ace Science 133 (2007) 23–34www.elsevier.com/locate/cis

Advances in Colloid and Interf

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

Vuk Uskoković ⁎, Miha Drofenik

Jožef Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia

Available online 27 February 2007

Abstract

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 surfactantmolecules, and uniformly distributed within a non-polar, oil phase. Although their role in serving as primitive membranes for encapsulation ofprimordial self-replicating chemical cycles that anticipated the very origins of life has been proposed, their first application for ‘parent(hesis)ing’chemical reactions with an aim to produce ‘templated’ 2D arrays of nanoparticles dates back to only 25 years ago. Reverse micelles have sincethen been depicted as passive nano-reactors that via their shapes template the growing crystalline nuclei into narrowly dispersed or even perfectlyuniform nano-sized particles. Despite this, numerous examples can be supported, wherefrom deviations from the simple unilateral correlationsbetween size and shape distribution of reverse micelles and the particles formed within may be reasonably implied. A rather richer, dynamical roleof reverse micelles, with potential significance in the research and design of complex, self-assembly synthesis pathways, as well as possibleadoption 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

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 232. The need to reevaluate the functional representation of reverse micelles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 243. Examples of chemical ‘butterfly effects’ in reverse micelle-assisted syntheses . . . . . . . . . . . . . . . . . . . . . . . . . . . . 264. The example of nickel–zinc ferrite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 275. The example of lanthanum–strontium manganite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 286. Correlations with the biological context . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 307. Future directions in the application of reverse micelles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 308. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

1. Introduction

Reverse micelles present multi-molecular self-assemblyentities formed as dispersed colloid phases of microemulsionsat 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 usingreverse micelles [2]. Numerous other nanostructured materials,ranging from metallic catalysts [3–8] to semiconductorquantum dots [9–11] to various ceramic materials [12–16],silica and gold coated nanoparticles [17–21], latexes andpolymer composites [22–24], double-layered nanoparticles [25]and even superconducting materials [26,27] have been preparedsince then by means of reverse micelle technique. However, the

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 yellowspheres denote counterions. Note that the surfactant head groups do not completely shield aqueous interior of the modeled reverse micelle (b). (For interpretation of thereferences to colour in this figure legend, the reader is referred to the web version of this article.)

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explanation models that are typically invoked in the frame ofsuch experiments ordinarily refer to purely ‘templating’ role ofreverse micelles. As inert ‘nano-cages’, they are conceived asonly limiting the growth of precipitated nuclei, so that the initialnarrow dispersion of micelles in relation to their sizes andshapes becomes reflected on a similar uniformity of theeventually produced nanoparticles. Through referring tonumerous deviations from such an oversimplified picture, thisreview will challenge an idea according to which the only rolethat reverse micelles have in the processes of preparation ofnanoparticles is their ‘templating’ effect and superimposition ofspherical shapes upon the growing nuclei.

2. The need to reevaluate the functional representation ofreverse micelles

Reverse micelles are typically depicted as spherical nano-droplets, uniformly capped with a monolayer of surfactantmolecules (Fig. 1a), and isotropically distributed within an oilphase. However, a recent attempt to model the structure of areverse micelle resulted in an image of a multi-molecularaggregate wherein surfactant head groups did not completelyshield aqueous interior of the modeled micelle (Fig. 1b),indicating the need to reevaluate the typical representations ofmicelles as perfectly surfactant-capped and overstaticallyconfigured molecular aggregates [28].

The field of reverse-micellar synthesis of nanostructuredmaterials is permeated by representations of passive and solelytemplating role of the micelles in the course of particleformation processes. Simple, parametric correlations areroutinely used to predict and explain the particle size out ofthe initial microemulsion structure. Most notably, Pileni et al.proposed that the size of particles obtained by precipitation inreverse-micellar microemulsions based on sodium bis(2-ethylhexyl) sulfosuccinate (AOT) as a surfactant ought to beequal to 1.5 times the water-to-surfactant molar ratio innanometers [29,30]. Carpenter et al. suggested that the size ofprecipitated particles in reverse micelles that comprise cetyl-trimethylammonium bromide (CTAB) as a surfactant should be

equal in nanometers to the water-to-surfactant molar ratio of theparent microemulsion [31]. Although the former relationshipwas verified only for certain compositions of specific AOT-based microemulsions and particles prepared within [30], it hasbeen frequently mistaken as corresponding to all types ofmicroemulsions and particles [32].

