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Supramolecular Colloidosomes Based on Tri(dodecyltrimethylammonium) Phosphotungstate: A Bottom-Up Approach Loïc Leclercq,† Adrien Mouret,† Pierre Bauduin,‡ and Véronique Nardello-Rataj*,† †

Université Lille Nord de France, Université Lille 1, EA 4478, Chimie Moléculaire et Formulation, F-59655 Villeneuve d’Ascq, France Institut de Chimie Séparative de Marcoule, UMR 5257, CEA/CNRS/UM2/ENSCM, BP 17171, CEA Marcoule, F-30207 Bagnols-sur-Cèze, France

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ABSTRACT: Hybrid tri(dodecyldimethylammonium) phosphotungstate ([C12]3[PW12O40]) amphiphilic nanoparticles self-assemble in situ at the water/toluene interface to form stable water-in-oil (W/O) Pickering emulsions (droplet size ≈ 20 μm). These emulsions are used as a template for the preparation of colloidosomes (ϕ ≈ 5 μm), which are produced solely through the self-assembly properties of the [C12]3[PW12O40] nanoparticles into a “fused” phase on the water-drop surface in contact with toluene. The structure of the emulsions has been determined using optical and cross-polarized light microscopy, while the colloidosomes have been characterized by scanning electron microscopy (SEM). The structure as well as the aggregation behavior of these nanoparticles has been investigated. Small- and wide-angle X-ray scattering (SWAXS) and transmission electron microscopy (TEM) experiments have revealed a lamellar organization of the inorganic polyoxometalate anions because of the van der Waals interactions between the alkyl chains of the organic cations. According to the solvent, the internal molecular arrangement inside the nanoparticles can be modified: in water, the nanoparticles tend to aggregate in a lamellar structure, whereas in toluene, the nanoparticles are “fused” or coagulated.

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INTRODUCTION Increasing attention has been devoted to microtechnology in recent years. Indeed, this field opens new perspectives in every area of the human development from electronic toward medicine. For example, nanometer- to micrometer-sized particles with well-defined electrical and mechanical properties are used in new contrast agents for imaging, encapsulation, and controlled release of drugs. Accordingly, an abundant and always increasing number of microencapsulation processes can be found in the literature.1 On the other hand, solid colloidal particles, such as modified silica,2,3 metallic oxides and hydroxides,4,5 cyclodextrins,6,7 or polymers,8,9 have been shown to be able to stabilize emulsions, providing the so-called Pickering emulsions. Indeed, such particles can adsorb irreversibly at the water−oil interface, leading to emulsions even more stable than those obtained with classical molecular surfactants.7 The nanosized particles can have different shapes, such as spheres or rods.10−13 Under appropriate conditions, the colloidal particles used for Pickering stabilization can also form microcapsules, called colloidosomes.2−9 Velev et al. have reported for the first time the production of colloidosomes in 1996, although the name was coined much later by Dinsmore et al., because of the analogy with liposomes.14,15 Colloidosomes are assembled from colloidal particles, which are anchored at the interface of oil© 2014 American Chemical Society

in-water or water-in-oil (W/O) emulsions. Generally speaking, it is easier to assemble colloidosomes from W/O emulsions, because they are closer to the interface, reducing the concentration of particles needed and producing shells with a better organization.16 The template (i.e., the oil droplet) determines the size of the colloidosomes. Two key parameters are important to consider for the formation of colloidosomes from Pickering emulsions. The first one is the wettability of the particles by the water and oily phases. Indeed, the particles that are partially hydrophobic are better stabilizers because they are partially wettable by both liquids, and therefore, they anchor better to the surface of the droplets. To avoid some restriction in the kind of colloidal particles that can be used, it is possible to tune the particle wettability by either chemical modification14 or surfactant adsorption.17 The second key parameter is the mechanism used to lock the particles together. Particles can be fused together according to three main approaches: (1) heating the system near the melting or glass temperatures of the colloidal particles to induce aggregation,18 (2) introducing a second phase that glues the particles together,19 or (3) inducing a covalent cross-linking from the oil or water phases.20 Received: January 29, 2014 Revised: April 29, 2014 Published: April 30, 2014 5386

