Shape-Tunable Biphasic Janus Particles as pH-Responsive

We report a simple and robust strategy to prepare pH-responsive biphasic Janus particles composed of polystyrene/poly(2-vinylpyridine) (PS/P2VP) homop...
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Shape-Tunable Biphasic Janus Particles as pH-Responsive Switchable Surfactants Kang Hee Ku,† Young Jun Lee,† Gi-Ra Yi,‡ Se Gyu Jang,§ Bernhard V. K. J. Schmidt,∥ Kin Liao,⊥ Daniel Klinger,*,# Craig J. Hawker,*,% and Bumjoon J. Kim*,† †

Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 34141, Republic of Korea ‡ School of Chemical Engineering, Sungkyunkwan University, Suwon 16419, Republic of Korea § Applied Quantum Composites Research Center, Korea Institute of Science and Technology (KIST), Jeonbuk 55324, Republic of Korea ∥ Max Planck Institute of Colloids and Interfaces, 14424 Potsdam, Germany ⊥ Department of Mechanical Engineering, Khalifa University, Abu Dhabi, United Arab Emirates # Freie Universität Berlin, Königin-Luise Str. 2-4, Berlin 14195, Germany % Materials Research Laboratory, University of California, Santa Barbara, Santa Barbara, California 93106, United States S Supporting Information *

ABSTRACT: We report a simple and robust strategy to prepare pHresponsive biphasic Janus particles composed of polystyrene/poly(2vinylpyridine) (PS/P2VP) homopolymers that are able to control the reversible formation, breakage, and switching of Pickering emulsions depending on their geometry- and pH-dependent hydrophilic−lipophilic balance. The chemical stability of these PS/P2VP Janus particles was tuned through the incorporation of cross-linkable benzophenone units along the backbone of the homopolymers. By employing these stabilized particles as emulsifiers for toluene and water, a facile transformation of emulsion types (i.e., from toluene-in-water to water-in-toluene emulsions) was achieved by adjusting the pH of the aqueous phase. More importantly, this pH-dependent switching behavior and associated stability of the emulsions could be actively controlled by adjusting the relative size ratio of PS to P2VP. When the PS volume fraction (ϕPS) was between 0.33 and 0.67, a wide range tuning of emulsion phase including rapid and reversible pH-triggered emulsion inversion was achieved by the Janus surfactants. Finally, incorporation of iron oxide nanoparticles facilitated magnetic separation of oil droplets from O/W emulsions and recovery of the Janus particles, which represents a considerable advantage for these systems.

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INTRODUCTION Stimuli-responsive switchable Pickering emulsions have played a pivotal role in the recent development of emulsion-based technologies,1−3 such as oil extraction and recovery,4,5 emulsion polymerization,6,7 and heterogeneous catalysis.8,9 Conventionally, such dynamic properties are imparted into emulsions through molecular surfactants, which are able to change their packing parameter and/or their hydrophilic−lipophilic balance (HLB) in response to environmental conditions.10−12 This is mostly realized by (polymeric) organic surfactant molecules which typically undergo conformational changes as a function of temperature,13−15 pH,16−19 CO2,20,21 and light.22−24 While this molecular surfactant approach shows great potential for the development of emulsions with controllable dynamic properties, such as externally triggered inversion of emulsion type (i.e., oil-in-water (O/W) and water-in-oil (W/O)), achieving a similar level of control with particle-based surfactants remains © XXXX American Chemical Society

challenging. This can be mainly attributed to the limited tunability of the amphiphilic nature and packing parameter of isotropic spherical particles.3,25−28 In addressing this challenge, we were drawn to biphasic Janus particles which have shown great promise as Pickering emulsion stabilizers.3,18,29−32 In contrast to isotropic particles, the asymmetric surface wettability of biphasic Janus particles renders them amphiphilic, thus providing a remarkable reduction in interfacial tension and affording much stronger interfacial adsorption.33 Importantly, the shape and volumetric ratio between the two particle hemispheres dictate the curvature of the fluid−fluid interface. In analogy to the packing parameter for molecular surfactants, the shape of particle Received: November 6, 2017 Revised: November 11, 2017

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DOI: 10.1021/acs.macromol.7b02365 Macromolecules XXXX, XXX, XXX−XXX

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Scheme 1. Schematic Illustration of the Preparation of Amphiphilic PS/P2VP Janus Particles by Solvent Evaporation-Induced Phase Separation of P(S-r-4VBOBP) and P(2VP-r-4VBOBP) in Oil-in-Water Emulsions.

investigated. Upon varying the pH of the aqueous phase from pH 2 to 8, the Janus particles stabilized reversal forms of Pickering emulsions (W/O type above neutral pH, and O/W type below pH 5). At near pH 5, we observed coalescence of the emulsions into two phases. Such pH-responsive switching behavior of emulsions was realized only for biphasic Janus particles with PS volume fractions (ϕPS) between 0.33 and 0.67. Importantly, transition of emulsion type was achieved within a few seconds and fully reversible over continuous cycles of pH variations. The potential of these new particle stabilizers for emulsion-based technologies was demonstrated by loading magnetic nanoparticles (NPs) into the P2VP hemisphere of the Janus particles, allowing removal of oil droplets from water and recovery of the surfactant particles by centrifugation.