As a response to such an oversimplification, numerous casesof experimental deviations from the proposed correlations werereported [5,6,33,34]. It is not only that water-to-surfactant molarratio in reverse-micellar ranges of the given microemulsionphase diagrams does not correspond to micellar sizes in directproportion in all cases, but the very same small-angle X-rayscattering (SAXS) characterization technique that was reliedupon in defining the mentioned relationship between water-to-surfactant molar ratio and the size of produced particles [30,35–37], has shown that micellar radii in the same AOT/isooctane/water system change in response to an addition of smallamounts of compounds solubilized in the microemulsion [29].Experimental results indicate that the size of reverse micellesdepends not only on water-to-surfactant molar ratio, but also onidentity of all included microemulsion components, theirrespective concentrations, pH, temperature and ionic interac-tions caused by introduced electrolytes or inherently dissociatedmolecular species [1]. Also, the particle formation processesnecessarily affect the structure of a parent emulsion, resulting ina feedback interaction that ends as either a form of phasesegregation or a metastable state in cases when isotropiccolloidal dispersion structure is preserved.

It has been known that phase diagrams of microemulsionsderived with and without the presence of the prepared materialor any other additional component may be drastically different[38]. Therefore, in light of such mutual transformations, theconcept of ‘templating’ as translation of shapes and sizes ofself-assembled organic species onto the structure of nucleatedand grown crystallites looks as if it needs to be reevaluated,particularly in the area of reverse-micellar preparation ofmaterials where the phrases like ‘nano-cages’, ‘nano-templates’or ‘nano-reactors’ seem to dominate the explanations of particleformation mechanisms.

Table 1Macroscopic and nanoscopic variables in the microemulsion-assisted andparticularly reverse-micellar synthesis of nanoparticles

Macroscopic parameters Nano-sized parameters

Identity of includedchemical species

Static, size and shape distribution of micelles

Microemulsioncomposition

Aggregation number

Water-to-surfactantmolar ratio

Dynamic interaction, rates and types of mergingand dissociation of micelles

pH

Ionic strengthDistribution of charged entities around dispersedparticles

Dissolved speciesconcentrations Surfactant film curvature and head-group spacing

Method and rate ofintroduction of species Effective Coulomb repulsion potential

Temperature and pressureAging times Van der Waals, hydrogen and

hydrophobic interactionsMethod and rate of stirringHomogeneous or

heterogeneous nucleation Screening length

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For the most surfactant-mediated syntheses, connectionbetween morphology of the surfactant aggregates and theresulting particle structure is more complex (than simplyrelating the average size and shape of the micelles to size andshapes of the precipitated particles) and affected by almostirreducible conditions that exist in the local microenvironmentsthat surround the growing particles [39]. These molecular-levelvariables are subject to change with macroscopically manipu-lated experimental conditions, as is shown in Table 1.Composition, pH, concentration of the reactants, ionic strengthand heat content are some of the experimentally modifiedvariables that co-influence this local environment. As chemicalreactions and physical transformations caused by aging takeplace within a colloid and its corresponding microenviron-ments, many of these factors are subject to change. Thedecoupling of effects that belong to each specific macroscopicmodification of the system presents one of the biggestchallenges in the practical field of colloid science.

Another oversimplified idea in the area of reverse-micellarsynthesis of nanoparticles is that the size of the producedparticles is supposed to be equal to the size of the micelles thatcap and limit the growth of individual crystallization nuclei.Despite such a picturesque representation of the processes ofparticle formation inside the so-called ‘nano-reactors’ (i.e.‘water pools’) of reverse micelles, numerous cases wherein thevariations in the produced particle sizes could not have beencorrelated with sizes of the reverse micelles, were reported[35,40]. The size of colloid units or any other relevant propertyof a colloid system can be considered as dependent not uponany single internal variable, but only on the complex inter-actions that are conditional for their existence. Many casessupport the idea that the reasons for the frequent mismatch ofthe properties and quantities derived by different experimentalmethods do not result from errors inherent in the experiments,but are evidential of a fundamental shortcoming in the singleparameter models [41]. Such a situation is highly reminiscent of

numerous attempts to infer hydrophobic interactions frommolecular-scale surface areas alone, even though bulk drivingforces and interfacial effects compete in determining hydro-phobic effects in any particular case [42]. As a more reasonableexplanation, the dynamic interaction among colloid aggregateshas since lately been generally considered as the most importantfactor that influences the morphology and the properties of thefinal reaction products [43]. However, since dynamic interac-tion of colloid multi-molecular aggregates, such as micelles,cannot be yet directly observed in real-time conditions, indirecttechniques are usually applied in order to evaluate both staticand dynamic properties of the corresponding media. In theapproximations (introduced in order to overcome the limitationsof characterization techniques in terms of sampling, experimenttime scales, etc.) and different implicit presuppositions ofvarious such techniques are present the reasons for a frequentmismatch [44,45] between the concluded properties attributedto the same systems by using different experimental methods.

