Magnetic-Patchy Janus Colloid Surfactants for ... - ACS Publications

Subscriber access provided by READING UNIV

Article

Magnetic-Patchy Janus Colloid Surfactants for Reversible Recovery of Pickering Emulsions Hyeri Kim, Jangwoo Cho, Jaehong Cho, Bum Jun Park, and Jin Woong Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b15894 • Publication Date (Web): 15 Dec 2017 Downloaded from http://pubs.acs.org on December 21, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Materials & Interfaces is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 18 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

ACS Applied Materials & Interfaces

Magnetic-Patchy Janus Colloid Surfactants for Reversible Recovery of Pickering Emulsions Hyeri Kim†, Jangwoo Cho†, Jaehong Cho†, Bum Jun Park‡,*, Jin Woong Kim†,§,* †

Department of Bionano Technology, Hanyang University, Ansan 15588, Republic of

Korea; ‡ Department of Chemical Engineering, Kyung Hee University, Yongin 17104, Republic of Korea; § Department of Chemical and Molecular Engineering, Hanyang University, Ansan 15588, Republic of Korea

KEYWORDS: Janus microparticles, Site-specific patching, Magnetic colloid surfactants, Reversible recovery, Pickering emulsion

ABSTRACT We present a straightforward and robust method for the synthesis of Janus colloid surfactants with distinct amphiphilicity and magnetic responsiveness. To this end, hydroxylfunctionalized amphiphilic Janus microparticles are synthesized by seeded monomer swelling and subsequent photo-polymerization. By incorporating controlled amounts of hydroxyl groups on poly (styrene-co-vinyl alcohol) seed particles, we adjust the interfacial tension between the seed polymer and the poly (tetradecyl acrylate) secondary polymer (γ13). From theoretical and experimental observations, we verify that when γ13 is tuned to ~8.5 mN/m in a medium with controlled solvency, which corresponds to the 0.6 volume fraction of ethanol in water, the particles bicompartmentalize to form oval or ellipsoidal Janus microparticles with controllable bulb dimensions. We also show that bulb site-specific patching of magnetic nanoparticles

can

be

achieved

using

the

electrostatic

interaction

between

the

polyethylenimine-coated bulb surface and the polyvinylpyrrolidone-stabilized Fe2O3 nanoparticles. Finally, we demonstrate that our magnetic-patchy Janus microparticles can assemble at the oil-water interface, enabling magnetic-responsive reversible recovery of Pickering emulsions. 1

ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 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

Page 2 of 18

1. Introduction Janus particles, which possess two distinct anisotropies in compositions or physicochemical properties, have been used widely for the fabrication of complex soft materials, such as anisotropic building blocks for supra-nanostructures, optical and bioelectrical sensors, and switchable display devices.1-4 Currently, Janus particles, ranging from a few nanometers to hundreds of micrometers in size, are being synthesized using a variety of sophisticated synthesis technologies,5 including phase separation during polymerization,6-7 drop-based microfluidic technologies,8-10 topo-selective surface modification of particle monolayers,11 and co-jetting of polymer fluids in a high electric field.12 For giving chemical anisotropy to the particles, it is critical to drive phase separation in a confined particle phase, which is usually induced either by phase immiscibility or by the entropic effect of a gel phase.13 Taking advantage of this, Janus particles can have two distinct compartments, thus exhibiting bifunctionality. For instance, if one of the compartment surfaces is decorated with functional nanoobjects, a Janus particle system with selected functionalities, such as surface wettability and catalytic activity, can be developed.14 In this process, it is important to design a particle morphology with lower surface free energy for given interfaces. Various synthetic methods have been developed to fabricate Janus particles with asymmetric bifunctional surfaces. They include emulsion templating,15-16 particle lithography,17 and glancing-angle vapor deposition.18 The anisotropic patchy particles synthesized using these techniques may find new applications in photonics,19 sensors,20 switching displays,21 and drug delivery.22 Despite the promising applications of Janus particles, their synthetic procedures are either complicated or time-consuming, which eventually lowers productivity, thus diminishing price competitiveness.23 Moreover, in some cases, because the size distribution of synthesized Janus particles is between hundreds of micrometers to a few millimeters, their response to either thermal energy or applied external stimuli gets too slow. To solve these problems, utilization of batch production is inevitable. A uniform Janus morphology should also be imparted to the particle with little deviation in the particle size, so that they can assemble at the interfaces without any structural defects. These challenges are widespread both in academia and in industry, making it necessary to develop a Janus particle synthesis technology capable of position-selective surface functionalization, which can be scaled up for mass production. 2


