Two-dimensional H-1-Si-29 heteronuclear correlation NMR spectroscopy confirms that both ethoxy and TMS groups are chemically bonded to the silica surfaces. For the single-pulse spectra, a procedure to calibrate the absolute H-1, C-13, and Si-29 NMR signal intensities with external references was developed. The quantification procedure is validated by the internal consistency between the H-1, C-13, and Si-29 results and the agreement between the measured sample mass and that predicted from the NMR data. The quantitative speciation data on the aerogel samples show that the silica surface is covered by a monolayer of ethoxy, TMS, and silanol groups. Silanol groups are progressively replaced by TMS groups with increasing modification time, and the TMS content has a strong effect on the density of the final aerogel.