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Current Opinion in Colloid & Interface Science 9 (2004) 264 – 278
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Microemulsion dynamics and reactions in microemulsions
y
M.A. López-Quintela a,*, C. Tojo b, M.C. Blanco a, L. Garcı́a Rio a, J.R. Leis a
a
Department of Physical Chemistry, University of Santiago de Compostela, E-15782 Santiago de Compostela, Spain
b
Faculty of Sciences, University of Vigo, E-36200 Vigo, Spain
Available online 11 August 2004
Abstract
Microemulsions are very versatile reaction media which nowadays find many applications, ranging from nanoparticle templating to
preparative organic chemistry. On one hand, for the synthesis of nanomaterials microemulsions represent a well-established technique that
can be used to fine control the particle size of many inorganic and organic materials, as well as latexes. On the other hand, the
thermodynamical stable and microheterogeneous nature of microemulsions, used as reaction media, induces drastic changes in the reagent
concentrations, and this can be specifically used for tuning the reaction rates. In particular, amphiphilic organic molecules can accumulate and
orient at the oil – water interface inducing regiospecificity in organic reactions. In this review, we will show the recent tendencies of the use of
microemulsions for the preparation of nanoparticles, and also as particularly interesting organic reaction media.
D 2004 Elsevier Ltd. All rights reserved.
Keywords: Microemulsions; Inverse micelles; Nanomaterials; Latexes; Core – shell nanoparticles; Microheterogeneous reaction media; Pseudo-phase model;
Water – oil interface properties; Catalytic properties
1. Introduction
Microemulsions have been used as chemical reactors
because of their special interfacial properties allowing an
intimate contact, at nanoscale level, of hydrophilic and
hydrophobic domains. The dynamic character of these
nano-reactors is one of the most important features, which
has to be taken into account for a comprehensive understanding of chemical reactions carried out in these media.
However, this important fact is usually overlooked. Scheme
1 shows a picture that is useful to understand the role that
the microemulsion dynamics can have on the chemical
reaction.
By microemulsion dynamics, we mean the fact that the
domains are not static ones, but are in continuous movement and collision with each other. In each collision,
material interchange can takes place. The whole process
of motion – collision exchange can be characterised through
a parameter sex, which is characteristic of each kind of
y
Madrid, March 11th, 2004: En memoria de todas las vı́ctimas
inocentes de éste y todos los demás actos de violencia (In memory of all the
innocent victims of this and all other violent actions).
* Corresponding author. Tel.: +34-981-595998; fax: +34-981-595012.
E-mail address: [email protected] (M.A. López-Quintela).
1359-0294/$ - see front matter D 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.cocis.2004.05.029
microemulsion. Understanding how the chemical reaction
proceeds depends on the ratio between the characteristic
chemical reaction time, sr, and sex. When the reaction is
very slow in comparison with the microemulsion dynamics,
sr/sexH1, the reaction ‘‘sees’’ the microemulsion as a static
object, and a pseudo-phase model can be applied. In
contrast, for chemical reactions with sr/sex< or c1, the
dynamics of the microemulsion has to be taken into account
to explain the chemical reaction.
2. Microemulsion dynamics
If one assumes that microemulsion domains are formed
by spherical droplets, the characteristic droplet’s collision
time in microemulsions can be easily calculated assuming
that the droplets diffuse through a continuous medium
with viscosity, g. Then, the collision rate constant is given
by kD=(8/3)kBT/gc109 M1 s1 for a typical low viscosity solvent. Because usually the droplet’s volume
fraction Uc0.1, i.e., [droplet]c103 M for a typical
droplet’s size c10 nm, the encounter rate constant kenc106
s1. Thus, the average collision time (encounter time) is
senc1 As.
It is well known that not all droplets’ collisions are
effective for material exchange. This can be taken into
M.A. López-Quintela et al. / Current Opinion in Colloid & Interface Science 9 (2004) 264–278
account introducing an encounter rate factor, c, which for a
particular material (reactant), depends on the film flexibility [1], i.e., kex=cken. For rigid films like AOT microemulsions, cc10 3 —that is, only 1 in each 1000
collisions is effective for the reactants’ exchange [2].
However, for flexible films, this value can reach up to
cc101 [1]. Then the microemulsion exchange characteristic time sex is in the range c10 As<sex<1 ms.
According to Scheme 1, reactions with a half-life
reaction time, sr, much larger than 0.01 –1 ms are assumed to occur in a ‘‘static’’ pseudo-phase system, in
which the exchange of material does not play any role in
the kinetics (region III, Scheme 1). However, for reactions
with a half-life reaction time (sr) far below 0.01 –1 ms,
the droplet’s exchange is the control factor of the kinetics
(I). This occurs only for reactions that are near diffusioncontrolled. The interplay of droplet’s exchange and chemical reaction has to be taken into account for reactions
with sr near sex (II). Therefore, to understand chemical
reactions in microemulsions, there is a need to know
which model is most suitable for a particular reaction.
However, there are not many available data for this
classification, and any study in this direction will be of
valuable importance. Because film flexibility is mainly
determined by the surfactant, it is convenient to classify
reactions located in regions I and II based on the
surfactants being used. This will restrict the sex values,
corresponding to each surfactant, to a small range that, in
turn, depends on other variables (such as cosurfactant, oil,
droplet’s size) and the specific reactant to be exchanged.
A more direct comparison of the different experimental
results can then be achieved.
To simplify this revision, we will first refer to reactions
leading to the formation of nanoparticles in Section 3.
Because of the possible influence of the microemulsion
dynamics, we will make a classification of reactions
according to the surfactants being used. In Section 4, we
concentrate on other types of reactions, which can be
explained by a pseudo-phase model, and therefore will
be classified according to the type of the reaction.
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3. Formation of particles in microemulsions [3]
3.1. Ionic surfactants
3.1.1. AOT microemulsions
Sodium bis(2-ethylhexyl) sulfosuccinate is an anionic
surfactant commonly known as AOT. When a small
amount of AOT is dissolved in organic solvents, thermodynamically stable reverse micelles are formed. These
micelles consist of a hydrophilic core compartmentalised
by the hydrophilic head group of the AOT, and with the
hydrophobic alkyl tails extending into the nonpolar continuous phase solvent.
3.1.1.1. Growth versus stabilising mechanisms. The size
of the micelle core is described by the molar ratio of water
to surfactant molecules in solution, W=[water]/[surtactant].
For materials such as CdS, ZnS and AgCl, it has been
observed that the particle size is controlled by the size of
the micelle, i.e., reverse micelles act as a template.