Page 3 of 18 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


Herein, we introduce a straightforward and robust method for fabrication of magneticpatchy Janus microparticles (JMPs) as a colloid surfactant that enables reversible recovery of Pickering emulsions. The JMPs synthesized in our study are distinguished by their asymmetric dual functionality, amphiphilicity and magnetic responsiveness. In order to provide the JMPs with a specific adsorption surface, we synthesize amphiphilic JMPs with one bulb surface having hydroxyl groups. For this, we employ the seeded monomer swelling and subsequent polymerization technique.24 This synthetic approach has the advantage of being suitable for mass production, because it is based on a batch technology. In addition, we propose a facile approach that leads to tight tuning over the adsorption area of JMPs by controlling the monomer swelling ratio. Magnetic NPs are selectively adsorbed onto the designated surface, enabling the synthesis of magnetic-patchy JMPs.25 Finally, we demonstrate that magnetic-patchy JMPs can assemble to form structurally stable Pickering emulsions while displaying an external magnetic field-responsive recovery behavior.

2. Experimental Section 2.1. Materials. Styrene, polyvinylpyrrolidon (PVP, Mn = 40,000 gmol-1), ethanol (EtOH, anhydrous, ~99.5%), poly (vinyl alcohol) (PVA, Mw = 13,000-23,000 gmol-1, 87-89% hydrolyzed), ethylene glycol dimethacrylate (EGDMA, 98%), 1-hydroxycyclohexyl phenylketone (Irgacure 184, 99%), fluorescein isothiocyanate (FITC), Pluronic F-127 (Poloxamer 407) and polyethyleneimine (PEI, Mn = 60,000 gmol-1, 50 wt% in H2O) were purchased from Sigma Aldrich (USA). 2, 2’-Azobis (isobutyronitrile) (AIBN, 98%) and vinyl acetate (VAc, 99%) were purchased from Junsei (Japan). Tetrdecyl acrylate (TA) was purchased from TCI (Japan). All chemicals were reagent grades and used without further purification. Deionized doubled distilled water was used in all the experiments.

2.2. Synthesis of PS-co-PVAc seed particles. PS-co-PVAc seed particles were synthesized using dispersion polymerization.24 Styrene (4 ml), VAc (1 ml), PVP (1.0 g, a stabilizer), and AIBN (0.05 g, an initiator) were dissolved in EtOH (50 ml, 200 proofs) in a 100 ml round bottom flask. The reaction mixture was purged with nitrogen for 5 min to remove oxygen. Then, the polymerization was carried out at 70 °C in an oil bath while stirring at 60 rpm for 48 h. After the polymerization, the particles were washed repeatedly via centrifugation with 3



Page 4 of 18

EtOH and EtOH/water mixture (1/1, v/v), respectively, to remove residual monomers and additives. Lastly, the PS-co-PVAc particles were stored in the EtOH/water mixture (2/1, v/v). The concentration of the particles was set to 10 % by mass.

2.3. Synthesis of amphiphilic JMPs. The PS-co-PVAc seed particles were converted into polystyrene-co-polyvinyl alcohol (PS-co-PVA) using saponification, which was carried out under basic conditions for 8 h at room temperature. The PS-co-PVA seed particles were swollen with a mixture of TA, EGDMA, and Irgacure 184 (0.06 g) in the presence of PVA (2 wt%) and Pluronic F127 (2 wt%) in an EtOH/water solution for 5 h at room temperature. In this step, we adjusted the monomer swelling ratio against the PS-co-PVA seed particles by changing the total monomer volume of TA and EGDMA from 0.2 ml to 0.5 ml (TA/EGDMA = 8/2, v/v). The diameter of the PS-co-PVA particles increased during the swelling step, while maintaining monodispersity of particle size. The monomers in the swollen particles were photo-polymerized by UV irradiation for 5 min at room temperature, which induced phase separation between the PS-co-PVA seed phase and the secondary polymerized PTA phase. PS-co-PVA/PTA JMPs were then washed repeatedly with a mixture of EtOH/water (1/1, v/v) to remove remaining monomers and additives. To confirm amphiphilicity of JMPs, we selectively reacted the hydroxy groups on the PS-co-PVA seed particles and the bulb of JMPs, respectively, with FITC at room temperature for 12 h. Excess FITC was completely removed by repeated washing with water by centrifugation for more than 5 times.

