TEMPLATE BASED SYNTHESISSynthesis, Mechanism and Pathway

<< SOL-GEL TECHNIQUES:DEFINITIONS, GENERAL MECHANISM, INORGANIC ROUTE

MICROEMULSION TECHNIQUES:Significance of Packing Parameter >>

Chapter - 6

TEMPLATE BASED SYNTHESIS

B.Kuppan

INTRODUCTION

To make a jar, a piece of wood of the desired shape is first fabricated, and layer of

clay is applied to the wood. Heat treatment of the clay/wood composite at high

temperature generates a ceramic jar. During the heat treatment, clay is transformed to

a ceramic material and the wood is burned off leaving the empty space in the

resulting jar. When this process is scale down to nanometer regime, it is basically the

template synthesis process [1].

Fig. 6.1. Schematic representation of template synthesis (reproduced from ref. 1)

One class of templates are surfactants that are used to produce mesoporous materials.

It is true to say one of the most exciting discoveries in the field of material synthesis

over the last 15 years is the formation of mesoporous silicate and alumino silicate

molecular sieves with liquid crystal templates. This family materials generally called

as M41S family (MCM-41, MCM-48 and MCM-50) [2].

These M41S family

materials can be prepared by using cationic surfactants as the templates, and other

kind of mesoporous materials also can be prepared by using non-ionic surfactants as

the templates. These family materials are called as SBA-n family [3].

The above mentioned mesoporous molecular sieves also can be used as templates

to prepare ordered mesoporous carbon by using sucrose as the carbon precursors.

These mesoporous silicates, aluminosilicates and mesoporous carbon materials are

6.2

Template Based Synthesis

more important in the field of catalysis because of its high surface area, narrow pore

size distribution and large number of surface functional groups. The surfaces of these

materials can also be tuned depending on the applications.

Another kind of template based synthesis is to prepare freestanding, non-oriented

and oriented nanowires, nanorods or nanotubes. The fabrication of metal nanowires

had potential applications in the microelectronic industry and in particular, for

interconnection in electronic circuits. The procedure is based on metal displacement

reaction leading to the growth of metal nanowires into the pores. The galvanic

displacement reaction for the synthesis of core /sheet nanostructured materials has

been investigated in the literature [4].

Nanostructured materials have attracted much interest because of their unique

properties. In fact, due to their structure features and size effects, they show physical

properties that are different from bulk materials. Many methods have been developed

for the fabrication of nanowires. Among these methods template synthesis is

considered as quite useful, because it can be used for the preparation of different

types of nanostructures. The high order degree of porous structure of anodic alumina

membrane (AAM) (consisting in a close packed array of columnar hexagonal cells,

each containing a central cylindrical pore normal to the surface), makes it an ideal

template for the fabrication of nanostructured materials, suitable for applications in

optoelectronics, sensors, magnetic memories and electronic circuits [5].

Synthesis, Mechanism and Pathway

A large number of studies have been carried out to investigate the formation and

assembly of mesostructures on the basis of surfactant self-assembly. The initial liquid

crystal template mechanism first proposed by Mobil's scientists is essentially always

"true" because the pathways basically include almost all possibilities. Two main

pathways that is cooperative self assembly and "true" liquid crystal templating

process, seems to be effective in the synthesis of ordered mesostructures.

Surfactants for the formation of templates

The term surfactant is a blend of "surface acting agent" surfactants are usually

organic compounds that are amphophilic in nature. Amphophilic means they contain

Synthetic Strategies in Chemistry

6.3

both hydrophobic groups (tails) and hydrophilic groups (heads). Therefore they are

soluble in both organic solvents and water.

Generally a clear homogeneous solution of surfactants in water is required to get

ordered mesostructures. Frequently used surfactants can be classified into cationic,

anionic and non-ionic surfactants.

Table 6.1. Cationic surfactants

(Reproduced from ref.7)

6.4

Template Based Synthesis

Quaternary cationic surfactants, CnH2n+1N (CH3)3Br (n = 8-22), are generally efficient

for the synthesis of ordered mesoporous silicate materials. Commercially available

CTAB (cetyltrimethylammonium bromide) is often used. Gemini surfactants,

multiheadgroup surfactants, and recently reported cationic fluorinated surfactants can

also be used as templates to prepare various mesostructures. Frequently used cationic

quaternary ammonium surfactants are shown in the table below

First reports of mesoporous silica from Mobil's company, cationic surfactants

were used as structure directing agents (SDAs). Cationic surfactants have excellent

solubility, have high critical micelle temperature (CMT) values and can be widely

used in acidic and basic media. But they are toxic and expensive.

Anionic surfactants include carboxilates, sulphates, sulfonates, and phosphates;

recently a kind of lab-made anionic surfactant terminal carboxylic acids is used to

template the synthesis of mesoporous silicas with the assistance of aminosilanes or

quaternary

aminosilanes

such

3-aminoprpoyltrimethoxysilane

(APS)

N-

trimethoxylsilylpropyl-N, N, N-trimethylammonium chloride (TMAPS) as co-

structure directing agents (CSDAs).

