Article Cite This: Langmuir XXXX, XXX, XXX−XXX
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Direct Observation of Carbon Nitride-Stabilized Pickering Emulsions Chenhui Han,† Qianling Cui,‡ Peng Meng,† Eric R. Waclawik,† Hengquan Yang,§ and Jingsan Xu*,† †
School of Chemistry, Physics and Mechanical Engineering, Queensland University of Technology, Brisbane, QLD 4001, Australia State Key Laboratory for Advanced Metals and Materials, School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China § School of Chemistry and Chemical Engineering, Shanxi University, Taiyuan 030006, China ‡
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S Supporting Information *
ABSTRACT: Pickering emulsions are emulsions stabilized by solid particles located at surfaces/interfaces of liquid droplets that have promising applications for drug delivery and in nanomaterials synthesis. Direct observation of Pickering emulsions can be challenging. Normally, cryoelectron microscopy needs to be used to better understand these types of emulsion systems, but cryofreezing these emulsions may cause them to lose their original morphologies. In this work, we demonstrate that graphitic carbon nitride (g-C3N4) can stabilize oil-in-water (o/w) emulsions, with hexane illustrated as a typical oil phase. The g-C3N4stabilized emulsions can act as an excellent platform for in situ study of emulsifying behavior from the mechanical point of view. Owing to its large lateral size and blue, stable fluorescence, the locations and motions of the gC3N4 stabilizer can be finely in situ monitored by light microscopy, fluorescence microscopy, and confocal microscopy. Accordingly, we illustrate two stabilizing configurations of the g-C3N4 particles with respect to the emulsion droplets under static conditions. Further, we demonstrate the capability to manipulate emulsion droplets and investigate their response to external forces. We perform real-time observations of the g-C3N4 particles and the emulsion droplets that move in the continuous phase and study their adsorption kinetics toward each other. Finally, the π−π interaction between the stabilizer and aromatic liquid phase (e.g., toluene) is considered and studied as an influencing factor on emulsifying behavior.
1. INTRODUCTION Emulsions have been widely explored and used in industrial processes and in daily consumables, including the petroleum, food, pharmaceutical, and cosmetic industries. Traditionally, emulsions are stabilized by molecular surfactants that can be dissolved in either water or oil. Concerns regarding the widespread use of surfactants are that they are difficult to recover, and their intensive use may impact the environment with the possibility of inducing cell damage or causing tissue irritation to living bodies. Pioneering work conducted by Ramsden and Pickering revealed that emulsions can also be stabilized by solid particles, rather than surfactants or polymers.1,2 Pickering emulsions usually feature superior stability and low toxicity and hold many potential applications, such as nanocomposite synthesis via Pickering emulsion polymerization,3,4 interfacial reactions for improved catalytic efficiency,5−7 as well as potential drug-delivery systems or in biosensing.8 Among various solid compounds used as Pickering emulsifiers (e.g., silica,9,10 Laponite clays,11−13 graphene oxide (GO) sheets,14,15 and metal oxides16−19), GO can be regarded as a two-dimensional, colloidal stabilizer and used in emulsion polymerization for generating polymer− GO nanocomposites.14,15 SiO2 nanoparticles are the most popular Pickering-type stabilizer due to their high homoge© XXXX American Chemical Society
neity, well-controlled particle size, and surface wettability. As such, SiO2 nanoparticles have frequently been deployed to produce Pickering emulsions for different applications, and also as a model agent to study the stabilization mechanism of Pickering emulsions more generally.9 Normally, the SiO2 stabilizer is small compared to the emulsion droplets (∼100 nm vs ∼100 μm), where the emulsion stabilization mechanism is considered to involve steric hindrance against droplet− droplet coalescence, provided by SiO2 aggregates at the interface.20 However, direct observation and proof of this stabilization mechanism is still lacking because visualization of the nanosized SiO2-stabilized emulsions requires freezefracture electron microscopy, which precludes in situ monitoring of the solid emulsifiers, the emulsion droplets, and their movements. Moreover, the emulsions may lose their original morphology upon freezing, so information obtained by this process is limited. Therefore, it is desirable to find a material platform to directly observe Pickering emulsion systems, to verify and investigate the stabilization configuration. The ideal material platform would allow the detection Received: July 11, 2018 Revised: August 3, 2018 Published: August 4, 2018 A
DOI: 10.1021/acs.langmuir.8b02347 Langmuir XXXX, XXX, XXX−XXX
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Figure 1. Hexane-in-water emulsion stabilized by g-C3N4: (a) droplet size of different g-C3N4 concentrations, (b) average droplet size variation against aging time under different g-C3N4 concentrations, and (c) emulsifying efficiency under different g-C3N4 concentrations. (d) Dynamic hexane−water interfacial tension, with 0.05 and 0.25 mg/mL g-C3N4 dispersed in water, respectively.
