The (100) face of β-cristobalite covered by geminal hydroxyls
has been taken as a model for the silica surface
in order to describe the hydrogen bonding and the condensation of the
surface hydroxyls of silica. Periodic
Hartree−Fock calculations have been performed for slabs of five
atomic layers. The optimized orientation
of the hydroxyls corresponds indeed to hydrogen bonds at the surface.
As a result, the surface atoms are
slightly more negative and the hydroxyls have a specific orientation:
one pointing outside the surface plane
and the other one directed toward the bulk. The H-bond energy is
evaluated to 25 kJ·mol-1. The
dehydration,
leading to siloxane bridges, is endothermic by ≈200
kJ·mol-1. It must be accompanied by a
surface
restructuring implicating more than only the first silicon and oxygen
layers.
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