synthesis of materials within reverse micelles

Transcription

June 2, 2005 12:33 00700
Surface Review and Letters, Vol. 12, No. 2 (2005) 239–277
c World Scientific Publishing Company
SYNTHESIS OF MATERIALS WITHIN REVERSE MICELLES
VUK USKOKOVIĆ∗ and MIHA DROFENIK†
Jožef Stefan Institute, Jamova 38, 1000 Ljubljana, Slovenia
†
Faculty of Chemistry and Chemical Engineering, Smetanova 17, 2000 Maribor, Slovenia
∗
[email protected]
Received 1 March 2005
Reverse micelles as nanosized aqueous droplets existing at certain compositions of water-in-oil
microemulsions are widely used today in the synthesis of many types of nanoparticles. However, without a rich conceptual network that would correlate the properties and compositions of
reverse micellar microemulsions to the properties of to-be-obtained particles, the design procedures in these cases usually rely on a trial-and-error approach. As like every other science, what
is presently known is merely the tip of the iceberg compared to the uninvestigated vastness still
lying below. The aim of this article is to present readers with most of the major achievements
from the field of materials synthesis within reverse micelles since the first such synthesis was
performed in 1982 until today, to possibly open up new perspectives of viewing the typical problems that nowadays dominate the field, and to hopefully initiate the observation and generation
of their actual solutions. We intend to show that by refining the oversimplified representations
of the roles that reverse micelles play in the processes of nanoparticles synthesis, steps toward
a more complex and realistic view of the concerned relationships can be made.
The first two sections of the review are of introductory character, presenting the reader with
the basic concepts and ideas that serve as the foundations of the field of reverse micellar synthesis of materials. Applications of reverse micelles, other than as media for materials synthesis,
as well as their basic structures and origins, together with experimental methods for evaluating their structural and dynamic properties, basic chemicals used for their preparation and
simplified explanations of the preparation of materials within, will be reviewed in these two
introductory sections. In Secs. 3 and 4, we shall proceed with reviewing the structural and
dynamic properties of reverse micelles, respectively, assuming that knowledge of both static
and dynamic parameters of microemulsions and changes induced thereof, are a necessary step
prior to putting forth any correlations between the parameters that define the properties of
microemulsions and the parameters that define the properties of materials synthesized within.
Typical pathways of synthesis will be presented in Sec. 5, whereas basic parameters used to
describe correlations between the properties of microemulsion reaction media and materials
prepared within, including reagent concentrations, ionic strength, temperature, aging time and
some of the normally overlooked influences, will be mentioned in Sec. 6. The whole of Sec. 7
is devoted to reviewing water-to-surfactant molar ratio as the most often used parameter in
materials design by performing reverse micellar synthesis routes. The mechanisms of particle
formation within precipitation synthesis in reverse micelles is discussed in Sec. 8. Synthesis of
composites, with special emphasis on silica composites, is described in Sec. 9. All types of materials, classified according to their chemical compositions, that were, to our knowledge, synthesized
by using reverse micelles up-to-date, will be briefly mentioned and pointed to the corresponding references in Sec. 10. In Sec. 11, some of the possible future directions for the synthesis of
nanostructured materials within reverse micelles, found in combining reverse micellar syntheses
∗
Corresponding author.
239
June 2, 2005 12:33 00700
240
V. Uskoković & M. Drofenik
and various other synthesis procedures with the aim of reaching self-organizing nanoparticle
systems, will be outlined.
Keywords: Nanomaterials; reverse micelles; review; synthesis.
1. Introduction
Reverse micelles as nanoscale hydrophilic cavities of microemulsions have been known since the
1960s, but these diverse multimolecular structures
were for the first time used as nano-templates for
materials synthesis (of monodispersed Pt, Pd, Rh
and Ir particles) in 1982.1 After these pioneering researches, many different materials comprising
de-agglomerated and monodispersed particles
(Fig. 1) have been prepared2–6 by using reverse
micelles. Reverse micellar synthesis of materials
belongs to the class of wet materials synthesis procedures, and exhibits, in general, all the advantages
that usually accompany other wet approaches to
materials synthesis. Excellent control of the final
powders’ stoichiometries with possibilities of obtaining homogeneity and mixing on the atomic scale,
narrow particle sizes distributions, negligible
contamination of the product during the homogenization of the starting compounds, low energy consumption, low aging times and simple equipment,
are some of the ordinary qualities of wet syntheses,
especially when compared to high-temperature traditional routes for the synthesis of common ceramic
and metallic materials. Improved control of the particle sizes, shapes, uniformity and dispersity are
additional general advantages of reverse micellar
synthesis compared to other, bulk wet approaches.
Materials resulting from wet powder preparation
might have extremely small sizes, which implies a
number of potential advantages, such as lower sintering temperatures in case of the preparation of
ceramic materials. In cases where high-temperature
treatment cannot be completely avoided by roomtemperature aging procedures in reverse micelles,
the formation of nanoparticles with high specificsurface area can enable calcination temperatures
to be set lower compared to traditional, solid-state
approaches.
Organized self-assembled surfactant phases have
recently received a lot of attention as reaction and
templating media, but have for a long time been used
(a)
(b)
Fig. 1. (a) La–Ni oxalate nanoparticles5 and (b) silica
particles6 as obtained by reverse micellar synthesis in
microemulsion.
for many other purposes:
• As wash and cleaning systems — since they facilitate the solubilization of both hydrophobic and
hydrophilic components at the same time and at
reduced temperatures.
• As separators — due to selective solubilization of
certain molecules.
• As lubrication compounds.
• As pharmaceuticals.
June 2, 2005 12:33 00700
Synthesis of Materials within Reverse Micelles
• As fuels — due to improved fuel atomization and
evaporation of water which increases the heat and
temperature of combustion.
• As a medium in crude oil exploitation — due to
their surface activity.
• As catalyzers7,8 or inhibitors9 of biochemical
enzyme-driven reactions — due to compartmentalization of reactants and products as well as
changes in activity and substrate specificity of
enzymes due to alteration in solubilized enzymes
conformations when accommodating the micellar
interior structure.
• In preparative organic chemistry, in order to overcome reactant solubility problems due to the ability of microemulsions to solubilize both polar and
nonpolar substances and to compartmentalize and
concentrate reactants.10
• For storing bioactive chemical reagents.11
• As a cell membrane-mimetic medium for the study
of membrane interactions of bioactive peptides.
Encapsulating a protein in a reverse micelle and
dissolving it in a low-viscosity solvent can lower
the rotational correlation time of a protein and
thereby provide a novel strategy for studying proteins in a variety of contexts.12 Since it was
noted that denaturation of proteins can be prevented in reverse micelles,13 these self-organized
multimolecular assemblies have been used as lifemimicking systems,14 and as such, have received
large interest after the proposition of hypotheses that self-replicating biochemical reactions of
primordial planetary life were initiated in reverse
micelles made of glycerine molecules, palmitic,
stearic and oleic acid, which existed at air–water
interfaces close to the shores of some ancient
seas.15 Treating reverse micelles as active and even
as the most basic structures of life was especially
emphasized after the discovery of the possibility of
initiating self-replication of reverse micelles due to
a reaction ocurring within micellar structures.16,17
2. Basic View of Reverse Micelles,
Microemulsions and Their
Potentialities Within Materials
Synthesis
Reverse micelles exist at certain compositional range
of water-in-oil microemulsions. Microemulsions — a
term coined by J. H. Schulman in 195918 — are
transparent thermodynamically stable dispersions
241
of two immiscible liquids containing appropriate
amounts of surfactant. Amphiphilic substances, to
which surfactants belong, possess significantly distanced hydrophilic and hydrophobic parts within
their molecules, and, therefore, each of the two
parts of the surfactant molecule has its preferent
solvent. Two mutually immiscible components are
usually water and an alcohol, hydrocarbon or,
lately, environmentally-viable supercritical carbon
dioxide,19,20 which proves to be of extreme convenience due to solving the problems of difficult
separation and removal of solvent from products
in conventional reverse micellar syntheses. Surfactant monolayers separate water and oil domains and
hence reduce the unfavorable oil–water contact. In
contrast to macroscopic emulsions which are thermodynamically unstable, nanosized microemulsion
droplets are formed spontaneously and, although
the reverse micellar systems are heterogenous on a
molecular scale, they are equilibrium phases and are
thus thermodynamically stable. Since the interactions between polar head-groups of the surfactant
molecule and the interactions between the nonpolar
tails favor only aggregates of a very specific size and
molecular configuration, microemulsions typically
have narrow droplet size distributions. The droplet
uniformity of microemulsions is especially important,
having a direct effect on the distribution of resulting
particle sizes during precipitation reactions.
Colloidal particle formation is a complex process, which involves interplay between nucleation,
nanocrystal formation, intermediate growth as well
as eventual coagulation and flocculation, that all
depend on the specific interactions between ionic and
molecular species within microemulsion. The success of the reverse micellar procedure for materials
synthesis is closely related to the fact that particle
nucleation can be initiated simultaneously at a large
number of locations within reverse micelles, with the
nucleation sites well isolated from each other due to
the presence of surfactant films that may act as stabilizers of the formed particles. Normally, monodisperse particles are formed only when the nucleation
and growth stages are strictly separated, which is a
property of reverse micelle material synthesis due to
the uniform nanodroplet structure and specific intermicellar interactions.
Reverse micelle phases are tiny droplets of
water, encapsulated by surfactant molecules and thus
June 2, 2005 12:33 00700
242
physically separated from the oil phase (Fig. 2).
Simplified representation of the reverse micellar
preparation of particles takes that aqueous “pools” of
the reverse micelles act as nanoreactors for performing simple reactions of synthesis, and that the sizes
of the microcrystals of the product are directly determined by the sizes of these pools.21–23 It is possible
to control the sizes of reverse micelles by controlling
the parameter w, defined as molar ratio of water-tosurfactant. The higher the w, the larger the water
pools of the micelles and the nanoparticles formed
within, and vice versa. Although such a correspondence between the size of the synthesized nanoparticle and the parameter w was approved in many
experiments, it has been put into question lately,
since within a number of performed syntheses a
(a)
(b)
Fig. 2. A drawing of (a) a reverse micelle and (b) a
more realistic model of reverse micelle.35 Blue spheres
represent surfactant’s head-groups, whereby smaller
yellow spheres denote counterions. Note that the surfactant head groups do not completely shield the aqueous
interior of the modelled reverse micelle (b).
similar correspondence could not be established.
Therefore, the dynamic interaction among micelles
has since lately been generally considered as the most
important factor that influences the morphologies
and properties of the final products.24
Particle size control results from regulation of
the nucleation site size and the particle growth rate.
Within reverse micellar materials synthesis, these
two factors are often implicitly conveyed to reverse
micelle sizes and material exchange between reverse
micelles, respectively. It has been observed that
many properties of the synthesized powders can be
controlled, and thus designed by using proper conditions, regarding primarily the composition of a parent microemulsion. Control over quantum states of
the particles or interparticle spacing can lead to novel
mesoscopic properties of materials, which are sometimes very different from those of their atomic and
bulk counterparts.25,26 Not only it is possible to synthesize nanosized uniform particles, but the obtained
particles might also be extremely well dispersed,2
which is a key property for some applications. Even
though reaction kinetics are neglected in many models since the intermicellar exchange is slow and governs the growth of particles, enhancement of reaction
rates is also one of the known possible advantages of
micellar synthesis routes.24,27
Since nanosized reverse micelles cannot yet
be directly observed in their dynamic interactions within a nonpolar medium, indirect techniques are usually applied in order to evaluate
certain, both static and dynamic properties of the
micelles. In the approximations and different implicit
pre-suppositions of various such techniques lie the
answer to the sometimes pronounced impossibility
of matching28 the concluded properties attributed
to the same systems by using different experimental methods.
Various indirect experimental methods may be
used for the evaluation of the microemulsions’
phase diagrams, but generally, transparency measurements, conductivity measurements29 and various
dynamic spectroscopy methods are the most often
used. Scattering methods have in general become
standard methods for probing microemulsions due to
nondestructivity and versatility of such approaches.
Since reverse microemulsion systems typically have
June 2, 2005 12:33 00700
a high degree of optical clarity, the transition from
the translucent, bright color of microemulsion to
a turbid, opaque or viscous white solution might
visually (with the naked eye) be interpreted as the
breakdown of the microemulsion.30,31 However, it
was shown that the solubilization limits obtained by
titration are different from those obtained by contacting the organic phase with an excess aqueous
phase,32 and such a difference was ascribed to the
fact that in the titration method, the composition of
water pools is fixed by the composition of the titrant,
while in the contacting method, the composition of
the water pools depends on the exchange of ions
between the reverse micelles and the excess aqueous phase.32 Conductivity measurements are often
used to routinely characterize microemulsion systems
and determine the maximum amount of water that
can be introduced into the system with maintaining
the given water-in-oil system33 in stable condition.
Upon addition of water into the system, conductivity increases due to percolation of charges through
the droplet clusters, all until continuous introduction
of water into the system makes the microemulsion
unstable and eventually induces phase separation
resulting in sharp decrease in conductivity.
Since reverse micelles are most often regarded
as “nano-templates” or “nano-reactors” for the
synthesis of nanosized particles, special emphasis in the field of their investigation is, from the
point of view of materials synthesis, placed on
the attempts to determine their structural parameters, that is primarily micellar sizes and shapes
as well as spatial distributions of these parameters.
Beside many attempts to model reverse micellar
structures,34–36 initiated by the pioneering attempts
by Brown and Clarke,37 within indirect experimental methods that are regularly used in order to
determine or evaluate different properties of reverse
micelles, are included: light scattering (LS)38 and
small angle neutron scattering (SANS) studies,39–42
small angle X-ray scattering (SAXS),41 quasi-elastic
light scattering (QELS),43 infrared spectroscopy,
including FTIR,44,45 picosecond IR pump-probe
spectroscopy46 and near-IR spectroscopy,11 dielectric
spectroscopy,47,48 fluorescence light and visible
light scattering measurements,49 X-ray scattering
techniques,50,51 ESR spectroscopy,52 freeze-fracture
243
etching
transmission
electron
microscopy,53
53,54
NMR,
measurements of diffusion coefficients,55
conductometric
measurements29,33,56,57
spectrophotometric measurements,57,58 shear and
extensional viscosity measurements,59–61 photon
correlation spectroscopy,62 fluorescence quenching
measurements,63 dynamic light scattering,46 Raman
spectroscopy,64 and DSC measurements.45 In general, spectroscopy has been a primary tool for investigating the structures of reverse micelles. Combined
with time-resolved methods, the spectroscopic methods can also yield dynamic information.35
The principle factors for explaining structural
changes in microemulsions are surfactant shape,
entropy, energy terms, as well as solvent properties such as ionic force and pH. Some of the most
important quantities used for defining surfactants
activities within microemulsion systems are: critical micelle concentration (CMC) — defined as the
minimal concentration of surfactant molecules above
which micelles are formed; and aggregation number
— the number of surfactant molecules per micelle.
The phase diagram of CTAB/1-hexanol/water
microemulsion is shown in Fig. 3.65 The balance
between electrostatic or polar interactions, geometry packing factor and topology mostly determine
the geometry of the colloidal structure formed. The
region denoted as L2 in Fig. 3, typical of its low
water-to-oil phase ratio, belongs to the phase compositions at which reverse micelles exist as multimolecular structures that the microemulsion comprises.
Beside reverse micellar structures, membraneous
structures, bilayer (lamellar) and cubic liquid crystals, sponge phases, hexagonal rod-like structures,
and uninverted (regular) micellar structures can be
formed by varying the composition of the given, most
often three- or four-component microemulsion system. The general remark is that microemulsion systems are very difficult to treat when the number of
their components exceeds three. The use of normal
micelles,66–68 bicontinuous structures,69,70 water-inoil-in-water pseudovesicular structures71 or highly
percolated pearl-like structures72 for the materials
synthesis are also receiving more and more attention.
AOT [sodium bis(2-ethyl hexyl) sulfosuccinate]
and CTAB (cetyltrimethylammonium bromide) have
been two mostly used surfactants for materials
June 2, 2005 12:33 00700
244
Fig. 3.
Phase diagram of CTAB/1-hexanol/water microemulsion.65
Fig. 4. Molecular structures of some of the most commonly used surfactants as components of microemulsions
for materials synthesis.
synthesis (Fig. 4), but surfactants such as dodecyl
penta(oxyethylene)
ether
(C12E5),
ndodecyloctaoxyethylene glycol monoether (C12E8),
cetylbenzyldimethylammonium chloride (CBAC),
didodecyl-dimethylammonium bromide (DDAB),14,73
sorbitan monooleate,74 and sodium dodecylbenzenesulfonate (NaDBS)75 have all been used often. Surfactants can be ionic in nature — like anionic AOT
or cationic CTAB, or non-ionic (in which case they
can be zwitterionic or dipolar) — like Triton X-100
[polyoxyethylene(10)isooctylphenyl ether],76 polyoxyethylene(4) lauryl ether (known as Brij30),77
pentaoxyethylene-glycol-nonyl-phenyl ether (known
as Igepal-CO520)78 or the various mixtures of
poly(oxyethylene)5 nonylphenol ether (NP-5),79
poly(oxyethylene)9 nonylphenol ether (NP-9)80 and