mechanistic study of Pickering-type systems. First, the as-used g-C3N4 possesses a sheetlike morphology with lateral sizes up to tens of micrometers and hence they can be readily observed by optical microscopy, in an ambient environment. Second, gC3N4 is intrinsically photoluminescent, a property resulting from its conjugated framework that causes it to emit blue light upon UV irradiation.33 This character endows fluorescence microscopy (FM) a viable technique to more precisely and clearly locate the g-C3N4 and meanwhile exclude the disturbance of possible solid impurities. Moreover, different from conventional SiO2 colloidal nanoparticles, g-C 3N4 typically possesses a nonuniform, layered morphology, so the study of its emulsifying properties can significantly supplement the understanding of Pickering-type stabilization. In this work, we employed g-C3N4 as a model Pickering emulsifier and studied its stabilization configurations by in situ observation of the emulsions through light microscopy (LM) and fluorescence microscopy (FM). Furthermore, the movements of the g-C3N4 particles and the emulsion droplets were detected in real time, providing solid evidence for understanding how the emulsions dynamically stabilized. We also found that the emulsifying function can be different, depending on the type of organic phase, aromatic solvents in particular, presumably owing to the π−π interactions between the conjugated framework of g-C3N4.
and recording of the dynamic changes of emulsifiers and emulsion droplets and thereby assist in understanding the nonequilibrium stability of Pickering emulsions. Over the past decade, the unique, metal-free semiconductor g-C3N4 has been extensively studied in many areas, including energy conversion and storage,21,22 optoelectronics,23−25 and catalysis.22,26,27 g-C3N4 possesses a layered structure with each layer consisting of tri-s-triazine units bridged by nitrogen atoms.28 In fact, g-C3N4 is also rich in primary/secondary amines attached to the conjugated framework. In addition to applications in the fields mentioned above, the physicochemical properties of g-C3N4 in liquid phases are beginning to attract increasing attention from researchers. For instance, gC3N4 can be readily processed into few-layered, aqueousdispersed nanoparticles via liquid-assisted exfoliation, which leads to blue-shifted photoluminescence.29,30 Zhang and coworkers reported that after hydrolyzing g-C3N4 in base solution, the as-obtained nanofiber dispersion can form a hydrogel network triggered by CO2, which is presumably covalently bonded by connecting the amine groups from different g-C3N4 frameworks.31 Very recently, our team has found that g-C3N4 can behave as a surface-active material at the liquid−liquid, water−solid, and water−air interfaces.32 In particular, g-C3N4 can act as a straightforward stabilizer for Pickering emulsions, generated by simply shaking a mixture of g-C3N4 aqueous dispersion along with a hydrophobic organic solvent.32 In this context, we consider that g-C3N4-stabilized emulsions can be an excellent platform for direct observation and
2. EXPERIMENTAL SECTION 2.1. Materials. Cyanuric acid (Sigma-Aldrich), melamine (SigmaAldrich), toluene (AR, Fisher Scientific), hexane fraction (Ajax B
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Figure 2. Hexane-in-water emulsions stabilized by 0.25 mg/mL g-C3N4: (a) LM and (b) corresponding FM images. Blue color indicates the fluorescence emitted by g-C3N4, denoting the exact position of the g-C3N4 particles. Images of the emulsion taken by a confocal microscope: (c) normal black-white picture and (d) the corresponding 3D confocal picture, where the colors indicate the height. Finechem), p-xylene (Sigma-Aldrich), and decane (Sigma-Aldrich) were used as received without further purification. Ultrapure water (18.2 MΩ cm, Synergy UV Water Purification System, Merck Millipore) was used for all of the emulsion preparation. 