Microemulsion-based synthesis of nanocrystalline materials · 2017-06-18 · Microemulsion-based synthesis of nanocrystalline materials Ashok K. Ganguli,* Aparna Ganguly and Sonalika
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Microemulsion-based synthesis of nanocrystalline materials
Ashok K. Ganguli,* Aparna Ganguly and Sonalika Vaidya
Received 23rd February 2009
First published as an Advance Article on the web 22nd September 2009
DOI: 10.1039/b814613f
Microemulsion-based synthesis is found to be a versatile route to synthesize a variety of
nanomaterials. The manipulation of various components involved in the formation of a
microemulsion enables one to synthesize nanomaterials with varied size and shape. In this
tutorial review several aspects of microemulsion based synthesis of nanocrystalline materials have
been discussed which would be of interest to a cross-section of researchers working on colloids,
physical chemistry, nanoscience and materials chemistry. The review focuses on the recent
developments in the above area with current understanding on the various factors that control
the structure and dynamics of microemulsions which can be effectively used to manipulate the size
and shape of nanocrystalline materials.
1. Introduction
The large number of unusual properties and applications
associated with nanomaterials has triggered enormous interest
among scientists from varied fields of research especially due
to the interdisciplinary nature of this subject. Nanoscience and
nanotechnology today is practised by chemists, biologists,
physicists, material scientists and engineers who have put
in tremendous efforts to understand new phenomena and
develop technologies in this field. A major contribution to
the development of this field has been made by chemists
working primarily on the theme to design and control of
nanostructures and also to functionalize them using both
low-temperature solution-based routes and high-temperature
(thermodynamic) methods. The microemulsion method is one
among the various low-temperature routes to tailor nano-
particles. The term ‘microemulsion’ was first coined by
J. H. Schulman in 1959,1 and since then its use has grown
considerably and has received justified acclaim from the
nanomaterials community. There have been several important
reviews published on this subject especially during 1993–2006
by Pileni,2 Eastoe,3 Lopez-Quintela,4 Capek,5 Holmberg,6 and
Uskokovic.7 In this review, we have given a brief introduction
to the concepts and principles involved in microemulsions and
their applications to nanomaterial synthesis. We then build on
the information available in the previous reviews and focus
on the developments in the past ten years, especially discussing
the current understanding on the various factors controlling
the structure and dynamics of microemulsions, and their
manipulation to control the synthesis of nanocrystalline
powders and related systems. In spite of the significant work
carried out earlier in this field, many aspects of microemulsion
Department of Chemistry, Indian Institute of Technology, Hauz Khas,New Delhi 110016, India. E-mail: ashok@chemistry.iitd.ernet.in;Fax: 91-11-26854715; Tel: 91-11-26591511
Ashok K. Ganguli
Prof. Ashok Kumar Ganguliobtained his PhD from theIndian Institute of Science,Bangalore in 1990. He sub-sequently worked at DupontCompany, Wilmington, USAand Ames Laboratory, IowaState University, USA. beforejoining IIT Delhi in 1995where currently he is a fullprofessor. His interests are inthe synthesis and properties ofnanocrystalline materials,complex metal oxides withdielectric and superconductingproperties and polar inter-
metallics. He has published over 125 papers in internationaljournals and around 15 in conference proceedings and books, andwas awarded the Materials Research Society of India Medal for2006, and the Chemical Research Society of India medalfor 2007.
Aparna Ganguly
Ms Aparna Ganguly obtainedher BSc (Hons) in chemistryfrom Sri VenkateshwaraCollege, University of Delhiin 2002. Later she obtainedher MSc in chemistry fromUniversity of Delhi withspecialisation in physicalchemistry in 2004. Currentlyshe is working as a joint PhDstudent of Prof. A. K. Ganguli(IIT Delhi) and Dr T. Ahmad(Jamia Millia Islamia) onmicroemulsion routes tosynthesize functionalisednanostructures.
474 | Chem. Soc. Rev., 2010, 39, 474–485 This journal is �c The Royal Society of Chemistry 2010
TUTORIAL REVIEW www.rsc.org/csr | Chemical Society Reviews
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based synthesis are yet to be understood, to cater to the
increasing demands of precisely tailored nanomaterials. Hence
a lot of excitement prevails among scientists to explore its vast
horizon. This review attempts to put into perspective the
understanding available at present, and to foresee future
developments, which may become popular and meaningful
in the coming years.
1.1 Colloidal solutions
The use of colloidal gold had started from the ancient Roman
period to colour glasses red, mauve or yellow by varying the
concentration of gold. Paracelsus, the alchemist of 16th
century is claimed to be the first to have prepared a gold
colloid (Aurum Potabile). Inspired by his work, Michael
Faraday8 prepared a gold colloid solution in 1857. The finely
divided gold particles exhibited different optical properties
which Faraday recognized as being dependent on the size of
the particles. Colloidal solutions have since become a topic
of intense research due to their interesting properties and
applicability. To prevent the particles from aggregating,
stabilizers such as citrate ions are added which are adsorbed
on the surface of the particles inducing a surface charge and
hence repulsion from other particles to prevent agglomeration.
This is a kind of electrostatic stabilisation. Steric stabilisation
can be achieved with bulky organic molecules being present on
the metal surface providing a protective shield, as is the case
with surfactants. The stabilizer should coordinate to the
particle strongly enough to prevent agglomeration but should
also be easily removable from the metal surface.
Colloidal solutions have found application in synthesis of
novel materials, plastics and ceramics, and more recently in
nanotechnology. Due to their optical and electronic properties
they find use as biosensors, especially gold colloids, which are
being extensively studied for this purpose.9 Among other
important applications, silver colloids have been found to
have anti-bacterial activity, which is being exploited in textiles.
1.2 Surfactant aggregates
The word surfactant is derived from ‘‘surface active agent’’
and is known to reduce the interfacial tension between two
immiscible phases. They are mostly organic molecules with
a polar head group (hydrophilic) and a long alkyl chain
(hydrophobic part). Depending on the size of these two chains,
an empirical number, Hydrophilic–Lipophilic Balance (HLB),
has been assigned to the surfactants. It is a measure of the
degree to which it is hydrophilic or lipophilic. Griffin proposed
an HLB scale for non-ionic surfactants and the HLB number 1
was assigned to the most lipophilic molecule while 20 was
assigned to the most hydrophilic molecule. Various methods
have been described in literature to calculate the HLB number
of the surfactants. For instance, HLB values of polyhydric
alcohol fatty acid esters can be calculated using eqn (1) in
which Mh is the weight of the hydrophobic group and Mw is
the molecular weight.