However, time-resolved studies on the formation of Cu
nanoparticles by Cason et al. [4..] have shown that the
final particle size in AOT/alkane micelles is independent
of W, although the particle growth rate is a function of W
and the bulk solvent type. This leads us to propose an
alternate view: that the sizes are largely controlled by
solvent stabilisation of the particles, and the surfactant
acting as a stabilising ligand [5]. This argument has also
been proposed by Kitchens et al. [6], in their study of the
solvent effects on the growth rate and final particle size of
copper metallic nanoparticles. Their experimental results
support the assumption that the surfactant has a double
influence: on the particle growth and the stabilisation
process. Initially, the surfactant provides an initiation site,
the micelle core, for the reduction of the metal followed by
particle growth through intermicellar exchange. At the
latter end of the particle growth, the surfactant acts as a
stabilising ligand with a weak interaction between the
metal particle and the surfactant head group. The authors
concluded that growth rate and particle size are inversely
Scheme 1. Reaction time (sr) versus microemulsion dynamics (sex).
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related, where a decrease in growth rate corresponds to a
larger particle size at a specific W value. Increased
interaction between the solvent and the surfactant tails
results in a more stable micelle system and enhanced
ability to stabilise larger particles while reducing the
intermicellar exchange. In contrast, the micelle core templating effect, observed in CdS, ZnS and AgCl, is
accounted for because these nanoparticles are not pure
metals and it is possible that a weaker interaction between
the particle surface and the ionic headgroup of the surfactant exists, thus inhibiting the steric stabilization of particles larger than the micelle core. In each of these cases,
the obtained particle sizes rarely exceeded the size of the
micelle core. The surfactant-stabilized nanoparticle droplets
do not only act as microreactors, but also inhibit the
aggregation of particles, because the surfactants can be
adsorbed onto the particle surface when the particle size
approaches the water pool size (for capping, see, e.g. Ref.
[3]). Manna et al. [7] reported the possibility that the
strong interactions between Au and SH groups of AOT
help to form a ‘‘three-dimensional self-assembled monolayer’’ onto the particle surface, which controls the growth
and stabilisation of the nanocrystals.
Different stabilising agents have been used to inhibit the
nanoparticle growth in AOT microemulsions. As an example,
trioctyl phosphine oxide (TOPO) has been used to stabilize
iron nanoparticles [8], and dodecanthiol [9] and bis(2-ethylhexyl)amine (BEA) [10] were likewise used to stabilize
cadmium sulfide nanoparticles. The stabilization mechanism
consists in the rapid formation of a chemically bonded layer
of oriented molecules on the nanoparticle surface. Formation
of a thin layer of SiO2 on different nanoclusters offers a new
challenge in synthesis. By changing the thickness of the shell
and the particle radius, the overlap of the wave functions and
band gap can be changed, which represents a major interest in
the semiconductor field. Therefore, silica-coated nanoparticles have attracted the attention of many researchers in
recent years. As an example, the size of Ag particles and the
thickness of the coating can be controlled by manipulating the
relative rates of the hydrolysis and condensation reactions of
tetraethoxysilane [11] within the microemulsion. Silica nanoparticles have also been used to cover a ZnFe2O4 magnetic
core [12..] and rhodium nanoparticles [13].
3.1.1.2. Cosurfactant. Marchand et al. [14.] have studied
the synthesis of MoSx particles in AOT/water/n-heptane
microemulsions. The addition of NP-5, as a nonionic
cosurfactant, at a small concentration compared to that of
AOT, leads to a substantial decrease of the mean micellar
size, resulting in a significant decrease of the nanoparticle
size. This can be attributed to a higher fluidity of the
interfacial film and a higher mean curvature of the droplets.
Therefore, the cosurfactant increases the fluidity of the
interface and thus the kinetics of the intermicellar exchange,
which in turns ensures a more homogeneous repartition of
reactants among droplets. NP-5 has a cycloalkane hydro-
phobic chain and introduces a discontinuity in the interfacial
film of the water pool, thus promoting the intermicellar
exchange and leading to smaller though more numerous
particles. Indeed, a higher intermicellar exchange implies a
higher consumption of the reactants during the nucleation
stage, which means that less are left for the growth stage.
Similar results were found by Bagwe and Khilar [15], who
used NP-5 as a cosurfactant to synthesise Ag particles in
AOT/n-heptane/water.
3.1.1.3. Supercritical CO2 microemulsions. Increasing attention has been paid to the synthesis of nanoparticles in
water-in-supercritical CO2 microemulsions [16 –18]. One
problem of using conventional water-in-oil microemulsions
for nanoparticle synthesis is the separation and removal of
solvent from products. Supercritical carbon dioxide used as
a solvent offers several advantages such as fast reaction
speed, rapid separation and easy removal of solvent from
nanoparticles. This method has been used to produce Ag
and Cu [16] and CdS and ZnS [18] nanoparticles. Hydrogenation of olefins catalysed by Pd nanoparticles in a
water-in-CO2 microemulsion has also been reported by
Ohde et al. [17].
The selectivity coefficients for the counterion exchange
in the water – AOT – heptane microemulsion interface were
determined by using a pseudo-phase ion exchange formalism [19]. Theoretical results have been successfully compared to quenching of the RuL34 luminescence emission
measurements.
Finally, the catalytic activity of metallic particles synthesised in AOT microemulsions has been a field of high
activity [8,17,20 –22]—see Section 4.3.
3.1.1.4. Preparation of bimetallic particles. Chen et al.
[23. – 25] have reported the synthesis of Au – Ag [23.], Au –
Pd [24] and Pd – Pt [25] bimetallic nanoparticles in water/
AOT/isooctane microemulsions. The following mechanism
is proposed: for Au – Ag [23.] and Au – Pd [24], the reduction rate is so large that almost all of the ions are reduced
before the formation of nuclei. Then, the atoms start to
aggregate to form the nuclei. Since the nucleation rate of Au
is much faster than that of Ag [23.] or Pd [24], the nuclei of
the bimetallic system should be mainly formed from Au
atoms, and the composition of the nuclei might have a
higher Au concentration than that of the feeding solution,
i.e., Au might act as the seed for the formation of the
bimetallic particle. All nuclei might be formed almost at the
same time. After that, Ag or Pd atoms codeposite onto the
nuclei and grow to their final sizes. The faster growth rate of
Au than Ag or Pd leads to the enrichment of Ag or Pd in the
outer layer of the bimetallic nanoparticle. For the case of
Pd – Pt [25], the formation rate of Pd nanoparticles is faster
than Pt nanoparticles, but the difference is less than Au –Ag
and Au – Pd particles. Consequently, the nucleus might
contain both Pd and Pt codeposited at a similar deposition
rate, so that a homogeneous alloy structure is obtained. It is
M.A. López-Quintela et al. / Current Opinion in Colloid & Interface Science 9 (2004) 264–278
interesting to note that the same Pd – Pt synthesis using a
different reduction agent, i.e., different chemical reaction,
leads to a different final nanoparticle size.