Anionic surfactants

Fig. 6.2. Representation of anionic surfactants (reproduced from ref. 7)

Synthetic Strategies in Chemistry

6.5

Table 6.2. Non-ionic surfactants (reproduced from ref. 7)

Non-ionic surfactants are available in a wide variety of different chemical structures.

They are widely used in industry because of attractive characteristics like low price,

nontoxicity, and biodegradability. In addition the self assembling of non-ionic

surfactants products mesophase with different geometries and arrangements. They

become more and more popular and powerful in the syntheses of mesoporous solids.

The syntheses that largely promote the development of mesoporous materials are

simple and reproducible. A family of mesoporous silica materials has been prepared

with various mesoporous packing symmetries and well defined pore connectivity.

6.6

Template Based Synthesis

Cooperative Self-Assembly of Surfactant and Silica Source to Form

Mesostructure

This pathway is established on the basis of the interaction between the silicates

surfactants to from inorganic-organic mesostructure composites.

Cooperative Self-Assembly Mechanism

Fig. 6.3. schematic diagram of the co-operative self assembly of silicate-surfactant

mesophase. (reproduced from ref. 6)

A layer to hexagonal mechanism (folded sheet mechanism) was postulated by Kuroda

and co-workers, according to which the mesostructure is created from a layer

kanemite precursors. Silicate polyanions such as silicate oligomers interact with

positively charged groups in cationic surfactants driven by Coulomb forces. The

silicate species at the interface polymerized and cross-link and further change the

Synthetic Strategies in Chemistry

6.7

charge density of the inorganic layers. With the proceeding of the reaction, the

arrangement of the surfactants and the charge density between inorganic and organic

species influence each other. Hence the compositions of the inorganic-organic

hybrids differ to some degree. It is the matching of charge density at the

surfactants/inorganic species interfaces that govern the assembly process. The final

mesosphere is the ordered 3D arrangement with the lowest interface energy. The

transformation of the isotropic micellar solutions of CTAB into hexagonal or lamellar

phase when mixed with anionic silicate oligomers in highly alkaline solutions were

indeed detected through a combination of correlated solution state 2H,

C, and

nuclear magnetic resonance (NMR) spectroscopy, small angle x-ray scattering

(SAXS), and polarized optical microscopic measurements. The mechanism in

different surfactant systems has been studied using NMR technique.

This cooperative formation mechanism in non-ionic surfactant system was

investigated by in-situ techniques. Goldfarb and co-workers investigated the

formation mechanism of mesoporous silica SBA-15, which are templated by triblock

copolymer P123 (EO20PO20EO20)

by using direct imaging and freeze-fracture

replication cryo-TEM techniques, in situ electron paramagnetic resonance (EPR)

spectroscopy, and electron spin-echo envelope modulation (ESEEM) experiments.

They found a continuous transformation from speriodial micelles into threadlike

micelles. Bundles were then formed with dimension that are similar to those found in

the final materials. The elongation of micelles in a consequence of the reduction of

polarity and water contact within the micelles due to the adsorption and

polymerization of silicate species. Before the hydrothermal treatment, the majority of

PEO chains insert into silicate frameworks, which generates micropores after removal

of templates. Moreover they found that the extent of the PEO chains located within

the silica micropores depended on the hydrothermal aging temperature and Si/P123

molar ratio. The formation dynamics of SBA-15 studied by Flodstrom et al. on the

basis of time-resolved in situ 1H NMR and TEM investigations. They observed four

stages during the cooperative assembly, which are the adsorption of silicates on

globular micelles, the association of globular micelles into floes, the precipitation of

floes, and the micelle-micelle coalescence. Khodakov et al. proposed a structure with

6.8

Template Based Synthesis

a hydrophobic PPO core and a PEO water silicate corona in the first stage. The

cylindrical micelles pack into the domains. At the same time, solvents are replaced by

condensed silicate species.

These mechanisms consider the interactions on the surfactants/inorganic species

interfaces. Monnier and Huo et al. gave a formula of the free energy in the whole

process.

Ginter +

Gwall +

Gintra +

Gsol

In which

Ginter associated with interaction between the inorganic walls and

Gwall is the structural free energy for the inorganic frameworks,

surfactant micelles,

Gintra is the van der Waals force and conformational energy of the surfactant, and

Gsol is the chemical potential associated with the species in solution phase.

Gsol can be

For surfactant-templating assembly mesostructured silicates,

regarded as a constant in a given solution system. Therefore, the key factor is the

interaction between surfactant and inorganic species, such as the matching of charge

density. The more negative

Ginter is, the more easily the assembly process can be

proceeded.

Elaborate investigations on mesoporous materials have been focused on

understanding and utilizing the inorganic-organic interactions. Table 6.3 lists the

main synthesis routes and the corresponding surfactants and classical products.