poly(oxyethylene)12 nonylphenol ether (NP-12).
Cationic surfactants such as CTAB have been shown
not only to be effective in retaining the powder’s
properties that are the direct consequence of deagglomeration of particles — such as superparamagnetism is, for instance81 — but also to be effective
at condensing and thereby accelerating the transport of genetic material (DNA) across biological
membranes.82 Molecular structures of some of the
commonly used surfactants for the reverse micellar
materials synthesis, are shown in Fig. 4.
Whereas AOT/isooctane/water83 has been
the major AOT-based microemulsion for the
materials synthesis, AOT/cyclohexane/water, AOT/
n-heptane/water42,84 and AOT/toluene/water85
have been used as well. Many different microemulsions based on CTAB have been used: CTAB/
CTAB/2-octanol/water,1,87
hexanol/water,53,86
June 2, 2005 12:33 00700
CTAB/1-butanol/n-octane/water,29,69,88,89 CTAB/
and
CTAB/nn-pentanol/hexane/water,90,91
butanol/isooctane/water.92 It was suggested that
n-pentanol is, in combination with CTAB, a better co-surfactant compared to n-butanol since it has
a stronger van der Waals interaction with CTAB,
which ensures the formation of more compact and
stable interfacial film protecting nanoparticles from
aggregation and nonuniform growth.90 Hexane is
denoted as a desirable organic solvent since, due
to its high vapor pressure, it may induce the selforganization of monodisperse monocrystals upon
dewetting and/or rapid solvent evaporation and
even the formation of energetically unfavorable 1D
structures.90
Complex processes of particles formation in solution, which involve nucleation, growth, coagulation
and flocculation are largely influenced by the presence of surfactant and its ability to compartmentalize
reactants as is the case in reverse micellar microemulsions. This role may largely alter not only properties
of the final product such as atomic arrangement,
morphology, ground transition and product states by
affecting the step velocity and rate of crystal growth
in certain preferred directions,93 but by changing the
pathway of the reaction it may promote the formation of a product of different identity compared to
diluted aqueous solution94 or to the reverse micellar medium comprising different surfactants.93 Only
surfactant as an additional component in the precipitation reaction is often used in order to increase
the specific surface area of the products and stabilize
the colloidal solution,95–98 but using ligand or complexing stabilizing agents to inhibit particle growth
is a much older procedure than the microemulsion
technique.99 Surfactant additives have been shown
to act like mobile impurities and thus affecting the
step velocity and rate of crystal growth in certain
preferred directions.100 Compared to this approach,
reverse micelles present more complex structures
that medium for the wet synthesis comprises, since
in these systems nanosized droplets of water possess a significant individuality and specific organization, which is supposed to reflect on potentially
unique and monodispersed properties of the materials synthesized within them. Mixed surfactants
have also been used for the reverse micelle preparation of nanoparticles, and are shown to be especially promising for the formation of silica-coated
245
nanoparticles101,102 and 1D nanostructures,103,104
such as nanotubes, nanowires, nanorods, nanobelts
and tree-like superstructures, but the effect of any
used mixture is generally unpredictable. The components in a surfactant mixture could enhance the
efficiency of processes due to synergy or even antagonism among them under different conditions.105 In
general, due to the enormous complexity of physicochemical influences, the synthesis of materials within
microemulsions today relies more on trial-and-error
approaches and the tendency to reproduce experiments than on the formal predictions of morphology, dispersity and various other properties, prior
to the performance of the corresponding synthesis
procedures.
3. Structural Properties of Reverse
Micelles
Micellar structures are often presented as being
raspberry-like, with hydrophilic charged head-groups
closely packed to each other and hydrocarbon chains
stretched towards the center of the micelle. This picture is thought to be wrong for two reasons. First,
owing to electrostatic repulsion it is not possible to
spontaneously pack up to a hundred charged entities
close to each other, even if the counterion binding
is taken into account. Second, the conformation of
all tails being stretched and almost lined up would
lead to an enormous local pressure. As an example,
an NMR study showed that the uninverted micelles
formed by SDS have about one-third of their surface
covered by hydrocarbon tails.49
In an attempt to model reverse micelle by computer simulation, nearly all counterions of a reverse
micelle comprising 70 AOT molecules, 70 Na+ counterions and 525 water molecules, resided at the interface. The surfactant head-groups did not completely
shield the aqueous core from the nonpolar exterior,
and some water molecules were trapped between
head-groups in contact with the nonpolar phase. This
model suggests that it should be possible to catalyze a reaction of an insoluble probe at the micellar
interface.35
FTIR studies suggest that the water interior
of reverse micelles has a multilayered structure,
consisting of the so-called interfacial, intermediate
and core water. The interfacial layer is composed
of water molecules that are bounded directly to
June 2, 2005 12:33 00700
246
the surfactant’s polar head-groups; the intermediate layer consists of the next few nearest-neighbor
water molecules that can exchange their state with
interfacial water; and the core layer is found at
the interior of the water pool and has the properties of bulk water.44 It was reported that five
water molecules are tightly bound per one CTAB
molecule,52 whereby by using NMR measurements
it was realized that only two molecules of water are
tightly bound to one molecule of AOT.106 Thus, if
parameter w < 6 − 10, the water which occupies the
interior of a reverse micelle is highly structurized due
to association with the polar head-groups of the surfactant molecules, and the conditions for a typical
design of the particles in reverse micelles are said to
be not available.73,107 On the other hand, if w > 10,
micelles have a free water core with bulk water solvent characteristics.
Reverse micelles of various surfactants can solubilize different amounts of water. AOT reverse
micelles are known for their ability to enclose very
large amounts of water; parameter w ranges from
0 to 70 for many systems.35 Exactly in its ability
to solubilize large amounts of water while retaining
spherical micellar shapes in a variety of hydrophobic
organic solvents lies the reason for the often use of
AOT as a surfactant for the application of materials
synthesis.108 However, compared to AOT-based systems, CTAB reverse micellar systems, due to higher
flexibility of surfactant film that gives rise to a higher
exchange dynamics of the micelles, enable significantly higher solubilization capacities of high concentration aqueous salts.28
In contrast to CMC, micellar size varies with
various factors in a manner which is complex and
at present still difficult to predict.28 A large variety of different techniques, including classic light
scattering, viscosity measurements, tracer diffusion,
NMR and quasi-elastic light scattering have been
performed in order to obtain information about the
size distribution of micelles for given parameters such
as w or weight percentage of water or surfactant.
Though some of these methods gained very precise
results according to their performers, a clear criticism was raised regarding the reliability of these
methods.28 For a certain system, NMR measurements have clearly indicated that the distribution
of micelle size is narrow and somewhat asymmetrical around the average value,28 whereby the SANS
measurement results have shown that reverse
micelles are spherical and monodisperse at low water
content, whereas with increase in water content, the
micelles increase in size and polydispersity.109
With an increase in water content of a microemulsion system, the size of the reverse micelles increases
as well. In general, aggregation number together with
micelle radius increases with w. Other influences on
the aggregation number of micellar configurations
are dependent on the whole system used. Thus, it was
reported that the aggregation number showed little
dependence on the concentration of the surfactant
or the addition of salt (up to 2.3 M of sodium azide,
the salt consisting of azide ion — N=N=N− , which,
due to its small size and high charge is likely to dive
into the core of the investigated reverse micelles),44
whereby it was observed that size of the reverse
micelle is largely influenced by the ionic strength of
its aqueous pools. The aggregation number of CTAB
micelles increased from 81 to 121 with the addition
of NaOH in the concentration of 0.01 M, and micelle
shapes were found to change from spherical to elliptical with the addition of NaOH.110
As expected, micelle size increases with increasing alkyl chain length of the amphiphile. In several
cases, the growth in size is small and corresponds
to a retention of the same micellar shape, but in
others, shape changes must be invoked to explain
the data. Surfactants with longer tails will according
to theory have a lower CMC and a larger aggregation number than analogues with shorter tails; also,
counterions that are more strongly bound to the surfactant will induce a lower CMC and a higher aggregation number.49 Whereas the CMC depends little
on head-group structure and counterion (ionic surfactant in a solution dissociates on surfactant ion
and counterion), micelle size can vary by orders of
magnitude.
The aggregation number calculated for linear surfactants with C12 , C14 and C16 atoms by using
a theoretical model49 were 55, 75 and 95, respectively. These results are in agreement with experimental findings for the corresponding alkylsulphate
surfactants, whereby this model predicts aggregation numbers that are too small for the longest
alkyltrimethylammonium surfactant. This is due to
a combination of the bulkier head-group of this surfactant as well as a change in shape of the aggregate
from a spherical to a more prolate reverse micelle.
June 2, 2005 12:33 00700
However, the main point of the calculation is still
present — within a series of surfactants differing
in hydrocarbon chains length only, the aggregation
number should increase with increasing tail length.
Closely related to the fact that a single entity or
variable cannot be invoked for any scientific explanation, the size of the reverse micelles is not dependent upon any single variable, but on the complex
interactions which are conditional for their existence.
Thus, the microemulsion droplet sizes in CTABbased reverse micellar microemulsion were in the
range of 30–70 nm111 at 55◦ C, whereby spectroscopic studies have shown that size of the CTAB
reverse micelles is in the order of 1–4 nm for different water percentages.63 Reverse micelles based on
CTAB ranging in length from 10 to 1000 nm were
also found to coexist together.53
Albeit expectations to establishing direct links
between thermodynamic stability of reverse micelles
in microemulsions and their low polydispersity in
size, predictions based on theoretical calculations
have shown that micelles are broadly distributed in
size with polydispersity index equal to 2.106 Increase
in water content results not only in a nonlinear
increase in the average diameter of the nanodroplets
and the synthesized particles within, but in a more
enhanced polydispersity as well.3 For the cases of
growth to very long, rod-like micelles, the polydispersity becomes large. For many systems, the low degree
of polydispersity is explained by the fact that after
the increase in association constant up to a certain
aggregation number there is a region of marked anticooperativity with the equilibrium constant decreasing with aggregation number.
Until recently it was thought that only reverse
micelles spherical in shape exist. However, it
has been observed that the probe motion within
reverse micelle becomes more anisotropic with
increase in water content.52 It was proposed
that a slight anisotropic rotational motion of the
probe in micelles, as detected, for instance, by
electron spin resonance measurements, might be
explained by the formation of cylindrical aggregates. Generally, spherical micelles at low surfactant concentration (close to CMC) might, due to
aggregate–aggregate interactions, alter their geometries at higher monomer concentrations. For micellar systems showing the sphere-to-rod transition,
the anti-cooperativity, which lies in head-group
247
repulsions, is partly eliminated this way. As shown
by the case of CTAB, the elimination of headgroup repulsions can be brought about by counterions which may approach the charged groups
closely or intercalate between them, or by certain
solubilizates which are located in the head-group
region.28
Linear surfactants tend to form ellipsoidal
micelles more often than branched surfactants, which
almost always form spherical micelles. Thus, for
instance, a linear surfactant cetyltrimethylammonium 4-vinylbenzoate forms viscoelastic solutions in
water containing cylindrical micelles of 4 nm in diameter and thousands of nanometers long,40 whereby
AOT-based micelles are known to exist only in spherical shapes. In general, the micelles formed by surfactants with tails of moderate length (approximately
C10 − C16 ; note that both CTAB and AOT are C16
surfactants) are thought to be spherical or nearly
spherical — at least close to CMC.
One of the major challenges within modern surfactant science is drawing a precise link between
the geometry of surfactant self-assemblies and the
final structures of the synthesized materials.112 Many
cases in which acicular particles were unexpectedly
produced by reverse micellar synthesis were ascribed
to the templating effect of worm-shaped reverse
micelles.4,66,113 Reverse micelles based on CTAB as
the surfactant are known to produce such effects, so
another CMC at which spherical-to-worm-like transition occurs is ascribed to such microemulsion systems. It is also known that an alkali substance,110
salts66 or co-surfactant,53,114 such as 1-hexanol28 or
an alkane,66 might induce a spherical-to-worm-like
transition. Elliptical and spherical CTAB micelles
are known to be able to coexist.53,59 However,
adding water to the CTAB/water/n-pentanol/nhexane microemulsion always results in an increment
of the reverse micellar radii, whereby the micelle
shapes remains spherical.91
While the formation of rod-like micelles is well
demonstrated for several cases, especially when
CTAB is used as a surfactant, other geometries of large non-spherical micelles have not been
observed yet, which is in agreement with theoretical
predictions.28 Meanwhile, the computer simulation
of a reverse micelle (w = 10) formed from a novel
double-chained phosphate surfactant in CO2 , including 1616 water molecules, 160 surfactant molecules,
June 2, 2005 12:33 00700
248
160 counterions and 6991 CO2 molecules, showed
that the modeled reverse micelle is not restricted to
a spherical shape.35 Molecular interactions drive the
formation of the reverse micelles, leading to a range
of shapes.
Sphere-to-rod transitions in CTAB micelles at
higher concentrations have been reported in both the
presence and the absence of added salts for surfactant concentrations exceeding 0.10 M.51,56 The concentration at which these transitions occur depends
on the property measured. By using absolute SAXS
technique it was shown that at c ∼ 0.05 M, CTAB
micelles are spherical, and that at room temperature the sphere-to-rod transition occurs right above
0.05 M, at 50◦ C at 0.17 M and at 70◦ C at 0.25 M.51
There is a number of evidence indicating a transition from closely spherical to very long rod-like
aggregates for CTAB micelles at a concentration of
0.2–0.3 M.28 Increasing the temperature from 30
to 50◦ C, as well as substituting Cl− for Br− as
counterion, eliminates the transition. Addition of
small amounts of simple solubilizates, such as benzene or a long-chain alcohol (hexanol, octanol, etc.),
may markedly facilitate the transition to rod-like
micelles, whereby alkanes have no effect. Decreasing the alkyl chain length considerably increases
the transition concentration or eliminates the transition completely.28 By using SANS studies it was
shown that micelles of CTAB molecules in 0.2 M solution were ellipsoidal with semi-minor axis of 2.12 nm
and semi-major axis of 5.62 nm, whereby aggregation number was 186 and fractional charge 0.09.39
Other SANS studies have concluded that CTAB
micelles are ellipsoidal in shape, have an aggregation number of 177, and that in the presence of
hydrotropes the aggregation number increases dramatically with decrease in the fractional surface
charge of the micelles.115 Certain calculations related
with QELS measurements of CTAB micelles in aqueous solutions, and the diffusion of mesoscopic optical probes through the same solution have implied
extensive micellar growth and failure of the spherical
micelle assumption.43 It has also been reported that
CTAB forms rod-shaped micelles in aqueous systems
above the second CMC of 0.3 M and assembles into
hexagonal liquid crystals above 1.1 M.116
Sizes and shapes of the reverse micelles are
dependent on temperature. When the temperature
increases, the aggregation number was found to
increase as well.44 Although the CMC of ionic surfactants is insensitive to temperature changes, the
tendency to form micelles different from the spherical increases with decreasing temperature. The size
of the reverse micelles comprising nonionic surfactants increases with increasing temperature,28 such
that sometimes at temperatures just above the room
temperature, radii of the micelle increases together
with broadening of the micelle size distribution
and increased excluded volume effects in magnitude, which can be explained by considering the
sphere-to-rod or sphere-to-disk transitions of the
micelles.55 It is thought that two processes which
disrupt microemulsions, that is colescence and Ostwald ripening, accelerate at higher temperatures.
Coalescence of reverse micelles is Brownian motiondriven and hence is more present at higher temperatures. On the other side, Ostwald ripening, the
disruption of emulsions by the growth of larger
droplets at the expense of smaller ones, is driven by
Kelvin effect (a high curvature of small droplets creates a high internal Laplace pressure, which consequently increases the vapor pressure of the emulsified
monomers). The smaller the droplet is, the greater
the tendency for the droplet to shrink and disappear,
since the Laplace pressure increases as droplet diameter decreases. It has been shown that L2 range is
widening with increase in temperature.117
One common feature of reverse micellar systems is the relative lack of stability even without encapsulated salts. One way of minimizing this
phenomenon is including a co-surfactant in the
reverse micellar system. It was even noted that
for CTAB-based reverse micelles, a co-surfactant is
necessarily required for the formation of the stable microemulsion.118 The incorporation of short
chain alkanols makes reverse micelles more stable and
in addition, the presence of alkanols decreases the
aggregation number of the surfactant molecules and
the diameter of the reverse micelles.113 N-butanol
or SDS (sodium dodecyl sulfate) are often used as
co-surfactants, increasing the polarity of the surfactant and helping to stabilize the reverse micelle
solutions.73,119 N-butanol as a co-surfactant is used
together with CTAB to help decrease the fraction of the micellar head-group that is neutralized
and thereby increase the stability of the micelles.
June 2, 2005 12:33 00700
Without the addition of a co-surfactant, the amount
of free water available to carry on the reactions
is greatly reduced, as most of the water is locked
in the head-groups of surfactant molecules. When
using 1-hexanol as a co-surfactant in the system
CTAB/isooctane/water, the encapsulated enzyme’s
half life was increased 45-fold.118 As an oil-phase,
a non-branched alcohol has been considered as an