2.4. Synthesis of PVP-stabilized Fe2O3 NPs. PVP-coated Fe2O3 NPs were synthesized according to a previously reported method.26 Briefly, 0.0322 g of PVP was dissolved in 40 ml of distilled water and the resulting solution was heated up to 80 °C under vigorous mechanical stirring. Then, 0.86 g of FeCl2·4H2O and 2.36 g of FeCl3·6H2O were dissolved into the mixture. The mixture was purged with nitrogen gas to completely remove oxygen. During this process, an orange-colored initial reaction solution gradually turned into a brownish-black colloidal dispersion. The mixture was reacted for 30 min at 80 °C. The colloidal dispersion thus produced was then cooled to room temperature and washed by the addition of excess EtOH/water. The Fe2O3 NPs were re-dispersed and stored in EtOH. The zeta-potential of Fe2O3 NPs in water was measured as -10 mV (ELS-Z, Otsuka, Japan). The 4


Page 5 of 18 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


sizes of the nanoparticles were determined from analysis of transmission electron microscope (TEM) images.

2.5. Production of Fe2O3 NPs-patchy JMPs. To induce effective patching of Fe2O3 NPs onto hydroxyl groups-functionalized bulb surface of JMPs, we treated it with PEI. First, 0.45g of JMPs was dispersed in the solution of EtOH/water (9 ml, 5/4, v/v). 0.045g of PEI was also prepared by dissolving it in 0.5 M NaCl solution (10 ml). Then, the PEI solution was added dropwise to the dispersion of JMPs at room temperature. After complete addition, the mixture was tumbled for 12 h with a rotation speed of 60 rpm at room temperature. The removal of remnant PEI was conducted by repeated centrifugation with a mixture of EtOH/water (1/1, v/v). Finally, PEI-coated JMPs were stored in EtOH/water (1/1, v/v) at room temperature. To give magnetic responsiveness to JMPs, we selectively patched Fe2O3 NPs onto the PEI-coated bulb surface. For this, a dispersion containing 1 wt% Fe2O3 NPs was added dropwise to the dispersion of PEI-coated JMPs over 10 min while vigorously stirring the mixture at room temperature. Then, the mixture was tumbled for 12 h with a rotation speed of 60 rpm. After removal of excess Fe2O3 NPs by repeated centrifugation with EtOH/water (1/1, v/v), the Fe2O3 NPs-patchy JMPs were stored at room temperature. 2.6. Fabrication and recovery of Pickering emulsion drops. The aqueous continuous phase was prepared by dispersing 0.5 g of Fe2O3 NPs-patchy JMPs in water. n-hexadecane was selected as an oil phase with the concentration to 20 vol% against the total emulsion mass. First, oil-in-water emulsion drops were produced via mild sonication (Power Sonic 510, Hwashin, Korea) for 5 min at room temperature. The Pickering emulsion drops were observed with a bright-field microscope (Axio Vert. A1, Carl Zeiss, Germany). The contact angle of JMPs at the oil-water interface was also observed with the bright-field microscope and determined by using ‘Image J’ software. The recovery behavior of Fe2O3 NPs-patchy JMPs-stabilized emulsions was observed by direct monitoring the drop recovery behaviors under external magnetic field via photography (EOS 700D, Cannon, Japan). In this process, we applied 5000 G of magnetic field at room temperature.

5



Page 6 of 18

2.7. Characterizations. The shape and morphology of the particles were observed with a bright-field microscope (Axio Vert. A1, Carl Zeiss, Germany). The Janus phase of the particles was examined with a fluorescent microscope (Axio Vert. A1, Carl Zeiss, Germany). In this case, the hydroxyl group on the PS-co-PVA seed phase was covalently labelled with a fluorescence probe, FITC. Surface topologies of bare JMPs and magnetic-patchy JMPs were observed with a scanning electron microscope (SEM, S-4800, Hitachi, Japan) and a highresolution transmission electron microscope (HR-TEM, JEN 2100F, JEOL, Japan). The particle size was determined from direct analysis of SEM and TEM images by counting and analyzing more than 100 particles. The PVP-stabilized magnetic nanoparticles were analyzed with an X-ray photoelectron spectrometer (XPS, Theta Probe, Thermo Fisher Scientific, USA). After patching of magnetic NPs, the degree of magnetization of Fe2O3-patchy Janus microparticles was measured using a vibrating sample magnetometer (VSM, Lakeshore, USA).

