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Triggering the Mechanical Release of Mineralized Pickering Emulsions-Based Capsules Marion Baillot, Ahmed Bentaleb, Eric Laurichesse, Véronique Schmitt, and Rénal Backov Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.5b04687 • Publication Date (Web): 31 Mar 2016 Downloaded from http://pubs.acs.org on April 1, 2016

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Manuscript prepared as a full paper for submission to Langmuir

Triggering the Mechanical Release of Mineralized Pickering Emulsions-Based Capsules Marion Baillot,1 Ahmed Bentaleb,1 Eric Laurichesse,1 Véronique Schmitt1,* and Rénal Backov1,* 1

Université de Bordeaux, CRPP-UPR CNRS 8641, 115 Avenue Albert Schweitzer, 33600

Pessac, France. [email protected], [email protected] Keywords

Abstract Taking the benefit of Pickering based emulsions and sol-gel chemistry, we synthesized mineralized Pickering emulsions-based capsules constituted of a dodecane core and siliceous shell. In order to trigger the oily core mechanical release we first made the use of one step polycondensation synthetic path, reaching limited shell thickness from 43 nm to 115 nm with a resistance against the application of an external pressure from 0.5 to 6 MPa. When addressing a sequential mineralization route, we were able to reach both better shell homogeneity and higher values of shell thickness from 85 nm to 135 nm associated with a shell breaking pressure varying from 1.2 to 10 MPa. In this last configuration shell homogeneity and thickness are acting cooperatively towards enhancing the shell mechanical toughness and the associated effective breaking pressure of the dodecane@SiO2 core shell particles. 1 ACS Paragon Plus Environment

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INTRODUCTION Nowadays micro-encapsulation is a wide spread technology-encompassing sectors as pharmaceutics, cosmetics, food, textile, agriculture for chemicals) and in all sectors where confinement and delivery are required.1 Encapsulated species are very various depending on the application: drugs, fragrances, flavours, dyes, pesticides... Both the capsules shapes and morphologies have been the object of an increasing complexity going from mononuclear (core-shell), polynuclear (several cores within a single surrounding shell) and matrix types (species are embedded in a continuous phase). Thereby, tremendous capsule morphologies have been generated as proteins vehicles,2 cyclodextrins,3 thermally gated liposomes,4 concentrated lamellar vesicles,5 double emulsions,6,7,8 colloidosomes,9,10 silica shell microcapsules,11,12,13 thermo-sensitive PNIPAM-Silica nanocapsules,14 thermo-sensitive hydrogel micropsheres,15 PNIPAM-Polylactide microsphere16, magnetic nanomaterials17,18,19, polyamide capsules20,21 and so forth. In all these cases, the delivery is associated either with a slow and progressive release of the capsule content mainly governed by the Fick's diffusion or with an induced release promoted through the application of an external stimulus. In this last configuration, we have proposed the synthesis of wax surrounded by a silica shell, labelled Wax@SiO2 capsules, that exhibit the ability to open and release at once their content upon application of a soft thermal treatment.22 Particularly, the protocol is easy and very versatile as it can be applied on various oils with advantage of a direct transfer toward mass production. Later on, we extended the process to double water-in-wax-in-water or wax-inwater-in-oil

emulsions

to

elaborate

respectively

Water@Wax@SiO223

and

Wax@Water@SiO224 multi-cargo type of capsules. This strategy presents the advantage of obtaining capsules bearing multiple compartments addressing thereby higher potential for multi-tasks delivery while segregating the hydrophobic and hydrophilic moieties when trapped within the capsules, preventing thus their interaction prior their delivery.

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Based on these previous studies, we propose herein a more systematic and fundamental study on the synthesis of monodisperse oil@SiO2 core-shell capsules where sol-gel chemistry allows tuning the shell thickness and homogeneity while employing direct Pickering-based emulsions25,26 to control the particle diameters. The fine design of both core-shell particle diameters and their shell thicknesses allows conveniently triggering the mechanically-induced opening of the shell and thereby the release of their oily content. Overall, to tune both particle diameters and shell thicknesses we have employed the integrative chemistry synthetic path,27,28,29 that is to say the fine coupling of soft matter and soft chemistry to design advanced functional architectures. The main idea is to organize inorganic skeleton within the geometric space taking advantage of multi-scale templates; combination of self-assembled surfactants and larger templates as emulsion droplets,30,31 air-liquid foams,32,33,34 latex beads,35,36 and so forth. METHODS Chemicals All chemical reactants were purchased from Sigma-Aldrich and used as received, without further purification. Aerosil silica nanoparticles A380 (diameter 7 nm) were provided by Evonik. The oil is dodecane, ReagentPlus®, purity ≥99%, from Aldrich. Tetraethoxyorthosilicate (Si(OEt)4, TEOS), purity >99%, is the employed sol-gel alkoxide. Cetyltrimethylammonium bromide ((C16H33)N(CH3)3Br, CTAB), purity ≥98%, is a cationic surfactant. Chlorhydric acid, 37% was used to set the acidic pH. Particle Functionalization Silica nanoparticles are bearing a strong hydrophilic character, therefore they do not adsorb at oil-water interfaces. To favor the stabilization of droplets, we need to confer a more


