Article pubs.acs.org/Langmuir
Micrometer-Sized Gold−Silica Janus Particles as Particulate Emulsifiers Syuji Fujii,*,† Yuichi Yokoyama,† Yuki Miyanari,† Takafumi Shiono,† Masanori Ito,‡ Shin-ichi Yusa,‡ and Yoshinobu Nakamura†,§ †
Department of Applied Chemistry, Faculty of Engineering, Osaka Institute of Technology, 5-16-1 Ohmiya, Asahi-ku Osaka 535-8585, Japan ‡ Graduate School of Engineering, University of Hyogo, 2167, Shosha, Himeji, Hyogo 671-2280, Japan § Nanomaterials Microdevices Research Center, Osaka Institute of Technology, 5-16-1 Ohmiya, Asahi-ku Osaka 535-8585, Japan S Supporting Information *
ABSTRACT: Micrometer-sized gold−silica Janus particles act as an effective stabilizer of emulsions by adsorption at the oil−water interface. The Janus particles were adsorbed at the oil−water interface as a monolayer and stabilized near-spherical and nonspherical oil droplets that remained stable without coalescence for longer than one year. Gold and silica surfaces have hydrophobic and hydrophilic features; these surfaces were exposed to oil and water phases, respectively. In contrast, bare silica particles cannot stabilize stable emulsion, and completed demulsification occurred within 2 h. Greater stability of the emulsion for the Janus particle system compared to the silica particle system was achieved by using the adsorption energy of the Janus particles at the oil−water interface; the adsorption energy of the Janus particles is more than 3 orders of magnitude greater than that of silica particles. Suspension polymerization of Janus particlestabilized vinyl monomer droplets in the absence of any molecular-level emulsifier in aqueous media led to nonspherical microspheres with Janus particles on their surface. Furthermore, polymer microspheres carrying Au femtoliter cups on their surfaces were successfully fabricated by removal of the silica component from the Janus-particle stabilized microspheres.
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within precisely defined multicomponent micelles,38 and a method involving the crushing of hollow spheres.24 Interest in Pickering emulsions stabilized with Janus particles is increasing. Binks and Fletcher39 theoretically predicted that the adsorption of an amphiphilic Janus particle adsorbed to an oil/water interface would be stronger than that of a particle with homogeneous surface wettability. Ikeda and co-workers reported that asymmetrically modified Janus silica (SiO2) particles could stabilize toluene-in-water emulsions by adsorbing at the oil−water interface.20 Weitz and co-workers demonstrated that amphiphilic snowman-shaped Janus particles worked as an efficient particulate emulsifier for oil-in-water emulsions.21 Okubo and co-workers23 succeeded in stabilizing Pickering-type oil-in-water emulsions using mushroom-shaped Janus particles as a particulate emulsifier. Chen et al. used submicrometer-sized Janus nanosheets as an emulsifer and stabilized both oil-in-water and water-in-oil emulsions.24 In contrast to particles exhibiting uniform surface wettability, the surface of the Janus particles are compartmentalized into two sections possessing different wettability. Therefore, the Janus particles combine both the Pickering effect and the amphiphilicity of a classical molecular-level surfactant. Con-
INTRODUCTION Emulsifying properties of fine particles have been recognized for more than a century.1,2 Particles attach to the oil−water interface and stabilize emulsions, producing Pickering emulsions. In general, particulate emulsifiers offer more robust and reproducible formulations and lower toxicity profiles compared to conventional molecular-level surfactants.3 Particulate emulsifiers are likely to become more widely used in the future because of the importance of emulsions for various industrial application such as food and cosmetics. Stable emulsions have been obtained using organic polymer particles,4−7 inorganic particles,8−12 nanocomposite particles,13−15 and bionanoparticles.16,17 Colloidal particles consisting of two surfaces possessing different chemical properties, called Janus particles, have attracted attention because of their anisotropic wettability and optical, electronic,and magnetic properties.18 By taking advantage of these properties, Janus particles have been used in diverse applications such as particulate emulsifiers,19−24 imaging nanoprobes,25,26 and self-motile colloidal materials.27 Several techniques have been developed to synthesize Janus colloidal particles,18 including a polymerization-induced phaseseparation method,28,29 formation of colloid clusters,30,31 stamping and sputtering of materials/chemical modification onto selected regions of colloids,32−34 controlled assembly of materials in microfluidics,35−37 cross-linking of components © XXXX American Chemical Society
Received: February 23, 2013 Revised: March 29, 2013
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dx.doi.org/10.1021/la400697a | Langmuir XXXX, XXX, XXX−XXX
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monolayer on the glass substrate. A thin Au layer (thickness, 100 nm) was deposited on the 2D colloidal crystal by vacuum deposition using a supermini vacuum coater SVC-700 (Sanyu Denshi Co., Ltd.) equipped with an SQM-160 rate/thickness monitor (Inficon). The Janus particles were released from the glass substrate surface by sonication in water using a Bransonic 221 instrument (Yamato). For details of the procedure, see Supporting Information. Characterization of Au-SiO2 Janus Particles. Optical Microscopy (OM). A drop of an aqueous dispersion of the Janus particles was placed on a microscope slide and observed using an optical microscope (Shimadzu Motic BA200; Shimadzu Corp., Kyoto, Japan) fitted with a digital system (Shimadzu Moticam 2000). Scanning Electron Microscopy. Scanning electron microscopy (SEM; Keyence VE-8800, 1−12 kV) studies were conducted on dried samples. The SiO2 and Au-SiO2 particles were observed without Au sputter coating. Number-average diameters of the SiO2 particles and Janus particles (n = 100) were determined from the SEM images. Contact Angle Measurement. Contact angles for water or oil droplets (10 μL) placed on the glass (Matsunami, 18 mm × 18 mm) and Au substrates were determined 3 s after setting the water or oil droplets using an Excimer SImage02 apparatus at 25 °C. Gold-coated glass (100 nm Au thickness) prepared by vacuum deposition was used as the Au substrate. The glass substrate was cleaned by soaking in 30 wt % aqueous NaOH for 60 min followed by copious rinsing with distilled water and drying at 25 °C for 5 h. The glass substrate was used as a model surface of SiO2. X-ray Photoelectron Spectroscopy (XPS). For XPS analyses, glass slides with and without an Au coating (100 nm thickness) were used. The samples were mounted onto sample stubs using conductive tape. The XPS measurements were obtained using an XPS spectrometer (Axis Ultra) with a monochromated Al Kα X-ray gun. Base pressure was