3. Results and Discussion For the fabrication of magnetic-patchy colloid surfactants with a monodisperse particle size distribution in the micrometer scale, we synthesized surface area-designated JMPs using the seeded monomer swelling and polymerization (Fig. 1). In a typical synthesis procedure, highly monodisperse PS-co-PVAc seed particles were produced by using dispersion polymerization. Seed particles with high size monodispersity could be obtained due to the nucleation and growth mechanism during dispersion polymerization in a polar solvent.27 The copolymerization of VAc with styrene increased the size of the resultant PS-co-PVAc particles from 2.5 µm to 3.5 µm, because the increased solubility of growing polymer chains in the polar medium generated less primary particles (Fig. S1).27 A decrease in the particle size could be achieved by lowering the medium solubility. When 2.5 vol% of water is added to the EtOH medium, the diameter of the PS-co-PVAc seed particles could be adjusted to ~2.5 µm and their coefficient of variation in particle size was less than 0.03 (Fig. 2A). The PVAc in the seed copolymer phase was then converted into PVA using saponification under a basic condition (pH 13) (Fig. 2B). We confirmed the presence of hydroxyl groups on the surface of PS-co-PVA seeds by labeling them with FITC (see the inset of Fig. 2B). 6


Page 7 of 18 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


Figure 1. Schematic illustration of the fabrication of amphiphilic JMPs consisting of PS-coPVA and PTA bulbs. Subsequently, the PS-co-PVA seed particles dispersed finely in a mixture of EtOH/water (3:2, v/v) with the aid of polymeric stabilizers were swollen with a mixture of TA, EGDMA, and Irgacure 184 (Fig. 2C). The diameter of PS-co-PVA particles increased during monomer swelling, while maintaining their monodispersity in particle size. It was very interesting to observe that phase separation already occurred upon incorporation of monomers into the dispersion of seed particles. The curved interface between the PS-co-PVA seed and monomer phases was generated with a curvature range of 0.2-0.8 (Fig. S2). After UV polymerization, amphiphilic JMPs were produced (Fig. 2D). Successful incorporation of hydroxyl groups on the PS-co-PVA bulb was confirmed by site-specific labeling of FITC thereon (see the inset of Fig. 2D). The diameter of the seed particles increased by 2 µm as compared with the length of major axis of JMPs obtained after seeded polymerization (Fig. 2E-G). Even though polymerization of the monomer phase reduced the interfacial tension difference between two polymer phases, a slightly curved interface could be still observed with a curvature of ~0.3. In addition, the resulting particles showed a slightly oval or ellipsoid particle morphology, depending on the monomer swelling ratio. These results imply that the particle morphology of JMPs is affected by the combination of the interfacial tension between the two polymer phases forming the JMP and the interfacial tension present in the continuous phase with which each polymer phase is in contact.

7



Page 8 of 18

Figure 2. Bright field microscope images of (A) PS-co-PVAc seed particles, (B) PS-co-PVA seeds after saponification (the inset is a fluorescence image of PS-co-PVA seeds labeled by FITC), (C) TA monomers-swollen PS-co-PVA seed particles, and (D) monodisperse amphiphilic JMPs formed after UV polymerization (the inset is a fluorescence image of JMPs half-labeled by FITC). SEM images of (E) PS-co-PVA seed particles and (F) amphiphilic JMPs. (G) Particle size distributions of (a) PS-co-PVA seed particles and (b) the JMPs (based on the diameter of major axis). To understand in detail the formation mechanism of JMPs, we tried to predict the particle morphology by calculating and minimizing the total surface energy of interfaces between the two polymer phases, i.e., PS-co-PVA (1) and PTA (3), and between each polymer and the continuous medium, i.e., the mixture of EtOH/water (2) (see the detailed calculation method in Supporting Information)28. However, due to the uncertainty of estimation of the surface tension between the polymers (γ13), we estimated the particle morphology with varying values of γ13 and the results were compared to the morphology of experimentally synthesized particles. The phase diagram in Fig. 3A indicates different morphologies in response to the initial volume ratio of EtOH (VR) in water prior to mixing and the value of γ13. In the lower bright and upper dark grey regions, the particles adopt a core-shell or occluded structure and 8