HLB ¼ 20 1�Mh
Mw
� �ð1Þ
For fatty acid esters (Tween type), the HLB value can be
calculated using eqn (2) where E is the weight percentage of
oxyethylene and P is the weight percentage of polyhydric
alcohol.
HLB = (E + P)/5 (2)
Pasquali et al. in their studies has developed different
equations to calculate the HLB number for various kinds of
surfactants.10 The HLB value of the surfactant depends on its
structure and thus decides its action in the solution. The
application of the surfactant can be predicted from its HLB
number, for example w/o type of emulsions can be formed
using a surfactant with low HLB number while o/w emulsions
can be formed with surfactants having a high HLB number.
The other factor which is important when surfactants are
discussed is the critical micellar concentration (CMC). At
low concentrations, the surfactant dissolves in the aqueous
phase but when the concentration exceeds the critical micellar
concentration (CMC), the surfactant molecules organize
spontaneously to form aggregates such as micelles, vesicles
etc. Formation of such micelles is an entropy driven process.
Water molecules in the liquid state can be considered to have a
3-D structure of hydrogen bonds similar to ice with cavities. In
liquid water, there is always an equilibrium existing between
the destruction and formation of hydrogen bonds, which
results in movement of free water molecules through cavities.
In the presence of a hydrocarbon, the cavities are occupied by
the hydrocarbon molecules that results in restricted movement
of water; consequently water molecules surrounding the
hydrophobic solute become more ordered. During micellization,
there is a transfer of non-polar surfactant chains from an
ordered aqueous environment to the hydrocarbon-like
environment of the micelles, resulting in the disordering of
water molecules surrounding the non-polar molecules, thereby
increasing the entropy of the system and stabilizing the
microemulsion. In a micelle the hydrophobic tail of the
surfactant points towards the core while the polar head group
forms an outer shell. Such an assembly maintains a favourable
contact with water. Micelles can thus solubilize significant
amounts of non-polar molecules attributed to the hydro-
phobic core inside. Similarly, surfactants or amphiphiles may
Sonalika Vaidya
Ms Sonalika Vaidya obtainedher BSc (Hons) in chemistryfrom Hindu College, Univer-sity of Delhi in 2002. Sheobtained second position inthe university at her under-graduate level. She has beenawarded the Rastogi Awardin all the three years of herundergraduate studies forholding first position at collegelevel. Later she obtained herMSc in Chemistry from IITDelhi in 2004 after which shejoined for her PhD and iscurrently working on core–shell nanostructures inProfessor Ganguli’s group.
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aggregate in non-polar organic solvents (bulk), with less
amount of water, wherein the structural organization is just
the opposite of what is observed for micelles and thus they are
referred to as reverse or inverse micelles. The aqueous interior
of these reverse micelles allows the dissolution of polar
moieties.
The third factor associated with surfactant is the surfactant
packing parameter (Ns) which depends mainly on the volume
of the polar head group and the length of the hydrocarbon
chain and is given mathematically by the ratio Ns = V/alcwhere, V is the effective hydrocarbon volume, a is the surface
area of the headgroup of the surfactant, and l is the fully
extended chain length. These parameters control the varying
forces which play a key role in the formation of surfactant
aggregates. The collective effect of the forces acting simulta-
neously on different molecules (water, surfactant and oil) and
also on different parts of the same molecule ultimately deter-
mine the structure of the microemulsions. The effect is more
pronounced on the surfactant molecules as they are located
at the interface of the immiscible oil and water mixture.
Repulsive hydrophilic forces on the head group of the
amphiphile are balanced by the attractive hydrophobic forces
acting at the water–hydrocarbon interface and the repulsive
steric forces between the chains. The surface area of the
headgroup of the surfactant, a, can be determined by the
first two forces. The steric chain–chain and oil penetration
interactions acting within the hydrocarbon interior determine
the effective hydrocarbon volume, V, and fully extended chain
length l. By simple geometry, the critical radius of curvature R
can be determined in which the molecules pack together within
the aggregate. Calculations of packing parameter require
constraints to be included for proper treatment of the
assembly of the surfactant aggregates.11 For a spherical
micelle the radius of the hydrocarbon core, R is given by eqn (3).
R = 3V/a (3)
Since the radius of a spherical micelle cannot exceed a
certain critical length, lc (fully extended chain length of the
hydrocarbon), so from this equation, it can be deduced that
when V/alc 4 1/3, the formation of spherical micelles is
prohibited, giving a critical condition for the formation of
sphere as V/alc = 1/3. For cylinders, planar bilayers
and inverse aggregates, this parameter is 0.5, 1 and 41,
respectively. Fig. 1 shows schematic representations of various
surfactant aggregates.
The knowledge of the factors discussed above (HLB number,
CMC and Ns) enables one to choose surfactants for desired
applications especially in the synthesis of nanomaterials with
controlled size and shape.
1.3 Microemulsions
A microemulsion is a thermodynamically stable dispersion of
two immiscible liquids in the presence of an emulsifier or
surfactant. They are characterized by ultra-low interfacial
tension, large interfacial area and capacity to solubilize both
water and oil components. Microemulsions are of use in oil
recovery, pharmaceutics, cosmetics, detergency, lubrication
etc. They are categorized as water-in oil (w/o) microemulsions
when the water is dispersed homogenously in an organic
media with the help of the surfactant and oil-in-water (o/w)
microemulsions, where oil is dispersed in water. The water-in-
sc-CO2 (sc = supercritical) microemulsion is another class of
microemulsion added more recently.3
Though a microemulsion appears to be homogenous
macroscopically; distinct phases can be seen at the microscopic
levels. Among the various classes of microemulsion, the w/o
microemulsion has been extensively studied. These are
important due to their application in the synthesis of inorganic
nanoparticles. There is a subtle difference between the terms
w/o microemulsion and reverse (inverse) micelles. Fig. 2 shows
a schematic diagram of a typical reverse micelle. Aggregates
containing a small amount of water (below Wo = 15; where
Wo = [H2O]/[surfactant]) are usually called reverse micelles
whereas microemulsions correspond to droplets containing a
large amount of water (Wo 4 15).2 Since the reverse micellar
region is stabilized only in certain regions of the ternary phase
diagram, a complete understanding of the phase diagram is
required to exploit its advantages for synthesis purposes.
Keeping the relative concentration of any two constituents
(oil/surfactant/water/co-surfactant) fixed, a four-component
pseudo-ternary phase diagram (Gibbs triangle) can be
obtained using the titration method. Three parameters are
required to define completely the four component microemulsion
system, [water]/[surfactant] ratio (Wo), [co-surfactant]/[surfactant]
Fig. 1 Schematic representation of organized aggregates of
surfactants: (a) normal micelles, (b) reverse micelles, (c) cylindrical
micelle, (d) planar lamellar phases, (e) onion-like lamellar phases and
(f) interconnected cylinders. Fig. 2 A typical structure of a reverse micelle in a non-polar phase.