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lipophilic character to the particles being thus partially wetted by both fluids. By adsorption of a very low amount of surfactant, unable to stabilize the emulsion by itself, the hydrophilic surface of silica particles is rendered partially hydrophobic. Emulsification process Functionalized particles are dispersed in water at ambient temperature (around 20°C) and the oily phase is added dropwise while maintaining vigorous stirring for 1min at 1600 rpm, using an Ultra-Turrax homogenizer T25 equipped with a S25 N-25F rotor head. Ultra-Turrax apparatus is frequently used as homogenizer to produce emulsions. It is equipped with a rotor/stator head that induces both droplets deformation and sample recirculation. In order to get narrower droplet size distributions, the premix, obtained previously, is transposed into a high-pressure homogenizer microfluidizer MS110 from microfluidics during 30 s at a given pressure varying from 31 to 95 MPa. After high-pressure homogenization, emulsions have been kept at rest for the limited coalescence phenomenon37 to take place, this issue will be also detailed further within the results and discussion section. The amount of Aerosil A380 particles sets the drop average size, taking advantage of the above mentioned limited coalescence process. Pickering emulsions mineralization Prior applying the sol-gel process, emulsions are diluted from 20 to 2 wt % by adding CTAB solution at various concentrations (1.0, 0.7 and 0.5 wt % of CTAB with respect to the added aqueous solution). We checked that addition of surfactant after emulsification does not alter the emulsion. In order to catalyze the TEOS hydrolysis-condensation, while optimizing heterogeneous condensation at the oil-water interface, we adjust the pH close to 0, below the silica isoelectric point (pH 2.1). At this stage, the emulsion final volume is 95 mL with an oil fraction equal to 2 wt%. TEOS is then added dropwise at different amounts (2.5, 5.0 and 7.5 4 ACS Paragon Plus Environment

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mL). During the addition of TEOS, the solution is placed under magnetic stirring at 550 rpm, a stirring unable to modify the drop size distribution. Therefore, the total oil-water interface area to be covered by the shell to be synthesized is kept constant. Then stirring is decreased to 450 rpm and left overnight at room temperature (around 20 °C). Products obtained are washed in pure water by cycles of centrifugation-replacement of the supernatant by pure waterredispersion, centrifugation is performed at 7000 g during 15 min at 20 °C. Washing is performed in order to stop the mineralization, remove possible homogeneous nuclei and all traces of free surfactant. The process is repeated several times until no foam is observed in the centrifugation tube. The mineralization process has been reproduced in triplicate to check the reproducibility. After the washing steps a second mineralization may be performed as followed. Assynthesized oil@SiO2 core-shell particles are diluted at 2 wt % into a CTAB solution at various concentrations (0.05, 0.10, 0.17, 0.25, 0.33 and 0.50 wt %) while pH is adjusted by addition of HCl solution close to 0. Addition of TEOS is made dropwise and the amount is the same as the one employed during the first mineralization step (2.5, 5.0 or 7.5 mL). Stirring is carried out under the same conditions as for the first mineralization, but during 3 days as the CTAB concentrations are decreased when compared to the first mineralization step, CTAB being known to catalyze polycondensation kinetics. A third mineralization, identical to the second one, can be proceeded. Final core-shell particles are labeled hereafter as followed x-SiO2-y(w)-z where, "x" is related to the starting emulsion droplet diameters, "y" corresponds to the total amount of TEOS employed during the full synthetic path while "w" corresponds to the number of mineralization steps 1, 2 or 3 and "z" depicts the final shell thickness. This nomenclature is shortened, when the discussion only deals with emulsion (absence of y, w and z) for example. Characterization techniques 5 ACS Paragon Plus Environment

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Emulsions were observed optically using an inverted optical microscope Zeiss Axiovert X100 and the images were recorded using a Hitashi video camera and Scion image program. Optical microscopy was used to evaluate the emulsions size distributions over 100 drops. This statistical evaluation has been preferred to static light scattering to avoid creaming inside the measuring cell. Scanning electron microscopy (SEM) pictures were captured with a Hitachi TM-1000 apparatus. For a better resolution and to estimate the shell thickness of obtained capsules, SEM was also performed with a Hitachi S2500, which allows increasing the resolution to the nanometer scale. The suspensions were previously dried at ambient temperature or were lyophilized using an Alpha 2-4 LD Plus freeze-dryer after freezing during one night at -80 °C. All samples were gold coated before observation. Diffractograms of X-ray scattering were recorded on a Bruker Nanostar apparatus equipped with a Hi-Star detector, from the same company, to obtain the structure type on the shell when we varied the CTAB concentration. The λ = 1.5418 Å radiation of a Copper source (Siemens), operated at 40 kV and 35 mA, was selected. The sample-to-detector distance fixed close to 0.25 m, is checked, using Silver Behenate as standard. From the Gaussian width of the first order Bragg peak of Silver Behenate, we estimate a resolution width (FWHM) ∆q ≈ 2.0 x 10-2 Å-1. To reach higher scattering wave vector values, (ranging typically from 0.1 to 0.25 Å-1), we used a custom-made instrument with a Copper rotating-anode-based set up from Rigaku. On the contrary with the Nanostar system, the collimation flight path is placed under vacuum. All samples are introduced into cylindrical quartz capillaries with a nominal diameter 1.5 mm that are further flame sealed. Helium pycnometry was used to determine the density of the silica shells by measuring the pressure change in a calibrate volume. The results were recorded on a Accupyc 1330 of Micromeritics whose operates on Mariotte laws of gas: P1V1 = P2V2 where V1 is the cell volume, V2 the expansion volume, P1 the pressure in the cell and P2 the pressure in the


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expansion circuit. In fact: Vsample = Vempty cell-V1. The volumes of the empty cell, like the expansion volume, are known after the instrument calibration. In consequence, the density measure is determined by: d = msample / Vsample. RESULTS and DISCUSSION The elaboration of capsules is based on a four steps process as described on Fig. 1. A dodecane-in-water direct emulsion is prepared by emulsification of dodecane progressively introduced into an aqueous suspension of silica nanoparticles rendered previously partially hydrophobic. Indeed, the surface of silica Aerosil A380 particles is negatively charged, as all silica networks. This specificity is induced as during sol-gel process occurs "Si-O-Si" oxobridges within the bulk of the native clusters while the silanol groups "Si-OH" are repulsed at the outer surface of the growing clusters.38 This morphogenesis scenario is thus providing to silica particles both an acidic character and thereby external negatively charged surfaces when spread in water. Thus, the CTAB cationic surfactants will electrostatically adsorbed thanks to its cationic head group, conferring the adequate wetting character upon particles. In order to maintain the specific coverage of 25 nm2 per CTAB molecule at the silica surface, the amount of surfactant is fixed with respect to the total mass of particles to cover. For 7 nm-sized particles, the CTAB/silica weight ratio has to be equal to 19.10-3. Various quantities of particles from 0.34 to 1.36 g are used to vary the droplet sizes keeping the CTAB/SiO2 weight ratio at the constant value of 19.10-3. We would like to recall that we make the assumption that all the surfactant is consumed to adsorb at the silica surface and this low CTAB concentration is by far inefficient at stabilizing an emulsion. Indeed, we emulsifed dodecane with an aqueous solution of CTAB whose the concentration is equivalent to the CTAB solution used to functionalize the silica nanoparticles (1/5 CMC knowing that the critical micellar concentration, CMC, is equal to 0.92 mM). An emulsion was formulated and stabilized during few minutes before breaking. In another experiment, to estimate the amount 7 ACS Paragon Plus Environment

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of non-adsorbed surfactant, we centrifuged and removed the functionalized particles of the CTAB/Silica solution, and measured the surface tension of the supernatant (≈68 mN/m). This value close to the pure water surface tension (72mN/m) reveals that the cationic surfactant concentration in water after particle functionalization was around 1x10-6 M, a concentration much too low for efficient emulsion stabilization. The native emulsion exhibits a narrow drop size distribution (size monodispersity).

Figure 1. Scheme of the whole process used to obtain the capsules. Step 1: Electrostatic adsorption of CTAB at silica nanoparticles surface. Step 2: Progressive incorporation of dodecane in a dispersion of functionalized silica nanoparticles in water to obtain an emulsion Step 3: Limited coalescence phenomenon leading to monodisperse Pickering emulsion. Step 4: Silica shell mineralization around the dodecane droplets. In order to reach this specificity, we take advantage of the limited coalescence phenomenon, that makes the coalescence to stop when the oil-water interface covered by the nanocolloids is optimized.37 Limited coalescence phenomenon consists in producing a large excess of oil-water interface compared to the interfacial area that can be covered by solid particles. Hence, the system must be formulated with a very small amount of solid particles irreversibly anchored at the dodecane-water interface (the particles desorption energy being very large compared to kBT: 2350 kBT at 25°C). When the agitation is stopped, partially uncovered drops coalesce in order to reduce the total amount of oil-water interface. The coalescence process stops as soon as the oil-water interface is sufficiently covered. The resulting emulsions are characterized by narrow drop size distributions and are stable over months. Moreover, adjusting the amount of particles allows tuning the mean drops size: the 8 ACS Paragon Plus Environment

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more particles, the smaller the drop size as coalescence stops at an earlier stage. Thus, exploiting the phenomenon and varying the amount of particles, we produced monodisperse emulsion from 4.0 to 11.5 µm (Fig. 2) called 4.0-SiO2, 7.0-SiO2 and 11.5-SiO2.

Figure 2. a-c) Optical microscopy images of Pickering emulsions dispersed in water with various amount of particles in wt % with respect to oil: a) 6.8 wt %, b) 3.4 wt % and c) 1.7 wt %. Scale bars = 20 µm. d) Corresponding size distributions of the three emulsions; the means diameters are 4.0 µm – 4.0-SiO2 (red solid line), 7.0 µm – 7.0-SiO2 (blue dashed line) and 11.5 µm – 11.5-SiO2 (green dotted line). Another advantage of using the limited coalescence phenomenon is that in this particle-poor regime, drop sizes are independent of the applied pressure during emulsification process, only depending on the amount of particles employed (Fig. S1). Also an important feature, dealing with Pickering emulsions, produced by the limited coalescence phenomenon is that, for geometric considerations,37 the drops size is inversely proportional to the amount of particles keeping the dispersed phase amount constant. By measuring the mean diameter of the droplets thanks to a statistic evaluation by optical microscopy, we can calculate the surface-averaged diameter, D[3,2], defined by:


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D[3,2] =

∑ ∑

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(1)

Therefore, the plot of the inverse of the oil droplets mean size versus the amount of colloids normalized by the oil amount should be a linear function. Regarding Fig. 3 we can assess that the droplet sizes are driven by the so-called limited coalescence phenomenon. This can also be observed on Fig. 3 that shows how the oil droplet diameters are decreasing when increasing the amount of silica particles employed to stabilize the oil-water interface. Again this effect is indeed expected as higher amount of solid colloids are able to stabilize larger oilwater interface areas corresponding to smaller diameters at constant oil amount. Assuming that all particles are adsorbed, we can defined the surface coverage, C, as the ratio between the interface area that the particles may cover and the total interfacial area Sinterf (which is directly linked to the average drop size D[3,2] through: Sinterf = 6.Vd / D[3,2] where Vd is the dispersed phase volume). A geometrical relation links the amount of particles and the drop size37 (2): C=

.[,] . .

