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Self-Assembled Polyoxometalates Nanoparticles as Pickering Emulsion Stabilizers Loic Leclercq, Adrien Mouret, Severine Renaudineau, Véronique Schmitt, Anna Proust, and Véronique Nardello-Rataj J. Phys. Chem. B, Just Accepted Manuscript • Publication Date (Web): 04 May 2015 Downloaded from http://pubs.acs.org on May 4, 2015

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The Journal of Physical Chemistry

Self-Assembled Polyoxometalates Nanoparticles as Pickering Emulsion Stabilizers Loïc Leclercq,a Adrien Mouret,a Séverine Renaudineau,b Véronique Schmitt,c Anna Proust,b and Véronique Nardello-Rataj,a* a

Université de Lille, Sciences et Technologies, UCCS UMR CNRS 8181, F-59655 Villeneuve d’Ascq

Cedex, France. * Corresponding author: E-mail:[email protected]. Tel: +33 (0)3 20 33 63 69. b

Sorbonne Universités, UPMC Univ Paris 06, UMR CNRS 8332, Institut Parisien de Chimie

Moléculaire, F-75005 Paris, France. c

Centre de Recherche Paul Pascal, Université de Bordeaux, UPR CNRS 8641, 115 Avenue du Dr.

Albert Schweitzer, F-33600 Pessac, France. RECEIVED DATE ABSTRACT. We easily produced a series or polyoxometalate (POM) nanoparticles by taking benefit from electrostatic attractions between various POMs and alkylammonium cations. These self-assembled supramolecular nanoparticles are fully characterized in terms of shape, nanostructure and physicochemical properties. The nanoparticles differences are discussed on the basis of the chemical composition of the initial POM. Moreover such particles have the ability to stabilize water-in-oil Pickering emulsions. Using a gel-trapping technique coupled to atomic force microscopy (AFM) observations, we determined their affinity towards oil by the contact angle of adsorbed nanoparticles. We show that the emulsion droplet size and stability can directly be linked to the nanoparticles hydrophobicity which is tuned by the charge localization and molecular packing of POMs with the

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ammonium cations. Such particles are of special interest as they open large possibilities for Pickering interfacial catalysis. KEYWORDS. Amphiphilic polyoxometalates nanoparticles/ Pickering emulsion/ Self-assembly/ Particle interfacial adsorption/ Contact angle measurement. Introduction In 1991, Wuest et al. proposed to call tectons the molecules which play the role of building blocks in a self-assembled ordered supramolecular structure.1 Tectons form crystals, liquid crystals, organic gels and monolayers.2-4 The concept of tecton was later extended to ionic liquids based on imidazolium or thiazolium salts due to the strong intermolecular forces (H-bonds, aromatic stacking and electrostatic interactions) that lead to an extended 3D supramolecular network in the solid state.5,6 This network is maintained in liquid and gas phases7 and can be used to include ionic and organic compounds in the solid state or in solution.8,9 In parallel, the term "tecton" has been ascribed to polyoxometalate (POMs) and cationic surfactant ionic self-assemblies with liquid crystal properties.10 More recently, it has been shown that monodisperse and spherical solid nanoparticles (Ø ≈ 35 nm) resulting from the electrostatic interaction of phosphotungstate anions, [PW12O40]3-, with dodecyltrimethylammonium cations, abbreviated as [C12], in water, exhibit a well-organized structure based on a 3D supramolecular network.11 Unlike classical tectons, the self-assembly is finite but under appropriate conditions, it can be extended to form more complex structures such as supramolecular colloidosomes.12 The structure of the [C12]3[PW12O40] nanoparticles has been described as a local lamellar organization of the inorganic POM anions as a result of the combined effects of van der Waals interactions, electrostatic, hydrophobic and steric forces. All these contributions induce the spontaneous formation of spherical supramolecular nanoparticles in water and ensure the cohesion as well as the shape of the final structure. The effect of structural parameters of the cationic moiety have been thoroughly investigated and rationalized11,13 but not the nature of POM. Indeed, POMs exist in a wide variety of structures. As examples, the phosphomolybdate, [PMo12O40]3-, and the silicotungstate, [SiW12O40]4-, anions have the same structure ACS Paragon Plus Environment

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as [PW12O40]3-, displaying the so-called "Keggin type" with the generic formula [YM12O40]n-, where Y is the heteroatom (most commonly P, Si, or B), M is the addenda atom (most common Mo or W).14 The Keggin anions are particularly relevant for the investigation of the driving forces leading to nanoparticle formation because they offer some versatility in terms of structure (sphericity, charge number and localization, etc.) and thanks to the variety of transition metals likely to be incorporated in their structure, they can be used as catalysts for oxidation involving oxygen or hydrogen peroxide as oxidants.15-17 However, the main issue of classical POM salts (typically sodium and potassium) is their incompatibility with the hydrophobic substrates (e.g. alkenes, sulfides, long chains alcohol, fatty acids, esters, etc.). To overcome this drawback, porous crystalline framework18-20 and cationic micelles or substrates can be used to load POMs.21,22 In such conditions, recyclable quasi-homogeneous catalysts were obtained. An alternative is the use of POM-based hybrid catalysts as emulsifiers to increase the contact area between two immiscible phases during the reaction. These systems can be classified into two groups: the hybrids with electrostatic interactions (e.g. combination of alkylammonium and POMs23-28) and the hybrids with covalent bonds between the organic and inorganic components.29,30 Liu et al. have recently written an interesting review on this topic.31 The first family is very attractive because no specific synthetic steps are required. However, in these systems, the possible formation of a stable emulsion at the end of the reaction can complicate the separation of the two phases and obviously the recovery of the catalytic surfactant from the reaction mixture. Another alternative consists in resorting to "Pickering Interfacial Catalysis" (PIC)32,33 using solid amphiphilic POM nanoparticles able to stabilize emulsions as recently shown with [C12]3[PW12O40] amphiphilic catalytic nanoparticles which were successfully applied to hydrogen peroxide-based oxidations.11,32 Such catalytic Pickering emulsions, which are emerging as novel platforms for the design of efficient biphasic catalytic systems, offer a large water/oil interface while combining the advantage of the biphasic and heterogeneous catalysis without their drawback (e.g. catalyst leaching or long demixion time). As the concepts of "catalytic amphiphilic nanoparticles" and PIC are very promising for the future development of new green reaction media, it becomes essential to go deeper in the rationalization of ACS Paragon Plus Environment

