Environmental Engineering Reference
In-Depth Information
Hydrolysis
Si
OR
+
HOH
Si
OH
+
ROH
Water
condensation
Si
OH
+
Si
OH
Si
O
Si
+
HOH
Alcohol
condensatio
n
Si
OH
+
Si
OR
Si
O
Si
+
ROH
FIGURE 8.1
Sol-gel process begins with hydrolysis of alkoxysilane. Silanol can condense with another silanol or an alkoxy
group to form a Si-O-Si linkage.
condensation, resulting in the three-dimensional, porous siloxane network that constitutes
the gel. The solvent in the interstitial space is then removed by evaporation to create solids
that have various nanoscale to microscale morphologies, possessing chemical functional
groups based on the choice of precursor. A wide range of materials can be produced; from
dense glasses, to highly ordered porous materials, to ultralow density aerogels.
8
The sol-
gel process is a convenient method to prepare hybrid inorganic-organic materials with
unique properties both in terms of chemical composition and physical microstructure.
There is a substantial variety of organically modiied alkoxysilanes possessing different
types of chemical groups, which facilitates a wide diversity of materials to be prepared
using a common synthetic scheme. Bridged precursors are a subset of organically modi-
ied silanes where two silicon centers are tethered by an organic group (RO)
3
-Si-Y-Si(OR)
3
,
where Y is the bridging group and -OR is the polymerizable alkoxide group. The advantage
to such a structure is that polymerization can radiate from two directions and the bridge
group can act as a structure directing unit.
9,10
Polycondensation of bridged precursors have
been reported and subsequently used to create nanostructured materials.
11,12
Control of
the sol-gel processing conditions (e.g., solvent, catalyst, aging temperature) can be used
to tailor the texture of the resulting sol-gel-derived solids, which is advantageous when
designing new materials.
13
Highly animated materials have been created by using molecu-
lar self-assembly as a structure-directing process to control the nanomechanical properties
of polymeric organosilicate.
14
The resulting material is commercially available as Osorb.
Osorb is a nanoengineered sol-gel-derived material that rapidly swells about four to ive
times its dried volume and swells up to eight times its dry mass in nonpolar solvents, but not
water. Before the discovery of Osorb, sol-gel-derived solids were characterized as inelastic
and not able to swell after preparation. The highly animated nature of Osorb is derived
from the polymerization of a bridged organosilane precursor bis(trimethoxysilyethyl)
benzene (Figure 8.2) under carefully controlled conditions, including the use of THF as a
OCH
3
OCH
3
Si
OCH
3
H
3
CO Si
OCH
3
OCH
3
FIGURE 8.2
BTEB structure.