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Figure 1. Typical TEM image of [C12]3[PW12O40] nanoparticles after dispersion in water (10−4 M) at different magnifications. The size distribution of the nanoparticles is presented (the average particle size is 35.2 ± 6 nm). A schematic representation of the molecular arrangement in the lamellar organization of the [PW12O40] anions inside the nanoparticle is also presented. This sketch is idealized because it is known from the SWAXS spectra shown in Figure 3 that neither the POM nor the alkyl chain parts present a crystalline arrangement.

anion is maintained here by van der Waals interactions between the alkyl tails of the ammonium cations. Figure 1b shows the lamellar-packing arrangement of the POM units and their schematic representation, where the alkyl chains of the cations are overlapped to generate this particular arrangement. Indeed, in this arrangement, the [PW12O40] anions are trapped between the ammonium bilayers. In other words, the van der Waals interactions between the alkyl chains of the ammonium cations act as a supramolecular organic matrix, which allows for the stabilization of the nanoparticles in combination with entropic forces. Indeed, pure water molecules adopt a structure that maximizes the entropy because of a dynamic three-dimensional network of water molecules. The non-hydrogen-bonding part of the cationic surfactants linked electrostatically to the POM unit partially disrupts this dynamic network, creating a cavity that is unable to interact with the water molecules. The water molecules rearrange themselves around the hydrophobic surface portion, to minimize the number of disrupted hydrogen bonds. When more than one cavity is present, the surface area of disruptions is high, meaning that there are fewer mobile water molecules. To maximize the entropy of the system, the hydrophobic parts are expelled to form a cage structure around them with a smaller surface area than the total surface area of the various cavities.25 Therefore, the nanoparticle formation is primarily entropy-driven via the hydrophobic effect and secondarily enthalpy-driven via non-covalent attractive forces, such as electrostatic and dispersion forces. All of these interactions ensure the cohesion inside the nanoparticles, unlike other nanoparticles that require sophisticated methods. It is noteworthy that the speed and time of addition of H3[PW12O40] to an aqueous solution of [C12][OH] as well as stirring and temperature have no influence on the size or structure of the nanoparticles. The formation of uniform size particles is noteworthy and may be due to electrostatic effects. We propose the following process as a possible explanation for the formation of nanosized particles. The formation of particles here is the result of the strong electrostatic attraction between the soft negatively charged POM, bearing three negative charges, and the cationic surfactant. A stoichiometric charged compensation leads to the formation of an uncharged complex of the type [C12]3[PW12O40] that shows locally a lamellar structure observed by small- and wide-angle X-ray scattering (SWAXS) and TEM. During the growth process of the particles, some defects may appear in the lamellar arrangement and may lead to the appearance of surface charges that would

In this paper, the microcapsules are obtained according to a new approach, complementing the classic route to assemble colloidosomes from Pickering emulsions and allowing for the development of colloidosome-like shells without applying harsh methods to lock the colloidal particles together. The colloidosomes are based on amphiphilic polyoxometalate nanoparticles, resulting from the electrostatic coupling of [PW12O40] anions and dodecyltrimethylammonium cations ([C12]). In the presence of water and an aromatic oil, such as toluene, these nanoparticles give stable Pickering emulsions,21 which have been shown to be particularly efficient catalytic reaction media for hydrogen-peroxide-based oxidations. Here, we report especially on the internal structure of the nanoparticles and the solvent effects on this structure. As results, the particles can be self-assembled in the Pickering emulsion to allow an easy fabrication of novel hybrid colloidosomes with a shell of self-assembled nanoparticles made of tri(dodecyldimethylammonium) phosphotungstate ([C12]3[PW12O40]).

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RESULTS AND DISCUSSION Synthesis of the [C12]3[PW12O40] Nanoparticles. The preparation of [C12 ]3 [PW12 O 40 ] nanoparticles is quite straightforward and based on the anion metathesis between H3[PW12O40] and [C12][OH] in aqueous solution. Almost instantaneously, a colorless precipitate of tri(dodecyldimethylammonium) phosphotungstate, [C12]3[PW12O40], was formed. A typical transmission electron microscopy (TEM) image of the spherical nanoparticles formed is presented in Figure 1. The nanoparticles are isotropic (i.e., low aspect ratio) in shape (Figure 1a). These results illustrate the formation of quasi-spherical nanoparticles through the combination of dodecyldimethylammonium cations, [C12], and phosphotungstate anions, [PW12O40]. Figure 1a also shows a histogram of the particle size distribution, with a mean particle diameter of 35.2 ± 6 nm. More than 80% of the particles are in the size range from 23 to 44 nm, indicating the spontaneous size selectivity during the formation process mainly driven by van der Waals interactions.21 The local arrangement of [C12]3[PW12O40] inside the nanoparticles can be determined thanks to TEM analysis. As previously reported, the structure of these inside nanoparticles is unusual and specific.21 Indeed, contrary to the structure of [NH4]3[PW12O40],22−24 in which the [PW12O40] units are connected by the acidic protons, each polyoxometalate (POM) 5387

