6961
J. Phys. Chem. 1993,97, 6961-6973
FEATURE ARTICLE Reverse Micelles as Microreactors M. P. Pileni UniversitL P. et M. Curie, Laboratoire S.R.S.I. Batiment F(74), 4 place Jusieu, 75005 Paris, France, and Centre d’Ltudes nuclsaires de Saclay, D.R.E.C.A.M.-S.C.M., 91 191 Gif sur Yvette, France Received: January 22, 1993; In Final Form: April 6, 1993
In this paper an extensive introduction to reverse micellar properties is given. It is shown that reverse micelles can be used as microreactors to produce either well-defined nanosized crystallites or chemically modified enzymes. Solubilization of macromolecules induces either the gelation of droplets or a percolation process followed by a phase transition. In the latter case this could favor the extraction of product reactions catalyzed by enzymes. Finally, a summary gives some prospective aspects of the use of such water-in-oil droplets.
Introduction
Surfactants, S, dissolved in organic solvents form spheroidal aggregates called reverse micelles.’ They can be formed both in the presence and in the absence of water. However, if the medium is completely free of water, the aggregates are very small and polydisperse. The presence of water is necessary to form large surfactant aggregates. Water is readily solubilized in the polar core, forming a so-called “water pool”, characterized by w, the water-surfactant molar ratio (w = [H20]/ [SI). The aggregates containing a small amount of water (below w=15) are usually called reverse micelles whereas microemulsions correspond to droplets containing a large amount of water molecules (above w=15). The spontaneouscurvature for reverse micelles correspondsto the energetically favorablepacking configurationof the surfactant molecules at the interface.Z4 It depends basically on the molecular geometry. The surfactant molecule can be represented as a truncated cone (Figure 1A) whose dimensions are determined by the respective ranges of hydrophilicand hydrophobicparts. Then if u is the surfactant molecular volume, u the area per polar head, and I the length of the hydrophobicpart, the number v/ul, called the “surfactant parameter” or “packing parameter”, gives a good idea of the shape of the aggregates formed spontaneously. In “dry” reverse micelles (without water addition) and water-indroplets, this parameter, v/ul, has to be above three and equal to one, respectively.4 Assuming water-in-oildroplets are spherical, the radius of the sphere is expressed as
R = 3V/2 where R, V, and 2 are the radius, the volume, and the surface of the sphere. Assuming that the volume and the surface of the droplets are governed by the volume of the water molecules and by the surfactant molecules (all located at the interface), respectively, the water pool radius, R,, is similar to the sphere radius (R=R,) and can be expressed as where V,, is the volume of water molecules. Various experimentsSconfirm the linear variation of the water pool radius with the water content and, at a given water content (Figure 2), the invariance of the water pool radius with the polar volume fraction (insert, Figure 2). 0022-3654/93/2097-696 1%04.00/0
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Figure 1. Shape of surfactant molecules forming reverse micelles.
One of the surfactants often used to form reverse microemulsions is sodium bis( 2-ethylhexyl)sulfosuccinate, usually called Aerosol OT or AOT (Figure 1B). From the geometrical model developed above, the water pool radius follows the relation
R,
(A) = 1 . 5 ~
For AOT-water-isooctane solution, this relation- has been tested by SAXS and SANS (Figure 2). The differences observed between SANS and SAXS are due to the scattering by the surfactant polar head group in the latter. So as the size of the droplet increases,the micellar concentration decreases while the water content, w, increases. From the geometrical model developed above, the average location of solute in a reverse micellar solution can be deduced.6 The contribution of the solute (Figure 3) to the interface or to 0 1993 American Chemical Society
Pileni
6962 The Journal of Physical Chemistry, Vol. 97, No. 27, 1993
- 60
-
50
-40
Figure 2. Variation of the water pool radius of the droplet with the water content, tested by SAXS ( 0 )and SANS (+) and from a kinetic study using the hydrated electron as a probe (0)in AOT-water-isooctane
solution. [AOT]=0.1 M.
A
C
B
D
the hydrated electron formed in reverse micelles9Jois observable at relatively high water contents (above w=15). It can be seen from Figure 3D that the hydrated electron is formed in unfilled micelles and does not react with a guest molecule which creates its own aggregate (filled micelles). By contrast, quencher (or solutes) located either in (or at) the interface, or inside the water pool, reacts with the hydrated electron. So, from the ability of the hydrated electron to react with the solute, it is possible to discriminate between various locations of solutes in reverse micelles. It must be pointed out that the geometrical model developed above neglects interaggregate interactions and applies only to dilute dispersions. Structural evolution is dominated by both repulsive and very short-range attractive interactions between the micelles."-2' Several parameters such as solvent, temperature, number and size of droplets, and addition of solutes have to be taken into account. In the case of large attractive interactions, formation of infinite clusters appears with a way for ions to migrate all over the system. This, called the percolation process, is followed by a phase separation well before the emulsification failure. The Van D i j k g r o ~ pshows ~ ~ l that ~ a divergenceof the static dielectric permittivity and a sharp increase in the conductivity are attributed to a percolation transition. The percolation threshold corresponds to the maximum of the permittivity and to the onset of conductivity. Van Dijk developed a clustering model for reverse micelles in which the "dipole-dipole" interactions contribute to the polarizability of one microemulsion droplet. Percolation theories have been mainly developed on the basis of lattice models.22-26In this case, the critical behaviors of some physical properties such as dc conductivity, ~ ( 4 )and , static permittivity, cs(4), are described as the following:
44)
CAP)
E
Figure 3. Model of location of probes in reverse micelles (the ball
represents the solute). the polar volume induces an increase either in the interfacial area, d2, or in the polar volume, dV. The water pool radius expressed as a sphere (R,=3( V+dV)/Z+dZ) changes by increasing either dV or dZ. So a probe located in the bulk phase or at the external interface does not change the water pool radius (dV=d2=0) (Figure 3A,B). A probe located at the interface induces a decrease in the water pool radius due to an increase in the interface, d2, with a constant volume, V (Figure 3E). In contrast, an increase in the water pool radius is observed for a probe located in the water pool due to the increase in the volume, dV, with a constant surface, Z (Figure 3C,D). This model has been tested by SAXS using various reactants68 in AOT-waterisooctane. However, when the solute creates its own aggregate (Figure 3F), there are two micellar populations. The SAXS technique does not allow one to differentiate between an increase in the polydispersity and the appearance to two populations of filled and unfilled micelles. This case can be resolved by using the hydrated electron as a probe and the solute as a quencher:
-
-
I9-9Pl"
for 9 < 9,
for 9 > 4pand 9 < 9,
I4-9JS
through critical exponents t and s close to the percolation threshold 9, where an infinite aggregate exists. The s and t values determined from static measurements are similar to the theoretical predictions: t 2 and s 0.6-0.8.The critical exponents s describing the behavior of conductivity below and above the percolation threshold are not the same. This has been attributed to the dynamic character of the droplet clusters. Due to their small size ( # 1-10 nm), the reverse micellar are subject to Brownian motion. They collide continuously, and a small fraction of droplets exist on short-lived dimers with exchange of the water content. These dimers again form new droplets. As a result of the coalescence and decoalescence sequence, probe molecules solubilized in the water pools are redistributed over the micellar population. The distribution of probe in micellar solution follows a Poisson distribution, and the kinetic decay is a complex rate law:'OJ"33
-
-
where [PI and [PI0 are the probe concentrations a t time t and zero time, respectively, [Q] is the quencher concentration, ko is the first-order rate constant governing the probe decay in the absence of the quencher Q, k, is the quenching rate constant, k, is the bimolecular exchangerateconstantfor water pool collisions, and fi is the average number of quenchers per water pool ( A = [Q]/[RM]), where [RM] is the reverse micellar concentration. Assuming that the micelles are monodisperse spheres and the water molecules are all located in the micellar water pool, the radius of the water pool is given by Rw3= 3w [SI 10-3/55 [RM]4~
By using the hydrated electron as a probe and nitrate ion as a