Unlike some of the surfactant-templating syntheses that canbe considered as structurally transcriptive (a copying or castingas in the cases of some porous inorganics [46]), ‘templating’ ofcrystallization processes within fine and sensitive, advancedcolloid systems such as microemulsions and particularly reversemicelles can be regarded first as synergistic and only then asreconstitutive [47]. Despite the fact that only spherical orelliptical micelles have been detected and theoreticallypredicted so far, beside spherically shaped particles, variousother exotic morphologies, including nanorods, nanofilaments,acicular particles, star-shaped patterns etc, were prepared byrelying on this method. When a microemulsion-assistedsynthesis of copper nanocrystals was performed in the presenceof sodium fluoride, sodium chloride, sodium bromide or sodiumnitrate, small cubes, long rods, larger cubes and variety ofshapes resulted, respectively [48]. Variations of salt identitiesand concentrations in another case of preparation of coppernanocrystals 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 extendedcrystalline nanofibers aligned to form superstructures, wherebya precipitation of barium chromate in the same microemulsionsystem resulted in primary cuboids aligned to linear ‘cater-pillars’ or rectangular mosaics [47]. In the case of synthesis ofcalcium phosphates, variations in relative concentrations of themicroemulsion components resulted in various differentmorphologies, ranging from co-aligned filaments to amorphousnanoparticles, hollow spheres, spherical octacalcium phosphateaggregates of plate-shaped particles, and elongated plates ofcalcium hydrogen phosphate dehydrate [51]. Moreover, in thefirst historical report on the synthesis of materials in reverse-micellar media [2], it was observed that size of the preparedplatinum, rhodium, palladium and iridum particles was alwaysin the range of 2–5 nm, independently on surfactant, water andreactant concentrations applied in the experiments [52].

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

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active in defining the reaction pathways that take place in theirpresence, thus influencing the very chemical identities of the finalproducts [53]. Specific intermolecular interactions at thehydrophilic sides of surfactants surrounding the aqueous cores,intense local electric fields and significant level of cooperativeweak molecular forces that modify the local microenvironmentscomparing to bulk conditions, as well as specific structure andsolvent properties of water at close interfacial distances, areproposed to have catalytic effects on the rates of chemical changes[54,55].

The behavior of liquid molecules confined in nano-sizedspaces or at solid–liquid interfaces in general, due to surface-induced structuring, significantly differs from their behaviorwithin a bulk system [56]. Fourier-Transform Infrared (FTIR)spectroscopic studies have indicated that the water interior of areverse micelle has a multilayered structure, consisting ofinterfacial, intermediate and core water. The interfacial layer iscomposed of water molecules that are directly bounded by polarhead groups of a surfactant; the intermediate layer consists ofthe next few nearest-neighbor water molecules that canexchange their state with interfacial water; and the core layeris found at the interior of the ‘water pool’ and has the propertiesof bulk water [57]. Depending on the size of reverse micelles,available water may have significantly different solventproperties, ranging from highly structurized interiors with littlemolecular mobility [58,59] to free water cores that approximatebulk water solvent characteristics.

Different water structures may also dissolve differentamounts of gases, which can drastically influence the reactionpathways, particularly in the cases where oxidation or reductionreactions by means of dissolved gases comprise crucial steps inpreparation procedures, as the numerous cases of ferric oxidesand complex corrosion phenomena may illustrate [1]. Theaccumulated gases are significantly present at hydrophobicinterfaces [60] comparing to the typical range of dissolved gasconcentration in water at normal pressure and temperature(∼5 ·10−3 M). Fine variations in the experimental outcomesdepending on the gas effects have been noticed [53], and therewere cases where certain effects, which depended on manyparameters, disappeared on removal of the dissolved gas [60].

Interfacial self-association mechanisms can also be quitedifferent depending on the surface wettability. As a biologicalexample, the rate of blood coagulation tends to increase with anincrease in water-wettability of the tube surface [61]. Also, self-assembly processes that occur during the drying steps ofsynthesis procedures involve complex competition between thekinetics of evaporation and the time scales with which solvatednanoparticles diffuse on a substrate, and due to the specific roleof hydrophobic interactions and a variety of ways to nucleateevaporation may lead to unexplored territories in the field ofnovel design [42]. Anyhow, treating water as a continuummedium in both theoretical approaches (such as in theframework of DLVO theory) and explanation of experiments,altogether with disregarding its fine interactions with gases,salts and electromagnetic fields may in future indeed cause everincreasing difficulties in attempts to explain fine variations fromthe ranges of expected results.

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

As far as the current state-of-the-art is concerned, it isexceedingly difficult to predict the outcomes of experimentalsettings aimed to produce novel fine structures and morphol-ogies by means of reverse-micellar methodology, and the mostattractive results in this practical field come from trial-and-errorapproaches. There are many evidences that slight changes in thelimiting conditions of particle synthesis experiments canproduce significant differences as the end results [1]. Thefollowing examples may illustrate such a proposition and enrichone's belief in crucial sensitivity and subtleness of the materialdesign procedures that involve wet environments and colloidalphenomena in general.