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ratio (Po) and [solvent]/[surfactant] ratio (No). Information
regarding the different surfactant aggregates forming the
phases can be obtained by using experimental tools such as
small angle neutron scattering (SANS), small angle X-ray
scattering (SAXS), NMR self diffusion, freeze fracture TEM
(FFTEM) or conductivity measurements. To preserve the w/o
microemulsion structure, it is critical to determine the water
emulsification failure boundary (wefb). It is near the failure
boundary where the formation of discrete water droplets in a
continuous oil phase is expected. The boundary (wefb) is a
measure of the maximum amount of water that can be
solubilised in the oil phase at a constant temperature. Here
the saturated w/o microemulsion coexists with the excess of
water. We will discuss in detail how various parameters can be
controlled for the design of the nanostructures. From the point
of view of applications, reverse micelles are important due to
their role as nanoreactors where the synthesis of nanoparticles
can be carried out with sufficient precision by controlling the
components of the ternary/quaternary system.
1.4 Microemulsions as nanoreactors
Microemulsion-based synthesis is a powerful method where
expensive or specialized instruments are not needed, contrasting
to the case for several physical methods such as plasma
synthesis, ball milling, chemical vapour deposition etc.
The product obtained is microhomogeneous as the desired
stoichiometry is maintained inside the water pools. Metallic
nanoparticles,12,13 semiconductor quantum dots,14 polymeric
nanoparticles,3 ceramics15 etc are a few examples of nano-
materials synthesized using reverse micelles. The reverse
micelles collide among themselves to exchange the reactants
and then again break apart. This coalescence process is critical
since it is only through this mechanism that the reactants,
solubilised in individual reverse micelles (nanoreactors), come
in close contact and undergo homogenous mixing. While
decoalescence ensures the presence of the protective coating
of the amphiphile for the controlled nucleation and growth it
also prevents aggregation. On mixing the microemulsions, the
reverse micelles containing the reactants, collide with each
other forming a water channel which results in the formation
of a transient dimer. Once such a dimer is formed, intermi-
cellar exchange of the reactants take place and thus nucleation
starts at the micellar edges with the well known growth process
‘‘from the boundary to core’’. It is known that most ionic
reactions are very fast compared to the lifetime of a dimer and
hence the reaction starts instantly which can account for the
nucleation starting at the micellar edge. Further growth occurs
around this point, with more reactant fed in via intermicellar
exchange. The boundary for the core growth mechanism was
experimentally shown by Li et al. using TEM and has been
illustrated in a review by Eastoe and co-workers.3
These reverse micelles (referred to as nanoreactors) favor the
formation of small crystallites with a narrow size distribution.
A schematic diagram of the reaction dynamics for a binary
system is given in Fig. 3. The intermicellar exchange rate can
be characterized by a parameter, tex, which is specific to the
type of microemulsion chosen.4 Along with intermicellar
exchange time, the time required for the chemical reaction,
tr (occurring inside the reverse micelles) is also critical. The
ratio tr/tex determines the kinetics of the chemical reaction
inside the micelle. An encounter rate factor, g, depending on
the film flexibility, affects the exchange rate constant kex. This
value varies from 10�3 for a rigid interface (for AOT) to 10�1
for more flexible films. Thus, the characteristic exchange time,
tex for the reverse micelles fall in the range of 10 mso tex o 1 ms.
This rate can be controlled via the interfacial fluidity of
the surfactant membrane which will be discussed later.
Simulations to elucidate mechanisms affecting the droplet
exchange, growth and size have been studied for a better
understanding.16 From the simulations we understand that
both the droplet exchange and the chemical reaction are
important to understand the underlying mechanism. A variety
of experiments may be designed in order to determine the
intermicellar exchange rate. Quenching of a cytochrome in
presence of dyes can be studied for the determination of the
exchange rate which is based on the fact that quenching is
exchange limited. The rate is dependent on the water content
and the type of quencher used. Other techniques which
measure the micellar diffusion coefficient are light scattering,
quasi-elastic light scattering, neutral scattering, voltametry,
and 1H pulse-gradient-stimulated-echo NMR.
Based on the use of w/o microemulsions our group has
successfully synthesized metal nanoparticles,17 dielectric15 and
magnetic oxides,18,19 and more recently ternary and quaternary
oxides such as Ca- and Sr-doped LaMnO3.20 The methodology
involves the use of as many microemulsions as the number of
reacting ions. For example in the synthesis of Sr doped
LaMnO3, a quaternary oxide, four microemulsions are
required. Three of these microemulsions contain a metal ion
each and the fourth contains the precipitating agent. An
alternative to this method for binary compounds is the single
microemulsion method. One of the desired reactants is
solubilised in the reverse micelle while the other one is added
directly to it. A variety of nanomaterials have been synthesized
by the above microemulsion methods and studied for their
Fig. 3 Mechanism showing the intermicellar exchange for the
formation of nanoparticles.
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properties.3 Single molecule magnets or coordination polymers
are yet another class of nanomaterials which have been
successfully obtained by this method. Uniform cubes of size
B15 nm of Prussian blue were synthesized with the help of
AOT reverse micelles.21 The ability to fine tune the particle size
and morphology by using this method is largely due to the
various parameters which can be altered to provide flexibility
for suitable tailoring of the products.
