Study of Molecular Interactions and Dynamics in Thin Silica Surface Layers by Proton Solid-State NMR Spectroscopy

We present results from proton double-quantum magic-angle spinning NMR spectroscopy in support of tight molecular contacts of the modifier bis(triethoxysilylpropyl) tetrasulfane with the surface of precipitated, amorphous silica. While chemical bonding cannot be proven directly, its presence is indicated by through-space dipolar contacts between surface-OH groups and all aliphatic protons of the modifier, as well as strongly anisotropic motions of the surface-bound molecules. Silica−modifier contacts are further only found when the samples were heated during preparation. Only small amounts of ethanol were seen to leave the silica surface upon modification. A good part of the ethanol set free upon condensation becomes bound to the silica surface, which is supported by the presence of tight contacts in silica treated with pure ethanol. The anisotropy of molecular motions of the surface-bound molecules is characterized by the estimation of proton dipole−dipole coupling constants from double-quantum sideband patterns and build-up curves. Self-condensed samples of the modifier either exhibit very different dynamic properties or show no indication of contacts between hydroxyl and aliphatic protons.