Page 9 of 18 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


an entirely phase-separated morphology, respectively. In the middle light grey region, particles form a partial engulfing morphology, such as Janus, dumbbell, or truncated sphere. In this diagram, each phase boundary linearly decreases as the VR increases. As schematically exemplified in Fig. 3B, the phase 3 tends to be encapsulated in the phase 1 due to the relatively larger value of γ23 than that of γ12. In contrast, as the γ13 value increases, the phase 3 tends to be more exposed to the phase 2 and, eventually, the phase separation between the two polymer phases occurs in the conditions of sufficiently large values of γ13. The particle morphology experimentally obtained at VR ≈ 0.6 (Fig. 2F) adopted the partial engulfing morphology with a clear boundary between the two polymer phases and resembled the one in Fig. 3B (e), in which γ13 ~8.5 mN/m was used in the calculation. Notably, the other particles have an occluded morphology at VR ≈ 0.2, a partially occluded engulfing morphology at VR ≈ 0.4, and a phase separated one at VR ≈ 0.8 (Fig. S4). These experimental observations are consistent with the results estimated at γ13 ~ 8.5 mN/m, as indicated by the diamond symbols in Fig. 3A. The correspondence between the theoretical predictions and the experimental results highlights the fact that the formation of JMPs is determined by the total free energy gained by combination of the interfacial tensions between the phases (see also supporting information).

9



Page 10 of 18

Figure 3. (A) Theoretical prediction of particle shape with varying values of ߛଵଷ under experimental fluid conditions at different volume ratio of ethanol in water (VR). The horizontal dashed line is a guideline at ߛଵଷ = 8.5 mN/m. The experimental results marked as diamond symbols are shown in Fig. S4. (B) Calculated geometries of particles in the six conditions indicated as circle characters from (a) to (f) in the phase diagram. The advantage of using our synthetic approach was that the degree of Janusity, which is defined as the relative dimension ratio of the compartmented bulb against the whole particle,24 can be precisely controlled. In our study, the degree of Janusity corresponds to the horizontal length of a PTA bulb against the whole horizontal length of a JMP (D/D0), as illustrated in the inset of Fig. 4A. The degree of Janusity could be regulated in the range of 0.3-0.46 by varying the monomer swelling ratio against the PS-co-PVA (Fig 4A). Below D/D0 ≈ 0.3, phase mixing occurred to form a homogeneous particle phase during polymerization because of favorable thermodynamic miscibility at such low solute concentrations. Above D/D0 ≈ 0.46, uniform swelling of monomers into PS-co-PVA particles was not observed, thus producing polydisperse particles after polymerization. Uniform JMPs could be obtained in the D/D0 range of 0.3-0.46. As D/D0 becomes larger than 0.3, oval particles were formed due to the low volume of PTA bulb. As D/D0 approaches 0.46, the oval particles changed to symmetrical elliptical particles, meaning the bulbs of the same volume were formed (see the insets of Fig. 4A). In the proper D/D0 range that formed uniform JMPs, the overall particle size, which was expressed by the length of long axis, increased in accordance with the increase in the monomer swelling ratio (Fig 4B). We observed that utilization of hydroxyl functionalized PS-co-PVA seeds made it easier to form JMPs. When the JMPs were synthesized using non-saponified PS-co-PVAc seeds, the spherical Janus particle morphology was generated at much higher monomer swelling ratio, thus making control of the degree of Janusity more difficult (Fig. S5). This is because the PS-co-PVAc seeds are less hydrophilic than the PS-co-PVA seeds, leading to the lower phase separation between PS-co-PVAc seeds and the PTA phase.

10


Page 11 of 18 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


Figure 4. (A) Changes in degree of Janusity by varying TA monomer swelling ratio against PS-co-PVA seed particles. The inset images are JMP morphologies at different D/D0. (B) Particle size variation of JMPs by regulation of TA monomer swelling ratio. After synthesis of amphiphilic JMPs composed of a PS-co-PVA bulb and a PTA bulb, site-specific patching of Fe2O3 NPs onto the PS-co-PVA bulb surface was conducted. To induce a strong interaction between the target bulb surface and Fe2O3 NPs, we treated the PSco-PVA bulb surface with PEI, thereby strengthening the binding force via electrostatic attraction (Fig. 5A).29 Without this pretreatment, no site-specific patching of Fe2O3 NPs on the JMPs was achieved. Instead, we observed either whole adsorption or non-adsorption of Fe2O3 NPs (Fig. S6). By selective adsorption of Fe2O3 NPs onto the PEI-coated PS-co-PVA bulb, distinct bifunctionality, amphiphilicity and magnetic responsiveness, could be endowed to the JMPs.30 In our study, it is essential to take advantage of the electrostatic attraction between PVP-stabilized Fe2O3 NPs and PEI-coated PS-co-PVA bulb surface. The electrostatic stabilization of Fe2O3 NPs by adsorption of PVP was confirmed by assignment of characteristic peaks for N 1S, N 1S_1, and N 1S_2N obtained from XPS spectra of PVPstabilized Fe2O3 NPs (Fig. 5B). Measurement of zeta-potential, detected by -10 mV, confirmed the presence of negative surface charges, thereby giving the Fe2O3 NPs-patchy bulb surface hydrophilic properties, and an overall amphiphilic nature. Also, the magneticpatchy JMPs synthesized in this study exhibited chemical anisotropy as well as a controlled degree of magnetization (Fig. 5C-E). 11