Replacement of manganese ions with nickel ions in an ex-periment of reverse-micellar precipitation synthesis of a mixedzinc–ferrite resulted in the production of spherical particles inthe former case [62] and acicular ones in the latter [63,64].When bromide ions of cetyltrimethylammoniumbromide(CTAB) surfactant were in a synthesis of barium-fluoridenanoparticles replaced by chloride ions (CTAC), identity of thefinal product was no longer the same, whereas a replacement of2-octanol with 1-octanol significantly modified crystallinity ofthe obtained powder [35]. Various choice of precipitation agentscan often result in distinctive morphologies obtained [65,66].

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

It has been evidenced that even the method of stirring insome of the microemulsion-assisted procedures of preparationof nanoparticles can have decisive influence on some of thefinal particle properties. Thus, using a magnetically coupled stirbar during an aging of a dispersion of particles influencedcrystal quality and in some cases resulted in a different crystalstructure as compared with non-magnetically agitated solutions[40]. In case of a synthesis of organic nanoparticles in reversemicelles, the use of magnetic stirrer led to the formation ofnanoparticles larger in size comparing to the particles obtainedwith using ultrasound bath as a mixer, even though no changesin particle size were detected on varying solvent type,microemulsion composition, reactant concentrations and evengeometry and volume of the vessel [67].

Changes in the sequence of introduction of individualcomponents within a precursor colloid system could result indifferent properties of the final reaction products [68]. Such aproperty is directly related to the fact that microemulsions, likeall colloid systems, do not present thermodynamically equilib-rium phases that spontaneously form, but are thermodynami-cally unstable and only due to the existence of large interfacialenergies that are stronger than thermal energy, kT, their order ispreserved.

Changes in size of a volume where the particle preparationprocesses take place – as occurs when the transition from small-scale research units to larger industrial vessels is attempted –can lead to extensive variations in some of the properties of thesynthesized material [69]. For instance, absorptivity of

Fig. 2. X-ray diffraction patterns of the powders synthesized by the same precipitation procedure, with (upper) and without (lower) reverse-micellar microemulsion.The peaks denoted with an S are ferrite-derived spinel reflections, whereas the peaks denoted with a D are δ-FeOOH-derived.

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cadmium-sulfide particles dramatically changed when theamounts of the microemulsions used in the synthesis procedurewere tripled [70]. Also, when two same chemical procedures fora colloid synthesis of nanoparticles were performed in closedand open, otherwise identical vessels, perfectly uniformspherical particles were yielded in the former case, wherebyelongated particles of similarly narrow size distribution wereproduced in the latter [69].

Numerous examples of unexpected effects of reverse micelleson the kinetics of encapsulated reactions may be provided as well.As amatter of fact, whereas kinetic conditions in ordinary solutionsmay reasonably be approximated as continuous, dynamics ofsolvation effects and reaction kinetics can – depending on thestructure of the microheterogeneous colloid system – largely varyin different local microenvironments, effectively producing sig-nificantly complex outcomes. Slight changes in micellar dispersitytowards wider polydisperse distributions have, for instance, beenshown as capable of triggering the processes of Ostwald ripeningof the colloid particles that result in complete phase segregation[71]. The dynamics of solvation effects can drastically changewith an interfacial distance, which may prove to be a significanteffect in the cases of chemical reactions performed within micellaraggregates.

The rate constants of chemical reactions performed withinmicellar aggregates include the effects of Brownian diffusion ofreverse micelles, droplet collision, water channel opening,complete or partial merging of micelles, diffusion of reactantsand the chemical reaction, as well as fragmentation of transientdimers or multimers (wherein the slowest step determines thetemporal aspect of the overall process of synthesis) [72], rang-ing from the order of magnitude of nanoseconds for diffusion-controlled intermicellar reaction to an order of miliseconds forintermicellar exchange of reactants [43]. However, despite thefact that dynamic response in colloid systems is typically muchslower compared to their bulk counterparts [28], extremely fastresponses may be favorable under certain conditions, as can beillustrated by numerous examples of catalytic effects producedby the influence of micellar encapsulation [32,54,55,73] and

exchange [43,74,75] of reactants. For example, the rate con-stant of the hydrolysis of acetylsalicylic acid in the presence ofimidazole catalyst increased by 55 times when the reaction wasperformed in AOT/supercritical ethane microemulsion com-pared to the aqueous buffer [74]. Numerous other AOT-basedmicroemulsions have been shown to possess catalytic effectsupon particular hydrolysis reactions [75]. It has also beenreported that the rate of oxidation of Fe2+ and a subsequentformation of needle-shaped FeOOH particles by spontaneousair oxidation is from 100 to 1000 times faster in reverse micellesthan in a bulk solution, regardless of the differences insurfactant or other conditions [73]. In the case of certain ironcomplexes, a two to tenfold increase in the rate of dissociationwas correspondingly measured in comparison to pure aqueoussolution [32].