Stucky and co-workers proposed four general synthetic routes, which are S+I-, S-

I+, S+X-I+, and S-X+I- (S+ = surfactant cations, S- = surfactant anions, I+ = inorganic

precursors cations, I- = inorganic precursors anions, X+ = cationic counter ions, and

X- = anionic counterions). To yield mesoporous materials, it is important to adjust the

chemistry of the surfactants headgroups, which can fit the requirement of the

inorganic components. Under basic conditions, silicate anions (I-) match with the

surfactant cations (S+) through Coulomb forces (S+I-). The assembly of the polyacid

anions and surfactant cations to "salt"-like mesostructures also belongs to S+I-

interaction. In contrast to this, one of the examples of S+I- interaction occurs between

cationic keggin ion (Al137+) and anionic surfactants like dodecyl benzenesulfonate

salt.

Synthetic Strategies in Chemistry

6.9

The organic-inorganic assembly of surfactants and inorganic precursors with the same

charge is also possible. However, counter ion is necessary. For example, in the

syntheses of mesoporous silicates by the S+X-I+ interaction, S+ and I- are cationic

surfactants and inorganic anionic precursors, and X- can be halogen ions (Cl-, Br-, and

I-), SO42-, NO3-, etc. In strongly acidic medium, the initial S+X-I+ interaction through

Coulomb forces or more exactly, double-layer hydrogen bonding interaction,

gradually transforms to the (IX)-S+ one. It was the first time that the mesoporous

silica was synthesized under a strongly acidic condition. Here anions affect the

structures, regularity, morphologies, thermal stability, and properties of mesoporous

silicas. The Hofmeister series of the anions are one of the possible reasons that

change the hydrolysis rates of the silicate precursors and the micellar structures.

Schematic illustration of the two types of interactions between APS (A) or TMAPS

(B) and anionic surfactant headgroups.

Fig. 6.4. interactions between APS (A) or TMAPS (B) and anionic surfactant

headgroups (reproduced from ref. 7)

Compared with those cationic surfactants, the repulsive interaction between anionic

surfactants and silicate species fails to organize ordered mesostructures. Concerning

the charge matching effect, Che et al. demonstrated a synthetic route to create a

family of mesoporous silica structures under basic conditions by employing anionic

6.10

Template Based Synthesis

surfactants as SDAs APS or TMAPS as CSDAs. This route can be described as an "S-

N+I-" pathway, where N+ are cationic amino groups of organoalkoxysilanes. Fig. 6.4

gives the schematic illustration of interactions between amino groups and anionic

surfactant head groups. The negatively charged headgroups of the anionic surfactants

interact with the positively charged ammonium sites of APS or TMAPS electrostatic

ally through neutralization. The most efficient surfactant is possibly terminal

carboxylic acid. The co-condensation of tetraethylorthosillane (TEOS) with APS or

TMAPS and assembly with surfactants occur to form the silica framework.

Table 6.3. Synthesis Routes to Mesoporous Materials with the Emphasis on Silicates

(reproduced from ref.7)

route

interactions

symbols

conditions

classical products

S+I-

S+, cationic surfactants

electrostatic

basic

MCM-41, MCM-48

I-, anionic silicate species

Coulomb

MCM-50,

SBA-6

force

SBA-2, SBA-8

FDU-2, FDU-11

FDU-

S-I+

S-, anionic surfactants

electrostatic

aqueous

mesoporous alumina

I+, Transition metal ions

Coulomb

Force

S+X-I+ electrostatic

S+, cationic surfactants

acidic

SBA-1, SBA-2

I+, cationic silicate species

SBA-3

Coulomb

force, double X-, Cl-, Br-, I-, SO4-, NO3-

layer H bond

S-N+I- electrostatic

S-, anionic surfactants

basic

ASM-n

N+, cationic amino groups

Coulomb

Synthetic Strategies in Chemistry

6.11

I-, anionic silicate species

Force

S-X+I- electrostatic S-, anionic phosphate

basic

W, Mo oxides

Coulomb

surfactants

force, double I-, transition metal ions,

layer H bond X+, Na+, K+, Cr3+, Ni2+

SoIo H bond

So, nonionic surfactants

neutral

HMS, MSU,

disordered

(NoIo)

No, organic amines

Worm-like

Io, Silicate species, aluminate

mesoporous

species

silicates

SoH+X-I+ electrostatic So, nonionic surfactants

acidic

SBA-n (n=11,

Coulomb

12, 15, 16)

I+, silicate species

FDU-n (n=1, 5)

Force,

X-, Cl-, Br-, I-, SO42-, NO3-1

KIT-5 and KIT-6

double,

layer

H bond

No...I+

No, organic amines

coordination

acidic

Nb, Ta oxides

I+, transition metal

bond

S+-I-

S+, cationic surfactants

covalent

basic

mesoporous

I-, silicate species

silica

bond

Liquid Crystal Template Pathway

In this pathway true or semi-liquid crystal mesophases are involved in the surfactant

assembly to synthesize ordered mesoporous solids. Attard and co-workers

synthesized mesoporous silicas using high concentrations of non-ionic surfactants as

6.12

Template Based Synthesis

templates. The condensation of inorganic precursors is improved owing to the

confined growth around the surfactants and thus ceramic-like frameworks are formed.