3.1.2. CTAB microemulsions
Cetyl trimethylammonium bromide (C16H33 – (CH3)3 –
N+Br, CTAB) provides a very flexible film, which gives
rise to a high exchange dynamic of the micelles. In recent
years, an increasing number of works used CTAB as
surfactant because of CTAB reverse micellar systems,
which show remarkably more solubilization capacity of
high concentration aqueous salt solution than AOT-based
systems.
Using water-CTAB-n-octane microemulsions, with 1butanol as cosurfactant, Porta et al. [26] obtained smaller
Au nanoparticles as the reactant concentration increases.
They discussed the possibility that alcohol behaves as
capping agent at high concentrations [27]. As expected,
higher pentanol contents favours exchange dynamics and
leads to larger particles, even larger that the original droplet
diameter. Husein et al. [28] obtained similar results for the
synthesis of silver chloride in water – dioctyldimethylammonium chloride –n-decanol-isooctane. They explained these
results assuming that, at high alcohol content, the particle
size increases due to decreasing of the interactions between
the nanoparticles and the stabilising surfactant layer. In
addition, these authors also found that the particle size
decreases as the concentration of AgNO3 increases. This
result is explained, assuming that smaller particles are
formed when there is a larger number of nuclei. At a fixed
surfactant concentration and a fixed W, increasing the
amount of reactant reduces the Ag+ ion occupancy number.
More droplets carrying a AgCl concentration higher than the
critical nucleation concentration are formed, and the rate of
nucleation becomes less dependent on the intermicellar
exchange of solubilizate. The larger number of nuclei
provides more seeds for particle growth and results in
particles with smaller diameter.
Chen and Wu [29.] studied the influence of both reactants, metal salt and reducing agent, on the final Ni nanoparticle size in a water/CTAB/n-hexanol microemulsion. At
a constant NiCl concentration, the size of Ni nanoparticles
decreases with the increase of hydrazine concentration and
then approaches to a constant value. This fact can be
explained from the influence of reduction rate on the
nucleation. Collisions between several atoms must occur
for a nucleation—with this probability at a much lower rate
than the probability for collisions between one atom and a
nucleus already formed. Once the nuclei are formed, the
growth would be superior to nucleation. In addition, if all of
the nuclei were formed almost at the same time and grew at
the same rate, nanoparticles would be monodisperse. Thus,
the number of nuclei formed at the very beginning determines the number and size distribution of the obtained
particles. At low hydrazine concentration, the reduction rate
is slow, and only few nuclei are formed at the initial period
267
of the reduction. Atoms formed at a latter period will collide
with the nuclei already formed, and larger particles are
obtained. As hydrazine concentration increases, the enhanced reduction rate favours the generation of considerably
higher number of nuclei, and leads to smaller Ni nanoparticles. When the concentration ratio of hydrazine to NiCl
is large enough, the reduction is much faster than the
nucleation. The nucleation rate is not further raised and
the number of nuclei is held constant with the increase of
hydrazine concentration. Then, the size of the nanoparticles
remains constant. The effect of NiCl concentration on final
size was also investigated [29.]. In contrast to previous
results [26,28], Ni size increases as the NiCl concentration is
increased. However, experimental conditions are hardly
comparable because a hydrazine excess is used.
ZrO2 – Y2O3 nanoparticles has been obtained in a CTAB/
hexanol/water microemulsion by Fang and Yang [30]. In their
report, these authors noted a peculiar behaviour: nanoparticle
size distribution is narrowed down in two cases, increasing
water content at fixed surfactant concentration or decreasing
the surfactant content at fixed W. Both cases led to larger
droplets. The greater number of metallic ions existing in a
large droplet would cause more nuclei to form in a reverse
micelle. When a water pool, containing free metallic atoms,
fuses with such a reverse micelle, these nuclei will grow up at
the same speed. As the droplet sizes increases (because of the
increase of the water content), the surfactant film becomes
thinner, thus accelerating the exchange process. The high
exchange rate could lead to a uniform nucleation and growth
process. For water pools with smaller sizes, nucleation only
occurs in a little number of micelles at the very beginning of
the precipitation reaction, because most of them do not
contain enough metal ions to form a critical nucleus. As a
result, due to diffusion, new nuclei will form as a function of
time. Particles already existing and newly emerging will grow
at a different rate, causing a broad size distribution.
This kind of microemulsions have also been used to
produce other materials: spinel ZnAl2O4 [31], perovskite
LaMnO3 [31], bioceramic hydroxyapatite [32], cerium oxide
[33], and coated materials like, SiO2 coated Pt, Pd and Pt/
Ag [34]. Finally, the catalytic activity of nanoparticles
synthesised in CTAB microemulsions was also studied [34].
3.1.3. PFPE microemulsions
The anionic surfactant perfluoropolyethercarboxylic acid
was converted to its ammonium salt by reaction with excess
ammonium hydroxide. PFPE – NH4 has an average formula
of [CF3O(CF2CF(CF3)O)f3CF2COO][NH4]+.
This surfactant was recently used in water-in-carbon
dioxide microemulsions [35 –38] to produce Ag nanoparticles [35] and Ti [36].
3.1.4. Sodium lauryl sulfate
Sodium lauryl sulfate (SLS) was used by Wang et al. [39]
to synthesise PbS nanoparticles by a sonochemical method.
A probable mechanism for the formation of nanocrystalline
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M.A. López-Quintela et al. / Current Opinion in Colloid & Interface Science 9 (2004) 264–278
PbS particles in a toluene-in-water microemulsion with the
inducement of ultrasound irradiation is proposed.
Finally, the catalytic activity of metallic particles synthesised in Triton X microemulsions has also been reported
[44,46,47].
3.2. Nonionic surfactants
Nonionic surfactants are very common in the literature,
and are mainly used to prevent possible counterion
interactions.
3.2.1. Triton X microemulsions
Triton X-100 [Polyoxyethylene(9)4-(1,1,3,3-tetramethylbutyl)phenyl ether] has been used to prepare different kind
of nanoparticles: CeO2[40], Ce1x – ZrxO2 [40], Ce – Tb
mixed oxides [41], Al2O3 [42], Y2O3:Eu3+ [43], TiO2
[44], silver halides [45]. Zhang and Chan [46] studied
the formation of Pt– Ru bimetallic nanoparticles using a
water-in-oil reverse microemulsion of water/Triton X-100/
propanol/cyclohexane. The composition in the Pt – Ru
nanoalloy is found to be the same as that in the original
precursor solution. They noted that the bimetallic particle
size is relationally largest at higher precursor concentration, but it showed a plateau at very low and very high
metal salt concentrations. This size dependence on concentration of precursor was similar to that reported by
Chen and Wu [29.] for Ni nanoparticles prepared with a
cationic surfactant. They concluded that the size of nanoparticles appears to be limited by nucleation at low
concentration and limited by collisions with hydrazine
droplets at high precursor concentrations. It is interesting
to note that they proposed the existence of highly improbable multiple collisions. Zhang and Chan [47] also studied
the synthesis of Pt – Co nanoparticles using the same
microemulsion. They noted that larger particles would be
formed if contact between two Pt– Co containing microemulsions occurred before their individual contacts with
the microemulsion carrying the reducing agent.