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their bottom-up elaboration driven by supramolecular interactions and in their ability to stabilize Pickering emulsions. Hence, in the present work, we focused our attention, for the first time, on the relationships between the nature of the POM tectons, the nanoparticle structures, their adsorption at the interface and the emulsions properties. Dynamic light scattering, optical, electron and atomic force microscopies were used to correlate the emulsion properties (type of emulsion, drop size and stability), the POM characteristics (size, shape, electronic density, localization and value of partial negative charge on the bridged oxygen atoms of polyoxoanion and the supramolecular interactions between the POM and the ammonium tectons), the nanoparticles structure (size, shape), nanoparticle wettability (contact angle at the oil/water interface) and interface rigidity. It is noteworthy that the consecutive bottom-up supramolecular organization (POM and ammonium tectons → nanoparticles → interface → Pickering emulsion) is followed step by step throughout the manuscript. Experimental Section 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 by two freeze-pump-thaw cycles before use. All emulsifications were performed using Ultra-turrax (11 500 rpm, 60 seconds, IKA, T10 basic). Curable polydimethylsiloxane (PDMS) oil, namely Sylgard 184 (Dow Corning), was used as polymerizable oil for particle contact angle measurements. The native POMs are prepared using published method: K3[PW12O40],34 K4[PW11VO40],35 K5[PW10V2O40],35 K6[PW9V3O40],35

H3[PMo12O40],36

H4[PMo11VO40],37

H5[PMo10V2O40],37

Na3[AsW12O40],36

K4[SiW12O40],38 K5[BW12O40],36 and K5[CoW12O40].39 Note that all syntheses were made in the presence of argon. Synthesis and characterization of the nanoparticles All syntheses were prepared from the methodology developed in our previous work.11,12 The ten types of obtained particles are referred to as [C12]3[PW12O40], [C12]4[PW11VO40] [C12]5[PW10V2O40] [C12]6[PW9V3O40], [C12]3[PMo12O40], [C12]4[PMo11VO40], [C12]5[PMo10V2O40], [C12]3[AsW12O40], [C12]4[SiW12O40], [C12]5[BW12O40], [C12]6[CoW12O40]. The general procedure as well as the characterization by 1H, 13C, 31P NMR and elemental analyses is detailed in Supporting Information. Nanoparticle morphology and size characterization.


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The nanoparticles were examined using a FEI Tecnai G2-20 twin transmission electron microscope and Hitachi S4700 scanning electron microscope. Before analysis, the powders were dispersed in water. Two drops of water containing the nanoparticles were then deposited on a carbon-copper grid. Images were analyzed with ImageJ software (National Institutes of Health, USA) to obtain the size of the droplets. The electrophoretic properties as well as the size of the nanoparticles were determined at 25 ± 0.05 °C using a Zetasizer Nano-ZS (Malvern Instrument). Light-scattering cells of 10 mm were used. All measurements were performed at 173°. An automatic process was performed with a 10-runs method for each sample. The particle sizer and its attendant software provided first the time-correlation function of the scattered intensity. The decrease of this correlation function with displacement time can be used to extract information about the diffusion coefficient of a particle or droplet in solution. The measured diffusion coefficient can be used to calculate a hydrodynamic radius of the droplet using the Stokes– Einstein equation. The data are first analyzed by cumulant analysis to obtain an average diffusion coefficient and subsequently by CONTIN analysis in order to obtain information about the entire distribution of the particle size. X-ray scattering. X-ray scattering experiments were performed with a home-made device at CRPP. The beam characterized by at wavelength of 1.5418 Å and energy of 8 keV is obtained from a RIGAKU MicroMax 007HF generator with a rotative copper anode coupled to a confocal mirror Osmic Max-Flux. The Mar345 from MARRESEARCH detector with a 180 mm diameter is placed at a distance of 240 mm from the 1.5 mm diameter glass capillary tube containing the sample. The setup configuration gives access to scattering angle ranging from 0.6 to 20.5°. Film procedure and characterization. The contact angle measurements were performed using the gel trapping technique (GTT) first proposed by V. N. Paunov40 and adapted by Destribats et al.41 Nanoparticles are formed in situ by pouring simultaneously the two solutions containing the POM anions and the ammonium salt. After 1 min of equilibration, polymerizable silicone oil (Sylgart 184) was deposited at the top of aqueous phase to produce a macroscopic oil-water interface. This allows imaging individual particles at the interface, and makes it possible to measure the angle. Indeed, aggregation of nanoparticles and/or large amounts of particles at contact prevents the AFM tip to reach the nude interface and therefore the protrusion height of the particle embedded in the film is not accessible. After curing the PDMS layer at room temperature for 48h, it was peeled off the aqueous phase. The films containing the nanoparticles were observed by means of a multimode MMAFM-2 Atomic Force Microscope (from Digital Instruments) using the tapping-mode. Images were analyzed with the device software (Nanoscope III 5.12r3) to obtain the height of nanoparticle. ACS Paragon Plus Environment

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Emulsification procedure and emulsion characterization. Ammonium polyoxometalate nanoparticles (50 mg) were dispersed in a mixture of toluene (0.75 mL) and water (2.25 mL) and this system was emulsified with an Ultra-turrax (11500 rpm, 1 min, IKA, T10 Basic). 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 to ± 0.1 °C the temperature. Images were analyzed with ImageJ software (National Institutes of Health, USA) to obtain the size of the droplets. The distribution function is obtained by treatment of experimental data with log-normal function (OriginPro 8®, USA). Rheological Measurement. Dynamic viscoelasticity measurements of toluene/water emulsions were performed using a Malvern Kinexus rheometer (Malvern Instrument) equipped with a plate-plate-geometry with a 1 mm gap diameter of 40 mm. A maximal constraint was fixed at 50 Pa. The frequency ranged from 0.1 to 100 Hz. A plateau is observed between 0.1 and 10 Hz. Freeze Fracture TEM observations. Transmission electron microscopy (TEM) observations were performed after preparation of replica by Freeze-Fracture. The replica were obtained by first pressing a drop of the sample between the two copper slides of a sandwich holder and then freezing by quick plunging the holder into liquid propane at the temperature of liquid nitrogen. Frozen samples were then fractured by mechanical separation of the two copper slides. The freshly fractured surface was shadowed by deposition of platinum at an angle of 45° and mechanically reinforced by carbon deposition at an angle of 90°. The platinum-carbon replica were washed several times with water and then left in dimethylformamide before being rinsed in water and finally dried. Then the replicas were transferred into a TEM Hitachi H600 apparatus for imaging. Results and Discussion Preparation of the [C12]y[POM] nanoparticles. Recently, we have reported that spherical [C12]3[PW12O40] nanoparticles are spontaneously formed by ionic metathesis from [H]3[PW12O40] and [C12][OH] in aqueous solution.11,12 Based on this finding, the scope of nanoparticles has been extended to other Keggin POMs while examining the influence of (i) the partial or total substitution of the W atoms by V or Mo atoms, respectively, and (ii) the replacement of the central P atom by Si, As, B and Co atoms. Ten novel n-dodecyltrimethylammonium polyoxometalates, abbreviated as [C12]y[POM], were thus prepared by the same procedure as reported ACS Paragon Plus Environment

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for [C12]3[PW12O40] (see ESI and Figure 1). The method has the advantage of being very simple and efficient since the nanoparticles instantaneously and quantitatively precipitate. For some POMs, depending on the commercial availability of the precursors, the sodium or potassium salts were used instead of the acid forms. As evidenced with [PW12O40]3-, the nature of the POM precursor does not impact the nanoparticle formation as well as its form, nature and size. These results are in perfect agreement with the HSAB theory: hard cations (H+, Na+, K+) interact with hard anions (OH-) and soft anions ([POM]) form stable complexes with soft cations ([C12]). This reflects that the negative charge of POMs is delocalized among a large number of oxygen sites42 and that the electrostatic interactions between the ammonium cations and the POM anion are strong inside the POM nanoparticles.

