NMR Investigation of Water in Mesoporous Silica Materials With Different Surfaces

The influence of the hydrophilicity of mesoporous silica on wetting with water was investigated by comparing a standard hydrophilic MCM‐41 and a partially hydrophobicized material obtained by trimethylsilylation of MCM‐41. The materials were characterized using X‐ray diffraction, N ₂ physisorption, thermogravimetric analysis, infrared spectroscopy, as well as ²⁹ Si and ¹³ C solid‐state nuclear magnetic resonance spectroscopy. Trimethylsilylation leads to a reduction of the pore diameter (determined using density functional theory) from 3.8 to 3.6 nm. Using proton solid‐state nuclear magnetic resonance (NMR) under magic‐angle spinning, it was found that the filling of these narrow pores with increasing amounts of water follows an axial mode, independent of surface hydrophilicity. The axial model of pore‐filling is supported by the coexistence of two characteristic ¹ H NMR water signals, one from physisorbed water molecules, which can exchange protons with silanol groups, and one from bulk‐like water in completely filled pores, where water molecules are in exchange between sites at the pore wall and sites closer to the center of the pore.