surfactants determines the overall stability and type of the Pickering emulsion.33−35 Therefore, controlling the Janus particles’ shape and amphiphilicity by external stimuli would allow creating responsive surfactants capable of changing the nature of the emulsion phase on demand. However, the development of such stimuli-responsive biphasic Janus particle surfactants is still in its infancy, and only few examples have demonstrated the ability to stabilize and break emulsions as response to external triggers.18,36 The stimuli-induced inversion of Janus particle-stabilized emulsions was recently reported by Lee et al. for the first time.18 In their elegant approach, amphiphilic Janus particles with reversible pH-dependent HLB were developed by seeded emulsion polymerization. This work represents a starting point to gain a deeper understanding of the interfacial behavior of stimuli-responsive Janus particle surfactants. Especially, further elucidating the effect of changing particle shape (e.g., volumetric ratio of the two particle hemispheres) on the emulsion type and stability is of high interest. In combination with the triggered change in amphiphilicity, such investigations will give structure−property relationships crucial for the development of biphasic Janus particles as new responsive stabilizers for emulsion-based technologies. To allow systematic investigations, a facile, onestep method is required to prepare a library of soft Janus particles with control over shape and amphiphilicity. Another important challenge in the development of such a procedure is ensuring structural stability of the Janus particles to enable their utilization as emulsion stabilizers. Because of the location of the particles at the fluid−fluid interface, the respective hemispheres of the particles have to be covalently cross-linked to prevent dissolution. On the basis of these considerations, we demonstrate a simple and versatile method for the synthesis of stimuliresponsive biphasic Janus particles, which are able to efficiently facilitate pH-triggered, reversible phase inversion. Janus particles were prepared via phase separation of polystyrene (PS) and poly(2-vinylpyridine) (P2VP) based polymers upon solvent evaporation from the emulsion droplets. This method allows the facile adjustment of the volume fractions of the two hemispheres by simply changing the mass ratio between PS and P2VP. Cross-linking was achieved by utilization of benzophenone-functionalized PS and P2VP copolymers for the particle assembly and subsequent irradiation with UV light. Following this procedure, a series of biphasic Janus particles were obtained with different volume ratios of PS to P2VP ranging from 4:1 to 1:4. The influence of the Janus particle shape on their ability to stabilize Pickering emulsions of toluene and water was

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RESULTS AND DISCUSSION The development of stimuli-responsive Janus particles as stabilizers for switchable Pickering emulsions is based on the following considerations. For the preparation of a library of biphasic Janus particles composed of PS and P2VP, we aimed to use a facile synthetic approach based on solvent evaporation from emulsified polymer solutions to induce phase separation (see Scheme 1). To stabilize the soft particles against dissolution and enable their use as emulsifiers, UV-curable benzophenone moieties are proposed to be incorporated by random copolymerization of styrene or 2-vinylpyridine with 4vinylbenzyloxybenzophenone (4VBOBP) to yield poly(styrener-4-vinylbenzyloxybenzophenone) (P(S-r-4VBOBP)) and poly(2-vinylpyridine-r-4-vinylbenzyloxybenzophenone) (P(2VP-r4VBOBP)). After cross-linking of the particles under UV light, the structurally stabilized particles were investigated with respect to their ability to change the type of emulsion from water and toluene mixtures. The emulsion properties are assumed to be adjustable in response to pH changes in solution, which should facilitate facile inversion of the emulsions. At neutral pH, only the PS side of the particles is assumed to be swollen by toluene, increasing the size of the PS domain and generating W/O emulsions. By contrast, at low pH (below 3), the P2VP side of the particles should also swell significantly due to protonation of P2VP, which increases the size of the P2VP domain and produces O/W emulsions. Given that the biphasic Janus particles undergo such pH-induced amphiphilicity and morphological changes, we expect phase inversion of emulsions from W/O to O/W types. The preparation of biphasic Janus particles follows a versatile and facile strategy where solutions of polymer blends are confined in oil-in-water emulsion droplets and phase separation B

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Figure 1. (a) SEM image, (b) TEM image, and (c) size distribution of PS/P2VP biphasic Janus particles (PS:P2VP = 1:1) produced by emulsion encapsulation and evaporation. (d) TEM and (e) SEM images of biphasic Janus particles dried from toluene dispersion. (f) TEM and (g) SEM images of biphasic Janus particles dried from aqueous solution at pH 2. P2VP chains were stained by iodine vapor.