After the condensation, the organic templates can be removed by calciation,

extraction, etc. The inorganic materials "cast" the mesostuructures, pore sizes, and

symmetries from the liquid crystal scaffolds. Direct templating of microemulsion

liquid-crystal mesophases were used to synthesis mesoporous silicas from butanol-

water-copolymer silica ternary system [6, 7].

Silica/surfactant = 1/0.27

calcination

As-synthesized

As-synthesized MCM-41

(hexagonal structure)

MCM-41

Silica/surfactant =

1/0.60

calcination

MCM-48

As-synthesized MCM-48

(cubic structure)

Fig. 6.4. possible mechanism pathways for the formation of M41S family by liquid

crystal template method (reproduced from ref. 2).

Evaporation Induced Self Assembling Techniques (EISA)

Evaporation induced self-assembly mechanism

The evaporation induced self assembling technique is one of the best techniques to

prepare the nanomaterials. The successful synthesis of mesoporous silica films, the

EISA method is engaged to prepare ordered mesoporous polymer and carbon

materials. The EISA method is strategy that avoids the cooperative assembling

Synthetic Strategies in Chemistry

6.13

process between the precursors and the surfactant template. Therefore, the cross-

linking and thermopolymerization process of the resols separate from the assembly.

Fig. 6.5. Scheme for the preparation of ordered mesoporous polymer resins and

carbon frameworks from the surfactant templating process of EISA. (reproduced from

ref. 8)

Compare to hydrothermal synthesis the EISA method is easier and can produce the

mesoporous resins and carbon in a wider synthetic range, including pH values,

surfactant and phenol/template ratio.

The choice of organic precursors is essential for the EISA of the organic organic

templating process. The polymerization of inorganic precursors should be low enough

to form a moldable inorganic-organic framework at the initial assembly stage of

inorganic species with organic surfactants. Highly ordered mesostructures can be

formed. The inorganic framework is rigid. Therefore, the mesophase can be

6.14

Template Based Synthesis

solidified, and the surfactant can be easily removed by calcinations or extracted with

ethanol.

The synthesis procedure includes five major steps (Fig. 6.5), which are the

preparation of resol precursors, the formation of ordered hybrid mesophases by

organic-organic self assembly during the solvent evaporation, thermopolymerization

of the resols around the templates to solidify the ordered mesophases, the removal of

the templates, and carbonization of the resin polymer frameworks to the homologous

carbons [8].

Template Synthesis of Metal Nanostructures

Fig. 6.6. Scheme of the arrangement used for the fabrication of copper nanowires

into the AAM template (reproduced from ref. 5)

Copper nanowires are prepared by using aluminium foil as active metal having a

thickness of 1.5 mm. In order to deposit copper inside the channels of anodic alumina

membrane (AAM), prior to cementation a thin conductive layer of Au was sputtered

on one side of the AAM using a conventional sputter coater to make this surface

electrically conductive. A portion of AAM was mounted onto the aluminum support

by means of a conductive paste and delimited by an insulating film. Since the

displacement deposition process needs an electrolytic contact of the active metal

Synthetic Strategies in Chemistry

6.15

surface, a small area of the aluminum support was exposed to the deposition solution.

A scheme of the arrangement used for the fabrication of copper nanowires is reported

in Fig. 6.5. This arrangement was placed horizontally in a beaker and covered with

25 ml of a 0.2 M copper sulphate and 0.1 M boric acid solution having pH 3. The

experiments were conducted at room temperature. The surface area of the AAM

exposed to the deposition solution was of the order of 1 cm2. A fresh solution was

used for each experiment. Experiments were carried out for different times of

deposition (from 7 h to 7 days) [5].

Preparation of Carbon Nanotubes

Fig. 6.7. Schematic representation of carbon nanotubes (reproduced from ref. 9)

Alumina membrane is used as template for the preparation of carbon nanotubes; here

polyphenyl acetylene is used as a carbon source for the preparation of carbon

nanotubes.

contains

only

carbon-hydrogen

bonds.

The

polyphenyl

acetylene/alumina composite was prepared by adding 10 ml of 5% w/w polyphenyl

acetylene in dichloromethane to the alumina membrane applying vacuum from the

bottom. The entire polymer solution penetrates inside the pores of the membrane was

dried in vacuum at 373 K for 10 min. the composite was then polished with fine

neutral alumina powder to remove the surface layers and ultrasonicated for 20 min to

6.16

Template Based Synthesis

remove the residual alumina powder used for polishing. The composite was

carbonized by heating in Ar atmosphere at 1173 K for 6 h at heating rate of 10 K/min.

This resulted in the deposition of carbon on the channel walls of the membrane. The

carbon/alumina composite was then placed in 48 % HF to free the nanotubes. The

nanotubes were washed with distilled water to remove HF [9].