[PMo11VO40]4[PMo10V2O40]5-

[POM]y-

[C12]y[POM]

[PW12O40]3[PMo12O40]3-

[PW 11VO40]4[PW 10V2O40]5[PW 9V3O40]6-

[AsW12O40]3[SiW12O40]4[BW 12O40]5[CoW12O40]6-

Figure 1. [C12]y[POM] catalytic amphiphilic nanoparticles prepared by ionic metathesis. Characterization of the dry [C12]y[POM] nanoparticles. i) Sizes and shapes. The POM nanoparticles have been fully characterized by transmission electron microscopy (TEM, Figure 2). As shown on the TEM images of Figure 2, the POM nanoparticles are spherical except the [C12]6[PW9V3O40]-based nanoparticles which provide a needle-like structure. However, some irregularities in the spherical forms can be observed with other members of the series [C12]6[PW12-xVxO40] when x = 1 and 2. To extract quantitative data from the TEM images such as the sphericity or the size distribution of the nanoparticles, image analyses using Image J software (NIH, ACS Paragon Plus Environment

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USA) were carried out.43 The area (A) and perimeter (P) of the 2D projection of each nanoparticle available from the images were thus determined allowing the calculation of the circularity (C), aspect ratio (AR), roundness (R), solidity (S) and eccentricity (ε). x=1

x=2

x=3

x = 3, Y = As

x = 4, Y = Si

x = 5, Y = B

x = 6, Y = Co

[C12]3+x[PW12-xVxO40]

x=0

[C12]3+x[PMo12-xVxO40]

a

[C12]x[YW12O40]

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Figure 2. Typical TEM images of the [C12]y[POM] nanoparticles, scale bar = 100 nm. All these parameters provide a good description of the nanoparticle morphology. The circularity (C) is a measure of the particles shape relative to a perfect circle (see Eq. 1). A perfect circle has a circularity of 1 while an irregular particle has a circularity value closer to 0. C = 4π

A P2

(1)

The aspect ratio (AR), defined as the ratio between the major axis and the minor axis (a/b), is a measure of anisotropy or elongation. The roundness R is calculated from Eq. 2. R > 0.6 indicates high roundness, 0.4 < R < 0.6, medium roundness, and R < 0.4, low roundness. ACS Paragon Plus Environment

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R=4

A πa 2

(2)

Solidity (S) is defined as the ratio between the area (A) and the convex area (CA). It is noteworthy that CA is the area enclosed by an imaginary "string" that wraps the nanoparticles. A solidity of 0 reflects a rough particle edge while a value of 1 corresponds to a smooth particles edge. The eccentricity ε is calculated using Eq. 3. The eccentricity values range from 0 (sphere) to 1 (ellipse).

 1    AR 

2

ε = 1− 

(3)

If C, AR and R tend to 1 and ε tends to 0, the nanoparticles can be considered as spherical and their diameter, Ø, can be estimated from Eq. 4. ØTEM = 2

A

π

(4)

According to the IUPAC definition, the equivalent diameter of a non-spherical particle, Ø’TEM, (i.e. when A and R tend to 0, AR tends to infinite and ε tends to 1) is equal to a diameter of a sphere of equivalent area and it can be calculated using Heyt and Diaz formula (Eq. 5).44 Ø’TEM = 1.55

A 0.625 P 0.25

(5)

All the calculated size and shape descriptors derived from the analysis of the TEM images are reported in Table 1. From a general point of view, all the nanoparticles have a spherical shape for which 0.73 < C < 0.92, R > 0.83, AR tends to 1 and ε is < 0.52, except [C12]6[PW9V3O40]. The average sizes of the nanoparticles, obtained from the TEM images, are comprised between 35 to 61 nm ([C12]6[PW9V3O40] excluded). Each modification of the nanoparticle size can be related to the [C12]y[POM] units that constitute the nanoparticles. Firstly, we examined the triply charged anions, [PM12O40] (M = W or Mo), which contain a central P heteroatom. These two Keggin POMs have a similar size (diameter ≈ 1 nm) and share similar structures, but differ in their composition with respect to the peripheral metal centers. Moreover, for both compounds, the negative charges are delocalized among a large number of oxygen


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sites (see above). However, the size of dry nanoparticles increases when W is replaced by Mo (35 vs. 55 nm).

Table 1. Size and shape descriptors derived from the analysis of the TEM images of the [C12]y[POM] nanoparticles.

[C12]y[POM]

Ca

ARa

Ra

Sa

[C12]3[PW12O40]

0.92

1.05

0.95

0.95 0.31

35

[C12]4[PW11VO40]

0.86

1.12

0.90

0.95 0.43

60

[C12]5[PW10V2O40]

0.89

1.12

0.89

0.94 0.43

61

[C12]6[PW9V3O40]

0.27

8.14

0.15

0.78 0.99

392c

[C12]3[PMo12O40]

0.90

1.14

0.89

0.94 0.42

55

[C12]4[PMo11VO40]

0.94

1.12

0.90

0.90 0.43

51

[C12]5[PMo10V2O40]

0.86

1.19

0.85

0.90 0.43

57

[C12]3[AsW12O40]

0.89

1.19

0.85

0.92 0.49

42

[C12]4[SiW12O40]

0.73

1.20

0.86

0.91 0.47

54

[C12]5[BW12O40]

0.77

1.11

0.90

0.93 0.42

51

[C12]6[CoW12O40]

0.91

1.24

0.83

0.90 0.52

57

a

εa

ØTEM (nm)b

The standard deviation is estimated at ± 2%. b The standard deviation is estimated at ± 10%. Calculated from eq. 5 and corresponding to an equivalent diameter Ø'TEM.

c

This behavior can be explained with the HSAB theory. Indeed, the ammonium is a "soft" cation as well as the POM (see above). According to Izumi et al., the [PW12O40]3- is "softer" than [PMo12O40]3-.45 Based on the HSAB theory, a strong pairing of [PW12O40]3- with the [C12] cations can be expected, leading to smaller nanoparticles. A similar argument can be invoked for the size increase observed from

[PW12O40]3- to [AsW12O40]3- (35 vs. 42 nm). Based on the work of Izumi et al., the softness of isocharged POMs, which only differ by their central atom in a given group of the periodic table, decreases within a column going from top to bottom because of a strong inward polarization.45 Therefore, [PW12O40]3- is more able to interact with soft ammonium cations than [AsW12O40]3- and provide smaller nanoparticles. In a second time, we examined whether the charge of the POM might modify the size of the nanoparticles. For instance, within the series [C12]y[YW12O40] with different central heteroatoms (Y = P, Si, B, and Co and y = 3, 4, 5 and 6 respectively), the charge density of the ACS Paragon Plus Environment