optimal choice, but the use of diesel oil (together
with CTAB as a surfactant, 1-butanol, 1-propanol,
1-octanol or 1-pentanol as a co-surfactant) — a complex mix of aliphatic alkanes with chain lengths
of up to C32 — was also noticed.30 In relation
to the application of microemulsions for materials
synthesis, it is generally recommended that surfactant and aqueous fluid be each from about 1–30%
by weight of the total system and the maximum
total amount of surfactant and water be up to 50%
(preferably to 30%). The amount of co-surfactant
is preferably between 25 and 75% by weight of
composition.73
Penetrating into the micellar interface, cosurfactant molecules change the mean distance
between the polar head-groups of surfactant
molecules and thus reduce the electrostatic repulsion
between the head-groups, promoting the sphericalto-worm-like micelle transition.59,114 This penetration may increase the volume of the micelle core,
which is equivalent to increasing effective head-group
(hydrophilic) cross-sectional area. It is well-known
that the curvature of a micellar aggregate (whose
aqueous side is concave rather than convex, being
an essential property of reversed micelles), which is
dependent on the interfacial tension between micelle
and oil phase,34 is strongly influenced by the ratio of
the effective head-group cross-sectional area to the
effective cross-sectional area of the aliphatic chain.
By decreasing this ratio, the surfactant aggregate
shapes should follow the trend: spheroidal micelle
→ worm-like (rod-shaped) micelles → bilayer structures → reverse structures.59 When the alcoholic
co-surfactant chain length decreases for a given
micellar radius, the thickness of the penetrated layer
is increased and the interaction between micelles is
therefore stronger, because attractive intermicellar
potential depends on the volume of the interlapping
region.62 The volume of this region increases with an
increase of micellar radius as well.
249
4. Dynamic Structure of Reverse
Micelles
In order to understand processes in which the synthesis of materials within reverse micelles takes place,
it is useful to outline the basic dynamic interaction
among reverse micelles. In Fig. 5, a schematic representation of certain processes ocurring within reverse
micellar microemulsion as a medium for the synthesis reaction, is shown. On one side, the reverse
micellar systems are extremely dynamic in nature
and such dynamic character lies at the basis of their
thermodynamical stability. On the other hand, the
appearances of reverse micelles which emerge from
numerous experimental results are one of a labile and
sensitive multimolecular configurations.107
Surfactants associate into liquid-like aggregates
that possess rapid molecular motions often on different time scales. The complexity of the system is
due to the highly dispersed and heterogeneous nature
of the overall phase. A microemulsion system during the synthesis procedure comprises a variety of
species such as surfactant molecules, solutes in the
aqueous phase, species which may be preferentially
distributed in one of the phases, and the solid particles formed in reaction followed by nucleation. Some
of the basic interactions in such a system include
the exchange of surfactant molecules between reverse
micelles and the bulk phase, the formation–breakup
of dimers, trimers, and lower-level aggregates which
may be present in relatively small concentrations,
intermicellar fusion–fission of reverse micelles, and
effects involving solute species such as intramicellar
kinetic reactions, particle nucleation, and growth via
intramicellar attachment and intermicellar exchange
of aqueous phase contents.24
For reactions in microemulsions involving reactant species completely confined within the dispersed
water droplets, a necessary step prior to the chemical reaction is the exchange of reactants by the coalescence of two droplets.120 Microemulsions have a
dynamic structure wherein the droplets of the dispersed phase diffuse through the continuous phase
and collide with each other. These collisions are
inelastic because droplets coalesce and temporarily
merge with each other, but subsequently break to
form separate droplets again, so that the average size
and number of the micelles remain the same as a
function of time.120 Surfactant film flexibility is one
June 2, 2005 12:33 00700
250
Fig. 5.
Schematic representation of entities and dynamic physicochemical effects existing in a reverse micellar system.24
of the key parameters in determining the intermicellar exchange and particle growth processes, since the
interdroplet exchange of the particles growing inside
the reverse micelles is inhibited by the inversion of
the film curvature in the fused dimer which, in turn,
depends on the film flexibility.99
The intermicellar exchange of content leads to
a distribution of material throughout the system,
and a mathematical description of this phenomenon
requires the use of population balances. Partial opening of the surfactant layer or interfacial transfer of
aqueous pool solubilizates present close to the micellar surface can result in a partial exchange of material. Under equilibrium conditions, the size and the
aggregation number of the reverse micelles are dictated by thermodynamics and equilibrium transport
between micelles and bulk phase. As is evident from
experimental reports, even the molecules of different coexisting species follow distributions of their
occupancy numbers, defined as the number of particle species solubilized in the micelle core. For noninteracting and random kinetics, the distribution is
known to be Poissonian, whereas geometric and binomial distributions can result under other conditions.
This feature clearly leads to a situation different from
continuous conditions.
The typical time scales that have been reported24
for different mechanistic effects in polar reverse
micellar systems are: 1–10 ns for diffusion-controlled
intermicellar reaction, 50–100 ns for the lifetime of
a pair of reactants confined to the micelle core,
300–500 ns for the formation of an encounter complex
via diffusion on the micellar surface, 1–100 ns for a
diffusion-controlled entry of a surfactant molecule
June 2, 2005 12:33 00700
onto the micellar surface from the bulk phase,
0.1–1 µs for diffusion-controlled collision of micellar
aggregates, and 0.1–1 ms for intermicellar exchange
to occur. While micellar re-orientation and the
molecular diffusion around the micelle occur on
the nanosecond scale, the local motion of spherical micelles is very fast (1–40 ps), compared to reorientation of rod-shaped aggregates, which is seen
as the third contribution to the relaxation times.121
The retention time of water molecules on the surface
of reverse micelles lies within a nanosecond in order
of magnitude.107 Although reverse micelles have been
studied in detail and are well characterized, especially on AOT systems, the mechanistic details of
reactions of finite rates, ultrafine particle formation,
and growth seem to have not been studied through
a single consistent mathematical model.24
There are many unanswered questions and challenges concerning role and behavior of surfactant
within microemulsion during the materials synthesis. Even if a universal relationship or a set of
relationships are developed between the structure of
surfactants and their behavior at interfaces, the question regarding their dynamics when adsorbed layers
are perturbed, as in the case of nanoparticles formation, will remain open due to specific interaction with
reactants and products of the synthesis procedure.105
It was shown that two populations of reverse micelles
exist depending on whether they encapsulate probe
molecules or not,13 which is an apparent sign of the
feedback effects of product properties on the structure of reverse micelles, neglected in models that
are built on implicit adoption of chemically inert,
nano-templating role of reverse micelles, and normally based on the introduction of parameter w as
the major influence on product’s properties. Reference 99, which is often cited, concerns the independence of the particle size of the synthesized Pt, Rh,
Pd and Ir, always in the range of 2–5 nm,1 on the
surfactant, water amount and reactant concentration
used in the experiments. Therefore, particle sizes can
be seen as controllable by solvent stabilization of the
particles, whereas the surfactant acts as a stabilizing
agent.10
There is a large attention paid to developing
a convenient theoretical framework for correlating
complex combinations of compositional, structural
and interactional parameters with morphological
properties of synthesized particles. Due to their
251
nonlinear nature, colloidal spheres are in general
difficult to model in their dynamic interactions.
Contrary to expectations based on DLVO theory122
(according to which, only few static charges on
colloidal particles’ surfaces can cause repulsions
strong enough to keep them stably separated), it
has been observed that like-charged colloidal particles sometimes attract each other.123 The attraction between aggregates is found to increase when
the micellar size increases, the alcohol chain length
decreases, the polar head-area increases and the
molecular volume of oil increases.38,124 Attractions
seem to be favored by highly charged spheres in
very low salt concentrations.123 Basically, the balance of forces acting on three energy scales —
Van der Waals attraction, randomizing influence
of thermal energy, and hierarchy of electrostatic
interactions among highly-charged colloidal particles and the single-charged simple ions around them
determines all static and dynamic properties of
microemulsions.
It is generally thought that since being confined to closed micellar structures, water molecules
have restricted motions and their diffusion slow and
approximately equal to the diffusion of the droplets.
If all the surfactant molecules are located at the
water — oil interface, they will obviously diffuse
with equal rates as the droplets too.121 However,
from comparing experimental results on the diffusion
behavior of various components in CTAB and watermediated reverse micellar microemulsion, it can be
realized that water has the highest self-diffusion coefficient (D = 2×10−9 m2 · s−1 ) — higher than bromide
ion (around 7 × 10−10 m2 · s−1 ), surfactant (around
8 × 10−11 m2 · s−1 ) and the least diffusive solubilizates with D ranging from 7 × 10−11 m2 · s−1 to
7 × 10−12 m2 · s−1 depending on the concentration.
Water, bromide ions and surfactant molecules had
diffusion coefficients almost independent on the concentration above CMC value.125
NMR measurements126,127 have shown that
mobility of the ionic surfactant’s molecular chain
progressively increases from its head-group to the
terminal methyl group. However, this is not always
the case since mobility of 2-ethyl side chain in AOT
molecule is very restricted. Anyhow, the general
opinion is that mobility of different parts of long surfactant molecules is independent on the surfactant’s
nature and presence of a co-surfactant. As water
June 2, 2005 12:33 00700
252
content in a four-component water-in-oil AOT-based
microemulsion is increased, mobility of surfactant’s
polar head-group progressively decreases. This effect
was ascribed to an increased order in interfacial layer
as well as to the decreased freedom for inner movement in the case of a larger number of molecules,
including the polar head when water content is
decreased. The largest increase in mobility was
observed at low water content when water molecules
were engaged in hydratation of sodium ions and
sulphonate head, and when the water core with properties of bulk water had not yet been formed.
Beside exchanging molecules of reactants by
direct coalescence of micelles, it is necessary to take
into account the exchange of molecules between
phases other than only interiors of aqueous nanodroplets. This exchange might in the general case
be considered so as to occur between bounded, interfacial states and free, bulk states. NMR investigations have revealed that the exchange time of
Na+ ion between the bounded and the free state in
AOT-based water-in-oil microemulsion is less than
10−4 s.128 However, the same time was reported to
lie in nanosecond range.129 On the other hand, investigations based on electric field jump130 concluded
that the retention time of I− ion in Stern’s layer is
about 10−7 s.
NMR investigations have concluded that the typical time for the exchange of alcohol molecules
between the interfacial layer, the continuum phase
and/or the dispersed phase (depending on its affinity) is a lot less than 10−4 s.107 The addition of NaCl
Fig. 6.
in dispersed aqueous phase leads to the decrease
in an exchange rate of alcohol molecules,131 probably due to an increase in compactness of surfactant
monolayers induced by an increase in ionic strength
of aqueous phase. The exchange rate of neutral
probe molecules between the interfacial monolayer
and the aqueous core of reverse micelle was estimated
to ∼ 107 /sec in AOT/heptane/water microemulsion
system at w = 31.132 The relaxation time of the
exchange of surfactant molecules between the interfacial monolayer and aqueous phase in oil-in-water
microemulsions was experimentally determined to
be 3 × 10−8 s,133 whereby proof for the existence
of the surfactant’s exchange between two separate
phases within water-in-oil microemulsions do not
yet exist. NMR researches have shown that movement of surfactant is largely limited to the interfacial space between the oil and water phases.107 It is
also known that the fluidity of the surfactant’s monolayer might be increased by decreasing the length of
alcohol co-surfactant’s hydrocarbon chain. Typical
times for the rearrangement of surfactant monolayers in birefringent microemulsions are estimated to
> 0.1 µs.134
Two major processes during which exchange
of reactants between reverse micelles takes place
are shown in Fig. 6. Solutes can be exchanged
between reverse micelles either by processes of coalescence of reverse micelles — during which temporary
merging of aqueous droplets occur (fusion) —
that subsequently fragment (fission), or by partial
fragmentation of droplets with consequent loss of
Processes of exchange of reactants (others than via continuous phase) in reverse micellar microemulsion.107
June 2, 2005 12:33 00700
aqueous solubilizates, that might later merge with
other droplets (coagulation).
There are many indications that the merging of
reverse micelles as well as fast intermicellar exchange
of their aqueous contents are diffusion-controlled and
are independent on the surfactant’s nature.135 The
time needed for single fusion of reverse micelles to
occur is estimated at ∼ 10−6 s, whereby the time
constant for the collision of reverse micelles with
subsequent merging is between 108 and 109 /Msec
in respect to the droplet concentration. A model
according to which intermicellar material exchange
occurs through a water channel formed at the point
of temporary merging of reverse micelles (temporary
dimer) has been considered.107
It has been discovered that compartmentalization of reactants in reverse micelles decreases
reaction rates due to the coupling of chemical
reaction and the rate of micellar mergings. However, there are many cases of catalytic effects
derived from the compartmentalization of reagents,
including the increase of the rate constant of
the hydrolysis of acetylsalicylic acid in the presence of imidazole catalyst by 55 times when
the reaction was performed in AOT/supercritical
ethane microemulsion compared to the aqueous
buffer,136 and the reaction of acid hydrolysis of
phthalomohydroxamic acid in AOT/isooctane/water
microemulsions.27 Kinetic analyses of the bimolecular rate constant of exchange ke of reaction
have concluded that rate constant ke is complex
since the reaction of exchange comprises Brownian diffusion of reverse micelles leading to collisions,
collision of two droplets, water-channel opening
(merging), diffusion and chemical reaction between
reactants, as well as fragmentation of a transient
dimer. The slowest step of these basic rate processes will scale the temporal aspects of the overall process of synthesis,137 and the second stage of
this complex reaction is decisive for limiting reaction rate. The rate constant ke takes an approximate
value of 107 /sec at room temperature.138 Therefore,
approximately one in a thousand collisions result
in a temporary merging of dispersed aqueous nanodroplets with exchange of reactants. However, such
253
effectiveness of reactants’ exchange is said to be valid
only for rigid surfactant films, such as AOT-formed
reverse micelles.10 For flexible films, such as the ones
that CTAB forms, one in ten collisions can give
rise to micellar content exchange.10 Large activation
entropy of this process is ascribed to the fact that
after the temporary merging of reverse micelles (that
is, the formation of activated state) some of surfactant molecules from the interfacial layer are released
to one of the separate phases. The lifetime of the
transient dimer cannot be longer than a few µs since
a longer lifetime would lead to the separation of
phases due to the uncontrolled droplets ripening.
Uneven fragmentation of the previously merged
droplets leads to an increased polydispersity of
reverse micelles. However, modification of the system ought to result in a decrease in polydispersity. In the case of microemulsion systems based
on CTAB and pentanol, large variations in reverse
micelles sizes were noticed, but an addition of alkane
led to a decrease in polydispersity, which was most
significant for the alkanes with long hydrocarbon
chains.107 Since larger ke implies that microemulsion acts more like a critical system, it is thought
that there must exist a correlation between the
increase in ke , the polydispersity of reverse micelle
sizes, their mutual interaction and the critical behavior of microemulsion. It has been observed that
a low reaction rate may lead to very large particles irrespective of the exchange protocol of micellar
content.139 The limiting situations concerning chemical reactions occurring between reactants enclosed in
reverse micelles — merging-controlled and reactioncontrolled cases are shown in Fig. 7.
Co-surfactant leads to improved chemical reaction rates within reverse micellar microemulsions. It
has been noted that the addition of benzylalcohol as
a co-surfactant increased ke by almost 20 times.140
The rate constant ke is dependent on the length
of alcoholic co-surfactant’s hydrocarbon chain —
it increased when 1-hexanol as a co-surfactant was
replaced by 1-pentanol.140 The value of ke is largely
dependent on the oil hydrocarbon chain’s length —
decrease in ke by a factor of 10 was noticed when
moving from dodecane to hexane.107 The micellar exchange rates of cyclohexane, heptane, and
decane in AOT-based microemulsions are approximately 106 , 107 and 108 M−1 · s−1 , respectively.137
In some cases, such as for AOT/n-heptane/water
June 2, 2005 12:33 00700
254
of the enhancement of reaction rates is not definitely known, but it is widely accepted that an electrostatic effect in the aqueous phase of the reverse
micelle is one of the reasons for the acceleration of
reactions. The properties of local reaction media are
quite different from those of the bulk solutions as a
consequence of the intense local electric fields, affecting all the relevant parameters that modulate the
reaction rates.10 Specific intermolecular interactions
at the hydrophilic sides of surfactants surrounding
the aqueous cores and specific water structure in
this region are proposed to have catalytic effects on
the rates of chemical changes.141 However, reaction
kinetics are neglected in many models because intermicellar material exchange is relatively slow, and it
plays a major role in particles growth.24
5. General Synthesis Procedures
Fig. 7. Limiting situations for the kinetics of chemical
reactions taking place within reverse micelles.107
system at w = 33, ke was only slightly smaller
(∼ 1.5 × 109/Msec) than the rate of droplet collisions
(∼ 109/Msec), which suggests that in that case reactants exchange between reverse micelles is no longer
limited by the rate of interfacial layer opening, but
primarily by the diffusion of droplets.107
Even though kinetic equations describing reactions of synthesis include factors such as merging