Preparation of Porous Solids by Template Method

Ordered mesoporous carbon has been prepared by using mesoporous silica as the

template, and this mesoporous silica can be prepared by using CTAB, P123,

surfactants as the templates.

Mesoporous silica SBA-15 is prepared by the following method, P123 non-ionic

surfactant was dissolved in 2 M HCl solution, and it was stirred for 1 h then TEOS

was added as the silica source, this mixture was stirred until TEOS were completely

dissolved. The mixture was placed in oven at 373 K for 48 h. The product was filtered

and washed with distilled water and dried at 333 K for 6 h and then SBA-15 was

calcined at 823 K for 6 h in air atmosphere.

Fig. 6.8. XRD patterns of SBA-15 and CMK-3 (reproduced from ref. 10)

Synthetic Strategies in Chemistry

6.17

The calcined SBA-15 was impregnated with aqueous solution sucrose containing

sulfuric acid, this mixture was heated to 373 k to 433 K for 24 h. then the

carbonization was completed by pyrolysis with heating to typically 1173 K under N2

atmosphere for 6 h at the rate of 5 K/min. the carbon-silica composite obtained was

washed with 1 M NaOH solution or 5 Wt % hydrofluoric acid at room temperature, to

remove the silica template. The template free carbon product thus obtained was

filtered, washed with ethanol and dried at 393 K.

In Fig. 6.9 the low angle XRD conforms the mesoporous structure and to support this

mesoporous structure the transition electron microscopy images shows the presence

of ordered mesoporous structure [10].

Fig. 6.9 TEM and selected area electron diffraction (SAED) images of CMK-3

(reproduced from ref.10)

SUMMARY

Strategy aimed at the controllable synthesis has been focused on the control of micro,

meso and macroscale, including synthetic methods, architecture concepts, and

fundamental principles that govern the rational design and synthesis. In this chapter,

synthesis mechanisms and the corresponding pathways are first demonstrated for the

synthesis of mesoporous silicates from the surfactant-templating approach. Virtually

all mesoporous silicates begin with an understanding of the interaction between

organic surfactants and inorganic species, as well as among themselves.

6.18

Template Based Synthesis

Soft templating approach is one of the most general strategies now available for

crating nanostructures. The assembly of surfactants and silicates species is normally

carried in solutions or the interface to allow the required driving force for the

formation of nanostructures. Relying on sol-gel, solution and surface chemistry, there

is great potential to explore novel strategies for mesostructures, especially a strategy

that can utilize interfacial tension. Items that attract attention also include the control

of weak interaction such as the hydrophobic interaction between the assembling

components. In view of the fact that surfactant self-assembly can occur with

components larger than 1 nm continuous to be challenge and an interest in condensed

matter science.

REFERENCES

1. J. Lee, J. Kim, T. Hyeon, Adv. Mater., 18 (2006) 2073.

2. C. T. Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vartuli, J. S. Beck,

Nature, 359 (1992) 710.

3. D. Zhao, J. Feng, Q. Huo, N. Elosh, G. H. Fredirckson, B. F. Chemlka,

G. D. Stucky, Science, 279 (1998) 548.

4. G. Cao, D. Liu, Adv. Colloid. Interface. Sci. 136 (2008) 45

5. R. Inguanta, S. Piazza, C. Sunseri, Eleectrochem. Commun., 10 (2008) 506

6. A. Corma, Chem. Rev. 97 (1997) 2373.

7. Y. Wan, D. Zhao, Chem. Rev. 107 (2007) 2821.

8. Y. Meng, D. Gu, F. Zhang, Y. Shi, L. Cheng, D. Feng, Z. Wu, Z. Chen,

Y. Wan, A. Stein, D. Zhao, Chem. Mater., 18 (2006) 4447.

9. M. Sankaran, Ph.D Thesis, Indian Institute of Technoloyg Madras, 2007.

10. S. Jun, S. Joo, R. Ryoo, M. Kruk, M. Jaroniec, Z. Liu, T. Ohsuna,

O. Terasaki, J. Am.Chem. Soc., 122 (2000) 712.

Chapter - 7

MICROEMULSION TECHNIQUES

Ch. Venkateswara Rao

The main focus of this chapter is to give brief introduction to the microemulsion systems,

formation of microemulsions, how those microemulsions can be used for the preparation of

various types of nanoparticles and the dominant factors that influence the nanoparticles

preparation in the microemulsion.

INTRODUCTION

In 1943, Hoar and Schulman first reported that oil can be dissolved in bulk water or water in

bulk oil with the aid of surfactant to produce a clear homogeneous solution. The oil phases are

simple long-chain hydrocarbons and the surfactants are long-chain organic molecules with a

hydrophilic head (usually an ionic sulfate or quarternary amine) and lipophilic tail. The generally

used oil phases are cyclohexane, decane, heptane and surfactants are cetyltrimethylammonium

bromide (CTAB), sodium bis(2-ethylhexyl) sulfosuccinate (AOT).