(2)

where mparticles is the mass of particles, dp their diameter and ρp their density. We can then estimate the average number of particle layers n by n = C/0.9, where 0.9 corresponds to the coverage for a dense monolayer. In our case, we have determined that there are four layers of particles covering the oil droplets. The details of the method allowing this determination may be found in Destribats et al.39 The result is consistent with the results obtained by Arditty et al.37 Indeed in their paper, these authors used two stirring methods: either low energy manual shaking or high energy with a high jet homogenizer. They found that particles adsorbed either as a monolayer or with an average number of ten layers respectively. Such a difference can be


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explained by the aggregated states of particles prior to emulsification. In the present study the number of layers is intermediate. Once the direct emulsions are prepared, they are diluted at 2 wt % in an aqueous solution containing various concentration of CTAB (0.5, 0.7 and 1.0 wt %) and 15 wt % HCl (37 %) into a glass beaker. CTAB is used here to catalyze the Si(OH)4 polycondensation. On a chemical point of view, the silica particles used to stabilize the emulsion and quasi irreversibly adsorbed at the oil-water interfaces, will serve as nucleation sites for the shell formation during the last step of sol-gel-based mineralization process. 0.30 0.25

1/D[3.2] (µm-1)

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0.20 0.15 0.10 0.05 0.00

0

20

40

60

mparticles / moil (mg/g)

80

Figure 3. Evolution of the inverse diameter mean diameter (1/D) as a function of the mass of particles with respect of the oil mass for a dodecane-in-water emulsion; 11.5-SiO2 (green diamond), 7.0-SiO2 (blue square) and 4.0-SiO2 (red circle). Indeed when dealing with nucleation and growth two types of nucleation can be distinguished, namely, homogeneous and heterogeneous. The homogeneous nucleation requires an additional surface energy because it occurs in the bulk solution with the creation of new interfaces between the native nuclei and the solution. Based on pre-existing nuclei at interfaces, the heterogeneous nucleation will intrinsically minimize the nucleation enthalpy. Therefore, heterogeneous nucleation will be preferentially promoted by the presence of silica nanoparticles at the oil-water interfaces. HCl is used to set the pH below the silica isoelectric point IP (IP ≈ 2.1), in order first to catalyze the hydrolysis-condensation of TEOS at the oil11 ACS Paragon Plus Environment

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water interface. The second reason of being at pH far below the silica IP is to balance the dual inorganic skeleton fractal-Euclidian character. At pH values close to 0, the system exhibits a good fractal-Euclidian balance of the on growing inorganic skeleton, this is to say fractal enough to grow uniformly from the droplets surface (rather than punctual growth) while bearing at the same time good Euclidian characteristics that favors good mechanical properties of the on-growing silica shell.38 The mineralization is initiated by incorporating 5 mL of TEOS as alkoxide monomer. The first step of mineralization consists in the TEOS hydrolysis to Si(OH)4 and subsequent diffusion of the former at the oil-water interfaces. In the second step, these entities polycondense at these same interfaces, leading still to discrete objects. After droplets mineralization has occurred, a down-shift of the diameters distribution is observed (Fig. 4d). This down-shift is increasing with the capsule diameters. Nevertheless, the final capsules are still monodisperse in size (Fig. 4d). This diameter decrease, occurring during the sol-gel process, cannot be explained by oil droplet compression, as liquids are incompressible. Indeed the oil droplets size reduction is likely promoted by the TEOS hydrolysis. Actually the hydrolysis of one molecule of TEOS generates one molecule of silicic acid Si(OH)4 and four molecules of ethanol. This ethanol release in the water phase, likely increases the dodecane solubility in the aqueous phase composed of water, ethanol and CTAB surfactant. Dodecane therefore migrates into the aqueous phase reducing thereby the oil droplet volume. 50

Volume fraction (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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40 30 20 10 0

2

4

6

8

10

12

14

Droplets size (µm)


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Figure 4. Comparison of size distributions of emulsions before mineralization (thin lines): 11.5-SiO2 (green dotted), 7.0-SiO2 (blue dashed) and 4.0-SiO2 (red solid); and capsules after mineralization (thick lines): 11.5-SiO2-5.0(1) (green dotted), 7.0-SiO2-5.0(1) (blue dashed) and 4.0-SiO2 -5.0(1) (red solid). See text for the definitions of the various volumes.