4. The example of nickel–zinc ferrite

When the chemical procedure of preparation of δ-FeOOH isperformed in the presence of CTAB/1-hexanol/water reverse-micellar microemulsion of particular composition, nickel–zincferrite is obtained instead [53], as can be observed from Fig. 2.Faster rates of oxidation and slower rates of precipitationwhen thesynthesis is performed in reverse micelles rather than in bulkconditions, are suggested as the reason for the difference inchemical identities of the final products. The reason for the fasterrate of Fe2+ oxidation in reverse micelles compared to the bulkconditions might lie in the atypical structure of water as a solventin reverse micelles. Oxidation of initial Fe2+ ions is generallyregarded as the first step in nucleation of precipitating, ferrite orferric-oxide phases [76]. It was suggested that the increase inhydrogen bonding between water molecules in a thin layerneighboring to surfactants may favor the transfer of electronsfrom Fe2+ to Fe3+ by a tunnelling effect [54], whereby the excesselectrons will be consumed in aqueous solution to producehydroxide ions in the presence of dissolved oxygen. The oxidationof Fe2+ with the decomposition of H2O is, by consideringthermodynamic data, proven to be an energetically favorable

Fig. 3. XRD patterns of the as-dried powder synthesized by hydroxide co-precipitation procedure in solution (a), of the same powder calcined at 450 °C (b) and 600 °C(d) for 2 h in air, and of the sample co-precipitated within hydroxide approach in reverse micelles and calcined at 450 °C in air for 2 h (c).

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process [77], and the solvent properties of reverse-micellar watermay significantly influence the process of oxidation and,therefore, the crystallization of novel ferric-oxide phases.

We have previously shown that slight changes in thecomposition of the parent reverse-micellar microemulsionmay result in significantly changed physical properties of theprepared powders [78]. In case of the investigated synthesis ofnickel–zinc ferrite particles of specific composition, weunexpectedly arrived to areas in the phase diagram of CTAB/1-hexanol/water microemulsion where drastic increases inspecific magnetization resulted from otherwise identicalpreparation procedures [79]. A material with average particlesize of 10 nm and specific magnetization of 50 emu/g (which isabout two-thirds of the magnetization that sintered andcommercial nickel–zinc ferrites possess), was prepared byemploying such a technique at almost the room temperaturewith less than an hour of aging time [79]. This effect wasexplained by referring to the particular composition of theparent microemulsion employed, wherein micellar percolationeffects that led to efficient redistribution of micellar contentswere pronounced. Depending on whether the encapsulatedreactions were initiated by diffusion of one of the reactantsthrough the oil phase or by collision, merging and micellarcontent exchange, the final product could end up with havingsignificantly different properties [64,78,79]. Similar as in thefield of evaluation of environmental and toxicological effects ofnanoparticles where small variations in chemical structure orparticle size may lead to drastic differences in the investigatedoutcomes [80], the formulations of overgeneralized concepts inthe field of reverse-micellar synthesis of materials are proven asexceedingly difficult in light of such sensitivity of the finaloutcomes upon seemingly negligible variations in the initialconditions of the synthesis experiments.

5. The example of lanthanum–strontium manganite

The following example related to reverse micelle-assistedpreparation of lanthanum–strontium manganites may offer

significant insights into how different mechanisms of formationof identical compounds may proceed with and without thepresence of reverse micelles [81]. Similar as in the case of nickel–zinc ferrite, performing identical chemical procedures in bulkconditions and in the presence of reverse micelles resulted indifferent chemical identities of the final powders. Whereasprecipitation of precursor cations in the form of oxalates fromaqueous solutions was limited by the formation of [Mn(C2O4

2−)NO3

−] coordination complexes (hence aqueous–alcoholic solu-tions had to be employed), such an effect was absent whenidentical reaction was performed within reverse micelles ofCTAB/1-hexanol/water microemulsion. Whereas strong bases,such as NaOH, could in aqueous solution yield precipitate thatwould form the desired monophase manganite upon annealing,and weak bases, such as (CH3)4NOH, could not raise pH tosufficient level that would induce the subsequent solid-stateformation of manganite compound, completely different situationwas observed in the case of precipitation in reverse micelles.Whereas strong bases led to disruption ofmicroemulsion structureand phase segregation, the use of (CH3)4NOH as precipitatingagent resulted in sufficiently high pH levels that favored thecomplete precipitation of cations and eventual formation of puremanganite products.

The difference in the annealing mechanism of the formationof bulk-prepared and microemulsion-assisted-prepared LaSr-manganite powders – after the precursor cations wereprecipitated in forms of hydroxides [82] – can be observed bycomparing the X-ray diffraction (XRD) patterns presented inFig. 3. Whereas in case of the bulk synthesis, the growth ofSrCO3 crystallites comprising the as-dried powder as well as thetransformation of La(OH)2 into La2O2CO3 is evident fromcomparing the XRD patterns (a) and (b), the transformation ofqualitatively identical as-dried powder as prepared in micro-emulsion into an amorphous, more homogeneous transientcomposition, is obvious by comparing XRD patterns (a) and (c).Both powders after heating for 2 h in air at ≥600 °C yieldmanganite perovskite samples. However, whereas the changesin crystal structure, going from tetrahedral to orthorombic

Fig. 4. Dependencies of the average particle size (d) and crystal lattice parameter (a) on the calcination temperature in the bulk manganite-synthesis case (left), and ofthe average particle size vs. calcination temperature for the sample synthesized by using reverse micelles (right).

