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Advances in Colloid and Interface Science 133 (2007) 23 34


Reverse micelles: Inert nano-reactors or physico-chemically active guides of the capped reactions

Vuk Uskoković , Miha Drofenik

Jožef Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia

Available online 27 February 2007


Reverse micelles present self-assembled multi-molecular entities formed within specific compositional ranges of water-in-oil microemulsions. The structure of a reverse micelle is typically represented as nano-sized droplet of a polar liquid phase, capped by a monolayer of surfactant molecules, and uniformly distributed within a non-polar, oil phase. Although their role in serving as primitive membranes for encapsulation of primordial self-replicating chemical cycles that anticipated the very origins of life has been proposed, their first application for parent(hesis)ingchemical reactions with an aim to produce templated2D arrays of nanoparticles dates back to only 25 years ago. Reverse micelles have since then been depicted as passive nano-reactors that via their shapes template the growing crystalline nuclei into narrowly dispersed or even perfectly uniform nano-sized particles. Despite this, numerous examples can be supported, wherefrom deviations from the simple unilateral correlations between size and shape distribution of reverse micelles and the particles formed within may be reasonably implied. A rather richer, dynamical role of reverse micelles, with potential significance in the research and design of complex, self-assembly synthesis pathways, as well as possible adoption of their application as an aspect of biomimetic approach, is suggested herein.

© 2007 Elsevier B.V. All rights reserved.

Keywords: Colloids; Microemulsion; Nanomaterials; Reverse micelles; Review




Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .



The need to reevaluate the functional representation of reverse micelles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .



Examples of chemical butterfly effectsin reverse micelle-assisted syntheses . . . . . . . . . . . . . . . . . . . . . . . . . . . .



The example of nickelzinc ferrite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .



The example of lanthanumstrontium manganite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .



Correlations with the biological context . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .



Future directions in the application of reverse micelles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .



Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .


References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .


1. Introduction

Reverse micelles present multi-molecular self-assembly entities formed as dispersed colloid phases of microemulsions at particular compositional ranges thereof [1]. In 1982

Corresponding author.

E-mail address: vuk21@yahoo.com (V. Uskoković).

0001-8686/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.cis.2007.02.002

Boutonnet et al. first reported synthesis of a material via using reverse micelles [2]. Numerous other nanostructured materials, ranging from metallic catalysts [38] to semiconductor quantum dots [911] to various ceramic materials [1216], silica and gold coated nanoparticles [1721], latexes and polymer composites [2224], double-layered nanoparticles [25] and even superconducting materials [26,27] have been prepared since then by means of reverse micelle technique. However, the


V. Uskoković, M. Drofenik / Advances in Colloid and Interface Science 133 (2007) 2334

Fig. 1. A drawing of a reverse micelle (a) and a computational model (b) of reverse micelle [28]. Blue spheres represent surfactant head groups, whereby smaller yellow spheres denote counterions. Note that the surfactant head groups do not completely shield aqueous interior of the modeled reverse micelle (b). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

explanation models that are typically invoked in the frame of such experiments ordinarily refer to purely templatingrole of reverse micelles. As inert nano-cages, they are conceived as only limiting the growth of precipitated nuclei, so that the initial narrow dispersion of micelles in relation to their sizes and shapes becomes reflected on a similar uniformity of the eventually produced nanoparticles. Through referring to numerous deviations from such an oversimplified picture, this review will challenge an idea according to which the only role that reverse micelles have in the processes of preparation of nanoparticles is their templatingeffect and superimposition of spherical shapes upon the growing nuclei.

2. The need to reevaluate the functional representation of reverse micelles

Reverse micelles are typically depicted as spherical nanodroplets, uniformly capped with a monolayer of surfactant molecules (Fig. 1a), and isotropically distributed within an oil phase. However, a recent attempt to model the structure of a reverse micelle resulted in an image of a multi-molecular aggregate wherein surfactant head groups did not completely shield aqueous interior of the modeled micelle (Fig. 1b), indicating the need to reevaluate the typical representations of micelles as perfectly surfactant-capped and overstatically configured molecular aggregates [28].