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density of the alkylammonium organic counterpart. The experiment was performed on a lab setup that enables us to access not only the small-angle (q < 5 nm−1) region but also the wide-angle (q > 10 nm−1) region, such as in classical powder diffraction experiments, but here, the spectra were collected in transmission. Consequently, the distance scale covered here in a single experiment ranged from angstroms to ≈30 nm. As depicted in Figure 3, in the intermediate q range between the small- and wide-angle regions (5−10 nm−1), i.e., in real space between around 0.6 and 1.2 nm, an intense signal is observed. This contribution to the spectrum is attributed to the scattering of the POM itself, which has a diameter of ≈1 nm. This signal is relatively independent of the nature of the solvent, and no significant difference is observed with the dry powder. In the wide-angle region, large signals are observed for the samples containing a solvent. This region corresponds to the interatomic correlations and is here mainly due to the contribution of the solvent. In the spectra of the dry powder, the scattered intensity is rather low and flat. The absence of Bragg peaks in this region informs us of the low crystallinity of the nanoparticles. In the low q range, the scattered intensity shows a q−4 dependence, which is related to the scattering of a well-defined interface between two media of different scattering length densities, i.e., the nanoparticle/solvent or nanoparticle/ air interfaces. The interesting part in the SWAXS spectra concerning the internal molecular arrangement inside the nanoparticles ranges between 1 and 5 nm−1. In this region, the scattering spectrum shows a broad and intense peak at q10 values ranging from 1.85 to 2.1 nm−1 depending upon the nature of solvent used (see Table 1). A peak of lower intensity

limit the particle growth. The particles show a measurable negative ζ potential (−35.5 mV) that is in agreement with this possible process that limits the particle growth. This electrostatic effect may be substantial and may be at the origin of the rather monodisperse size repartition that is obtained. In a recent work, Coulomb explosion was given as a possible explanation of the formation of large monodisperse vesicles obtained with metallacarborane compounds;26,27 this type of effect may be involved as well here in the formation of nanoparticles. During our several attempts to observe the nanoparticles, we have surprisingly noticed that the size distribution of the nanoparticles increases in aromatic solvents (e.g., toluene) compared to water (Figure 2). Indeed, the mean particle

Figure 2. Typical TEM images of [C12]3[PW12O40] nanoparticles (10−4 M) after dispersion in (a) water and (b) toluene. The scale bar corresponds to 100 nm.

diameter is about 51.5 ± 10 nm in toluene versus 35.2 ± 6 nm in water. This difference can be due to the greater or lesser ability of the solvent to penetrate the nanoparticles (see below). The incorporation of toluene between the alkyl chains of the cations distorts the nanoparticles that results in a non-spherical shape. Influence of the Solvent on the Internal Structure of the [C12]3[PW12O40] Nanoparticles. To obtain better insight into the solvent effect on the internal structure of the nanoparticles, SWAXS experiments were performed on the [C12]3[PW12O40] nanoparticles (Figure 3). Indeed, SWAXS is sensitive to electron density inhomogeneity in the sample. The contrast in the nanoparticles studied here is basically observed between the POM, which is an electron-rich species, mainly because of the presence of tungsten atoms, and the low electron

Table 1. Values of the Lamellar Peaks (q10 and q20), Layer Thickness (d*), and Variation of the Layer Thickness (Δd*) Observed for [C12]3[PW12O40] Nanoparticlesa dry particles water toluene m-xylene

q10 (nm−1)a

q20 (nm−1)a

d* (nm)b

Δd* (nm)c

2.13 2.10 1.96 1.86

4.2 4.2 4.0 3.9

2.95 2.99 3.20 3.38

0 0.04 0.25 0.43

a c

Determined from SWAXS spectra. bCalculated from d* = 2π/q10. Calculated as Δd* = d* − d* (dry particles).

