Microemulsion-based synthesis of nanocrystalline materials · 2017-06-18 · Microemulsion-based...

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: [email protected];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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http://dx.doi.org/10.1039/B814613F

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.

This journal is �c The Royal Society of Chemistry 2010 Chem. Soc. Rev., 2010, 39, 474–485 | 475

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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


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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.

References

1 T. P. Hoar and J. H. Schulman, Nature, 1943, 152, 102–103.2 M. P. Pileni, J. Phys. Chem. C, 2007, 111, 9019–9038.3 J. Eastoe, M. J. Hollamby and L. Hudson, Adv. Colloid InterfaceSci., 2006, 128–130, 5–15.

4 M. A. Quintela, C. Tojo, M. C. Blanco, L. Garcıa Rio andJ. R. Leis, Curr. Opin. Colloid Interface Sci., 2004, 9, 264–278.

5 I. Capek, Adv. Colloid Interface Sci., 2004, 110, 49–74.6 K. Holmberg, J. Colloid Interface Sci., 2004, 274, 355.7 V. Uskokovic and M. Drofenik, Adv. Colloid Interface Sci., 2007,133, 23–34.

8 M. Faraday, Philos. Trans. R. Soc. London, 1857, 147, 145–181.9 S. Chah, M. R. Hammond and R. N. Zare, Chem. Biol., 2005, 12,323–328.

10 R. C. Pasquali, M. P. Taurozzi and C. Bregni, Int. J. Pharm., 2008,356, 44–51.

11 J. N. Israelachvili, D. J. Mitchell and B. W. Ninham, J. Chem.Soc., Faraday Trans. 2, 1976, 72, 1525–1568.

12 M. Boutonnet, J. Kizling, P. Stenius and G. Maire, Colloids Surf.,1982, 5, 209–225.

13 K. Naoe, C. Petit and M. P. Pileni, Langmuir, 2008, 24, 2792–2798.14 C. Petit, P. Lixon and M. P. Pileni, J. Phys. Chem., 1990, 94,

1598–1603.15 A. K. Ganguli, T. Ahmad, S. Vaidya and J. Ahmed, Pure Appl.

Chem., 2008, 80, 2451–2477.16 O. Myakonkaya and J. Eastoe, Adv. Colloid Interface Sci., 2009,

149, 39–46.17 J. Ahmed, K. V. Ramanujachary, S. E. Lofland, A. Furiato,

G. Gupta, S. M. Shivaprasad and A. K. Ganguli, Colloids Surf.,A, 2008, 331, 206–212.

18 A. Ganguly, R. Kundu, K. V. Ramanujachary, S. E. Lofland,D. Das, N. Y. Vasanthacharya, T. Ahmad and A. K. Ganguli,J. Chem. Sci., 2008, 120, 521–528.

19 J. Ahmed, T. Ahmad, K. V. Ramanujachary, S. E. Lofland andA. K. Ganguli, J. Colloid Interface Sci., 2008, 321, 434–441.

20 T. Ahmad, K. V. Ramanujachary, S. E. Lofland andA. K. Ganguli, J. Chem. Sci., 2006, 118, 513–518.

21 S. Vaucher, M. Li and S. Mann, Angew. Chem., Int. Ed., 2000, 39,1793–1796.

22 M. Boutonnet, Curr. Opin. Colloid Interface Sci., 2008, 13, 270–286.23 I. Lisiecki, J. Phys. Chem. B, 2005, 109, 12231–12244.24 A. B. Smetana, J. S. Wang, J. Boeckl, G. J. Brown and C. M. Wai,

Langmuir, 2007, 23, 10429–10432.25 W. Zhang, X. Qiao and J. Chena, Mater. Sci. Eng., B, 2007, 142,

1–15.26 P. Setua, A. Chakraborty, D. Seth, M. U. Bhatta, P. V. Satyam

and N. Sarkar, J. Phys. Chem. C, 2007, 111, 3901–3907.27 A. R. Malheiro, L. C. Varanda, J. Perez and H. M. Villullas,

Langmuir, 2007, 23, 11015–11020.28 A. K. Ganguli and T. Ahmad, J. Nanosci. Nanotechnol., 2007, 7,

2029–2035.29 A. J. Zarur and J. Y. Ying, Nature, 2000, 403, 65–67.30 L. Xiong and T. He, Chem. Mater., 2006, 18, 2211–2218.

