MICROEMULSION TECHNIQUES:Significance of Packing Parameter

<< TEMPLATE BASED SYNTHESISSynthesis, Mechanism and Pathway

SYNTHESIS BY SOLID STATE DECOMPOSITION:DECOMPOSITION METHODS >>

7.10

Microemulsion Techniques

(ii)

Isooctane, being bulkier with a larger molecular volume, cannot penetrate the

surfactant tails so efficiently, thereby leading to a more fluid interface and thus faster

growth rates.

Although these ideas provide plausible explanations of the phenomena they remain

controversial.

Measurement of the surfactant film rigidities in microemulsions show that the solvent

type has only a minor effect. Solvent molecular volume may also explain the observed

change in final particle size. It was proposed that a more stable micelle system arises from

greater interactions between the solvent and surfactant tails which in turn leads to an

enhanced ability to stabilize larger particles. Any increase in rate of intermicellar exchange

will result in a higher rate of growth comparable to nucleation, hence is likely to generate

systems with lower polydispersity. Effect of solvent on the size of the Ag nanoparticles is

given in Table 7.4.

Table 7.4. Effect of solvent on the absorption spectra of silver nanoparticles synthesized in

reverse micelles of AOT (data taken from ref. 22)

System

Particle size (nm)

6.0

AOT/decane

22.0

AOT/heptane

5.4

AOT/cyclohexane

Surfactants and co-surfactants

Various studies showed that the choice of surfactant is critical to the size, shape and stability of

the particles. The most commonly used surfactant is the anionic AOT, although a variety of

common cationic surfactants are also frequently employed, such as CTAB or DDAB (di-n-

didodecyldimethylammoniumbromide) and non-ionics Triton X-100, polyoxyethylene (5)

nonylphenyl ether (NP-5) or polyoxyethylene (9) nonylphenyl ether (NP-9). For some systems

co-surfactants (intermediate chain length alcohols, such as n-butanol or n-hexanol) are also

employed.

The Evolving Synthetic Strategies in Chemistry

7.11

Among the anionic surfactants that form reverse micelles, the best known are the systems

derived from AOT (sodium 1,4-bis-2-ethylhexylsulfosuccinate) in different non-polar media.

The reasons are as follows:

(i)

AOT has a well-known V-shaped molecular geometry, giving rise to stable reverse

micelles without co-surfactant.

(ii)

AOT has the remarkable ability to solubilize water with values of W (W =

[H2O]/[AOT]) as large as 40-60 depending on the surrounding non-polar medium, the

solute and the temperature; however the droplet size depends only on the water

amount, W. The bulk properties of water (polarity, viscosity, hydrogen bond ability,

etc.) either inside the pool (free) or at the interface (bound) change with W.

It is also known that addition of co-surfactant can reduce the surfactant concentration in

microemulsion preparation. Normally, low molecular weight alcohols, such as n-butanol can be

used for this purpose. Their short hydrophobic chain and terminal hydroxyl group is known to

enhance the interaction with surfactant monolayers at the interface, which can influence the

curvature of the interface and internal energy. The amphiphilic nature of co-surfactants could

also enable them to distribute between the aqueous and oil phase.

In general, it was concluded that the addition of a co-surfactant leads to a higher fluidity of

the interfacial film, thus increasing the rate of intermicellar exchange (but also leading to a

higher curvature of the droplets), so smaller particles.

Surfactant Concentration

When the amount of water and oil is kept at fixed values, an increase of the amount of surfactant

will increase the number of droplets. It means that the number of metal ions per droplet will

decrease and consequently the size of the particles. The morphology of reverse micelles is

different with the surfactant concentration. At different concentrations, the surfactant molecules

can form various molecule aggregations (Fig. 7.2).

7.12

Microemulsion Techniques

Fig. 7.2. Structures of different micelles (reproduced from ref. 26)

It was found that the change of micelle structures had an energetic barrier. The main points are

given as follows:

(i)

At a low surfactant concentration, only spherical micelles appear in solution.