of micelles which carry reactants, that must happen prior to the reaction, increases in rates of chemical reactions taking place within reverse micellar
systems are known phenomena, and have recently
been almost taken as the general advantage of this
method.24 It has been reported that the rate of oxidation of Fe2+ and subsequent formation of needlelike FeOOH particles by spontaneous air oxidation
is from 100 to 1000 times faster in reverse micelles
than in a bulk solution, regardless of the differences
in surfactant or other conditions.52 In the case of
certain iron complexes, a two to tenfold increase
in the rate of dissociation was measured in comparison to the pure aqueous solution.121 The cause
One way to perform reverse micellar synthesis of
materials is to produce one parent microemulsion
and then to successively let reactants to diffuse into
the interior of reverse micelles and react. The major
problem with this so-called single-microemulsion
approach is that all reactants do not react at
approximately same conditions defined by their
physical surroundings, but significant concentration gradients are involved. The second problem is
that the composition of the microemulsion is gradually changed as solutions of different reactants
are successively introduced, which might induce
significant transitions in micellar or some other
multimolecular structural properties. Therefore, a
multi-microemulsion approach, within which separate microemulsions of the same compositions are
prepared for every reactant involved in the synthesis, is used most often in order to overcome
the problems of the single-microemulsion approach
and achieve a better control over synthesis parameters. Schematic illustrations of single- and multimicroemulsion approaches to the materials synthesis,
are presented in Fig. 8. It has been observed that
the latter, so-called multi-microemulsion approach
yields finer particles when compared to the single
microemulsion approach.80,142 The particle size distribution was also found to be different when synthesis is performed by single- and double-microemulsion
approaches. The particle size distribution had a
June 2, 2005 12:33 00700
255
Fig. 8. Schematic illustration of various stages in the growth of nanosized particles in reverse micelles in multimicroemulsional approach (up) and single-microemulsional approach (down).121
Gaussian symmetrical bell shape for the singlemicroemulsion approach, whereby bimodal distribution was attributed to the powder synthesized by the
double-microemulsion approach.142
Whereas the procedures based on separately dissolving the reagents within dispersed reverse micelles
are used most frequently in the context of materials
synthesis, the procedures according to which a type
of ion that is to become incorporated in the final
product, is initially introduced as the constituent
of the surfactant molecules, are also often followed.
CdS nanoparticles were synthesized by using Cdbased surfactant, such as cadmium bis(ethyl-2-hexyl)
sulfosuccinate.143,144 Ultrafine ZnS and CdS particles were, as well as CaCO3 particles,145 prepared in
reverse micelles by using extractant-metal ion surfactant complex as a metal ion source.146 In the case
of the preparation of metallic particles, the reduction reactions are initiated, whereby in the case of
the preparation of ceramic particles, usually a precipitation agent in the form of an acidic or alkali
agent is used and further thermal treatment is performed; otherwise, an oxidation reaction is initiated
as well.147,148
Disruption of micelles in the reaction mixture
might be carried out by introducing alcohols in
excess, causing the nanoparticles to precipitate.
The major problem of the recovery procedure is
caused by high surface energy of ultrafine particles, which makes them coagulate irreversibly when
reverse micelles are destroyed without any protection treatment,149 such as surface modification of the
particles. Removing the surfactant caps surrounding
the individual, well-dispersed particles most often
yields highly agglomerated samples even though
the particles were well dispersed in their parent
microemulsion. Centrifugation is mostly used for the
separation of precipitate from the liquid phase, but
magnetic separation by a permanent magnet has
been used as well,150,151 just like filtration, freezedrying or phase separation by cooling the reverse
microemulsion.152 To separate CTAB from sensitive polypeptide nanoparticles, gel permeation chromatography (GPC) was used.111 Washing of the
precipitate in order to remove residual surfactant
and oil-phase molecules might be done with water,
ethanol, chloroform or the mixture of two. Since most
surfactants including CTAB and AOT are soluble in
these solvents, their molecules are together with byproducts to be separated from the particles of the
desired product.
Even though the calcination is sometimes a
necessary step to obtain fine crystalline samples, in situ syntheses of nanocrystalline as-dried
powders have been frequently reported. Nanocrystalline MnZn-ferrites117,148,153–156 [Fig. 9(a)],
NiZn-ferrites147,157,158 [Fig. 9(b)], TiO2 159 were
synthesized in situ at room temperature as well
as a number of various metal particles such as Fe 29
or Bi.160 Metallic particles can be obtained much
June 2, 2005 12:33 00700
256
(a)
of surfactant molecules acting as cages for growing
crystallites which thereby reduce the average size
of the particles during the collision and aggregation
process76 is seen as the major reason for such a difference in particle sizes and crystallinity.
Since many chemicals are included in the ordinary microemulsion synthesis (where surfactant, oil
phase and co-surfactant are normally environmentally degradable162 ), one of the most important challenges of future researches is to develop a way to
recycle163 the components which are included in the
synthesizing procedure, so that literally the same
reverse micelles might be used in the repeating cycles
of materials production. Many ‘green’ options are
nowadays available, with lecithins, bile salts, dipotassium glycerrhizinate, biocompatible solvents like
Transcutol, glycofurol, ethanol and isopropanol, and
surfactants like poloxamers and polysorbates being
routinely used in microemulsion-assisted syntheses
of drug nanoparticles.164 Since the synthesis reactions proceed in stable reverse micellar microemulsions only in the dilute solution of precursors, the
result is a low yield of nanomaterial, which places the
problem on the economic feasibility of the method.
6. General Overview of the
Influences
(b)
Fig. 9. (a) In-situ-obtained nanocrystalline MnZnferrite117 and (b) NiZn-ferrite161 in CTAB/1-hexanol/
water reverse micellar microemulsion, with the latter
having saturation magnetization of 50 emu/g, which is
approximately two-thirds of a value that the same stoichiometric samples obtained by the traditional solid-state
route exhibit.
easier in situ by using reverse micelles compared to
ceramic materials, since they are usually prepared
in such a way that they are the direct products of
the precipitation reaction between the dissociated
cationic species and a reduction agent. The samples synthesized in the reverse micelles are often of
better crystallinity when compared to the samples
synthesized in the bulk aqueous phase. The existence
There are many factors that might be discerned as
having an effect on the properties of the produced
powders within the synthesis procedure used. Within
this paragraph, we shall number and discuss some
of them.
Besides parameter w (that will be discussed in
detail in the following chapter), mentioned earlier
as a major variable in the design of materials that
are to be prepared in reverse micelles, particle sizes
might in certain cases be controlled by varying the
amount of metal ion in each reverse micelle — more
metal ions will cause larger particles to grow because
of diffusion.73 It was generally proposed that an
increase of particle size might be induced by increasing the reactant concentration, whereas a decrease
of particle size can be induced when one of the
reactants is increased in excess until a plateau is
reached at high excesses.99 However, one of the major
problems with microemulsion-assisted material processing is the effect of the reactants and products
on the stability of the microemulsion, particularly
June 2, 2005 12:33 00700
the metals concentration in the aqueous phase. It
is mentioned that the ion concentration is, after
water-to-surfactant ratio, the second most important parameter to control the particle’s size.21 As
has been long known in the literature, the introduction of a small amount of a species highly insoluble
in the continuous phase into the emulsion droplets
substantially increases the stability of the droplets
with respect to the Ostwald ripening process.165 As
droplets shrink and lose dispersed monomer, the
concentration of the insoluble species within the
droplet grows. The desire of the droplets to maintain osmotic equilibrium by maintaining the same
osmotic pressure (same concentration of insoluble
species within each of the droplets) competes against
Ostwald ripening. Droplets cannot shrink and disappear because of the insoluble species, and so the
number concentration of droplets, along with the
number concentration of insoluble species within
the droplets, remains constant (in the absence of coalescence processes). Electrolyte addition in ionic systems in general gives an increased micelle size; in
most cases the effect is rather small, while in others
dramatic changes may occur. Adding [Ru(bpy)3 ]2+
to small CTAB reverse micelles was found to change
the reverse micelle water pool size distribution from
monodisperse to bimodal.44
The influence of electrolyte addition on the
reverse micellar solution phase L2 obviously depends
on the interaction of individual components, and to
date defies any general conclusions. Some researches
have demonstrated narrowing the L2 range in
DDAB/dodecane/water microemulsion with an increase in salinity,166 which is consistent with observed narrowing of the L2 range in CTAB/1-hexanol/
water microemulsion.117 There were also results
according to which extensions of one-phase regions in C9 H19 C6 H4 O(CH2 CH2 O)9.7 H/cyclohexane/
water microemulsion were not affected by the addition of NaCl.167
Salts have been found to affect both aggregation
and micelle formation. When the ionic strength of
the aqueous phase is increased, the water uptake
will decrease dramatically, and the type of ions
has a great effect on the water uptake.53 Aggregation number, CMC and critical micelle temperature all depend on ionic strength. Studies of
AOT reverse micelles show that introducing electrolytes such as NaCl reduces aggregation number
257
and micelle radius. Others report that at low concentrations (10−3 –10−2 M) ions do not alter reverse
micelles appreciably.44 It was reported that adding
salt to an ionic micellar solution will decrease
the CMC and increase the aggregation number
owing to the screened electrostatic repulsion.49
With a decreased electrostatic repulsion between the
charged head-groups of surfactant, it is possible to
pack the surfactant head-groups closer to each other,
with a subsequent increase in aggregation number.
Besides expectations that the higher concentration of salt precursors within reverse micelles tends
to stabilize the micelle structure, because of the suppression of surfactant head-groups, which causes the
head-group area to decrease and the packing ratio to
increase, inducing the formation of more stabilized
microemulsions, some authors have found out that
excessively high salt concentrations tend to drive
the alcohol (co-surfactant) into the oil-phase and
some studies even suggested that this would limit
the salt concentration at which microemulsions can
form at around 0.4 M in the aqueous phase.168 Nevertheless, microemulsions, albeit with extremely soluble sodium nitrite and alkanes with a variety of
chain lengths comprising the oil phase, were formed
at concentrations in the order of ∼ 3.5 M.168 The
solubilization capacity is dependent on the nature
of the surfactant as well, since, as has been already
mentioned, it was shown that CTAB-based reverse
micelles have higher solubilization capacities of high
concentration aqueous salt compared to AOT-based
systems.53
Beside w and pH, the particle size and
morphology is greatly influenced by the ratio of surfactant to co-surfactant. In the CTAB/n-butanol(cosurfactant)/isooctane/water microemulsion system,
the higher the ratio of CTAB to n-butanol was, the
smaller were the particles obtained. Therefore, it
was proposed that the key factor affecting the particle size is the interfacial property rather that the
size of the microemulsion droplets.169 An increase
in particle size could be obtained by directing an
increase of the surfactant film flexibility, which might
be achieved not only by approaching the microemulsion instability phase boundaries or by changing the
droplet size, but by increasing the amount of cosurfactant (alcohols) or changing the chain length
of the oil or co-surfactant as well.99 An increase
in the chain length of oil results in an increase
June 2, 2005 12:33 00700
258
in the value of intermicellar exchange rate coefficient due to the fact that as the chain length
of the oil increases it becomes increasingly coiled
and therefore its penetration in the surfactant layer
becomes more difficult, resulting in a stronger mutual
interaction between surfactant tails compared to
the intensity of interaction between surfactant tails
and oil molecules.137 The co-surfactant in a quaternary microemulsion system may have a more significant effect on the interfacial properties than the
oil phase in a ternary one.169 It was shown that in
CTAB/n-hexanol/water microemulsion, n-hexanol
acts mainly as the continuous oil phase, although it
also affects the interfacial properties. It was observed
that the average size of the nanoparticles increased
with the increase in the n-butanol/CTAB (cosurfactant/surfactant) weight ratios, except when
the weight ratio of n-butanol/CTAB was below 0.5
(gelation occurred). It was also revealed that the
variation of the weight ratio of n-butanol/CTAB
affected only the interfacial properties and not the
size of reverse micelles.169 The presence of alcohol
in the CTAB/hexane/pentanol/water microemulsion
is shown to be an important factor in regulating
the size distribution of the synthesized nanoparticles, acting on the particles growth by influencing
the flexibility of the interfacial film.170
Different particle morphologies can in some
cases be achieved simply by choosing NaOH over
NH4 OH.76,116 However, in the presence of a large
amount of strong base, the total ionic strength of the
water pool increases, which causes instability in the
microemulsion system.76 The ability of a strong base
to promote the hydrolytic decomposition of an ionic
surfactant will contribute to the destabilization of
the surfactant aggregates77 and possible subsequent
phase separation of the microemulsion system.3 In
order to keep the pH value of the precipitation
at the same level during the process, a computercontrolled constant-pH apparatus was used in some
experiments.171 However, it is important to note that
water pool properties of reverse micelles — local viscosity, local polarity, local acidity — can be substantially different compared to the effective macroscopic
properties of an overall measured system.10 Investigations of local pH in reverse micelles by using pHsensitive probes concluded that when NaOH and HCl
were used for pH adjustments, an almost constant
intensity ratio over a wide pH range was obtained,
suggesting that the water pools of microemulsions,
due to a large number of polar surfactant headgroups localized at the oil–water interface, may have
buffer-like action.172
The temperature of the synthesis procedure influences the properties of the synthesized powders. It
was shown that higher temperatures induce better
crystallinities of the synthesized powders.154,156 The
reason for this effect might be found not only in an
increased thermal motion of the active species, but
by the fact that the micellar structures change as
well. Average radii of AOT-formed micelles increase
by approximately 50% when the temperature is
increased from 20◦ C to 38◦ C.46 Similar findings were
applied to CTAB-117 and PEGDE-based173 systems.
On the other hand, SANS studies have concluded
that both AOT shell thickness and the inner water
pool in the AOT/n-heptane/water system reversibly
decreased with increasing temperature.42 Contrary
to expectations, an increased temperature did not
increase the reduction rate of Pt4+ with H2 in the
CTAB/octanol/water system.1
Albeit the existence of many cases where in a
matter of moments after the key reaction was initiated, particles stable for months were obtained,174
the average size of the particles can sometimes be
tuned by setting the desired time between the mixing of the microemulsions containing the reacting
species, and their disruption.21 By prolonging this
time from one minute to two hours, the average diameter of the particles, as observed by TEM, changed
from 13 to 35 nanometers. On reducing the reaction time, the morphologies of the same particles
evolved from perfect cubic shapes to more spherical ones with rounded edges.21 On the other hand,
prolonging the reaction time can sometimes lead to
a change from cubic to spherical-shaped particles.175
Within an in situ synthesis of MnZn-ferrite particles,
it was observed that the average particle size did not
significantly change in the aging time range between
1 min and 3 h, whereas ∼ 25% increase in size was
detected after aging of the powder in microemulsion
up to 26 h.156
Aging time variations can be applied in order to
induce self-organizing effects on particles assembly
scale. During an aging time of 60 days, the particles
of PANI/TiO2 composite were shown to self-organize
in reverse micelles of AOT/iso-octane/water and
CTAB/hexanol/water microemulsions into either
June 2, 2005 12:33 00700
sea urchin-like or needle-like shapes.176 Four-day
aging time of prepared Prussian blue nanoparticles in microemulsion led to the formation of highly
ordered cubic superlattices.177 By letting the reaction microemulsion mixtures age for more than
10 min in the process of preparation of CaCO3 and
BaCO3 , aggregation of the already-formed spherical
nanoparticles is initiated, leading to the slow formation of networks (after aging of up to 1 h) and
solid wires (after aging of up to 48 h).178 Introducing an external magnetic field to the process
of evaporating deposition of cobalt particles prepared in reverse micelles, led to the availability of
designing the arrangements of the nanocrystals into
either hexagonal patterns of micrometer-sized dots
or labyrinthine 3D superlattices, depending on the
intensity of the applied magnetic field and the evaporation rate of the previously prepared particles.144
A similar procedure, but without the use of magnetic
field, was successfully applied in obtaining “supracrystals” made of silver nanoparticles self-organized
in 3D superlattices.179
Some of the often skipped influences might
present decisive parameters in the synthesis procedures. Bunker et al. have found that tripling the
amount of microemulsions used in the synthesis procedure leads to significant changes in the obtained
CdS particles absorptivity.180 It was found that the
procedure of passing N2 through the reaction mixture not only protects critical oxidation but also
reduces the particle size by 25% when compared with
methods without removing the oxygen.181 Purging of
inert gases through the reaction mixture is especially
important when easily oxidized elements are present,
as is the case for the synthesis of ferrites147,157 and
other iron-comprising compounds.182 The matter of
purity is significant, since any introduction of foreign substances might induce heterogenous nucleation with avoiding the desired monodispersity and
uniform morphology of the synthesized particles.
Sequence according to which adding of microemulsion components into the mixture is done is also
shown to be important.75 It was found that even
stirring microemulsion with a magnetically coupled