Feature Article quencher,1° the water pool radius is deduced at various water contents. Figure 2 shows a good agreement between the water pool radiusdeduced from kinetic and S A X S and SANS techniques (Figure 2). The average location of quenchers in reverse micelles can be deduced from the variation of the quenching rate constant of the solutes differing in their location.35 A theoretical model36 compares the rate constants of a reactant freely diffusing inside the sphere and the other fixed either at the center or at the surface of the sphere. In the first case a dependence of the kinetic rate constant on R-3 whereas in the latter case on R 2 . 5 is found. This has been experimentallyconfirmed35for quenchers locatedinside the water pool such as nitrate ion, ribonuclease at pH 9, or succinylatedcytochrome c (highly negatively charged, -lo), the hydrated electron quenching rate constant varies with Rw-3(or w3) whereas for quenchers located at the interface such a copper ions, native cytochrome c, of ribonuclease at pH 4, the quenching rate constant depends on Rw-2(or w2). Reverse micelles serve as host to solute such as proteins and enzymes. The presence of organicsolventshas a deleteriouseffect on enzymeactivity and stability. However, shieldingthe enzyme by a surfactant monolayer from the continuous organic phase induces higher reactivity comparedto that in aqueous solution.3741 Chemical enzyme modifications can be performed by using reverse micelles.42 To prevent denaturation, the enzyme is solubilized in reverse micelle, and species especially reacting with lysine residues for example are added in the bulk phase. Residues in contact with the interface react, keeping those participating at the active center protected against chemical reaction. The modified enzymes, after extraction, keep their activity. Reverse micelles are used in material sciences or in biotechnology: (i) New Inorganic Materials. In aqueous solution, polymerization of monomers induces the formation of either gels of flocculated polymer. Similar reactions performed in water-inoil droplet microemulsions induce the formationof 100-nm latexes, characterized by high polymerization rates.4346 After polymerization polymer particles, with one polymer chain on the average, are obtained. Instead of using monomer solubilized in the water droplets, the polymerization of surfactants forming reverse micelles favors formation of 2-10-nm latexes.47 Because of the exchange processes between droplets, described above, reverse micelles can be used to synthesize size-controlled crystallites by carrying out coprecipitation or chemical reaction in the water pools. The behavior of such reactions differs in aqueous and micellar solutions. In the bulk aqueous phase, because of the difference in the solubilities of reactants and products, the mixture of some compounds induces flocculation of bulk material. Reverse micelles prevent growth and then flocculation of the material. In these past few years several groups have studied the formation of various types of clusters such as semiconductors,4’J-54metallic clusters,55J6 carbonate aggregates, magnetite particle^?^ or colloidal alumina in reverse microemulsions. (ii) Biotechnology Research. Enzymatic reactions are mainly performed in aqueous solution. The conversion of nonpolar substrates is impossible. The use of reverse micelles allows one to convert either water- or oil-soluble substrates into products. As the matter of fact, during the water pool exchange, nonpolar compounds are in contact with the enzyme favoring the catalytic reaction.27-31 It is well-known that enzymes such as chymotrypsin catalyze either hydrolysis in aqueous solution or amino acid condensation in the absence of water. Because of the polar character of the enzymes, the latter reaction cannot easily be performed. In reverse micelles, the amount of water is controlled and the enzyme is stable in the water droplets. This property favors the catalysis of amino acid condensation at low water content58whereas the hydrolysis reaction takes place at high water content. These processes have not be used in industry because of thedifficultyin extractingthe product from the micellar solution
Th e Journal of Physical Chemistry, Vol. 97, No. 27, 1993 6963 and keeping the enzymestill active. This problem could be solved if the enzyme could induce a phase transition. Synthesis of Nanocrystallites We have chosen to present an example of coprecipitation reaction (formationof CdS semiconductors) and one of a chemical reduction (formation of copper metallic clusters) performed in AOT-water-isooctane solutions. The results presented here can be extended to other semiconductorsor metallic clusters such as C d x Z n d ? g AgzS, and AgPOsemiconductorsor silver metallic clusters.56 Colloidal dispersionsof copper metal exhibit broad regions of absorption in the ultraviolet-visible range due to interband transitions and are characteristic propertiesof the metallic nature of the particles. From Mie’s theory,6l the absorption spectra are simulated, for various particle sizes, and are compared to the absorption spectrum in the bulk phase. For copper metallic particles with a diameter below 4 nm, the absorption band is broad. By increasing the size of the particles, the absorption band is better resolved with a peak centered at 570 nm. The shape of the absorption spectrum depends on the cluster sizes: the decrease in the size induces a strong decrease in the 570-nm peak. Cadmium sulfide suspensions are characterized by an absorption spectrum in the visible range. In the case of small particles, a quantum size effect62d6is observed due to the perturbation of the electronic structure of the semiconductor with the change in the particle size. For a CdS semiconductor, as the diameter of the particles approaches the excitonic diameter, its electronic properties start to change.6566 This gives a widening of the forbidden band and therefore a blue shift in the absorption threshold as the size decreases. This phenomenon occurs as the crystallite size is comparable or below the excitonic diameter of 50-60 A. In a first approximation a simple “electron-hole in a box” model can quantify this blueshift with thesizevariation.62~~~65 Thus, the absorption threshold is directly related to the average size of the particles in solution. Change in the Size of the Crystallites with Water Content. I. Metallic Particles.55 Hydrazine as a reducing agent is added to micellar solution containing copper ions in aerated solution. A change in the water content of the micelle, w, is obtained by adding H 2 0 to the micellar solution before the reaction takes place. Figure 4 shows the absorption spectra of the colloidal particles, electronic microscopy pictures, and the histograms observed 5 h after adding the reducing agent, for reverse micelles with various w values. At low water content, a continuous absorption spectrum with a shoulder at 570 nm is observed. Upon increasing the water content, the 570-nm band characteristic of a copper cluster appears progressively. The electron microscopy pictures show an increase in the size of the particles from 2 to 10 nm upon increasing the water content from 1 to 10. At water contents above 10, the size of the particles remains unchanged but the polydispersity increases. Electron diffractograms compared to a simulated diffractogram of bulk metal copper are in good agreement. This indicates a crystalline face-centered-cubic (fcc) structure with a lattice constant of 3.61. The increase in the water content induces an increase in the number of copper ions which react with the reducing agent. This favors the growth of the average particles. At relatively high water content (above w= lo), copper ions are totally hydrated and free water molecules are present. This favorsdiffusionof copper ions inside the droplet. Electrostatic interactions between the head pblar groups of the surfactant and copper ions oppose the hydration energy. The difference in these energies remains constant upon increasing the water content which keeps the particle size constant. 2. Semiconductor Particles.48-s4 Figure 5 shows a red shift in the absorption spectra of CdS on increasing the water content.
6964 The Journal of Physical Chemistry, Vol. 97, No. 27, 1993
Pileni
wavebngth (m)
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300
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Figure 4. Absorption spectrum, histogram, and electron microscopy picture of copper metallic particles obtained at various water contents in AOTwater-isooctane solution [AOT]=O.l M, [Cu(AOT)2]=10-* M, and [N2H4]= 3 X M.
This is attributed to an increase in the average size of particles with the water content. Below the absorption onset several shouldersare observed (Figure 5) and can be clearly recognized in the second derivative (inserts, Figure 5). Theseweak absorption bands correspond to the excitonictransitions. This clearly shows a narrow size distribution.54 At low water content the first excitonic peak is well resolved and is followed by a bump. The second derivativeshows a very high intensity of this bump (insert, Figure 5A). With small crystallites, according to the data previously published,s4 several bumps due to several excitonic peaks are expected. The insert in Figure SA shows only one bump. This is due to the fact that the others are blue-shifted and are not observable in our experimentalconditions. By increasing the water content, that is to say, by increasing the size of the particles, several bumps are observed (insert, Figure 5B). The intensity of these bumps decreases with the water content, w (inserts, Figure 5). This indicates a decrease in the number of
excitonictransitions with the sizeof the particle and is in agreement with the theoretical calculations previously published for the Q-particles. The polydispersity strongly depends on the preparation mode.67 From the relation between62 the absorption onset and the size of CdS particle, the average radius, r, s deduced. Figure 6 shows a strong change in the size of the particle with the relative ratio of cadmium and sulfide ions (x = [CdZ+]/[S-]). The biggest sizes are obtained for x = 1 and the smallest for x = 2. It can be noticed that the size of CdS is always smaller when one of the two reactants are in excess (x = 1/4, */2,2). This confirms that the crystallization process is faster when one of the species is in e~cess.5~ The crystallites have been extracted from reverse micelles, and CdS powder is obtained with an unchanged value in the size distribution. Electron microscopy has been performed on the powdered semiconductor dispersed in pyridine.59 Figure 7A,B shows an increase of the particle size with the water content. The
The Journal of Physical Chemistry, Vol. 97, No. 27, 1993 6965
Feature Article 80
w=5
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electron diffraction pattern indicates that the particles keep a ZnS crystalline structure (fcc) with a lattice constant equal to 5.83 A. Changes in Oxidation States of Copper Metallic Clusters. By using sodium borohydride as the reducing agent, in the absence of oxygen, an absorption spectrumcharacteristic of copper metallic particles is observed. Figure 8 shows a decrease in the size of the particles from 28 to 3 nm upon increasing the water content. At low water content (w = 3 and 5 ) , the particles are homogeneously dispersed. Upon increasing the water content from w = 4 to w = 8, the particles become progressivelyassociated. From electron diffraction patterns for particles formed in micelles with low water content (