(ii)

When the surfactant concentration in the solution reaches a well-defined saturation

value, i.e., second critical micelle concentration (second CMC), the energetic barrier

will be overcome and the micelle structure will change from a spherical micelle to

other special structure, and then the micelle will be steady again at a new

concentration range. For example, if the concentration of surfactant attains to 40

50%, the spherical micelle turns into rod-shaped or column-shaped micelle.

Furthermore, the micelle is also able to self-assemble into layer or liquid crystal

structure.

The special micelles formed at different concentrations of surfactants can be usually used as

effective structure-directing agents to prepare nanoparticles with desired morphologies. Thus, the

micelle formed by the surfactant with a proper concentration can offer an appropriate growth

condition for nanoparticles.

The Evolving Synthetic Strategies in Chemistry

7.13

Nature of the Precipitating Agent (reducing agent)

The main point that should be followed in the selection of suitable reducing agent in the

preparation of nanoparticles is it must be stable in an aqueous environment and does not react

with the other components of the reverse micelle system. As a general rule, a fast nucleation

process will result in the production of small particles.

In most of the cases, water soluble sodium borohydride and hydrazine (usually N2H4.2HCl or

N2H4.H2O) are used as effective reducing agents. Eventhough bubbled H2 gas results in the

reduction of metal particles, the kinetics is not desirable, particularly at room temperature. As it

was mentioned earlier, faster exchange between the reactants and fast nucleation process will

give smaller particles. So both sodium borohydride (NaBH4) and hydrazine are efficient reducing

agents for most of the transition metal salts. The reduction process is in this case completed

instantly and is very fast in comparison to pure hydrogen.

When increasing the concentration of hydrazine while the concentration of metal salt is kept

constant, a decrease in the particle size is observed. This was shown when Ni particles were

prepared in a microemulsion containing cetyltriammonium bromide (CTAB) as surfactant, n-

hexanol as oil phase, water as aqueous phase and hydrazine as reducing agent at a temperature of

350 K (Capek, 2004). The diameter of the nickel particles decreases when the ratio of the

hydrazine to nickel chloride concentrations increases. The diameter of the particles reaches a

constant value when this ratio is above 10.

Effect of the Micellar Template

It was claimed that particle shape can be controlled by using micellar templates. A simple

surfactant/water/oil system can produce many different self-assembly structures: by changing

composition, one can obtain spheres (reverse micelles or micelles), cylinders, interconnected

cylinders and planes, termed lamellar phases, which also can re-organise into onion-type

structures. Hence in theory many possible nanoparticle structures could be grown inside these

different shaped templates. Despite plenty of work, there is still much controversial debate on

these aspects.

Influence of ion/molecular adsorption

Anion species added as electrolyte is important for generating different shapes of metal

nanocrystals. The initial micellar shape is shown to be largely unaffected by these additives. For

example, in the case of copper nanoparticle systems, a large excess of hydrazine favours disk

7.14

Microemulsion Techniques

over spherical particles. In both cases, Pileni et al. (2003) postulate that selective adsorption of

molecules or ions on to facets of the nanocrystal affects growth in certain directions, explaining

the apparent preference for certain shapes.

It was also known that pH affects the shape of nanocrystals, for example, nanostructured

NiZn ferrites (Uskokovic et al., 2005). When the pH is lower, needle-like nanocrystals are

formed, whereas other spheres are observed at higher pH. One possible reason for this is due to

an increased number of hydroxyl ions at higher pH which eliminate the sulphate and bromide

ions, hampering their ability to promote uniaxial growth.

SUMMARY

The most remarkable features of the microemulsion technique are: (i) particle size and

composition can be controlled to a great extent and a narrow particle size distribution can be

obtained and (ii) Bimetallic particles can be obtained at room temperature.

A large number of different nano-materials can be synthesised in water-in-oil microemulsions

and reverse micelles. Particle growth has shown to be strongly dependent on intermicellar

exchange rates. The resultant particle size appears to be dependent on dominant parameters,

namely solvent type, surfactant/co-surfactant type, concentration of the reagents and composition

via [water]:[surfactant] ratio, W.