2.2. Synthesis of g-C3N4. g-C3N4 was synthesized using the cyanuric acid−melamine supramolecular precursor according to a previous report.34 Typically, a 1:1 molar ratio of cyanuric acid and melamine were mixed in water and shaken for 24 h at room temperature. Then, the obtained milky suspension was centrifuged and washed with water three times and dried at 50 °C under vacuum. Afterward, the precursor powder was annealed in capped crucibles at 550 °C for 4 h at a heating rate of 2.3 °C/min in nitrogen atmosphere. After cooling down, the yellow g-C3N4 powder was collected. 2.3. Preparation of Pickering Emulsions. g-C3N4 aqueous suspensions of different concentrations were prepared by sonicating gC3N4 solid in ultrapure water for 5 min. The suspensions were then mixed with an organic solvent (e.g., hexane, 1/1 mL) in a vial (5 mL) and sealed by a cap with a Teflon gasket. The liquid mixture was shaken by hand for about 30 s to produce Pickering emulsions. The quoted concentrations of g-C3N4 represent concentrations in the dispersed aqueous phase. The o/w emulsions with toluene, decane, and p-xylene as the oil phase were prepared by a similar method. 2.4. Interfacial Tension Measurement. The dynamic water− hexane interfacial tension was measured by the pendant drop method (OCA 25, Dataphysics, Germany). A pendant drop of g-C3N4 aqueous dispersion (0.05 and 0.25 mg/mL) was formed at the end of a needle immersed in the hexane oil phase. The aging of the interface was monitored by measuring the interfacial tension of a drop over 50 min. 2.5. Observation of Pickering Emulsions. The microscope (LM, FM, and confocal) images and the video of the manipulating process on droplets were obtained through observing a water-diluted emulsion placed in the middle of two pieces of glass slides separated
by a distance of 1 mm. During the manipulating process, a thin copper wire (0.1 mm in diameter) was used as a probe to touch and manipulate emulsion droplets. A Nikon Eclipse Ni light microscope, a Nikon Eclipse Ti inverted epif fluorescence microscope, and a Nikon A1R Confocal microscope were used to image the emulsions. The stability of the emulsions has been evaluated by monitoring the size variation of at least 300 droplets against aging in a bulk sample (1 mL of emulsion). 2.6. Contact Angle Measurement. The contact angle was measured on a g-C3N4 disk (diameter, 13 mm; thickness, ca. 1 mm) made from approximately 1 g of g-C3N4 particles. The powder was compressed into a disk in a steel die with a pressure of 6 × 108 N/m2. The sessile water droplet method was performed on an FTA200 Contact Angle Analyser. The picture was taken by a camera and analyzed using Fta32 Video 2.0 software. Nine independent measurements were performed and mean values presented.
3. RESULTS AND DISCUSSION 3.1. Static Observation of Emulsions. The g-C3N4 emulsifier was synthesized using the approach of heating a cyanuric acid−melamine supramolecular precursor, resulting in aggregates with a thickness of 15−30 nm and lateral sizes of up to ∼10 μm (Figure S1).34 The UV−vis absorption and photoluminescence spectra of g-C3N4 bulk powder and aqueous dispersion were measured and are recorded in Figure S1d,e. Typically, Pickering emulsions were generated by manually shaking a mixture of hexane and g-C3N4 aqueous dispersion in a vial (Figure S2). The emulsions were then transferred to a homemade setup (Figure S3) for direct LM and FM observation. We noted that oil-in-water (o/w) emulsions were formed under a quite low g-C3N4 concentration (0.25 mg/mL), with the oil droplets having sizes of C
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Figure 3. (a) LM and (b) the corresponding FM image of emulsions prepared by 2 mg/mL aqueous g-C3N4 dispersion and hexane. (c) Threedimensional confocal image of the emulsion covered by g-C3N4. The colors indicate the height of the g-C3N4. (d) Schematic illustration of two configurations of g-C3N4-stabilized emulsions.
50−150 μm. The formation of o/w type of emulsions agreed with the contact angle measurements, which illustrates that gC3N4 was predominantly hydrophilic and had a contact angle of ca. 58° (Figure S4).9 Figure 1a shows that the average droplet size of the freshly prepared emulsions decreased from 125 to 75 μm when the g-C3N4 concentration increased from 0.05 to 2 mg/mL, agreeing with the normal property of Pickering emulsions.35 The stability of the emulsions was examined by monitoring the droplet size variation against different aging times in an ambient environment. We observed that the Pickering emulsion stabilized by 0.05 mg/mL g-C3N4 started to collapse after 2 weeks, while the emulsions remained stable for at least 3 months when the concentration of g-C3N4 reached 0.25 mg/mL or higher (Figure 1b). Further, the emulsifying efficiency ij y remaining g ‐ C N concentration in water jj = 1 − orignial g ‐ C N3 4concentration in water × 100%zzz o f g 3 4 k { C3N4 was determined to be above 90% for all concentrations (Figure 1c), suggesting the strong capability of g-C3N4 for emulsification. The dynamic hexane−water interfacial tension with g-C3N4 dispersed in the water phase was monitored by the pendant drop method. As shown in Figure 1d, the hexane−water interfacial tension was measured over 50 min. As a reference, the pure water−hexane interfacial tension was measured and determined to be 47.5 mN/m, which is in agreement with previous reports.36 The presence of g-C3N4 lowered the interfacial tension to 32 and 24 mN/m for concentrations of 0.05 and 0.25 mg/mL, respectively, indicating the surface activity of g-C3N4. Besides, the interfacial tension did not reach
(
equilibrium even after 50 min, suggesting slow partitioning of the g-C3N4 particles to the hexane−water interfaces, probably owing to their large hydrodynamic sizes.37 It can be seen in the LM and FM pictures (Figure 2a,b) that the surfaces of droplets were rarely completely covered by gC3N4 particles when the concentration of g-C3N4 in the dispersing phase was relatively low. Instead, the g-C3N4 particles aggregated at the droplet interfaces, while only a small amount of g-C3N4 was nonuniformly adhered to the droplet surface. This observation was confirmed by threedimensional (3D) confocal imaging (Figure 2c,d). The results clearly show that emulsion droplets were not necessarily fully covered by the g-C3N4 to gain stability; rather, islands of gC3N4 particles at droplet junctions provided the steric barrier required to prevent coalescence between neighboring droplets, resulting in exceedingly high emulsifying efficiency (Figure 1c). This phenomenon significantly differs from the commonly proposed stabilization geometry for Pickering emulsions that a dense (or sparse) particle layer must evenly form around the dispersed droplet to act as the barrier to coalescence.38−40 To exclude the possible effect of surface-active small molecules being associated with the g-C3N4, two control experiments were carried out: (1) the pristine g-C3N4 powder was thoroughly washed with acetone and ethanol and then dried in a vacuum oven. The obtained g-C3N4 was then dispersed in water to form a suspension. We found that this pretreated g-C3N4 retained its emulsifying capacity (Figure S5a). (2) The pristine g-C3N4 was first dispersed in water by sonication and then the g-C3N4 powder was fully removed by centrifugation and filtering. The aqueous supernate did not
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Figure 4. (a−c) LM images of the emulsion droplets manipulated by a copper wire. The emulsion was prepared by 0.25 mg/mL g-C3N4 dispersion and hexane. (d) LM image of g-C3N4 aggregated between two droplet interfaces.