The field of reverse-micellar synthesis of nanostructured materials is permeated by representations of passive and solely templating role of the micelles in the course of particle formation processes. Simple, parametric correlations are routinely used to predict and explain the particle size out of the initial microemulsion structure. Most notably, Pileni et al. proposed that the size of particles obtained by precipitation in reverse-micellar microemulsions based on sodium bis(2- ethylhexyl) sulfosuccinate (AOT) as a surfactant ought to be equal to 1.5 times the water-to-surfactant molar ratio in nanometers [29,30]. Carpenter et al. suggested that the size of precipitated particles in reverse micelles that comprise cetyltrimethylammonium bromide (CTAB) as a surfactant should be

equal in nanometers to the water-to-surfactant molar ratio of the parent microemulsion [31]. Although the former relationship was verified only for certain compositions of specific AOTbased microemulsions and particles prepared within [30], it has been frequently mistaken as corresponding to all types of microemulsions and particles [32].

As a response to such an oversimplification, numerous cases of experimental deviations from the proposed correlations were reported [5,6,33,34]. It is not only that water-to-surfactant molar ratio in reverse-micellar ranges of the given microemulsion phase diagrams does not correspond to micellar sizes in direct proportion in all cases, but the very same small-angle X-ray scattering (SAXS) characterization technique that was relied upon in defining the mentioned relationship between water-to- surfactant molar ratio and the size of produced particles [30,3537], has shown that micellar radii in the same AOT/isooctane/ water system change in response to an addition of small amounts of compounds solubilized in the microemulsion [29]. Experimental results indicate that the size of reverse micelles depends not only on water-to-surfactant molar ratio, but also on identity of all included microemulsion components, their respective concentrations, pH, temperature and ionic interactions caused by introduced electrolytes or inherently dissociated molecular species [1]. Also, the particle formation processes necessarily affect the structure of a parent emulsion, resulting in a feedback interaction that ends as either a form of phase segregation or a metastable state in cases when isotropic colloidal dispersion structure is preserved.

It has been known that phase diagrams of microemulsions derived with and without the presence of the prepared material or any other additional component may be drastically different [38]. Therefore, in light of such mutual transformations, the concept of templatingas translation of shapes and sizes of self-assembled organic species onto the structure of nucleated and grown crystallites looks as if it needs to be reevaluated, particularly in the area of reverse-micellar preparation of materials where the phrases like nano-cages, nano-templates or nano-reactors seem to dominate the explanations of particle formation mechanisms.

V. Uskoković, M. Drofenik / Advances in Colloid and Interface Science 133 (2007) 2334


Table 1

Macroscopic and nanoscopic variables in the microemulsion-assisted and particularly reverse-micellar synthesis of nanoparticles

Macroscopic parameters

Nano-sized parameters

Identity of included

Static, size and shape distribution of micelles

chemical species




Aggregation number





Dynamic interaction, rates and types of merging

molar ratio

and dissociation of micelles





Distribution of charged entities around dispersed

Ionic strength


Dissolved species




Surfactant film curvature and head-group spacing

Method and rate of



introduction of species

Effective Coulomb repulsion potential

Temperature and pressure



Aging times

Van der Waals, hydrogen and

Method and rate of stirring

hydrophobic interactions

Homogeneous or



heterogeneous nucleation

Screening length




For the most surfactant-mediated syntheses, connection between morphology of the surfactant aggregates and the resulting particle structure is more complex (than simply relating the average size and shape of the micelles to size and shapes of the precipitated particles) and affected by almost irreducible conditions that exist in the local microenvironments that surround the growing particles [39]. These molecular-level variables are subject to change with macroscopically manipulated experimental conditions, as is shown in Table 1. Composition, pH, concentration of the reactants, ionic strength and heat content are some of the experimentally modified variables that co-influence this local environment. As chemical reactions and physical transformations caused by aging take place within a colloid and its corresponding microenvironments, many of these factors are subject to change. The decoupling of effects that belong to each specific macroscopic modification of the system presents one of the biggest challenges in the practical field of colloid science.