Silica-coated iron oxide nanoparticles were studied by
Santra et al. [48], using different precipitating agents
(NH4OH or NaOH) and surfactants (Triton X, Brij 97 and
Igepal). They found that the ultrasmall synthesized particles
(<5 nm) aggregate in different morphologies. Results are
explained, assuming that the extent of surfactant adsorption
onto the particle surface varies depending on the experimental conditions. The more ordered structures observed
with Brij 97 are attributed to its longer hydrophobic chain
(compared to Triton X and Igepal), promoting a more
ordered particle aggregation, due to stronger hydrophobic – hydrophobic interactions between the oleyl groups
attached to adjacent nanoparticles. The authors do not
exclude the influence of the ultrasounds used during the
reaction on the adsorption process. Different results were
obtained by Tartaj and Serna [49]. These authors found that
the nature of the surfactant (Igepal CO-720 or Triton X100) did not significantly affect the microstructure of the
prepared iron oxide nanoparticles, using cyclohexane as oil
and n-hexanol as cosurfactant.
3.2.2. NP-5, NP-9 and NP-12 microemulsions
Poly(oxyethylene)5 nonylphenol ether (NP-5), poly(oxyethylene)9 nonylphenol ether (NP-9) and poly(oxyethylene)12 nonylphenol ether (NP-12) have been recently
used to prepare different particles, such as hydroxyapatite
[50], metallic bismuth [51], and also applied in catalytic
activity studies [52,53].
Rh nanoparticles have been synthesised in NP-5/cyclohexane microemulsion [54..]. At a fixed RhCl3 and
surfactant concentration, and fixed W value, ultrafine Rh
particles were obtained using different reducing agents
(H2, NaBH4 and N2H4). Different final particle sizes were
obtained, depending on the reactants’ nature.
Pt –Ru bimetallic nanoparticles have been prepared by
using these nonionic surfactants [52]. Silica-coated Rh
nanoparticles was also reported [54..].
3.2.3. Brij microemulsions
Brij30, a nonionic surfactant with a short hydrocarbon
chain (polyoxyethylene(4) lauryl ether: H 3 (CH 2 ) 11
(CH2CH2O)4OH), was used to study the immobilization
of ZnS nanoparticles synthesised in microemulsions to
silica [54..], and to prepare silica coated iron oxide
[48].
In a very interesting paper, Grasset et al. [12..]
compared various surfactants for the synthesis of silicacoated zinc ferrite nanoparticles. The coating is achieved
by using a ferrofluid-in-oil microemulsion to which tetraethoxysilane (TEOS) is added. They show that the most
uniform and spherical coatings are achieved by using
Brij30 or a mixture of AOT and Brij30 (50/50 wt.%) as
the surfactant phase, being the mixture of surfactants the
optimal one. In this way, relatively monodisperse particles
in the range 40– 80 nm with a ferrite core of 4 –6 nm are
obtained. Results also show that poorer coatings are
obtained using pure AOT or SDS (with added propanol)
surfactants. Although the authors attribute this different
behaviour of surfactants to differences in the flexibility
and interfacial tension of the surfactant film, a definite
explanation is still missing.
3.2.4. Igepal microemulsions
Pentaoxyethylene-glycol-nonyl-phenyl ether, commonly
known by Igepal-CO520, is a nonionic surfactant used by
Bae et al. [55] to study the influence of [water]/[TEOS]
molar ratio on the final size of Pd and Pd/SiO2. They
concluded that the particle size and the thickness of the
coating can be controlled by manipulating the relative
rates of the hydrolysis and condensation reaction of
TEOS.
Interesting contribution on the influence of W on the
nanoparticle size was given by Nanni and Dei [56].
M.A. López-Quintela et al. / Current Opinion in Colloid & Interface Science 9 (2004) 264–278
Special attention has been paid to the study of magnetic
properties of different nanoparticles synthesised in this kind
of microemulsions: silica-coated CoFe2O4 and MnFe2O4
spinel ferrite nanoparticles [57], silica-coated iron oxide
particles [49] and iron oxide-doped alumina nanoparticles
[58].
3.2.5. Other surfactants
3.2.5.1. PEG. Polyethylene glycol (PEG) has been used to
obtain Cs-doped alumina nanoparticles [59] and bariumstabilised alumina nanoparticles [60].
3.2.5.2. Span – Tween 80. Span –Tween 80, a commercial
mixture of sorbitol monooleate and polysorbate 80, was
used to prepare TiO2 nanoparticles in microemulsions [61].
The main factor affecting nanoparticle sizes and the physical
properties of the nanoparticles was the W ratio.
3.2.5.3. Polyoxyethylene 4 lauryl ether. Polyoxyethylene
4 lauryl ether was used to prepare Pd, Pt and Pt/Pd nanoparticles, which showed a high catalytic activity [62].
3.2.5.4. Polyoxyethylene 15 cetyl ether. Polyoxyethylene
15 cetyl ether was used by Tago et al. [63] as a surfactant to
obtain SiO2-coated CeO2 nanoparticles. They observed that
the type of particle-forming agents affects the efficiency of
silica coating on the CeO2 nanoparticles.
3.2.5.5. Epikuron 170. Epikuron 170 is a lecithin (min.
67% phosphatidylcholine) used as a surfactant to prepare
nimesulide, a molecule of pharmaceutical interest [64]. The
size seems to be independent of either the W ratio or the
concentration of the active compound. Debuigne et al. [64]
proposed that the constancy of the size suggests that the size
is controlled by thermodynamic stabilization of the nanoparticles with the surfactant molecules.
3.2.5.6. S-1670. S-1670 is a nonionic surfactant of food
grade sucrose fatty acid ester which was used to prepare
CdS and PbS nanoparticles by Khiew et al. [65]. This
surfactant is a commercial food grade additive, and
provides a suitable microenvironment for the preparation
of materials with narrow size distribution and high
monodispersity.
3.3. Polymerization reactions
The use of microemulsions to prepare latexes with
particle sizes below approximately 100 nm is a very important topic with many potential applications in drug delivery,
microencapsulation, etc. For this reason, a considerable
amount of activity has been conducted in this area (see,
e.g., Refs. [66,67]).
Although there is currently no general scheme for the
kinetics of polymerization in microemulsions, the Morgan–
269
Kaler model—and its extension [68]—provide a sufficiently
reliable general description. This model has been used to
predict the evolution of nanoparticle size of hexyl methacrylate in a microemulsion of water, n-hexyl methacrylate,
dodecyltrimethylammonium bromide (DTAB) and didodecyldimethylammonium bromide (DDAB) [69..].