However, the generality of the factors that affects the shape of particle remains to be established.

REFERENCES

1. K. Tapas and A. Maitra, Adv. Colloid Interface Sci., 59 (1995) 95.

2. J.J. Silber, U.A. Biasuttia, E. Abuinb and E. Lissi, Adv. Colloid Interface Sci., 82 (1999)

189.

3. T. Tadros, P. Izquierdo, J. Esquena and C. Solans, Adv. Colloid Interface Sci., 108-109

(2004) 303.

4. S. Eriksson, Ulf Nylén, S. Rojas and M. Boutonnet, Applied Catalysis A: General 265

(2004) 207.

5. T.P. Hoar and J.H. Schulman, Nature 152 (1943) 102.

6. M.-P. Pileni, Nature 2 (2003)145.

7. T. Tago, T. Hatsuta, K. Miyajima, M. Kishida, S. Tashiro and K. Wakabayashi, J. Am.

Ceram. Soc. 85 (2002) 2188.

The Evolving Synthetic Strategies in Chemistry

7.15

8. S. Rojas, F.J. Garcia-Garcia, S. Jaras, M.V. Martinez-Huerta, J.L. Garcia Fierro and M.

Boutonnet, Applied Catalysis A: General 285 (2005) 24.

9. M. Boutonnet, J. Kitzling and P. Stenius, Colloids. Surf. 5 (1982) 209.

10. A.P. Herrera, O. Resto, J.G. Briano and C. Rinaldi, Nanotechnology 16 (2005) S618.

11. L. Xiong and A. Manthiram, Solid State Ionics 176 (2005) 385.

12. V. Raghuveer, P.J. Ferreira, A. Manthiram, Electrochem. Commun. 8 (2006) 807.

13. P.S. Khiew, S. Radiman, N.M. Huang, Md Soot Ahmad and K. Nadarajah, Mater. Lett.

59 (2005) 989.

14. J. Eastoe and A.R. Cox, Colloids Surf A Physicochem. Eng. Asp. 101 (1995) 63.

15. Ch. Venkateswara Rao and B. Viswanathan, J. Phys. Chem. C 111 (2007) 16538.

16. V. Raghuveer, Keshav, Kumar and B.Viswanathan, Indian J. Eng. Mater. Sci. 9 (2002)

137.

17. A. Bumajdad, M.I. Zaki, J. Eastoe and L. Pasupulety, Langmuir 20 (2004)11223.

18. S. Xing, Y. Chu, X. Sui and Z. Wu, J. Mater. Sci. 40 (2005) 215.

19. E. Carpenter, J.A. Sims, J.A. Wienmann, W.L. Zhou and C.J. O'Connor, J. Appl. Phys.

87 (2000) 5615.

20. R. Schomäcker, M.-J. Schwuger and K. Stickdorn, Chem. Rev. 95 (1995) 849.

21. V. Uskokovic and M. Drofenik, Colloids Surf. A Physicochem. Eng. Asp. 266 (2005)168.

22. R.P. Bagwe and K.C. Khilar, Langmuir 16 (2000) 905.

23. C. Petit, P. Lixon and M.P. Pileni, J. Phys. Chem. 97 (1993) 12974.

24. I. Capek, Adv. Colloid Interface Sci., 110 (2004) 49.

25. Taleb, C. Petit and M.P. Pileni, Chem. Mater. 9 (1997) 950.

26. J.H. Fendler, Chem. Rev. 87 (1987) 877.

27. W.L. Zhou, E.E. Carpenter, J. Sims, A. Kumbhar and C.J. O'Connor, Mater. Res. Soc.

Symp. Proc. 581 (2000) 107.

28. C.-H. Lu and H.-C. Wang, J. Mater. Chem. 13 (2003) 428.

29. C. Liu, A.J. Rondinone and Z.J. Zhang, Pure Appl. Chem. 72 (2000) 37.

Table of Contents: