Chemistry Reference
In-Depth Information
d
n
1
r
3
n
g
|
5
Chart 8.12
3
.
Figure 8.6
SEM images of the dried samples prepared under acidic (a) and basic (b)
conditions; SEM images of calcined samples prepared under acidic (c) and
basic (d) conditions.
Reprinted with permission from ref. 53. Copyright 2000 American
Chemical Society.
species were not adsorbed onto cationic aggregates of 26 and no fibrous
structure was observed.
The chiral aggregates of organogelators 27 and 28 (Chart 8.12) containing
ClO
4
-
as the counteranion were successfully employed as chiral templates in the
sol-gel polymerisation.
54
The metal (Ta, V) alkoxides afforded tubular helical
Ta
2
O
5
and V
2
O
5
structures. The helices of metal oxide fibres could be
controlled by the chirality of the cationic amphiphile.
In all of the previous examples discussed above, positively charged gelators
have been primarily used as template for sol-gel condensation because the
corresponding gel fibres will electrostatically attract the negatively charged
precursors produced at the initial stage of the polymerisation. In this context,
Suzuki and coworkers described the preparation of TiO
2
nanostructures in
organogels based on uncharged gelators. The sol-gel polymerisation of
Ti(O
i
Pr)
4
in the organogels of 29 (Chart 8.13) containing propylamine as a
catalyst resulted in the formation of TiO
2
nanotubes.
55
Propylamines reacted
with the -COOH groups in 29, resulting in the formation of charged nanofibres
and sol-gel polymerisation took place on the nanofibres (Figure 8.7). Whereas
the negatively charged gelator 30 produced uniform sized TiO
2
nanoparticles as
the polymerisation on the negatively charged nanofibres of gelator 30 is in-
hibited due to the electrostatic repulsion between nanofibres and sol-gel
precursors resulting in their inability to act as a template.
