Article pubs.acs.org/Langmuir
Fabrication of a Composite Colloidal Particle with Unusual Janus Structure as a High-Performance Solid Emulsifier Xiaohui Meng,†,‡ Yinyan Guan,†,‡ Zhengdong Zhang,§ and Dong Qiu*,† †
Beijing National Laboratory for Molecular Sciences, State Key Laboratory of Polymer Physics and Chemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing, 100190, China ‡ Graduate University of Chinese Academy of Sciences, Beijing, 100190, China § National Institute of Metrology, Beijing, 100013, China S Supporting Information *
ABSTRACT: Core−shell particles with cross-linked core and shell were used as seed particles to produce composite Janus particles. It was found that when the shell has distinctly higher cross-linking degree than the core, Janus particles with very unusual structures can be obtained. These particles have two parts, with one part embraced partially or entirely by the other part, adjustable by parameters such as phase ratio or cross-linking degree. On the basis of experimental observations, a possible mechanism for the formation of such unusual Janus particles has been proposed. Janus particles with arms are used to emulsify water−toluene mixtures, forming oil-in-water (O/W) emulsions at very high internal phase content with rather low concentration of particles. Nonspherical emulsion droplets were observed, indicating that these Janus particles are likely to jam at the interface, forming a strong protecting layer to stabilize emulsions.
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INTRODUCTION In recent years, anisotropic colloidal particles have received increasing attention because of their potential applications in optical device,1−3 structural fluid,4−7 and atomic/molecular models.8,9 In general, the synthesis of particles with complex morphology is not straightforward because the minimization of surface tension favors a spherical morphology. This obstacle may be overcome by controlled clusterization,10,11 microfluidics,12−14 or random coagulation method.15 Spontaneous phase separation in seeded polymerization is a more facile and effective strategy to produce anisotropic colloidal particles, through which particles with a number of morphologies, such as “dumbbell”, “tripe rod”, “triangle”, “cone”, “diamond”, and even water molecule-shaped,8,16−21 have been prepared at relatively large scale. The resultant particles not only have complex morphologies, but also have multicomponents,22,23 opening the possibility of postmodification on selected regions through chemical or physical interactions. In this approach, the size of the newly formed polymer phase is tunable by changing the monomer/seed ratio or cross-linking density, etc. However, each compartment of the anisotropic particle still tends to keep a smooth and spherical-like morphology, regulated by the minimization of specific interfacial area. Only recently dumbbell Janus particles with smooth and coarse surfaces on different sides were achieved by using seed particles made of clusters of small primary particles, where the seed particles remained coarse while the newly formed phase appeared smooth.24 Theoretical calculations have indicated that the adsorption © 2012 American Chemical Society
energy of anisotropic particles depends not only on particle size but also on their shape and surface roughness.25−27 Therefore, a simple method to control the morphology, including shape, roughness, and size of anisotropic particles, is still highly demanding, especially for their application as solid emulsifiers, where the particles need to be adsorbed at the interface. In this study, we report a facile way to control the overall morphology of the resultant composite particle in seeded polymerization. The emulsification performance of these complex anisotropic particles has been briefly evaluated.
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EXPERIMENTAL SECTION
Materials. Styrene (St), tert-butyl acrylate (TBA, purchased from Sigma-Aldrich) and divinyl benzene (DVB, obtained from J&K Chemistry) were treated by being passed through basic alumina columns to remove inhibitors before use. Recrystallized 2,2′azobisisobutyronitrile (AIBN, purchased form Sinopharm Chemistry) and potassium persulfate (KPS, purchased from Alfa-Aesar) were used as initiators. Methanol, ethanol, toluene, formic acid (Beijing Chemistry), tetraethoxysilane (TEOS), polyvinylpyrrolidone (PVP K30, Shantou, China), and sodium dodecyl sulfate (SDS, Aladdin, China), were used as received. Pure water was generated by an ELGA Purelab system, with a resistance greater than 18.2 MΩ. Synthesis of Cross-Linked Spherical Polystyrene Particles. Cross-linked spherical polystyrene particles (cPS) were prepared by Received: June 13, 2012 Revised: August 2, 2012 Published: August 6, 2012 12472
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mg.mL−1 was diluted into 2 mL by water. Then toluene was charged into water. After a gentle shake, a stable emulsion was obtained. Characterization. Transmission electron microscopy (TEM) images were obtained using a Hitachi 800 transmission electron microscope operated at an accelerator voltage of 100 kV. Scanning electron microscopy (SEM) images were collected on a JEOL JSM6700 scanning electron microscope operated at an accelerator voltage of 5 kV. Polarizing optical microscopy images were obtained with an Olympus optical microscope.
