Chemistry Reference
In-Depth Information
The basic synthetic strategy involves the conden-
sation of an appropriate monomer (for silica species
this is typically (EtO)
4
Si (tetraethyl orthosilicate,
TEOS) or a salt of the SiO
4
2-
anion). The condensa-
tion is carried in the presence of a surfactant, which
exists as micellar aggregates before the start of reac-
tion or, more typically, forms micellar aggregates as
the hydrolysis reaction proceeds. The micellar aggre-
gates generally are thought to be rod-like and the
inorganic precursors assemble around these rods,
condensing to give a silica framework encasing the
micelles (Fig. 7.1). Depending on the nature of the
surfactant, these silica-micelle tubes can align with
each other to differing degrees, with charged surfac-
tants appearing to give the highest long-range order.
In the final stage of the synthetic procedure, the tem-
plate is removed from the composite material
formed, leaving what is essentially an amorphous
silica with regular cylindrical pores where the micel-
lar template was.
The fact that the pores are formed by the removal
of template allows the control of pore size by simply
adjusting the synthesis conditions to expand or con-
tract the micelle. This can be done by a number
of well-established methods (Fig. 7.2). Apart from
changing the chain length of the surfactant, the addi-
tion of co-surfactants or micelle-swelling agents can
change the pore diameter by an enormous extent.
Co-surfactants are typically short-chain alcohols that
become incorporated in the outer layer of the
micelle (the palisade layer). This allows the surfac-
tant tail to curl up somewhat, reducing the head-to-
tail distance and contracting the micelle. Similarly,
the use of aromatics such as mesitylene can have one
of two effects. In solvent mixtures of low polarity the
aromatic molecules can partition in the palisade
layer, again causing contraction of micelles and
lower pore size. On the other hand, in highly polar
mixtures in which the aromatic is insoluble, the aro-
matic will partition in the centre of the micelle and
thus minimise solvent-aromatic interactions. This
means that the micelle swells and the pore diameter
is increased. This effect was utilised in the initial
work of Beck
et al
., who prepared materials with
pore sizes of up to 10 nm by this route. Similar phe-
nomena have been demonstrated using alkanes as
swelling agent.
The choice of surfactant is an important feature of
the synthetic design. A variety of approaches have
been developed and these have been categorised into
an overall framework by Stucky
et al
. [36,37]. The
possible systems that have been defined are indicated
in Fig. 7.3.
The most commonly utilised methods involve
three groups of surfactants—the quaternary ammo-
nium salts (S
+
), the neutral amines (S
o
) and the block
copolymers (N
o
). The block copolymers are typi-
cally poly(ethylene glycols) endcapped with long
alkyl chains or poly(ethylene glycol-co-propylene
glycol-ethylene glycol) triblock polymers. The choice
Fig. 7.1
Idealised mechanism for the synthesis of
micelle-templated materials.