stir bar during the powder’s aging time can influence crystal quality and in some cases result in a
different crystal structure as compared with nonmagnetically agitated microemulsions.183 In case of the
synthesis of organic nanoparticles in reverse
259
micelles,174 the use of a magnetic stirrer led to the
formation of nanoparticles larger in size compared
to the particles obtained with using ultrasound bath
as a mixer, even though no changes in particle size
were detected on varying solvent type, parameter w,
reactant concentrations and even geometry and volume of the vessel. Particle size can be controlled by
decreasing the exchange of materials between different micelles, which might be accomplished by lowering the temperature or slowing the agitation of the
solution, which will in return slow the intermicellar
exchange of materials, giving either smaller particles
or equally-sized particles over a longer time scale.73
All of the processing conditions which affect the
formation of a nanocompound, i.e. its microstructure
and stoichiometry and therefore its potential application, may be summarized as follows. Besides the
identities of surfactant (the size of hydrophilic headgroup, ionic or neutral, branched or narrow, etc.),
oil phase (oil chain length, primarily), co-surfactant
if included (its structure, branching, chain length,
etc.), metal salts, precipitating agent and/or other
reactants, there are: curvature free energy, determined by the elastic constant and the curvatures
of the surfactant film;99 concentration of reactants
within reverse micelles; pH; the sequence of introducing the components in the micellar mixture; single- or
multiple-microemulsion synthesis approach; overall
composition of microemulsion, which is usually
decoupled to molar ratio of water-to-surfactant
described by parameter w and/or surfactant-to-cosurfactant ratio; aging time of the precipitate in
microemulsion; intensity and mechanism of stirring;
a method for separation of precipitated particles
from the liquid phase; purity of used substances and
incorporation of impurities during the synthesis; and
temperature under which various processes within
micelles proceed.
However, it is necessary to mention that tendencies to outline universal relationships which
govern the processes of materials synthesis are disobeyed by many cases in which changing only
one ion in the complex microemulsion had dramatic qualitative effects on the product’s properties. When only Mn2+ ions were replaced by Ni2+
ions,particles of mixed zinc-ferrites had completely
different morphologies. Thus, the synthesized MnZnferrite117 comprised spherical nanosized particles,
whereas particles of NiZn-ferrite were of acicular
June 2, 2005 12:33 00700
260
shapes.157,94 When bromide ions of CTAB surfactant were replaced by chloride ions, the product’s identity was not the same.87 When 2-octanol
was replaced by octanol, much higher background
noise was observed within XRD measurements of
the synthesized powders.87 The inability to discern and predict the influences of all the species
involved in the synthesis procedure and provide
a set of relationships between properties of the
microemulsion used and the potentially designed
properties of the synthesized materials, is the reason why the method of synthesis discussed herein
had relied mostly on the trial-and-error approach in
the past.
7. Influence of Water Content
Over the past two decades, a great deal of research
within the reverse micellar method for the materials synthesis was undertaken to discern the various parameters which significantly influence the
properties of the synthesized materials. By precise
evaluation of the effects of all the major influences within the method, through control over processing parameters the desired material properties
might be designed. The parameter, which is in this
light regarded as the most important, is water-tosurfactant molar ratio, described by parameter w.
The higher the parameter w, the larger the water
pools of the reverse micelles, and if the micelles are
imagined as nanosized templating cages, then the
particle sizes might be seen to increase with the
increase of w value. However, as we shall see, there
are many cases of synthesis for which reversed correspondence or no linear correspondence at all has
been concluded.
It was found that reverse micelle size generally
increases linearly with parameter w as well as with
water content of the microemulsion. The relation
between water core radius r of reverse micelle in
nanometers and parameter w is in the case of AOTformed reverse micelles found to be very simple: r =
1.5w.22,50 Even though this relation between the size
of the water pool radius of the reverse micelles and w
was confirmed by SANS and SAXS measurements,50
it is sometimes mistakenly identified121 with all
reverse micelles, whereas it was concluded only for
certain types of AOT-based microemulsions. Beside
increasing the volume of the reversed micelles with
the water content when surfactant concentration is
kept constant and decreasing the volume of the
reversed micelles with the surfactant content when
the water concentration is kept constant, it is known
that if both the surfactant and water are increased in
a constant ratio, the volume of the reversed micelle
will not change, but its numbers will increase.44
In the case of reverse micelles formed by CTAB
molecules, it was reported that the reverse micelle
size is equal to parameter w.23 There were also indications that parameter w might be taken to influence
the particle size distribution.4
It was reported that at constant surfactant
concentration S [mol/dm3 ], the radius r [Å] of
the droplets encapsulated by surfactant monolayers
increased linearly with the volume fraction of the dispersed phase φ and could therefore be readily controlled through this parameter184 :
r = (4.98 × 103 )φ/As S
where As [Å2 ] is the area occupied by the surfactant
at the droplet surface. However, this relation would
provide a templating relation for control of particle
size only in cases where the microemulsion structure is not perturbed during the reaction. In cases
where transfer of the components through intermicellar diffusion and exchange processes between
reverse micelles permits fast exchange of both solutes
and surfactant between droplets, the result will be
an uneven growth from which the structure of the
droplet template may be altered during the reaction or be totally destroyed as a result of phase
separation.
Today, it is known that it is not only synthesized
particle sizes that do not show in all cases linear
dependencies on parameter w, but the very size of the
reverse micelles also do not follow the same dependency. Experimental results indicate that the size of
reverse micelle depends not only on parameter w,
but also on temperature, organic solvent (oil phase),
type of surfactant, its concentration and present
electrolytes. The same SAXS researches which have
come to the conclusion that dependency of reverse
micelle radii on w is parametric,50 have shown that
reverse micelle radii in AOT/isooctane/water system vary upon addition of small amounts of compounds solubilized in the microemulsion.22 It was
noted that the water core radius of microdroplets
of water/AOT/toluene microemulsion changes from
June 2, 2005 12:33 00700
1 to 1.6 nm when parameter w changes from 5
to 10.185 As the concentration of the surfactant
decreases, the amount of water that can be solubilized decreases too.44 Some studies have shown that
the smallest Stokes radii, which correspond to the
droplet size, were obtained between 15◦ C and 18◦ C
in case of AOT-formed microemulsions, and that
there was no relation between the Stokes radii of
the microemulsion and AOT content of the system.2
It was shown that the structure and composition of
an elementary water droplet enclosed by surfactant
molecules remained unchanged up to a water volume
fraction per composition equal to 0.3, but this observation generally refers only to microemulsions where
attractive forces are not very strong.62
It was very often found that the synthesized particles are of greater average particle size than the
size of their parent reverse micelles as derived from
parameter w.185 Within a synthesis of silica particles,
as the parameter w increased from 0.7 to 2.3, the particle size decreased (and the size distribution became
narrower) not in monotonous manner.6 Within the
reverse micelle synthesis of BF2 nanoparticles, the
particle size was larger than the swollen micelle
size,87 which demonstrates that the microemulsion
does not consist of hard spheres that strictly limit
particle growth. In case of microemulsion synthesis of AlPO4 -5 fibers, the dimensions of the crystals
produced were many times larger than the typical dimension of microemulsion droplets.183 Directly
proportional correspondence of reverse micelle sizes
and average particle size turned to inversely proportional at higher water-to-surfactant ratios.186
Within the synthesis of Pd particles in CTAB/
n-butanol/isooctane/water microemulsion, the average diameters of the produced particle also decreased
slightly with the increase in water content when the
weight ratio of surfactant-to-isooctane was fixed.169
The following explanation was assigned to this phenomenon: increase in w pushes the composition of
the microemulsion toward the solubilization curve,
and therefore to increased intermicellar interaction.
Increases in intermicellar interaction will promote
the aggregation of the particles contained in the
interacting reverse micelles. Therefore, it can be
clearly concluded that in addition to the size of
microemulsion droplets (described by parameter w),
there must be other factors affecting the size of the
produced nanoparticles.
261
Today, it is generally known that for the most
surfactant-mediated synthesis applications, the connection between the morphology of the surfactant aggregates and the resulting particle structure
is much more complex (than the simple connection between the sizes and shapes of the micelles
to the sizes and shapes of the produced particles), being affected by hardly reducible conditions
prevailing in the local microenvironment surrounding
the growing particle. These include the pH, composition, and ionic strength, as well as surfactant headgroups spacing. As reaction takes place within these
microenvironments, many of these factors are liable
to change. The decoupling of each of these effects still
remains a challenge, and provides interesting new
opportunities in this field.112
8. Mechanism of Particle Formation
Synthesis of materials in reverse micelles might
be considered as an ordinary precipitation or
(co-)precipitation method of synthesis, but which
takes place within a very specific environment. Compared to many other wet syntheses such as sol-gel,
within reverse micelle synthesis, the precursor ions
incorporated in form of a colloidal precipitate and
not a complex compound, must be obtained. One of
the major problems related to the co-precipitation
method of synthesis is the inability to control supersaturation of the precipitating system which in turn
leads to very poor control over the distribution of
particle sizes, that is normally much more broadened
than in the cases of reverse micellar syntheses.29 Due
to narrowly dispersed reverse micelles with templatelike properties, the synthesis of materials within
reverse micelles very often overcome these problems.
The particle formation in a microemulsion can
occur either through collisions between microdroplets which carry reactants within their aqueous cores (multiple microemulsion approach) or
by diffusion of the hydrophilic precipitating reactant to the microemulsion droplets containing the
to-be-precipitated reactant (single microemulsion
approach). Although the surfactant walls of the
microdroplets act as cages for the growing particles and thereby reduce the average size of the particles, the exchange of the precursor ions between
the microdroplets by either a diffusion process or
June 2, 2005 12:33 00700
262
a collision of microdroplets affects the particle formation, resulting in a similar narrow particle size
distribution.185 Droplet collisions and rapid intermicellar exchange of their water content are processes
which are mainly controlled by diffusion and are also
dependent on the nature of the surfactant.135
Wide size distribution of the synthesized particles was ascribed to the fact that for reverse micelles
with smaller size, nucleation will occur only in a
small number of micelles at the beginning of the
precipitation reaction because most of them do not
contain enough ions to form a critical nucleus.53
Since the typical ion concentrations per reverse
micelle are too small to form critical nuclei, the formation of the final precipitated particles requires the
aggregation of precursor atoms from several different reverse micelles. Therefore, due to diffusion, new
nuclei will form as a function of time, which will cause
a different growth rate of particles, and subsequently
a broad size distribution. Size distribution can be
narrowed by increasing the concentration of reagents,
as was proven in case of the synthesis of Ni nanoparticles when increased hydrazine concentration gave rise
to enhanced reduction rate that led to the generation
of a higher number of nuclei and, therefore, to the
formation of smaller and more uniform particles.187
When using a co-surfactant to increase the fluidity of
the interface and therefore the kinetics of the intermicellar exchange, a more homogeneous repartition
of reactants among micelles could be ensured, thus
promoting the formation of smaller, but more numerous particles.188 By using a Monte Carlo model, Jain
et al. showed that as the efficiency of the intermicellar exchange increased, the resultant particle
size increased,139 whereas within the synthesis of Ag
nanoparticles in AOT-based reverse micelles, it was
shown that the particle size decreases at higher intermicellar exchange rates.137 By studying the solvent
effects on copper particles prepared in AOT reverse
micelles, the authors concluded that growth rate and
particle size are inversely related, so that a decrease
in growth rate leads to a larger particle size at a
specific w value.189 From this point of view, the
dynamic intermicellar interaction might be considered as the major, but thoroughly complex influence
on the properties of the synthesized particles.
One of the often skipped questions with regard
to materials synthesis in reverse micelles is the matter of influence of the nucleation and crystallization
of particles, as well as of their subsequent interaction on the structure of reverse micelles. The tendency for the particles to agglomerate sometimes
exceeds the forces which keep reverse micelles dispersed within the oil phase and complete or localized
phase separation occurs. Researches based on steadystate and time-resolved emission spectroscopy as a
function of water content came to the conclusion
that encapsulation of a relatively small particle or
molecule (around 1.2 nm) within a large reverse
micelle (with diameter of 3 nm and w > 10) does
not perturb the nanodroplet structure, but on the
other hand, an incorporation of a similarly sized particle within a small reverse micelle (with diameter
of 1.5 nm and w ∼ 5) results in a redistribution of
surfactant and water.58 Three possible hypotheses
were proposed in order to explain the final states of
the particles synthesized within reverse micelles.190
According to the first hypothesis, nanoparticles are
in the organic phase and are in direct contact with
the polar heads of the surfactant, whereas the surfactant tails are in organic phase. According to the
second hypothesis, nanoparticles are surrounded by
a layer of water, whereas according to the third
hypothesis, nanoparticles are surrounded by surfactant tails that point their polar heads toward the
water.
If the microemulsion becomes unstable, mostly
by exceeding the limiting parameter w for given
oil/surfactant ratio, the aggregation of particles
takes place, which leads to the formation of nonuniform particles regarding their size and shape as
well as to the formation of multicore particles in
the process of composite synthesis.185 It was mentioned that in the case of AOT/isooctane/water
microemulsion system, the extreme size limits of
5 nm and 30 nm exist, where the size distribution
increases due to the problems in micelle stability
and crystallization.151 Also, depending on the nature
of the surfactant and the type of interaction, the
amphiphilic molecule may either stabilize the inorganic particles or induce significant aggregation. It
has been demonstrated, for instance, that positively
charged particles flocculate upon addition of anionic
surfactants and that, on further addition, the particles re-disperse because of the formation of surfactant bilayers.135
When hydrothermal and reverse micellar
methods for producing zincophosphate crystals were
June 2, 2005 12:33 00700
compared, it was noticed that the crystal growth
process was about an order of magnitude slower
within the reverse micellar procedure, as compared
to the hydrothermal method.191 The surface of the
hydrothermally obtained crystals was found to have
multiple layers and terraces, whereas the crystals
obtained from the reverse micellar method were
atomically smooth, which suggests a much finer
mechanism for the formation of particles within
the reactions coupled with obviously morphologyregulating intermicellar exchange and surfactantmediated growth control.
Ultrathin nanowires (Fig. 10) have recently been
produced by using the so-called catanionic reverse
micelles that are formed by mixed cationic–anionic
surfactants, whereby the molar ratio between the
mixed surfactants is expected to play a key role
in the synthesis of such 1D nanostructures.103 Only
mixtures with an excess of either cationic or anionic
surfactants, when ratio of two oppositely charged
head-groups is far from 1:1, have been shown to
form catanionic micelles.192 The length of V2 O5
nanowires was shown to be possible to be tuned by
controlling the aging time of the synthesized particles in microemulsion,193 which is consistent with
the findings in the case of the formation of ZnS
nanowires.194 Parameter w also largely influenced
the process of the formation of ZnS nanowires in
C12 E9 /cyclohexane/water reverse micelles. It was
shown that acicular NiZn-ferrite particles are formed
within CTAB/1-hexanol/water microemulsion only
within the precipitating pH value range of 8–11.94
Colloidal particles with an electric or magnetic
dipole moment were theoretically predicted to selfassemble into flexible chains, and these assemblies
were recently experimentally observed.195 The production of such pearl-like microstructures today
surely presents a significant challenge for materials
design by using self-assembly templating amphiphile
structures.
Surfactant aggregates associate (or dissociate)
with time scales that can range over several orders
of magnitude. If the time scale associated with the
particle formation is small compared to the time
scale of such transformations, intermediate, nonequilibrium aggregate structures might afford additional
opportunities for templated materials synthesis. Fast
kinetics associated with several solution-based precipitation processes might be ideally suited for this
263
(a)
(b)
Fig. 10. BaCrO4 nanowires103 as obtained from catanionic reverse micelles consisting of (a) water, decane,
undecylic acid and decyl amine and (b) molecular
sieve fibers of AlPO4 -5183 as obtained from cetylpyridinium chloride/butanol/toluene/water reverse micellar
microemulsion.
application. The control of solution conditions and
surfactant architecture to exploit nonequilibrium
surfactant morphologies presents an interesting scientific challenge.112 The potential of reverse micellar microemulsions in the field of nanotectonics —
controlled construction of organized matter using
nanoparticle building blocks — is significant and is
expected to bring about new challenges in materials
research.
June 2, 2005 12:33 00700
264
9. Synthesis of Composites within
Reverse Micelles
Reverse micellar microemulsion medium is a
dynamic system where colliding micelles coalesce,
exchanging their content and splitting. This property provides conditions for performing the desired
chemical reactions of synthesis in a range of new