(q20) is observed at exactly 2 times the q10 value. This set of peaks corresponds to a lamellar structure constituted by layers of closed packed [C12]3[PW12O40] units, with the alkyl chains of the surfactant part being in an extended conformation.21 From q10 values, the interlamellar spacing can be evaluated as d* = 2π/q10 (see Table 1). For the dry nanoparticles, d* (2.95 nm) corresponds to the sum of the length of the surfactant, i.e., around 1.95 nm in an extended conformation, and the size of the POM (1 nm). Consequently, the molecular arrangement in the lamellar structure, as proposed in Figure 1b, is confirmed. In water, the SWAXS spectrum is not altered; therefore, the interlamellar spacing remains as in the dry powder, meaning that water has no effect on the internal structure of the nanoparticles that are simply dispersed in water. On the opposite, in the presence of toluene and m-xylene, the lamellar peaks (q10 and q20) are significantly shifted to lower q values, meaning that the interlamellar distance increases (see Table 1). This swelling corresponds to an increase in d* of 0.25 and 0.43 nm, respectively, for toluene and m-xylene, which is in the order of the solvent molecular size. This effect can then be

Figure 3. SWAXS of [C12]3[PW12O40] nanoparticles dispersed in different solvents: water (purple), toluene (green), and m-xylene (red). The blue curve corresponds to the dry nanoparticles (powder). 5388

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Figure 4. TEM images of [C12]3[PW12O40] nanoparticles in water: (a) 10−4 M, (b) 10−3 M, (c) 10−2 M, (d) 10−1 M, (e) 5 × 10−1 M, and (f) schematic representation of the aggregation process.

Figure 5. TEM images of [C12]3[PW12O40] nanoparticles in toluene: (a) 10−4 M, (b) 10−3 M, (c) 10−2 M, and (d) 5 × 10−1 M.

Figure 6. Variation of transmission and backscattering at 25 °C versus the sample height and time of the [C12]3[PW12O40] nanoparticles (10−2 M) dispersed in water (a and a′), dispersed in toluene (b and b′), and representation of the aggregation/clarification/sedimentation or creaming process (T, transmitted light; BS, backscattered light).

performed a study of the aggregation process in water and toluene. We have already reported on the aggregation properties of [C12]3[PW12O40] nanoparticles in water.28 Here, we compared the aggregation of [C12]3[PW12O40] nanoparticles dispersed in water or toluene at various concentrations. Two drops of each dispersion were deposited on a carbon−copper grid and analyzed by TEM. As depicted in Figures 4 and 5, the nanoparticles tend to aggregate. It is clearly shown that the aggregation behavior of [C12]3[PW12O40] nanoparticles is totally different in water and toluene. Indeed, in

interpreted as the intercalation of the solvent molecules in the lamellar structure, which is not surprising because toluene and m-xylene are good solvents of the surfactant alkyl chains. The difference in the swelling observed between toluene and mxylene can be attributed to the difference in their molecular size. Influence of Solvents on the Aggregation and Dispersion of Nanoparticles. It is likely that the solvent effect on the nanoparticle internal structure is also observed on the local structure at the nanoparticle surface. Indeed, we 5389