dispersion polymerization. In a typical dispersion polymerization of St, the mixture of monomer St, cross-linker DVB, initiator AIBN, stabilizer PVP, and solvent methanol was kept at 70 °C for 12 h with magnetic stirring under nitrogen atmosphere. cPS seed particles were isolated by centrifugation (8000 rpm, 5 min) after the polymerization. These particles were washed twice with ethanol followed by water before further use. Seeded Polymerization for Core−Shell Seed Particles (cPS@ cPTBA). In a typical experiment, 2.8 mL cPS dispersion (with a solid content of 0.18 g.mL−1) was added into a 50 mL flask equipped with a reflux condenser and diluted to 30 mL with pure water, followed by the addition of 0.6 mL SDS aqueous solution (10 mg.mL−1). After 5 min agitation (300 rpm), the mixture of 200 μL TBA and 20 μL (or 3.5 μL) DVB was injected. After deoxygenation by bubbling with nitrogen for 30 min, the system was heated to 70 °C. Then 300 μL KPS aqueous solution (10 mg.mL−1) was added to initiate polymerization. The reaction was allowed to proceed for 12 h with a magnetic stirring (350 rpm). The final particles were isolated by centrifugation (8000 rpm, 5 min) after the polymerization and washed twice with ethanol before further use. Hydrolysis of Core−Shell Seed Particles. The core−shell cPS@ cPTBA seed particles was partially hydrolyzed to obtain a hydrophilic surface, through a method modified from Furukawa.28 cPS@cPTBA particles (0.5 g) were dispersed in 30 mL of ethanol/water mixture (5:1, v:v), then 6 mL of formic acid (88 wt %) was charged. After a 12 h heat treatment, the seed particles were isolated by centrifugation (8000 rpm, 5 min) and washed thoroughly with water. The cPS@ cPTBA seed particles used in this study are summarized in Table 1.
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RESULTS AND DISCUSSIONS The Preparation of Core−Shell Seed Particles. The core−shell particles were prepared by seeded polymerization of TBA on cPS particles obtained through dispersion polymerization as described in the Experimental Section. The cPS particles were fairly monodisperse, with an average diameter of around 850 nm (Figure 1a). After seeded polymerization with TBA, spherical particles (cPS@cPTBA) with larger size were obtained (Figure 2b). The cPTBA shells were partially hydrolyzed with formic acid. Since the hydrolyzed shell is abundant with carboxyl groups, TEOS can be selectively hydrolyzed in the shell, to form a layer of colloidal silica, as shown in Figure 1c. However, the core−shell structure was not observed by TEM (Figure 1c, inset), probably due to the large overall size of the composite particles. After removal of the polymer phase by calcination, hollow particles were obtained (Figure 1d), verifying the core−shell structure formed in the above seeded polymerization. The Preparation of Anisotropic Composite Particles from Core−Shell Seed Particles. Composite anisotropic colloidal particles were obtained through seeded polymerization from the cPS@cPTBA particles (Seed 1, Table 1). Figure 2 shows SEM images of resultant anisotropic particles. It is clearly seen that part of the anisotropic particles has a rough surface, which is embraced by two arms from a particle with a smooth surface. To the best of our knowledge, it is the first time colloidal particles with such morphology have been obtained. Given the fact that the size of rough sphere shown in Figure 2 is similar to Seed 1, we suppose that the rough sphere shown in Figure 2a was a seed, and the rest is newly formed polymer phase. This hypothesis was confirmed by control experiments through adjusting the monomer/seed ratio (Figure 3). The size of the heteromorphous phase increases with monomer/seed ratio, while that of the sperical side of the anisotropic particle remains unchanged, suggesting that the heteromorphous part is the secondary polymer phase. The growth of these composite particles was also followed by dynamic light scattering (DLS), and the details can be found in the Supporting Information. The selective hydrolysis of TEOS on the obtained anisotropic particles has been used to further confirm the above argument. TEOS was found only to hydrolyze on the coarse spherical part in the anisotropic particles, where a clear layer of colloidal silica was observed (Figure 4). This strongly indicates that the coarse spherical particle is the seed, and the part with two arms is the newly formed phase. It also confirms that the two parts in anisotropic particles have different surface properties, i.e., they are indeed Janus particles. The morphology of the heteromorphous phase is tunable not only by monomer/seed ratio (Figure 3), but also by crosslinker concentration in the swelling monomer mixture. Figure 5 shows the images of anisotropic particles obtained with different cross-linker (DVB) concentrations. Obviously, the size of the newly formed phase aside from the seed particle increases with DVB concentration. The two arms embracing