Page 12 of 18

Figure 5. (A) Schematic illustration for selective patching of Fe2O3 NPs on JMPs. (B) N 1S XPS spectra of PVP-stabilized Fe2O3 NPs: (a) N 1S, (b), N 1S_1, and (c) N 1S_2. The inset is TEM image of PVP-stabilized Fe2O3 NPs. SEM images of Fe2O3 NPs-patchy JMPs: (C) SEM images of Fe2O3 NPs-patchy JMPs. Fe2O3 NPs/JMPs = 1/4 (w/w) and (D) Fe2O3 NPs/JMPs = 1/2 (w/w). (E) Controlled degree of magnetization of Fe2O3 NPs-patchy JMPs: (a) neat JMPs, (b) Fe2O3 NPs/JMPs = 1/4 (w/w), and (c) Fe2O3 NPs/JMPs = 1/2 (w/w). To show the practical applicability of magnetic-patchy JMPs, we assembled them to produce Pickering emulsions. Their assembly at the water-oil interface was directly verified by using JMPs tagged with fluorescence probe (Fig. 6A, Fig. S7).31 Since the JMPs had the designated amphiphilicity, which is determined by the degree of Janusity, each bulb of JMPs was wet by the compatible liquid phase: the hydrophilic magnetic-patchy bulb by the water phase and the hydrophobic PTA bulb by the oil phase (Fig. 6B-C). The contact angle of magnetic-patchy JMPs at the oil-water interface decreased from 127° to 90° with an increasing degree of Janusity. Perfect wetting of the particles could be obtained when using 12


Page 13 of 18 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


the JMPs with D/D0 ~0.5, which shows a good agreement with the reference (Fig. 6D).32 Moreover, when the contact angle is close to 90°, we were able to obtain an improved stability of Pickering emulsions (Fig. 6E). Most importantly, the Pickering emulsion made with magnetic-patchy JMPs could be recovered with ease by simply applying a magnetic field. We also confirmed that the recovered emulsion drops could be redispersed in water without any aggregation and coalescence under mild mechanical stirring conditions, which demonstrates well the development of a recyclable Pickering system (Fig. 6F, Fig. S8).

Figure 6. (A) Fluorescence microscope image of JMPs-stabilized Pickering emulsion drops. For this observation, JMPs covalently labeled with FITC were used. (B) Microscope image of Pickering emulsion drops stabilized by Fe2O3 NPs-patchy JMPs. (C) Microscope image of Fe2O3 NPs-patchy JMPs assembled at the oil-water interface (D/D0 = 0.46). (D) Contact angles of Fe2O3 NPs-patchy JMPs at the oil-water interface. (E) Drop viability of Fe2O3 NPspatchy JMPs-stabilized Pickering emulsion drops at 50 °C: D/D0 = 0.3 () and D/D0 = 0.46 (). (F) Reversible recovery of Fe2O3 NPs-patchy JMPs-stabilized Pickering emulsions: (a) before and (b) after recovery and (c) redispersion of recovered Pickering emulsions. 13



Page 14 of 18

4. Conclusions In conclusion, we introduced a truly effective synthetic method for the production of magnetic-patchy JMPs, in which seeded monomer swelling and polymerization were employed in a batch reaction. Characterization showed that the JMPs consisted of a hydroxyl-functionalized PS-co-PVA bulb and a hydrophobic PTA bulb and the degree of each bulb dimension could be controlled by simply changing the monomer swelling ratio. Moreover, we successfully demonstrated that Fe2O3 NPs could be bulb-site specifically patched on the PS-co-PVA bulb surface, thus enabling the development of a Janus partice system having amphiphilicity as well as magentic responsiveness. Thanks to these unique particle properties, the magnetic-patchy JMPs could not only stabilize Pickering emulsion drops, but also imparted excellent magnetic responsiveness against the applied external magnetic field. These characteristics highlight that the magentic-patchy JMPs fabricated in our study can be used as smart colloid surfactants that are able to recover oils with high yields in response to the applied magnetic field, thus exploring a variety of applications in personal care, food, and oil recovery industries.