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followed by the increase in La stoichiometric proportion (due togradual incorporation of La3+ from oxycarbonate transientcompound into the manganite phase) and the decrease in Mnproportion (due to the compensation of charges), with XRD-determined average particle size kept constant (Fig. 4a), arenoticed with the further increase in the temperature ofcalcination in case of the bulk-synthesized sample, a linearincrease in average particle size with calcination temperature(the mean value of crystal lattice parameter being constant at0.5474 nm) is noticed in case of the microemulsion-assisted-synthesized sample (Fig. 4b), obviously due to more homoge-neous re-crystallization processes inherent in the annealingtransformation of the latter as-dried composition into themanganite phase. Therefore, besides different mechanisms ofmanganite formation up to 600 °C, the effect of the furtherlinear increase in magnetization with annealing temperature(observed in both cases) is attributed to thoroughly differentmechanisms: rearrangement of crystal structure in the bulk–

Fig. 5. Normalized XRD patterns of the samples synthesized using oxalate co-precip500 °C (a, b), 700 °C (c) and 1000 °C (d) for 2 h in air.

synthesis case, and grain growth in the microemulsion–synthesis case.

In case of the synthesis of the same compound byprecipitation of precursor cations in form of oxalates, thecomparison between microemulsion-assisted and the bulk caseyields thoroughly opposite observations [83]. Namely, theprocess of the manganite formation follows more homogeneousroute when the approach in the bulk solution is followed,comparing to the microemulsion-assisted procedure. In case ofthe bulk synthesis, a mostly amorphous transient structure isdetected at 500 °C (Fig. 5a), whereby after annealing at thesame conditions, transient phases of La2O2CO3 and cubicMn2O3 are detected in case of the microemulsion synthesis(Fig. 5b). The formation of the manganite is completed after theheat treatment at ≥1000 °C in case of the latter approach(Fig. 5d), whereby 700 °C is proven to be sufficient temperaturefor the desired manganite formation in case of the synthesis inhydroalcoholic solution (Fig. 5c).

itation approach in bulk solution (a, c) and in reverse micelles (b, d), annealed at

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6. Correlations with the biological context

Reverse micelles have recently been proposed as candidatesfor the most primitive membranes that hosted the first planetaryself-replicating chemical reactions that became the precursors ofliving processes in the evolution of life [84]. Apart from the useof reverse micelles in preparative organic chemistry forcompartmentalization and selective solubilization of reactants,separation of products [55] and phase transfer [85], they havebeen used in the field of biochemistry both for storing bioactivechemical reagents [86] and as catalyzers [87,88] or inhibitors[89] of biochemical enzyme-driven reactions. Because encap-sulating a protein in a reverse micelle and dissolving it in a low-viscosity solvent can lower the rotational correlation time of aprotein and thereby provide a strategy for studying proteins inversatile environments [90], reverse micelles are used as a cellmembrane-mimetic medium for the study of membraneinteractions of bioactive peptides [91]. The observations thatdenaturation of proteins can be prevented in reverse micelles[92] have spurred even more interest in the application of theseself-organized multi-molecular assemblies as either drug-delivery carriers or life-mimicking systems [93]. Such abiomimetic role of reverse micelles has been further instigatedby the discovery of possibility of initiating self-replication ofreverse micelles due to reactions occurring within micellarstructures [94,95]. As a matter of fact, positioning reversemicelles right at the interface between the domains of ‘living’and ‘non-living’ may present a crucial shift in improvedunderstanding of their function and bioimitative utilization ofsuch knowledge for practical purposes.

Such a widening shift in understanding of the roles of reversemicelles in materials synthesis experiments goes together withthe current trend of thinking according to which neither lipidmembranes are seen anymore as passive matrices for hostingbiomolecular reactions [96], confirming that cellular activitiesare in large extent controlled by lipids in addition toconventional protein-governed mechanisms [97]. Although itis known that chemical self-replication reactions need a sort ofprotection membrane to selectively absorb the influences of theenvironment, that is to say require “a sophisticated cradle to belulled in” [98], how these protective mesophases indeed ‘sing’presents a challenge for the future investigations.

