Amphiphilic Pickering Emulsifiers Based on Mushroom-Type Janus

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Amphiphilic Pickering Emulsifiers Based on Mushroom-Type Janus Particles E. Passas-Lagos and F. Schüth* Max-Planck-Institut für Kohlenforschung, Kaiser-Wilhelm-Platz 1, D-45470 Mülheim an der Ruhr, Germany S Supporting Information *

ABSTRACT: Iron-based mushroom-type Janus particles consisting of a poly(sytrene-codivinylbenzene) and a silica moiety both with controllable morphologies were successfully synthesized on the gram scale and investigated as surfactants for Pickering emulsions. Two oil−water model systems, namely toluene−water and vegetable oil−water, were stabilized, giving mainly water-in-oil (w/o) emulsions. By varying several parameters, including Janus particle morphologies and the oil−water ratio, fine-tuning of the emulsion systems was possible; it was even possible to invert the continuous phase to an oil-in-water (o/w) system. Furthermore, the emulsions were stable against coalescence and sedimentation and could be easily separated by centrifugation or a strong magnet. The synthesized mushroom-type Janus particles are suitable for creating Pickering emulsions and can be used as building blocks for creating nanostructures with tailored properties for specific applications.

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by-layer self-assembly,32,33 the Pickering emulsion method,34,35 surface-initiated free-radical polymerization,36 and in situ click chemistry.11,12 Even though Janus particles have proven themselves to be excellent candidates for stabilizing emulsions,25,37−40 several challenges have to be faced. Regarding potential industrial use, a facile and highly reproducible synthesis should give the desired structures in applicable amounts and high purity, using nontoxic starting materials; furthermore, the particles have to be resistant against solvents and harsh conditions. Finally, complete control over the colloid morphology (particle size ≤200 nm) to positively influence the emulsion properties, even introduce surface functionalization and porosity to individual hemispheres, is highly desirable. Magnetic mushroom-type Janus nanoparticles synthesized by Feyen et al.,41 composed of a hydrophobic polystyrene and a hydrophilic silica hemisphere, appear to be very promising in meeting the requirements discussed above. In this contribution we further investigate these particles and their application in stabilizing emulsions. The morphology is controlled by changing synthetic parameters, which also controls the amount of Janus particles obtained in the process. These mushroom-type Janus particles were used in the stabilization of water-based emulsions, using toluene and vegetable oil as model compounds. Different parameters, including shaking time, particle concentration, particle morphology, and oil−water volume ratio, were investigated in order to study the influence of these parameters on emulsion properties.

INTRODUCTION In 1907 the British chemist Percival Spencer Umfreville Pickering observed that emulsions of two immiscible liquids could be stabilized by solid particles due to their adsorption onto the interface between the two phases.1 Since then, several applications making use of solid-stabilized emulsions have been described in multiple articles and patents, spanning various industrial and scientific fields.2−5 They allow for emulsifier-free cosmetic formulations, in which finely dispersed particles are generating oil-in-water6 or water-in-oil emulsions.7 A method for petroleum recovery by injecting emulsions stabilized by not dissolved, partially oleophilic solid particles was described by Bragg.8 Particle-stabilized emulsions are used as a driving fluid for stripping off hydrocarbons from porous formations. The type of particle to be used as solid surfactants has shifted over the years from rather simple colloids (e.g., silica)9 to more complex superstructures10 with anisotropic shapes and surface chemistry, the so-called Janus particles.11−14 It has been demonstrated that Janus particles with amphiphilic structures are very suitable as surfactants in water-based emulsions as they are adsorbed onto the water−oil interface with their hydrophobic hemispheres wetted by the oil phase and hydrophilic hemispheres wetted by the aqueous phase.15,16 The particle character is determining the stabilized emulsion system. A more hydrophilic character will give oil-in-water, a more hydrophobic character water-in-oil emulsions.8,16−18 Several particle parameters, namely the particle size,19 particle concentration,20 oil/ water volume ratio,19,21 pH,22 salt concentration,23 and solvent type,24 have been found to be crucial for the final properties.18,25 Many strategies have been proposed for the fabrication of Janus particles, including polymer self-assembly,26−28 deposition by surface coating of evaporated metal particles,29−31 layer© XXXX American Chemical Society

Received: April 3, 2015 Revised: June 11, 2015

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DOI: 10.1021/acs.langmuir.5b01198 Langmuir XXXX, XXX, XXX−XXX

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Langmuir Scheme 1. Schematic Description of Procedure for the Synthesis of Mushroom-Type Janus Structures

