- K. Saalwächter, M. Krause, W. Gronski.
Molecular interactions and dynamics in thin silica surface layers by proton
solid-state NMR spectroscopy.
Chem. Mater. 16, 4071-4079 (2004).
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 exhibit either very different dynamic properties or show no
indication of contacts between hydroxyl and aliphatic protons.