The clear homogeneous

solution is called as microemulsion. According to the nature of the bulk solvent used in the

microemulsion, they are designated as water-in-oil or oil-in-water microemulsions. The

microemulsion is primarily distinguished from the emulsion not by being composed of smaller

droplets but by being subjected to a restrictive condition that it is thermodynamically stable.

MICROEMULSION FORMATION AND NANOPARTICLES PREPARATION

In general, water and oil are not miscible. But the amphiphilic nature of the surfactants such as

CTAB makes them miscible. The surfactant molecules form a monolayer at the interface

between the oil and water, with the hydrophobic tails of the surfactant molecules dissolved in the

oil phase and the hydrophilic head groups in the aqueous phase. It leads to the formation of

microemulsion. So the microemulsion is defined as a system of water, oil and surfactant. This

system is an optically isotropic and thermodynamically stable. At macroscopic scale, a

microemulsion looks like a homogeneous solution but at molecular scale, it appears to be

heterogeneous. Even though it is optically isotropic; it cannot be properly described as a solution.

The internal structure of the microemulsion at a given temperature is determined by the ratio of

its constituents. The structure consists either of nanospherical monosized droplets or a

7.2

Microemulsion Techniques

bicontinuous phase. The different structures of a microemulsion at a given concentration of

surfactant are shown in Fig 1. It indicates that

(i)

At high concentration of water, the internal structure of the microemulsion consists of

small oil droplets in a continuous water phase (micelles), known as o/w

microemulsion.

(ii)

With increased oil concentration, a bicontinuous phase without any defined shape is

formed.

(iii)

At high oil concentration, the bicontinuous phase is transformed into a structure of

small water droplets in a continuous oil phase (reverse micelles), known as a w/o

microemulsion.

Depending on the type of surfactant used to form microemulsion, size of the different

droplets will be varied from 5-100 nm. It is also evident that the microemulsion system is

sensitive to temperature. It can be seen in Fig. 7.1 that the increase in temperature will

destroy the oil droplets while the decrease in temperature will destroy the water droplets.

Fig. 7.1 The microscopic structure of a microemulsion at a given concentration of surfactant as

function of temperature and water concentration (reproduced from ref. 20).

Significance of Packing Parameter

The shape of micellar aggregates and the formation of microemulsion can be understood from

the packing parameter of surfactant molecule used for the microemulsion formation. Packing

The Evolving Synthetic Strategies in Chemistry

7.3

parameter is defined as v/a.l, where v is the volume of hydrocarbon of the surfactant, a is the

polar head area and l is the fully extended chain length of the surfactant. The packing parameter

value can be 1, <1 and >1. When the packing parameter value (or ratio v/a.l) is greater than 1, the

aggregate curvature will be toward the water. This corresponds to a situation where the oil is

penetrating the surfactant tails and/or the electrostatic repulsion between the charged head group

is low. When the packing parameter value (or ratio v/a.l) is less than 1, it corresponds to a

situation where the electrostatic repulsion is larger and/or the oil is not penetrating the surfactant

tails. It implies that (i) when oil is solubilized in hydrophilic micelles, one can observe the

formation of o/w microemulsions for v/a.l<1; (ii) when water is solubilized in hydrophobic

micelles, one can observe the formation of w/o microemulsions for v/a.l>1.; and (iii) When

v/a.l≈1, lamellar phases or bicontinuous microemulsions are observed.

Based on the experimental results, it is observed that the micelles will be in spherical shape

when the packing parameter is less than 1/3. The packing parameters for cylinders and planar

bilayers are 0.5 and 1, respectively. In the case of reverse micellar structures, the packing

parameter is greater than 2 for cylinders as well as spherical micelles. It was observed that

reverse micelles will be in cylinder shape up to v/a.l≤2 and spherical shape when v/a.l>3.

Oil-in-water (o/w) microemulsions are monodisperse. Water-in-oil (w/o) microemulsion

solutions are mostly transparent, isotropic liquid media with nanosized water droplets that are

dispersed in the continuous oil phase and stabilized by surfactant molecules at the water/oil

interface. These surfactant-covered water pools offer a unique microenvironment for the

formation of nanoparticles. They not only act as micro-/nano-reactors for processing reactions

but also exhibit the process aggregation of particles because the surfactants could adsorb on the

particle surface when the particle size approaches to that of the water pool. As a result, the

particles obtained in such a medium are generally uniform in size and shape (i.e., monodisperse).

APPLICATIONS OF MICROEMULSIONS

1. Synthesis of Metal Nanoparticles

Precipitation of metal particles in the Water-in-oil (w/o) microemulsion solutions (reverse

micellar system) has been found to be the simple methodology for the preparation of

nanoparticles.

7.4

Microemulsion Techniques

The method of nanoparticle preparation consists in mixing of two microemulsions carrying the

appropriate reactants (generally metal precursor and reducing agent) in order to obtain the

desired particles. It is represented in Scheme 7.1.