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in i-PrOH (173 mL). The size of the SiO2 moieties of the materials was controlled by the amount of silica precursor (0.3, 1.0, 2.9, or 14.5 g). The solution was stirred for 17 h at room temperature. Finally, the obtained products were washed three times with ethanol by the procedure described by Feyen, redispersed in mQ-H2O, and freezedried. Thus, obtained particles are named Fe3O4@DVB-X&SiO2-Y, with X the mol % of PDVB content in the polymer hemisphere and Y the size of the resulting SiO2 moiety in nanometers. Varying DVB Content in Polymer Matrix. The content of crosslinker DVB was gradually increased from 2% to 40%, as it improves the resistance of the final monomer matrix to solvents and harsh experimental conditions. The synthesis procedure was kept the same as described above. For example, in the synthesis of Fe3O4@PSD colloids containing 40 mol % DVB, the monomer mixture used contained 10.88 mmol of styrene, 8.19 mmol of DVB, and 1.48 mmol of GMA monomer (Table 1). To ensure consistent experimental

EXPERIMENTAL METHODS

Materials. Ammonium peroxodisulfate (≥98.0%), ammonium hydroxide solution (28 wt %) in water, iron(II) chloride tetrahydrate (≥99.0%), iron(III) chloride hexahydrate (≥99.0%), styrene (≥99%) divinylbenzene (technical grade, 80% mixture of isomers), glycidyl methacrylate (98.0%), L-lysine, tetraethyl orthosilicate (TEOS) (≥99%), isopropanol (chromasolv), Rhodamine 6G (dye content ∼95%), and toluene (99.8%) were obtained from Sigma-Aldrich. Vegetable oil (100% sunflower oil) was purchased at a local supermarket. 16-Heptadecenoic acid (HDA) was prepared following the synthesis of Mirviss et al.42 The final product was obtained with a purity of 96% determined with ESI-MS. All chemicals were used as received, except for divinylbenzene and styrene, which were freshly distilled at 50 °C under vacuum before use to remove the inhibitors. Synthesis of Fe3O4@PSD&SiO2 Mushroom-Type Janus Structures. The colloids were prepared following the synthetic route described by Feyen et al.41 (Scheme 1), yielding high purity mushroom-type Janus particles. However, compared to the original synthesis, some parameters were changed as detailed in the following in order to influence the final nanoparticle morphologies. In a first step, magnetite particles were prepared using a modification of the synthetic route described by Massart et al.43 based on coprecipitation of Fe(II) and Fe(III) chlorides in basic solution. All steps were performed in argon atmosphere. In a typical synthesis 5.0 mmol of FeCl3·6H2O and 2.5 mmol of FeCl2·4H2O were dissolved in 10.0 mL of Millipore water (18.2 MΩ cm). This solution was injected dropwise into an aqueous solution of ammonium hydroxide (2 wt % in mQ-H2O) at 90 °C under vigorous mechanical stirring (600 rpm). After 30 min the formed black material was collected with a strong magnet. Stabilization of the iron oxide particles in aqueous media was achieved by adding a mixture of 1.1 mmol of 16heptadecenoic acid dissolved in 5.0 mL of an aqueous ammonium hydroxide solution (2 wt %). After 1 h of stirring at 50 °C a stable dispersion containing Fe3O4@HDA (the “@” character indicates that the species left of it is encapsulated by the species on the right) particles (9.0 ± 5 nm) was obtained. In the second step, for the synthesis of Fe3O4@PSD colloids, typically 100 mg of the magnetite colloids described before was stirred with 19.83 mmol of styrene, 0.44 mmol of divinylbenzene (DVB), and 1.60 mmol of glycidyl methacrylate (GMA) at 50 °C for 1 min. Then 142 mL of warm ammonia solution (50 °C, 2 wt %) was added to the two-phase mixture. The black dispersion was quickly (within 10 min) heated up to 70 °C. Once the solution reached this temperature, 0.17 mmol of (NH4)2S2O8 dissolved in 2 mL of H2O Millipore water was added under vigorous stirring to initiate the polymerization reaction. After 17 h, a stable gray dispersion was obtained, containing Fe3O4 nanospheres immobilized on PSD polymer. The influence of 16-HDA on the final polymer sphere size was investigated by altering the concentration of 16-HDA added to the monomer mixture before polymerization from 1 to 14.4 mM. In the final step, mushroom structures were produced by asymmetric SiO2 growth on the accessible magnetite nanoparticles, which were immobilized on the surfaces of the PSD spheres. Typically 1.0 g of Fe3O4@PSD material was diluted in 160.0 mL of ammonia solution (2% NH3 in water) and stirred in 60 mL of isopropanol (iPrOH) for 1 h at room temperature. Then 242 mL of i-PrOH, premixed with concentrated ammonia solution (9.7 mL of 28% NH3 in water), was added, followed by a direct addition of TEOS dissolved

Table 1. Content of DVB in the Monomer Mixtures DVB content (%)

styrene (mmol)

DVB (mmol)

GMA (mmol)

2 10 20 30 40

19.83 17.59 15.18 12.77 10.88

0.44 2.18 4.17 6.14 8.19

1.60 1.56 1.52 1.49 1.48

conditions, the total number of magnetite nanoparticles used during the polymerization is determined with TEM analysis as a fix point to calculate the amounts of necessary monomer. Because of this reason, the overall molar amounts listed in Table 1 vary slightly. For simplification these materials were named after their cross-linker content Fe 3 O 4 @DVB-X, with X as the mol % content of polydivinylbenzene in the polymer. Emulsification Experiments. It is important to define a way for the emulsification process that is cheap and readily available. For this purpose a shaker (IKA KS 130 basic) was chosen, allowing precise control of the shaking speed. In a typical emulsification experiment 35 mg of Fe3O4@DVB-X&SiO2 Janus particles was added in a 10 mL glass sample vial. Next, 2 mL of water and 2 mL of oil were added, and the resulting mixture was emulsified in the shaker for 50 min at 800 rpm at room temperature. Two systems were investigated, namely toluene−water and vegetable oil−water. Toluene was chosen because generation of oil−water emulsions with toluene is difficult. Second, vegetable oil (100% sunflower oil) was chosen, as it is found often in industrial uses. The influence of different factors on creating these emulsions was investigated, such as the shaking time, particle concentration, oil/water ratio, and particle morphology of individual hemispheres. Characterization. Fe3O4@PSD&SiO2 Mushroom-Type Janus Structures. The morphology of the particles was investigated with transmission electron microscopy (TEM) using a Hitachi HF 7100. All samples were prepared on lacey carbon film supported by a copper grid. Dynamic light scattering (DLS) measurements were conducted on a Malvern Instruments Zetasizer Nano-ZS. Emulsions Stabilized with Fe3O4@PSD&SiO2 Janus Structures. The created emulsions were studied with an optical microscope (Olympus BX41) attached to a live camera (Soft Imaging Systems Color View 3). One drop of the investigated emulsion was applied on B


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Langmuir a microscopic slide and overlaid with a cover glass. To determine if the final system of the emulsions stabilized with mushroom-type Janus particles is an oil-in-water (o/w) or water-in-oil (w/o) emulsion, an organic dye only soluble in water and fluorescing when exposed to UV radiation (namely Rhodamine 6G) was chosen. 1−2 drops of a 2.1 mM dye solution were added to the Janus/oil−water mixtures prior to emulsification. The resulting samples were investigated with the optical microscope to identify the phase containing the organic dye. For a more distinctive result, the room was completely darkened, and the sample attached to the microscope was exposed to UV radiation with a wavelength of 366 nm. Because of the small amounts of dye in the sample, the fluorescence had to be made visible by increasing the light exposure time of the attached camera to 20 s.

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RESULTS AND DISCUSSION Synthesis of Mushroom Nanostructures Fe3O4@ PSD&SiO2. The wet-chemistry procedure illustrated in Scheme 1 led to a successful large-scale synthesis of colloidal Janus nanoparticles. Figure 1A shows TEM images of the Fe3O4@

Figure 2. Influence of stabilizing agent/surfactant 16-HDA concentration on the Fe3O4@DVB-X polymer sphere size; fitted curve y = 103.68x−1 + 29.97; R2 = 0.986 21.