Knowing that by actively regulating the flow of chemicalsbetween the cell and its surroundings and conducting electricimpulses between nerve cells, biological membranes play a keyrole in cell metabolism and transmission of information withinan organism highlights the practical significance of investiga-tions oriented towards reproducing or at least approaching areproduction of such an organizational complexity in artificialcolloid systems. Also, knowing that malignant cells have sig-nificantly different surface properties comparing to normal cells[99], maybe the transition of focus in apoptosis research awayfrom the genetic code disruption as the sole key influencetowards information transmission mechanisms that involvemembrane mediation would herein beneficially switch themajor scope to the cellular epigenetic network and finally tomore holistic biological and biomedical perspectives. Such an

integrative view at cellular structures may be further instigatedby the recent findings that a large percentage of body cells(cardiac muscle cells, in particular) is, similar as the aforemen-tioned reverse micelle model [28], in a ‘membrane-wounded’state, suggesting that continuous protective barrier is notessential for cell functioning [100]. Also, if the cytoplasmaticmedium is, instead as an ordinary solution, considered as acolloid gel, rich with interfaces between water and intracellularproteins, polysaccharides, nucleic acids and lipid membranes,then an array of interesting characteristics related to water-retaining properties of cellular gel matrices may be reasonablyarrived to, similar as in the case of uninvestigated influenceof unusual structure and solvent properties of water confinedin reverse-micellar regions.

Both self-organization phenomena in living organisms andself-assembly effects of amphiphilic mesophases are governedby multiple weak interactions, such as hydrogen bonds, hydro-phobic and hydrophilic interactions, van der Waals forces, saltbridges, coordination complexes (forces involving ions and li-gands, i.e. ‘coordinate–covalent bonds’), interactions amongπ-electrons of aromatic rings, chemisorption, surface tension,and gravity [101]. Whereas the traditional field of chemistrydeveloped by understanding the effects of covalent, ionic andmetallic bonding forces, an extension of the same approach toweak intermolecular forces is nowadays suggested as a naturaldirection for achieving future prosperity within the practicalaspect of the field of chemistry [102]. With attaching a moresignificant role to reverse micelles in the prospect of advancedstructural design, a general shift towards approaching morecomplex supramolecular architectures may be expected in thisarea of research and utilization of self-assembly phenomenaas well.

7. Future directions in the application of reverse micelles

As far as the future directions in the application of reversemicelles in the field of materials synthesis are concerned, thefollowing approaches may be outlined. Because of theemphasized uniqueness of particular designed structures andcompositions within specific parent microemulsions, thedevelopment and application of highly specific and growth-directing surfactants especially suitable for particular chemicalcompositions, crystal structures and intended morphologiesmay be expected in future [103]. In any case, the futureprosperity in the use of reverse micelles and microemulsions forinducing practical self-assembly phenomena depends on thecombined synergetic efforts of application of basic principles ofcolloid chemistry (mostly based on the simple framework ofDLVO theory), trial-and-error approaches, employment ofdiverse advanced microscopy techniques, and theoreticalprediction of specific molecular recognition effects.

Unlike ordinary emulsions, microemulsions do not requirehigh shear rates for their formation and may due to potentialexistence of fine and diverse metastable colloid states exhibit awide range of inherent multi-molecular configurations [104],including either regular or reverse micelles of various ovalshapes (spherical, cylindrical, rod-shaped), vesicular structures,

31V. Uskoković, M. Drofenik / Advances in Colloid and Interface Science 133 (2007) 23–34

bilayer (lamellar) and cubic liquid crystals, columnar, mesh andbicontinuous mesophases, cubosomes (dispersed bicontinuouscubic liquid crystalline phase), sponge phases, hexagonal rod-like structures, spherulites (radially arranged rod-like micelles),multiphase–substructured configurations (such as water in oil inwater droplets, for example), highly percolated pearl likestructures, supra aggregates that comprise various substructuredcombinations of microemulsion aggregates, as well as numer-ous transient configurations. The application of complex nonequilibrium phases and such transient configurations betweenreverse-micellar and various other inherent multi-molecularself-assembled entities could provide the basis for growth ofnumerous attractive novel morphologies. As a matter of fact,each particular point in a microemulsion phase diagramcorresponds to specific local conditions for physico-chemicaltransformations that take place therein and result in uniquematerial structures ‘templated’ in each of these cases [78].

Combinations of reverse-micellar or any other microemul-sion method of synthesis with other processing methods,including hydrothermal synthesis [40], ultrasonic and UV irra-diation [105,106], pH-shock wave method [107] and flame-spraying [108], have been investigated. Merging of two ormore preparation techniques into one can due to synergy ef-fects lead to multiple advantages, such as improved control ofthe stoichiometry of the final product (as adopted from sol–gelmethod) and extremely fine and controllable grain size (asacquired from reverse-micellar synthesis) in the examples[109,110] of the combination of sol–gel and reverse-micellarapproaches to materials synthesis. Low yields, surfactant-contamination and difficulties arising out of the attempts toseparate precipitated powders in the form of non-agglomeratedparticles – serious obstacles of the microemulsion-assistednanoparticle preparation procedures – were overcome byfeedstocking flame-spraying apparatus with nanoparticlestogether with their parent microemulsion [108], at the sametime transcending poor control of particle size and shape, whichis a typical drawback of the conventional flame-sprayingmethods of synthesis. Combining reverse-micellar synthesis ofcobalt particles with their subsequent evaporating deposition inexternal magnetic field led to the formation of large-scale 3Dsuperlattices of cobalt nanocrystals [111]. Silver nanorodsencapsulated by polystyrene were prepared by combination ofreverse-micellar, gas antisolvent, and ultrasound techniques[112], whereby specific carbon nanotubes were prepared bydirect introduction of in situ prepared catalytically active Co andMo particles by a reverse-micellar method [113,114].