In order to check the validity of this explanation, we put in contact dodecane, previously colored by Sudan I for a better observation, with an aqueous solution of CTAB (15 CMC) containing or not ethanol. The ethanol concentration in the aqueous phase is fixed to be equivalent to the amount of ethanol produced during the hydrolysis of 5.0 mL of TEOS. The samples are shaken to accelerate equilibrium. When adding 0.25 wt% of dodecane to the aqueous phases with or without ethanol different scenari are observed. In the first case, ethanol free, just after stirring turbidity is visible due to the formation of an emulsion that rapidly breaks in less than 5 minutes leading to a colored supernatant phase. The aqueous phase coexists with an excess of oil. On the contrary, the same experiment with the aqueous phase containing ethanol, exhibits no turbidity after stirring, the phase is limpid, and no colored supernatant is visible showing that 0.25 wt% of dodecane is soluble in the aqueous phase (Fig. S2). Therefore dodecane is more soluble in the ethanol-containing aqueous phase than in the ethanol-free aqueous phase. As the oil droplet diameter corresponds to the volume cubic root, the same loss of volume percentage will induce higher effect over the diameters of large droplets when compared with smaller ones. Indeed this effect of ethanol-inducedsolubility of "lighter-than-water non-aqueous phase liquids" (LNAPLs as dodecane, toluene or octane) in water is well known and employed as an ethanol-water blend flooding strategy of LNAPLs recovery.40 Finally, the native capsules can either be dried at ambient temperature or lyophilized without altering their integrity. We checked that the silica shell growth is completed while covering fully the dodecane core. Beyond we have studied the influence of CTAB and TEOS concentrations over the shell homogeneity and thickness. 13 ACS Paragon Plus Environment

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Influence of CTAB concentration over the materials morphologies The present role of CTAB is to catalyze the Si(OH)4 kinetics of condensation, as it was the case for the generation of Pickering emulsion-based macrocellular foams.39 This cationic surfactant has also been employed previously to stabilize the oil-water interface of direct concentrated emulsion where silica heterogeneous nucleation was promoted at the oil-water interface.31 In the present case, by decreasing the amount of CTAB in the bulk phase (concentrations 1.0, 0.7 and 0.5 wt %), we observed the influence of CTAB on the material final morphologies. Using scanning electron microsopy (SEM), additional aggregates clearly appeared on the shell outer surface (Fig. 5a-c) for larger amounts of CTAB.

Figure 5. (a-c) SEM pictures of dried capsules 7.0-SiO2-5.0 obtained from 7.0-SiO2 with various concentration of CTAB during the mineralization step. (a) 1.0 wt % CTAB; (b) 0.7 wt % CTAB; (c) 0.5 wt % CTAB. Scale bars = 10 µm. (d) Corresponding SAXS spectra; 1.0 wt % (top thick), 0.7 wt % (medium thin), 0.5 wt % (down very thin). 14 ACS Paragon Plus Environment

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X-ray diffraction of these crystals attached on the outer surface of the silica shells shows their hexagonal structure (Fig. 5 d), in agreement with the meso-structured crystals of silica labeled MCM-4141 obtained when using cationic lyotrope mesophase as sol-gel templating agents. When decreasing the amount of CTAB, (from Figure 5a to 5c) we can observe that both their numbers and sizes decreased. Considering the top and medium diffractograms of the Figure 5d, they exhibit a first strong peak at a wave vector of q0 = 0.12 Å-1, two harmonics can be observed at wave vectors equal to q1 = 0.20 Å-1 and q2 = 0.24 Å-1 respectively, with a ratio q1/q0 equal to √3 and 2 respectively showing a hexagonal structure. Thus the q0 wave-vector is attributed to, d100 (d100 ≈ 52 Å) of hexagonal order of mesoscopic voids and q1 and q2 are the two harmonics (110 and 200 reflection lines) that can be observed respectively at wave vectors √3q(100) and 2q(100) that depict without ambiguity the hexagonal nature of the silica meso-structure. The unit cell can be calculated a ≈ 60 Å (2.d100/√3).42 Consequently, CTAB is promoting a second heterogeneous nucleation above the silica shell, generating MCM41-like protuberances on the shell. This feature is diminishing when decreasing the CTAB concentration. Thus, for the lower CTAB concentration (Figure 5d bottom) we observe only the main peak slightly shifted to lower wave vector (0.11 Å-1) without harmonic at higher wave vector. This remnant single broad diffraction peak, that corresponds to a less organized vermicular-type meso-structure intrinsic of the silica shell, is indeed similar to what we observed earlier for water@silica capsules obtained from reverse emulsions.13 From figure 5c some protuberances are still visible but not in a sufficient density to induce a detectable hexagonal SAXS signal. Influence of TEOS concentration over the shell thickness We have determined the effect of TEOS concentration over capsules shell thicknesses. Observations are based on SEM pictures, the shell capsules seem to be homogeneous, except 15 ACS Paragon Plus Environment

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for the 11.5-SiO2 starting emulsion due to the high CTAB concentration that promotes external protuberances as just described. This is even more visible for smaller capsules observed by SEM (11.5-SiO2-5.0(1), 7.0-SiO2-5.0(1) and 4.0-SiO2-5.0(1)) previously washed with a solution of tetrahydrofuran (THF) to empty the capsules by dissolution of the inner oil (Figure 6).

Figure 6. (a-c) SEM pictures of dried and empty capsules (11.5-SiO2-5.0(1), 7.0-SiO2-5.0(1) and 4.0-SiO2-5.0(1)) obtained previously from 11.5-SiO2, 7.0-SiO2 and 4.0-SiO2 emulsions depicted in Fig. 3. Scale bars = 10 µm. While maintaining the emulsion mean size constant (4.0 or 7.0 or 11.5 µm), we varied the TEOS concentrations (2.5, 5.0 and 7.5 mL). Still, observation suggests an increasing shell