observed that the sphere size decreases with increasing 16-HDA concentrations following a reciprocal function. The reason for this trend can be found in the emulsion polymerization mechanism,44 as the monomer droplet size is determined by the amount of surfactant molecules present in the system. The more 16-HDA molecules are available that can locate on the monomer droplet surface, the smaller will be the resulting monomer droplet, thus making the final droplet size (dD) antiproportional to the surfactant concentration (c16‑HDA) with dD ∼ Ac16‑HDA−1 and A as a constant. It is important to note that a purity of 16-HDA above 95% is crucial for the successful synthesis of the desired nanostructures. In foregoing experiments, 16-HDA with visible side products and or remaining solvent in the NMR spectrum (estimated purity of 80−90%) did not lead to a successful formation of Janus nanostructures due to a complete agglomeration of magnetite particles in the polymerization step. Increasing the amount of lower purity 16-HDA during the polymerization helps to counter this effect yet leads to inferior Janus structures. Influence of TEOS. The silica hemisphere size in the final particle is increasing with the concentration of TEOS used during the Stöber process. This is to be expected, since the consumption of the silica precursor is more or less complete, and the silica only grows on the existing particles under wellchosen conditions. The size range could be chosen freely between 30 and 300 nm by adjusting the TEOS concentration. Even high amounts of TEOS did not lead to a complete encapsulation of the polymer spheres. For the emulsification experiments a fixed polymer sphere size of 90 nm was chosen. In order to investigate the influence of Janus particles with different silica morphologies on the emulsification experiments later on, differently sized SiO2 hemispheres (50, 90, 110, and 200 nm) were added to the polymer spheres by adjusting the TEOS amount. As expected for the Stöber process, the silica hemispheres (sphere volume) grow proportionally with increasing TEOS concentration used following a cube root function (Figure 3). The reason for this trend can be found in the Stöber process mechanism itself.44 At first the SiO2 beads grow very fast with the TEOS concentration, but diameter growth slows down at higher sizes, since the amount of precursor (cTEOS) consumed scales with the third power of the diameter orexpressed the other way rounddSiO2 = AcTEOS1/3 and A as a constant. To avoid misunderstandings, the silica hemisphere size will be indicated in the nomenclature when

Figure 1. TEM images of (A) Fe3O4@DVB-2 polymer spheres and (B) Fe3O4@DVB-2&SiO2 mushroom-type Janus particles, with a depicted vial containing the final material.

DVB-2 spheres with single iron oxide particles immobilized on the polymer surface. When the stirring period of the monomer/ magnetite phases was kept below 1 min, the magnetite particles were located at the surface of the polymer spheres. When the mixture was stirred for at least 1 h, all Fe3O4 particles were found completely encapsulated inside the polymer spheres, in agreement with the findings of Feyen.41 The particles with exposed magnetite on the polymer surface were used as seed material to synthesize Fe 3O4 @DVB-2&SiO2 mushroom structures of high purity via spatially selective silica coating, as shown in Figure 1B. It is important to note that nucleation of silica only occurs on the surface of accessible Fe3O4, not on the polymer surface. The results of TEM measurements were corroborated by dynamic light scattering analysis of the material, showing that particles obtained in every step are isolated and not aggregated and thus stable in suspension. Interparticle growth or attachment between particles of the same kind was not observed. Roughly 1 g of magnetic, ochreyellow material was collected (Figure 1B) from a typical batch. By changing different parameters during the synthetic route, complete control over the final particle morphology was achieved. Influence of 16-HDA. The polymer sphere size is directly influenced by the concentration of stabilizing agent 16-HDA present during the polymerization due to its role as surfactant in the process. Several series of experiments were conducted in order to obtain a reliable procedure that allows tuning of the polymer sphere sizes. Polymer spheres could be reproducibly produced in the size range between 35 and 125 nm. Figure 2 shows the PSD sphere sizes obtained for different 16-HDA concentrations used during the polymerization. It can be C


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Figure 3. Influence of TEOS concentration during Stöber process on the outcome of final SiO2 hemisphere size. Corresponding TEM images of mushroom-type Janus structures with concentrations of TEOS used (from left to right): 2.1, 7.0, 20.2, and 99.4 mmol/L; fitted curve y = 43.08x1/3; R2 = 0.988 67.

needed like Fe3O4@DVB-X&SiO2-Y with Y the SiO2 sphere size in nanometers. Influence of DVB Content in the Polymer. With future experimentation in mind, it appears important to make the final mushroom-type nanostructures more resistant against solvents and/or harsh experimental conditions. The weak spot of the whole structure is the polymer moiety, as pure polystyrene is resistant neither to various solvents nor to temperatures exceeding 100 °C.45 As already 2% of cross-linker DVB was incorporated in the polymer matrix, the DVB amount was gradually increased from 2% to 10, 20, and 40%. In all cases the synthesis was successful, yielding seed material to be used in the Stöber process. Figure 4 shows the TEM images obtained for samples prepared with different amounts of DVB. However, the increase of DVB monomer in the polymerization influences the process leading to the immobilization of magnetite nanoparticles on the polymer surface: The higher the amount of DVB, the more magnetite nanoparticles are encapsulated inside the polymer matrix and are thus not accessible as sites for silica nucleation, shown through Figure 4A,C,E,G. This leads to lower mushroom nanostructure yields as the count of isolated polymer spheres increases, seen through Figure 4B,D,F,H. The increase to 10% of DVB in the polymer matrix is only affecting slightly the quality of the obtained Janus structures. One can see that the number of isolated PSD spheres has increased (Figure 4D), though the number of mushroom-type structures is roughly the same. Silica hemispheres obtained for materials with 20% DVB content (Figure 4F) are significantly larger in size, as the same amount of TEOS precursor has to be divided among a very small number of surface accessible iron oxide sites. Janus nanostructures with 40% DVB content are rarely observed when analyzed with the TEM, mostly isolated Fe3O4@DVB-40 and SiO2 spheres are found (Figure 4H). This problem can be circumvented by decreasing the polymer sphere size and/or by increasing the size of the iron oxide particles used in the polymerization process. To illustrate this, α-Fe2O3 (hematite) nanoparticles (100 ± 25 nm)41 were used instead of Fe3O4 in the synthetic steps to obtain αFe2O3@PSD spheres with 20% DVB content in the polymer depicted in Figure 5A. TEM measurements revealed that all PSD spheres were successfully nucleated during Stöber process, giving α-Fe2O3@DVB-20&SiO2 mushroom structures (Figure 5B). Because of their size, the hematite particles cannot be fully

