doi:10.1016/j.cis.2007.02.002Advances in Colloid and Interf
Reverse micelles: Inert nano-reactors or physico-chemically active guides of the capped reactions
Vuk Uskokovi , Miha Drofenik
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 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)ing’ chemical reactions with an aim to produce ‘templated’ 2D 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
Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 2. The need to reevaluate the functional representation of reverse micelles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 3. Examples of chemical ‘butterfly effects’ in reverse micelle-assisted syntheses . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 4. The example of nickel–zinc ferrite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 5. The example of lanthanum–strontium manganite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 6. Correlations with the biological context . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 7. Future directions in the application of reverse micelles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 8. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
1. Introduction
Corresponding author. E-mail address: vuk21@yahoo.com (V. Uskokovi).
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 [3–8] to semiconductor quantum dots [9–11] to various ceramic materials [12–16], silica and gold coated nanoparticles [17–21], latexes and polymer composites [22–24], double-layered nanoparticles [25] and even superconducting materials [26,27] have been prepared since 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 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.)
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explanation models that are typically invoked in the frame of such experiments ordinarily refer to purely ‘templating’ role 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 ‘templating’ effect 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 nano- droplets, 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 cetyl- trimethylammonium 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 AOT- based 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 [30,35– 37], 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 interac- tions 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 ‘templating’ as 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.
Table 1 Macroscopic and nanoscopic variables in the microemulsion-assisted and particularly reverse-micellar synthesis of nanoparticles
Macroscopic parameters Nano-sized parameters
Static, size and shape distribution of micelles
Microemulsion composition
Aggregation number
Water-to-surfactant molar ratio
Dynamic interaction, rates and types of merging and dissociation of micelles
pH
Dissolved species concentrations 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
hydrophobic interactionsMethod and rate of stirring Homogeneous or
heterogeneous nucleation Screening length
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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 manipu- lated 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 microenviron- ments, 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 inter- actions 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 hydro- phobic 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 interac- tion 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]), ‘templating’ of 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 ‘cater- pillars’ or 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 reverse- micellar media [2], it was observed that size of the prepared platinum, rhodium, palladium and iridum particles was always in the range of 2–5 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
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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 solid–liquid interfaces in general, due to surface- induced 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 pool’ and 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 ·10−3 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, self- assembly 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 morphol- ogies 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 ex- periment of reverse-micellar precipitation synthesis of a mixed zinc–ferrite 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 equilib- rium phases that spontaneously form, but are thermodynami- cally 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 small- scale 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
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 the amounts of the microemulsions used in the synthesis procedure were tripled [70]. Also, when two same chemical procedures for a colloid synthesis of nanoparticles were performed in closed and open, otherwise identical vessels, perfectly uniform spherical particles were yielded in the former case, whereby elongated particles of similarly narrow size distribution were produced in the latter [69].
Numerous examples of unexpected effects of reverse micelles on the kinetics of encapsulated reactions may be provided as well. As amatter of fact, whereas kinetic conditions in ordinary solutions may reasonably be approximated as continuous, dynamics of solvation effects and reaction kinetics can – depending on the structure of the microheterogeneous colloid system – largely vary in different local microenvironments, effectively producing sig- nificantly complex outcomes. Slight changes in micellar dispersity towards wider polydisperse distributions have, for instance, been shown as capable of triggering the processes of Ostwald ripening of the colloid particles that result in complete phase segregation [71]. The dynamics of solvation effects can drastically change with an interfacial distance, which may prove to be a significant effect in the cases of chemical reactions performed within micellar aggregates.
The rate constants of chemical reactions performed within micellar aggregates include the effects of Brownian diffusion of reverse micelles, droplet collision, water channel opening, complete or partial merging of micelles, diffusion of reactants and the chemical reaction, as well as fragmentation of transient dimers or multimers (wherein the slowest step determines the temporal 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 for intermicellar exchange of reactants [43]. However, despite the fact that dynamic response in colloid systems is typically much slower compared to their bulk counterparts [28], extremely fast responses may be favorable under certain conditions, as can be illustrated by numerous examples of catalytic effects produced by 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 of imidazole catalyst increased by 55 times when the reaction was performed in AOT/supercritical ethane microemulsion com- pared to the aqueous buffer [74]. Numerous other AOT-based microemulsions have been shown to possess catalytic effects upon particular hydrolysis reactions [75]. It has also been reported that the rate of oxidation of Fe2+ and a subsequent formation of needle-shaped FeOOH particles by spontaneous air oxidation is from 100 to 1000 times faster in reverse micelles than in a bulk solution, regardless of the differences in surfactant or other conditions [73]. In the case of certain iron complexes, a two to tenfold increase in the rate of dissociation was correspondingly measured in comparison to pure aqueous solution [32].