31 A. Bumajdad, J. Eastoe, M. I. Zaki, R. K. Heenan andL. Pasupulety, J. Colloid Interface Sci., 2007, 312, 68–75.

32 N. N. Nassar and M. M. Husein, J. Colloid Interface Sci., 2007,316, 442–450.

33 R. Koole, M. M. Schooneveld, J. Hilhorst, C. M. Donega,D. C. Hart, A. Blaaderen, D. Vanmaekelbergh and A. Meijerink,Chem. Mater., 2008, 20, 2503–2512.

34 Y. Yang, L. Jing, X. Yu, D. Yan and M. Gao, Chem. Mater., 2007,19, 4123–4128.

35 M. Darbandi, R. Thomann and T. Nann, Chem. Mater., 2005, 17,5720–5725.

36 A. R. Loukanov, C. D. Dushkin, K. I. Papazova, A. V. Kirov,M. V. Abrashev and E. Adachi, Colloids Surf., A, 2004, 245, 9–14.

37 C.-W. Lu, Y. Hung, J.-K. Hsiao, M. Yao, T.-H. Chung, Y.-S. Lin,S.-H. Wu, S.-C. Hsu, H.-M. Liu, C.-Y. Mou, C.-S. Yang,D.-M. Huang and Y.-C. Chen, Nano Lett., 2007, 7, 149–154.

38 I. Lisiecki, M. Walls, D. Parker and M. P. Pileni, Langmuir, 2008,24, 4295–4299.

39 S. Vaidya, A. Patra and A. K. Ganguli, J. Nanopart. Res., 2009,DOI: 10.1007/s11051-009-9663-5.

40 L. F. Xi and Y. M. Lam, J. Colloid Interface Sci., 2007, 316,771–778.

41 P. P. Ingole, P. M. Joshi and S. K. Haram, Colloids Surf., A, 2009,337, 136–140.

42 M. Li, H. Schnablegger and S. Mann, Nature, 1999, 402, 393–395.43 S. Vaidya, P. Rastogi, S. Agarwal, S. K. Gupta, T. Ahmad,

A. M. Antonelli, K. V. Ramanujachary, S. E. Lofland andA. K. Ganguli, J. Phys. Chem. C, 2008, 112, 12610–12615.

44 X. Zhang, Y. Xie, F. Xu and X. Tian, J. Colloid Interface Sci.,2004, 274, 118–121.

45 N. Pinna, K. Weiss, H. Sack-Kongehl, W. Vogel, J. Urban andM. P. Pileni, Langmuir, 2001, 17, 7982–7987.

46 S. Devaraj and N. Munichandraiah, J. Solid State Electrochem.,2007, 12, 207–211.

47 D. Chen, G. Shen, K. Tang, Z. Liang and H. Zheng, J. Phys.Chem. B, 2004, 108, 11280–11284.

48 Y.-W. Chiu and Michael H. Huang, J. Phys. Chem. C, 2009, 113,6056–6060.

49 M. A. Michaels, S. Sherwood, M. Kidwell, M. J. Allsbrook,S. A. Morrison, S. C. Rutan and E. E. Carpenter, J. ColloidInterface Sci., 2007, 311, 70–76.

50 R. Ranjan, S. Vaidya, P. Thaplyal, M. Qamar, J. Ahmed andA. K. Ganguli, Langmuir, 2009, 25, 6469–6475.

51 M. Chen, Y. Wu, S. Zhou and L. Wu, J. Phys. Chem. B, 2008, 112,6536–6541.

52 V. Uskokovic and M. Drofenik, Surf. Rev. Lett., 2005, 12,239–277.

53 A. Szczes, J. Cryst. Growth, 2009, 311, 1129–1135.54 J. Eastoe and D. Sharpe, Langmuir, 1997, 13, 3289–3294.55 M. G. Spirin, S. B. Bricklin and V. F. Razumov, J. Colloid

Interface Sci., 2008, 326, 117–120.56 T. Charinpanitkul, A. Chanagul, J. Dutta, U. Rangsardthong and

W. Tanthapanichakoon, Sci. Technol. Adv. Mater., 2005, 6,266–271.

57 K. E. Marchand, M. Tarret, J. P. Lechaire, L. Normand,S. Kasztelan and T. Cseri, Colloids Surf., A, 2003, 214, 239–248.

58 M. L. Curri, A. Agostiano, L. Manna, M. D. Monica, M. Catalano,L. Chiavarone, V. Spagnolo and M. Lugara, J. Phys. Chem. B, 2000,104, 8391–8397.

59 J. P. Cason, M. E. Miller, J. B. Thompson and C. B. Roberts,J. Phys. Chem. B, 2001, 105, 2297–2302.


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