Figure 4. TEM images of (A) Fe3O4@DVB-2, (C) Fe3O4@DVB-10, (E) Fe3O4@DVB-20, (G) Fe3O4@DVB-40 polymer spheres and (B) Fe3O4@DVB-2&SiO2, (D) Fe3O4@DVB-10&SiO2, (F) Fe3O4@DVB20&SiO2 (H) Fe3O4@DVB-40&SiO2 mushroom-type Janus particles.

Figure 5. TEM images of (A) α-Fe2O3@DVB-20 polymer spheres, including a SEM microphotograph clearly showing surface accessible Fe2O3 surface, and (B) α-Fe2O3@DVB-20&SiO2 mushroom-type Janus particles.

encapsulated by the monomer droplets, leaving surface accessible sites for silica nucleation after the polymerization. In the emulsification experiments later on toluene was used as nonpolar compound, which is capable of dissolving pure polystyrene. In order to check whether this could be a problem, 35 mg of Fe3O4@DVB-2&SiO2 material (polymer matrix containing 2% DVB) was dispersed in toluene via ultrasonication and shaken for 24 h at 800 rpm and RT. After D


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Figure 6. Left: influence of shaking time (Fe3O4@DVB-2&SiO2-90 material used). Right: influence of emulsifier concentration (Fe3O4@DVB2&SiO2-50 material used) on the stabilization of a 1:1 toluene−water mixture (50 min shaking). The depicted picture shows the emulsion corresponding to the data point marked with an asterisk.

character, the overall polarity is very low due to the small polar magnetite particle size compared to the nonpolar PSD sphere size. This effect was confirmed by the fact that it is nearly impossible to disperse a dried sample of Fe3O4@DVB-2 material in water. On the other hand, it is very easy to disperse it in both toluene and vegetable oil. For the toluene−water system the lower Janus particle polarity appears to have no effect on the outcome of the emulsion, probably due to the low polarity of toluene itself. For the vegetable oil−water system, on the other hand, the low particle polarity prevents emulsification so that the two phases remain separated (Figure S3-A2). The fact that Fe3O4@DVB-2 particles do not emulsify the vegetable oil−water system could be attributed to the vegetable oils higher polarity compared to toluene. Toluene/Water System. Mixtures of 1:1 toluene/water systems (2 mL each) were successfully stabilized with Fe3O4@ DVB-X&SiO2-Y mushroom Janus particles. Already after 5 min of shaking at 800 rpm, emulsion droplets become visible to the naked eye, roughly 0.5−1.5 mm in size (Figure S4), which decrease to the micrometer scale with longer shaking times (Figure S5). Initially not all the water phase is emulsified but with increasing shaking times, more and more water is stabilized until a complete emulsification of water as droplets (discontinuous phase) in toluene (continuous phase) is reached (50 min shaking time) depicted in Figure S5E. The water droplets cannot be seen anymore by the naked eye, indicating a droplet size in the lower micrometer scale. An increase of shaking time over 50 min did not improve the outcome of the emulsion (Figure 6, left). With the analytical methods described above the droplets were determined to belong to a water-in-oil (w/o) system (Figure 7). Investigations with an optical microscope revealed that the smallest observable emulsion droplets have a size of 1 μm (Figure S6) and are growing in number with increasing shaking times, reaching a limit at 50 min of shaking. Emulsification experiments on toluene/water systems were also recently described in the literature46,47albeit with different types of particles, including those with switchable properties. The focus of these studies was the switching from w/o to o/w emulsions and the catastrophic inversion between these two. The droplet sizes in these studies, however, were significantly bigger. Because of the small droplets in the current study, our system appears to be interesting for phase transfer catalysis experiments, where high interfacial areas are required, and which is currently being explored in our laboratory. This potential