4. The example of nickel–zinc ferrite
When the chemical procedure of preparation of δ-FeOOH is performed in the presence of CTAB/1-hexanol/water reverse- micellar microemulsion of particular composition, nickel–zinc ferrite is obtained instead [53], as can be observed from Fig. 2. Faster rates of oxidation and slower rates of precipitationwhen the synthesis is performed in reverse micelles rather than in bulk conditions, are suggested as the reason for the difference in chemical identities of the final products. The reason for the faster rate of Fe2+ oxidation in reverse micelles compared to the bulk conditions might lie in the atypical structure of water as a solvent in reverse micelles. Oxidation of initial Fe2+ ions is generally regarded as the first step in nucleation of precipitating, ferrite or ferric-oxide phases [76]. It was suggested that the increase in hydrogen bonding between water molecules in a thin layer neighboring to surfactants may favor the transfer of electrons from Fe2+ to Fe3+ by a tunnelling effect [54], whereby the excess electrons will be consumed in aqueous solution to produce hydroxide ions in the presence of dissolved oxygen. The oxidation of Fe2+ with the decomposition of H2O is, by considering thermodynamic 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 water may significantly influence the process of oxidation and, therefore, the crystallization of novel ferric-oxide phases.
We have previously shown that slight changes in the composition of the parent reverse-micellar microemulsion may result in significantly changed physical properties of the prepared powders [78]. In case of the investigated synthesis of nickel–zinc ferrite particles of specific composition, we unexpectedly arrived to areas in the phase diagram of CTAB/ 1-hexanol/water microemulsion where drastic increases in specific magnetization resulted from otherwise identical preparation procedures [79]. A material with average particle size of 10 nm and specific magnetization of 50 emu/g (which is about two-thirds of the magnetization that sintered and commercial nickel–zinc ferrites possess), was prepared by employing such a technique at almost the room temperature with less than an hour of aging time [79]. This effect was explained by referring to the particular composition of the parent microemulsion employed, wherein micellar percolation effects that led to efficient redistribution of micellar contents were pronounced. Depending on whether the encapsulated reactions were initiated by diffusion of one of the reactants through the oil phase or by collision, merging and micellar content exchange, the final product could end up with having significantly different properties [64,78,79]. Similar as in the field of evaluation of environmental and toxicological effects of nanoparticles where small variations in chemical structure or particle size may lead to drastic differences in the investigated outcomes [80], the formulations of overgeneralized concepts in the field of reverse-micellar synthesis of materials are proven as exceedingly difficult in light of such sensitivity of the final outcomes upon seemingly negligible variations in the initial conditions of the synthesis experiments.
5. The example of lanthanum–strontium manganite
The following example related to reverse micelle-assisted preparation of lanthanum–strontium manganites may offer
significant insights into how different mechanisms of formation of identical compounds may proceed with and without the presence of reverse micelles [81]. Similar as in the case of nickel– zinc ferrite, performing identical chemical procedures in bulk conditions and in the presence of reverse micelles resulted in different chemical identities of the final powders. Whereas precipitation of precursor cations in the form of oxalates from aqueous 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 when identical reaction was performed within reverse micelles of CTAB/1-hexanol/water microemulsion. Whereas strong bases, such as NaOH, could in aqueous solution yield precipitate that would form the desired monophase manganite upon annealing, and weak bases, such as (CH3)4NOH, could not raise pH to sufficient level that would induce the subsequent solid-state formation of manganite compound, completely different situation was observed in the case of precipitation in reverse micelles. Whereas strong bases led to disruption ofmicroemulsion structure and phase segregation, the use of (CH3)4NOH as precipitating agent resulted in sufficiently high pH levels that favored the complete precipitation of cations and eventual formation of pure manganite products.
