The Fumed Silica Surface: A Study by NMR

High-resolution solid-state NMR techniques were used to
investigate the surface structure of Cab-O-Sil
fumed silica. 1H NMR results obtained from CRAMPS,
MAS-only and relaxation studies reveal the existence of
both hydrogen-bonded silanols and isolated silanols on the Cab-O-Sil
surface. A systematic dehydration study of
fumed silica was carried out, with results on the quantity of each type
of silanol on the surface at various dehydration
stages. 29Si CP-MAS experiments, including CP
spin dynamics studies and various other relaxation studies,
were
employed to probe hydrogen bonding and the local structural
environments of various hydroxyl groups of silica
surfaces. 29Si CP-MAS experiments on
water-treated and deuterium-exchanged Cab-O-Sil indicate the existence
of
interparticle silanols and internal silanols in fumed silica.
1H and 29Si NMR show that for fumed
silica both isolated
and hydrogen-bonded silanols are present on the surface of an untreated
sample, in contrast to the case of silica gel,
where all silanols of an untreated sample are hydrogen
bonded.