recovering the mushroom particles via centrifugation, TEM measurements revealed that the structures were not dissolved by toluene (Supporting Information Figure S1) and could be reused for further experimentation. Amphiphilic Pickering Emulsifiers Based on Mushroom-Type Janus Particles. Because of their amphiphilic character (PSD sphere is hydrophobic/silica sphere hydrophilic), mushroom-type Janus particles should be highly suitable for the use in Pickering emulsions, as they are expected to have high adsorption energy onto the oil−water interphase. In the following, the effects of the oil-phase compound (toluene, vegetable oil), particle concentration, shaking time, oil−water ratio, and silica sphere size were investigated. Preliminary results showed that after 50 min of shaking at 800 rpm the emulsification could not be further improved, independent of which compound was used for the oil phase. Emulsification with Bare Fe3O4@PSD Spheres. It is important to note that the bare Fe3O4@DVB-X spheres with surface accessible magnetite particles (obtained directly after the polymerization) are anisotropic Janus particles themselves. They set a good baseline for comparison with the new behavior given by the obtained mushroom-type Janus structures. The PSD spheres were purified, freeze-dried, and used to emulsify oil−water systems, applying the conditions described above. As seen in Figure S2-A2, bare Fe3O4@DVB-2 spheres perform extremely well in the emulsification of a toluene−water mixture (giving w/o emulsions) even better compared to the Fe3O4@ DVB-2&SiO2 particles depicted in the same figure. A trend can be observed where the grade of emulsification is dramatically increasing with decreasing silica hemisphere size. The reason for this effect can be found in at least two variables. First, the Fe3O4 particle is much smaller in volume than the used silica sphere sizes. This means that for a sample with 8.75 mg/mL pure Fe3O4@DVB-2 material the number of individual Janus particles is by a factor of 1.2 higher than in the same amount of a sample containing Fe3O4@DVB-X&SiO2-50 particles with 50 nm large silica hemispheres. In agreement with literature,46 a higher amount of colloid structures usually has a positive effect on the emulsification of water droplets in toluene. The small size of the magnetite particles should have no effect on the possible number of Janus structures located on a single w/o emulsion droplet, as in this case the theoretical number is limited by the fixed PSD hemisphere size of 90 nm. The second variable could be the change of total particle polarity. Even though the Fe3O4@DVB-2 structure has an amphiphilic E


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mL (Figure S8). These findings are summarized in Figure 6 (right). As mentioned before, the size of the silica hemisphere on the mushroom-type nanoparticles can be easily adjusted with the correct amount of TEOS used during the Stöber process. The influence of silica sphere sizes on the emulsions was investigated, and the results are shown in Figure 8 (left). All created emulsions for 1:1 oil−water systems were found to be w/o systems. It was observed that with a growing silica sphere size the quality of emulsification became lower and lower (see Supporting Information Figure S3, parts A2, B2, C2, and D2), until the point where no emulsification was observed at all with Fe3O4@DVB-2&SiO2-200 (Figure S2-E2). These results can be explained with the very nature of Pickering emulsions.44 In a w/o system water droplets have to be stabilized with the hydrophilic side of the particle (silica hemisphere) penetrating the water droplet. The hydrophobic side is located in the continuous oil phase. As long as the silica moiety is smaller than the polymer moiety, in total a droplet appears to be hydrophobic. This means, the fraction of hydrophobic patches exposed to the oil phase is crucial for a w/o droplet to be dispersed in nonpolar medium. With increasing silica hemisphere size, the droplet appears to be more and more hydrophilic, leading to lower emulsification grades or no emulsification at all, as the different polarities inhibit the dispersion of w/o droplets in the oil phase. To put it into perspective, the fraction of polar surface on the particles used for the emulsification process was calculated to be 2.7, 23.6, 50.0, 59.9, and 83.2% respectively for polar hemisphere sizes of 15 (bare Fe3O4 particle), 50, 90, 110, and 200 nm. Furthermore, the total number of Janus particles in 8.75 mg/ mL of a sample with 200 nm silica spheres is going to be lower than the number of particles in 8.75 mg/mL of a sample with 50 nm small silica spheres. A calculated estimate based on the volume of each Janus structure reveals that the number of particles for the latter case is roughly 10 times higher. This influence appears to be less relevant, because as shown in Figure S7 even minimal amounts of mushroom-type particles are sufficient to create at least emulsion droplets visible to the naked eye (0.5−2 mm). Another effect can be found in the largely increased polarity of the whole Janus particle, resulting in a loss of amphiphilic character. In order to explore if the emulsions generated with mushroom-type Janus particles (giving mainly w/o systems) could be reversed to an o/w system, the oil−water ratio was