Both weak soft-tech potentials for structuring self-assembledproducts into functional systems of hierarchical organizationand inherent limitations in the resolution of lithographictechniques in nanostructural design may be overcome byconstructive coupling of the soft-tech production of fine-structured materials with hard-tech assembly methodologies.Excellent achievements have been recently reported by relyingon such an approach of combining ‘bottom–up’ and ‘top–down’ methodologies [46,115–118]. Langmuir–Blodgett films[119], obtained by coupling self-assembled orientation ofmolecules at air–water interfaces with a technique for their

deposition on solid substrates, present a classic example of suchcomplementary synthesis/processing methodology. In thatsense, layer-by-layer (LbL) techniques comprising adsorptionof oppositely charged polyelectrolytes on a solid surface ofsynthesized particles in reverse micelles were used to overcomedifficulties arising out of the inabilities to carry out sequentialreactions inside the same reverse micelles in order to obtainmultilayered composites [120].

The assembling of particles formed in the processes ofreverse-micellar and, in general, microemulsion-assisted syn-theses into precisely tailored, supra-nanocrystalline 3D struc-tures, presents an important challenge, whereas in situ reactionsin well-organized amphiphilic matrices present only one steptowards this goal [39]. Self-assembly parallelism and theselective patterning precision of lithographic and etchingtechniques can be united in a multitude of hybrid techniquesfor the production of fine structures [121]. External fields, suchas electric and magnetic fields, heat gradients or single layershearing, can induce unexpected orderings depending on theintensities and directions of the field relative to the suspensioncell [122], and may be used to hierarchically organize particlesinto 3D matrices. On the other hand, electrospraying, electro-coalescence and other methods that involve various externalfields, may be used for ink-jet spraying, fluid atomization, phaseand particle separation, thus improving the functionalizationalcontrol of the self-assembled fine structures [123].

To sum up, reverse-micellar and other microemulsionalsystems can provide complex interfaces that can support parallelreactions leading to surprisingly complex outcomes, and theirrelatively stable existence in thermodynamically metastable statescan support significant modifications of the product structures bythe pure influence of aging treatment. However, small yieldsobtained due to employing extremely small concentrations andexpensively complex environments used in most of the cases,altogether with the fact that increasing the space of options forproduction of various end results via extremely fine variations ofcertain experimental parameters comes at the price of increasedsensitivity of initial experimental settings that lead to reproducibleoutcomes, provide implicit difficulties within such an approach toadvancedmaterials synthesis. The future prosperity of applicationof reverse micelles, microemulsions and other self-assemblingamphiphilic matrices in advanced structural design will in largeextent depend on the successful global balancing of these prosand cons.

8. Conclusions

The presented results may suggest that the role that re-verse micelles play in ‘parenting’ materials formation pro-cesses is more intricate than purely ‘templating’ one. Reversemicelles have been shown as capable of significantly mod-ifying the reaction pathways that take place in their presence.Instead of being considered as chemically inert nano-reactors,reverse micelles may be regarded as complex multi-molecularentities that could be under specific conditions activelyengaged in the chemical pathways of the formation of givenmaterials.

32 V. Uskoković, M. Drofenik / Advances in Colloid and Interface Science 133 (2007) 23–34

The application of reverse micelles for materials synthe-sis purposes could be, therefore, acknowledged in part as abiomimicking approach to advanced structural design. Thepresented examples of pronounced sensitivity of processes thatemploy reverse-micellar effects in materials processing mayinitiate an apprehension of their active physico-chemical role,presumably similar to the primitive biological membranes.Reverse micelles can arise as an important step for the practicalfield of colloid science oriented towards reaching highly organizedand ultra-sensitive functional structures and ‘templating’ environ-ments. The consequence of such convergence between biologicalfeatures and self-assembly design is that with increasing thecomplexity of advanced nanofunctional devices, an increasedsensitivity of the intended products, manifested either asirreproducibility of synthesis procedures or exceptional functionalsensitivity towards slightest environmental effects, will startappearing as a significant problem. However, knowing that everyadvantageous challenge always has its risky side as well, such anintricate situation could be,with a lot of effort involved, turned intoan optional range of convenient and potentially fruitful outcomes.

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