2. Synthesis of nanocrystalline materials using
reverse micelles
2.1 Metal nanoparticles
Boutonnet et al. in 1982 used for the first time reverse micelles
as a template to synthesize metal nanoparticles of Pt, Rh, Pd
and Ir.12 Many different materials comprising deagglomerated
and monodispersed metal nanoparticles have been synthesized
thereafter due to their interesting optical, magnetic and
electrical properties. Colloids of coinage metals such as Au,
Ag and Cu have especially been of interest due to their
interesting optical properties attributed to the surface plasmon
resonance. Since the surface plasmon resonance (SPR) of Au
lies in the visible region and is easy to visualize, the chemistry
of gold nanoparticles and its applications (imaging, biosensing
etc.) have been of tremendous interest. In an earlier review on
metal nanoparticles by Capek et al.,5 some aspects of the
synthesis of nanomaterials using microemulsions and the
parameters (such as the reducing agent concentration/water
content) that affect the final particle size for the metal nano-
particles, have been discussed. In an investigation by Pileni
et al. on metallic Pd nanoparticles, transformation of spherical
nanoparticles to worm-like nanostructures was observed on
increasing the water content.13 Recently Boutonnet has
reviewed22 the developments in the microemulsion synthesis
of nanoparticles especially for catalytic applications. Detailed
investigation by Isabelle23 on metal nanoparticles elucidates
the role of water content, capping agent, and concentration
of reducing agent on the shape and size of copper
nanoparticles.23 Control over the size of gold nanoparticles
(from 2.2 to 6.6 nm) formed by using AOT-based reverse
micelles has been achieved by controlling the reaction
temperature from �15 to 40 1C24 (Fig. 4). The advantages
offered by the microemulsion method over the other methods
have been highlighted in the study on the synthesis of
nanosized particles.25 In order to explore the scope of other
polar organic solvents as the reaction media, methanol has
been employed instead of water in an AOT/heptane system.26
Though solvation dynamics (through steady-state absorption
and fluorescence spectroscopy) and light scattering studies
have been carried out on some of these reverse micelles, there
exists a need to better understand the design and stability of
such complex microemulsion systems, which are necessary for
more intelligent tailoring of nanostructures. It should be noted
that the properties of microemulsions with polar organic cores
do not depend on the Wo value, making them very different in
comparison to those with aqueous cores. Bimetallic alloy
nanoparticles exhibit many improved properties over
their single counterparts, which makes them commercially
important, and nanoparticles of Fe/Pt,27 and Cu/Ni17 have
also been synthesized in low sizes using the microemulsion
method. The scope of the microemulsion method can thus be
expanded and employed for a wide range of metal (elemental)
and alloy nanostructures.
2.2 Metal carboxylate nanorods
An important and challenging area of research in nanoscience
and nanotechnology is to obtain anisotropic nanostructures
(nanorods, nanofibres and nanowires) which have received
increasing interest due to their potential applications in
nanodevices. The template method is very effective for the
fabrication of one-dimensional nanostructures of a desired
material. Two different classes of templates are normally
defined (hard and soft templates). Carbon nanotubes and
porous alumina fall in the category of hard templates and
can be used to control the size, shape and alignment of the
synthesized material. The soft template method uses various
types of microemulsions and micelles in which the reaction is
restricted inside the micellar core and the shape and size can be
tuned by the structure of the polar core. Most of such
syntheses lead to spherical particles, which however under
certain conditions may aggregate in the form of ellipsoids,
cubes, triangles or higher-order nanostructures (needles, fibres
and nanotubes). In addition single-crystalline anisotropic
nanostructures may also be obtained by the microemulsion
method. Although nanorods or nanowires have smaller
surface area as compared with nanoparticles, they exhibit a
great advantage in device fabrication. Soft chemistry using
surfactants and bidentate ligands has been exploited to
synthesize such one-dimensional nanostructures. Nanorods
of transition-metal oxalates21,28 have been obtained by the
reverse micellar route using CTAB (cetyltrimethylammonium
bromide) as the surfactant. Details of these nanorods have
been given in Table 1. It was observed that the metal oxalate
rods have a negative surface charge. It is thus proposed
that the formation of these nanorods is facilitated by the
templating effect of the cationic surfactant (CTAB) molecules,
which align themselves on the linear arrangement of the metal
and the ligand formed, as shown schematically in Fig. 5. It is
observed that in the absence of the cationic surfactant, only
spherical particles could be obtained. Most of these nanorods
Fig. 4 Transmission electron micrographs of gold nanoparticles
obtained at different temperatures. Reprinted with permission from
ref. 24. Copyright 2007, American Chemical Society.
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exhibit properties similar to their bulk counterparts with
lowering of the magnetic transition temperature (Table 1).
A TN of 45 K has been observed for nickel oxalate dihydrate,
contrary to the bulk value of 50 K, and a transition at 15 K for
manganese oxalate dihydrate which is somewhat higher than
the bulk value of 2.6 K. Detailed analysis of the above studies
show that the 1 : 1 stoichiometric ratio of ligand (carboxylate)
to the metal ion is critical for the formation of nanorods.
Metal ions with higher oxidation states (Ce3+, Zr4+) require
larger content of the carboxylate ions (41 : 1) and result in
spherical particles (not rods) with the divalent carboxylate
ligand.15,18 The metal–carboxylate nanorods were found to be
suitable precursors for the formation of metal and metal oxide
nanoparticles.
2.3 Metal oxide nanoparticles
Metal oxide nanoparticles have applications in various industries
which include paints, pigments, cosmetics, batteries, electronics,
pharamaceutics, magnetic and optical devices.
Microemulsion synthesis of oxides, especially used as
heterogeneous catalysts, has been studied in detail.22 Zarur
et al. developed processes to synthesize nanocrystalline metal
(non noble) oxides of high surface area as catalysts for
combustion. Despite the different hydrolysis rates of the metal
alkoxides, chemical homogeneity could be maintained in
presence of the reverse micelles.29 Microemulsions have also
been used to obtain ultra-low sized (B5 nm) tungsten oxide
with high surface area at a much lower temperature compared
to the conventional techniques.30 Eastoe et al.31 have obtained
nanocrystalline oxides using mixed surfactants (DDAB and
Brij 35) systems. The mixed surfactant systems were found to
have improved thermal stability and large water solubilisation
capacity of the microemulsion compared to the commonly
used anionic surfactant, AOT. The stability of the micro-
emulsion structure on addition of additives such as
mono-, di- and trivalent metal ions, along with high concen-
trations of the precipitating agents, was studied from
scattering studies. The microemulsion structure remains
unperturbed with both soluble and insoluble additives. A
complete study elucidating the effect of mixing time, surfactant
concentration, water-to-surfactant molar ratio and precursor
concentration on copper oxide nanoparticles was carried out
by Nassar et al.32 The ionic strength of the water pools and the
degree of interaction of the surfactant head group was found
to affect the uptake of nanoparticles.32 The increase in
the nanoparticle uptake was attributed to higher occupancy
number, coupled with a rigid interface which promotes the
intermicellar nucleation and growth, resulting in higher
uptake. The reverse-micellar approach was used to synthesize
simple binary oxides such as CeO2,15 ZrO2,
15 Fe2O328 to
complex ternary oxide nanoparticles such as BaTiO3,15
SrZrO3,15 LaMnO3
20 etc. Our group has been actively
involved in the synthesis of ternary metal oxides with inter-
esting dielectric properties such as strontium titanates15
(SrTiO3 and Sr2TiO4), barium titanates15 (BaTiO3 and Ba2TiO4),
lead titanate15 (PbTiO3) and many other titanates and
zirconates15 using the microemulsion method with Tergitol
as a surfactant (non-ionic). The route developed avoids the use
of expensive alkoxides such as Ba-alkoxides, Sr-alkoxide etc.