As examples of recent works performed in this area, we
note that:
Tauer et al. [70] have investigated the polymerization
kinetics by calorimetry and DLS. The results have been
explained by the classical nucleation theory.
Cosurfactants effects on the microemulsion polymerization of styrene have been studied by Puig et al. [71].
Although the kinetics of polymerization slows down in
the presence of alcohol, no particle trend was noticed on
the particle size.
Xu et al. [72] have prepared polystyrene microlatex using
a polymerizable surfactant and CTAB. In this way,
monodisperse latex particles with diameters ranging from
50 to 80 nm could be obtained. The dependence of the
particle concentration, the latex particle size and the
copolymer molar mass on the polymerization time is
discussed in conjunction with the effect of the monomer
concentration.
Larpent et al. [73] showed that polymerisation of a
reactive monomer in microemulsions followed by postfunctionalisation allows the synthesis of ligand-functionalized nanoparticles in the 15 – 25 nm diameter range.
Nanosize polymers of about 60 nm of PMMA have been
obtained by Zhang et al. [74] at relatively low surfactant
concentrations and with a relatively high polymer content
(c30 wt.%). Jiang et al. [75] have also prepared PMMA
in the 20– 40 nm range.
4. Microemulsions as reaction media
In recent years, microemulsions have been evaluated as
reaction media for a variety of chemical reactions. In
preparative organic chemistry, microemulsions have been
used to overcome reactant solubility problems due to the
ability of microemulsions to solubilize both polar and nonpolar substances and to compartmentalize and concentrate
reactants. Recent reviews from Holmber et al. [76,77] show
that microemulsions can be regarded as an alternative to
biphasic systems with added phase transfer agent. By
combining microemulsion and phase transfer approaches,
very high reactions rates have been obtained. Organic
molecules with polar and nonpolar regions will accumulate
at the oil – water interface of microemulsions. They will
orient at the interface in such a way that the polar part of the
molecule extends into the water domain and the non-polar
part extends into the hydrocarbon domain. This tendency for
orientation at the interface can exploited to induce regiospecificity in an organic reaction. In fact, bromination of
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phenols and anisols gives a higher para/ortho ratio than
conventional bromination. The strong preference for para
substitution can be of preparative interest. Photocyclodaddition of 9-substituted anthracenes is also controlled by
using water-in-oil microemulsions as reaction media [78].
Photoirradiation of most of the substituted anthracenes
incorporated in the microemulsions almost exclusively
yielded the head-to-head photocyclomers.
4.1. Chemical reactivity in microemulsions
Experimental evidences and models about the effect of
such systems on chemical reactivity should parallel the use
of microemulsions as reaction media. In particular, the
microheterogeneous nature of microemulsions induces severe changes of reagent concentrations. Their local concentrations can increase or decrease in relation to their bulk
concentrations, thus allowing the tuning of reaction rates.
Also, the properties of local reaction media are quite
different from those of the bulk solutions as a consequence
of the intense local electric fields. These fields affect all the
relevant parameters that modulate the reaction rates. In
order to carry out a quantitative interpretation of the
influence of the microemulsion on the reactivity, it is
necessary to know the concentrations of the reagents in
the various pseudo-phases/microenvironments of the system
and the corresponding rate constants. A kinetic model based
on the formalism of the pseudo-phase was devised to
explain the reactivity in water in oil microemulsions. In
the pseudo-phase model for homogeneous microemulsions,
in which the internal structure may be oil-in-water droplets, bicontinuous, or water-in-oil droplets, the whole
solution is divided into oil, surfactant film, and water
regions (pseudo-phases) with the surfactant lining the
boundary between the oil and water regions. Each region
is treated as a separate phase or pseudo-phase, and the
partitioning of components between the regions depends
on their free energies of transfer between the pseudophases. The rate of transfer or reactants is assumed to be
much faster than the observed rate of the reaction (region
III, Scheme 1). In other words, distribution of the reagents
is always described by equilibrium constant throughout the
course of the reaction.
4.1.1. Solvolytic reactions
Solvolysis reaction rates are usually written as unimolecular rate constants and they are the simplest example to
be studied in microemulsions. Solvolysis of substituted
benzoyl chlorides was studied in AOT/isooctane/water
microemulsions [79..]. In order to apply the pseudo-phase
model, it was assumed that benzoyl chlorides are only
present at the interface and the oil pseudo-phase. This is
quite reasonable, when one takes into account their low
solubility in water. This distribution (Scheme 2) yields to
the conclusion that the only pseudo-phase, where benzoyl
chlorides and water are present, is the interface of the
microemulsion and hence the reaction should take place at
the interface.
Kinetic results allow several conclusions: (i) Reaction
rate changes with W as a consequence of the changes of
Scheme 2.
M.A. López-Quintela et al. / Current Opinion in Colloid & Interface Science 9 (2004) 264–278
271
Scheme 3.
the physical properties of the interface. (ii) Rate constant
for high water content, Wi50, is much lower than in bulk
water due to the insufficient hydration of the interface. (iii)
Influence of W on the rate constant is determined by the
reaction mechanism. For benzoyl chlorides following a
dissociative mechanism (path a in Scheme 3 ), rate constant increases with W, whereas for associative mechanisms, rate constants increase when W decreases (path c in
Scheme 3).
When comparing rates of solvolysis of benzoyl fluoride,
chloride and bromide in AOT based microemulsions [80.],
the ratio kBr/kF decreases 40 times when W decreases from
50 to 2. Such decrease of the ratio kBr/kF agrees with a more
efficient associative solvolytic mechanism on decreasing the
water content of the microemulsion.
Recent studies on solvolysis of benzoyl chlorides and 4nitrophenylchloroformiate in CO2-induced microemulsions
[81.] of (EO)27(PO)61(EO)27 (P104; EO=ethylene oxide,
PO = propylene oxide)/p-xylene/CO2/H2O show that the
observed rate constant of both substrates increase significantly with W, and that W has a larger influence on the
hydrolysis of benzoyl chloride. The different influence of W
on the two reactions can be explained in terms of the
different reaction mechanisms. Bunton et al. studied the
decarboxylation reaction. . .(CTPABr) and AOT microemulsions in CCI4 analysing the relationship between rate
constants and water properties [82]. With CTPABr, decarboxylation is much faster than in water, and its addition
slows the reaction. The anionic substrate in the interior of
CTPABr microemulsions interacts with the CTPA+ headgroup, which assists charge delocalisation in the transition
state, and reactions at Wi0 are faster than those in water by
a factor of approximately 106. With AOT, decarboxylation
of 6NBIC has a rate similar to that in water and there is no
catalysis, with 0«!<W<10—indicating that decarboxylation
takes place in the water pool and does not involve the
anionic surfactant, which consistent with repulsive interactions between anionic 6NBIC and the AOT headgroups.