Figure 7. Optical micrographs of a toluene-H2O 1:1 mixture containing 8.75 mg/mL Fe3O4@DVB-2&SiO2-110 material, dyed with Rhodamine 6G: (A) with natural light; (B) exposed to UV radiation in the dark.

application is supported by the fact that the very different chemistries of the two moieties of the Janus particles allow individual functionalization. It is important to note that all obtained 1:1 oil−water emulsions were w/o systems, stable for at least 24 h. By using the emulsification method described above, Fe3O4@DVB-2&SiO2-50 material was used to create emulsions containing different concentrations of Janus particles. The results of these experiments are shown in Figure 6 (right). As expected, the grade of emulsification is decreasing with decreasing concentration of particles used to create the emulsion. The lowest concentration used (1.5 mg/mL) is just sufficient to emulsify only some water droplets in the size range of 0.5−2 mm, which are visible to the naked eye at the toluene/ water interface (Figure S7). By increasing the concentration to 3 mg/mL and further, no toluene/water interface is visible anymore, only water droplets that are emulsified in the toluene phase (Figure S8). The water droplets at low emulsifier concentrations are sinking to the bottom of the vial and are not fully dispersed throughout. This is attributed to the water droplet size at low Janus-particle concentration. Even though droplets of ≤1 μm are formed, there are still larger droplets present. For these droplets the density of the emulsified material has an effect on determining the final position of the droplet in the vial. This explanation is backed up by the fact that with increasing amounts of emulsifier the water droplets become smaller and can be dispersed more easily throughout the continuous phase. All emulsions observed were w/o systems. Nevertheless, the effect of improving the emulsion by increasing the amount of particles is limited, as no difference was observed with particle concentrations of 8.75 and 12.5 mg/

Figure 8. Left: influence of silica hemisphere size on the stabilization of a 1:1 toluene−water mixture. Right: influence of the toluene−water ratio on the outcome of the emulsion. F


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Figure 9. Left: influence of Fe3O4@DVB-2&SiO2-50 concentration on the stabilization of a 1:1 vegetable oil−water mixture with 2 h standing. Right: influence of silica hemisphere size on the stabilization and on 1:1 vegetable oil−water emulsion with 2 weeks standing.

interphase (Figure S10). After 2 h of standing no further deemulsification could be observed; even after 2 weeks no significant change occurred to the final appearance of the emulsion. On closer inspection of Figure S10-E, emulsion droplets with a size of 0.5−1 mm are visible, with a layer of smaller droplets on top. It appears as if the water droplets are located in the vegetable oil phase indicating a w/o emulsion. This fact was confirmed by optical microscopy analysis, using the UV active dye Rhodamine 6G as described before (Figure S11). Furthermore, the optical microphotographs are revealing that the smallest observable emulsion droplets are in the size range of 1−2 μm (Figure S12), comparable to the toluene/ water system (Figure 7 and Figure S6). These droplets are formed initially in great numbers, and the size distribution is rather monodisperse. However, the droplets start to coalesce as soon as the shaking is stopped. A stationary state is reached after 2 h. The best emulsifying sample for the vegetable oil− water system Fe3O4@DVB-2&SiO2-50 (see Figure S3-B2) was used to create emulsions containing different concentration of Janus particles, depicted in Figure 9 (left). As mentioned before, the vegetable oil/water systems are created rather easily, making it more difficult to make assumptions on the initial emulsification process. However, in terms of completeness the emulsification results directly after the shaking process (50 min) are shown in Figure S13. As expected, the ochre-yellow color is becoming darker with increasing concentration of mushroom particles. The results after 2 h of standing are depicted in Figure S14. As expected, the effectivity of emulsion stabilization is increasing with the amount of emulsifier used in the system. Almost a complete emulsification of the water phase is reached, with the droplets dispersed throughout the continuous vegetable oil phase, at high particle concentration (Figure S14-E). Even if the standing time was increased beyond 2 weeks, only slight changes in the appearance of the emulsion were observed (see Figure S3-B2, which is the same sample as Figure S14-D, but after standing for 2 weeks). Figure S14 also reveals that the limit where all the water in the system is emulsified is between 4.38 and 8.75 mg/ mL of mushroom particle concentration (Figure S14-C and -D). When the mushroom particle amount is further increased, the average droplet size is decreasing (in agreement with the findings of the toluene−water system) as the droplets are more easily dispersed throughout the continuous oil phase. Even at a high particle concentration of 12.5 mg/mL no Janus particles were found dispersed in the oil phase (Figure S14-E), as

