Bismuth Oxyhalides are crucial to a number of
applications including BiOCl in cosmetics and
in cracking of butane and BiOI as
“components for color filters in transparent
nanocomposite materials”14
. Like other
nanosized particles, their shape, size and size
distribution are crucial to their applications in
“catalysis, electronics, miniaturization, and
ceramics14”.
Reverse microemulsions are often used in the
preparation of nanoparticles to avoid broad
particle size distributions that often result
from precipitation. A reverse microemulsion
differs from an emulsion in that the tail
groups of the surfactants orientate outwards
into the continuous phase and their head
groups towards the dispersed phase.
A relatively new method of using
microemulsions to synthesis the nanoparticles
is through a multi-microemulsion route. Such
a route involves sets of microemulsions with
the same water, oil and surfactant used and
similar ratios of the 3 components, but with
each of the reactants for the nanoparticles
dissolved in the aqueous phase of different
microemulsions. Intermicellar exchange of
reactants present in the reverse micelles
subsequently occurs and the nanoparticles
form. (See illustration on next page.) Studies
show that “the multi-microemulsion routen
yields finer particles and narrow size
distributions.15”
Fig 2. Multi-microemulsions route of synthesising nanoparticles.
Courtesy of Henle(2007)14
By varying the concentrations of the salt
solutions in the aqueous phase and the water
to surfactant ratio, Henle and team (2007)
managed to characterize and formulate the
relationship between those conditions and
the particle size in the range of 3 to 22nm;
allowing them to manipulate characteristics of
the nanoparticles such as “the band gap,
absorption edge, and color of BiOI
nanoparticles”.
Further on, processing and preparations of
other nano-sized substances also employed
the use of microemulsions such as for
nanocrystalline hydroxyapatite16 and for
single crystal BaMoO4 nanofibers17
.
The ability to narrow the size distribution,
create finer nanoparticles, and control their
size using microemulsions also found an
application in the study of reaction kinetics in
catalytic processes. Some reaction kinetics
and mechanism are dependent on the catalyst
and/or its size and well-defined catalysts in
the nanometer range deposited on supports
are very often indispensible to study them.
It has been found that water-in-oil
microemulsions are particularly advantageous
over other methods of preparing nanoparticle
catalysts on supports, such as electron beam
lithography, colloidal lithography and spincoating, as they “can be formed at
atmospheric pressure and at room
temperature and that large sample volumes
relatively easily can be obtained.18” This
process involves adding support to the
microemulsion suspension and subsequently
destabilising the microemulsion by the
addition of a destabilizer such as
tetrahydrofurane which removes the
surfactant.
surfactantsandmicroemulsions 