Table 1. Information of the Core−Shell Seed Particles Used in This Work cross-linking degree Seed Seed Seed Seed
0 1 2 3
core (wt %)
shell (wt %)
hydrolysis in the shell
1 1 1 1
10 10 1.7 1.7
no yes no yes
Preparation of Anisotropic Composite Colloids from cPS@ cPTBA Seed Particles. Composite anisotropic colloids were prepared by a method modified from what is described by Weitz.16 In a typical experiment, 20 vol % of seed particles were dispersed in a 2.5% w/v PVA (99.8−100% hydrolyzed, 100 000 g·mol−1) aqueous solution. A 20 vol% monomer emulsion was also prepared in a 2.5% w/v PVA aqueous solution by homogenizing at 8000 rpm and mixed with the seed particles dispersion. The monomer emulsion consisted of TBA, DVB, and initiator AIBN (0.5 wt % of monomer). The reaction mixture was tumbled at a speed of 40 rpm for more than 12 h at room temperature to allow the seed particles to swell. To minimize secondary particle formation during polymerization, hydroquinone aqueous solution (0.05 wt % of TBA) was added in the continuous phase. Polymerization was performed by tumbling again at 150 rpm for 10 h at 70 °C in a water bath. The resultant particles were separated by centrifugation and thoroughly washed with ethanol, then redispersed in water. Preparation of Anisotropic Silica/PTBA Composite Colloids. Anisotropic colloids (0.1 g) with distinct surface roughness were dispersed in 30 mL of ethanol containing 0.5 mL of aqueous ammonia (28 wt %). After 5 min agitation (300 rpm), 50 μL TEOS was charged into the dispersion. A sol−gel process was carried out under stirring for 4 h at room temperature. After centrifugation and washing with ethanol, the anisotropic silica/PTBA composite colloids were obtained. Emulsification Using the Composite Anisotropic Colloids. Before the emulsification, the composite anisotropic colloids were washed carefully to remove the stabilizer and linear polymers. A 0.7 mL composite colloidal dispersion with a concentration of 24 12473
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Figure 1. (a) SEM image of cPS obtained by dispersion polymerization. (b) cPS@cPTBA core−shell particles obtained by seeded polymerization from cPS with the following recipe: 0.4 g of cPS seed, 200 μL of TBA, 20 μL of DVB, 0.6 mL of SDS, and 30 mL of H2O. (c) SEM and TEM images of cPS@cPTBA particles with silica shell after the hydrolysis of TEOS. (d) Hollow silica spheres from composite particles in panel c by calcination at 600 °C for 3 h in air.
Figure 2. SEM images, top view (a) and side view (b), of composite anisotropic particles obtained from Seed 1. The monomer/seed ratio is 6:1, and the DVB concentration is 3 vol% of TBA.
Figure 3. SEM images of anisotropic particles produced at different monomer/seed ratios from Seed 1. The monomer/seed ratio (w/w) of panels a and b are 2.5:1 and 4:1, respectively.
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Figure 4. SEM images before (a) and after (b) selective hydrolysis of TEOS on resultant anisotropic colloidal particles (the anisotropic colloidal particles used here correspond to Figure 3b).
Figure 5. Janus particles with different cross-linker concentrations obtained from Seed 1. The DVB concentrations in a, b, and c are 0, 2, and 3 vol% of TBA, respectively.
Figure 6. Janus colloidal particles obtained with different seeds. The seeds used in a, b, c, and d are Seed 0, Seed 1, Seed 2, and Seed 3, respectively. The monomer/seed ratio and DVB concentration for all four cases are 4:1 and 2 vol% of TBA, respectively.
Most commonly, simple dumbbell-like particles would be produced in the absence of shell or in the presence of hydrophilic shells,21,23 rather than the complex anisotropic particles obtained in this study. We therefore suppose that cross-linking degree and surface property in the shell would have an important effect. Figure 6 shows the SEM images of Janus particles obtained with different kinds of seed particles.