emulsion was further observed by confocal microscopy, and the 3D confocal image clearly exhibits the spatial distribution of the g-C3N4 particles (Figure 3c). It is worth noting that the Pickering emulsions stabilized by g-C3N4 (0.25 mg/mL or higher) all showed remarkable stability. On the basis of these results, we propose a stabilization mechanism for Pickering emulsions under static conditions, with the g-C3N4 emulsifier present in two configurations. As described by the scheme in Figure 3d, when the g-C3N4 concentration is relatively low, the oil droplets require only partial coverage of g-C3N4 to inhibit coalescence, with the solid particles located at the interfaces of the droplets (scheme i). With increased g-C3N4 concentration, more g-C3N4 particles adsorb to the droplet surfaces and eventually achieve full coverage (scheme ii). 3.2. Manipulation and Dynamic Observation of Emulsions. Real-time observation can be used to uncover the behavior and stabilization mechanism of Pickering emulsions; however, due to the invisibility of the normal emulsifier particles (mainly due to their small size and nonfluorescence) under a light microscope, this remains a great challenge. In the present work, we were able to conduct dynamic monitoring of the g-C3N4-stabilized emulsion system, taking advantage of the relatively large size of the g-C3N4 sheets. In the first set of experiments, we used a thin copper wire (diameter, 100 μm) to manipulate oil droplets under LM. As illustrated in Figure 4a, we used the probe to carefully touch one of the droplets (with a diameter of ∼240 μm). Interestingly, the droplet did not break after contacting the wire, but attached to the wire instead. Next, a significant deformation occurred to this droplet when we tried to pull
show any activity for emulsification (Figure S5b). These results confirmed that g-C3N4 can behave as an effective emulsifier. Vignati et al. reported silica colloid (ca. 500 nm)-stabilized emulsions with particle coverages as low as 5% and attributed the stabilization mechanism to a kinetic effect, i.e., the Brownian motion and the associated redistribution of nanoparticles.41 For the case of g-C3N4 aggregates described here, however, the surfactant sizes of several tens of micrometers makes it unlikely that a Brownian motion-based stabilization mechanism occurred. Besides, we did not observe Browniantype motion during LM imaging. Therefore, we consider the high stability of g-C3N4 Pickering emulsions to originate from two effects: first, the hexane−water interfacial tension was lowered owing to the presence of g-C3N4; second, and more importantly, a steric barrier was provided by the interfacial adsorption of the solid particles such that the adjacent droplets would not coalesce over a long time course. We consider that these effects should be suitable for a range of Pickering emulsions, including those stabilized by silica, clays, and a variety of metal oxides. In another set of experiments, the concentration of g-C3N4 in the dispersion was increased to 2 mg/mL and the obtained emulsion was observed. As can be seen in Figure 3a, the droplets’ surfaces had a “powdery” appearance due to the adhesion of a large amount of g-C3N4. The visualization of gC3N4 coverage could be further improved by FM, which clearly imaged the full coverage of the emulsion droplets by g-C3N4 particles (Figure 3b), as indicated by the blue emission. Interestingly, the excess g-C3N4 preferred to be located in the conjoining regions of droplets, forming networks that bridged the droplets and contributed to emulsion stability.42 The E
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Figure 5. Continuous picturing of emulsion droplets and g-C3N4 particles using LM: (a) one droplet moving, (b) g-C3N4 sheets moving, and (c) one droplet sliding accompanied by g-C3N4 shifting on another droplet’s surface. The emulsion was made by shaking 0.25 mg/mL g-C3N4 dispersion and hexane. All scale bars represent 100 μm.
