Chemistry Reference
In-Depth Information
isolated
terminal
H-bounded
internal
3737 cm
-1
3550 cm
-1
3715 cm
-1
3660 cm
-1
H
H -
O
Si
O-H….
O-H
...O-H
O
O-H….O-H...
O-H
Si- O
H
Si
Si Si Si
Si Si Si
ν
OH
(
+
)H
2
O
ν
δ
3715
3660
3550
5270
3737
0.1
(
ν
+
δ
)OH
0.5
δ
H
2
O
4520
1630
5400
5000
4600
4200
3500
3000
2500
2000
1500
(cm
-1
)
(cm
-1
)
Figure 17.2 (a) Infrared spectra (transmission mode) of pristine silica in the 1400-6000 cm
1
range; dotted line, spectrum recorded at room atmosphere; full line, spectrum recorded after
outgassing the sample under vacuum during 3 hours at 100
C. Reproduced by permission J.
Phys. Chem. C 2008, 112, pp. 1094310951, American Chemical Society
area, the concentration of molecules at the surface (n
H2O
per nm
2
) can be calculated
(Table 17.3). The decrease is less abrupt and leads to conclude that the fluorination
improves the hydrophobic character of silica.
Table 17.2 Determination by infrared spectroscopy of adsorbed water in pristine
silica and in samples fluorinated at room temperature with various F
2
/Ar molar
ratios. Fluorination conditions as in Table 17.1
n H
2
O(
m
mol g
1
)
n H
2
O per nm
2
Sample
Bulk H
2
O (wt.%)
SiO
2
8.0
4800
5.7
10F2-25
2.9
1650
2.3
20F2-25
1.0
550
0.9
50F2-25
1.2
650
1.3
60F2-25
0.8
450
0.8
70F2-25
1.2
650
1.4
80F2-25
1.7
950
2.1
Figure 17.3 shows IR spectra for dried silica after outgassing under vacuum of 10
4
Pa at
150 C for 3 hours. The intensity of the
(OH) band of the isolated Si-OH group at
3737 cm
1
becomes weaker upon fluorination, showing the lower concentration of isolated
hydroxyl groups. For the highest F contents the
(OH) band of isolated silanol group finally
vanishes whereas a small part of the one of H-bounded species (3660 cm
1
) persists.
Quantification of the dependence of the residual content of surface hydroxyl groups upon
the F
2
percentage was obtained from the area of the characteristic
þ
d (OH) band of silanol