is induced by solvent evaporation.37−42 As a result, the particle shape and polymer phase separation can be thermodynamically controlled by tuning (i) the interfacial energy between polymer solution and the surrounding aqueous phase and (ii) the interfacial energy between the polymers during phase separation within the droplets.42−45 The selection of surfactant molecules and solvents is therefore critical for controlling overall shape and internal morphologies (Figures S1 and S2). Figures 1a and 1b show scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images of the PS/P2VP biphasic Janus particles (containing 5 mol % 4VBOBP) obtained by emulsification of a chloroform solution of the two functionalized homopolymers (500 μL, 1 wt %) into an aqueous surfactant solution (5 mL, 0.1 wt % cetyltrimethylammonium bromide (CTAB)). During evaporation of the chloroform from the emulsion at room temperature, these polymers separate into two phases within the particle, resulting in a dumbbell-shaped biphasic Janus particle where the major axis was in a range of 210−1200 nm in length with an average diameter of 585 nm (Figure 1c). The balanced interfacial energy between the chloroform solution of PS/P2VP and the aqueous CTAB solution resulted in negative spreading parameters for PS, P2VP, and CTAB with the contact angles of θPS = 92° and θP2VP = 118°, which generated dumbbellstructured Janus particles.46,47 The ability of the Janus particles to stabilize Pickering emulsions crucially depends on the ability of the two hemispheres to swell in the respective phases at the oil− water interface. Therefore, structural stability of the particles has to be ensured and dissolution was prevented via lightinduced cross-linking. To this end, a series of biphasic Janus particles were prepared using random copolymers of P(S-r4VBOBP) and P(2VP-r-4VBOBP) containing different molar fractions of cross-linkable 4VBOBP units (0−15 mol %; see Table S1 for the polymer information). The chemical structures, molecular weights, and detailed synthetic procedures for P(S-r-4VBOBP) and P(2VP-r-4VBOBP) are provided in the Supporting Information. Investigations on the emulsionstabilizing properties of the Janus particles revealed that the

incorporation of 5 mol % 4VBOBP results in an optimum cross-linking density that allows sufficient swelling without losing structural integrity (see Figure S3). Therefore, in subsequent experiments, random copolymers with 5 mol % 4VBOBP units were selected for particle fabrications. To investigate the ability of biphasic Janus particles to stabilize toluene/water emulsions, it is crucial to understand how particles swell at the toluene−water interface. Therefore, particles were dispersed in toluene or water, cast onto TEM grids and silicon wafers and dried quickly, and imaged by TEM and SEM. Particles dispersed in toluene contained a significantly expanded PS region, such that the swollen PS was spread out on the substrate, while the hydrophilic P2VP portion maintained its shape due to the low solubility of P2VP in toluene (Figures 1d,e). Dumbbell-shaped particles were observed in the TEM images of the particles cast from an aqueous solution (pH ∼ 7), as shown in Figure 1b. The SEM and TEM images of particles cast from acidic solutions (pH 2), in which the P2VP is protonated, revealed significant swelling of the P2VP region relative to the particles at neutral pH (Figures 1f,g).48,49 These results demonstrate the ability to control the degree of swelling for the P2VP domain of biphasic Janus particles at the toluene−water interface by simply adjusting the pH of the aqueous solution. In addition to the good swelling properties of PS in toluene, this should enable to tune the amphiphilicity of the Janus particles at the oil−water interface of Pickering emulsions. Having demonstrated structural integrity upon selective and pH-dependent swelling of the different hemispheres, the biphasic Janus particles were then employed for the preparation of highly stable Pickering emulsions of toluene/water (1:1 vol/ vol). A vortex mixer was used to emulsify 1 mL of toluene and 1 mL of aqueous suspension containing 1 wt % biphasic Janus particles and the oil-soluble dye Nile Red (added to facilitate optical characterization of the emulsions). Since our biphasic Janus particles are composed of two sides with significantly different preferences for toluene and water, they were expected to assemble at the toluene−water interface, oriented perpendicular to the interface.30,50 As shown in Figure 2a, the C

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Figure 2. Photograph, fluorescence microscopy image, and schematic illustration of Pickering emulsions formed from toluene and water (1:1 vol/ vol) at (a) pH 7 and (b) pH 2, showing water-in-toluene and toluene-in-water emulsions, respectively. The oil phase contains 0.01 wt % Nile Red for visual contrast.