The difference in the annealing mechanism of the formation of bulk-prepared and microemulsion-assisted-prepared LaSr- manganite powders – after the precursor cations were precipitated in forms of hydroxides [82] – can be observed by comparing the X-ray diffraction (XRD) patterns presented in Fig. 3. Whereas in case of the bulk synthesis, the growth of SrCO3 crystallites comprising the as-dried powder as well as the transformation of La(OH)2 into La2O2CO3 is evident from comparing the XRD patterns (a) and (b), the transformation of qualitatively identical as-dried powder as prepared in micro- emulsion into an amorphous, more homogeneous transient composition, is obvious by comparing XRD patterns (a) and (c). Both powders after heating for 2 h in air at ≥600 °C yield manganite perovskite samples. However, whereas the changes in 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 of the 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 to gradual incorporation of La3+ from oxycarbonate transient compound into the manganite phase) and the decrease in Mn proportion (due to the compensation of charges), with XRD- determined average particle size kept constant (Fig. 4a), are noticed with the further increase in the temperature of calcination in case of the bulk-synthesized sample, a linear increase in average particle size with calcination temperature (the mean value of crystal lattice parameter being constant at 0.5474 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 annealing transformation of the latter as-dried composition into the manganite phase. Therefore, besides different mechanisms of manganite formation up to 600 °C, the effect of the further linear increase in magnetization with annealing temperature (observed in both cases) is attributed to thoroughly different mechanisms: rearrangement of crystal structure in the bulk–
Fig. 5. Normalized XRD patterns of the samples synthesized using oxalate co-precip 500 °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 by precipitation of precursor cations in form of oxalates, the comparison between microemulsion-assisted and the bulk case yields thoroughly opposite observations [83]. Namely, the process of the manganite formation follows more homogeneous route when the approach in the bulk solution is followed, comparing to the microemulsion-assisted procedure. In case of the bulk synthesis, a mostly amorphous transient structure is detected at 500 °C (Fig. 5a), whereby after annealing at the same conditions, transient phases of La2O2CO3 and cubic Mn2O3 are detected in case of the microemulsion synthesis (Fig. 5b). The formation of the manganite is completed after the heat treatment at ≥1000 °C in case of the latter approach (Fig. 5d), whereby 700 °C is proven to be sufficient temperature for the desired manganite formation in case of the synthesis in hydroalcoholic 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 candidates for the most primitive membranes that hosted the first planetary self-replicating chemical reactions that became the precursors of living processes in the evolution of life [84]. Apart from the use of reverse micelles in preparative organic chemistry for compartmentalization and selective solubilization of reactants, separation of products [55] and phase transfer [85], they have been used in the field of biochemistry both for storing bioactive chemical 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 a protein and thereby provide a strategy for studying proteins in versatile environments [90], reverse micelles are used as a cell membrane-mimetic medium for the study of membrane interactions of bioactive peptides [91]. The observations that denaturation of proteins can be prevented in reverse micelles [92] have spurred even more interest in the application of these self-organized multi-molecular assemblies as either drug- delivery carriers or life-mimicking systems [93]. Such a biomimetic role of reverse micelles has been further instigated by the discovery of possibility of initiating self-replication of reverse micelles due to reactions occurring within micellar structures [94,95]. As a matter of fact, positioning reverse micelles right at the interface between the domains of ‘living’ and ‘non-living’ may present a crucial shift in improved understanding of their function and bioimitative utilization of such knowledge for practical purposes.
Such a widening shift in understanding of the roles of reverse micelles in materials synthesis experiments goes together with the current trend of thinking according to which neither lipid membranes are seen anymore as passive matrices for hosting biomolecular reactions [96], confirming that cellular activities are in large extent controlled by lipids in addition to conventional protein-governed mechanisms [97]. Although it is known that chemical self-replication reactions need a sort of protection membrane to selectively absorb the influences of the environment, that is to say require “a sophisticated cradle to be lulled in” [98], how these protective mesophases indeed ‘sing’ presents a challenge for the future investigations.