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Keggin-type POMs increases linearly with the decreasing valence of the central heteroatom. All these Keggin POMs share a similar structure but differ in their composition with respect to the central heteroatom. From the values of ØTEM reported in Table 1, it seems that increasing the negative charge of the POM species (i.e. the ammonium cation number) leads to an increase in the nanoparticle size. However, we can note a significant difference from a triply charged POM to a quadruply charged one while this difference becomes less important for higher-charged Keggin-type POMs as if a limit value is reached. The charge of the Keggin anion can also be increased by replacing some of the higher-valent metal centers by lower-valent ones (e.g. MoVI or WVI by VIV). Thus, introduction of vanadium atoms in the initial [PM12O40]3- (M = Mo or W) units increases the charge of the POM and hence, the number of dodecyltrimethylammonium counterions. Here again, within the series [C12]3+x[PW12-xVxO40] (x = 0 to 3) series, we observe a substantial increase of the nanoparticle size from x = 0 to x = 1 while this increase is very slight from x = 1 to x = 2. It is noteworthy that the most spherical nanoparticles are obtained for [C12]3[PW12O40] which are fine, uniform and well defined (S = 0.95) with a mean diameter of 35 nm. On the other hand, [C12]6[PW9V3O40] (x = 3) form needle-like particles with an equivalent diameter Ø'TEM of 392 nm (see Figure 1 and Table 1). Between these two extreme behaviors, the circularity tends to decrease whereas the average size increases. However, evolution of the shape behavior is not monotonous and a sharp transition is observed between [C12]5[PW10V2O40] and

[C12]6[PW9V3O40] (e.g. the aspect ratio, AR, increases from 1.12 to 8.14). This behavior can be related to the surface charge distribution that becomes more asymmetric with the number of vanadium atoms.46 As a result, the Coulomb forces become more directional and the arrangement in a spherical shape is unfavorable because of the steric hindrance between the ammonium chains. Moreover, a more distorted geometric structure and several isomers are obtained for some substituted POMs.46 Indeed,

[PM11VO40]4- has only one structure whereas at least 5 position isomers can be identified for [PM10V2O40]5-.47 Accordingly, the surface charge distribution is different for all the isomers and the nanoparticle shape is also affected by the presence of isomer mixture. Finally, we find that the solidity is always higher than 0.90 except for [C12]6[PW9V3O40] (0.78). As S is related to the nature of the particle ACS Paragon Plus Environment

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edge, the cylindrical [C12]6[PW9V3O40] nanoparticles have a rougher edge than the spherical ones.

[C12]6[PW9V3O40] nanoparticles have probably a more "crystalline" structure than the others. ii) Internal structure. The internal structure of [C12]6[PW9V3O40] nano-needles has been investigated by X-ray scattering (XS) and compared to the structure of [C12]3[PW12O40] nanoparticles. Since both compounds possess electron-rich and electron-poor domains (POM anions and alkylammonium cations, respectively), XS is a useful tool to determine their internal structure. The experiments have been performed with a home-made lab setup which provides access not only to a part of small angles (SAXS, 1 to 5 nm-1) but also to a very small portion of wide angles (WAXS, q > 10 nm-1). Hence, the distance scale covered in a single experiment ranged from 1 Å to ≈ 6.3 nm, which is sufficient to access the arrangement inside the particles (Figure 3). 6.3

1.2 0.6

d (nm) 102

[PW12O40]

[C12]

10

I (a.u.)

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1.1 nm

q10 1

q30 q-4

1.9 nm

q20

10-1

10-2 0.1

1

10

q (nm-1)

Figure 3. X-ray scattering spectra of dry [C12]3[PW12O40] nanoparticles and [C12]6[PW9V3O40] nanoneedles (red and grey, respectively). The purple curve shows the full spectrum of [C12]3[PW12O40] published in reference 11. The right scheme represents the molecular organization inside a nanoparticle.

The spectrum of the [C12]3[PW12O40] nanoparticles is in good agreement with the published data. It is noteworthy that the absence of sharp scattering peaks in the investigated region informs on the low crystallinity of these nanoparticles. The intense signal observed at q30 value (6.2 nm-1, i.e. in real space 1.1 nm) results from the scattering of the POM units (Ø ≈ 1 nm). The most informative part of the ACS Paragon Plus Environment

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spectra is between 1 and 5 nm-1 and concerns the internal molecular arrangement of the particles. In this region, the scattering spectrum shows a broad and intense peak at q10 (2.1 nm-1) and a moderate peak at q20 = 2 × q10. This set of peaks corresponds to a local lamellar structure.11,12 From q10 values, the layer thickness can be evaluated as d* = 2×π/q10. For [C12]3[PW12O40], d* = 2.95 nm and corresponds to the sum of the trimethyldodecylammonium length, i.e. around 1.95 nm in an extended conformation, and of the size of the POM (1 nm). Consequently, the [PW12O40]3- units are entrapped between the ammonium alkyl chains (see the scheme in Figure 3). The XS spectrum of the non-spherical [C12]6[PW9V3O40] particles seems to indicate a similar POM arrangement but with some differences. Indeed, SAXS peaks reveal the emergence of well-defined Bragg peaks related to the crystallinity increase of these needlelike particles. Moreover, all peaks shift towards the lower q values. The shift of q30 is due to a small increase of the POM size itself while the shifts of q10 and q20 are related to a very small increase of the layer thickness (3.06 vs. 2.95 nm for [C12]6[PW9V3O40] and [C12]3[PW12O40], respectively). Since the number of cations is doubled between these two compounds and since the layer thickness increase is quasi-negligible, a better molecular packing is then obtained, explaining the crystallinity increase for

[C12]6[PW9V3O40]. Moreover, it is noteworthy that all other particles exhibit the same POM arrangement. This observation supports their mechanism of formation, which has been previously proposed for the [C12]3[PW12O40] nanoparticles and which is here extended to other Keggin POM nanoparticles.12 Through van der Waals and electrostatic interactions, combined with entropic forces, the alkylammonium chains act as a supramolecular organic matrix allowing the stabilization of the nanoparticles. This organization is in line with the [C16H33N(CH3)3]y[POM] structures discussed in the literature.48,49

Physicochemical properties of the [C12]y[POM] nanoparticles in water and biphasic system. i) [C12]y[POM] nanoparticles in aqueous solution. To get a better insight into the aqueous behavior of the nanoparticles, dynamic light scattering (DLS) experiments were performed. Since the nanoparticles tend to aggregate in aqueous solution, they were generated in situ by anion metathesis by