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thickness all results are summarized within the Table 1. However, the addition of TEOS is bearing one major drawback. As observed previously, high TEOS concentration is inducing high silicic acid (Si(OH)4 molecular hydrophilic precursor) concentration within the surrounding continuous water phase that promotes the generation of monoliths rather than discrete capsules. Table 1. (a-b) Summary of shell thickness for different amount of TEOS and diameter droplets. (a) For one mineralization step; (b) for consecutive mineralization steps. a) Materials 4.0-SiO2-2.5(1)-45 4.0-SiO2-5.0(1)-59 4.0-SiO2-7.5(1)-66 7.0-SiO2-2.5(1)-63 7.0-SiO2-5.0(1)-73 7.0-SiO2-7.5(1)-89 11.5-SiO2-2.5(1)-85 11.5-SiO2-5(1)-102 11.5-SiO2-7.5(1)-115 b) Materials 4.0-SiO2-7.5(3)-86 4.0-SiO2-15.0(3)-98 4.0-SiO2-22.5(3)-117 7.0-SiO2-7.5(3)-84 7.0-SiO2-15.0(3)-92 7.0-SiO2-22.5(3)-105 11.5-SiO2-7.5(3)-103 11.5-SiO2-15(3)-120 11.5-SiO2-22.5(3)-134

Droplet diameters (µm) 4.0 4.0 4.0 7.0 7.0 7.0 11.5 11.5 11.5

Total amount of TEOS (mL)

Shell Thickness (nm)

2.5 5.0 7.5 2.5 5.0 7.5 2.5 5.0 7.5

45 ± 24 56 ± 25 66 ± 23 63 ± 24 73 ± 22 89 ± 22 85 ± 35 102 ± 40 115 ± 41

Droplet diameters (µm) 4.0 4.0 4.0 7.0 7.0 7.0 11.5 11.5 11.5

Total amount of TEOS (mL) 7.5 15.0 22.5 7.5 15.0 22.5 7.5 15.0 22.5

Shell Thickness (nm) 86 ± 18 98 ± 13 117 ± 16 84 ± 19 92 ± 17 105 ± 15 103 ± 14 120 ± 14 134 ± 12

One can notice from Table 1 that there is no proportionality between the amount of added TEOS and the final shell thickness for a given droplets diameter. When the amount of added TEOS is tripled the thickness is only increased by a factor 1.4 at best. This shows that there is 17 ACS Paragon Plus Environment

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no preservation of the TEOS volume involved in the shell synthesis. This is to say that part of the added TEOS is not involved into the silica polycondensation at the oil water interface. Two main reasons can explain this feature. First TEOS is a hydrophobic oil, when added into the emulsion part of the TEOS will migrate preferentially into the dodecane oily phase prior to the hydrolysis, once a first thin SiO2 shell is generated the TEOS still trapped within the oily phase will not be hydrolyzed into Si(OH)4, being thus inactive for the polycondensation process. Secondly, we have to remember that part of the TEOS is consumed in promoting the MCM-41 type nodules. An idea to circumvent this effect would be to increase further the TEOS concentration, thus the Si(OH)4 concentration, but also the amount of released ethanol. As said above by increasing the TEOS concentration, we also promote the gelation of the full continuous phase generating a monolithic material, rather than addressing the polycondensation specifically at the oil/water interface, thus obtaining discrete capsules. Also, when considering the Table 1 we can see that overall, the capsule diameters vary from 4.0 to 11.5 µm while the silica shell thicknesses vary from 45 to 115 nm. At that point it is interesting to compare the present evolution of thickness versus diameter with data obtained previously for water@silica capsules13 generated through surfactant-based reverse emulsion and wax@silica capsules generated from Pickering-based direct emulsion of wax.22 This comparison is proposed on Figure 7 a).


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2.5

b) 0.7

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0.6

Voccupied / Voptimal

a)

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0.3

0.5

0.2

0.0 0

10

20

30

40

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6

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VTEOS (mL)

Figure 7. a) Evolutions of the capsules shell thicknesses versus capsules diameters obtained from surfactant-based reverse emulsions (data from Ref. 13), oil-based Pickering emulsions from the present study, wax-based Pickering emulsions (data from Ref.17). b) Comparison of ratio Voccupied / Voptimal evolution versus the TEOS volumes for, 4.0-SiO2 (black square), 7.0-SiO2 (red circle) and 11.5-SiO2 (blue diamond) after their mineralization. Lines are used as a eyes guide. First, in all cases (direct or reverse emulsions) we can notice a homothetic relation between the capsule diameters and the shell thicknesses. Secondly, for the same capsule diameters the shell thicknesses obtained with the Pickering-based direct emulsions are smaller than the ones reached with surfactant-stabilized reverse ones, providing different slopes of the homothetic relation. We would like to underline that the ratios between the surface of oil droplets to be covered and amount of added TEOS are roughly equal in both cases, thereby the observed discrepancy is not related to this parameter. Also pH conditions are identical, strongly acidic where hydrolysis kinetics is faster than the polycondensation one. Indeed when considering reverse emulsions, the droplets are constituted of water, thus once TEOS diffuses at the oilwater interface, hydrolysis will occur and native Si(OH)4 will migrate into the water droplets acting thereby as Si(OH)4 canister where the shell polycondensation at the oil-water interface 19 ACS Paragon Plus Environment