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water, the nanoparticles tend to aggregate in a lamellar structure, whereas in toluene, the nanoparticles are fused or coagulated. In water, the structure of the nanoparticles is maintained (compare Figures 4e and 5d) and the aggregation takes place at a long scale. In contrast, a good alkyl chain solvent (e.g., toluene) leads some alkyl chains to point out of the nanoparticles. In this case, a fusing of the particles is observed. Therefore, a major effect in the control of the aggregation/self-assembly process of the nanoparticles is the penetration degree of the solvent across the organic matrix according to its molecular structure and polarity. To obtain better insight into the dispersion stability of the nanoparticles in water or toluene, we have performed multiple light scattering (MLS) to detect concentration variation in the colloid by scanning the whole height of the sample in transmission and backscattering (Figure 6). In the two solvents, the transmitted and backscattered light fluxes are modified with the time compared to the initial scan (baseline). In water, there is a clear increase of the transmission signal at the bottom of the sample because of a creaming process. The phase thickness of the transmission peak is gradually increased following the time. In 2 h, the phase thickness is about 50% of height of the sample (purple arrow). The backscattering curves show an increase of the signal at the top of the sample bottle because of an increase of the nanoparticles (black arrow) and an opposite behavior in the bottom of the sample because of clarification (gray arrow). In toluene, the instability process is totally different. Indeed, it can be clearly seen that the transmission peak gradually increases from top to bottom, and after 2 h, the phase thickness is about 60% of height of the sample (purple arrow). The backscattering profile clearly reveals a sedimentation process (the backscattering signal increase in the bottom of the sample; see black arrow) associated with a clarification at the top of the sample (the backscattering signal decrease; see gray arrow). This process is accompanied by a clear migration of the particles from the top to the bottom of the sample (purple arrow). These experimental results are in good agreement with TEM and SWAXS data. Indeed, in water, the interlamellar spacing between the POMs inside the nanoparticles remains the same as for the dry powder, whereas toluene penetrates into the nanoparticles, leading to their size increase. This toluene insertion results in the “fusion” of the nanoparticles, which become heavier than in water, resulting in a sedimentation process. In water, because the nanoparticles are also selfassembled but without “fusion” and because the nanoparticles are hydrophobic, the particles migrate to the surface. It is noteworthy that the destabilization process is faster in water than in toluene at the beginning (compare the initial transmission variation in panels a and b of Figure 6). Emulsification and Colloidosome Formation. As previously reported,21 stable W/O Pickering emulsions can be obtained in situ when water and toluene (water/toluene = 3, v/v) are emulsified in the presence of 0.5 wt % [C12]3[PW12O40] nanoparticles. Figure 7 shows a picture of the whitish emulsion analyzed by optical microscopy. As the volume fraction of the drops approaches close packing, they deform into polyhedral shapes and flattened areas of contact between droplets appear.29 After dilution into toluene (Figure 7b), spherical droplets are obtained and the [C12]3[PW12O40] nanoparticles are adsorbed at the interface because the birefringence is located on the surface droplets (Figure 7c). The average droplet size of the stable Pickering emulsion has been estimated around 20 ± 2 μm.

Figure 7. Toluene/water/[C12]3[PW12O40] emulsion (23.8:71.4:4.8 wt %, 11 500 rpm, 60 s) and microphotographs of the (a) concentrated emulsion, (b) diluted emulsion in toluene (dilution factor of 1/8), and (c) diluted emulsion under cross-polarization.

The physicochemical properties of the [C12]3[PW12O40] nanoparticles (i.e., the fusing of the particles in toluene and lamellar organization of the nanoparticles in water) combined with its ability to form Pickering emulsions make such a system of particular interest for the formation of colloidosomes. The basics of the method are illustrated in Figure 8. The synthesis

Figure 8. Schematics of the method for preparation of [C12]3[PW12O40] colloidosomes.

involves two steps: (i) water and toluene phases are emulsified in the presence of POM nanoparticles to produce a stable W/O Pickering emulsion at 25 °C, and (ii) the vaporization of the solvents under air-drying or vacuum provides colloidosomes. To investigate the packing and ordering of the particles at the oil−water interface, scanning electron microscopy (SEM) experiments were performed. The toluene/water/ [C12]3[PW12O40] emulsion (23.8:71.4:4.8 wt %, 11 500 rpm, 60 s) was deposited under air-drying on a aluminum stub (1.3 cm radius). The colloidosomes could thus be directly observed using conventional SEM after vaporization under vacuum or air-drying (Figure 9). This experiment confirmed that the spherical structures, observed with the optical microscope, are Pickering W/O emulsion droplets (Figure 9). However, if the evaporation is too fast (vacuum treatment), the colloidosomes are easily destroyed; i.e., collapsed colloidosomes are formed (panels a, b, and c of Figure 9). Indeed, the [C12]3[PW12O40] nanoparticles are held together by hydrophobic bonding (see above). As a result, we were able to observe a crack-opened shell, which reveals that the nanoparticles were fused on the water-droplet surface because of the presence of the continuous phase (see above). In contrast, with an appropriate evaporation, we can observe intact colloidosomes (panels a′, b′, and c′ of Figure 9). A careful look at the structure of the colloidosome shell reveals the presence of fused [C12]3[PW12O40] nanoparticles. This illustrates that the [C12]3[PW12O40] nanoparticles are suitable 5390