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mixing aqueous solutions of [X]y[POM] (10-4 M) and [C12][OH] (y x 10-4 M) at 25 °C (see ESI, Figure S1). In all cases (except [C12]6[PW9V3O40]), the auto-correlation functions can be described by a single exponential decay reflecting the presence of mono-sized nanoparticles. The self-diffusion coefficient of nanoparticles (Dobs) can be calculated by fitting the auto-correlation functions using the cumulant analysis. The hydrodynamic diameter (Øh) of the particles can be calculated from Dobs by the StokesEinstein equation (i.e. with a spherical model). Two measurements were performed at t = 0 and 1h to highlight the aggregation behavior. Moreover, the ζ-potential was also simultaneously investigated to evaluate the surface charge of the particles. Indeed, the electric potential ζ of a particle is determined at a location away from the particle surface called the shear plane. The potential measured at this plane is related to particle movement in its water environment. The sizes derived from DLS measurements (Øh) as well as the ζ-potential of the nanoparticles are reported in Table 2. The sizes of [C12]3[POM] nanoparticles dispersed in water are in good agreement with those of dry particles (compare ØTEM and Øh at t = 0). This is also true for [C12]4[PW11VO40] and [C12]4[SiW11VO40]. On the opposite, the size of higher-charged POMs, i.e. [C12]5[POM] and [C12]6[POM] (except [C12]6[PW9V3O40]), in water are higher than that of the dry nanoparticles. This effect might be interpreted as the intercalation of some water molecules around the POM anions leading to a swelling of nanoparticles in an aqueous environment. Indeed, water molecules are strongly associated with the POMs as indicated by the elemental analyses (see ESI). In the presence of water, we can assume that water molecules are able to partially hydrate some of the most accessible POM units without a clear modification of the lamellar arrangement inside the nanoparticles.12 This partial hydration is promoted with the higher-charged POMs leading to the aggregation of these nanoparticles. Indeed, the size of [C12]3[POM] is independent on time (i.e. no or slight aggregation) whereas the [C12]4, [C12]5 and [C12]6[POM] sizes increase as a function of time (i.e. clear aggregation process). It is noteworthy that for the [C12]3+x[PM12-xVxO40] (M = W or Mo and x > 2), the numerous isomers can be also invoked in the aggregation process. Indeed, the unequal repartition of the surface charge density can promote penetration of water molecules inside the nanoparticles leading to an easy aggregation process. ACS Paragon Plus Environment

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Table 2. Physicochemical properties of the [C12]y[POM] nanoparticles in water and biphasic system (hydrodynamic diameters, Øh, ζ-potential and three-phase contact angle, θ). b

[C12]y[POM]

ØTEM (nm)

[C12]3[PW12O40]

Øh (nm)b

ζ (mV)c

ØAFM (nm)b

θ (°)d

t=0

t = 1h

t=0

t = 1h

35

33

33

-37.3

-37.8

39

153

[C12]4[PW11VO40]

60

63

65

-28.9

-28.1

60

158

[C12]5[PW10V2O40]

61

93

198

-26.6

-52.5

72

168

[C12]6[PW9V3O40]

392e

147e

361e

-22.9

-63.1

-f

-f

[C12]3[PMo12O40]

55

47

48

-47.6

-48.2

65

171

[C12]4[PMo11VO40]

51

139

191

-27.1

-48.6

79

167

[C12]5[PMo10V2O40]

57

240

394

-22.4

-49.5

84

162

[C12]3[AsW12O40]

42

46

49

-20.7

-21.5

47

157

[C12]4[SiW12O40]

54

58

65

-29.1

-34.9

63

163

[C12]5[BW12O40]

51

62

85

-15.9

-21.6

92

172

[C12]6[CoW12O40]

57

101

223

-1.3

-16.8

73

175

a

In-situ preparation ([X]y[POM] = 10-4 M, [C12][OH] = y×10-4 M, Vtot = 2 mL, T = 25 °C). b The standard deviation is estimated at ± 10%. c The standard deviation is estimated at ± 5%. d The standard deviation is estimated at ± 1%. e Calculated as equivalent diameter for non-spherical particles. f Not determined due to the absence of spherical nanoparticles. The evolution of the ζ-potential is more difficult to rationalize. Indeed, the formation of particles results from the electrostatic attraction between the negatively charged POMs and the cations leading to the formation of an uncharged complex of the type [C12]y[POM]. During the growth process of the particles, some defects may appear in the lamellar packing and may lead to the appearance of surface charges that would limit the particle growth. The nanoparticles show negative ζ-potentials from -63.1 to -16.8 mV. These values are in agreement with this possible process that limits the particle growth. It is noteworthy that this electrostatic effect may be at the origin of the monodisperse size distribution. However, since the size of the resulting particles as well as the defects are intrinsic to the [C12]y[POM] nature, we cannot rationalize in detail the data obtained here. However, the nanoparticles that exhibit an aggregation behavior in water show more negative ζ-potentials after 1h. This behavior is directly related


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to the stability of dispersions. Indeed, the magnitude of the ζ-potential indicates the degree of electrostatic repulsion between adjacent charged particles in the dispersion. For the nanoparticles, a high

ζ-potential will confer electrical stability (i.e. the dispersion resists aggregation) but when the ζpotential is small, attractive forces may exceed this repulsion and the nanoparticles tend to aggregate to acquire greater stability. Moreover, the extent of the ζ-potential deviation between the initial and final time is directly related to the charge number bearing by the POM (see Table 2). ii) [C12]y[POM] nanoparticles in oil/water biphasic system. The water/oil behavior was investigated by the measure of the three-phase contact angle of the nanoparticles adsorbed at the oil-water interface. The method used in the present study is based on the modified Paunov gel trapping technique.40,41 To avoid aggregation, the nanoparticles were generated in situ as for the DLS experiments. The three-phase contact angle, θ, is determined from the AFM image using the following relation: h r

 

θ = arccos − 1

(6)

where h is the average height of the nanoparticle protruding from PDMS film and r is the average radius of the nanoparticles. In order to avoid the use of size determined by TEM and DLS, obtained in a dried state or in an aqueous medium, the radius of the nanoparticles is obtained by the following relation: r=

a2 + h2 2h

(7)

where a is the apparent radius formed by the nanoparticle at the PDMS surface (see ESI, Figure S2). As equations 6 and 7 are valid only for spherical particles, the contact angle of the needle-like

[C12]6[PW9V3O40] particles could not be determined. The diameters, ØAFM = 2r, and the contact angles of the [C12]y[POM] spherical nanoparticles obtained from AFM images are given in Table 2. Note that ØAFM evolve similarly to the diameters obtained by TEM (Table 2). A small discrepancy between TEM and AFM can be observed with a systematic larger nanoparticle size at the oil-water interface (AFM). This can be explained either by some aggregation or, more likely, by penetration of PDMS inside the nanoparticles. Indeed, we have previously reported that aromatic solvents (e.g. toluene) penetrate into ACS Paragon Plus Environment