Scheme 7.1. Proposed mechanism for the formation of metal particles by the microemulsion

approach (reproduced from ref. 24)

At the beginning of reaction, the attractive van der Waals force and the repulsive osmotic force

between reverse micelles lead to collisions of micelles. It results in the interchange of the

reactants (in general, metal ions and reducing species) solubilized in two different reverse

micelles respectively. As a result, the initial monomeric metal nuclei begin to form and grow.

When the exchange of reactants is fast in the water droplets, metal ions reduce and the metal

nuclei grow quickly. It remains unchanged when the particle size reaches a certain size. It

corresponds to the thermodynamically stabilized species in the presence of microemulsion. Due

The Evolving Synthetic Strategies in Chemistry

7.5

to the possible aggregation, the final size of silver particles is generally larger than that of the

water cores. Once the particles attain the final size, the surfactant molecules are attached to the

surface of particles and stabilize and protect them against further growth. The dynamic exchange

of reactants such as metallic salts and reducing agents between droplets via the continuous oil

phase is strongly depressed due to the restricted solubility of metal salts in the oil phase. This is a

reason why the attractive interactions (percolation) between droplets play a dominant role in the

particle nucleation and growth in the water-in-oil (w/o) microemulsion reaction medium.

2. Synthesis of Metal Alloys

Various metal alloys can be prepared in the similar way as described in scheme 1. It consists of

mixing the reverse micelle containing two or three metal precursors and another reverse micelle

contain reducing species. Example: formation of Pt-Ru (1:1) alloy. In order to prepare the alloy

nanoparticles, both Pt4+ and Ru3+ will be taken in 1:1 atomic ratio to form reverse microemulsion

I. Reverse microemulsion II will be prepared by using NaBH4 reducing agent. When both the

reverse microemulsions are mixed together, reduction of Pt4+ and Ru3+ takes place by BH4- ions

and results in the formation of Pt-Ru (1:1) nanoparticles (Xiong and Manthiram, 2005). In the

similar way, Pt-Fe nanoparticles in different ratios (1:1, 1:2 and 1:3) can be prepared (Carpenter

et al., 2000). In the same manner, various metal chalcogenides also can be prepared by taking

two micelle solutions containing the desired ions prepared separately and then rapidly mixed.

In conventional methods of preparation, the reaction temperature should be high to promote

alloy formation. As a result, the formed particles will be large in size. Moreover, the shape of the

particle cannot be controlled. Since the reaction takes place inside the micellar cores,

(i) the enormous amount of heat that is generated within the micellar cores during the reduction

process is enough to form alloy of desired composition.

and (ii) both size, shape and composition can be controlled.

3. Synthesis of Metal Oxides

The synthesis of oxides from reverse microemulsion relies on the co-precipitation of one or more

metal ions. It is almost similar in many respects to the precipitation of oxides from aqueous

solutions. But the precipitation occurs within the micellar cores so that particle size as well as

shape can be controlled. In a typical process, precipitation of hydroxides is induced by addition

of a reverse micelle solution containing dilute NH4OH to a reverse micelle solution containing

aqueous metal ions at the micellar cores. Alternatively, dilute NH4OH can simply be added

7.6

Microemulsion Techniques

directly to a micelle solution of the metal ions. The precipitation of the metal hydroxides is

typically followed by centrifugation and heating in the presence of oxygen to remove water

and/or improve crystallinity. In general, it is represented as follows:

A2+ + 2B2+ + OH- (excess) → AB2O4 + xH2O↑

Where A and B are metals.

In this way, simple metal oxides and multicomponent metal oxides can be prepared. Some of the

examples of nanomaterials synthesized in w/o microemulsions are given in Table 7.1.

Table 7.1. Nanomaterials formed in w/o microemulsions

Nanoparticle system

Example

Reference

Metals/Metal alloys

Rojas et al., 2005

Boutonnet et al., 1982

Taleb et al., 1997

Herrera et al., 2005

Pt-Ru

Xiong and Manthiram, 2005

Fe-Pt

Carpenter et al., 2002

Pd-Co-Au

Raghuveer et al., 2006

ZnS

Metal chalcogenides

Khiew et al., 2005

PbS

Eastoe et al., 1995

RuSe

Venkateswara Rao and Viswanathan, 2007

Pb2Ru2O7

Metal oxides

Raghuveer et al., 2002

CeO2

Bumajdad et al., 2004

CoFe2O4

Liu et al., 2003

LiNi0.8Co0.2O2

Lu et al., 2000

Core-shell nanoparticles

Fe3O4/SiO2

Tago et al., 2002

Fe/Au

Zhou et al., 2000

The Evolving Synthetic Strategies in Chemistry

7.7

4. Synthesis of Core-Shell Nanoparticles

Some of the metallic nanoparticles like Fe, Co, Ni are susceptible to rapid oxidation. This

problem can be largely circumvented by coating the nanoparticles with gold or other inert

metals. The technique for applying gold coatings on metal nanoparticles is reasonably

straightforward and simply adds an additional step to the reverse micelle synthesis. In this

process, a water-soluble gold salt (HAuCl4) is dissolved and dispersed in a separate reverse

micelle solution that is then added to the metal-containing reverse micelle solution that has

already been reduced with an excess of BH4-. The aqueous AuCl4- ions encapsulate the metal

particles and are subsequently reduced by forming a metallic gold shell around the metal

particles.