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Figure 9. SEM images of [C12]3[PW12O40] colloidosomes observed after evaporation of toluene and water core by vacuum treatment (collapsed structures = a, b, and c) and observed after evaporation of the toluene by air-drying treatment (intact structures = a′, b′, and c′). Note that panel c′ represents the surface of the colloidosomes. emulsifications were performed using Ultraturrax (11 500 rpm, 60 s, IKA, T10 basic). Characterization Methods. The 1H, 13C and 31P nuclear magnetic resonance (NMR) spectra were recorded using an Advance 300 Bruker spectrometer, at 300.13, 75.49, and 121.49 MHz, respectively. Chemical shifts are given in parts per million (ppm) (δ) and measured relative to residual solvent for 1H and 13C and to external reference (H3PO4) for 31P{1H} NMR. Infrared (IR) analyses were performed with a Fourier transform infrared (FTIR) spectrometer PARAGON 500 (PerkinElmer) using KBr wafers containing 5 wt % product by scans of 64 with a resolution of 1 cm−1. ζ potentials were measured using a Zetasizer nanosizer (Nano ZS, Malvern). Synthesis of Tri(dodecyltrimethylammonium) Phosphotungstate [C12]3[PW12O40] Nanoparticles. Dodecyltrimethylammonium bromide [C12][Br] (19.5 mmol) was dissolved in water (100 mL). The aqueous solution of [C12][Br] was eluted on an hydroxide ion-exchange resin to obtain an aqueous solution of [C12][OH]. An aqueous solution of H3[PW12O40] (around 6.5 mmol, 10−4 M) was added dropwise (3 mL/min) to the aqueous [C12][OH] solution (3 × 10−4 M) until reaching pH 7 at 25 °C under dry argon and vigorous magnetic stirring (1500 rpm). The colorless precipitate of tri(dodecyltrimethylammonium) phosphotungstate ([C12]3[PW12O40]) formed within a few minutes was washed with water and lyophilized. For all synthesized salts, no melting points were observed between 20 and 260 °C. It is noteworthy that the synthesis was made in the presence of argon to eliminate oxygen. 1 H NMR (300 MHz, DMSO-d6, 20 °C, TMS) δ (ppm): 0.82−0.89 (m, 3H, CH3), 1.18−1.31 (m, 18H, CH2), 1.66 (m, 2H, N−CH2− CH2), 3.03 (s, 6H, NCH3), 3.24 (m, 4H, NCH2). 13C NMR (75 MHz, DMSO-d6, 20 °C) δ (ppm): 14.4, 22.5, 22.6, 26.2, 28.9 29.2, 29.3, 29.4, 29.5, 31.8, 52.6, 65.8. 31P{1H} NMR (121 MHz, DMSO-d6, 20 °C) δ (ppm): −15.6. IR (KBr): ν̃ (cm−1): 800 (W−Oc−W), 889 (W− Ob−W), 982 (W−Od), 1080 (P−Oa), 1476 (C−H), 2850 (C−H), 2952 (C−H). Anal. calcd for (C45H102N3, PW12O40), 2.5(H2O): C, 14.98%; H, 2.99%; N, 1.16%; P, 0.86%; W, 61.15%. Found: C, 15.39%; H, 2.99%; N, 1.35%; P, 0.80%; W, 60.79%. Yield: >99%. Nanoparticle Characterization. The nanoparticles were examined using a FEI Tecnai G2-20 twin transmission electron microscope. Before analysis, the powders were dispersed in solvents (water or toluene). Two drops of dispersion containing the nanoparticles were then deposited on a carbon−copper grid. SWAXS. SWAXS data were obtained using a XENOCS setup. The X-ray beam originates from a Mo GENIX source. The Kα radiation (λ = 0.71 Å) is selected using a multi-layered curved mirror (one reflection), focusing the beam at the infinite. The size of the beam (less than 1 mm2) in front of the sample is defined by scatterless slits provided by FORVIS.30 Sample and empty cell transmissions are

for building strong microcapsule membranes. It is noteworthy that the obtained structures are stable upon redispersion in water.