The particle size for BaTiO3 at 900 1C was found to beB35 nm
(Fig. 6) A weak tetragonal distortion was concluded based on
Raman studies (inset of Fig. 6) and a weak ferroelectric
transition was observed in the dielectric measurements. The
dielectric constant (e) was found to increase with sintering
temperature for BaTiO3 being 520 after sintering at 1100 1C.
The dielectric constant for 30–40 nm sized particles of
SrTiO3 was found to be 90 with a dielectric loss of 0.08 at
100 kHz. Dielectric oxides such as BaZrO3, SrZrO3, PbZrO3
Fig. 5 Mechanism depicting the formation of nanorods in the
presence of a cationic surfactant. The TEM image shows nanorods
of nickel oxalate dihydrate.
Table 1 Transition metal oxalate hydrate (MC2O4�nH2O) nanorods
M Diameter/nm Length/nm TN/K
Cu 130 480 TIPa
Ni 250 2500 45Mnb 100 2500 15Zn 120 600Co 300 6500 21Fe 70 470 27
a TIP= temperature independent paramagnetism. b n=0 (anhydrous).
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and a solid solution of Ba1�xPbxZrO3 were synthesized at low
temperature.15 An increase in grain size was observed with
increase in lead content for Ba1�xPbxZrO3 (0 r x r 1). The
increase in dielectric constant with Pb content was also
observed up to x = 0.50, however, a further increase in
lead content decreases the dielectric constant. Using metal
carboxylates as precursors (optimized in our laboratory)
several commercially important binary oxides have been
obtained. Nanorods of cobalt and iron oxalate dihydrate were
decomposed in different environments (nitrogen, hydrogen
and vacuum) to obtain the nanoparticles of various cobalt19
(Co3O4, CoO and metallic Co nanoparticles) and iron oxides28
(Fe2O3 and Fe3O4). Thus the microemulsion method proves to
be a versatile route for the synthesis of various types of
ceramics (including ternary and quaternary oxides).
2.4 Core–shell nanostructures
The water-in-oil (W/O) microemulsion system in conjunction
with the Stober synthesis and silane coupling method has been
used for the preparation of silica-coated, metallic, magnetic
and semiconductor nanocrystals. Synthesis of core–shell
nanostructures with semiconducting cores enables one to
overcome the drawback that arises due to the presence of bent
surfaces, dangling bonds, photo-oxidation. Different app-
roaches have been followed by scientists to form shell over
semiconducting nanoparticles. The first approach is to coat
these nanoparticles with an insulator such as silica or titania.
Recently silica coating over PbSe33 (Fig. 7), quantum dots
of CdS and CdTe34 and core–shell –shell nanostructures of
CdSe@ZnS@silica35 have been synthesized. The coating of
silica causes a red shift in the PL maximum of the semi
conductor. The second approach involves coating the
semiconductor with another semiconductor of wide band
gap. The most widely studied core–shell material of the above
type is the CdS/ZnS core–shell nanostructures.36 The shell
thickness was varied by using two different methodologies,
one by increasing water content, at a given precursor concen-
tration and second by increasing the precursor concentration
at a given water content. It was found that the nanoparticle
diameter and the shell thickness increase linearly with increase
in the water content. The diameter was also found to increase
with the precursor concentration. An increase of the absorp-
tion and photoluminescence intensity with increase in ZnS
shell thickness was observed which suggested better quantum
efficiency of the core–shell. There are numerous examples in
the literature where w/o microemulsions have been used to
synthesize magnetic core–shell37 nanostructures and bimetallic
core–shells.38 Our group has recently shown an increase in the
PL intensity indicating surface enhanced Raman scattering
(SERS) activity for Ag@TiO2 core–shell nanostructures
synthesized using w/o microemulsions. The homogeneity of
the shell was more pronounced when the shell forming
agent was changed from titanium isopropoxide to titanium
hydroxyacylate.39
2.5 Metal chalcogenide nanoparticles
Pileni and co-workers are among the first few to synthesize
metal chalcogenides using microemulsions.14 Since then,
microemulsions have been extensively used to synthesize
quantum dots, as the size distribution of these nanoparticles
can be successfully controlled. Of all the known chalcogenides,
CdS has been most conveniently synthesized using reverse
micelles and widely studied for its properties. CdSe is another
important semiconductor widely used in photoelectric devices.
Nanorods of CdSe have been synthesized using the anionic
surfactant AOT.40 A more recent study shows that quantum
dots of 1-hexanethiolate capped a-Cu2�xSe, a superionic
conducting phase, were synthesized at room temperature in
Triton X-100 water-in-oil microemulsions.41
2.6 Interesting morphologies using reverse micelles
Pileni and Eastoe in their reviews have thrown light on the
various parameters that govern particle size and shape.2,3 The
micellar template is found to influence the particle growth.
Along with the control over the size of the nanoparticle
synthesized, a control over the morphology is equally viable.
Compositional changes in the water-in-oil surfactant systems
can thus bring about a change in the shape of the surfactant
aggregates. Spherical (reverse micelles or micelles), cylinders,
interconnected cylinders and planes, termed as lamellar
phases, can thus be obtained. From Pileni’s illustrations on
Ag and Cu nanocrystals it is certainly evident that water-in-oil
Fig. 6 TEM micrograph of BaTiO3 sintered at 900 1C. The inset
shows the Raman spectrum of 35 nm BaTiO3 indicating weak
tetragonal distortion.
Fig. 7 TEM image of PbSe QDs in silica spheres. The inset shows
magnification of the same sample. Reprinted with permission from
ref. 33. Copyright 2008, American Chemical Society.
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microemulsions can be used to obtain specific shapes.
Additives such as NaCl and KCl favour specific adsorption
on the facets of the nuclei leading to various shapes. Not only
the concentration but also the type of ions play a major role.
For instance chloride ions induce remarkable changes during
the growth process of elongated copper crystals whereas I�
and F� ions do not cause any change in the nanocrystal
shape.23 The ionic selective adsorption is a complex process
not only related to the crystallographic nature of the surface
but also related to the surface energies, that vary with the
precursor involved, which give rise to varied morphologies.
The coupled effect of self assembly and the modified synthesis
leading to the designing of different types of nanostructures of
barium chromate has been reported by Mann et al.42 They
report the formation of ordered prismatic nanoparticles,
nanofilaments and a two-dimensional superlattice by simply
altering the molar ratio of reactants.