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the nanoparticles leading to their size increase. From the contact angle measurements, it can be deduced that all the spherical POM nanoparticles exhibit a contact angle higher than 90°, between 153 and 175 nm, indicating a preferential wettability of the particles by the oil. Their lipophilicity is directly related to the alkylammonium cation number as well as the self assembly of tectons leading to the nanoparticle formation (see above). Indeed, higher-charged Keggin-structured POMs show a propensity to induce an increase of the contact angle. Moreover, the aggregation observed in aqueous dispersion (see above) can also explain not only the size effect but also the increase of the contact angle. Indeed, in contact with oil at the oil-water interface, some alkyl chains point outside of the nanoparticles, conferring them a higher hydrophobicity, which is not surprising because oil is good solvent of the surfactant alkyl chains.12

Pickering emulsions stabilized by [Cn]y[POM] nanoparticles. i) Emulsion drop size and stability. Stable water-in-oil Pickering emulsions can be formulated in situ when water and toluene (3/1 v/v) are emulsified in the presence of 1.7 wt.% [C12]3[PW12O40] nanoparticles and under vigorous stirring (11 500 rpm, 60 min, see Figure 4 for a typical microphotograph of this emulsion).11,12 The mean diameters of emulsion droplets, Ødropets, obtained under same experimental conditions, are reported in Table 1 for all [C12]y[POM] nanoparticles. To estimate the emulsion stability, we have also reported the mean diameters of droplets after 15 and 90 days. The initial size of the droplets is between 13 and 63 µm. In a given family (i.e. [C12]3+x[PW12xVxO40],

[C12]3+x[PMo12-xVxO40] and [C12]y[YW12O40] series), the droplets size tends to increase with

the POM charge (i.e. the ammonium number). This behavior can be directly related to the nanoparticles size at the oil/water interface (see ØAFM in Table 2). As the nanoparticle size increases, the same number of particle exhibits a higher surface area leading to a higher ability to cover the interface (see below). However, consistency between droplets and nanoparticles size within the different series is not always obtained: it is just a general trend. Indeed, for the emulsion stabilized by [C12]3[PW12O40],

[C12]3[PMo12O40] and [C12]3[AsW12O40], the initial droplet sizes are similar (16, 13 and 19 µm), while the diameters of the particles at the interface, ØAFM, are 39, 65 and 47 nm, respectively. This behavior


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can be related to the contact angle. The measured contact angles are 153, 171 and 157° for

[C12]3[PW12O40], [C12]3[PMo12O40] and [C12]3[AsW12O40], respectively. As a consequence, the best dispersion is obtained with the particles having the largest contact angle (i.e. [C12]3[PMo12O40]) which leads to smaller droplets. The most stable emulsions are obtained with [C12]3[PW12O40] and

[C12]5[PMo10V2O40] while [C12]6[PW9V3O40] gives the less stable one, for which, after 15 days, the coalescence process results in the formation of an excess lower water phase (≈ 50% of the initial total volume) in equilibrium with the Pickering emulsion.

Table 3. Average droplet size (Ødroplets), elastic modulus (G’), degree of surface coverage by particles (τ) and emulsion elasticity (σ) of Pickering emulsions stabilized by [C12]y[POM] nanoparticles.a

[C12]y[POM]

Ødroplets (µm)b

G’ (Pa)c

τd

σ (mN/m)

0 day

15 days

90 days

[C12]3[PW12O40]

16

14

24

711

0.91

55.36

[C12]4[PW11VO40]

23

39

43

1340

0.50

149.90

[C12]5[PW10V2O40]

34

54

75

3352

0.81

553.99

[C12]6[PW9V3O40]

53

78e

107e

248

-f

-f

[C12]3[PMo12O40]

13

42

103

542

1.14

34.64

[C12]4[PMo11VO40]

15

35

49

863

0.64

63.49

[C12]5[PMo10V2O40]

27

24

29

1534

0.94

202.72

[C12]3[AsW12O40]

19

48

59

624

0.95

57.68

[C12]4[SiW12O40]

24

48

97

441

0.80

51.44

[C12]5[BW12O40]

63

106

118

658

2.20

201.23

[C12]6[CoW12O40]

31

78

125

2410

1.52

362.16

a

Water-in-toluene total emulsion (3/1 v/v) stabilized with [C12]y[POM] nanoparticles (1.7 wt. %) after emulsification (11 500 rpm, 60 sec.) at 25 °C b Droplet size distribution profile estimated from microphotograph. The average droplet size is obtained after fitting with log-normal function (OriginPro 8®). The standard deviation is estimated at ± 10%. c The standard deviation is estimated at ± 2%. d The standard deviation is estimated at ± 3%. e Biphasic medium composed of an emulsified and a water phase. f Not determined due to the absence of spherical nanoparticles. The destabilization of the [C12]6[PW9V3O40]-based emulsion is a consequence of the needle-like shape of the particles (see above). For all the other nanoparticles, emulsions slowly coalesce during the first 15 days but then remain relatively stable because of the well-know limited-coalescence process associated ACS Paragon Plus Environment

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with their desorption and plausible aggregation.50 The coalescence process is well correlated to the POM charge ([C12]3+x[PMo12-xVxO40] series excluded) because of the increase of the contact angle with the number of ammonium counter-cation. As the evolution is opposite for the [C12]3+x[PMo12-xVxO40] series, the stability order is also reversed. Finally, it is noteworthy that the size change (i.e. the coalescence rate) is more important for high contact angles. These observations strengthen the correlation between the structure and the charge of POMs, their hydrophobic behavior and their ability to form and to stabilize the Pickering emulsions. At this step, it is noteworthy that the nanoparticles, formed by the self-assembling of surfactants and POMs, are held together by non-directional interactions (electrostatic and van der Waals). To explain the strong electrostatic interactions between surfactants and POMs, we must consider that the specificity of ionic interactions in solution is dependent on both the cation and the counteranion. Indeed, based on the HSAB theory: hard cations interact with hard anions and soft anions form stable complexes with soft cations. This theory can be extended from interactions between ions to interactions between anions and surfactant head-groups. In this context, the ammonium headgroup of [C12] is a soft cation, strongly interacting with large soft POM anions, thereby releasing weakly bound water, efficiently screening electrostatic interactions, decreasing the headgroup surface area, and increasing the packing parameter. The increase of the packing parameter of [C12] cation promotes the lamellar organization observed inside the nanoparticles (see above). In addition to these electrostatic forces, van der Waals interactions (i.e. London dispersion forces) also promote the lamellar organization. Indeed, London forces between all alkyl tails of the cationic surfactants represent a significant part of the total interaction force inside the nanoparticles, even though they are generally weaker than ionic bonds they are maximized by the number of carbon bearing by the cation. However, it is possible that, at the water/toluene interface, the POM nanoparticles may decompose due to the decrease of both ionic and London dispersion interactions. Indeed, the water molecules are able to solvate the POM units whereas the toluene is able to penetrate inside the nanoparticles (see above). However, in biphasic systems (nanoparticles in water or in toluene), the nanoparticle structure is kept (compare ØTEM, Øh and ØAFM in Table 2). Indeed, only a ACS Paragon Plus Environment

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swelling of nanoparticles is observed. In order to prove the cohesion of the nanoparticles, FTIR, Raman, and

