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Current Opinion in Colloid & Interface Science 9 (2004) 264 ­ 278 www.elsevier.com/locate/cocis

Microemulsion dynamics and reactions in microemulsions

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´ ´ M.A. Lopez-Quintela a,*, C. Tojo b, M.C. Blanco a, L. Garcia Rio a, J.R. Leis 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

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 MÀ1 sÀ1 for a typical low viscosity solvent. Because usually the droplet's volume fraction Uc0.1, i.e., [droplet]c10À3 M for a typical droplet's size c10 nm, the encounter rate constant kenc106 sÀ1. 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

´ Madrid, March 11th, 2004: En memoria de todas las victimas ´ ´ inocentes de este y todos los demas 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. Lopez-Quintela). 1359-0294/$ - see front matter D 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.cocis.2004.05.029

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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 À3 --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 cc10À1 [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.

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 com4À pared to quenching of the RuL3 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

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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+BrÀ, 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

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, SiO2À 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]À[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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PbS particles in a toluene-in-water microemulsion with the inducement of ultrasound irradiation is proposed. 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], Ce1Àx ­ 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.

Finally, the catalytic activity of metallic particles synthesised in Triton X microemulsions has also been reported [44,46,47]. 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].

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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

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.

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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 OHÀ 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 OHÀ ions when the water content of the microemulsion decreases, which means an increase in OHÀ 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

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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 2À (NH3)5pz]2+and S2O8 in 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.

microemulsions prepared with CTAB in n-dodecane/CHCl3 and isooctane/n-hexanol has been obtained [95.] for 2,4,6+ trimethylbenzenediazonium (1-ArN2 ) and 2,6-dimethyl-4+ hexadecylbenzenediazonium (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

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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 molÀ1 at 15 jC to 39 kJ molÀ1 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

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 Tecno´ logia (Projects BQU2002-01184 and MAT2002-00824) and Xunta de Galicia (Projects PGIDT03-PXIC20905PN and PGDIT03-PXIC20907PN)) is gratefully acknowledged.