Figure 3. TEM images and size distributions of biphasic Janus particles with different PS to P2VP volume ratios: (a) 4:1, (b) 2:1, (c) 1:1, (d) 1:2, and (e) 1:4. Scale bars in the TEM images represent 1 μm. (f) Geometry of PS/P2VP Janus particles is described to calculate the aspect ratio and packing parameter (Ppacking) of the Janus particles.

the vial, and residual aqueous solution was observed at the bottom of the vial, indicating the formation of an O/W emulsion (Figure 2b). Red droplets of toluene in water were clearly observed at pH 2 in the fluorescent microscopy image. The pH-dependent switch between emulsion types can be attributed to the selective swelling of the P2VP domain of the biphasic Janus particles in acidic conditions.32,50 Notably, the average size of the O/W emulsion droplets (17.6 μm) was significantly smaller than that of the W/O emulsions (165 μm), which suggests that the curvature of the oil−water interface of the emulsions stabilized by biphasic Janus particles at pH 2 is much higher than those formed at neutral pH. This feature can be attributed to the pronounced swelling of the protonated hydrophilic P2VP domain by the aqueous phase relative to that

partially cross-linked biphasic Janus particles produced waterin-toluene emulsions at neutral pH. Since the density of toluene is lower than that of water, the water droplets settled on the bottom of the vial and excess toluene remained on the top of the emulsions. To confirm the emulsion type (i.e., W/O vs O/ W), the emulsions were further characterized by fluorescence microscopy. Dark emulsion droplets in the fluorescence microscopy image in Figure 2a indicate the encapsulation of water in toluene (W/O emulsions). Emulsion droplets ranged from 21 to 273 μm in diameter, with an average diameter of 165 μm. Interestingly, when the pH was reduced to 2 by addition of hydrochloric acid (HCl) to the aqueous phase, the phase of Pickering emulsions inverted into a toluene-in-water emulsion. The emulsion formed at pH 2 floated to the top of D

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Figure 4. Fluorescence optical microscopy images of Pickering emulsions produced by shape-controlled biphasic Janus particles at various pH conditions. Janus particles with different volume ratios of PS to P2VP spheres [(a, PS:P2VP = 4:1, ϕPS = 0.8), (b, 2:1, ϕPS = 0.67), (c, 1:1, ϕPS = 0.5), (d, 1:2, ϕPS = 0.33), and (e, 1:4, ϕPS = 0.2)] were used to emulsify the mixture of toluene and aqueous solution under various pH values ranging from pH 2 to pH 8. The volume ratio of oil and water phases is kept at 1:1, and the oil phase contains 0.01 wt % Nile Red. Scale bars in the TEM images represent 500 nm.

biphasic Janus particle is defined as AR = (RPS + RP2VP + d)/ (RPS + RP2VP), where RPS and RP2VP are the radius of the spherical hydrophobic PS and hydrophilic P2VP portions of the particle, respectively, and d is the distance between the center of the two separated spheres.34 The packing parameter, which is typically used to characterize the shape of molecular surfactants and the size ratio of hydrophilic and hydrophobic segments within a molecular surfactant, can be defined as V/ πRP2VP2l.55 The wettability of the two sides of the biphasic Janus particles is defined by the three-phase contact angles of the hydrophilic P2VP and hydrophobic PS spheres (θPS and θP2VP). The PS/P2VP interface appears to be almost planar in the TEM images due to staining, while the actual PS/P2VP interface is a spherical surface. Therefore, we obtained accurate values of the contact angles (θPS and θP2VP) at the interface of Janus particles, which were predicted by computer simulation, as shown in Figure S5.34,46 It should be noted that while broad size distributions were observed for the Janus particles, the values of AR, Ppacking, and contact angles were identical regardless of the size of the particles; these values were found to depend only on the PS/P2VP ratio. The symmetric biphasic Janus particles with a ϕPS of 0.5 had the highest AR (1.26). As the asymmetry of Janus particle increased (ϕPS = 0.2 or 0.8), the AR value decreased to 1.22. In addition, as the ϕPS increased from 0.2 to 0.8, Ppacking increased from 0.77 to 1.13 with the increase of θPS from 73° to 97° and the decrease of θP2VP from 130° to 111°. Further geometric information for the shape-controlled Janus particles is provided in Table S2. The effects of AR, Ppacking, θPS, and θP2VP on determining the phase and stability of Pickering emulsions will be discussed in a subsequent section.

of the PS domain in toluene, generating more stable and smaller emulsion droplets.18,51 To demonstrate the importance of the balanced amphiphilicity of the particle surfactants for producing the emulsions, PS/P2VP Janus particles were used to emulsify different types of oil−water systems. The stable, pHresponsive emulsions can be produced with the solvents as oil phase that enable the selective swelling of PS (i.e., benzene, xylene, and chlorobenzene) as described in Figure S4. The power of this synthetic approach to pH-responsive Janus particles is the ability to precisely tune the shape and chemical composition, leading to control over the emulsion-stabilizing properties of the particles. For example, the relative domain size of the biphasic Janus particles can be simply controlled by tuning the volume ratio of P(S-r-4VBOBP) and P(2VP-r4VBOBP) in the solutions used for particle formation, which in turn determines the particle swelling and wettability at the water/toluene interface as well as the pH-responsiveness of the emulsions. To investigate the influence of Janus particle size, shape, and geometry on the emulsion properties, five different batches of biphasic Janus particles were prepared by adjusting the PS/P2VP ratio from 4:1 to 1:4 to tune the volume ratio of the respective domains (Figure 3). Notably, we observed the formation of biphasic Janus particles regardless of the PS/P2VP ratio. This is attributed to the negative spreading parameters for PS, P2VP, and CTAB from the balanced interfacial energy between the chloroform solution of PS/P2VP and the aqueous CTAB solution.46,47,52−54 To correlate the geometric characteristics of Janus particles to their interfacial behavior, the values of aspect ratio (AR) and packing parameter (Ppacking) were evaluated from the TEM images for each sample (100+ particles) as described in Figure 3f. The AR of the asymmetric E