the seed particle grew longer and eventually merged together, forming a complete circle on the seed particle (Figure 5c). Interestingly, even at low cross-linker concentration, Janus particles still showed different surface roughness on different parts, which might be a consequence of internal stress during the polymerization-driving phase separation process. 12475
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polymerization (Figure 7b). A newly formed phase became more and more evident with polymerization going on (Figure 7c to 7e) until 4 h of polymerization (Figure 7f), where two arms became observable. After 5 h, further polymerization only resulted in an evident increase in the length of arms (Figure 7g,h). Emulsification Performance of the Janus Particles. As discussed earlier, these composite Janus particles are indeed amphiphilic, which therefore would be good solid surfactants to emulsify water/oil mixtures. Mixtures of two immiscible liquids, water and toluene, were tested as model systems. A stable emulsion (Figure 8b) was obtained with a gentle shake in the presence of the anisotropic particles shown in Figure 5b. Conductivity measurement showed that water was the continuous phase. Stable oil-in-water (O/W) emulsions can be achieved until a toluene concentration as high as 85 vol% (Figure 8c), above which, a separate oil phase was observed floating on emulsion (Figure 8d). Interestingly, even at such high oil-to-water ratio, still only O/W-type emulsions were formed, as suggested by conductivity measurements, whether adding water into oil or the other way around, suggesting that the Janus particles had a high hydrophilic−lipophilic balance (HLB) value in analogy to that of molecular surfactant. This seems sensible, since the hydrophilic part was coarse, thus favoring O/W-type emulsions. Unusual nonspherical emulsion droplets were observed in this study (Figure 8f,g), presumably due to the jamming of these Janus particles with complex morphology (arms) at the interface, making the relaxation of the droplets into a spherical shape difficult.15 Further studies on the emulsification performance of Janus particles with different arm length are still underway.
Arms can be seen in both cases (Figure 6a,b) where seeds with distinctly higher cross-linking degree in shell than core are used, with arms slightly longer when the shell is hydrophilic (Figure 6b), indicating that surface wetability is an important but not essential prerequisite for the formation of arms. On the contrary, when the cross-linking degree in the shell decreased, only simple dumbbell-like particles were obtained, whether the seed surface was hydrophilic or hydrophobic (Figure 6c,d). Therefore, it can be concluded that higher cross-linking degree in the shell is the essential factor for the formation of arms in the newly formed polymer phase. The Formation Mechanism of Arms in the Newly Formed Polymer Phase. Based on the above observations, a possible formation mechanism of arms in the newly formed polymer phase can be proposed (Scheme 1). At first, the Scheme 1. Illustration of the Formation Mechanism of Complex Anisotropic Colloids
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secondary polymer phase protrudes from the core−shell seed, forming anisotropic particles, as observed by many other researchers. The newly formed polymer phase increases as the polymerization goes on until a critical size, when further increase in the newly formed phase becomes difficult. The joint parts between the newly formed polymer phase and seed phase might be more favorable for further growth of new polymer phase, thus arms are formed. If there is enough monomer, the arms can grow longer and eventually merged as a circle embracing the seed particle, as observed in Figure 2. This above mechanism was confirmed by investigating the morphological evolution of formed particles at different polymerization intervals (Figure 7). At the beginning, the particles were spherical after the swelling process (Figure 7a). The particles became slightly ellipsoidal after a 15 min of
CONCLUSION In conclusion, we have successfully prepared composite Janus particles with unusual structures. In general, these particles have two parts, with one part embraced partially or entirely by another part, adjustable by parameters such as phase ratio or cross-linking degree. A possible mechanism for the formation of such unusual Janus particles has been proposed. These particles are amphiphilic, with a coarse hydrophilic part and a smooth hydrophobic part, and serve as good solid emulsifier in water− toluene mixtures, forming O/W emulsions at very high internal phase content with rather low concentration of particles. Nonspherical emulsion droplets were observed, indicating that these Janus particles are likely to jam at the interface, forming a strong protecting layer to stabilize emulsions.
Figure 7. SEM images of composite particles produced at different polymerization intervals obtained from Seed 1. The monomer/seed ratio is 4:1, and DVB concentration is 3 vol% of TBA. 12476
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Figure 8. Mixture of toluene, water, and anisotropic particles without shaking (a). Emulsion of water/toluene in the presence of anisotropic particles with different toluene concentrations: (b) 33 vol%, (c) 85 vol%, (d) 95 vol%. Optical microscope photos of emulsion: spherical (e) and nonspherical (f and g) droplets of water/toluene emulsion. The anisotropic particle concentration is 0.8 wt % of water for all cases.
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ASSOCIATED CONTENT
S Supporting Information *
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The DLS results for Janus particle structural evolution with monomer/seed ratio and cross-linker concentration are provided. This information is available free of charge via the Internet at http://pubs.acs.org/.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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REFERENCES
ACKNOWLEDGMENTS
This work was supported by the State Key Development Program of Basic Research of China (Project No. 2012CB933200), the National Natural Science Foundation of China (Project No. 91027032, 51173193), the National Key Technology R&D Program of China (Project No. 2011BAI02B05), and the Chinese Academy of Sciences (Grant No. KJCX2-YW-H19). 12477
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