AUTHOR INFORMATION Supporting Information The Supporting Information is available free of charge on the ACS Publications website and contains more information and details about the theoretical prediction of particle morphology, experimental confirmation of particle morphologies, and demonstration of the reversible recovery of Pickering emulsion drops.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] * E-mail: [email protected]

Author Contributions H. Kim and J. Cho equally contributed to this work. Notes 14


Page 15 of 18 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


The authors declare no competing financial interests.

ACKNOWLEDGMENT This research was supported by National Research Foundation of Korea (NRF) grants funded by the Korea government (MSIP) (No. 2008-0061891 and 2016R1A2B2016148).

REFERENCES 1. Glotzer, S. C., Materials science. Some Assembly Required. Science 2004, 306 (5695), 419-420. 2. Yin, S. N.; Yang, S.; Wang, C. F.; Chen, S., Magnetic-Directed Assembly from Janus Building Blocks to Multiplex Molecular-Analogue Photonic Crystal Structures. J. Am. Chem. Soc. 2016, 138 (2), 566-573. 3. Lu, C.; Liu, X.; Li, Y.; Yu, F.; Tang, L.; Hu, Y.; Ying, Y., Multifunctional Janus hematitesilica nanoparticles: mimicking peroxidase-like activity and sensitive colorimetric detection of glucose. ACS Appl. Mater. Interfaces 2015, 7 (28), 15395-15402. 4. Walther, A.; Müller, A. H. E., Janus particles. Soft Matter 2008, 4 (4), 663-668. 5. Walther, A.; Muller, A. H., Janus particles: synthesis, self-assembly, physical properties, and applications. Chem. Rev. 2013, 113 (7), 5194-5261. 6. Yoon, K.; Lee, D.; Kim, J. W.; Kim, J.; Weitz, D. A., Asymmetric functionalization of colloidal dimer particles with gold nanoparticles. Chem. Commun. 2012, 48 (72), 9056-9058. 7. Kim, J.-W.; Suh, K.-D., Monodisperse polymer particles synthesized by seeded polymerization techniques. J. Ind. Eng. Chem. 2008, 14 (1), 1-9. 8. Nie, Z.; Li, W.; Seo, M.; Xu, S.; Kumacheva, E., Janus and ternary particles generated by microfluidic synthesis: design, synthesis, and self-assembly. J. Am. Chem. Soc. 2006, 128 (29), 9408-9412. 9. Shah, R. K.; Kim, J.-W.; Weitz, D. A., Janus Supraparticles by Induced Phase Separation of Nanoparticles in Droplets. Adv. Mater. 2009, 21 (19), 1949-1953. 10. Yuet, K. P.; Hwang, D. K.; Haghgooie, R.; Doyle, P. S., Multifunctional superparamagnetic Janus particles. Langmuir 2010, 26 (6), 4281-4287. 11. Bao, Z.; Chen, L.; Weldon, M.; Chandross, E.; Cherniavskaya, O.; Dai, Y.; Tok, J. B. H., Toward Controllable Self-assembly of Microstructures: Selective Functionalization and 15