31

P NMR MAS were performed on the solid [C12]y[POM] nanoparticles before and after

emulsification and all spectra were identical. Moreover, analysis of the separated aqueous and organic phases of the emulsion by 1H and 31P NMR spectroscopy did not show the presence of ammonium and POMs in solution. If a dissociation of the nanoparticles would occur, it would not exceed 0.2 % otherwise it would have been detected by the performed analysis. Finally, all the physicochemical behavior dynamic elasticity and degree of surface coverage as well as the freeze fracture TEM image are typical of Pickering emulsions wherein the nanoparticles fusion are favored by the number of ammonium counter-cations present in the vicinity of the particle/solvent interface (see the section below and Figure 5). All these results as well as already published results are compatible with the non dissociation of the nanopaticles.11-13 However, it is important to note that very similar complexes to those used in the present article are used in the literature for catalysis in emulsion systems. All these systems used a molecular surfactant-POM complex to explain the stabilization of the emulsions.23-25 As presented above, even if the electrostatic interactions are important the London dispersion forces are also very important for the formation of the spherical nanoparticles. Indeed, the modification of POMs charges (i.e. the number of surfactant inside the structure) results in a slight decrease of the stability (i.e. the cohesion inside the nanoparticles decrease, see Table 2 and the corresponding discussion). From a similar point of view, surfactant association structures may be explained by a geometric analysis, i.e., by the values of the packing parameter, PP = V/a0l. Here V and l are the volume and length of the alkyl chain of the surfactant and they are usually calculated from V = (27.4+26.9Nc) Å3 and l = (1.54+1.265Nc) Å with Nc the carbon number in the hydrophobic chain of the surfactant. The quantity a0 is the area of the surfactant head group. In all publications, mentioned above, the surfactant-POM complexes used a surfactant with longer alkyl tails, typically C18H37N(CH3)3. As a0 is identical for the two surfactants, the packing parameter is higher for C18H37N(CH3)3 than for C12H25N(CH3)3. Moreover, the hydrophobicity is increased leading to the existence of molecular surfactant-POM complex due to a better affinity for the oil phase and, obviously, it becomes more soluble in oil phase. In consequence, the ACS Paragon Plus Environment

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molecular surfactant-POM complexes are able to place in the interfacial layer and to stabilize emulsions as archetypal molecular surfactants. It is noteworthy that we have already reported that Pickering emulsions are formed only with ammonium chain lengths between 10 and 14 carbon atoms.11 ii) Dynamic elasticity and degree of surface coverage for Pickering emulsions. The Pickering emulsions stabilized by amphiphilic POM nanoparticles have been firstly characterized in respect with their elastic modulus (G’) measured between 0.1 and 100 Hz. The elasticity of concentrated Pickering emulsions may be described by Mason's law (Eq. 8) valid for monodisperse emulsions.50 G ' = 1 .7

σ R

φ 2 (φ − φ rcp )

(8)

where R is the mean drop radius, φ the water volume fraction equal to 0.75 for all the samples and φrcp the random close packing equal to 0.64 and corresponding to the fraction where all the drops are in contact giving rise to emulsion elasticity. σ depicts the mechanical behavior of the interface and is equal to the interfacial tension for surfactant-stabilized emulsions. For Pickering emulsions, stabilized by particles that may interact, σ corresponds to the interfacial rigidity.50 In opposition to G', σ allows discussing the film mechanical behavior independently of the drop size. σ has been estimated for the various emulsions and is reported in Table 3. In a same series, a general trend appears. The film stiffness increases with the number of alkyl chains (except for [C12]6[PW9V3O40] which presents a needle-like structure). The key to understanding this behavior may in fact lie in how the nanoparticles are arranged and linked together at the interface. Indeed, the interfacial POM nanoparticles may increase interfacial elasticity because of the cohesiveness between the particles. As mentioned above, the penetration of toluene molecules into the particles results in the release of some alkyl chains allowing their interlocking.12 This interlocking may act as elastic "springs" in the interfacial layer leading to significant changes of emulsion elasticity. It is noteworthy that the nanoparticle fusion is favored by the number of ammonium counter-cations present in the vicinity of the particle/solvent interface. However, we can suppose that the penetration of toluene depends on the internal cohesive forces inside the nanoparticles,


21


thus the interlocking forces are different from one series to another. This organization is in good agreement with nanoparticle and colloidosome structures previously reported for [C12]3[PW12O40].12 To estimate the "affinity" of the nanoparticles for the water/toluene interface, we have determined the degree of surface coverage by particles (τ) (proportion of oil-water interface covered by the particles). Because the coalescence is stopped when a particular degree of surface coverage is reached, the diameter is inversely proportional to the total amount of nanoparticles (mp) at constant degree of surface coverage (τ).50

1

φ droplets

=

mp

(9)

4τρ p d pVd

where ρp and dp are, respectively, the weight, the volumetric weight and the diameter of nanoparticles, Vd is the total volume of the dispersed phase which remains constant over the experiment and Ødroplets is the average droplet diameter. Figure 4 provides an example confirming the validity of the previous relation.

1/ Ødroplets (µm-1 ×104)

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Page 22 of 34

3 2 1 0

Pickering emulsion

microphotograph

0

0.02 0.04 0.06 0.08 0.1 mp (g)

Figure 4. Evolution of the inverse diameter, 1/Ødroplets, with mp for water/toluene (3/1 v/v, 11 500 rpm, 60 sec., 25 °C) Pickering emulsion stabilized with [C12]3[PW12O40] nanoparticles. From the experimental slope of the curve, we deduce τ = 0.91.

As reported in Table 3, the three more stable emulsions based on [C12]3[PW12O40], [C12]5[PMo10V2O40] and [C12]3[AsW12O40] nanoparticles have a degree of coverage ranging from 0.85 to 0.95. In this case, the droplets are stabilized by a compact monolayer. Moreover, the droplets coverage is optimal and ACS Paragon Plus Environment

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these nanoparticles do not aggregate (see Øh in Table 2). In contrast, the surface coverage is less than 0.85 for the emulsions based on [C12]4[PW11VO40], [C12]5[PW10V2O40], [C12]4[PMo11VO40] and

[C12]4[SiW12O40]. One could expect that as the coverage is slightly smaller, coalescence of droplets may occur. However, the emulsion stabilized by [C12]4[PW11VO40] has the lowest surface coverage while its stability is relatively good. This can be accounted for by the fusion of the particles in the interfacial layer, this interlocking of the chains limiting the destabilization process. The last behavior is obtained for [C12]3[PMo12O40], [C12]5[BW12O40] and [C12]6[CoW12O40]-based emulsions with τ > 0.95. In these cases, we can also assume a fusion of the particles in the interfacial layer but, as the contact angle of these nanoparticles is higher, their affinity for the toluene phase increases leading progressively to the destabilization of the emulsions. To confirm the fusion of the nanoparticles in the interfacial layer, freeze fracture TEM experiments were performed on the emulsion stabilized by [C12]3[PW12O40] nanoparticles (Figure 5 and ESI Figure S3). Indeed, the fusion process is observed for all the nanoparticles placed in the interfacial layer but we suppose that the extent of the cohesion differs with the number of ammonium counter-cation. Moreover, Figure 5 clearly proves that the observed humps on the surface are the POM nanoparticles because the diameters are very close to the diameter of the dry nanoparticles.