and unpredictable manners. On the other hand,
this property causes instability of colloidal solutions over time. Freshly formed nanocrystalline particles protected from further growth in micelles,
have high surface energy and once micelles coalesce,
particles readily aggregate.196 Nanoparticles in ferrofluids, for instance, have surface energies higher
than 100 dyn/cm2 , so strong magnetic dipole–dipole
attractions between particles tend to provoke their
agglomeration.181 In order to prevent agglomeration
of nanoparticles and retain their nature of nanocrystals, an inert silica coating (Fig. 11) or a novel mesoporous silica matrix is used, offering excellent control of nanocluster size distribution and cluster morphology as well as high thermal stability associated
with the inorganic host structure.171,197 The insulating matrix greatly enhances the chemical stability of
the particles, and the wear and corrosion resistance
of the media.198
When present in small quantities (< 0.05%), the
silica component can impede grain growth through
impurity drag and precipitate drag mechanisms,
whereas when silica is present in larger quantities
(> 0.05%), a liquid phase may form.199 If the liquid
phase is not properly distributed, discontinous grain
growth is probable. However, if the liquid phase is
properly distributed, grain-growth impedement can
still occur if the diffusion path between grains has
been increased sufficiently to offset the increased
diffusion rates caused by the reactive liquid. The
coating with silica improves the structural order at
the surface of the magnetic nanoparticles in diminishing the surface magnetic anisotropy.77 By varying the thickness of the silica shell and the coated
particle radius, an overlap of the wavefunction and
band gap can be designed, which opens attractive
new possibilities in the semiconductor field.10 The
outer silica surface is biocompatible and functionalizable as well. DNA was successfully attached to
silica-coated magnetic nanoparticles.76 To additionally protect the nanoparticles from agglomeration,
(a)
(b)
Fig. 11. Silica-coated rhodium particles as synthesized from a reverse micellar C-15/cyclohexane/water
microemulsion.101
silica surface can be covalently modified into a
neutral or charged surface76 or additionally coated
with some biomolecules. Hydrophilic organic coatings such as albumin, dextran or hydroxemethylmethacrylate may enhance biocompatibility.165
June 2, 2005 12:33 00700
Alkoxides tetraethyl orthosilicate [Si(OC2 H5 )4 —
TEOS, known as tetraethoxysilane as well] or
calcium isopropoxide [Ca(O3 H7 )2 ] are mostly used as
precursors for SiO2 or CaO coatings.200–204 Immersing TEOS in water and ethanol results in a bubbly
structure of solid silicon in a matrix of an organic
compound (HOCH2 CH3 ), which is removed in a subsequent step.205 Because this process occurs slowly
at room temperature, acid or base catalyst is added
to the formulation. The amount and type of catalyst play key roles in the microstructural properties
of the final coating. After addition of TEOS into a
microemulsion mixture, the silica shell can be synthesized by gradually increasing the pH inside the
droplets with ammonia to catalyze the condensation of TEOS.77 Optimal pH value is found to be
between 11 and 12.116 When using ammonia as a
catalyst, more spherical morphologies are obtained
and therefore, ammonia is often referred to as a morphogenic catalyst.116 For higher ammonia concentrations, a minimum in particle size was observed at
intermediate w values, and it was suggested that this
trend might be associated with the aggregation of the
nuclei.206 Ammonia concentration was found to have
impact on particle sizes, where at relatively low w,
the particle size goes to a minimum as ammonia concentration is increased, but at relatively high w, the
particle size increases monotonically with ammonia
concentration.6
However, in the presence of a large amount
of strong base, the total ionic strength of the
water pool increases, which causes instability of
the microemulsion system. To avoid this problem,
it was suggested76 that a very small amount of
base be used, that is sufficient for the precipitation
reaction followed by the polymerization reaction of
TEOS. Ammonia also provides the particles with
a negative, stabilizing surface charge, which does
not enable aggregation of the particles.207 It was
found that at low ammonia concentrations, the particle stability is higher. On one hand, an increase
in the concentration of ammonia and water will
increase the surface charge through the dissociation
of silanol groups and will thus stabilize the particles. On the other hand, increasing the concentration
of ammonia and/or water will increase the concen−
and thereby decrease the
tration of NH+
4 and OH
207
If the particles are very
double layer thickness.
stable (e.g. at low concentrations of ammonia) the
265
aggregation stops relatively early and a larger number of silica-coated particles grow to a small final
size. Because in this case the silica layer that is
deposited after the aggregation is thin, the resulting
particles will not yet be very spherical and the surface will be rough. At high ammonia concentration
the aggregation continues longer and only a relatively
small number of particles becomes stable.
Silica-coated ceramic particles may be synthesized by the double-microemulsion approach, where
the first microemulsion contains aqueous solution
of the salt precursors and the second microemulsion,
that is to be introduced dropwise in the first one,
contains an alkali precipitant and TEOS. In this way,
mixing and sonication for 2 h together with a day of
aging and subsequent washing with ethanol and centrifugation yielded silica-coated magnetic particles.76
By the use of an external magnetic field during the
solidification process, magnetic nanoparticles embedded in silica matrix could be preferably oriented
in order to obtain an anisotropic macrostructure,
known as a magnetically textured system,208 which
has such properties as anisotropic magnetization and
optical anisotropy.
It was found that the diameter of silica-coated
particles was highly dependent on the water-tosurfactant molar ratio. Not only did the amount
of free water influence the relative rates of hydrolysis and condensation reactions, but it also played
a determinant role in colloidal stabilization. Nucleation involves the condensation of monomers via
intramicellar or intermicellar exchange polymerization reactions, whereas particle growth occurred
either by the addition of TEOS to the nuclei or by
particle aggregation.135 The major problem for the
reverse micellar preparation of silica-coatings is that
TEOS is an organophilic molecule and is therefore
more readily dissolved in the oil-phase alcohols than
in the aqueous droplets.77 TEOS is also known to be
incompatible with strong oxidizing agents.
Upon TEOS addition to the reverse micellar system, the formation of silica coating over alreadyformed nanoparticles involves a series of steps,206
which can be identified as:
(i) association of TEOS molecules within the
reverse micelles;
(ii) TEOS hydrolysis and formation of monomers;
(iii) nucleation;
June 2, 2005 12:33 00700
266
(iv)
(v)
(vi)
(vii)
(viii)
particle growth;
nuclei dissolution;
intermicellar exchange of monomers;
ionization of monomers; and
particle surface ionization.
The reaction temperature for hydrolysis and condensation of TEOS monomers should be kept below
25◦ C to avoid any complications due to the presence
of a possible thermally induced phase inversion,77
and after 48 h the reaction is normally completed.
It was suggested that the synthesis is silica-driven
and that the micelle-initiated nucleation is not the
dominant mechanism, but the nucleating site for the
porous silica coating is either oligomeric or polymeric silica.116 In most cases, not all ethoxy groups
from TEOS are hydrolyzed and therefore part of the
solvent remain trapped in the silica pores. In fact, it
was proven that several percent of the ethoxy groups
never leave the TEOS molecules.207
Besides the water-to-surfactant molar ratio and
concentrations of ammonia and water, the ionic
strength can be used to influence the final particle
size. The hydrolysis and subsequent condensation of
TEOS can be affected by the precursor salts introduced to the initial solutions of the gels. In the case
of nitrate salts, for instance, due to high solubility of
nitrates, the hydrating water from salts allows a total
hydrolysis of TEOS molecules and as a consequence
of that, the oxygen presence is low. On the contrary,
the chloride salts retain the water hydration retarding the hydrolysis process and promoting a rich oxygen ambient, sometimes convenient for the formation
of a different phase.209 The number of ions determines the particle size at which colloidal stability is
reached and when the aggregation stops; a smaller
number of growing particles results in a larger final
silica coating. In a certain experiment, the hydrolysis rate was unaltered by the addition of LiNO3 salt,
while the final radius of synthesized particles was significantly increased.207
When comparing the number of silica particles
with the number of surfactant aggregates, one silica
particle was produced out of circa 104 to 106 surfactant aggregates, depending on the parameter w. The
minimum number of hydrolyzed TEOS monomers
required to form a stable nucleus was estimated to
be 2, and the nucleation efficiency factor, i.e. the
probability of effecting nucleation in a reverse micelle
that contains enough monomers to produce a stable nucleus was found to increase with w for relatively low ammonia concentrations (1.6 wt%). A
decrease in the apparent nucleation efficiency factor
was observed at high w values with more concentrated ammonia, and this was attributed to silica
nuclei aggregation promoted by enhanced intermicellar collisions and intermicellar exchange.206 The
thickness of the SiO2 layer increases with increasing hydrolysis time, and therefore it can be designed
by controlling the hydrolysis time. In order to terminate TEOS hydrolysis, the micellar structure of
the solution containing SiO2 -coated nanoparticles
can be destroyed by adding an alcohol such as
propanol.101
Metallic nanoparticles synthesized in reverse
micelles have been coated with gold layer by the
reduction of Au3+ from HAuCl4 to Au via excess
hydrazine.150 The gold shell on the metallic particles in this case provides the functionality required to
form organized arrays on functionalized substrates.
Such so-called nano-onion structures comprise a solid
core grown in the first step, which serves as a
nucleation source for a functionalized shell.119 The
Au:Fe:Au nanocomposites produced by a sequential
reverse micelle technique have been shown to exhibit
the GMR effect.210 Gold has become a favored coating material because of the simple synthetic procedure and its chemical functionality.211
Ferrofluids as prosperious magnetic composites212
were prepared by using reverse micellar microemulsion.213 Magnetic nanoparticles in ferrofluid
composites are coated with surfactant monolayers
and are immersed within a carrier that might be an
organic solvent such as heptane, xylene, toluene, or
an inorganic solvent such as water, a hydrocarbon
(synthetic or petroleum) or any ester, polyglycol, or
styrene. Magnetic nanoparticles in such systems are
unable to aggregate and thus the superparamagnetic
nature of the whole system is retained.212 Oleic acid
derivatives have proven to be of great benefit in the
encapsulation reactions of colloidal magnetic particles through microemulsion polymerization. The
surface of magnetite colloid was first coated by a
monolayer of sodium monooleate and then stabilized by adsorption of SDBS surfactant.135 Reverse
micellar synthesis is especially convenient for this
sort of composites due to capability of tuning various magnetic properties — such as magnetization or
June 2, 2005 12:33 00700
coercivity — by control of simple parameters within
the given procedure of synthesis.
An interesting approach has been found in
the synthesis of ceramic — polymer composites,
within which nanosized spherical crystals of the
ceramic phase are incorporated into micron-sized
polymer particles. Some of the essential properties
of magneto–ceramic–polymer composites are its light
weight, good formability toughness and flexibility,
typical for plastics, that allow the preparation of
the products for magnetic circuits with complicated
shapes.214 The remarkable aspect of polymerization
in reverse micelles is the fact that the polymer
could form precipitates out from the solution with
the morphology of interconnected submicron-sized
spheres.215 It was claimed that micelles have a
templating effect, to fold chains as they are formed
to the resulting spheres. It was proposed that the
polymer chains grow around the micelle interface
and the collisions of reverse micelles with growing
chains result in chain linkages and the formation of
interconnected spherical particles. As a direct consequence of the assembly process of surfactants on
mineral surfaces, a hydrophobic interlayer can form
on the inorganic particles, into which monomers
can solubilize, and polymerization can subsequently
proceed. Whereas covalent binding through in situ
polymerization techniques allows the formation of
nanocomposite particles with hairy-like, polynuclear,
or core-shell morphologies, electrostatic coupling
or physicochemical adsorptions preferentially afford
heterocoagulated nanostructures with raspberrylike morphologies.135 P-ethylphenol (PEP) is used
as an organic precursor and polymerization of
these monomers was achieved by adding hydrogen peroxide with an oxidative enzyme (horseradish
peroxidase) in order to polymerize the phenols
encapsulated in the reverse micelles.215 Polymerization within reverse micelles95 yields oligomers of
controlled conjugation lengths and without occurrence of undesirable nano-oligomers coagulation or
interconnecting, and, therefore, the products of
such procedures might find a number of applications in electronics and photonics such as for controlled band gaps.216 In this sense, PPV [poly
(p-phenylenevinylene)] is an interesting option
because it is traditionally made by a base-catalyzed
reaction of a water-soluble salt monomer precursor
and has a number of applications due to its
267
electrical, nonlinear optical, electroluminiscent and
lasing properties.
The dispersion polymerization or microemulsion
polymerization in the presence of surfactant-coated
particles carries the risk of incomplete and nonuniform encapsulation. Reaction conditions must drive
all the to-be-coated particles to transfer uniformly
into the resulting polymer particle, or these particles must provide the only site for the precipitation of polymer. One way to potentially guarantee
the uniform loading of core particles into latex was
proposed: directly disperse the magnetic particles
into the monomer of interest, emulsify, and then
polymerize.165 Theoretically, the insolubility of the
polymer and magnetic particles in the oil continuum
should guarantee uniform loading of magnetite per
latex; however, no proof has been provided yet. Upon
polymerizing acrylamide monomers microemulsified
in oil, 40-nm-diameter polyacrylamide latex results.
Upon microemulsifying and precipitating iron salts
within water droplets in microemulsions, magnetic
nanoparticles result. A combination of the two
approaches results in magnetic nanoparticles encapsulated in hydrophilic polymers, with average diameters ranging from 80 to 320 nm.185 However, a
relatively polydisperse latex in this case contains
only 3.3 wt% (0.6 vol%) of magnetic particles. A
material with a greater concentrations of the core
phase might arise using the larger droplets in reverse
micellar microemulsions, created by using relatively
low amounts of surfactant, as opposed to the nanodroplets of microemulsions, created with significantly larger amounts of surfactant.165
It was demonstrated that the presence of the
coupling agent (MPS) was a prerequisite condition
for the encapsulation reaction within the synthesis
of silica/polystyrene colloids. The dispersion of core
particles, which is a major drawback of encapsulation reactions in microemulsion polymerization, can
be significantly improved with the aid of ultrasound.
In miniemulsion polymerization, an effective surfactant/hydrophobe system was used, and the role of
the hydrophobe was to prevent or retard coalescence
of the droplets and Ostwald ripening.135 The particle size was shown to be possible to be controlled by
water-soluble cross-linker concentration and surfactant/water ratio.217
When using micelle polymerization as a term,
surfactant molecules are regarded as monomers,
June 2, 2005 12:33 00700
268
whereas when using microemulsion polymerization
as a term, monomers are thought to be encapsulated within reverse micelles and polymerize
under surfactant restrictions. Surfactant monomers
(so-called surfmers) are used often in order to
gain nanoparticle–polymer composites with narrow
size and shape distribution. Since the pioneering
surfmer synthesis performed by Freedman et al.
who reported the first synthesis of vinyl monomers
which also functioned as an emulsifying agent,218
vast amount of literature has been dedicated to the
topic of polymerization of/in organized amphiphilic
assemblies.219 Polymerization of surfmers was
observed only at concentrations above CMC, proving
that micelle formation is a necessary precondition for
polymerization.219 Surfactants acting as surfmers are
CVDAC
(cetyl-p-vinylbenzyldimethylammonium
chloride, that might be photopolymerized too),
DDM-MEAC
[N,N-didodecyl-N-methyl-N-((2methacryloyloxy)ethyl)ammonium chloride] and
didecyldimethylammonium methacrylate.220 The
first synthesis of 2–5-nm-sized latex particles was
performed by using didecyldimethylammonium
methacrylate, a polymerizable surfactant which
forms reverse micelles in toluene.221
10. Survey of the Types of
Materials Synthesized by Using
Reverse Micelles
Materials of many different chemical compositions were synthesized within reverse micelles and
we shall list some of them. Applications of the
produced materials by using reverse micelles are
widespread in paints and surface coatings, catalysis,
separation media, drug delivery systems, highfrequency electronic components, etc. The possibility of obtaining materials with high surface
areas makes reverse micellar methods of synthesis viable for the production of catalytically
active transition metals, including Pt,222 Au,90
Ag,186 Pd,169 Rh,1,223 Ir,1 PtPd,224 AuAg,225
AuPd,226 PtRu,227 PtCo,228 Zr,142 Si,6 Ni,187 Ti,229
Bi,230 Cu,4,231 Se,84 Ce,232 Ce1-x -ZrxO2 ,233 Ce–Tb
mixed oxides,234 (Ce,Zr)O-x/Al2 O3 ,235 CeZrO4 ,236
CeZrO4 /Al2 O3 ,237 Pd/CexZr1-O-x(2)/Al2O3 threeway catalysts,238 LaMnO3 /CdS photocatalyst,239
and ferric sulfide catalyst for direct coal liquefaction (DCL).240 The metal nanoparticles produced
within reverse micelles are characterized by a higher
catalytic stability and effectiveness than classical
metal catalysts.108 Nanosized and narrowly sizedistributed reverse micelle aqueous cores open a
way to synthesize narrowly distributed semiconductors with marked quantum-size (Q-size) effects,
such as: CdS,24,149 PbS,241 ZnS,242 CdE (E = S,
Se, Te) semiconductors.243 BaF2 and BaF2 :Ce,87
BaF2 :Nd,244 AgBr,245 AgI,42,113 AgCl,246 Ag2 S,4
Cu2 S,247 MoSx ,188 CeF3 ,248 Bi2 Te3 ,249 ZnSe,250 B
alloys,251 Ni-, Co- and Fe- borides,252 tungstic
acid,253 Cu2 [Fe(CN)6 ],254 amorphous AlOOH255
were also synthesized within reverse micelles.
Many composites have been prepared, which
primarily include using silica coatings or matrices as additional component. Such materials are
silica-coated Co nanoparticles,256 Pt/silica,257
Pd/SiO2 particles,78 Cu/SiO2 ,258 Ag/SiO2 ,259
SiO2 -coated Rh nanoparticles,101 SiO2 -coated
CeO2 nanoparticles,260 SiO2 -coated Co-ferrite
and magnetite,102 SiO2 -coated Pt, Pd and
PtAg particles,261 SiO2 -coated NiFe permalloy
nanoparticles,312 silica-coated Fe2 O3 ,76 silica-coated
ZnFe2 O4 ,77 silica-coated CoFe2 O4 and MnFe2 O4 ,262
TiO2 /SiO2 composites,263 ZrO2 /SiO2 .264 The use of
gold as an additional component of composites produced in reverse micelles — exemplified by the cases
of preparation of gold-coated Fe nanoparticles,211