Another oversimplified idea in the area of reverse-micellar synthesis of nanoparticles is that the size of the produced particles is supposed to be equal to the size of the micelles that cap and limit the growth of individual crystallization nuclei. Despite such a picturesque representation of the processes of particle formation inside the so-called nano-reactors (i.e. water pools) of reverse micelles, numerous cases wherein the variations in the produced particle sizes could not have been correlated with sizes of the reverse micelles, were reported [35,40]. The size of colloid units or any other relevant property of a colloid system can be considered as dependent not upon any single internal variable, but only on the complex interactions that are conditional for their existence. Many cases support the idea that the reasons for the frequent mismatch of the properties and quantities derived by different experimental methods do not result from errors inherent in the experiments, but are evidential of a fundamental shortcoming in the single parameter models [41]. Such a situation is highly reminiscent of

numerous attempts to infer hydrophobic interactions from molecular-scale surface areas alone, even though bulk driving forces and interfacial effects compete in determining hydrophobic effects in any particular case [42]. As a more reasonable explanation, the dynamic interaction among colloid aggregates has since lately been generally considered as the most important factor that influences the morphology and the properties of the final reaction products [43]. However, since dynamic interaction of colloid multi-molecular aggregates, such as micelles, cannot be yet directly observed in real-time conditions, indirect techniques are usually applied in order to evaluate both static and dynamic properties of the corresponding media. In the approximations (introduced in order to overcome the limitations of characterization techniques in terms of sampling, experiment time scales, etc.) and different implicit presuppositions of various such techniques are present the reasons for a frequent mismatch [44,45] between the concluded properties attributed to the same systems by using different experimental methods.

Unlike some of the surfactant-templating syntheses that can be considered as structurally transcriptive (a copying or casting as in the cases of some porous inorganics [46]), templatingof crystallization processes within fine and sensitive, advanced colloid systems such as microemulsions and particularly reverse micelles can be regarded first as synergistic and only then as reconstitutive [47]. Despite the fact that only spherical or elliptical micelles have been detected and theoretically predicted so far, beside spherically shaped particles, various other exotic morphologies, including nanorods, nanofilaments, acicular particles, star-shaped patterns etc, were prepared by relying on this method. When a microemulsion-assisted synthesis of copper nanocrystals was performed in the presence of sodium fluoride, sodium chloride, sodium bromide or sodium nitrate, small cubes, long rods, larger cubes and variety of shapes resulted, respectively [48]. Variations of salt identities and concentrations in another case of preparation of copper nanocrystals also resulted in drastic morphological changes [49].

Although most of the particles produced in reverse-micellar, AOT-based microemulsion systems were spherical in nature [50], crystallization of barium sulfate resulted in extended crystalline nanofibers aligned to form superstructures, whereby a precipitation of barium chromate in the same microemulsion system resulted in primary cuboids aligned to linear caterpillarsor rectangular mosaics [47]. In the case of synthesis of calcium phosphates, variations in relative concentrations of the microemulsion components resulted in various different morphologies, ranging from co-aligned filaments to amorphous nanoparticles, hollow spheres, spherical octacalcium phosphate aggregates of plate-shaped particles, and elongated plates of calcium hydrogen phosphate dehydrate [51]. Moreover, in the first historical report on the synthesis of materials in reversemicellar media [2], it was observed that size of the prepared platinum, rhodium, palladium and iridum particles was always in the range of 25 nm, independently on surfactant, water and reactant concentrations applied in the experiments [52].

Far from being only inert constraints to the growth of crystallites, microemulsions were shown to be physico-chemically


V. Uskoković, M. Drofenik / Advances in Colloid and Interface Science 133 (2007) 2334

active in defining the reaction pathways that take place in their presence, thus influencing the very chemical identities of the final products [53]. Specific intermolecular interactions at the hydrophilic sides of surfactants surrounding the aqueous cores, intense local electric fields and significant level of cooperative weak molecular forces that modify the local microenvironments comparing to bulk conditions, as well as specific structure and solvent properties of water at close interfacial distances, are proposed to have catalytic effects on the rates of chemical changes [54,55].

The behavior of liquid molecules confined in nano-sized spaces or at solidliquid interfaces in general, due to surfaceinduced structuring, significantly differs from their behavior within a bulk system [56]. Fourier-Transform Infrared (FTIR) spectroscopic studies have indicated that the water interior of a reverse micelle has a multilayered structure, consisting of interfacial, intermediate and core water. The interfacial layer is composed of water molecules that are directly bounded by polar head groups of a surfactant; the intermediate layer consists of the next few nearest-neighbor water molecules that can exchange their state with interfacial water; and the core layer is found at the interior of the water pooland has the properties of bulk water [57]. Depending on the size of reverse micelles, available water may have significantly different solvent properties, ranging from highly structurized interiors with little molecular mobility [58,59] to free water cores that approximate bulk water solvent characteristics.