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CONCLUSION

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EXPERIMENTAL SECTION

A hybrid [C12]3[PW12O40] amphiphile, which stabilizes emulsions formed as a result of growth and adsorption of [C12]3[PW12O40] nanoparticles at the water−toluene interface, has been investigated. This phenomenon represents an interesting effect, where the adsorption of insoluble [C12]3[PW12O40] nanoparticles leads to a Pickering emulsion stabilized by solid particles. We have studied the structure as well as the aggregation behavior of [C12]3[PW12O40] nanoparticles, which self-assemble in situ to stabilize the W/O emulsions. The [C12]3[PW12O40] nanoparticles (d ≈ 35 nm) constitute a lamellar organization of the inorganic polyoxometalate anions because of the van der Waals interactions between the alkyl chains (C12) of the alkylammonium cations. However, the internal molecular arrangement inside the nanoparticles can be modified according to the penetration degree of the solvent. For example, in toluene, the intercalation of the solvent molecules into the lamellar structure allows for a clear increase of the average size from ±35 nm in water to ±50 in toluene. As a result, in water, the nanoparticles tend to aggregate in a lamellar structure, whereas in toluene, the nanoparticles are fused or coagulated (without heating). Under these conditions, the Pickering emulsions can be used as a winning platform for the formation of a novel type of colloidosomes. For the first time, colloidosomes are spontaneously produced by removing the oil from the [C12]3[PW12O40]-stabilized W/O emulsion by evaporation. We demonstrate here that the obtained structure is due to a consecutive bottom-up supramolecular organization. These new microcapsules are held together only by supramolecular interactions and can potentially find application in the development of new catalytic systems and extended to other POMs.

General Information. DMSO-d6 and all other chemicals were purchased from Aldrich. Distilled deionized water was used in all experiments. All solvents were degassed by bubbling nitrogen for 15 min before each use or two freeze−pump−thaw cycles before use. All 5391

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determined using an offline pin diode that can be inserted downstream of the sample. The sample to the 2D MAR345 is 750 cm. SWAXS corresponding to a q range [q = 4π/λ sin(θ/2), with θ being the scattering angle] between 0.02 and 2.5 Å−1 is recorded in one acquisition. Quartz capillaries are used as sample containers; usual corrections for background (empty cell and detector noise) subtractions were applied. Dispersion Stability Analysis. The stability of the [C12]3[PW12O40] nanoparticle dispersions was analyzed with the Turbiscan Lab Expert (Formulaction). The dispersions were placed in cylindrical glass tubes and subjected to Turbiscan Lab Expert stability analysis. The analysis of stability was carried out as a variation of transmission (ΔT) and backscattering (ΔBS) profiles. Measurements were carried out using a pulsed near-IR light-emitting diode (LED) at a wavelength of 880 nm for 2 h. Two different synchronous optical sensors received the light transmitted through and backscattered by samples at an angle of 180° and 45° with respect to the incident radiation, respectively. The two sensors scanned the entire height (≈35 mm) of the dispersion (15 mL) for 2 h. Experimental data were correlated in percentage to the light flux of two reference standards constituted by a polystyrene latex suspension (absence of transmission and maximum backscattering) and a silicon oil (maximum transmission and absence of backscattering). Emulsion Characterization. Microphotographs were obtained using an optical polarizing microscope (Standard 25 ICS, Zeiss) coupled with a charge-coupled device camera (Digital Still Camera, Sony) and with a LTS120 Analysa Peltier temperature stage capable of controlling the temperature to ±0.1 °C. Images were analyzed with ImageJ software (National Institutes of Health, Bethesda, MD) to obtain the size of the droplets. The distribution function is obtained by treatment of experimental data with log-normal function (OriginPro 8, OriginLab Corporation, Northampton, MA). Colloidosome Characterization. The colloidosomes were examined using a Hitachi S4700 scanning electron microscope. Two drops of the Pickering emulsion were then deposited on an aluminum stub (13 mm radius). The emulsion solvents were vaporized under vacuum or air-drying before analysis.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

The authors are grateful to the Fonds Européens de Développement Régional (FEDER) and the Agence Nationale de la Recherche (ANR, Project ANR-10-CD2I-01) for financial support and Ahmed Addad for performing TEM experiments with the Université Lille 1 facility (Centre Commun de Microscopie).

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