Knowing that by actively regulating the flow of chemicals between the cell and its surroundings and conducting electric impulses between nerve cells, biological membranes play a key role in cell metabolism and transmission of information within an organism highlights the practical significance of investiga- tions oriented towards reproducing or at least approaching a reproduction of such an organizational complexity in artificial colloid 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 away from the genetic code disruption as the sole key influence towards information transmission mechanisms that involve membrane mediation would herein beneficially switch the major scope to the cellular epigenetic network and finally to more holistic biological and biomedical perspectives. Such an
integrative view at cellular structures may be further instigated by 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 not essential for cell functioning [100]. Also, if the cytoplasmatic medium is, instead as an ordinary solution, considered as a colloid gel, rich with interfaces between water and intracellular proteins, polysaccharides, nucleic acids and lipid membranes, then an array of interesting characteristics related to water- retaining properties of cellular gel matrices may be reasonably arrived to, similar as in the case of uninvestigated influence of unusual structure and solvent properties of water confined in reverse-micellar regions.
Both self-organization phenomena in living organisms and self-assembly effects of amphiphilic mesophases are governed by multiple weak interactions, such as hydrogen bonds, hydro- phobic and hydrophilic interactions, van der Waals forces, salt bridges, coordination complexes (forces involving ions and li- gands, i.e. ‘coordinate–covalent bonds’), interactions among π-electrons of aromatic rings, chemisorption, surface tension, and gravity [101]. Whereas the traditional field of chemistry developed by understanding the effects of covalent, ionic and metallic bonding forces, an extension of the same approach to weak intermolecular forces is nowadays suggested as a natural direction for achieving future prosperity within the practical aspect of the field of chemistry [102]. With attaching a more significant role to reverse micelles in the prospect of advanced structural design, a general shift towards approaching more complex supramolecular architectures may be expected in this area of research and utilization of self-assembly phenomena as well.
7. Future directions in the application of reverse micelles
As far as the future directions in the application of reverse micelles in the field of materials synthesis are concerned, the following approaches may be outlined. Because of the emphasized uniqueness of particular designed structures and compositions within specific parent microemulsions, the development and application of highly specific and growth- directing surfactants especially suitable for particular chemical compositions, crystal structures and intended morphologies may be expected in future [103]. In any case, the future prosperity in the use of reverse micelles and microemulsions for inducing practical self-assembly phenomena depends on the combined synergetic efforts of application of basic principles of colloid chemistry (mostly based on the simple framework of DLVO theory), trial-and-error approaches, employment of diverse advanced microscopy techniques, and theoretical prediction of specific molecular recognition effects.
Unlike ordinary emulsions, microemulsions do not require high shear rates for their formation and may due to potential existence of fine and diverse metastable colloid states exhibit a wide range of inherent multi-molecular configurations [104], including either regular or reverse micelles of various oval shapes (spherical, cylindrical, rod-shaped), vesicular structures,
31V. Uskokovi, M. Drofenik / Advances in Colloid and Interface Science 133 (2007) 23–34
bilayer (lamellar) and cubic liquid crystals, columnar, mesh and bicontinuous mesophases, cubosomes (dispersed bicontinuous cubic liquid crystalline phase), sponge phases, hexagonal rod- like structures, spherulites (radially arranged rod-like micelles), multiphase–substructured configurations (such as water in oil in water droplets, for example), highly percolated pearl like structures, supra aggregates that comprise various substructured combinations of microemulsion aggregates, as well as numer- ous transient configurations. The application of complex non equilibrium phases and such transient configurations between reverse-micellar and various other inherent multi-molecular self-assembled entities could provide the basis for growth of numerous attractive novel morphologies. As a matter of fact, each particular point in a microemulsion phase diagram corresponds to specific local conditions for physico-chemical transformations that take place therein and result in unique material 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 or more preparation techniques into one can due to synergy ef- fects lead to multiple advantages, such as improved control of the stoichiometry of the final product (as adopted from sol–gel method) and extremely fine and controllable grain size (as acquired from reverse-micellar synthesis) in the examples [109,110] of the combination of sol–gel and reverse-micellar approaches to materials synthesis. Low yields, surfactant- contamination and difficulties arising out of the attempts to separate precipitated powders in the form of non-agglomerated particles – serious obstacles of the microemulsion-assisted nanoparticle preparation procedures – were overcome by feedstocking flame-spraying apparatus with nanoparticles together with their parent microemulsion [108], at the same time transcending poor control of particle size and shape, which is a typical drawback of the conventional flame-spraying methods of synthesis. Combining reverse-micellar synthesis of cobalt particles with their subsequent evaporating deposition in external magnetic field led to the formation of large-scale 3D superlattices of cobalt nanocrystals [111]. Silver nanorods encapsulated by polystyrene were prepared by combination of reverse-micellar, gas antisolvent, and ultrasound techniques [112], whereby specific carbon nanotubes were prepared by direct introduction of in situ prepared catalytically active Co and Mo particles by a reverse-micellar method [113,114].