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will not be limited. On the contrary with direct emulsions and as said above, part of the TEOS will not be consumed intrinsically for the shell mineralization being trapped into the oily droplets, being consumed for the mineralization of MCM-41 type nodules. Another obvious difference addressed by the figure 7a is the extent of the error bars. We can see that the error bars are much larger for the capsules emerging from emulsion stabilized with surfactant when compared with Pickering-based ones. Indeed this feature is induced by the limited coalescence occurring within Pickering emulsions that tends to increase the droplet diameters monodispersity. Due to the homothetic relation the enhanced monodispersty of the oily droplet diameters, encountered in the case of the Pickering-based emulsion, will have a direct consequence over the shell thickness dispersion, leading to smaller error bars. Considering the shell thicknesses versus the TEOS. We evaluated the shells volume Voccupied, knowing the droplet sizes and the shell thicknesses and compared it to the added TEOS amount VTEOS (Fig. 7b). In the ideal case, all the added TEOS would contribute to the shell, so we normalized the shell volume (using the measured densities) by the optimal volume Voptimal, defined as the shell volume that should be reached if the entire added TEOS would contribute to the shell. As all the added TEOS is quantitatively transformed into Si(OH)4 molecules, being subsequently quantitatively involved into the polycondensation. When assimilated to a sphere, we calculated the volume of a SiO4 tetrahedral,, VSiO4, as the Si-O bond length is equal to 1.85 Å. Knowing the amount of TEOS used and thus the number of TEOS molecules, nTEOS, the optimal volume, Voptimal is given by the following relation with NA the Avogadro constant: Voptimal = VSiO4 nTEOS .NA Thereby the ratio Voccupied / Voptimal would be at best equal to 1. Considering Fig. 7b we can observe that this ratio is always below 1 and is decreasing when increasing the TEOS amount and also when the droplet diameter decreases. This shows that only a part of TEOS is used for the shell and as the amount of TEOS increases higher amount of MCM-41 nodules are


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produced that are not intrinsically involved within the shell thickness. When increasing further the amount of added TEOS we can see that this tendency becomes less pronounced (the decreasing rates of Voccupied / Voptima is smaller) but, as noticed previously the TEOS concentration enters a zone where monolith-type materials are obtained rather than discrete droplets, the full continuous water phase starts of being mineralized. In order to by-pass the extrinsic TEOS consumption that does optimize the shell growth, we propose to sequentially add the TEOS rather than employing a one step addition.

Influence of sequential mineralization over the shell thickness As said previously the first mineralization allows formation of a thin shell where increasing the TEOS concentration with the idea of increasing the shell thickness is not efficient. We then intend to tune the shell morphology through sequential mineralization steps. Once the 7.0-SiO2-5.0(1)-73 capsules are prepared and recovered after the washing end in 10 mL of water, we add an aqueous solution containing various concentration of CTAB (0.05, 0.10, 0.17, 0.25, 0.33 and 0.50 wt %) and a constant 15 wt % HCl (37 %) into a glass beaker. Final volume at this point is 95 mL. Addition of TEOS is defined by the amount used at the first mineralization and repeated at each step and we fixed it at 5 mL. Optimal conditions for the mineralization required an adaptation of the CTAB concentration due to the influence of the surfactant on the materials morphologies as explained previously. The CTAB concentration must be decreased to avoid unexpected formation of MCM41 structure on the surface of the shell (Fig. S3). We found out that repeating only twice the mineralization step is not effective toward increasing significantly the shell thickness. Nevertheless the sequential synthesis improved the shells homogeneity. For a CTAB concentration of 0.17 wt %, we obtained welldefined capsules bearing better both homogeneous shell and up-graded resistance against the vacuum imposed within the SEM chamber. This is only under a third mineralization step that


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one can notice an increase of the shell thicknesses. Three sequential mineralization steps are thus necessary to obtain a tougher and thicker shell (Fig. 8).

Figure 8. (a-c) SEM pictures on dried capsules based on Pickering emulsions 7.0-SiO2 for consecutive mineralization steps and summary shell thicknesses. a) Capsules 7.0-SiO2-5.0(1) with tE shell = 73 ± 22 nm; b) capsules 7.0-SiO2-10.0(2), tE shell = 75 ± 17 nm; c) capsules 7.0SiO2-15.0(3), tE shell = 95 ± 15 nm. Scale bars = 10 µm. For materials obtained with other amount of TEOS (originally 2.5 and 7.5 mL became 7.5 and 22.5 mL after the third mineralization), the shell thickness followed the same trend; the evolution of the shell thicknesses versus number of mineralization steps is provided within Table 1.b). Helium pycnometry was used to evaluate the shell density of capsules after each mineralization step. For better results, capsules were washed with THF which was then 22 ACS Paragon Plus Environment

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removed by evaporation. Thus, the density measurements were made on empty and dried capsules and we calculated an increasing density from 1500 to 2200 g.cm-3 after the second mineralization (kept constant for the third). As there is no shell thickness increasing, we can conclude that the second process was used to make the shell denser instead of contributing toward the shell thickening. However, this does not explain the complete difference as we doubled and also tripled the TEOS amount. But, as said previously, the TEOS is not completely used during the sol-gel process. With these results we can confirm that one part of the monomer is used to densify the network, another is used again to form the MCM41-like structure, especially for the third mineralization step when the shell densities have been already increased. Mechanical-induced release of the capsules core contents The mechanical properties of the capsules have been evaluated by an indirect method.43 It consists in subjecting the monodisperse final core-shell particles to a centrifugal stationary field able to concentrate and compress the capsules just before the breaking point. So, the critical compressive stress, enable to induce the surrounding shell collapse, is evaluated. At the rotation of 1000 g, the steady state is reached after 3 hours. After this time, there is no more evolution of the height of the density function of the cream of capsules. This point is our reference for the stationary stress. Several accelerations (1000 to 24000 g) are applied on the synthesized capsules to determine the critical compressive stress. Previous SEM experiments have already brought some qualitative results concerning the shell mechanical resistance under vacuum for capsules mineralized once or three times. By centrifugation, we can quantify the input of mineralization steps over the shell mechanical strength and thus over the associated mechanical-induced release while plotting the breaking pressure of the shell versus the shell thicknesses (Fig. 9). In order to facilitate the observation of weak dodecane release, dye Sudan I has been added into the oily phase as a color witness. 23 ACS Paragon Plus Environment