Different water structures may also dissolve different amounts of gases, which can drastically influence the reaction pathways, particularly in the cases where oxidation or reduction reactions by means of dissolved gases comprise crucial steps in preparation procedures, as the numerous cases of ferric oxides and complex corrosion phenomena may illustrate [1]. The accumulated gases are significantly present at hydrophobic interfaces [60] comparing to the typical range of dissolved gas concentration in water at normal pressure and temperature ( 5 · 103 M). Fine variations in the experimental outcomes depending on the gas effects have been noticed [53], and there were cases where certain effects, which depended on many parameters, disappeared on removal of the dissolved gas [60].

Interfacial self-association mechanisms can also be quite different depending on the surface wettability. As a biological example, the rate of blood coagulation tends to increase with an increase in water-wettability of the tube surface [61]. Also, selfassembly processes that occur during the drying steps of synthesis procedures involve complex competition between the kinetics of evaporation and the time scales with which solvated nanoparticles diffuse on a substrate, and due to the specific role of hydrophobic interactions and a variety of ways to nucleate evaporation may lead to unexplored territories in the field of novel design [42]. Anyhow, treating water as a continuum medium in both theoretical approaches (such as in the framework of DLVO theory) and explanation of experiments, altogether with disregarding its fine interactions with gases, salts and electromagnetic fields may in future indeed cause ever increasing difficulties in attempts to explain fine variations from the ranges of expected results.

3. Examples of chemical ‘butterfly effects’ in reverse micelle-assisted syntheses

As far as the current state-of-the-art is concerned, it is exceedingly difficult to predict the outcomes of experimental settings aimed to produce novel fine structures and morphologies by means of reverse-micellar methodology, and the most attractive results in this practical field come from trial-and-error approaches. There are many evidences that slight changes in the limiting conditions of particle synthesis experiments can produce significant differences as the end results [1]. The following examples may illustrate such a proposition and enrich one's belief in crucial sensitivity and subtleness of the material design procedures that involve wet environments and colloidal phenomena in general.

Replacement of manganese ions with nickel ions in an experiment of reverse-micellar precipitation synthesis of a mixed zincferrite resulted in the production of spherical particles in the former case [62] and acicular ones in the latter [63,64]. When bromide ions of cetyltrimethylammoniumbromide (CTAB) surfactant were in a synthesis of barium-fluoride nanoparticles replaced by chloride ions (CTAC), identity of the final product was no longer the same, whereas a replacement of 2-octanol with 1-octanol significantly modified crystallinity of the obtained powder [35]. Various choice of precipitation agents can often result in distinctive morphologies obtained [65,66].

The following examples may illustrate the idea that often routinely neglected influences in the preparation procedures may leave significant traces on the properties of the final products.

It has been evidenced that even the method of stirring in some of the microemulsion-assisted procedures of preparation of nanoparticles can have decisive influence on some of the final particle properties. Thus, using a magnetically coupled stir bar during an aging of a dispersion of particles influenced crystal quality and in some cases resulted in a different crystal structure as compared with non-magnetically agitated solutions [40]. In case of a synthesis of organic nanoparticles in reverse micelles, the use of magnetic stirrer led to the formation of nanoparticles larger in size comparing to the particles obtained with using ultrasound bath as a mixer, even though no changes in particle size were detected on varying solvent type, microemulsion composition, reactant concentrations and even geometry and volume of the vessel [67].

Changes in the sequence of introduction of individual components within a precursor colloid system could result in different properties of the final reaction products [68]. Such a property is directly related to the fact that microemulsions, like all colloid systems, do not present thermodynamically equilibrium phases that spontaneously form, but are thermodynamically unstable and only due to the existence of large interfacial energies that are stronger than thermal energy, kT, their order is preserved.

Changes in size of a volume where the particle preparation processes take place as occurs when the transition from smallscale research units to larger industrial vessels is attempted can lead to extensive variations in some of the properties of the synthesized material [69]. For instance, absorptivity of

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