References and recommended reading

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ticles in a Winsor II microemulsion of diethyl ether/AOT/water. Langmuir 2002;18:8237 ­ 40. ` [10] Caponetti E, Pedone L, Martino DC, Panto V, Turco Liveri V. Synthesis, size control, and passivation of CdS nanoparticles in water/ AOT/n-heptane microemulsions. Mater Sci Eng C 2003;23:531 ­ 9. [11] Li T, Moon J, Morrone AA, Mecholsky JJ, Talham DR, Adair JH. Preparation of Ag/SiO2 nanosize composites by a reverse micelle and sol ­ gel technique. Langmuir 1999;15:4328 ­ 34. [12] Grasset F, Labhsetwar N, Li D, Park CC, Saito N, Haneda H, Cador .. O, Roisnel T, Mornet S, Duguet E, Portier J, Etourneau J. Synthesis and magnetic characterization of zinc ferrite nanoparticles with different environments: powder, colloidal solution, and zinc ferrite ­ silica core ­ shell nanoparticles. Langmuir 2002;18:8209 ­ 16. This is a rigorous and interesting study about the preparation of uniform silica coatings onto zinc ferrite nanoparticles using ferrofluids/surfactant/ heptane microemulsions. Experimental conditions for the whole process are clearly described, and reasons for the different steps used in the synthesis are given. [13] Tago T, Shibata Y, Hatsuta T, Miyajima K, Kishida M, Tashiro S, Wakabayashi K. Synthesis of silica-coated rhodium nanoparticles in reversed micellar solution. J Mater Sci 2002;37:977 ­ 82. [14] Marchand KE, Tarret M, Lechaire JP, Normand L, Kasztelan S, Cseri . T. Investigation of AOT-based microemulsions for the controlled synthesis of MoSx nanoparticles: an electron microscopy study. Colloid Surf, A Physicochem Eng Asp 2003;214:239 ­ 48. An interesting study about the influence of cosurfactant on the kinetics of the intermicellar exchange. [15] Bagwe RP, Khilar KC. Effects of the intermicellar exchange rate and cations on the size of silver chloride nanoparticles formed in reverse micelles of AOT. Langmuir 1997;13:6432 ­ 8. [16] Ohde H, Hunt F, Wai CM. Synthesis of silver and copper nanoparticles in a water-in-supercritical-carbon dioxide microemulsion. Chem Mater 2001;13:4130 ­ 5. [17] Ohde H, Wai CM, Kim H, Kim J, Ohde M. Hydrogenation of Olefins in Supercritical CO2 catalyzed by palladium nanoparticles in a water-in-CO2 microemulsion. JACS 2002;124:4540 ­ 1. [18] Ohde H, Ohde M, Bailey F, Kim H, Wai CM. Water-in-oil microemulsions as nanoreactors for synthesizing CdS and ZnS nanoparticles in supercritical CO2. Nano Lett 2002;2:721 ­ 4. [19] Goncalves SAP, De Pauli SH, Tedesco AC, Quina FH, Okano LT, Bonilha JBS. Counterion exchange selectivity coefficients at waterin-oil microemulsion interface. J Colloid Interface Sci 2003;267: 494 ­ 9. [20] Yadav OP, Palmqvist A, Cruise N, Holmberg K. Synthesis of platinum nanoparticles in microemulsions and their catalytic activity for the oxidation of carbon dioxide. Colloids Surf, A Physicochem Eng Asp 2003;221:131 ­ 4. [21] Liao M-H, Chen D-H. Characteristics of magnetic nanoaprticlesbound YADH in water/AOT/isooctane microemulsions. J Mol Catal, B Enxym 2002;18:81 ­ 7. ´ [22] Ingelsten HH, Beziat J, Bergkvist K, Palmqvist A, Skoglundh M, Qiuhong H, Falk LKL, Holmberg K. Deposition of platinum nanoparticles, synthesized in water-in-oil microemulsions, on alumina supports. Langmuir 2002;18:1811 ­ 8. [23] Chen D-H, Chen C-J. Formation and characterization of Au ­ Ag . bimetallic nanoparticles in water-in-oil microemulsions. J Mater Chem 2002;12:1557 ­ 62. An interesting report on the synthesis of bimetallic nanoparticles. [24] Wu M-L, Chen D-H, Huang T-C. Synthesis of Au/Pd bimetallic nanoparticles in reverse micelles. Langmuir 2001;17:3877 ­ 83. [25] Wu M-L, Chen D-H, Huang T-C. Preparation of Pd/Pt bimetallic nanoparticles in water/AOT/isooctane microemulsions. J Colloid Interface Sci 2001;243:102 ­ 8. [26] Porta F, Prati L, Rossi M, Scari G. Synthesis of Au(0) nanoparticles from W/O microemulsions. Colloids Surf, A Physicochem Eng Asp 2002;211:43 ­ 8. [27] Curri ML, Agostiano A, Manna L, Della Monica M, Catalano M,