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a)

b)

4-SiO2-2.5(1)-45 4-SiO2-5(1)-56 4-SiO2-7.5(1)-66 7-SiO2-2.5(1)-63 7-SiO2-5(1)-73 7-SiO2-7.5(1)-89 11-SiO2-2.5(1)-85 11-SiO2-5(1)-102 11-SiO2-7.5(1)-115

10

Breaking effective pressure π(MPa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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40 60 80 100 120 140 160

Shell thicknesses (nm)

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4-SiO2-7.5(3)-86 4-SiO2-15(3)-98 4-SiO2-22.5(3)-117 7-SiO2-7.5(3)-84 7-SiO2-15(3)-92 7-SiO2-22.5(3)-105 11-SiO2-7.5(3)-103 11-SiO2-15(3)-120 11-SiO2-22.5(3)-134

20 40 60 80 100 120 140 160

Shell thicknesses (nm)

Figure 9. Applied pressure versus shell thickness and shell homogeneity. a) Single mineralization step is made on droplet with several TEOS concentration 2.5, 5.0 and 7.5 wt% corresponding to square, circle and triangle respectively. b) Three sequential mineralization steps where overall the amount of TEOS is tripled. Red plain, blue half filled and green empty markers correspond respectively to droplet sizes of 4.0, 7.0 and 11.5 µm. From Fig. 9 it clearly appears that, whatever the synthetic path employed, one step mineralization (Fig. 9a) or sequential ones (Fig. 9b), the mechanical release is effective. Also, as a general trend it can be observed that, the higher the capsule diameters, the higher the pressure before rupturing. Focusing on Fig.9a we can notice that an increase of the shell thicknesses induces higher effective applied pressure before release of the oily core. Moreover, resistance of capsules is 100 times higher than similar Pickering emulsion (i.e. without mineralized shell) of a comparative size demonstrating the importance of the shell homogeneity for better resistance.43 Considering sequential mineralization steps (Fig. 9b), this behavior is still true but the effective applied pressures at which the capsules breaking occurs and thus the associated oily core is released are shifted toward higher values. This effect is induced by both the increase of 24 ACS Paragon Plus Environment

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shell thickness but certainly even more importantly by the enhanced homogeneity of the shell. Indeed from Fig. 9b we can observe that the amount of added TEOS only modifies the shell thickness over a reduced range (85 to 135 nm) but the scope of the effective releasing pressure is by far wider when compared with Fig. 9a. Thereby the shell homogeneity is an important parameter enhancing the shells mechanical strength and acting cooperatively with the shell thickness over the effective breaking pressure of the oil@SiO2 core shell particles. The silica nanoparticles were used to allow an anchoring of the sol-gel process, but they may have an effect on the morphology of the shell and associated mechanical strength. In order to assess the role of the silica nanoparticles on the shell mechanical strength, we produced capsules from a surfactant-stabilized emulsion. This emulsion is produced by preparing a crude premixed emulsion with an aqueous solution of CTAB concentrated at 30 wt% and then by applying a shear rate of about 7000s-1 by means of a Couette cell.44 The mean diameter of the obtained droplets is 7 µm. Then, the oil-water interface is mineralized, by the same process than for the Pickering emulsion, using 7.5 mL of TEOS. The resulting shell thickness is 65 ± 22 nm. Then the breaking pressure of these nanoparticles free capsules, π, is evaluated by the same method using centrifugation, and we determined that for those capsules the breaking value is 0.45 MPa (Fig. S4). For identical size and shell thickness, these surfactant-stabilized emulsion based-capsules should be compared to the 7.0-SiO2-2.5(1)-63 capsules from Pickering emulsions (π = 3 MPa). This experiment shows the contribution of the anchored nanoparticles to the mechanical strength. CONCLUSION Inhere we have taken the benefit of Pickering emulsions and their association with sol-gel chemistry to generate mineralized Pickering emulsions-based capsules constituted of an oily core and siliceous shell, while bearing a good monodispersity of their diameters. The goal was to control the shell thickness in order to trigger the pressure at which the capsules would 25 ACS Paragon Plus Environment

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break. We first made the use of one step polycondensation synthetic path, while reaching limited shell thickness of 115 nm at best as part of the TEOS precursor is either dissolved into the dodecane oily phase prior the hydrolysis or consumed in promoting the MCM-41 type nodules. The as-synthesized capsules delivered their content while applying external pressure from 0.5 to 6 MPa. When addressing a sequential mineralization route we were able to reach both better shell homogeneity and higher values of shell thickness up to 135 nm associated with a shell breaking pressure varying from 1.2 to 10 MPa. In this last configuration shell homogeneity and thickness are acting cooperatively toward enhancing the shell mechanical toughness and the associated effective breaking pressure of the oil@SiO2 core shell particles.


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ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications websiteat DOI: Starting oil droplet diameters versus the pressure applied, ethanol influence on solubility of dodecane in the water phase, evolution of the shell morphologies with the CTAB concentration, macroscopic observation of oil release (PDF)

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