4.1.2. Neutral molecule-anion reactions
The microheterogeneous distribution of microemulsions
allows the compartmentalisation of reagents, which is
clearly shown in the case of reactions between a neutral
molecule (distributed along the different microenvironments according to its hydrophobicity) and an anionic
nucleophile. Depending on the reagent hydrophobicity
and the charge of the head groups, it is possible to observe
large catalytic or inhibitory effects. Recent results obtained
for basic hydrolysis of 4-nitrophenylacetate in AOT-based
microemulsions [83] show that reaction rate in microemulsions is considerably lower than that observed in bulk
water because OH is exclusively present in the water
droplet. 4-Nitrophenylacetate is quite hydrophobic and this
means that only a small fraction is present in the water
droplet and hence a remarkable inhibition of hydrolysis is
observed. Bimolecular rate constants show that the hydrolysis rate increases on decreasing the water content. This
behaviour has been attributed to desolvation of OH ions
when the water content of the microemulsion decreases,
which means an increase in OH reactivity.
Hao [84] studied the basic hydrolysis of aspirin and 2,4dinitrochlorobenzene in CTABr/butanol/octane/water and
sodium dodecylsulfonate/butanol/styrene/water microemulsions. Experimental results show that hydrolysis rate is
greatly affected by the microstructure and the structural
transitions of microemulsions. Hydrolysis rates are higher
in water-in-oil microemulsions and decrease with the addition of water. The rates increase in bicontinuous microemulsions and decrease in oil-in-water microemulsions. The
transition points of the hydrolysis rates occurred at the two
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microemulsion structural transitions points from w/o to
bicontinuous and from bicontinuous to o/w. Nevertheless,
these results should be considered with caution because
when water content is increased, it is possible that the
distribution of reagents is being also affected—which could
in turn modify the reaction rates.
4.1.3. Cation-neutral molecule reactions
The acid hydrolysis of phthalomohydroxamic acid has
been studied in AOT/isooctane/water microemulsions [85].
Reaction rate is higher in microemulsions than in bulk
water due to compartmentalisation of reagents. It was
observed that reaction rate slows down when increasing
AOT concentration at constant W. This behaviour is due to
the competition of H+ and Na+ for binding to the interface
of the microemulsion droplets. It was also observed that
reaction rate becomes faster when moving from W=4 to
W=12. This behaviour is considered as a consequence of
changes in the properties of interfacial water when changing the microemulsion composition. The acid hydrolysis of
some tailor-made Schiff bases having flexible spacers
between aldimine groups and alkoxy groups at ortho or
para position in the benzene ring has been investigated in
anionic (SDS) and cationic (CTAB) microemulsions [86].
The change in reactivity, due to change in the spacer
length and position of the alkoxy group in the Schiff
bases, has been explained based on the location sites of
the reaction centre at different polarity pockets of the
reaction media.
4.1.4. Neutral molecule-neutral molecule reactions
Pseudo-phase model was also used in studies of many
bimolecular reactions. In this case, distribution of both
reagents among the different pseudo-phases should be
taken into account, and reaction should take place in those
where both reagents are present. Local concentrations of
reagents in each pseudo-phase should be defined in order
to analyse kinetic results. Successful explanation was
achieved for nitroso group transfer reactions [87] to amines
of very different hydrophobicity. In all cases, the results
assume that the reaction occurs at the interface between the
water droplet and isooctane and that the dependence of the
observed rate constants on the reaction conditions is
largely governed by the relative affinities of the amines
for the different phases present in the medium. The
hydrophobicity of the amines determines its distribution
between the three pseudo-phases and whether the rate
constants increase, decrease, or remain essentially constant
as the droplet size increases. A comparative study of
nitroso group transfer from N-methyl-N-nitroso-p-toluenesulfonamide to secondary amines with different hydrophobic character has been carried out in aqueous micelles,
vesicles and AOT-based microemulsions [88]. By comparing the rate constants in the three colloidal systems, it can
be suggested that the interface of the microemulsion is less
hydrophobic than the vesicular one and more than that of
the micelles of LTABr. The comparison of the bimolecular
rate constants at the interface of the microemulsion with
those in the aqueous medium show three types of behaviours. These behaviours are well differentiated according to
the hydrophobicity of the amines, which reflects its localisation in different zones of the interface of the microemulsion. These zones of the interface will have different
polarities and will consequently give rise to different
effects on the rate of the reactions.
Nitroso group transfer reactions were also used to study
kinetic behaviour in two different types of four component
microemulsions: (i) mixed quaternary microemulsions
(AOT/SDS/isooctane/water) and (ii) quaternary microemulsions with cosurfactant (TTABr/hexanol/isooctane/water).
For mixed microemulsions (AOT/SDS/isooctane/water),
both surfactants will be located at the interface isooctane/
water [89.], and it is possible to assume that the total
surfactant concentration is the sum of both. The effect of
addition of SDS to AOT/isooctane/water microemulsions is
similar to the addition of more AOT. Nevertheless, when
using this type of microemulsions as reaction media, we
should consider the increment of the interfacial volume
related with inclusion of the second surfactant. In the case
of quaternary microemulsions with cosurfactant, it should
be considered that this cosurfactant would be distributed
between the interface and the oil phase. The inclusion of
part of the cosurfactant in the interface implies an increment
of its volume and the corresponding local dilution of
reagents. Moreover, the solvation ability of alcohol (cosurfactant) at the interface implies a lower presence of water
and the corresponding lower polarity of the interface [90..].
Addition of cosurfactant also significantly changes the
continuous pseudo-phase. Reagents not present in the isooctane in AOT/isooctane/water are now present in the oil
phase formed by isooctane and part of the cosurfactant
(hexanol).
For nucleophilic aromatic substitution reactions [91], a
mechanistic change was reported. The rate-limiting step is
now the formation of the intermediate. For W=10 and at
high amine concentrations, the base catalysed reaction in the
benzene pseudo-phase predominates over the interface reaction. Thus, the observed rate constant decreases with AOT
concentration because of the reactant distributions. However, the reaction rate is accelerated at least 3 orders of
magnitude in benzene/BHDC/water microemulsions with
respect to the pure solvent, suggesting that the reaction
occurs at the interface. The catalytic effects of cationic
microemulsions have been considered as a consequence of
the interaction between the zwitterionic intermediate and the
ammonium head of BHDC.
Another example of a catalytic effect derived from
compartmentalisation of the reagents is the hydrolysis of
acetylsalicylic acid in AOT/supercritical ethane/water microemulsions in the presence of imidazole catalyst [92]. An
increase of the rate constant by 55 times was observed in
AOT/supercritical ethane microemulsions compared to the
M.A. López-Quintela et al. / Current Opinion in Colloid & Interface Science 9 (2004) 264–278
reaction in aqueous buffer. The reaction had a strong
dependence on the water content of micelles, which decelerated to a large extent when the water content was
increased. The acceleration of the reaction in microemulsions compared to aqueous buffer and the water content
dependence have been confirmed to be due to compartmentalization of the water soluble reactant and catalyst in the
aqueous core of the microemulsion.