8AuCl4- + 3 BH4- + 9H2O → 8Au + 3B(OH)3 + 21H+ + 32Cl-

The preparation of core-shell type structures is not limited to metals as the core or shell

materials. Combinations of precipitation, reduction and hydrolysis reactions can be performed

sequentially to produce oxides coated with metals, oxides coated with oxides, and so forth.

EFFECTS OF THE PARAMETERS ON THE FORMATION OF NANOPARTICLES IN

MICROEMULSION

The main parameters that influence the size of nanoparticle are molar ratio, W =

[water]/[surfactant], type of solvent employed, surfactant or co-surfactants used and

concentration of reagents. Recent investigations suggest that the particle shape also can be

affected by the influence of micellar template, added anions and molecular adsorption. But a

general method for controlling nanocrystal shapes through soft chemistry has not yet been found.

The major factors that influence the formation of nanoparticles are explained with examples

below.

Water-to-Surfactant Ratio, W

The size of the metallic particle will depend on the size of the droplets in the microemulsion. The

droplet size will be influenced by the water-to-surfactant ratio, W.

In general, the volume of water in microemulsions is proportional to the cubic radius of the

water core. And also the amount of surfactant which is present as a film around the water cores is

proportional to the surface area of the micro/nanodroplets. So the molar ratio of water to

surfactant (W) has a linear relationship with the radii of the water cores (Rw). It has been

observed in AOT microemulsion system that the water-pool radius increases with the water

7.8

Microemulsion Techniques

content. An increase of molar ratio, W at constant concentration of surfactant will increase the

average diameter of the droplets. Consequently, the obtained nanoparticles will be large in size.

Table 7.2. Characteristic of silver particles obtained in w/o microemulsion (data taken from ref.

23)

Reducing agent

Conc. of reducing

Particle size (nm)

agent (M)

2.5 x 10-4

NaBH4

2.5 x 10-4

2.7

NaBH4

1.0 x 10-5

NaBH4

7.5

1.0 x 10-4

4.5

7.5

NaBH4

2.5 x 10-4

5.5

NaBH4

7.5

1.0 x 10-4

6.0

NaBH4

2.5 x 10-4

7.0

NaBH4

For example, by dynamic light scattering technique, it has been confirmed that the water core

radius, Rw, is coincident with the equation Rw(nm) = 0.18W + 1.5 in a wide range of alkanes. For

example in the AOT-water-isooctane microemulsion system, it was found that the linear

relationship between Rw and W was Rw (Å) = 1.5W. Thus the linear relationship may be different

in various systems. Generally, low water content is favorable to form smaller microemulsion

droplets (reverse micelles) which yield fine nanoparticles with a narrow size distribution. On the

contrary, high water content is favorable to form bigger microemulsion droplets which are easy

to fabricate larger particles. The reason for the behaviour is explained as follows: Actually, at

low water content, the water solubilized in the polar core is bound by the surfactant molecules,

which increases the boundary strength and decreases the intermicellar exchange rate. As a result,

a decrease in water content induces formation of monodisperse nanoparticles with small particle

size. However, the bound water would turn into bulk water with an increase in water content,

which is benefit for the water pools to exchange their contents by collisions and makes the

chemical reaction or co-precipitation between compounds solubilized in two different reverse

micelles complete more quickly. Since the nature of bulk water is drastically different from that

of bound water, reactants would be rapidly transferred from one water core to another, and thus

The Evolving Synthetic Strategies in Chemistry

7.9

the resultant particle size is relatively big and the size distribution becomes relatively wide. The

variation of silver and Fe nanoparticles size with W is given in Tables 7.2 and 7.3 respectively.

Table 7.3. Characteristic of iron particles obtained in w/o microemulsion (Data taken from ref.

24)

[FeCl2]x104

[NaBH4]x103

Particle size

S. No

[AOT]

(mol/dm3)

(nm)

3.9

3.5

7.5

0.1

4.1

0.68

1.6

0.1

2.5

0.35

0.93

0.05

1.9

0.025

2.5

0.025

3.3

0.025

4.3

0.025

4.7

0.025

5.4

0.025

5.8

0.025

Solvent Effect

Particle size is affected by solvent type. This was shown initially by Pileni (2003) in a study on

silver nanoparticles, in which larger particles were seen (by TEM) to be formed in isooctane than

in cyclohexane. This is probably due to the significant difference in intermicellar exchange rate

constant between the two solvents - a factor of 10.

The change in growth rate has been explained in the following way:

(i)

Smaller and less bulky solvent molecules with lower molecular volumes such as

cyclohexane, can penetrate between surfactant tails and increase the surfactant

curvature and rigidity. The increased rigidity at the interface may lead to a slower

growth rate.

Table of Contents: