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Studying a New Type of Surfactant Aggregate (“Spherulites”) as Chemical Microreactors. A First Example: Copper Ion Entrapping and Particle Synthesis Fabienne Gauffre*,† and Didier Roux Centre de recherche Paul Pascal, Av. du Docteur Schweitzer, 33600 Pessac, France Received November 2, 1998. In Final Form: February 26, 1999 We report the first example of a chemical reaction in a new type of surfactant aggregates called “spherulites” or “onions”. Spherulites are spherical microdomains (200-1000 nm) of lamellar phase. They are prepared by means of shear of a lamellar phase under controlled conditions. Spherulites can be considered as surfactant vesicles with a dense multilamellar structure. Chemical reduction of copper(II) ions encapsulated into spherulites by hydrazine demonstrates the possibility of using this new surfactant assembly as chemical microreactors. We describe the process of encapsulation of copper(II) ions into spherulites. The use of a ligand-like surfactant proved to be efficient in preventing the leakage of these ions despite spherulite permeability to small hydrosoluble molecules. Encapsulation ratios up to 80% are obtained. When hydrazine is added to an aqueous suspension of Cu(II)-containing spherulites, hydrazine diffuses into the spherulites and chemical reduction of the entrapped ions occurs. The formation of small Cu2O particles (10-30 nm) is observed throughout the multilayered structure of the spherulites. The suspension of nanoparticles embedded in the surfactant structure is stable, and the nanoparticles can be recovered by destruction of the spherulites.

I. Introduction Organized surfactant assemblies such as microemulsions, micelles, hexagonal phases, cubic phases, monolayers, and vesicles are unique reaction media. Indeed, they can solubilize, concentrate, localize, and even organize the reactants. Moreover, they can alter all the thermodynamical and kinetical parameters that govern chemical reactions: dissociation constants, oxydo-reduction potential, chemical rates, and stability of reactants, products, and intermediates. These structures can even be stabilized by polymerization when reactive lipids are used.1-3 Many naturally occurring biological reactions such as photosynthesis4 take place in organized media, and the selforganization obtained with surfactants in solution has opened a systematic exploration of their use to control chemical reactions.5 Micellar catalysis is certainly one of the most known applications.6 More recently, inverted microemulsions (i.e., reversed micelle phases) have also been used to prepare nanoparticles.7-9 In micelles, one can encapsulate hydrophobic particles, while in inverted microemulsion phases it is the hydrophilic compound which may be situated in small size water droplets (typically in the range of 10 nm) dispersed into a continuous oil phase. In both cases however the characteristic exchange time between either micelles or † Present address: lcc, 205 rte de Narbonne, 31077 Toulouse Cedex, France. E-mail: [email protected].

(1) Ringsdorf, H.; Schlarb, B.; Venzmer, J. Angew. Chem., Int. Ed. Engl. 1988, 27, 113-158. (2) Srisiri, W.; Sisson, T. M.; O’Brien, D. F.; McGrath, K. M.; Han, Y.; Gruner, S. M. J. Am. Chem. Soc. 1997, 119, 4866-4873. (3) Sisson, T. M.; Lamparski, H. G., Ko¨lchens, S.; Elayadi, A.; O’Brien, D. F. Macromolecules 1996, 29, 8321-8329. (4) Fendler, J. J. Chem. Educ. 1983, 60, 872-876. (5) Fendler, J. Membrane Mimetic Chemistry: Characterization and Applications of Micelles, Microemulsion, Monolayers, Bilayers, Vesicles, Host-guest Systems and Polyions; Wiley: New York, 1982. (6) Fendler, J.; Fendler, E. Catalysis in Micellar and Macromolecular Systems; Academic Press: New York, 1975. (7) Lisiecki, I.; Pileni, M.-P. J. Am. Chem. Soc. 1993, 115, 7. (8) Pileni, M.-P. J. Phys. Chem. 1993, 97, 1-6973. (9) Boutonnet, M.; Kizling., J.; Stenius, P. Colloids Surf. 1982, 209.

water cores of microemulsions is of the order of a millisecond. Indeed, water or oil contents are mixed during exchange processes. As the time scale involved in such exchanges is of the same order of magnitude as the chemical reaction time, both these exchanges and the chemical kinetics control the reaction. In most of the cases, surfactant vesicles are long-lived entities that do not undergo exchange of their water content. For this reason, they were widely used as encapsulating systems essentially referred to as “liposomes” (which is the common name for vesicles made of lipids). Liposomes have a variety of applications ranging from stabilization of the active agent in skin care lotions to the controlled delivery of drugs.10,11 However, reactivity in liposomes is limited because of the aqueous nature of the internal core, the weak permeability of the membrane, and also the poor encapsulation ratio. Up to now, applications of liposomes for reactivity have mainly been limited to photochemical reduction of metallic salt dissolved in the aqueous core of vesicles or polymerized vesicles.12 When submitted to shear, lamellar phases can undergo structure transformations.13,14 Recently, it was observed that when it is sheared under proper conditions, a lamellar phase can reorganize into a phase of multilamellar vesiclessso-called onions or spherulites (Figure 1b)s compactly packed in space (Figure 1a, step 1).15-19 Spherulites in compact organization can be dispersed in (10) Lasic, D. Am. Sci. 1992, 20-31. (11) Kim, H.-H.; Baianu, I. Trends Food Sci. Technol. 1991, 55-61. (12) Kurihara, K.; Fendler, J. J. Am. Chem. Soc. 1983, 105, 2-6153. (13) Bohlin, L.; Fontell, K. J. Colloid Interface Sci. 1978, 67, 272283. (14) Montalvo, G.; Valiente, M.; Rodenas, E Langmuir 1996, 12, 5202-5208. (15) Diat, O.; Roux, D. J. Phys. II, 1993, 9-14. (16) Diat, O.; Roux, D.; Nallet, F. J. Phys. II, 1993, 1427-1452. (17) Roux, D.; Diat, O.; Patent, France, 1992. (18) Roux, D.; Gulik, T.; Dedieu, J.; Degert, C.; Laversanne, R. Langmuir 1996, 12, 4668-4671. (19) Van der Linden, E.; Hogervorst, W. T.; Lekkerkerker, H. N. W. Langmuir 1996, 12, 3127-3130.

10.1021/la981541t CCC: $18.00 © 1999 American Chemical Society Published on Web 04/29/1999

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Figure 2. Schematic representation of a typical shear diagram of a lamellar phase in the case of a system: water, dodecane, pentanol, and sodium dodecyl sulfate: zone I, membranes are roughly oriented in the velocity direction; zone II, spherulites; zone III, membranes are well oriented in the velocity direction.

Figure 1. Spherulite structure (b) and stages in the preparation of a dispersion of spherulites (a). (a) (1) Submitted to a controlled shear, the lamellar phase organizes in a close-packed organization of multilamellar vesicles called spherulites. (2) Spherulites can be dispersed in an excess amount of solvent. (b) Spherulites are made of a regular stack (n ) 50-1000 layers) of membranes separated by water layers.

an excess water to give a colloidal dispersion (Figure 1a, step 2). More recently, we have shown that these spherulites can be used to encapsulate chemicals.20-22 Our present purpose is to demonstrate the ability to localize the reaction inside the spherulites which could then be considered as microreactors. For this, we have chosen to carry out the well-known reduction of copper(II) ions to copper(I) oxide particles by hydrazine into spherulites. We will first present a brief description of the spherulite technology, then we will describe in more detail its application to the making of nanoparticles. Spherulite Technology. Spherulite technology was presented in detail elsewhere.15-17 However, we found it necessary to give here a brief summary of the physical principles of the making of spherulites for a better understanding of this system. Spherulite technology was born from the study of the structure of lyotropic lamellar phases under shear. A lyotropic lamellar phase is made of surfactant, water, and sometime an additional hydrophobic component (oil). It is a very common phase which is most often found for relatively high concentrations of the surfactant,23 while in certain special cases very dilute lamellar phases can be prepared due to long-range repulsive interactions between membranes.24 The struc(20) Gauffre, F.; Roux, D. Langmuir, to be published. (21) Gauffre, F. Utilisation de ve´sicules pre´pare´es par cisaillement d'une phase lamellaire comme microre´acteurs chimiques; Ph.D. Thesis, Bordeaux I, 1997. (22) Roux, D.; Gauffre, F. European Chemistry Chronicle 1998, 3 (2), 17-24. (23) Ekwall, P. Surfactants in Solution; Academic Press: New York, 1975. (24) Roux, D.; Nallet, F.; Safinya, C. in Micelles, Membranes, Microemulsions and Monolayers; Geldbart, W., Ben Shaul, A., Roux, D., Eds.; Springer-Verlag Inc.: New York, 1993; pp 303-346.

ture of the lamellar phase is that of a smectic A phase in the nomenclature of the liquid crystalline phases. It is composed of a stack of surfactant bilayers separated with water layers. When present, the hydrophobic compound swells the surfactant bilayer. Upon dilution with extra water, two main behaviors are generally observed.24 (a) The dilution of the lamellar phase is limited by a phase transition with an isotropic liquid phase (micellar phase or sponge phase). (b) The dilution of the lamellar phase is limited by a phase transition with another liquid crystalline phase. However, in a limited number of interesting cases the lamellar phase separates with excess water when it reaches a maximum of uptake. This is for instance the case when phospholipids are used as surfactant. To understand the effect of flow on such phases, we have studied using rheophysics methods the structure of lyotropic phases submitted to shear. Using a number of structural techniques under shear such as scattering techniques (light scattering,16 neutron,25 X-ray26) or dielectric measurements,27 it was possible to show that the effect of shear can be described using a shear diagram. This diagram, which can be considered as a generalization for out-of-equilibrium systems of phase diagrams, describes the effect of shear as a succession of stationary states of orientation separated by dynamic transitions. Indeed, while always remaining a thermodynamically stable lamellar phase, the phase experiences a series of transitions modifying the orientation of the lamellae in space so as to respect the direction of the shear. These different orientations correspond to an organization of the defects in space, which is different from one orientation to the other. It is a modification of what is named the “texture” of the phase. Consequently, it is not a phase transition but has to be considered as an instability. It is however different from the classical hydrodynamic instabilities observed when a fluid is submitted to shear because it does not involve length scale directly related to the size of the shear cells but rather some microscopic length scale more related to the intrinsic structure of the fluid. Figure 2 is a schematic representation of a typical (25) Diat, O.; Roux, D.; Nallet, F. J. Phys. IV 1993, 193. (26) Roux, D. In Theor. Challenges Dyn. Complex Fluids 1997, 203233. (27) Soubiran, L.; Roux, D.; Coulon, C. Europhys. Lett. 1995, 243.

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shear diagram obtained in the case of a lamellar phase made of water, dodecane, pentanol, and sodium dodecyl sulfate.16 At very low shear rate the phase is more or less oriented with the membrane parallel to the velocity; however defects remain in the velocity direction as well as in the vorticity direction. At high shear rate the orientation is basically very similar, but the defects in the velocity direction have disappeared. In the intermediate regime, a new and interesting orientation appears. The membranes are broken into pieces by the flow, and the phase organizes itself into a phase of multilamellar vesicles all of the same size. The name “onions” originates from this multilamellar structure. A freeze-fracture picture of the phase after shearing can be obtained.18 Onions fill up space, and their size is a function of the shear rate.15,16 Many other surfactant systems have now been studied and exhibit the same kind of behavior.28,29 In most cases, the spherulites size is inversely proportional to the square root of the shear rate. This is an easy way to experimentally control the size of the vesicles. Depending upon the formulation (i.e., the surfactants choice) and the shear rate, sizes ranging from 0.2 µm to more than 50 µm can be found. Not only is the oriented to onion transition an interesting physical instability, but it became a process to prepare well-controlled multilamellar vesicles. Indeed, starting from the onion phase under shear, if one stops very quickly the shear cell, it is possible to quench the onion structure. In this case, all the observations indicating the existence of onions can be done on the system at rest. A very slow evolution can be seen depending on the system. However, thermal fluctuations, which are the driving force that destroys the onions, are not enough in most cases to let the system evolve. Certain systems can be prepared that way and are stable for several months. Moreover, after stopping the shear, one can disperse the lamellar phase prepared under shear in an excess solvent. Depending upon the phase transition that the lamellar phase experiences upon dilution two scenarios may happen. If the lamellar phase changes phases under dilution, the onions are slowly dissolved into the excess solvent to give the corresponding equilibrium phase. However, in the interesting case where the lamellar phase is in equilibrium with excess water (e.g., no micelles formed), the onion phase remains in suspension in the water. One obtains consequently a monodisperse emulsion of lamellar phase. If an active molecule (a drug, a perfume, a protein, or any type of other molecule) is dissolved in the lamellar phase previous to shearing and then the obtained onions dispersed into excess water, one obtains a suspension of encapsulated molecules. The size of the microcapsule can easily be chosen by applying the appropriated shear rate. Just before dilution, the active molecules are encapsulated inside the multilamellar structure of the onions. However, depending upon thermodynamics and kinetics the encapsulated molecules will either stay inside the onions or leak out and equilibrate with the excess water. Without giving general rules, we have observed that small hydrophilic molecules (molecular weight below 500 g/mol) effectively leak out and equilibrate while hydrophobic molecules or those big enough remain mostly inside. Like liposomes, spherulites have rapidly found many applications as encapsulating systems. This is a patented process, and products using spherulites are now on the (28) Sierro, P.; Roux, D. Phys. Rev. Lett. 1997, 1496. (29) Pannizza, P.; Colin, A.; Roux, D.; Coulon, C. J. Phys. (Paris), in press.

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market.30 Because of their specific features, onions dispersed in excess water seem to be also quite attractive for their use as microreactors. Indeed, we have already seen that spherulites can solubilize large amounts of hydrophobic compounds as well as of hydrophilic compounds in their multilayered structure. Moreover the typical time for a molecule to diffuse through the onions may vary from minutes to an infinite amount of time depending upon the size of the molecule.31 In any case this leakage time is much longer that the chemical reaction time. It is then possible to separate in the chemical process the diffusion of species from the reaction itself. Therefore, it seems appealing to use such microreactors for controlling in space a chemical reaction. It is also possible to use such systems to solubilize hydrophobic compounds and to make them react on hydrophilic (or even hydrophobic) ones. Since onions are droplets of lamellar phase, encapsulated compounds might “see” an environment quite different from the bulk water phase. Recently we have been using onions to demonstrate the potential interest of using such a structure for chemistry.20-22 Two kinds of experiments have been reported elsewhere: in the first one, encapsulation of fatty acid in phospholipidic onions allowed to create a difference in pH, the pH inside being lower than that outside.20 In the second one, enzyme (phosphatase alcaline) encapsulated in spherulites proved to have no activity on its substrate dissolved in the bulk phase. Destruction of the spherulites allowed recovery of the whole enzymatic activity.32 The aim of the present work is to give experimental evidence that reactions can take place into spherulites, one of the most challenging applications being of course the synthesis of small particles. We choose as an example the reduction of copper sulfate by hydrazine. Localization of the reaction in the spherulites is possible because one of the reactants (copper ions) is maintained in the vesicles and the other one (hydrazine) can diffuse from the bulk into the spherulites. In the first section we report an efficient technique to entrap copper salt despite the high permeability of spherulites toward inorganic salts. Copper ions were bound to the membrane by the use of the surfactant Genamin TO20 (R-N[(C2H4O)x-H)][(C2H4O)y-H)], where x + y ) 2), which has a polar headgroup with ligand properties. Thanks to the large number of surfactant“ligand” bilayers, a large amount of copper ions can be encapsulated into the vesicles volume and upon dilution very limited leakage is observed. In the second section we describe the chemical reaction itself. Permeability of spherulite bilayers allows the diffusion of hydrazine into the spherulites, and reduction of the copper salt occurs inside the surfactant aggregates. The resulting copper(I) oxide nanoparticles are embedded in the spherical surfactant structure. They can be isolated by disrupting the template. In the present case the spherulites were destroyed by addition of acid to the medium, using the fact that the protonated surfactant forms micelles instead of a lamellar phase. Their structure and chemical composition are investigated by various techniques including electron microscopy, X-ray diffraction, and light-scattering techniques. Although the chemical reaction itself is not original, we believe that our work demonstrates the possibility of using spherulites as chemical microreactors. (30) The company CAPSULIS S.A. is using this technology. (31) Degert, C.; Gauffre, F.; Roux, D.; Milner, S.; Laversanne, R. To be submitted. (32) Bernheim, A.; Gauffre, F.; Roux, D.; Ugazio, S.; Viratelle, O. To be submitted.

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Figure 3. Experimental phase diagram of the GTO20/H2O/ CuSO4 system: LR, lamellar; L1, micellar; L2, crystalline. The dashed lines are approximate boundaries.

II. Preparation of Spherulites Encapsulating Cu(II) Ions The choice of the surfactant system (in other words the “formulation”) is of crucial importance. It has to satisfy the following criteria: (1) the surfactant gives lamellar phases in the presence of a high concentration of copper salt; (2) shearing lamellar phases of this system leads to the formation of spherulites; (3) encapsulated ions do not leak out from spherulites in water dispersions. This last point suggests either to use nonpermeable spherulites (for example by covering the external layer by a polymeric shell) or to strongly enhance the affinity between the ions and the surfactant bilayers of the spherulites. Note that in order to carry out a chemical reduction of the encapsulated ions, the reducing species has to diffuse through the membranes into the spherulites. Thus it seems preferable to hold back ions into spherulites by creating an enhanced affinity for the membrane (for instance by displacing a chemical equilibrium) rather than by kinetically blocking the diffusion through membranes. We used the commercial surfactant GenaminTO20 (GTO20). This surfactant is a fatty amine with the following formula R-N[(C2H4O)x-H)][(C2H4O)y-H)], where x + y ) 2. Most of the aliphatic chains contain 17-18 atoms of carbon. GTO20 forms lamellar phases with water on a large domain of concentrations. It also possesses an ethoxylated amino headgroup that proved to have a very high affinity with copper(II) ions because of the formation of coordination compounds with those ions. Moreover, GTO20 forms micelles in aqueous acid solutions (pH < 4) instead of a lamellar phase at higher pH values. This will be used in the following to destroy surfactant spherulites and recover the particles that were synthesized. Phase Diagram. Samples were prepared by diluting pure surfactant with copper sulfate aqueous solutions. We explored therefore different dilution lines corresponding to fixed values of the ratio water/CuSO4, i.e., to fixed values of the concentration C (in grams per liter) of the aqueous copper sulfate solution used to dilute the surfactant. The phase diagram (Figure 3) was constructed by microscopic examination of the samples between crossed polarizers and also by X-ray scattering measurements when possible. Only the position of the lamellar phase (LR) domain was quantitatively determined. It extends on a wide region of the phase diagram. The micellar domain (L1) is restricted to very low surfactant concentrations and the GTO20 crystalline phase (L2) to very high surfactant concentrations. The Bragg peak position on X-ray scattering spectra allows measurement of the repeating distance in LR phases. Figure 4 shows the swelling of a GTO20/H2O/ CuSO4 lamellar phase diluted by a copper sulfate aqueous solution (C ) 200 g/L). Two linear regions are clearly seen.

Figure 4. Swelling of a GTO20/H2O/CuSO4 lamellar phase along a dilution line: dilution of pure surfactant by an aqueous solution C ) 200 g/l. In the LR domain the smectic length d is proportional to 1/φ (φ is the volumic fraction in surfactant). The maximal smectic length dmax is reached when demixion occurs.

Figure 5. Orientation diagram of GTO20/H2O/CuSO4 lamellar phases (C ) 60 g/L): zone I, randomly oriented; zone II, spherulites. The orientation state corresponding to zone III could not be characterized.

Below 1/φ ) 1.8 exists a linear swelling of the LR phase corresponding to d ) δ/φ in the monophasic region. Above 1/φ ) 2 the spacing saturates to a value around 55 Å. This corresponds to a two-phase region where excess water solution is expelled from the fully swollen lamellar phase. From the slope of the first part of the curve we can get the membrane thickness: d ) 29 Å. For dilution lines over C ) 50 g/L, the maximum dilution of the lamellar phase can be determined by X-ray scattering measurements. For C ) 50 g/L the maximum repeating distance of the lamellar phase is dmax ) 110 Å. Below C ) 50 g/L, very diluted lamellar phases with repeating distance over 120 Å can be obtained. Nevertheless, in this case, due to the high dilution and weak concentration in copper, contrast was not sufficient to determine the maximum repeating distance by X-ray scattering measurements. Shear Diagram of the Lamellar Phase. The orientation states of GTO20/H2O/CuSO4 lamellar phases were observed under shear with a microscope, using a transparent plate-plate cell to apply the shear. This cell is made of two parallel glass plates separated by a 0.5 mm gap. The upper plate remains stationary while the lower plate can rotate at a constant velocity ω. In this geometry

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Figure 6. (a) Two different behaviors of spherulite diameter D vs γ˘ : squares, “large” size spherulites (C ) 63 g/L, φ ) 0.54); circles, “small” size spherulites (C ) 144 g/L, φ ) 0.54). (b) Linear relationship between the spherulites and (γ˘ )-1/2 for “large” size population of spherulites.

Figure 7. Spherulites diameter vs C (φ ) 0.51, γ˘ ) 107 s-1). Empty squares, spherulites in close-packed organization (before dispersion); dots, spherulites dispersed into water. The transition between the “large” and “small” size appears clearly.

the shear rate depends linearly on the distance R from the center of the cell: γ˘ ) Rω/e (where e is the gap depth), which allows observation of different shear rates at the same time. Regions corresponding to different shear rates are observed by moving the cell under the microscope. All

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Figure 8. Size distribution curves of spherulites dispersed in water before and after adding hydrazine: (a) “large” size (C ) 80 g/L, φ ) 0.51, γ˘ ) 107 s-1); (b) “small” size (C ) 154 g/L, φ ) 0.51, γ˘ ) 107 s-1).

Figure 9. Encapsulation ratio vs copper/surfactant molar ratio (C ) 200 g/L, φ ) 0.54, manual shear). Spherulite dispersion: 1 g into 40 g of water.

observations are made between crossed polars. The orientation diagram for lamellar phases on a dilution line C ) 60 g/L is reported in Figure 5. Two steady states have been characterized, which are called I and II. In zone I (low shear) we observe between crossed polarizers a microscopic texture typical of a partially oriented lamellar phase. In zone II, the observed homogeneous texture is typical of spherulites. When a laser beam is directed

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Figure 10. SEM micrographs of spherulites after reduction of encapsulated copper sulfate: (a) C ) 85 g/L, φ ) 0.54, γ˘ ) 107 s-1; (b) C ) 80 g/L, φ ) 0.51, manual shear.

through the cell in an area corresponding to zone II, an isotropic ring of scattering can be observed on a screen. The ring is due to the diffusion of light by a monodisperse population of spherulites of micrometer scale in the sample. The size of the ring is an increasing function of the shear rate, indicating that the spherulite diameter decreases when increasing the shear rate. When high shear rates are reached, another transition occurs leading to a different type of orientation. For most of the systems studied up to now, at very high shear rates the lamellar

phase is oriented in the direction of flow and transition between the spherulite domain and this oriented state usually passes through an area where those two states coexist. In the present case, we could not investigate the upper limit of zone II because of the development of instabilities at high shear. However we could observe the disappearance of the scattering peak at high shears by running light scattering experiments through a transparent Couette cell, as will be described later. This is the limit between regions II and III.

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Figure 11. TEM micrographs of spherulites after reduction of encapsulated copper sulfate: (a) C ) 144 g/L, φ ) 0.54, manual shear; (b) C ) 85 g/L, φ ) 0.54, γ˘ ) 107 s-1; (c) expanded picture.

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Spherulites Size Measurements. Spherulite diameters (0.3-10 µm) are determined by light-scattering measurements. Lamellar phases are sheared in a homemade transparent Couette cell that consists of two concentric cylinders separated by a 0.5 mm gap. The outer cylinder remains stationary while the inner one rotates at a constant velocity. Spherulite diameters between 0.25 and 1.5 µm were measured in a regular light scattering setup: the transparent Couette cell is placed in the setup, a laser beam is directed through the cell, and the scattered intensity is measured as a function of the scattering vector q. A peak is observed at a position q ) qmax, which gives the diameter of the spherulites D ) 2π/qmax. Diameters above 1.5 µm were determined by measuring the size of the scattering ring on a screen. Two different behaviors were observed depending on the copper concentration (Figure 6). (1) All samples on dilution lines below C ) 90 g/L exhibited a linear relationship between the spherulites diameter (typically between 1 and 10 µm) and 1/(γ˘ )1/2 (Figure 6b). This behavior has been previously described for different surfactant systems and is explained on the basis of dimensional arguments.16 (2) On the opposite, the behavior of the most concentrated dilution lines is quite unusual. The spherulites formed are smaller than those obtained in the previous case (D . 0.3 µm) and show very little size variation with the shear rate (Figure 6a). Spherulites corresponding to these two different behaviors will be called in the following “large” and “small” spherulites, respectively. Figure 7 shows the evolution of the size at a given shear rate (γ˘ ) 107 s-1) and volumic fraction in membrane (φ ) 0.51) for different values of C. The transition between the formation of large and small populations of spherulites appears clearly at approximately C ) 90 g/L. If we stop the shear, the spherulite structure does not evolve within days. Indeed, there is no noticeable evolution of the ring of scattering observed on a screen when a laser beam is passed through the sample. The sample can even be removed from the Couette cell without being destroyed. Spherulite Dilution. As was previously reported for some other surfactant systems, spherulites can be dispersed in an excess amount of solvent. Indeed, when water is added under gentle stir to the previous samples (φ ) 0.51, 10 < C < 220 g/L) dispersed spherulites can be observed under a microscope. Diameters were measured by dynamic light scattering. Figure 8 shows two typical size distribution curves corresponding to the large and small size populations (dashed lines); dotted lines show the size distribution after addition of hydrazine (see below). In this experiment, samples were sheared at the same shear rate γ˘ ) 107 s-1. Results are reported Figure 7 (dots), indicating that sizes remain unchanged for the “small” spherulites but are increased by about 40% for the “large” ones. This result can be qualitatively explained by considering the phase diagram. Indeed, the lamellar phases containing less copper sulfate are submitted to larger swelling upon dilution into water (see for example dilution from A to A′ on the phase diagram Figure 3), leading to a correlated swelling of the spherulites. On the opposite, the swelling of the most concentrated lamellar phases (for example from B to B′) is very slight; the spherulites are then very little modified by the dispersion. Large size spherulites also exhibit less stability while dispersed into water. We therefore used only samples highly concentrated in copper sulfate to run the particle synthesis experiments. Encapsulation Ratio Measurements. The encapsulation ratio τ is defined as the number of Cu(II) ions

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Figure 12. Evolution of aggregates size after HCl addition (pH