moving droplet. Alternatively, the emulsion droplets might remain still, but rather the g-C3N4 particles in the continuous phase floated around and were trapped onto the droplet surface (Figure 5b). For either pathway, the adsorption is an irreversible process owing to the associated decrease in surface energy.38 The in situ observation of the adsorption process (more scenarios are shown in Figures S6 and S7 in the Supporting Information) for both conditions is helpful to understand the emulsifying process, which involves the interaction and stabilization of multiple phases (oil, water, and solids). Modeling can be carried out in the future to further investigate the formation process of Pickering emulsions based on the presented phenomena. In another experiment (Figure 5c), we found that the droplet (A, as noted) inside the continuous phase appeared to slide around the larger droplet to which it was attached (B), supported by the interfacial g-C3N4 particles, as indicated by white arrows. This motion should be expected to result from the vibration of the continuous phase (induced by external disturbance), and the motion ceased after droplet A was linked to other droplets (D, and then C). Focussing on the g-C3N4 particles on the surface of the neighboring droplet (C), these particles simultaneously shifted toward A during the motion of droplet (A), and were eventually pinned to the interface, acting as the steric barrier between A and C (red arrows). Here, the spontaneous movement of g-C3N4 accompanying the movement of the nearby droplet reveals the kinetic nature of stabilization of Pickering emulsions, that is to say, the solid particle aggregates spontaneously shift their locations to the
away the wire. Noting that the opposite side of this droplet was adhered to several smaller droplets, this ability to physically deform the shape indicates that the droplet possesses certain elasticity (Figure 4b). By pulling the tip further away, the smaller droplets detached from their original sites and moved with the wire, as an aggregate, together with the controlled large droplet, as depicted in Figure 4c. These results demonstrate that the emulsion droplets are stable even against a strong, physical external force. Moreover, it can be deduced that the g-C3N4 emulsifier aggregates located at droplet junctions were connected and shared by the neighboring droplets. This was further confirmed by examining a highresolution LM image focused on the interfacial g-C3N4 particles (Figure 4d). Similar results were reported by Binks and co-workers using silica colloids as the emulsifier, which formed a monolayer at the junction and bridged the emulsion droplets, while a water-in-oil emulsion was generated in their case.39 In a separate set of experiments, we monitored the motions of oil droplets in real time, as well as the associated particle emulsifier upon minor disturbance. It can be seen in Figure 5a that one droplet was initially (0 s) floating in the continuous phase along with several g-C3N4 aggregates in the vicinity (indicated by the arrows). During the droplet’s random motion, a collision occurred and g-C3N4 sheets were subsequently adsorbed to the droplet (3 s, Figure 5a). Afterward, they moved together until being anchored to a cluster of droplets (11 and 28 s, Figure 5a). These pictures clearly exhibit how the g-C3N4 emulsifier was captured by the F
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Figure 6. LM, FM, and overlapped images of emulsions prepared using 0.5 mg/mL g-C3N4 aqueous dispersion with (a−c) hexane and (d−f) toluene. Blue color indicates the fluorescence from g-C3N4, which denotes the position of g-C3N4 particles. Digital photographs of emulsions made from shaking the mixture of (g) toluene−water and (h) hexane−water. For each mixture: left, g-C3N4 initially dispersed in water; middle, g-C3N4 initially dispersed in organic phase; right, g-C3N4 initially dispersed in organic phase and stirred at 1500 rpm for 20 min with water. Then, the mixture was shaken to make emulsions.