The synthesis of anisotropic structures of metal dicarboxylates
with the aid of cationic surfactants has been studied
in detail.19,28 Rods, cubes and spheres of Ni-oxalate were
designed by choosing appropriate microemulsion systems.43
On varying the oxidation state of the metal ion from+2 to +3
and the ligand (succinate instead of oxalate) led to, spherical
nanoparticles of iron succinate pentahydrate18 as opposed to
the rod shaped nanostructures obtained for the corresponding
metal oxalate. An unusual fish-bone type nanostructured
BaWO4 has also been synthesized44 (Fig. 8(a)).
Other unusual variations of structures have been achieved
by controlling the surfactant aggregates. Equilateral nano-
triangles of CdS (Fig. 8(b))45 and nanopetals of manganese
dioxide46 (Fig. 8(c)) have been designed using the templating
effect of the anionic surfactants. Studies have revealed the
importance of the reaction conditions on the morphology of
the final product. The concentration of the surfactant, water
content and reaction temperature were found to have a
pronounced effect on PbWO4 nanostructures (spheres, ellipsoids,
bipyramids and rod-like bundles) (Fig. 8(d)–(f)) using the
same anionic surfactant.47 Hexabranched germanium oxide
with carambola shape has been synthesized with the help of
surfactant template, however, the authors have reasoned
such formation owing to its crystal structure, the role of
microemulsion is still limited in controlling its size.48
3. Control of size and shape of nanocrystalline
structures
There have been may recent reviews in the literature which
focus on the parameters affecting the size and shape of reverse
micelles and thereby their role in governing the size and shape
of the synthesized product.2–4,6,7,25,49 Michaels et al. in their
review49 have given a quantitative model on the dependence
on size of non-ionic reverse micelles as a function of molecular
structure of the surfactant, the type of oil, the total concentration
of surfactant [NP], the ratio of surfactant to total surfactant (r),
the water to surfactant molar ratio (o), temperature, salt
concentration, and polar phase. It is to be noted that there
are no simple rules that decide the morphology of the synthesized
product. However, the film rigidity of the reverse micelles can
be controlled by changing the components involved in the
formation of w/o microemulsions, which consequently gives
nanomaterials of different size and shape. Several mechanisms
have been proposed which reflect the roles of solvent and
surfactant in controlling the size and shape of the designed
nanomaterials.44,50,51 In this review we have concentrated on
recent developments in the synthesis of nanomaterials that are
governed by four major factors viz. Wo, surfactant, solvent
and co-surfactant.
3.1 Effect of water-to-surfactant ratio (Wo)
The aqueous core of the reverse micelles plays a crucial role in
determining the size of the final product formed. The water
pool solubilizes the reactants and provides the stage where the
reaction occurs. An equation which relates the radius of the
reverse micelles (assuming the water-in-oil droplets to be
spherical) with the water content (Wo) is R = 3V/S where
R, V and S are the radius, the volume and the surface area
of the sphere, respectively. Thus, the particle size can be
controlled by varying the aqueous content. The relation of
the aqueous core to the surfactant concentration is given
by Wo = [H2O]/[surfactant]. By varying the Wo, one
effectively varies the concentration of the reactants. The water
Fig. 8 (a) SEM image of BaWO4 fishbone-like nanostructures.
Reprinted with permission from ref. 44. Copyright 2004, Elsevier.
(b) TEM image of nanotriangles of CdS nanocrystals, Reprinted with
permission from ref. 45. Copyright 2001, American Chemical Society.
(c) SEM image of nanopetals of MnO2, Reprinted with permission
from ref. 46. Copyright 2007, Springer-Verlag. FESEM images of
(d) ellipsoids, (e) bipyramids and (f) rodlike bundles of PbWO4.
Reprinted with permission from ref. 47. Copyright 2004, American
Chemical Society.
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solubilisation capacity of reverse micelles increases linearly
with an increase in surfactant concentration as reflected by the
relation Wo = [H2O]/[surfactant]. This was also theoretically
proposed by Michaels et al.,49 where they found that the
micellar size increases with increase in Wo, provided all the
other factors that govern the micellar size are kept constant.
The effect of Wo has been discussed in earlier reports.6,52
Uskokovic et al.52 discuss the parameters that govern the size
of the reverse micelles. The radius of reverse micelles (r) at
constant surfactant concentration S is given by eqn (5).
r = (4.98 � 103)j/AsS (5)
where j is the volume fraction of the dispersed phase which
can be controlled easily and As is the area occupied by the
surfactant at the droplet surface. This relation is applicable
where the microemulsion structure is not perturbed during the
reaction. Pileni’s group pioneered the studies2 related to the
control of size and shape of nanomaterials by varying Wo. An
increase in the size of copper nanoparticles with increasing
water content was observed, which saturates at Wo = 10–15.
Many reports exist that show the dependence of size of the
nanoparticles on the Wo parameter, though this corres-
pondence in the increase of particle size with increasing Wo
(found by many researchers) cannot be generalized. The
surfactant/nanomaterial affinity also influences the particle
size. A low affinity of the surfactant towards the metal centre
implies its inefficiency to control the growth, thus the increase
in the final size.23
Recently, the dependence of the size and shape of the copper
oxalate nanostructures on the aqueous content of the system
was studied wherein the nanostructures were synthesized using
a non-ionic surfactant (TX-100). An increase in the particle
size was observed with increase in the aqueous content.50
3.2 Nature of surfactants: size and charge
Surfactants play the crucial role in stabilizing the immiscible
oil/water phase by lowering the interfacial tension to form
microemulsions. The review articles by Pileni,2 and Eastoe
et al.3 are an informative source on the experimental and
theoretical aspects pertaining to the effect of surfactants
in controlling both the size and the shape. A variety of
surfactants categorized as cationic, anionic, non-ionic and
zwitterionic (having both positive and negative charges)
depending on the type of charge on their head group are
known. Two of most commonly used surfactants are the
anionic surfactant sodium bis(2-ethylhexyl) sulfosuccinate
(AOT) and the cationic surfactant cetyltrimethylammonium
bromide, (CTAB). Tergitol, TX-100, Igepal, NP-5 are a few
common non-ionic surfactants. Different types of surfactant
along with representative examples are given in Table 2.
CTAB (C16H33–(CH3)3N+Br�) provides a very flexible film
and thus a high exchange dynamics is observed in its micro-
emulsions allowing high reactant loading compared to the
AOT-based systems. Quite a large number of nanocrystalline
materials have been synthesized with desired morphology and
size using CTAB as the surfactant.
The anionic surfactant, AOT (bis(2-ethylhexyl) sulfosuccinate)
has two hydrophobic chains and is widely used in the synthesis
of nanomaterials via the microemulsion route. The versatility
of this surfactant is mainly due to the large stability region of
the reverse micelles in the ternary phase diagram. In the AOT
reverse micellar system it has been found that short chain
hydrocarbons penetrate the layer of AOT, forming a reverse
micellar shell, due to which the inter-micellar exchange is
reduced. Long chain hydrocarbons have strong intermolecular
interactions and hence get embedded in the AOT layer to form
an extra shell which allows easy inter-micellar exchange.