varied. It was expected that if the oil phase (toluene) volume would be significantly smaller than the water phase, an o/w emulsion could be created, as the continuous phase is usually the one taking up most volume in the system.44 The best stabilizing particles from the foregoing experiments used to create w/o toluene−water emulsions (Fe3O4@DVB-2&SiO250) were used to create emulsions with different ratios of toluene/water, depicted in Figure 8 (right). When the volume of the water phase is increased, a visual change can be observed, indicating the expected change to an o/w system between a water volume ratio of 0.5 and 0.75 (Figure S9-A2 and -B2). On the other hand, when increasing the fraction of the toluene phase, an interesting effect is observed: The w/o emulsions are still created, but the toluene phase appears to be containing dispersed mushroom Janus particles that are not used for the emulsification (Figure S9-C2 and -D2). The concentration of mushroom particles in the toluene phase appears to increase with decreasing volume of water used in the system. This effect was already observed while investigating the effect of increasing mushroom particle concentration in the emulsion (Figure S8). In the sample where 12.5 mg/mL of Fe3O4@DVB-2&SiO2-50 was used (Figure S8-E) the toluene phase on top appears slightly brown, indicating that not all the mushroom particles were used to create the emulsion droplets. The same observation is made when the amount of water in the system is decreased (Figure S9-C2 and -D2). This suggests that a saturation point is reached at which no more mushroom-type Janus particles are used to create emulsion droplets. Monodisperse emulsion droplets were found under these circumstances under an optical microscope, as the mushroom particles are too small to be visible to the eye. Vegetable Oil/Water System. Vegetable oil (100% sunflower oil) needs to be emulsified in many practical applications; thus, it is an appropriate compound to be tested as oil phase in mushroom particle stabilized emulsions. For reasons of comparability, the same 50 min shaking time, as used in the toluene/water experiments, was used for the vegetable oil as well. Longer shaking times showed no further improvement in emulsion quality. Surprisingly, in contrast to the toluene/water experiments, already 5 min of shaking time was sufficient to completely emulsify said mixture. On the other hand, the created emulsions would slowly separate over time to some extent when left standing, where both the oil and water phase become visible. The emulsion droplets are located at the oil/water G


Langmuir previously observed for the toluene−water system (Figure S8E). It was shown above that silica hemisphere sizes could be easily adjusted with the correct amount of TEOS used during the synthesis. The influence of silica hemisphere sizes on the emulsions with vegetable oil was investigated analogous to the toluene−water system. All emulsions were found to be w/o systems. As observed before, growing silica hemisphere sizes reduce the quality of emulsification (Figure 9 (right), also see Figure S3). In contrast to the toluene−water system, even Fe3O4@DVB-2&SiO2-200 had a moderate emulsifying effect (Figure S3-E2).

ABBREVIATIONS

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REFERENCES

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CONCLUSIONS Magnetic amphiphilic mushroom-type Janus structures were successfully synthesized in appreciable amounts. Properties of the materials could be completely controlled, so that the sizes of individual PSD and SiO2 hemispheres can be chosen at will. This provides interesting building blocks that can be tailored for a specific application. The DVB cross-linker content in the polymer matrix could easily be increased to 40 mol %, however, at the expense of Janus particle yields. Pickering emulsions of toluene−H2O and vegetable oil−H2O were successfully stabilized with these Janus particles, obtaining mainly w/o emulsions. It was demonstrated that the o/w system could be reversed to a w/o emulsion by increasing the fraction of the oil phase. The most effective emulsification was achieved with small silica hemisphere size, as particles with large SiO2 hemispheres are hindered in the formation of emulsion droplets by steric effects. In the future, possibilities of functionalizing both hemispheres of the Janus particle differently will be explored, as the individual moieties can be tuned to increase the amphiphilic character of the nanostructures and to incorporate additional functionality, which could be interesting for phase-transfer catalysis. ASSOCIATED CONTENT

S Supporting Information *

TEM image of Fe3O4@DVB-2&SiO2 particles after being recovered from 24 h shaking in toluene; photographs of oil− water emulsions stabilized with Fe3O4@DVB-2 and Fe3O4@ DVB-2&SiO2 nanostructures as well as optical microphotographs of said emulsions. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.5b01198.

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DVB, divinylbenzene; GMA, glycidyl methacrylate; PSD, poly(styrene-co-divinylbenzene).

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Article

AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (F.S.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Support by the ERC in the framework of the POLYCAT project and the Cluster of Excellence TMFB is gratefully acknowledged. We are also grateful for the basic funding provided by Max-Planck-Institut für Kohlenforschung. Special thanks to Alessia Padovani and Dr. Gurudas P. Mane for their contributions in the laboratory. H


Article

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Amphiphilic Pickering Emulsifiers Based on Mushroom-Type Janus

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