Page 16 of 18

Fabrication of Patterned Spheres. Chem. Mater. 2002, 14 (1), 24-26. 12. Lahann, J., Recent progress in nano-biotechnology: compartmentalized micro- and nanoparticles via electrohydrodynamic co-jetting. Small 2011, 7 (9), 1149-1156. 13. Shum, H. C.; Santanach-Carreras, E.; Kim, J. W.; Ehrlicher, A.; Bibette, J.; Weitz, D. A., Dewetting-induced membrane formation by adhesion of amphiphile-laden interfaces. J. Am. Chem. Soc. 2011, 133 (12), 4420-4426. 14. Du, J.; O'Reilly, R. K., Anisotropic particles with patchy, multicompartment and Janus architectures: preparation and application. Chem. Soc. Rev. 2011, 40 (5), 2402-2416. 15. Jiang, S.; Granick, S., Controlling the geometry (Janus balance) of amphiphilic colloidal particles. Langmuir 2008, 24 (6), 2438-2445. 16. Pardhy, N. P.; Budhlall, B. M., Pickering emulsion as a template to synthesize Janus colloids with anisotropy in the surface potential. Langmuir 2010, 26 (16), 13130-13141. 17. Yin, Y.; Lu, Y.; Gates, B.; Xia, Y., Template-assisted self-assembly: a practical route to complex aggregates of monodispersed colloids with well-defined sizes, shapes, and structures. J. Am. Chem. Soc. 2001, 123 (36), 8718-8729. 18. Pawar, A. B.; Kretzschmar, I., Multifunctional patchy particles by glancing angle deposition. Langmuir 2009, 25 (16), 9057-9063. 19. McConnell, M. D.; Kraeutler, M. J.; Yang, S.; Composto, R. J., Patchy and multiregion janus particles with tunable optical properties. Nano Lett. 2010, 10 (2), 603-609. 20. Yang, S.; Xu, F.; Ostendorp, S.; Wilde, G.; Zhao, H.; Lei, Y., Template-Confined Dewetting Process to Surface Nanopatterns: Fabrication, Structural Tunability, and StructureRelated Properties. Adv. Funct. Mater. 2011, 21 (13), 2446-2455. 21. Nisisako, T.; Torii, T.; Takahashi, T.; Takizawa, Y., Synthesis of monodisperse bicolored janus particles with electrical anisotropy using a microfluidic co-flow system. Adv. Mater. 2006, 18 (9), 1152-1156. 22. Baraban, L.; Makarov, D.; Streubel, R.; Monch, I.; Grimm, D.; Sanchez, S.; Schmidt, O. G., Catalytic Janus motors on microfluidic chip: deterministic motion for targeted cargo delivery. ACS Nano 2012, 6 (4), 3383-3389. 23. Nie, L.; Liu, S. Y.; Shen, W. M.; Chen, D. Y.; Jiang, M., One-pot synthesis of amphiphilic polymeric Janus particles and their self-assembly into supermicelles with a narrow size distribution. Angew. Chem. Int. Ed. 2007, 46 (33), 6321-6324. 16


Page 17 of 18 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


24. Kim, J. W.; Cho, J.; Cho, J.; Park, B. J.; Kim, Y. J.; Choi, K. H.; Kim, J. W., Synthesis of Monodisperse Bi-Compartmentalized Amphiphilic Janus Microparticles for Tailored Assembly at the Oil-Water Interface. Angew. Chem. Int. Ed. 2016, 55 (14), 4509-4513. 25. Shmueli, R. B.; Anderson, D. G.; Green, J. J., Electrostatic surface modifications to improve gene delivery. Expert Opin. Drug Deliv. 2010, 7 (4), 535-550. 26. Lee, H. Y.; Lim, N. H.; Seo, J. A.; Yuk, S. H.; Kwak, B. K.; Khang, G.; Lee, H. B.; Cho, S. H., Preparation and magnetic resonance imaging effect of polyvinylpyrrolidone-coated iron oxide nanoparticles. J. Biomed. Mater. Res. Part B. Appl. Biomater. 2006, 79 (1), 142-150. 27. Kim, J. W.; Kim, B. S.; Suh, K. D., Monodisperse micron-sized cross-linked polystyrene particles. VI. Understanding of nucleated particle formation and particle growth. Colloid Polym. Sci. 2000, 278 (6), 591-594. 28. Wang, Y.; Guo, B. H.; Wan, X.; Xu, J.; Wang, X.; Zhang, Y. P., Janus-like polymer particles prepared via internal phase separation from emulsified polymer/oil droplets. Polymer 2009, 50 (14), 3361-3369. 29. Rao, P. S.; Smitha, B.; Sridhar, S.; Krishnaiah, A., Preparation and performance of poly(vinyl alcohol)/polyethyleneimine blend membranes for the dehydration of 1,4-dioxane by pervaporation: Comparison with glutaraldehyde cross-linked membranes. Sep. Purif. Technol. 2006, 48 (3), 244-254. 30. Sharonova, A.; Loza, K.; Surmeneva, M.; Surmenev, R.; Prymak, O.; Epple, M., Synthesis of positively and negatively charged silver nanoparticles and their deposition on the surface of titanium. IOP Conf. Ser.: Mater. Sci. Eng. 2016, 116, 012009. 31. Jiang, Z. T.; Zhang, Q. F.; Tian, H. L.; Li, R., The reaction of allyl isothiocyanate with hydroxyl/water and beta-cyclodextrin using ultraviolet spectrometry. Food Technol. Biotech. 2006, 44 (3), 423-427. 32. Ryu, D. Y.; Shin, K.; Drockenmuller, E.; Hawker, C. J.; Russell, T. P., A generalized approach to the modification of solid surfaces. Science 2005, 308 (5719), 236-239.

17



Page 18 of 18

Table of Contents Graphic

18


Magnetic-Patchy Janus Colloid Surfactants for ... - ACS Publications

Recommend Documents