Both weak soft-tech potentials for structuring self-assembled products into functional systems of hierarchical organization and inherent limitations in the resolution of lithographic techniques in nanostructural design may be overcome by constructive coupling of the soft-tech production of fine- structured materials with hard-tech assembly methodologies. Excellent achievements have been recently reported by relying on such an approach of combining ‘bottom–up’ and ‘top– down’ methodologies [46,115–118]. Langmuir–Blodgett films [119], obtained by coupling self-assembled orientation of molecules at air–water interfaces with a technique for their
deposition on solid substrates, present a classic example of such complementary synthesis/processing methodology. In that sense, layer-by-layer (LbL) techniques comprising adsorption of oppositely charged polyelectrolytes on a solid surface of synthesized particles in reverse micelles were used to overcome difficulties arising out of the inabilities to carry out sequential reactions inside the same reverse micelles in order to obtain multilayered composites [120].
The assembling of particles formed in the processes of reverse-micellar and, in general, microemulsion-assisted syn- theses into precisely tailored, supra-nanocrystalline 3D struc- tures, presents an important challenge, whereas in situ reactions in well-organized amphiphilic matrices present only one step towards this goal [39]. Self-assembly parallelism and the selective patterning precision of lithographic and etching techniques can be united in a multitude of hybrid techniques for the production of fine structures [121]. External fields, such as electric and magnetic fields, heat gradients or single layer shearing, can induce unexpected orderings depending on the intensities and directions of the field relative to the suspension cell [122], and may be used to hierarchically organize particles into 3D matrices. On the other hand, electrospraying, electro- coalescence and other methods that involve various external fields, may be used for ink-jet spraying, fluid atomization, phase and particle separation, thus improving the functionalizational control of the self-assembled fine structures [123].
To sum up, reverse-micellar and other microemulsional systems can provide complex interfaces that can support parallel reactions leading to surprisingly complex outcomes, and their relatively stable existence in thermodynamically metastable states can support significant modifications of the product structures by the pure influence of aging treatment. However, small yields obtained due to employing extremely small concentrations and expensively complex environments used in most of the cases, altogether with the fact that increasing the space of options for production of various end results via extremely fine variations of certain experimental parameters comes at the price of increased sensitivity of initial experimental settings that lead to reproducible outcomes, provide implicit difficulties within such an approach to advancedmaterials synthesis. The future prosperity of application of reverse micelles, microemulsions and other self-assembling amphiphilic matrices in advanced structural design will in large extent depend on the successful global balancing of these pros and 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. Reverse micelles 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-molecular entities that could be under specific conditions actively engaged in the chemical pathways of the formation of given materials.
32 V. Uskokovi, 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 a biomimicking approach to advanced structural design. The presented examples of pronounced sensitivity of processes that employ reverse-micellar effects in materials processing may initiate 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 practical field of colloid science oriented towards reaching highly organized and ultra-sensitive functional structures and ‘templating’ environ- ments. The consequence of such convergence between biological features and self-assembly design is that with increasing the complexity of advanced nanofunctional devices, an increased sensitivity of the intended products, manifested either as irreproducibility of synthesis procedures or exceptional functional sensitivity towards slightest environmental effects, will start appearing as a significant problem. However, knowing that every advantageous challenge always has its risky side as well, such an intricate situation could be,with a lot of effort involved, turned into an optional range of convenient and potentially fruitful outcomes.
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Reverse micelles: Inert nano-reactors or physico-chemically active guides of the capped reactio.....
Introduction
The need to reevaluate the functional representation of reverse micelles
Examples of chemical ‘butterfly effects’ in reverse micelle-assisted syntheses
The example of nickel–zinc ferrite
The example of lanthanum–strontium manganite
Correlations with the biological context
Future directions in the application of reverse micelles
Conclusions
References