gold-coated FePtx and CoPtx particles,265 goldcoated Co, CoPt and CoPt3 magnetic alloys266 or
superparamagnetic magnetite-gold composites267 —
is noticed as well as the use of polymers in polymer
ceramics nanocomposites,86,215,217 including magnetite polymer composites,185,268 nanocomposites of
polyaniline with AgCl, BaSO4 and TiO2 .86,176 Composites of TiO2 /Fe2 O3 ,269 CdS particles coated with
ZnS,270 Co–Ag and Fe–Cu–Fe nanoparticles with
antiferromagnetic coupling between the Fe layers271
were also prepared within reverse micelles.
Beside using reverse micelles and other
dynamic nanostructures of self-assembling surfactant molecules in microemulsions to limit the particle size of dispersed systems formed by precipitation
in the case of inorganic particles, various approaches
have been used for lattices gained by polymerization reactions. Nanoscale polymerization reactions
within the reverse micellar confinement yield products with controlled conjugation lengths and, thus,
controlled band gaps for applications to electronics
June 2, 2005 12:33 00700
and photonics. In addition, control of the length
of the polymer blocks provides the prospects of
monodispersity, improved processability, and designable preparation of nanocomposites. Polymeric
nanosized particles (latexes) were first synthesized
by using the reverse micelle method221 and since then
many polymers, including poly(p-phenylenevinylene)
(PPV),216 were synthesized within reverse micellar microemulsions. Many procedures of synthesis concerning encapsulation of nanoparticles with
polymers,326 either by extracting the particles from
the microemulsion or by in situ polymerization, such
as in the case of the preparation of Bi/PMMA,160
CdS/polyacrilamide nanocomposites272 and BaSO4
in polymerized surfactants,273 were also reported in
the literature.
High-Tc superconductors YBa2 Cu3 O7-x 274–276
were prepared by this method as well as many
materials with exotic morphologies, such as V2 O5
nanowires,193 ZnS semiconductor nanowires,194
PbCl2 nanowires,277 Sb2 O3 and Sb2 O5 nanorods,278
BaCO3 single-crystal nanowires,279 single-crystal
SnO2 nanorods,280 CaCO3 , BaCO3 and CaSO4
nanowires,178
MnOOH
nanorods,281
BaSO4
nanoparticles and nanofilaments,282 star-shaped
CdS patterns,283 nanosized fluorescent hybrid silica (NFHS),284 fluorescent europium(III) chelatedoped silica nanoparticles,285 and silica-coated
terbium(III) chelate fluorescent nanoparticles.286
Ca(OH)2 ,3 CaCO3 145 and materials for biomedical
applications, such as calcium hydroxyapatite287,288
and Ca(PO4 )3 ,289 have also been produced within
reverse micelles. The pioneering nanoengineering via biocompatible microemulsion was performed by Debuigne et al. and was related to
the preparation of anti-inflammatory drug nimesulide by direct precipitation in reverse micellar
microemulsions comprising either Epikuron 170
(lecithin)/isopropyl myristate/water/n-butanol or
E170/isopropyl myristate/water/n-isopropanol190
and to the synthesis of cholesterol, Rhovanol and Rhodiarome in AOT/heptane/water,
Triton/decanol/water, and CTAB/hexanol/water
microemulsions.174
Many ceramic materials have been synthesized
by using this method: ZrO2 –Y2 O3 nanoparticles,53
barium hexaaluminate nanoparticles,152,290 zirconolite CaZrTi2 O7 ,291 CuHo2 O5 and Cu2 Er2 O5 ,88
CaSO4 ,292 ZnAl2 O4 ,89 YVO4 ,293 perovskite-type
269
LaNiO3 , La2 CuO4 and BaPbO3 ,5 Y2 O3 : Eu3+ ,294
Gd2 O3 : Eu3+ and Gd2 O2 S : Eu3+ ,295
Na2 PO4 and K2 PO4 ,296 calcium thiophosphate,297
zincophosphates191,298 as well as many oxides,
such as GeO2 ,299 ZnO,300 EuO,301 SnO,302
TiO2 ,303 ZrO2 ,85 CeO2 ,304 Fe2 O3 ,269 Al2 O3 ,305
iron oxide-doped alumina nanoparticles,306 Csdoped alumina nanoparticles,307 and bariumstabilized alumina nanoparticles.308 Reverse micelles
have shown to be feasible in producing magnetic
ceramics, such as manganites LaMnO3 89,309,69
and
LaSr-manganites,310,311
Li(Mn,Co)2 O4 ,74
ferrites
NiFe2 O4 ,79
MnZnFe2 O4 ,80,117,313,314
147,157,161
NiZnFe2 O4 ,
MnFe2 O4 ,75 SrFe2 O4 ,31,92
67,120
CoFe2 O4 ,
CoCrFe2 O4 ,315,316
BaFe2 O4 ,317
69
318
LaFeO3 , and Fe3 O4 .
Many magnetic materials, as Fe,29,319 Co,23 FeCo nanocrystals,182 Co,
CoPt and CoPt3 ,266 KMnF3 and NaMnF3 antiferromagnetic particles,83 were successfully synthesized
within reverse micelles.
11. Future Directions
After all that has been said, it useful to take a look
once again at the major conclusion which emerges
from this review. On one hand, reverse micelles are
useful multimolecular structures that give significant
opportunities for the design of nanostructured materials of well-defined and uniform properties. On the
other hand, reverse micelles cannot be considered
as nano-engineering reactor shells because their size
and permeability cannot yet be tuned with sufficient
precision.108 “Nanoreactor” does not sound as a convenient term at all for describing the function of
reverse micelles in microemulsion-assisted syntheses
of materials, since conventional microreactors imply
enabling precise control over the reactions and properties of the resulting nanomaterials, and it is still
difficult to achieve this aim at the nano level by using
the reverse micellar technique.
Combinations of the reverse micellar precipitation method or any other microemulsion method
with some other methods of synthesis — such as solgel,291,320,321 hydrothermal synthesis,183 ultrasonic
irradiation,322 UV irradiation,323 pH-shock wave
method,324 and flame-spraying325 — that have been
used lately, have presented many new potentialities
and advantages. Merging of two or more preparation
techniques into one can, due to synergy effects, lead
June 2, 2005 12:33 00700
270
to multiple advantages, such as improved control of
the stoichiometry of the final product (taken from
sol-gel method) and extremely fine and controllable
grain size (taken from reverse micellar synthesis) in
the examples290,327 of the combination of sol-gel and
reverse micellar approaches to materials synthesis.
Low yields, surfactant-contamination and difficulties
arising from the attempts to separate precipitated
powders in the form of non-agglomerated particles —
serious obstacles of the microemulsion-assisted
nanoparticle preparation procedures — were overcome by feedstocking flame-spraying apparatus
with nanoparticles together with their parent
microemulsion,325 at the same time transcending
poor control of particle size and shape, which is
a typical drawback of conventional flame-spraying
methods of synthesis. Combining reverse micellar
synthesis of cobalt particles with their subsequent
evaporating deposition in external magnetic field
led to the formation of large-scale 3D superlattices
(Fig. 12) of cobalt nanocrystals.144 Carbon nanotubes have as well, for instance, been prepared
by direct introduction of in situ prepared catalytically active Co and Mo particles by a reverse
micellar method.328,329 Silver nanorods encapsulated
by polystyrene were prepared by a combination
of reverse micellar, gas antisolvent, and ultrasound
techniques.330 In order to overcome difficulties arising out of the fact that it is not possible to carry out
sequential reactions inside the same reverse micelles
in order to obtain multilayered composites, layerby-layer (LbL) techniques comprising adsorption of
oppositely charged polyelectrolytes on a solid surface
of synthesized particles in reverse micelles have been
used.108
The assemblying of particles formed in the
processes of reverse micellar and, in general,
microemulsion-assisted syntheses into precisely tailored, supra-nanocrystalline 3D structures, clearly
presents an important challenge. In situ reactions in
well-organized and immobilized amphiphilic matrices present only one approach toward this goal. The
use of shear on liquid crystalline systems for instance,
can be used to produce the alignment of microscopic
domains into macroscopic regions in the direction
of shear, or other interesting morphological transitions. External stimuli provides great promise, and
it is predicted that in the future days, progress in
Fig. 12. Self-organized arrangements of cobalt nanoparticles obtained by combining the synthesis of the material within reverse micelles and evaporating deposition
thereof.144
using light, electric and magnetic fields to organize
particles into 3D matrices will be realized. A more
active and potentially richer approach is the use of
engineered peptides to direct the self-organization of
nanoparticles into technologically important structures. The real test of self-assembly and materials
synthesis will be the generation of novel functional
structures with unique and useful properties.112
Future research in the field of the materials synthesis within reverse micelles may not only help in
the fabrication of new inorganic materials and the
development of spatially confined and highly controllable synthetic approaches, but may also provide
us with new understanding of the chemical, physicochemical and biochemical processes that occur in a
confined environment at the nanometer scale, which
is significant for the fundamental understanding of
nature and life.
June 2, 2005 12:33 00700
References
1. M. Boutonnet, J. Kizling and P. Stenius, Coll. Surf.
5(3) (1982) 209–225.
2. D. O. Yener and H. Giesche, J. Am. Ceram. Soc.
84(9) (2001) 1987–1995.
3. A. Nanni and L. Dei, Langmuir 19 (2003) 933–938.
4. M. P. Pileni, A. Hammounda, I. Lisiecki, L. Motte,
N. Moumen and J. Tanori, Control of the size and
shape of nanoparticles, in Fine Particles Science
and Technology, ed. E. Pelizzetti (Kluwer Academic
Publishers, 1996), pp. 413–429.
5. L. M. Gan, L. H. Zhang, H. S. O. Chan, C. H. Chew
and B. H. Loo, J. Mater. Sci. 31 (1996) 1071–1079.
6. F. J. Arriagada and K. Osseo-Asare, J. Coll. Interface Sci. 211 (1999) 210–220.
7. Y. X. Chen, X. Z. Zhang, K. Zheng, S. M. Chen,
Q. C. Wang and X. X. Wu, Enzyme Microbial Technol. 23(3–4) (1998) 243–248.
8. P. P. Sun and Y. M. Zhang, Synth. Commun.
27(23) (1997) 4173–4179.
9. K. K. B. Lee, L. H. Poppenborg and D. C.
Stuckey, Enzyme Microbial Technol. 23(3–4) (1998)
253–260.
10. M. A. Lopez-Quintela, C. Tojo, M. C. Blanco,
L. Garcia Rio and J. R. Leis, Curr. Opin. Coll.
Interface Sci. 9 (2004) 264–278.
11. K. Carlile, G. D. Rees, B. H. Robinson, T. D. Steer
and M. Svensson, J. Chem. Soc., Faraday Transact.
92(23) (1996) 4701–4708.
12. C. R. Babu, P. F. Flynn and A. J. Wand, J. Biomol.
NMR 25(4) (2003) 313–323.
13. E. P. Melo, S. M. B. Costa, J. M. S. Cabral, P. Fojan
and S. B. Petersen, Chem. Phys. Lipids 124 (2003)
37–47.
14. G. G. Chang, T. M. Huang and H. C. Hung,
Proc. Natl. Sci. Counc. ROC B24(3) (1999)
89–100.
15. A. Tuck, Surv. Geophys. 23 (2002) 379–409.
16. P. A. Bachmann, P. Walde, P. L. Luisi and J. Lang,
J. Am. Chem. Soc. 113(22) (1991) 8204–8209.
17. P. A. Bachmann, P. L. Luisi and J. Lang, Chimia
45(9) (1991) 266–268.
18. J. H. Schulman, W. Stoeckenius and L. M. Prince,
J. Phys. Chem. 63 (1959) 1677–1680.
19. J. Liu, Y. Ikushima and Z. Shervani, Curr. Opin.
Solid State Mater. Sci. 7 (2003) 255–261.
20. M. C. McLeod, R. S. McHenry, E. J. Beckman and
C. B. Roberts, J. Phys. Chem. B107(12) (2003)
2693–2700.
21. C. J. O’Connor, V. Kolesnichenko, E. Carpenter,
C. Sangregorio, W. Zhou, A. Kumbhar, J. Sims and
F. Agnoli, Synth. Met. 122 (2001) 547–557.
22. M. P. Pileni, T. Zemb and C. Petit, Chem. Phys.
Lett. 118(4) (1985) 414–420.
23. E. E. Carpenter, C. T. Seip and C. J. O’Connor,
J. Appl. Phys. 85(8) (1999) 5184–5186.
271
24. U. Natarajan, K. Handique, A. Mehra, J. R. Bellare
and K. C. Khilar, Langmuir 12 (1996) 2670–2678.
25. R. H. Kodama, J. Magn. Magn. Mater. 200 (1999)
359–372.
26. P. Tartaj and C. J. Serna, Chem. Mater. 14 (2002)
4396–4402.
27. K. K. Ghosh and L. K. Tiwary, J. Dispersion Sci.
Technol. 22(4) (2001) 343–348.
28. B. Lindman and H. Wennerstrom, Micelles.
Amphiphile Aggregation in Aqueous Solution,
Topics in Current Chemistry, Vol. 87 (SpringerVerlag, Berlin, 1980).
29. F. Li and C. Vipulanandan and K. K. Moharty,
Coll. Surf. A223 (2003) 103–112.
30. S. L. Watt, D. Tunaley and S. Biggs, Coll. Surf.
A137 (1998) 25–33.
31. J. Fang, J. Wang, L. M. Gan, S. C. Ng, J. Ding
and X. Liu, J. Am. Ceram. Soc. 83(5) (2000)
1049–1055.
32. H. R. Rabie, M. E. Weber and J. H. Vera, J. Coll.
Interface Sci. 174 (1995) 1–9.
33. F. Li and C. Vipulanandan, Conductivity of waterin-oil microemulsion system, retrieved from University of Houston website at http://gem1.cive.
uh.edu.
34. D. Brown and J. H. R. Clarke, J. Phys. Chem. 92
(1988) 2881–2888.
35. N. E. Levinger, Science 298 (2002) 1722–1723.
36. S. Senapati and M. L. Berkowitz, J. Chem. Phys.
118(4) (2003) 1937–1944.
37. D. Brown and J. H. R. Clarke, J. Phys. Chem.
92(10) (1988) 2881–2888.
38. S. Brunetti, D. Roux, A. M. Bellocq, G. Fourche
and P. Bothorel, J. Phys. Chem. 87(6) (1983)
1028–1034.
39. V. K. Aswal and P. S. Goyal, Physica B245 (1998)
73–80.
40. C. C. Co, E. W. Kaler and S. R. Kline, Microstructure transformation during microemulsion and
micellar polymerizations, Research Highlights in
the monthly review paper of NIST Center for Neutron Research, world wide web.
41. V. K. Aswal, Bhabha Atom. Res. Centre Newslett.
237 (2003).
42. S. Tamura, K. Takeuchi, G. Mao, R. Csencsits,
L. Fan, T. Otomo and M. L. Saboungi, J. Electroanal. Chem. 559 (2003) 103–109.
43. N. V. Sushkin, Ph.D. dissertation, Worcester Polytechnic Institute (1997).
44. Q. Zhong, D. A. Steinhurst, E. E. Carpenter and
J. C. Owrutsky, Langmuir 18 (2002) 7401–7408.
45. G. Y. Xu, L. Zhang, S. L. Yuan, X. R. Huang and
G. Z. Li, J. Dispersion Sci. Technol. 22(6) (2001)
563–567.
46. T. Patzlaff, M. Janich, G. Seifert and H. Graener,
Chem. Phys. 261 (2000) 381–389.
June 2, 2005 12:33 00700
272
47. R. H. Cole, S. Mashimo and P. Winson, J. Chem.
Phys. 84(7) (1980) 786–793.
48. B. Gestblom and E. Noreland, J. Chem. Phys.
81(8) (1977) 782–788.
49. J. van Stam, S. Depaemelaere and F. C. De
Schryver, J. Chem. Edu. 75(1) (1998) 93–98.
50. M. P. Pileni, J. Phys. Chem. 97 (1993) 6961–6973.
51. F. Reiss-Husson and V. Luzzati, J. Phys. Chem.
68(12) (1964) 3504–3511.
52. F. Li, G. Z. Li, H. Q. Wang and Q. J. Xue, Coll.
Surf. A127 (1997) 89–96.
53. X. Fang and C. Yang, J. Coll. Interface Sci. 212
(1999) 242–251.
54. G. Lindblom, B. Lindman and L. Mandell, J. Coll.
Interface Sci. 34(2) (1970) 262–271.
55. T. Kato and T. Seimiya, J. Phys. Chem. 90 (1986)
3159–3167.
56. L. Wang and R. E. Verrall, J. Phys. Chem. 98
(1994) 4368–4374.
57. I. Benito, M. A. Garcia, C. Monge, J. M. Saz
and M. L. Marina, Coll. Surf. A125 (1997)
221–224.
58. J. J. Rack, T. M. McCleskey and E. R. Birnbaum,
J. Phys. Chem. B106 (2002) 632–636.
59. Z. Lin, J. J. Cai, L. E. Scriven and H. T. Davis,
J. Phys. Chem. 98 (1994) 5984–5993.
60. T. Kinugasa, A. Kondo, S. Nishimura, Y. Miyauchi,
Y. Nishii, K. Watanabe and H. Takeuchi, Coll. Surf.
A204 (2002) 193–199.
61. G. Montelvo, M. Valiente and E. Rodenas, Langmuir 12 (1996) 5202–5208.
62. D. Roux, A. M. Bellocq and P. Bothorel, Prog. Coll.
Polymer Sci. 69 (1984) 1–11.
63. E. Rodenas and M. Valiente, Coll. Surf. 62 (1992)
289–295.
64. J. Kim and M. Lee, J. Phys. Chem. A103(18)
(1999) 3378–3382.
65. P. Ekwall, L. Mandell and P. Solyom, J. Coll. Interface Sci. 35(2) (1970) 267–272.
66. Y. Liu, W. Wang, Y. Zhan, C. Zheng and G. Wang,
Mater. Lett. 56 (2002) 496–501.
67. C. Liu, A. J. Rondinone and Z. J. Zhong, Pure Appl.
Chem. 72(1–2) (2000) 37–45.
68. C. R. Vestal and Z. J. Zhang, J. Solid State Chem.
175 (2003) 59–62.
69. A. E. Giannakas, A. K. Ladavos and P. J. Pomonis,
Appl. Catal. B49 (2004) 147–158.
70. D. Walsh and S. Mann, Chem. Mater. 8(8) (1996)
1944–1953.
71. T. Hirai, S. Hariguchi, I. Komasawa and R. J.
Davey, Langmuir 13(25) (1997) 6650–6653.
72. H. Zhang and A. I. Cooper, Chem. Mater. 14 (2002)
4017–4020.
73. J. C. Linehan, J. L. Fulton and R. M. Bean, Process of forming compounds using reverse micelle
or reverse microemulsion systems, US Patent
5,170,172 (1998).
74. C. H. Lu and H. C. Wang, J. Eur. Ceram. Soc. 23
(2003) 865–871.
75. C. Liu, B. Zuo, A. J. Rondinone and Z. J. Zhang,
J. Phys. Chem. 104(6) (2000) 1141–1145.
76. S. Santra, R. Tapec, N. Theodoropoulou,
J. Dobson, A. Hebard and W. Tan, Langmuir 17
(2001) 2900–2906.
77. F. Grasset, N. Labhsetwar, D. Li, D. C. Park,
N. Saito, H. Haneda, O. Cador, T. Roisnel,
S. Mornet, E. Duguet, J. Portier and J. Etourneau,
Langmuir 18(21) (2002) 8209–8216.
78. D. S. Bae, K. S. Han and J. H. Adair, J. Mater.
Chem. 12(10) (2002) 3117–3120.
79. J. Fang, N. Shama, L. D. Tung, E. Y. Shin, C. J.
O’Connor, K. L. Stokes, G. Caruntu, J. B. Wiley,
L. Spinu and J. Tang, J. Appl. Phys. 93(10) (2003)
7483–7485.
80. J. Wang, P. F. Chong, S. C. Ng and L. M. Gan,
Mater. Lett. 30 (1997) 217–221.
81. T. Liu, L. Guo, Y. Tao, Y. B. Wang and
W. D. Wang, Nanostruct. Mater. 11(4) (1999)
487–492.
82. Y. Y. Luk and N. L. Abbott, Curr. Opin. Coll.
Interface Sci. 7 (2002) 267–275.
83. E. E. Carpenter, C. Sangregorio and C. J.
O’Connor, Mol. Cryst. Liq. Cryst. 334 (1999)
641–649.
84. J. A. Johnson, M. L. Saboungi, P. Thiyagarajan,
R. Csecsits and D. Meisel, J. Phys. Chem. B103
(1999) 59–63.
85. X. Li, C. K. Loong, P. Thiyagarajan, G. A. Lager
and R. Miranda, J. Appl. Crystalog. 33(1) (2000)
628–631.
86. X. M. Sui, Y. Chu, S. X. Xing and C. Z. Liu, Mater.
Lett. 58(7–8) (2004) 1255–1259.
87. R. Hua, C. Zang, C. Shao, D. Xie and C. Shi,
Nanotechnology 14 (2003) 588–591.
88. F. Porta, C. Bifulco, P. Fermo, C. L. Bianchi,
M. Fadoni and L. Prati, Coll. Surf. A160 (1999)
281–290.
89. A. E. Giannakas, T. C. Vaimakis, A. K. Ladavos,
P. N. Trikalitis and P. J. Pomonis, J. Coll. Interface
Sci. 259 (2003) 244–253.
90. F. Chen, G. Q. Xu and T. S. A. Hor, Mater. Lett.
57 (2003) 3282–3286.
91. G. Palazzo, F. Lopez, M. Giustini, G. Colafemmina
and A. Ceglie, J. Phys. Chem. B107 (2003)
1924–1931.
92. D. H. Chen and Y. Y. Chen, J. Coll. Interface Sci.
236 (2001) 41–46.
93. J. P. Wilcoxon and P. P. Provencio, J. Phys. Chem.
B103 (1999) 9809–9812.
94. V. Uskoković and M. Drofenik, Coll. Surf. A, submitted, 2005.
95. Y. Teraoka, S. Nanri, I. Moriguchi, S. Kagawa,
K. Shimanoe and N. Yamazoe, Chem. Lett. 10
(2000) 1202–1203.
June 2, 2005 12:33 00700
96. M. Mamak, G. S. Metraux, S. Petrov, N. Coombs,
G. A. Ozin and M. A. Green, J. Am. Chem. Soc.
125(17) (2003) 5161–5175.
97. K. Kurumada, S. Itakura, S. Nagamine and
M. Tanigaki, Langmuir 13(14) (1997) 3700–3705.
98. M. P. Pileni, N. Moumen, J. F. Hochepied,
P. Bonville and P. Veillet, J. Phys. IV France 7,
C1-505-8 (1997).
99. M. A. Lopez-Quintela, Curr. Opin. Coll. Interface
Sci. 8 (2003) 137–144.
100. B. R. Smith and A. E. Alexander, J. Coll. Interface
Sci. 34(1) (1970) 81.
101. T. Tago, Y. Shibata, T. Hatsuta, K. Miyajima,
M. Kishida, S. Tashiro and K. Wakabayashi,
J. Mater. Sci. 37 (2002) 977–982.
102. T. Tago, T. Hatsuta, K. Miyajima, M. Kishida,
S. Tashiro and K. Wakabayashi, J. Am. Ceram.
Soc. 85(9) (2002) 2188–2194.
103. H. Shi, L. Qi, J. Ma, H. Cheng and B. Zhu, Adv.
Mater. 15(19) (2003) 1647–1651.
104. B. A. Simmons, S. Li, V. T. John, G. L. McPherson,
A. Bose, W. Zhou and J. He, Nano Lett. 2(4) (2002)
263–268.
105. P. Somasundaran, J. Coll. Interface Sci. 256 (2002)
3–15.
106. G. Porte, D. Roux, J. F. Berret, S. Lerouge,
J. P. Decruppe, P. Lindner and L. Noirez, Rheology
of wormlike micelles, ITP Complex Fluid Program
3/27/02 (CNRS-Rhodia, 2002).
107. R. Zana and J. Lang, Dynamics of Microemulsions,
in Microemulsions: Structure and Dynamics, eds.
S. E. Friberg and P. Bothorel (CRC Press: Boca
Raton, 1987), pp. 153–172.
108. D. G. Shchukin and G. B. Sukhorukov, Adv. Mater.
16(8) (2004) 671–682.
109. P. van Beurten and A. Vrij, J. Chem. Phys. 74(5)
(1981) 2744–2748.
110. F. G. Sanchez and C. C. Ruiz, J. Lumin. 69 (1996)
179–186.
111. Z. Cui and R. J. Mumper, Pharm. Res. 19(7)
(2002) 939–946.
112. V. T. John, B. Simmons, G. L. McPherson and
A. Bose, Curr. Opin. Coll. Interface Sci. 7 (2002)
288–295.
113. S. Xu, H. Zhou, J. Xu and Y. Li, Langmuir 18
(2002) 10503–10504.
114. J. Yang, Curr. Opin. Coll. Interface Sci. 7 (2002)
276–281.
115. M. Bhat, Studies in surfactant science and
hydrotropy, Ph.D. Thesis, University Institute of
Chemical Technology, Mumbai, India.
116. R. I. Nooney, D. Thirunavukkarasu, Y. Chen,
R. Josephs and A. E. Ostafin, Chem. Mater. 14
(2002) 4721–4728.
117. D. Makovec, A. Košak and M. Drofenik, Nanotechnol. 15 (2004) S160–S166.
273
118. A. M. Goncalves, A. P. Serro, M. R. Aires-Barros
and J. M.S. Cabral, Biochim. Biophys. Acta 1480
(2000) 92–106.