[C12]3[PW12O40] nanoparticles

100 nm

Figure 5. Typical image of the Pickering emulsion droplet replica. Conditions: water/toluene (3/1 v/v), [C12]3[PW12O40] nanoparticles (1.7 wt. %), 11 500 rpm for 60 s, T = 25 °C. ACS Paragon Plus Environment

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Reversibility of supramolecular systems based on [C12]y[POM] nanoparticles. An inherent and essential property of supramolecular systems is their reversibility. This property is a key distinguishing feature between supramolecular systems and molecules. Indeed, unreactive molecules have the same composition in the solid state or in solution. In contrast, the environment in which building blocks are placed can be determinant on the existence of a supramolecular system.

[C12]y[POM] nanoparticles are formed from POM and ammonium tectons that self-assemble in Pickering emulsions by their adequate packing at the oil/water interface, as we supposed that nanoparticles as well as emulsions are held together by non-directional interactions (electrostatic, hydrophobic and steric forces), so we can evaluate their dissociation depending on intrinsic and extrinsic parameters. i) Dissociation of nanoparticles in elementary clusters. To performed the dissociation of

[C12]y[POM] nanoparticles, we tried to dissolve them in various solvents. As mentioned in our previous work, the particles are neither soluble in water nor in typical organic solvents such as alkanes, aromatics, chloroform, dichloromethane or acetonitrile in which the nanoparticles are more or less dispersed.11 However, they are completely soluble in DMSO and in DMF.13 Thus, a drop of a solution of

[C12]3[PW12O40] (1 mg/mL) was deposited on a grid and dried before observation by TEM (Figure 6).

2 µm

500 nm

Figure 6. TEM images of [C12]3[PW12O40] nanoparticles after solubilization in DMSO and evaporation. As shown on Figure 6, star-like crystalline structures are formed instead of spherical POM nanoparticles. Their mean diameter, estimated from the TEM images, is 1.1 ± 0.6 µm. We assume that ACS Paragon Plus Environment

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the star-like morphology is caused by multiple crystal twinning at the onset of growth. In parallel, the DMSO was removed under reduced pressure. Then, the dried powder was dispersed again in water under vigorous stirring (11 500 rpm, 12 h) before performing TEM analysis. The spherical nanoparticles of [C12]3[PW12O40] are reformed with the same diameter. The concept of self-assembly is thus proved. Indeed, we have demonstrated that the [C12]3[PW12O40] nanoparticles, after dissociation into its components (i.e. ionic elementary clusters) in DMSO, can be reconstituted in adequate medium (i.e. in water) leading to the reconstruction of the intact [C12]3[PW12O40] nanoparticles. This experiment of "reconstitution" shows that all necessary information for the unequivocal process of assembly of the

[C12]3[PW12O40] nanoparticles is contained in its components. ii) Dissociation of Pickering emulsions. We have already reported that the dissociation of Pickering emulsions stabilized by self-assembled particles is very easy.11,12 Indeed, a simple centrifugation of the emulsified medium can be used to obtain a triphasic medium (water, toluene and powder). TEM images, before and after emulsification, reveal that the nanoparticle morphology remains unchanged. Moreover, these nanoparticles can be easily re-emulsified.11,12 This observation clearly proves that an extrinsic parameter can disturb the Pickering emulsions and the nanoparticles can be re-obtained easily despite their fusion in the oil/water interfacial layer (see above).

Conclusion The POM amphiphilic nanoparticles can be seen as a special class of materials with properties easily foreseen and which result from the association of POM and ammonium tectons that engage in multiple strong interactions. This strategy, based on molecular recognition, self-assembly and self-organization, has led to a toolbox of fascinating nanomaterials. Within their structure, POM tectons control how their ammonium neighbors are positioned in a 3D space. This organization leads to the formation of the elementary clusters which form well-defined nanoparticles by self-assembly through the ammonium chains. Indeed, we have shown that the size and the shape of the particles are completely governed by the nature of the POM tectons and their association with the ammonium counter-cations. The key


25


parameters are the charge number as well as the localization and the value of partial negative charge on the bridged oxygen atoms of polyoxoanion (see Table 4).

Table 4. Relationships between the structure of the [C12]y[POM] tectons, the structure and the

Nanoparticles

POM tecton

physicochemical properties of the nanoparticles and their Pickering emulsions stabilized thereof.

Emulsion

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Structure

Symmetric

Asymmetric

Symmetric

Asymmetric

Charge localization

Localized

Delocalized

Localized

Delocalized

Charges number

3

4

5

6

Example

[PW12O40]3-

[PW11VO40]4-

[BW12O40]5-

[PW9V3O40]6-

Shape

Spherical

Internal structure

Lamellar

Aggregation

Moderate

Contact angle (°)

153

Stability

Stable

Interfacial rigidity

Weak

Pseudo-spherical Lamellar

Lamellar

Moderate to high 158

172 Moderate

High

Non-spherical Lamellar Very high Instable

Very high

Very weak

Pseudo Surface coverage

Monolayer

Partial

multilayer

However, the internal structure of the nanoparticles is completely independent of the size or the shape of the nanoparticles and it only results from the supramolecular self-assembly process based on electrostatic, van der Waals, hydrophobic and steric forces. All these interactions ensure the cohesion inside the nanoparticles unlike other nanoparticles that require sophisticated synthetic methods to obtain “covalent” nanoparticles. Moreover, the nanoparticles behavior (aggregation and contact angle) is also dictated by the POM tectons. In other words, there is a close relation between the structure of the POM tectons and the nanoparticles size and shape and their physicochemical properties. The nanoparticles association in water/toluene biphasic system induces the formation of Pickering emulsions in which ACS Paragon Plus Environment

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each particle is positioned predictably with respect to its neighbors (fusion process of ammonium chains between particles) and its hydrophobic behavior. As a consequence, a bottom-up transition of the properties is effected between the POM tecton, the nanoparticles structures and the properties of the resulting Pickering emulsions. This work is expected to be a new productive source of supramolecular catalytic and amphiphilic nanoparticles with programmed structures and properties, as well as of designed Pickering emulsions with higher degrees of organization to obtain new, smart and switchable catalytic media.

Acknowledgments. We are grateful to the ANR (Project ANR-10-CD2I-01), the Université de Lille and the Fonds Européens de Développement Régional (FEDER) for financial supports. The authors would like to thank A. Bentaleb, H. Saadaoui and I. Ly for their valuable help in particle characterization by XRay scattering, AFM measurements and freeze fracture electron microscopy observations respectively.

Supporting Information Available. Experimental data and supplementary figures. This material is available free of charge via the Internet at http://pubs.acs.org.


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Table of Contents Graphic


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Self-Assembled Polyoxometalates Nanoparticles ... - ACS Publications

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