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Figure 5. Reversibility of the pH-responsive surfactant behavior of Janus particles. (a) Fluorescence optical microscopy images showing the dynamic transition of Pickering emulsion phase by adjusting the pH of the aqueous phase. (b) Characterization of the size and type of Pickering emulsions formed during continuous cycling of the pH variation in the aqueous phase.

Figure 4 shows the fluorescence optical microscopy images of Pickering emulsions stabilized by a library of shape-controlled biphasic Janus particles at different pH conditions. The mean diameter (D) of the Pickering emulsion droplets was measured by optical microscopy, and the results are summarized in Table S3. The five different biphasic Janus particles prepared with different PS/P2VP ratios of 4:1 (ϕPS = 0.8, Figure 4a), 2:1 (ϕPS = 0.67, Figure 4b), 1:1 (ϕPS = 0.5, Figure 4c), 1:2 (ϕPS = 0.33, Figure 4d), and 1:4 (ϕPS = 0.2, Figure 4e) were used to investigate the effect of the particle shape on the pH-dependent phase behavior and stability of the toluene/water emulsions. For ϕPS = 0.8, i.e., a PS volume 4 times larger than the P2VP volume, stable W/O emulsions were produced due to the larger volume of the swollen hydrophobic PS chains in the toluene phase (Figure 4a, pH 6). Interestingly, in contrast to the case of symmetric particles (ϕPS = 0.5) (Figure 2b) where O/W emulsions were formed at pH 2, the W/O emulsions achieved by PS-dominant particles (ϕPS = 0.8) did not show any inversion of emulsion type over the entire pH range (2−8) (Figure 4a, pH 2). In this case, the swelling of P2VP chains under the acidic condition was insufficient to overcome the large volume occupied by the toluene-swollen PS domains. However, the stability of the W/O Pickering emulsions at pH 2 and pH 4 (D0.8,pH 2 = 237 μm, D0.8,pH 4 = 218 μm) was remarkably lower compared to those above pH 5 (D0.8,pH 6 = 110 μm, D0.8,pH 8 = 90 μm). In the opposite case, when the volume of the P2VP hemisphere was 4 times larger than that of PS (ϕPS = 0.2), O/W emulsions were formed. This emulsion type was stable from pH 2 up to pH 5 (D0.2,pH 2 = 12 μm, D0.2,pH 4 = 12 μm, D0.2,pH 5 = 62 μm) (Figure 4e, pH 2 to Figure 4e, pH 5). At pH 6 and 8, the emulsion became unstable and coalesced, as shown in the inset digital photograph in Figure 4e, pH 8. For this particle geometry, swelling of the PS domain by toluene was not sufficient to stabilize W/O emulsions, even for deprotonated P2VP domains. Significantly, only the biphasic Janus particles with ϕPS ranging from 0.67 to 0.33 enabled the dynamic inversion of the emulsion phases between pH 2 and 8. In this pH range, different types of Pickering emulsions (W/O and O/W types)

and instability points were observed. In all cases, O/W emulsions were observed for pH 2 and 4, whereas for pH 6 and 8, the phase inversion to W/O emulsions was observed. Near pH 5, the coalescence occurred, and neither W/O nor O/ W emulsions were stable. At a given pH condition, the stability of the emulsion droplets varied depending on the relative composition of the particles. For the W/O emulsions at pH 6, the average diameter of the water droplets decreased from 230 to 130 μm as ϕPS increased from 0.33 to 0.67. This indicated that W/O emulsion droplets become more stable with increasing PS content of the particles. At pH 2, O/W emulsions were produced in which the average diameter decreased from 110 to 15 μm as ϕPS decreased from 0.67 to 0.33, indicating the enhanced stability of O/W emulsion droplets. To gain deeper insight into the pH-dependent phase inversion of Pickering emulsions, we considered the configuration of the biphasic Janus particles at the oil−water interface. As described in the previous section, the equilibrium configuration of the Janus particles at the oil−water interface depends on the AR, contact angle (θP2VP, θPS), and, in particular, Ppacking.20,39 For example, the type of emulsion is strongly correlated with the Ppacking value of Janus particle surfactants (i.e., O/W emulsions for Ppacking < 1.0, and the W/O emulsions for Ppacking > 1.0). Asymmetric particles with ϕPS = 0.8 have Ppacking larger than 1.0 (Ppacking = 1.13). Therefore, the particles favor localization at the oil−water interface with high curvatures toward the oil phase, producing the W/O emulsion droplets. The swelling of PS domains by toluene will increase the effective volume of PS parts, leading to the increase of Ppacking up to ∼1.4. Therefore, no emulsion inversion can be observed even when the aqueous phase was changed to an acidic condition where the swelling of the P2VP region occurs. In other words, the increased size of the P2VP region is not sufficient to overcome the inherently large asymmetry, and the Ppacking of the particles is still larger than 1.0, producing stable W/O emulsions. In a similar fashion, for the particles with ϕPS = 0.2, the small value of Ppacking (Ppacking = 0.77) promotes the formation of O/W emulsion droplets over a wide range of pH F

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An additional benefit of this system is that various inorganic and organic reagents can be easily loaded into the biphasic Janus particles to produce composite particle surfactants with tailored functionality. This allows creation of functional emulsions or emulsion-based chemical nanoreactors.4,58−61 The strong affinity of P2VP for metal ion precursors can be used for loading the domains with inorganic nanoparticles.62−65 Moreover, small dye molecules or NPs functionalized with hydroxyl or carboxylic acid groups can also be introduced into the P2VP domains through hydrogen-bonding interactions.66−71 As a proof of concept, we incorporated magnetic NPs into the PS/P2VP biphasic Janus particles, which enabled simple separation of liquid droplets from the fluid mixture using magnetic fields. By modifying the surface of iron oxide (Fe3O4) NPs with a mixture of dodecanethiol and 6-mercapto-1hexanol, the Fe3O4 NPs could be successfully loaded within the P2VP domain of biphasic Janus particles, as shown in the TEM image (Figure 6a). These Fe3O4-containing particles were

values. Therefore, the expansion of the PS side by toluene at the toluene−water inteface is not enough to overcome the large difference between RP2VP and RPS, so Ppacking is still below 1.0. In contrast, the inversion between W/O to O/W emulsions was occurred by pH change for the Janus particles having a low asymmetry with ϕPS between 0.33 and 0.67, where 0.90 < Ppacking < 1.08. In this range, the increase of Ppacking of Janus particles by expansion of PS parts in toluene is expected to be ∼0.15, allowing Ppacking to be larger than 1.0 in a neutral solution. By contrast, a decrease in pH caused the substantial swelling of the P2VP parts by acidic water; thus, the Ppacking of the same particles decreased below 1.0. Therefore, the pHtriggered crossover of Ppacking value of Janus particle surfactants from above 1.0 to below 1.0 can explain the inversion of emulsion type from W/O to O/W induced by pH change. The stabilities of Pickering emulsion (i.e., droplet size) with different shapes of Janus particles can be also understood by estimating the curvature of the emulsion droplets from the Ppacking value (Figure 4). For example, when the ϕPS of particles increased from 0.33 to 0.8, the corresponding Ppacking increased from 1.05 to 1.4 in a neutral solution. The change in the curvature of W/O emulsions according to increase of Ppacking is calculated to be approximately 2.5 times.56 This estimation is consistent with the reduction in the droplet size of W/O emulsion from 242 μm (ϕPS = 0.33) to 90 μm (ϕPS = 0.8) at pH 8. In a similar way, the curvature of the O/W emulsions can be estimated from the Ppacking of particles in acidic condition. As the ϕPS of particles decreased from 0.67 to 0.2, the corresponding Ppacking is reduced from ∼0.95 to ∼0.5.57 The curvature of the O/W emulsion was calculated to 10-fold increase with this change,56 which is in good agreement with the decrease in the size of O/W emulsions from 110 μm (ϕPS = 0.67) to 12 μm (ϕPS = 0.2) at pH 2 condition. This result reveals that tailoring the shape and size of Janus particles is crucial to achieve appropriate amphiphilic balance to afford pHresponsive, reversible Pickering emulsions. High reversibility and stability of emulsions are prerequisites for the successful implementation of emulsion-based technologies. Therefore, we monitored the phase behavior and transitions of Pickering emulsions over multiple pH adjustment cycles. The pH of the aqueous phase of the emulsions was tuned by adding concentrated acidic and basic solutions (i.e., HCl and sodium hydroxide (NaOH)), followed by vigorous mixing with a vortexer to stabilize the emulsions at different pH conditions. Dramatic phase transitions of the emulsions were observed with excellent reversibility over multiple cycles (Figure 5). First, dark water emulsion droplets (W/O emulsion) were inverted to the reversed type, in which bright toluene emulsion droplets formed (O/W emulsion) by decreasing the pH of the aqueous phase to 2. Next, as shown in the third fluorescence optical microscopy images in Figure 5a, the emulsions reverted to W/O type after pH increase to 9. Ultrafast transition of emulsion type within a few seconds was reproducible over continuous cycles of pH tuning (Figure 5b). To evaluate the stability of the emulsion droplets during cycles, the size of Pickering emulsion droplets was measured by optical microscopy. The mean diameters (D) of W/O and O/W emulsion droplets were approximately the same, 160 and 18 μm, respectively, within a 10% error range, indicating excellent reproducibility over several cycles. Furthermore, the high stability of biphasic Janus particles was confirmed by SEM images (Figure S6), showing minimal change in size or shape over multiple pH cycles.

Figure 6. (a) Schematic illustration of magnetic manipulation and recovery of Pickering emulsions stabilized by Fe3O4-containing biphasic Janus particles. Digital image and grayscale reconstruction of the Pickering emulsions on a slide glass (b) before and (c) after magnetic separation. (d) Recovery of the magnetic Janus particles by centrifugation.

then utilized to stabilize O/W emulsions. The oil droplets were efficiently separated from the fluid mixture by simply placing a magnet under the emulsion as described schematically in Figure 6a. Digital images in Figures 6b,c were analyzed further by grayscale reconstruction to determine the recovery efficiency (see real-time movie in Supporting Information). From the image analysis, more than 95% of the toluene droplets were collected upon application of magnetic force. Additionally, the near-quantitative recovery of Janus particle surfactants was demonstrated. For this, the particle surfactants were collected by centrifugation after breaking the emulsion via adjusting the aqueous solution pH (Figure 6d). The facile recovery of the surfactants from the fluid mixture allows reusing the biphasic Janus particle surfactants for oil−water separation applications and thus represents a great environmental friendly and costeffective advantage over molecular-based surfactants.

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CONCLUSIONS Amphiphilic biphasic Janus particle surfactants composed of hydrophobic PS and hydrophilic P2VP with controlled shape and swelling behavior were simply prepared by evaporation of G

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Generation of Pickering Emulsions. Toluene containing 0.01 wt % Nile Red (1 mL) and aqueous suspension containing 1 wt % biphasic Janus particles (1 mL) were added to a glass vial and vortexed for 1 min. All emulsions were characterized 1 day later. To adjust the pH condition of the aqueous phase, 1.0 M NaOH and 1.0 M HCl solutions were used. After adding the HCl or NaOH solutions into mixtures of toluene and aqueous suspensions of biphasic Janus particles, the mixtures were vortexed for an additional 1 min. O/W or W/O Pickering emulsions (20 μL) were placed between two glass slides for characterization by fluorescence microscopy. Fabrication of Fe3O4 Nanoparticles-Containing Biphasic Janus Particles. Fe3O4 NPs were synthesized by a method published by Yun et al. with minor modifications.73 See Supporting Information for the detailed synthetic procedure. To produce Fe3O4-containing biphasic Janus particles, 2 mg of Fe3O4 NPs was mixed with 0.5 mL of a 1 wt % polymer solution (P(S-r-4VBOBP) and P(2VP-r-4VBOBP) in chloroform) and stirred for 12 h. The mixture of polymer and NPs in chloroform was emulsified with 0.1 wt % of CTAB solution (5 mL) by homogenization for 1 min, and the chloroform was evaporated by stirring the emulsions for 48 h in open vials at room temperature. After cross-linking of the particles under the UV light (254 nm) for 2 h, the particles were washed with DI water by repeated centrifugation/ redispersion cycles. Characterizations. NMR spectra were acquired on a Bruker AV 300 spectrometer with CDCl3 and tetramethylsilane as the internal reference. SEM images were acquired on a Hitachi S-4800 microscope. TEM was performed on either a Tecnai FEI T20 or a JEOL 2000 FX microscope. The samples were prepared by drop-casting particle suspensions onto silicon wafers for SEM and onto grids coated with a 20 nm thick carbon film for TEM followed by exposure to iodine vapor to selectively stain the P2VP domains of the particles. To image emulsion droplets using fluorescence microscopy, a drop of the emulsion was placed between two glass slides. Fluorescence optical microscopy was performed on a Nikon Eclipse 80i microscope.

solvent from polymer-containing emulsion droplets. A series of biphasic Janus particles with different ratios of PS to P2VP ranging from 4:1 to 1:4 were produced and used to stabilize toluene/water Pickering emulsions. The volume differences between the PS and P2VP domains in the particles and their packing parameter determined the activity and wettability of the particles at the toluene−water interface, which determined the emulsion type (i.e., W/O vs O/W) and stability. Inversion of Pickering emulsions, from W/O type above pH 6 to O/W type below pH 5, was achieved when the mismatch in the relative sizes of the PS and P2VP parts was small (i.e., PS/P2VP volume ratios from 1:2 to 2:1). Significantly, the inversion between emulsion types was achieved within seconds and was reversible over multiple pH cycles. Furthermore, efficient separation of the emulsions and high recovery of the Janus particle surfactants were demonstrated.

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EXPERIMENTAL SECTION

Materials. Azobis(isobutyronitrile) (AIBN, 98%) was purchased from Junsei Chemical Co. and purified by recrystallization from ethanol. Styrene (99%) and 2-vinylpyridine (97%) were purified in an alumina column two times. 4-Hydroxybenzophenone (98%), 4vinylbenzyl chloride (97%), sodium bicarbonate (99.7%), dimethylformamide (anhydrous, 99.8%), petroleum ether (anhydrous), 2phenyl-2-propyl benzodithioate (99%), CTAB (98%), toluene (anhydrous, 99.8%), chloroform (99%), and Nile Red (98%) were purchased from Sigma and used as received. Synthesis of 4VBOBP. 4VBOBP was synthesized according to a method reported in the literature.72 7.0 g of 4VBOBP was obtained with a 70% yield. 1H NMR (CDCl3, 300 MHz): δ (ppm) 5.14 (s, 2H, −CH2O−), 5.26, 5.29 (d, 1H, CH2CH−), 5.78, 5.80 (d, 1H, CH2 CH−), 6.68−6.78 (q, 1H, CH2CH−). Synthesis of P(S-r-4VBOBP) and P(2VP-r-4VBOBP). Styrene (or 2VP), 4VBOBP, 2-phenyl-2-propyl benzodithioate, and AIBN were mixed in THF and poured into a glass ampule. The mixture was degassed by consecutive cycles of freeze−pump−thaw. After polymerization for 48 h at 90 °C, the product was precipitated in methanol (for P(S-r-4VBOBP)) or 9:1 v/v mixture of hexane and ethyl acetate (for P(2VP-r-4VBOBP)) and vacuum-dried. The mole fraction of 4VBOBP was estimated by calculating the integration ratio of styrene or vinylpyridine units and 4VBOBP units from the 1H NMR spectra. The molecular weight (Mn), polydispersity index (PDI), and the mole fractions of 4VBOBP in each copolymer are summarized in Table S1. See the Supporting Information for detailed synthetic procedures and additional polymer characterization. Fabrication of Biphasic Janus PS/P2VP Particles with Controlled Shape. Amphiphilic biphasic Janus particles were synthesized from oil-in-water emulsions by slow solvent evaporation from emulsions containing PS and P2VP in the oil phase. First, an aqueous stock solution of CTAB (1 mg/mL) was prepared by heating to 40 °C for 1 h as the continuous phase. For the dispersed phase of the emulsion, desired amounts of P(S-r-4VBOBP) and P(2VP-r4VBOBP) were dissolved in chloroform to prepare a 1 wt % solution. Biphasic Janus particles with different volume ratios of PS and P2VP spherical domains were produced by varying the volumetric ratio of P(S-r-4VBOBP) and P(2VP-r-4VBOBP) solutions from 4:1, 2:1, 1:1, 1:2, and 1:4, respectively, while maintaining a total volume of 500 μL. Then, the polymer solution was emulsified in the aqueous CTAB solution (5 mL) using a homogenizer for 1 min. The chloroform was evaporated by stirring the emulsions for 48 h in open vials at room temperature. Subsequently, particle dispersions were stirred for 2 h under UV illumination (254 nm) for cross-linking. Next, the particles were washed with deionized (DI) water to remove the large excess of remaining surfactants by repeated centrifugation performed at 13 000 rpm for 10 min. The obtained particle dispersions were investigated by TEM and SEM. The swelling behavior of the cross-linked biphasic Janus particles was investigated by redispersing the dried particles in (i) toluene and (ii) diluted hydrochloric acid (HCl, pH 2) solution.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b02365. Detailed synthetic procedures for polymers and iron oxide nanoparticles as well as additional TEM, SEM, optical microscopy images, and geometric information on the biphasic Janus particles and Pickering emulsions (PDF) Real-time movie showing the magnetic separation of Pickering emulsions from mixtures of toluene and water (AVI)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (B.J.K). *E-mail: [email protected] (C.J.H.). *E-mail: [email protected] (D.K.). ORCID

Gi-Ra Yi: 0000-0003-1353-8988 Se Gyu Jang: 0000-0002-9969-7236 Bernhard V. K. J. Schmidt: 0000-0002-3580-7053 Craig J. Hawker: 0000-0001-9951-851X Bumjoon J. Kim: 0000-0001-7783-9689 Notes

The authors declare no competing financial interest. H

DOI: 10.1021/acs.macromol.7b02365 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

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ACKNOWLEDGMENTS This research was supported by the National Research Foundation Grant (NRF-2017K2A9A2A12000315), funded by the Korean Government. We acknowledge additional support for this work from the KETEP and the MOTIE of the Republic of Korea (No. 20163010012200) and from the Research Projects of the KAIST-KUSTAR and the CRH (Climate Change Research Hub) of KAIST. GRY acknowledges the support of the Gyeongi-Do Technology Development Program as “Development of smart textronic products based on electronic fibers and textiles (kitech IZ-17-0039)”. We acknowledge Dr. Hongseok Yun and Dr. Rachel Letteri for helpful discussions.

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DOI: 10.1021/acs.macromol.7b02365 Macromolecules XXXX, XXX, XXX−XXX