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

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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.

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 a matter 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 significantly 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], ranging from the order of magnitude of nanoseconds for diffusioncontrolled 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 constant 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 compared 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 reversemicellar microemulsion of particular composition, nickelzinc ferrite is obtained instead [53], as can be observed from Fig. 2. Faster rates of oxidation and slower rates of precipitation when 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

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

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).

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 nickelzinc 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 nickelzinc 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 lanthanumstrontium 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 nickelzinc 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(C2O42) NO3] coordination complexes (hence aqueousalcoholic solutions 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 of microemulsion 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 LaSrmanganite 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 microemulsion 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

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

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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).

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 XRDdetermined 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 homogeneous 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

synthesis case, and grain growth in the microemulsionsynthesis 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).

Fig. 5. Normalized XRD patterns of the samples synthesized using oxalate co-precipitation approach in bulk solution (a, c) and in reverse micelles (b, d), annealed at 500 °C (a, b), 700 °C (c) and 1000 °C (d) for 2 h in air.

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

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 encapsulating a protein in a reverse micelle and dissolving it in a lowviscosity 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 drugdelivery 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 livingand 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 singpresents 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 investigations oriented towards reproducing or at least approaching a reproduction of such an organizational complexity in artificial colloid systems. Also, knowing that malignant cells have significantly 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 aforementioned 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 waterretaining 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, hydrophobic and hydrophilic interactions, van der Waals forces, salt bridges, coordination complexes (forces involving ions and ligands, i.e. coordinatecovalent 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 growthdirecting 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,

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