´ M.A. Lopez-Quintela et al. / Current Opinion in Colloid & Interface Science 9 (2004) 264­278 [45] Xu S, Li Y. Different morphology at different reactant molar ratios: synthesis of silver halide low-dimensional in microemulsions. J Mater Chem 2003;13:163 ­ 5. [46] Zhang X, Chan K-Y. Water-in-oil microemulsions synthesis of platinum ­ ruthenium nanoparticles, their characterization and electrocatalytic properties. Chem Mater 2003;15:451 ­ 9. [47] Zhang X, Chan K-Y. Microemulsion synthesis and electrocatalytic properties of platinum ­ cobalt nanoparticles. J Mater Chem 2002; 12:1203 ­ 6. [48] Santra S, Tapec R, Theodoropoulou N, Dobson J, Hebard A, Tan W. Synthesis and characterization of silica-coated iron oxide nanoparticles in microemulsions: the effects of nonionic surfactants. Langmuir 2001;17:2900 ­ 6. [49] Tartaj P, Serna CJ. Microemulsion-assisted synthesis of tunable superparamagnetic composites. Chem Mater 2002;14:4396 ­ 402. [50] Bose S, Saha SK. Synthesis and characterization of hydroxyapatite nanopowders by emulsion technique. Chem Mater 2003;15: 4464 ­ 9. [51] Fang J, Stokes KL, Wiemann JA, Zhou WL, Dai J, Chen F, O'Connor CJ. Microemulsion-processed bismuth nanoparticles. Mater Sci Eng B 2001;83:254 ­ 7. [52] Liu Z, Lee JY, Han M, Chen W, Gan LM. Synthesis and characterization of PtRu/C catalysts from microemulsions and emulsions. J Mater Chem 2002;12:2453 ­ 8. [53] Althues H, Kaskel S. Sulfated zirconia nanoparticles synthesized in reverse microemulsions: preparation and catalytic properties. Langmuir 2002;18:7428 ­ 35. [54] Hanaoka T, Hayashi H, Tago T, Kishida M, Wakabayashi K. In situ .. immobilization of ultrafine particles synthesized in a water/oil microemulsion. J Colloid Interface Sci 2001;235:235 ­ 40. This is an interesting example of the influence of the chemical reaction rate on the final particle size. [55] Bae D-S, Han K-S, Adair JH. Synthesis and microstructure of Pd/ SiO2 nanosized particles by reverse micelle and sol/gel processing. J Mater Chem 2002;12:3117 ­ 20. [56] Nanni A, Dei L. Ca(OH)2 nanoparticles from w/o microemulsions. Langmuir 2003;19:933 ­ 8. [57] Vestal C, Zhang ZJ. Synthesis and magnetic characterization of Mn and Co spinel ferrite/silica nanoparticles with tunable magnetic core. Nano Lett 2003;3:1739 ­ 43. [58] Tartaj P, Tartaj J. Microstructural evolution of iron-oxide-doped alumina nanoparticles synthesized from microemulsions. Chem Mater 2002;14:536 ­ 41. [59] You Z, Balint I, Aika K. Synthesis of thermally stable Cs-doped alumina nanoparticles by microemulsion method. Chem Lett 2002; 1090-1. [60] Balint I, You Z, Aika K. Morphology and oxide phase control in the microemulsion mediated synthesis of barium stabilized alumina nanoparticles. Phys Chem Chem Phys 2002;4:2501 ­ 3. [61] Zhand R, Gao L. Preparation of nanosized titania by hydrolysis of alkoxide titanium in micelles. Mater Res Bull 2002;37: 1659 ­ 66. [62] Yashima M, Falk LKL, Palmqvist AEC, Holmberg K. Structure and catalytic properties of nanosized alumina supported platinum and palladium particles synthesized by reaction in microemulsion. J Colloid Interface Sci 2003;268:348 ­ 56. [63] Tago T, Tashiro S, Hashimoto Y, Wakabayashi K, Kishida M. Synthesis and optical properties of SiO2-coated CeO2 nanoparticles. J Nanopart Res 2003;5:55 ­ 60. [64] Debuigne F, Cuisenaire J, Jeunieau L, Masereel B, Nagy JB. Synthesis of nimesulide nanoparticles in the microemulsion epikuron/ isopropyl myristate/water/n-butanol (or isopropanol). J Colloid Interface Sci 2001;243:90 ­ 101. [65] Khiew PS, Radiman S, Huang NM, Ahmad MdS. Studies on the growth and characterization of CdS and PbS nanoparticles using sugar-ester nonionic water-in-oil microemulsion. J Cryst Growth 2003;254:235 ­ 43.