The structure of colloidal templates thus depends upon the
surfactant used and also controls the nanocrystal growth.2
We have earlier investigated the effect of various surfactants
on the morphology of nanomaterials. It is observed that
cationic surfactants lead to anisotropy and that nanorods of
several divalent metal carboxylates could be obtained.43
Fig. 9(a) shows the formation of nanorods of nickel oxalate
synthesized using the cationic surfactant CTAB. An isotropic
growth occurs on using the non-ionic surfactant TX-100
leading to spherical nanoparticles of (B5 nm). On changing
the non-ionic surfactant from TX-100 to Tergitol, larger cubes
(Fig. 9(b)) of size B50 nm are formed. The rigidity of the
surfactant plays a crucial role in guiding the morphology of
the product formed. The rigidity of the surfactants decreases in
the order TX-100 4 Tergitol 4 CTAB. Also it is expected
that the positively charged surfactants assemble on the surface
of the negatively charged nickel oxalate and thus favor the
anisotropic growth (rods). In the absence of such positively
charged surfactants, an isotropic growth leads to spheres and
nanocubes. Thus the choice of surfactant becomes critical to
the size, shape and stability of the particles synthesized. Recent
studies53 show the effect of different types of surfactant used in
controlling the morphology and crystal structure of calcium
carbonate. A high concentration of non-ionic surfactant
resulted in the formation of oblate sphere-like crystals of
vaterite, while reduction in the amount of Brij causes the
formation of a mixture of oblate sphere and needle-like
crystals of vaterite and aragonite, respectively.53 In the
Table 2 Classification of surfactants
Surfactant type Examples
Cationic Cetyltrimethylammonium bromide (C16H33N(CH3)3Br)Anionic SDS: Sodium dodecylsulfate (C12H25SO4Na),
AOT: Sodium bis(2-ethylhexyl) sulfosuccinate (C20H37O7SNa)Non-ionic Tergitol: C9H19(C6H4)(OCH2CH2)9OH,
TX-100: (CH3)3CCH2C(CH3)2C6H4(C2H4O)9.5OHZwitterionic Hexadecylsulfobetaine SB3-16: (C16H33N(CH3)2(CH2)3SO3)Natural/biosurfactant Rhamnolipids: e.g. RLL (a-L-rhamnopyranosyl-b-hydroxydecanoyl-b-hydroxydecanoate (C26H48O9)Switchable Amidines: e.g. C16H33NC(CH3)N(CH3)2
482 | Chem. Soc. Rev., 2010, 39, 474–485 This journal is �c The Royal Society of Chemistry 2010
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presence of DTAB, needle-like crystals in conjunction
with very few oblate sphere-like crystals and aggregates of
undefined shape are seen. In contrast to the above result, the
presence of anionic surfactant resulted in aggregates of
undefined shape.53 Not only the type, but also the surfactant
concentration also plays an important role in controlling the
shape of the synthesized product. Chen et al. in their study51
have tuned the morphologies of Ni complexes by changing the
concentration of the surfactant (AOT). On increasing the
AOT content, the shape evolved from spindles to ellipse-like
to cuboidal and finally cubes. Such formations were explained
on the basis of collisions and fusion of primary spherical
particles thereby increasing the intermicellar nucleation rate.
As a result, rearrangement of AOT molecules occur, which
lead to the assembly of surfactant bilayers between the
complex particles and thus the anisotropy. Self assembly and
Ostwald ripening further guides the morphology to spindles
and cubes. Recently we have also studied that the morphology
of copper oxalate monohydrate can be changed from rods to
cubes by changing the surfactant from cationic (CTAB) to
non-ionic (Tergitol).50 The aspect ratio of the nanorods also
depends on the length of the surfactant chain of the cationic
surfactant. The length and the diameter of the nanorods were
found to decrease with the decrease in the chain length of the
cationic surfactant50 from C-16 to C-14. The polar head group
also plays an important role in controlling the size of the
nanorods.50 Nanorods of lower aspect ratio were obtained
when the head group was changed to pyridinium ion
(cetylpyridinium bromide) instead of ammonium (CTAB)
which was attributed to the rigid surfactant layer formed by
the restricted orientation of the pyridinium ion.
3.3 Effect of solvent
The solvent plays an important role in the assembly of the
surfactant molecules and hence plays an important part in the
synthesis using w/o microemulsions. This has been discussed
by Pileni, Cason, Bagwe and Khilar and reviewed3 by Eastoe
and co-workers. The various interactions between the solvent
and the surfactant tails control the dynamics of nanoparticle
formation. A pronounced effect of the bulkiness of the solvent
molecules has been observed on the growth rate of the
nanoparticles. This is attributed to the variation in the
intermicellar exchange rate which is given by the degree of
interaction of the solvent molecules with the surfactant tails.
In particular, less bulky solvent molecules with lower molecular
volumes such as cyclohexane can penetrate between the
surfactant tails, increasing surfactant curvature and rigidity.
This increase in rigidity leads to a slower growth rate.43
Isooctane is an example of bulky molecule (large molecular
volume) and is unable to penetrate the surfactant tails
efficiently. This would lead to more fluid interface and thus
faster growth rates. Cyclohexane is a less bulky solvent and the
micellar exchange rate constant is estimated to be lower by a
factor of 10. However, there are some conflicting reports
shown by the studies done on surface rigidity by Eastoe
et al., where the solvents have a minimal effect.54 It appears
that the conclusions on the basis of film rigidity and interfacial
tension have to be considered along with the role played by the
surfactant. A comparative study of microemulsion-mediated
synthesis of nickel oxalate using solvents with varying lengths
of the hydrophobic tails shows that the length and the aspect
ratio of the nanorods are dependent on the bulkiness of
the solvent.43 With the increase in bulkiness of the solvent
molecule, the length of the nanorod increases. The solvent
bulkiness and the intermicellar exchange follows the order
n-hexane o cyclohexane o isooctane. Consequently the size
of the particles obtained also follows the same order. The role
of viscosity of the solvents on the growth kinetics of AgI
in AOT reverse micelles has been clearly observed by
Spirin et al.55 where the chain length (n) of the solvent
molecule has been varied from hexane to dodecane. The rate
of formation of nanoparticles is known to depend on the
collision frequency and efficiency of intermicellar exchange.
The collision frequency is defined by the diffusion constant K
(K = 8000RT/3Z, R is the universal gas constant, T is the
temperature and Z the dynamic viscosity of the medium).55
The intermicellar exchange is dependent on the rate of two
consecutive processes: (1) formation of the pair of the colliding
micelles and (2) formation of a channel between the two for
the exchange of reactants. One in 103–104 collisions results in
an effective exchange. The viscosity of the solvents increases
with increasing chain length of the solvent (0.307 cP for
hexane to 1.492 cP for dodecane). Thus according to the
above formula, a five-fold decrease in ‘K’ and hence in the
number of the intermicellar collisions, ‘N’ is expected as we go
Fig. 9 TEM micrographs for nickel oxalate dihydrate synthesized
using (a) CTAB/1-butanol/n-hexane and (b) Tergitol/1-octanol/
cyclohexane.
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from hexane to dodecane. Thus a decrease in the growth rate
with increasing chain length ‘‘n’’ is expected, which would
yield smaller nanoparticles. However, the expected decrease in
size from the viscosity relation was not observed experi-
mentally. Instead an increase in particle size was found with
increasing chain length ‘n’. This increase in size is attributed to
the presence of free water in the micellar pools which is further
dependent on the micelle size. At constant Wo, the size grows
with increasing chain length of the solvent molecule. However,
there is practically no free water for the small sized micelles
and the rate of particle growth is independent of ‘n’. Short
chain alkanes penetrate deeper into the micellar shell and
spread apart the surfactant molecules. This allows binding of
a greater amount of water molecules. As a result the relative
amount of free water inside the micelles decreases. Thus the
growth is largely controlled by the penetrability of the micellar
shells at initial stages while the size depends on the amount of
water in the micellar pool for higher Wo values. The particle
size of silver nanoparticles increased on changing the solvent
from decane to heptane but decreases for cyclohexane.25 The
aspect ratio of copper oxalate nanorods also changes when the
solvent was changed to cyclohexane and hexane using CTAB
when compared to our studies of copper oxalate nanorods
using isooctane as the solvent.
3.4 Effect of co-surfactant
For the appropriate packing of amphiphiles (surfactant molecules)
at the water–oil interface, surface active substances, in
addition to surfactants are often added. These are generally
short-chain alcohols or amines commonly referred to as
co-surfactants. The role of a co-surfactant is to lower the
interfacial tension between oil and water for the spontaneous
formation of surfactant aggregates. The addition of co-surfactant
is expected to reduce the surfactant concentration in the
microemulsion. Low molecular weight alcohols such as
butanol, due to its short hydrophobic chain and terminal
hydroxyl group, are expected to increase the interaction with
surfactant layers at the interface and thus influence the
curvature of the interface and hence the internal energy.56 In
a different study by Marchand et al.,57 the effect of
co-surfactant on the final particle size has been discussed
elaborately. However, contrary to the short chain alcohols
or amines generally used as co-surfactants, they have used
NP-5 (non-ionic surfactant) instead. The addition of NP-5 in
small amounts for the synthesis of MoSx using AOT as a
surfactant, leads to a substantial decrease in the average
micellar size. This is attributed to the higher fluidity of the
interfacial film, and a higher mean curvature of radius, which
in turn influences the intermicellar exchange. A high exchange
rate implies the higher consumption of reactants at the
nucleation stage, thus reducing the effective concentration
for further growth, and results in smaller nanoparticles. Direct
implication of the change of co-surfactant on the morphology
of ZnS has been studied by Charinpanitkul.56 However, no
attempt has been made to explain the results obtained. Curri
et al. have studied the effect of pentanol as a co-surfactant on
the synthesis of CdS nanoparticles using the CTAB/hexane/
water system.58 A pronounced effect was found on the size,
size distribution and stability of crystallites on addition of
co-surfactant. Pentanol is expected to increase the film
flexibility, thereby affecting the particle growth, and also its
absorption on the semiconductor surface stabilizes the
particles in solution by acting as a capping agent. The nature
as well as the amount of co-surfactant is found to be critical
while choosing the appropriate reverse micellar system. From
the time-dependent absorption studies for growth of copper
nanoparticles, Cason et al. found that at higher Wo, addition
of more than 1% octanol as a cosurfactant created an unstable
system by increasing the fluidity of the interface.59 As a result
the system ends up in phase separation with broken micelles.
On replacing octanol with 1-benzyl alcohol as a co-surfactant
in the same synthesis, an increased growth rate (at low Wo) is
observed with the addition of co-solvent, but the terminal
particle size is found to decrease. Our studies on the effect of
co-surfactant50 on the size of copper oxalate nanorods show
that the aspect ratio could be increased to 6.3 : 1 on changing
the co-surfactant from 1-butanol to 1-hexanol. Cubes of
dimension 80–100 nm and nanoparticles of size 8–10 nm
assembling to form nanorods, were obtained with 1-octanol
and 1-decanol as the co-surfactant, respectively. The increase
in the aspect ratio with increase in chain length is attributed
to the decrease in surfactant film flexibility which resulted
in rods with high aspect ratio. The change in morphology
with further increase in the chain length is possibly due
to the effective interaction of the alkyl chain of the
co-surfactant with the surfactant tail resulting in a more rigid
structure.50
Conclusions
We have reviewed several aspects of microemulsions, their
stability, versatility and flexibility towards the synthesis of
nanostructured materials. The field has grown considerably
from the initial synthesis of spherical metal nanoparticles in
the 1980s to the highly complex and multifunctional nano-
structures of today. We find that the understanding of the
subject has benefited tremendously due to the availability of
several new techniques to follow the dynamics of the processes
underlying the synthesis carried out in microemulsions. Since
there are several applications in high growth industries such as
cosmetics, food and pharmaceuticals, the future of micro-
emulsion based synthesis appears bright. However, there are
challenges which will dominate the research in the next decade.
The major challenge is the utilization and recycling of the used
solvents involved in these microemulsions systems which
currently restrict one to certain well understood and common
microemulsion systems in industry, even though technically
superior microemulsions (developed for academic purposes)
may be available. There is of course no doubt for high-end fine
chemicals. with stringent size and shape restrictions, for
which the microemulsion based synthesis will always be more
appropriate. Another direction of future development in this
area is foreseen in the use of natural and biosurfactants.
Finally the ability to control the release of drugs (pharmaceuticals
and cosmeceuticals) under appropriate stimulus will always
remain as a major theme in the future of microemulsion based
synthesis.
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Acknowledgements
A. K. G. thanks CSIR and DST (Govt. of India) for financial
assistance and IIT Delhi for facilities; S. V. thanks CSIR and
A. G. thanks UGC for fellowships.
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This journal is �c The Royal Society of Chemistry 2010 Chem. Soc. Rev., 2010, 39, 474–485 | 485
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