By study
ing
1H
![](/<font color=)
images/ent
it
ies/rarr.g
if">
29S
icross-polar
izat
ion dynam
ics of two untreated and two "dry" s
il
icagel samples (oneevacuated at 25
![](/<font color=)
images/ent
it
ies/deg.g
if">C and one evacuated at 200
![](/<font color=)
images/ent
it
ies/deg.g
if">C), we f
ind that allthe surface s
ilanols on the two untreated s
il
icasare hydrogen bonded, e
ither to the hydroxyl groups of adjacents
ilanol(s) or to water molecule(s). About 46%and47% of the gem
inal s
ilanols and 53% and 58% of the s
ingle s
ilanolsthat were hydrogen bonded only to water
in thetwo untreated s
il
icas become non-hydrogen bonded on the two "dry"s
il
ica surfaces, but the rema
inder of the s
ilanolsof the untreated s
il
icas (
i.e., those hydrogen bonded to others
ilanols) rema
in hydrogen bonded to other s
ilanolsupon dry
ing. The rat
io of the number of hydrogen-bond
ing s
ingles
ilanols to the number of hydrogen-bond
inggem
inal s
ilanols
is 17-to-1 for a F
isher s
il
ica surface evacuated at 25
![](/<font color=)
images/ent
it
ies/deg.g
if">C and 16-to-1 for a Baker s
il
ica surfaceevacuated at 200
![](/<font color=)
images/ent
it
ies/deg.g
if">C. These results can be expla
ined
in terms ofa general
ized s
il
ica surface model based on the
![](/<font color=)
images/g
ifchars/beta2.g
if" BORDER=0 ALIGN="m
iddle">-cr
istobal
ite crystal structure.