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[66] Candau F. In: Paleos CM, editor. Polymerization in Organized Media. Paris: Gordon & Breach, 1992. Chap. 4. [67] Antonietti M, Basten R, Lohmann S. Polymerization in microemulsions--a new approach to ultrafine, highly functionalized polymer dispersions. Macromol Chem Phys 1995;196:441 ­ 66. [68] De Vries R, Co CC, Kaler EW. Macromolecules 2001;34:3233 ­ 44. [69] Hermanson KD, Kaler EW. Kinetics and mechanism of the multiple .. addition microemulsion polymerization of hexyl methacrylate. Macromolecules 2003;36:1836 ­ 42. The Morgan ­ Kaler model, and its extended version, are used to examine the polymerization mechanisms and to predict the evolution of particle size. ´ ´ [70] Tauer K, Ramirez AG, Lopez RG. Effect of the surfactant concentration on the kinetics of oil in water microemulsion polymerization: a case study with butyl acrylate. C R Chim 2003;6:1245 ­ 66. ´ ´ [71] Puig JE, Mendizabal E, Delgado S, Arellano J, Lopez-Serrano F. Cosurfactant effects on the microemulsion characterization of styrene. C R Chim 2003;6:1267 ­ 73. [72] Xu XJ, Siow KS, Wong MK, Gan LM. Microemulsion polymerization of styrene using a polymerizable nonionic surfactant and a cationic surfactant. Colloid Polym Sci 2001;279:879 ­ 86. [73] Larpent C, Amigoni-Gerbier S, De Sousa Delgado A. Synthesis of metal-complexing nanoparticles by post-functionalisation of reactive nanolatexes produced by microemulsion polymerisation. C R Chim 2003;6:1275 ­ 83. [74] Zhang Y, Guo T, Hao G, Song M, Zhang B. Nanosize polymer latices made by microemulsion copolymerization: preparation and characterization. J Appl Polym Sci 2003;90:3625 ­ 30. [75] Jiang W, Yang W, Zeng X, Fu S. Structure and properties of poly(methyl methacrylate) particles prepared by a modified microemulsion polymerization. J Polym Sci 2004;42:733 ­ 41. [76] Holmber K. Organic reactions in microemulsions. Curr Opin Colloid Interface Sci 2003;8:187 ­ 96. [77] Hager M, Currie F, Holmber K. Organic reactions in microemulsions. ¨ Top Curr Chem 2003;227:53 ­ 74. [78] Wu D-Y, Zhang L-P, Wu L-Z, Wang B-J, Tung Ch-H. Water-in-oil microemulsions as microreactors to control the regioselectivity in the photocycloaddition of 9-substituted anthracenes. Tetrahedron Lett 2002;43:1281 ­ 3. ´ ´ [79] Garcia-Rio L, Leis JR, Moreira JA. Reactivity in water/oil micro.. emulsions. Influence of sodium bis/2-ethylhexyl)sulfosuccinate/isooctane/water microemulsions on the solvolysis mechanism of substituted benzoyl chlorides. J Am Chem Soc 2000;122:10325 ­ 34. Changes in the physical properties of microemulsions on changing W induce changes in the solvolysis reaction rates. As a consequence, the balance between associative and dissociative mechanisms is shifted to a more associative one on decreasing the water content. ´ ´ [80] Garcia-Rio L, Leis JR, Mejuto JC. Solvolysis of benzoyl halides in . AOT/isooctane/water microemulsions. Influence of the leaving group. Langmuir 2003;19:3190 ­ 7. The ionizing power of the interface of the microemulsions changes with W, and as a consequence also changes the solvolysis rate of benzoyl halides. The influence of the ionizing power on the solvolysis rate is for BrÀ>ClÀ>FÀ. [81] Zhang R, Liu J, He J, Han B, Wu W, Jiang T, Liu Z, Du J. Organic . reactions and nanoparticle preparation in CO2-induced water/P104/ p-xylene microemulsions. Chem Eur J 2003;9:2167 ­ 72. The different influence of W on the hydrolysis of benzoyl chloride and pnitrophenyl chloroformate can be ascribed to the different reaction mechanisms and the expected changes in the nucleophilicity and polarity of water in the microemulsions. [82] Bunton CA, Garreffa A, Germani R, Onori G, Santucci A, Savelli G. Relation between the IR spectrum of water and decarboxylation kinetics in microemulsions. Prog Colloid Polym Sci 2001;118: 103 ­ 6. ´ ´ ´ [83] Garcia-Rio L, Mejuto JC, Perez M. Modification of reactivity by changing microemulsion composition. Basic hydrolysis of nitro-

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