119. E. E. Carpenter, Ferrites: Proc. 8th Int. Conf. Ferrites (ICF 8), Kyoto/Tokyo, Japan (2000).
120. V. Pillai and D. O. Shah, J. Magn. Magn. Mater.
163 (1996) 243–248.
121. J. Sjöblom, R. Lindberg and S. E. Friberg, Adv.
Coll. Interface Sci. 95 (1996) 125–287.
122. S.
Bhattacharjee,
M.
Elimelech
and
M. Borkovec, Croatica Chem. Acta 71(4) (1998)
883–903.
123. D. G. Grier, Nature 393(6686) (1998) 621.
124. T. Dichristina, D. Roux, A. M. Bellocq and
P. Bothorel, J. Phys. Chem. 89(8) (1985)
1433–1437.
125. B. Lindman and P. Stilbs, in Microemulsions:
Structure and Dynamics, eds. S. E. Friberg and
P. Bothorel (CRC Press, Boca Raton, 1987),
119–152.
126. J. R. Hansen, J. Phys. Chem. 78 (1974) 256.
127. H. F. Eicke and P. E. Zinsli, J. Coll. Interface Sci.
65 (1978) 131.
128. M. Wong, J. K. Thomas and T. Nowak, J. Am.
Chem. Soc. 99 (1977) 4730.
129. B. Lindman and H. Wennerstrom, in Solution
Behavior of Surfactants, Vol. 1, eds. K. L. Mittal
and E. J. Fendler (Plenum Press, New York, 1982).
130. H. Grunhagen, J. Coll. Interface Sci. 53 (1975) 282.
131. J. Biais, B. Clin, P. Lalanne and B. Lemanceau,
J. Chim. Phys. 74 (1977) 1197.
132. H. Yoshioka, J. Coll. Interface Sci. 95 (1983) 81.
133. J. Lang, A. Djavanbakht and R. Zana, J. Phys.
Chem. 84 (1980) 145.
134. J. M. DiMeglio, M. Dvolaitzky, R. Ober and
C. Taupin, J. Phys. Lett. 44 (1983) L229.
135. E. Bourgeat–Lami, J. Nanosci. Nanotechnol. 2(1)
(2002) 1–24.
136. Z. Shervani and Y. Ikushima, J. New Chem. 26
(2002) 1257–1260.
137. R. P. Bagwe and K. C. Khilar, Langmuir 16 (2000)
905–910.
138. B. H. Robinson and P. D. Fletcher, Ber. Bunsenges.
Phys. Chem. 85 (1981) 863.
139. R. Jain and A. Mehra, Langmuir 20 (2004)
6507–6513.
140. S. Atik and J. K. Thomas, J. Phys. Chem. 85
(1981) 3921.
141. K. Inouye, R. Endo, Y. Otsuka, K. Miyashiro,
K. Kaneko and T. Ishikawa, J. Phys. Chem. 86(8)
(1982) 1465–1469.
142. J. Fang, J. Wang, S. C. Ng, C. H. Chew
and L. M. Gan, Nanostruct. Mater. 8(4) (1997)
499–505.
143. L. Motte, C. Petit, L. Boulanger, P. Lixon and M. P.
Pileni, Langmuir 8(4) (1992) 1049–1053.
June 2, 2005 12:33 00700
274
144. J. Legrand, A. T. Ngo, C. Petit and M. P. Pileni,
Adv. Mater. 13(1) (2001) 58–62.
145. K. Kandori, K. Kon-No and A. Kitahara, J. Coll.
Interface Sci. 122(1) (1988) 78–82.
146. H. Sato and I. Komasawa, J. Chem. Eng. Jpn.
33(2) (2000) 262–266.
147. V. Uskoković and M. Drofenik, J. Magn. Magn.
Mater. 284 (2004) 294–302.
148. A.
Košak,
D.
Makovec,
A.
Žnidaršič
and M. Drofenik, J. Eur. Ceram. Soc. 24 (2004)
959–962.
149. A. Agostiano, M. Catalano, M. L. Curri,
M. Della Monica, L. Manna and L. Vasanelli,
Micron 31 (2000) 253–258.
150. C. T. Seip and C. J. O’Connor, Nanostruct. Mater.
12 (1999) 183–186.
151. C. J. O’Connor, C. T. Seip, E. E. Carpenter, S. Li
and V. T. John, Nanostruct. Mater. 12 (1999)
65–70.
152. A. J. Zarur and J. Y. Ying, Nature 403 (2000)
65–67.
153. A. Košak, D. Makovec and M. Drofenik, Phys. Status Solidi. C1(12) (2004) 3521–3524.
154. A. Košak, D. Makovec and M. Drofenik, J.
Metastable Nanocryst. Mater. 23 (2005) 251–254.
155. A. Košak, D. Makovec, M. Drofenik and
A. Žnidaršič J. Magn. Magn. Mater. 272–276
(2004) 1542–1544.
156. A. Košak, D. Makovec and M. Drofenik, Mater. Sci.
Forum 453–454 (2004) 219–224.
157. V. Uskoković and M. Drofenik, Mater. Sci. Forum
453–454 (2004) 225–230.
158. V. Uskoković and M. Drofenik, Mater. Technol.
37(3–4) (2003) 129–131.
159. D. B. Zhang, L. M. Qi, H. M. Cheng and J. M. Ma,
Chin. Chem. Lett. 14(1) (2003) 100–103.
160. J. Fang, L. Stokes, J. Wiemann and W. Zhou,
Mater. Lett. 42 (2000) 113–120.
161. V. Uskoković and M. Drofenik, Surf. Rev. Lett. 12
(2005) 97–100.
162. M. C. Moran, A. Pinazo, L. Perez, P. Clapes,
M. Angelet, M. T. Garcia, M. P. Vinardell
and M. R. Infante, Green Chem. 6 (2004)
233–240.
163. Nano-Crystalline Materials Research, The Newsletter of the New York State Center for Advanced
Ceramic Technology 11.3 (1999).
164. A. A. Date and V. B. Patravale, Curr. Opin. Coll.
Interface Sci. 9 (2004) 222–235.
165. K. Wormuth, J. Coll. Interface Sci. 241 (2001)
366–377.
166. J. Sjoblom, R. Skurtveit, J. O. Saeten and
B. Gestblom, J. Coll. Interface Sci. 141(2) (1991)
329–337.
167. K. Shinoda and H. Takeda, J. Coll. Interface Sci.
32(4) (1970) 642.
168. S. L. Watt, D. Tunaley and S. Biggs, Coll. Surf.
A137 (1998) 25–33.
169. C. C. Wang, D. H. Chen and T. C. Huang, Coll.
Surf. A189 (2001) 145–154.
170. M. L. Curri, A. Agostiano, F. Mavelli and
M. Della Monica, Mater. Sci. Eng. C22 (2002)
423–426.
171. L. Zhang, G. C. Papaefthymiou and J. Y. Ying,
J. Appl. Phys. 81(10) (1997) 6892–6900.
172. M. Hasegawa, Langmuir 17 (2001) 1426–1431.
173. J. Kizling and P. Stenius, J. Coll. Interface Sci.
118(2) (1987) 482–492.
174. F. Debuigne, L. Jeunieau, M. Wiame and J. B.
Nagy, Langmuir 16 (2000) 7605–7611.
175. E. E. Carpenter, Synthesis of nanoparticles
using reverse micelles, www.darpa.mil/dso/future/
metamaterials/presentations/carpenter.pdf.
176. X. M. Sui, Y. Chu, S. X. Xing, M. Yu and C. Z.
Liu, Coll. Surf. A251(1–3) (2004) 103–107.
177. S. Vaucher, M. Li and S. Mann, Angew. Chem. Int.
Ed. 39 (2000) 1793–1796.
178. D. Kuang, A. Xu, Y. Fang, H. Ou and H. Liu,
J. Cryst. Growth 244 (2002) 379–383.
179. A. Courty, C. Fermon and M. P. Pileni, Adv. Mater.
13(4) (2001) 254–258.
180. C. E. Bunker, B. A. Harruff, P. Pathak, A. Payzant,
L. F. Allard and Y. P. Sun, Langmuir 20 (2004)
5642–5644.
181. D. K. Kim, Y. Zhang, W. Voit, K. V. Rao and
M. Muhammed, J. Magn. Magn. Mater. 225 (2001)
30–36.
182. N. C. Tansil, R. V. Ramanujan and H. F. Li, Trans.
Ind. Inst. Met. 56(5) (2003) 509–512.
183. J. C. Lin, J. T. Dipre and M. Z. Yates, Chem.
Mater. 15 (2003) 2764–2773.
184. D. C. Steytler, A. Gurgel and R. Ohly, Langmuir
20 (2004) 3509–3512.
185. P. A. Dresco, V. S. Zaitsev, R. J. Gambino and
B. Chu, Langmuir 15 (1999) 1945–1951.
186. P. Barnickel, A. Wokaun, W. Sager and H. F.
Eickel, J. Coll. Interface Sci. 148(1) (1992)
80–90.
187. D. H. Chen and S. H. Wu, Chem. Mater. 12 (2000)
1354–1360.
188. K. E. Marchand, M. Tarret, J. P. Lechaire,
L. Normand, S. Kasztelan and T. Cseri, Coll. Surf.
A214 (2003) 239–248.
189. C. Kitchens, M. C. McLeod and B. Roberts,
J. Phys. Chem. B107 (2003) 11331–11338.
190. F. Debuigne, J. Cuisenaire, L. Jeunieau,
B. Masereel and J. B. Nagy, J. Coll. Interface Sci.
243 (2001) 90–101.
191. R. Singh, J. Doolittle, M. A. George and P. K.
Dutta, Langmuir 18(21) (2002) 8193–8197.
192. H. Chakraborty and M. Sarkar, Langmuir 20 (2004)
3551–3558.
June 2, 2005 12:33 00700
193. N. Pinna, M. Willinger, K. Weiss, J. Urban and
R. Schlogl, Nano Lett. 3(8) (2003) 1131–1134.
194. Q. S. Wu, N. W. Zheng, Y. P. Ding and Y. D. Li,
Inorg. Chem. Commun. 5(9) (2002) 671–673.
195. S. A. Safran, Nature Mater. 2 (2003) 71–72.
196. J. Sims, A. Kumbhar, J. Lin, F. Agnoli,
E. Carpenter, C. Sangregorio, C. Frommen,
V. Kolesnichenko and C. J. O’Connor, Mol. Cryst.
Liq. Crys. 379 (2002) 121–130.
197. L. P. Li and G. S. Li, Hyperfine Interact. 128 (2000)
437–442.
198. S. H. Liou and C. L. Chien, Appl. Phys. Lett. 52(6)
(1988) 512–514.
199. C. R. Hendricks, V. W. R. Amarakoon and
D. Sullivan, Ceram. Bull. 70(5) (1991)
817–823.
200. Y. S. Cho, D. Schaffer, V. L. Burdick and V. R.
W. Amarakoon, Mater. Res. Bull. 34(14/15) (1999)
2361–2368.
201. K. Mandal, S. P. Mandal, P. Agudo and M. Pal,
Appl. Surf. Sci. 182 (2001) 386–389.
202. K. G. Brooks and V. R. W. Amarakoon, J. Am.
Ceram. Soc. 74(4) (1991) 851–853.
203. F. A. Selmi and V. R. W. Amarakoon, J. Am.
Ceram. Soc. 71(11) (1988) 934–937.
204. A. Chatterjee, D. Das, S. K. Pradhan and
D. Chakravorty, J. Magn. Magn. Mater. 127 (1993)
214–218.
205. G. Beckingham, Aerogels: their history, structure and application, http://members.tripod.com/
∼geobeck/frontier/aerogels.html.
206. F. J. Arriagada and K. Osseo-Asare, Coll. Surf.
A154 (1999) 311–326.
207. A. Van Blaaderen, J. Van Geest and A. Vrij, J. Coll.
Interface Sci. 154(2) (1992) 481–501.
208. F. Bentivegna, J. Ferre, M. Nyvlt, J. P. Jamet,
D. Imhoff, M. Canva, A. Brun, P. Veillet,
S. Visnovsky, F. Chaput and J. P. Boilot, J. Appl.
Phys. 83(12) (1998) 7776–7788.
209. S. Ponce-Castaneda, J. R. Martinez, F. Ruiz,
S. Palomares-Sanchez and O. Dominguez, J. SolGel Sci. Technol. 25 (2002) 29–36.
210. J. Wiggins, E. E. Carpenter and C. J. O’Connor,
J. Appl. Phys. 87(9) (2000) 5651–5653, Part 2.
211. J. Lin, W. Zhou, A. Kumbhar, J. Wiemann,
J. Fang, E. E. Carpenter and C. J. O’Connor,
J. Solid State Chem. 159 (2001) 26–31.
212. K. Raj, B. Moskowitz and R. Casciari, J. Magn.
Magn. Mater. 149 (1995) 174–180.
213. J. A. Lopez-Perez, M. A. Lopez-Quintela, J. Mira
and J. Rivas, IEEE Trans. Magn. 33(5) (1997)
4359–4362.
214. A. Kosturiak, J. Polavka, L. Valko, J. Slama,
A. Gruskova and M. Miglierini, J. Magn. Magn.
Mater. 153 (1996) 184–188.
215. N. S. Kommareddi, M. Tata, V. T. John,
G. L. McPherson, M. F. Herman, Y. S. Lee,
216.
217.
218.
219.
220.
221.
222.
223.
224.
225.
226.
227.
228.
229.
230.
231.
232.
233.
234.
235.
236.
237.
238.
275
C. J. O’Connor, J. A. Akkara, and D. L. Kaplan,
Chem. Mater. 8 (1996) 801–809.
M. Lal, N. D. Kumar, M. P. Joshi and P. N. Prasad,
Chem. Mater. 10 (1998) 1065–1068.
Y. Deng, L. Wang, W. Yang, S. Fu and A. Elaissari,
J. Magn. Magn. Mater. 257 (2003) 69–78.
H. H. Freedman, J. P. Mason and A. I. Medalia,
J. Org. Chem. 23 (1958) 76–82.
M. Summers and J. Eastoc, Adv. Coll. Interface
Sci. 100–102 (2003) 137–152.
T. Hirai, T. Watanabe and I. Komasawa, J. Phys.
Chem. B104 (2000) 8962–8966.
A. Hammouda, T. Gulik and M. P. Pileni, Langmuir 11 (1995) 3656–3659.
O. P. Yadav, A. Palmqvist, N. Cruise and
K. Holmberg, Coll. Surf. A221 (2003) 131–134.
T. Hanaoka, T. Hatsuta, T. Tago, M. Kishida
and K. Wakabayashi, Appl. Catal. A190 (2000)
291–296.
R. Touroude, P. Girard, G. Maire, J. Kizling,
M. Boutonnet, M. Kizling and P. Stenius, Coll.
Surf. 67 (1992) 9–19.
D. H. Chen and C. J. Chen, J. Mater. Chem. 12
(2002) 1557–1562.
M. L. Wu, D. H. Chen and T. C. Huang, Langmuir
17 (2001) 3877–3883.
X. Zhang and K. Y. Chan, Chem. Mater. 15 (2003)
451–459.
X. Zhang and K. Y. Chan, J. Mater. Chem. 12
(2002) 1203–1206.
W. E. Stallings and H. H. Lamb, Langmuir 19
(2003) 2989–2994.
J. Fang, K. L. Stokes, J. A. Wiemann, W. L. Zhou,
J. Dai, F. Chen and C. J. O’Connor, Mater. Sci.
Eng. B83 (2001) 254–257.
J. P. Cason and C. B. Roberts, J. Phys. Chem.
B104 (2000) 1217–1221.
S. Patil, S. C. Kuiry, S. Seal and R. Vanfleet,
J. Nanoparticle Res. 4(5) (2002) 433–438.
J. A. Rodriguez, J. C. Hanson, J. Y. Kim and
G. Liu, J. Phys. Chem. B107 (2003) 3535–3543.
A. B. Hungria, A. Martinez-Arias, M. FernandezGarcia, A. Iglesias-Juez, A. Guerrero-Ruiz, J. J.
Calvino, J. C. Conesa and J. Soria, Chem. Mater.
15(22) (2003) 4309–4316.
M. Fernandez-Garcia, A. Martinez-Arias, A. B.
Hungria, A. Iglesias-Juez, J. C. Conesa and
J. Soria, Phys. Chem. Chem. Phys. 4(11) (2002)
2473–2481.
A. Martinez-Arias, M. Fernandez-Garcia, A. B.
Hungria, J. C. Conesa and G. Munuera, J. Phys.
Chem. B107(12) (2003) 2667–2677.
A. Martinez-Arias, M. Fernandez-Garcia, A. B.
Hungria, J. C. Conesa and J. Soria, J. Alloy. Compound. 323 (2001) 605–609.
M.
Fernandez-Garcia,
A.
Martinez-Arias,
A. Iglesias-Juez, A. B. Hungria, J. A. Anderson,
June 2, 2005 12:33 00700
276
239.
240.
241.
242.
243.
244.
245.
246.
247.
248.
249.
250.
251.
252.
253.
254.
255.
256.
257.
258.
259.
260.
261.
J. C. Conesa and J. Soria, Appl. Catal. B31(1)
(2001) 39–50.
T. Kida, G. Q. Guan and A. Yoshida, Chem. Phys.
Lett. 371(5–6) (2003) 563–567.
D. B. Dadyburjor, T. E. Fout and J. W. Zondlo,
Catal. Today 63(1) (2000) 33–41.
A. J. I. Ward, E. C. O’Sullivan, J. C. Rang,
J. Nedeljkovic and R. C. Patel, J. Coll. Interface
Sci. 161(2) (1993) 316–320.
T. Hirai, H. Sato and I. Komasawa, Ind. Eng.
Chem. Res. 33 (1994) 3262.
C. B. Murray, D. J. Norris and M. G. Bawendi,
J. Am. Chem. Soc. 115(19) (1993) 8706–8715.
C. M. Bender, J. M. Burtlich, D. Baber et al.,
Chem. Mater. 12(7) (2000) 1969–1976.
M. I. Freedhoff, W. Chen, J. M. Rehm, C. Meyers,
A. Marchetti and G. McLendon, in Fine Particles
Science and Technology, ed. E. Pelizzetti (1996),
pp. 281–293.
T. Sugimoto and K. Kimijima, J. Phys. Chem.
B107 (2003) 10753–10759.
S. G. Dixit, A. R. Mahadeshwar and S. K. Haram,
Coll. Surf. A133(1–2) (1998) 69–75.
S. Q. Qiu, J. X. Dong and G. X. Chen, Powder
Technol. 113(1–2) (2000) 9–13.
E. E. Foos, R. M. Stroud and A. D. Berry, Nano
Lett. 1(12) (2001) 693–695.
F. T. Quinlan, J. Kuther, W. Tremel, W. Knoll,
S. Risbud and P. Stroeve, Langmuir 16(8) (2000)
4049–4051.
J. B. Nagy, I. Bodart-Ravet and E. G. Derouane,
Faraday Discuss. 87 (1989) 189–198.
J. B. Nagy, Coll. Surf. 35(2–4) (1989) 201–220.
A. K. Panda, S. P. Moulik, B. B. Bhowmik and
A. R. Das, J. Coll. Interface Sci. 235 (2001)
218–226.
S. P. Moulik, G. C. De, A. K. Panda, B. B.
Bhowmik and A. R. Das, Langmuir 15 (1999)
8361–8367.
Y. Berkovich, A. Aserin, E. Wachtel and N. Garti,
J. Coll. Interface Sci. 245 (2000) 58–67.
M. Wu, Y. D. Zhang, S. Hui, T. D. Xiao, S. Ge,
W. A. Hines and J. I. Budnick, J. Appl. Phys. 92(1)
(2002) 491–495.
D. S. Bae, K. S. Han and J. H. Adair, J. Am.
Ceram. Soc. 85(5) (2002) 1321–1323.
D. S. Bae, K. S. Han and J. H. Adair, J. Mater.
Sci. Lett. 21(1) (2002) 53–54.
D. S. Bae, S. W. Park, K. S. Han and J. H. Adair,
Met. Mater. Int. 7(4) (2001) 399–402.
T. Tago, S. Tashiro, Y. Hashimoto, K. Wakabayashi
and M. Kishida, J. Nanoparticles Res. 5 (2003)
55–60.
K. M. K. Yu, C. M. Y. Yeung, D. Thompsett
and S. C. Tsang, J. Phys. Chem. B107 (2003)
4515–4526.
262. C. Vestal and Z. J. Zhang, Nano Lett. 3 (2003)
1739–1743.
263. S. S. Hong, M. S. Lee, S. S. Park and G. D. Lee,
Catal. Today 87(1–4) (2003) 99–105.
264. P. Tartaj and L. C. De Jonghe, J. Mater. Chem.
10(12) (2000) 2786–2790.
265. C. J. O’Connor, J. A. Sims, A. Kumbhar, V. L.
Kolesnichenko, W. L. Zhou and J. A. Wiemann,
J. Magn. Magn. Mater. 226–230 (2001)
1915–1917.
266. A. Kumbhar, L. Spinu, F. Agnoli, K. Y. Wang,
W. L. Zhou and C. J. O’Connor, IEEE Trans.
Magn. 37(4) (2001) 2216–2218.
267. T. Kinoshita, S. Seino, K. Okitsu, T. Nakayama,
T. Nakagawa and T. A. Yamamoto, J. Alloy. Compound. 359 (2003) 46–50.
268. S. A. Gomez-Lopera, R. C. Plaza and A. V.
Delgado, J. Coll. Interface Sci. 240 (2001) 40–47.
269. T. Hirai, J. Y. Mizumoto, S. Shiojiri and
I. Komasawa, J. Chem. Eng. Jpn. 30(5) (1997)
938–943.
270. L. M. Qi, J. M. Ma, H. M. Cheng and Z. G. Zhao,
Chin. Chem. Lett. 6(11) (1995) 1013–1016.
271. M. A. Lopez-Quintela, J. Rivas, M. C. Blanco
and C. Tojo, in Nanoscale Materials, eds. L. M.
L. Marzan and P. V. Kamat, (Kluwer Academic
Plenum Publ, 2003), pp. 135–155.
272. Y. Ni, X. Ge, H. Liu, Z. Zhang and Q. Ye, Chem.
Lett. 9 (2001) 924–925.
273. M. Summers, J. Eastoe and S. Davis, Langmuir 18
(2002) 5023–5026.
274. P. Ayyaub, A. N. Maitra and D. O. Shah, Physica
C168(5–6) (1990) 571–579.
275. P. Ayyub and M. S. Multani, Mater. Lett. 10(9–10)
(1991) 431–436.
276. F. Li and C. Vipulanandan, IEEE Trans. Appl.
Supercond. 13(2) (2003) 3196–3198.
277. Q. S. Wu, N. W. Zheng and Y. P. Ding, Chem. J.
Chin. Uni. 22(6) (2001) 898–900.
278. L. Guo, Z. Wu, T. Liu, W. Wang and H. Zhu, Chem.
Phys. Lett. 318 (2000) 49–52.
279. L. M. Qi, J. M. Ma, H. M. Cheng and Z. G. Zhao,
J. Phys. Chem. B101(18) (1997) 3460–3463.
280. Y. Liu, C. Zheng, W. Wang, Y. Zhan and G. Wang,
J. Cryst. Growth 233 (2001) 8–12.
281. X. Y. Dong, X. T. Zhang, G. Cheng, Y. C. Li and
Z. L. Du, Acta Chim. Sin. 62(24) (2004) 2441–2443.
282. J. D. Hopwood and S. Mann, Chem. Mater. 9(8)
(1997) 1819–1828.
283. X. Mo, C. Wang, L. Hao, M. You, Y. Zhu,
Z. Chen and Y. Hu, Mater. Res. Bull. 36 (2001)
1925–1930.
284. H. H. Yang, H. Y. Qu, P. Lin, S. H. Li, M. T. Ding
and J. G. Xu, Analyst 128(5) (2003) 462–466.
285. Z. Q. Ye, M. Q. Tan, G. L. Wang and J. L. Yuan,
J. Mater. Chem. 14(5) (2004) 851–856.
June 2, 2005 12:33 00700
286. Z. Q. Ye, M. Q. Tan, G. L. Wang and J. L. Yuan,
Anal. Chem. 76(3) (2004) 513–518.
287. S. Bose and S. K. Saha, Chem. Mater. 15 (2003)
4464–4469.
288. J. Z. Liu, B. D. Gou, S. J. Xu and K. Wang, Prog.
Natural Sci. 12(8) (2002) 615–617.
289. S. Sarda, M. Heughebaert and A. Lebugle, Chem.
Mater. 11(10) (1999) 2722–2727.
290. A. J. Zarur, H. H. Hwu and J. Y. Ying, Langmuir
16 (2000) 3042–3049.
291. C. J. Barbe, R. Graf, K. S. Finnie, M. Blackford,
R. Trautman and J. R. Bartlett, J. Sol-Gel Sci.
Technol. 26 (2003) 457–462.
292. G. D. Rees, R. G. Evans, S. J. Hammond and B. H.
Robinson, Langmuir 15(6) (1999) 1993–2002.
293. L. Sun, Y. Zhang, J. Zhang, C. Yan, C. Liao and
Y. Lu, Solid State Commun. 124 (2002) 35–38.
294. M. H. Lee, S. G. Oh and S. C. Yi, J. Coll. Interface
Sci. 226(1) (2000) 65–70.
295. T. Hirai, T. Hirano and I. Komasawa, J. Coll. Interface Sci. 253(1) (2002) 62–69.
296. B. Delfort, L. Normand, P. Dascotte and L. Barre,
J. Coll. Interface Sci. 207(2) (1998) 218–227.
297. B. Delfort, A. Chive and L. Barre, J. Coll. Interface
Sci. 186(2) (1997) 300–306.
298. K. S. N. Reddy, L. M. Salvati, P. K. Dutta, P. B.
Abel, K. I. Suh and R. R. Ansari, J. Phys. Chem.
100(23) (1996) 9870–9880.
299. K. Kon-no, in Fine Particles Science and Technology, ed. E. Pelizzetti (1996), pp. 431–42.
300. J. Y. Liang, GuoLin, X. H. Bin, L. Jing, L. X.
Dong, W. Z. Hua, W. Z. Yu and J. Weber, J. Cryst.
Growth 252 (2003) 226–229.
301. G. Wakefield, H. A. Keron and P. J. Dobson et al.
J. Coll. Interface Sci. 215(1) (1999) 179–182.
302. K. C. Song and J. H. Kim, J. Coll. Interface Sci.
212(1) (1999) 193–196.
303. M. S. Lee, G. D. Lee and S. S. Hong, J. Indust.
Eng. Chem. 9(4) (2003) 412–418.
304. Z. Wu, J. Zhang, R. E. Benfield, Y. Ding,
D. Garndjean, Z. Zhang and X. Ju, J. Phys. Chem.
B106 (2002) 4569–4577.
305. Y. X. Pang and X. Bao, J. Mater. Chem. 12 (2002)
3699–3704.
306. P. Tartaj and J. Tartaj, Chem. Mater. 14 (2002)
536–541.
307. Z. You, I. Balint and K. Aika, Chem. Lett. 11 (2002)
1090–1091.
308. I. Balint, Z. You and K. Aika, Phys. Chem. Chem.
Phys. 4 (2002) 2501–2503.
309. M. Hayashi, H. Uemura, K. Shimanoe, N. Miura
and N. Yamazoe, Electrochem. Solid State Lett.
1(6) (1998) 268–270.
277
310. V. Uskoković, D. Makovec and M. Drofenik, Mater.
Sci. Forum 494 (2005) 155–160.
311. V. Uskoković and M. Drofenik, J. Mater. Sci., submitted (2005).
312. I. Ban, M. Drofenik and D. Makovec, Mater. Sci.
Forum 494 (2005) 161–166.
313. S. A. Morrison, C. L. Cahill, E. E. Carpenter,
S. Calvin and V. G. Harris, J. Appl. Phys. 93(10)
(2003) 7489–7491.
314. M. Drofenik, D. Lisjak and D. Makovec, Mater. Sci.
Forum 494 (2005) 129–136.
315. M. Han, R. Vestal and Z. J. Zhang, J. Phys. Chem.
B108 (2004) 583–587.
316. C. R. Vestal and Z. J. Zhang, Chem. Mater. 14(9)
(2002) 3817–3822.
317. M. Han, R. Vestal and Z. J. Zhang, J. Phys. Chem.
B108 (2004) 583–587.
318. S. Bandow, K. Kimura, K. Kon-no and A. Kitahara,
Jpn. J. Appl. Phys. 26(5) (1987) 713–717.
319. C. Y. Wang, W. Q. Jiqng, Y. Zhou, Y. N. Wang and
Z. Y. Chen, Mater. Res. Bull. 35(1) (2000) 53–58.
320. F. Meyer, A. Dierstein, C. Beck, W. Hartl,
R. Hempelmann, S. Mathur and M. Veith, Nanostruct. Mater. 12 (1999) 71–74.
321. P. Kluson, P. Kacer, T. Cajthaml and M. Kalaji,
J. Mater. Chem. 11(2) (2001) 644–651.
322. J. Huang, Y. Xie, B. Li, Y. Liu, J. Lu and Y. Qian,
J. Coll. Interface Sci. 236 (2001) 382–384.
323. K. Mallick, Z. L. Wang and T. Pal, J. Photochem.
Photobiol. A140(1) (2001) 75–80.
324. G. C. Koumoulidis, A. P. Katsouilidis, A. K.
Ladavos, P. J. Pomonis, C. C. Trapalis, A. T.
Sdoukos and T. C. Vaimakis, J. Coll. Interface Sci.
259(2) (2003) 254–260.
325. M. Bonini, U. Bardi, D. Berti, C. Neto and
P. Baglioni, J. Phys. Chem. B106 (2002)
6178–6183.
326. H. S. Wu and E. W. Kaler, Microemulsion polymerization systems and coated materials made therefrom, US Patent 5,539,047 (1996).
327. H. Althues and H. Kaskel, Langmuir 18 (2002)
7428–7435.
328. H. Ago, S. Ohshima, K. Uchida, T. Komatsu and
M. Yumura, Physics B323 (2002) 306–307.
329. H. Ago, S. Ohshima, K. Tsukuagoshi, M. Tsuji
and M. Yumura, Curr. Appl. Phys. 5(2) (2005)
128–132.
330. J. L. Zhang, Z. M. Liu, B. X. Han, T. Jiang, W. Z.
Wu, J. Chen, Z. H. Li and D. X. Liu, J. Phys. Chem.
B108(7) (2004) 2200–2204.

synthesis of materials within reverse micelles

Transcription

Similar documents

5,10,15,20-tetrakis(meso-hydroxyphenyl)porphyrin Loaded

PDF - Integraflow

Micelle Formation

Blaze orange SAR field shirt

Gas Well Deliquification

Die Cutting - Gaska Tape

Click here - Parishioners Federal Credit Union

Reverse Oil Painting

Intra-Limbal - Diversified Ophthalmics, Inc.

high performance innovation in firefighting

PDF - Innovare Academic Sciences