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, N2 physisorption, thermogravimetric analysis, infrared spectroscopy, as well as 29Si and 13C 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 1H 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.