4.1.5. Modification of the pseudo-phase model
The pseudo-phase model have been modified to take into
account the specificity of the reaction medium in explaining
the kinetic behaviour of the reaction between [Ru
(NH3)5pz]2+and S2O82in AOT/water/oil microemulsions
[93..]. New approaches incorporated to the model are: (i)
distribution of substrates between the interface and water
droplet is now described in terms of an equation similar to
Langmuir equation; (ii) the association of the substrate to
the interface is the result of two electrostatic contributions,
one dependent on the surface potential and the other which
is specific for each reaction.
4.2. Alteration of physical properties of the microemulsion
The water microdroplets formed in the central region of
the microemulsions provide a useful hydrophilic reaction
field. The reaction field effects of the water pool on various
photoreactions and thermal reactions and physical transitions have attracted considerable interest. Knowledge of the
water pool properties—local viscosity, local polarity, local
acidity, and their heterogeneous structure—is required to
fully understand the unique reaction systems in this hydrophilic nanospace. In this way, considerable interest has been
paid in recent years to studies focusing on the characterisation of composition and physical properties of the different pseudo-phases. Interfacial concentrations in aqueous
cationic micelles and microemulsions can be measured by
a phenyl cation trapping method developed by Chaudhuri
and co-workers [94]. Chemical trapping of bromide ions in
Scheme 4.
273
microemulsions prepared with CTAB in n-dodecane/CHCl3
and isooctane/n-hexanol has been obtained [95.] for 2,4,6trimethylbenzenediazonium (1-ArN2+) and 2,6-dimethyl-4hexadecylbenzenediazonium (16-ArN2+) tetrafluoroborates.
Quantitative analysis of the reaction products of 1-ArN2+and
16-ArN2+with water and bromide ion, the corresponding
phenol and bromo derivatives (Scheme 4), yielded the local
concentrations of Br-in the water pool and micellar interface
of CTAB microemulsions. Experimental data indicate that
the degree of counterion dissociation from CTAB microemulsions in n-dodecane/CHCl3 reaches a value of i0.2
above W=15. This value is very similar to that found in
aqueous micelles and may reflect an intrinsic property
related to specific, noncoulombic interactions between the
bromide ion and the tetralkylammonium headgroup.
The chemical trapping methodology has been applied to
study the interfacial composition of AOT/isooctane/water
microemulsions [96]. The obtained results demonstrate that
the interfacial regions of AOT-based microemulsions are
densely populated by the sulfosuccinate head-groups of
AOT and the interfacial concentrations of water are significantly lower than the molar concentration of bulk water.
Interfacial water concentration increases with W but the
maximum value obtained for W=44 is about 32 M, significantly lower than the value of 55.5 M for bulk water. This
difference is in keeping with studies of solvolysis reactions
at the interface of AOT-based microemulsiones, where the
reaction constant for high W values is significantly lower
than the corresponding value in bulk water.
Chemical trapping methodology have been applied by
Romsted and Zhang [97] to the determination of distribution
constants of tert-butylhydroquinone, TBHQ, in a fluid,
opaque, model food emulsion composed of the nonionic
emulsifier C12E6, octane and water. The distribution constants for partitioning of TBHQ between the oil and surfactant, and between the aqueous and surfactant film were
obtained by fitting the changes in the first order rate
constants with emulsifier volume fraction for the reaction
of 4-hexadecyl-2,6-dimethylbenzenediazomium ion with
TBHQ, by following the formation of the product hexadecyl-2,6-dimethylbenzene by HPLC.
The relationship between the pH of the aqueous solution
adjusted before solubilization and the water pool local pH
estimated with pH-sensitive probes has been studied in AOT/
heptane/water microemulsions. Hasegawa [98] has estimated
the local pH from the excitation spectra of the pH-sensitive
fluorescence probe, pyranine, solubilized into the water core
of the microemulsion as a function of the pH of the aqueous
solutions to be solubilized. This probe shows two distinct
excitation bands corresponding to the neutral and basic
forms. When NaOH/HCl was used for pH adjustment, even
if an acidic or alkaline aqueous solution was solubilized into
the microemulsion, the solubilized probes surprisingly
reported an almost constant intensity ratio over a wide pH
range. This result suggests that the water pools of AOT-based
microemulsions have buffer-like action. By using a highly
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water soluble model compound, disodium ethanedisulfonate
Et(SO3Na)2, Hasegawa show that this system also had a
buffer-like feature. Aqueous solutions with various pH
values were added to aqueous solutions of Et(SO3Na)2,
where the pH of the mixed solutions was kept at pHi5,
independent of the pH of the used aqueous solutions, in the
range pH=3 –10. The results led to the conclusion that a
considerable number of the AOT sulfonate groups localising
on the water/oil interface are responsible for the buffer-like
action. The buffer mechanism can be explained by the
suppressed ionic dissociation of the AOT sulfonate groups
in the reverse micellar aggregation state.
4.3. Catalytic process in microemulsions
Use of microemulsions as reaction media is compatible
with the simultaneous use of other catalysts. Oil-in-water
microemulsions based on a nonionic surfactant have been
used as reaction media to oxidize aqueous azo dyes [99].
Different combinations of manganese porphyrins and lipophilic acids were employed as oxidation catalysts. Phase
transfer catalysis can also be used in a microemulsion
system in which case a further acceleration of the reaction
may be obtained [100]. In biphasic systems, the role of the
phase transfer agent is to transfer the nucleophilic anion
from the aqueous to the organic phase. Once in the organic
phase, the nucleophile becomes highly reactive because (i)
the degree of solvation is low, and (ii) the large cations used
as phase transfer agents do not form strong ion pairs with
the nucleophile in the organic phase; thus, the anion behaves
as a ‘‘naked’’ ion. In the microemulsion approach, there is
no transfer of reagent from one environment to another.
Addition of phase transfer agents enhance catalytic efficiency of microemulsions in the reaction of lipophilic epoxide
with sodium sulfite; nevertheless, the mechanism of the
catalysis is still unclear. Alkenes with carbon numbers
higher than 8 are hydroformylated in the presence of
cobalt-based catalysts. An alternative for solving the problem of miscibility between oil (higher alkene) and water
(aqueous catalyst solution) is the addition of surfactants to
form microemulsions. Haumann et al. [101] have studied
the application of water-soluble catalysts based on cobalt for
the combined isomerization and hydroformylation of 7tetradecene or less reactive higher alkenes by using Rh
catalysts [102]. The catalyst is highly active and converts
the internal alkene into the corresponding branched aldehyde with high regioselectivity. In order to obtain linear
aldehydes from an internal alkene feedstock, cobalt-based
catalysts can be used. The cobalt catalyst allows isomerization of the double bond first, followed by hydroformylation.
More than 50% of the internal alkene is isomerised into a 1alkene before hydroformylation although there is no detectable concentration of 1-alkene. In order to increase the
linear to branched ratio of the obtained aldehydes, either
the reaction temperature had to be lowered or the excess of
ligand had to be increased.
Scheme 5.
In spite of the clear advantages in using microemulsions
as reaction media for reactions catalysed by metallic cations
and other catalysts, there is a lack of kinetic studies explaining the role of the microemulsion. Recently, Fanti et al.
[103..] have studied the hydrolytic reactivity of ligands
featuring a 6-alkylaminomethylpyridine or an N-alkylethylenediamine, as chelating subunits, in the presence of Cu(II)
in AOT/isooctane/water microemulsions. The substrates of
choice were the p-nitrophenyl esters of picolinic acid
(PNPP), of acetic acid (PNPA), and of diphenylphosphoric
acid (DPPNPP). In the presence of Cu(II) complexes of
hydroxy-functionalized ligands, such as 1a or 1b (Scheme
5), the cleavage of PNPP is a million-fold faster than in the
absence of Cu(II) and any ligand.
The effect of Cu(II) alone is much more important than
that observed in water solution; the rate accelerations at the
same Cu(II) concentrations being 2 orders of magnitude
larger. The difference can be ascribed to the favourable
partition of the metal ion and the substrate in the water pool
of the reverse micelle. The main characteristics of the
reaction mechanism are conserved, and as in normal micellar aggregates, the main source of the kinetic effects
observed is the high concentration of the reactants in the
aggregate core. However, in the case of reversed metallomicelles, the high concentration of organic species in the
water pool is determined by the presence of Cu(II) and by
the possibility of coordination with the metal ion rather than
by their hydrophilic or lipophilic nature.
In an attempt to clarify the reasons for a different
behaviour of catalysis by metallic cations in micelles and
microemulsions, a kinetic and thermodynamic study on the
complexation reaction of metallic cations by bidentate
ligands was recently reported. The complexation constants
of Ni2+ and Co2+ with pyridine-2-azo-p-dimethylaniline
(PADA) were investigated in AOT/isooctane/water microemulsions [104]. A complexation reaction is always the first
step in any process catalysed by Lewis acids. In all cases,
the values of the complexation constants are greater than
those obtained in bulk water. By applying the pseudo-phase
formalism, the microscopic complexation constants were
obtained showing that their values decrease as W increases
and are always lower than the values obtained in bulk water.
The interaction of interfacial water with the surfactant
headgroup in the microemulsion causes an increase in the
electronic density on the oxygen atoms of water and a
consequent increase in the interaction H2O. . .M2+, which
M.A. López-Quintela et al. / Current Opinion in Colloid & Interface Science 9 (2004) 264–278
in turn leads to a greater stabilisation of the ion with respect
to that in bulk water. As the water content decreases, this
interaction becomes stronger but the quantity of water
molecules available for hydration decreases. This situation
results in destabilisation of the ion and an increase in its
complexation capacity. Formation of Ni2+-PADA and Co2+PADA complexes have been kinetically studied in AOTbased microemulsions showing that the complexation rate
constants decrease as both AOT concentration and W
increase, being in all cases greater than in bulk water.
Analysis of the kinetic data shows that the complexation
mechanism in the microemulsion is compatible with that in
bulk water; the catalytic effects are derived from the
increase in the local reagent concentrations and the modification of water properties when W is varied.
In recent years, the widespread synthesis of nanosized
solid particles in w/o microemulsions has opened the possibility of new colloidal systems with potential catalytic
properties. Although their catalytic potential has been recognised, only a few research groups have so far studied the
kinetics of reactions catalysed by such particles. Spiro and de
Jesus [105 .. ] have shown that the oxidation of pMe2NC6H4NH2 by Co(NH3)5Cl2+ can proceed in a buffered
water/AOT/n-heptane microemulsion, and that it is strongly
catalysed by nanoparticles of palladium. However, the
reaction did not reach completion because of a side
reaction between the semiquinonediimine formed and the
surfactant. Kinetic results allow the authors to conclude
that the rate-determining step of the catalysis is probably
diffusion of Co(NH 3 ) 5 Cl 2+ ions through a layer of
adsorbed p-Me2NC6H4NH2 to reach the metal surface.
Electrons are then transferred via the metal from adsorbed
p-Me2NC6H4NH2 molecules to Co(NH3)5Cl2+ ions. Spiro
and de Jesus [106] have also studied the oxidation of
N,N,N,N-tetramethyl-p-phenylenediamine (TMPPD) by
Co(NH3)5Cl2+ ions catalysed by palladium nanoparticles
in an aqueous buffer/AOT/n-heptane microemulsions. The
activation energy of the catalytic reaction decreased from
97 kJ mol1 at 15 jC to 39 kJ mol1 at 40 jC. These
values are all greater than those recorded for the
corresponding p-Me2NC6H4NH2 reaction. Electrochemical
studies showed that the Co(NH3)5Cl2+ reduction current at
a rotating Pd electrode decreased in the presence of
micromolar amounts of added TMPPD. This finding,
together with the marked temperature variation of the
activation energy, indicated that adsorption of TMPPD
on the Pd particles affected the rate-determining step of
the catalysis. Slow diffusion of Co(NH3)5Cl2+ through
adsorbed TMPPD species is likely to be the slow step in
the catalysis followed by electron transfer via the metal
from the adsorbed diamine to the cobalt ion at the surface.
Using water/AOT/supercritical CO2 microemulsions,
Ohde and coworkers [17] showed that hydrogen gas can
cause reduction of a number of metal ions including Pd2+
dissolved in the water core of the microemulsion. After
reduction, the hydrogen gas can also serve as a starting
275
material for in situ hydrogenation in supercritical CO2. The
hydrogenation of 4-methoxycinnamic acid to 4-methoxyhydrocinnamic acid, hydrogenation of trans-stilbene to
1,2-diphenylethane and hydrogenation of maleic acid to
succinic acid were performed in supercritical CO2 microemulsions catalysed by Pd nanoparticles (size range of
about 5– 10 nm). Further studies [107.] have shown that
rhodium nanoparticles dispersed in CO2 microemulsions
are also effective catalysts for rapid hydrogenation of
arenes in supercritical CO2.
Acknowledgements
Financial support from Ministerio de Ciencia y Tecnologı́a (Projects BQU2002-01184 and MAT2002-00824) and
Xunta de Galicia (Projects PGIDT03-PXIC20905PN and
PGDIT03-PXIC20907PN)) is gratefully acknowledged.
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