hexane (which has very low polarity), polarity also contributes to the possible overall interaction between toluene molecules and g-C3N4 particles. Similar results were observed when comparing decane−water and xylene−water emulsions, as shown in Figure S9. In addition, it is worth mentioning that when the g-C3N4 concentration was at a very low level (e.g., 0.25 mg/mL), both the hexane and toluene droplets were barely covered by the particles (Figure S10a−c), with clusters of g-C3N4 particles primarily lying at the droplet interfaces. When the loading of gC3N4 in the solution reached a high level (2 mg/mL), the gC3N4 sheets were able to assemble into large aggregates over the entire droplet surface for both hexane and toluene (Figure S10d−f), in addition to staying at the interfacial areas. The interactions between g-C3N4 and toluene may also be reflected by another phenomenon. When changing the initial dispersion medium of g-C3N4 from water to toluene, an emulsion was not able to be formed anymore by just shaking the mixture (Figure 6g, middle), and the g-C3N4 particles tended to adsorb onto the bottle wall. We consider that this is because the preformed g-C3N4/toluene interface generated from π−π interaction and similar polarities of these two species could not be displaced by the g-C3N4/water interface if the shearing force was not strong enough. In this case, interface replacement could be expected to occur under elevated shear force.44 After vigorous stirring (1500 rpm for 20 min), a toluene−water emulsion was readily produced by shaking (Figure 6g, right). In contrast to this behavior, for the hexane− water system, emulsions were easily formed regardless of the
droplet junctions, playing a key role in the prevention of coalescence. The movement is probably induced by capillary force. It is worth highlighting that more events were captured and are displayed in Figure S8. These results correspond to the above results that g-C3N4 particles are preferably located at the interfaces of emulsion droplets to realize high stability (Figure 1). 3.3. Interaction between g-C3N4 Emulsifier and Oil Phase. Differing from conventional inorganic emulsifiers (e.g., SiO2 and Laponite clays), g-C3N4 possesses a π-conjugated structure that could strongly interact with certain aromatic organic solvents such as toluene. As illustrated by the LM and FM images (Figure 6a−c) of the emulsions, the droplet coating by g-C3N4 was quite low when using hexane as the oil phase, with g-C3N4 aggregating mainly at droplet interfaces. When toluene solvent was used instead, most of the oil droplets appear to be fully covered by g-C3N4 particles (Figure 6d−f). It is worth noting that the g-C3N4 concentration was the same (0.5 mg/mL) and o/w emulsions were generated for both cases. We ascribe the higher level of g-C3N4 coverage per droplet with toluene to the π−π interaction between the conjugated structure of g-C3N4 and the aromatic solvent. This attractive interaction is likely to enhance the affinity of the gC3N4 particles to the organic solvent. This type of interaction has previously been demonstrated by Antonietti and Wang in g-C3N4-catalyzed benzene transformations.43,44 Where no such π−π attraction force was present, as between g-C3N4 and hexane, a much lower surface coverage resulted. Recalling that toluene has a more similar polarity to g-C3N4 compared to G
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Figure 7. Scanning electron microscopy images of the g-C3N4 powders obtained by naturally drying (a, b) hexane−water emulsion and (c, d) toluene−water emulsion.
order in which g-C3N4 was dispersed, in water or hexane first (Figure 6h). The interaction between emulsion droplets and the g-C3N4 emulsifier might affect the geometry and therefore the assembly of solid particles on the droplet surface. Therefore, we examined the g-C3N4 particles that were obtained by naturally drying the emulsions. As shown in Figure 7a,b, the gC3N4 obtained from the hexane−water emulsion dried into a gallimaufry of aggregates. In comparison, the toluene−water system resulted in an interconnected, sheetlike g-C3N4 form. We believe that this difference can also be assigned to the π−π interaction between toluene and g-C3N4, as discussed above, and therefore, the g-C3N4 particles assembled and a smooth texture was maintained after evaporation of the liquids. Since the interactions between the solid stabilizers and the liquids on emulsification can be complicated, further work to understand this behavior is warranted.
interaction between stabilizer and organic phase on the emulsifying behavior was examined. By comparing emulsions formed in solvents that could form π−π interactions with the g-C3N4 stabilizer to those that cannot, this work provides significant insights into the interfacial (liquid−liquid) properties of g-C3N4 and the stabilizing mechanism of Pickering emulsions.
4. CONCLUSIONS In summary, we have investigated the stabilizing mechanism of g-C3N4-stabilized Pickering emulsions by in situ microscopy observations. Benefitting from the large sizes and photoluminescence of g-C3N4 particles, their positions and movements could be directly observed using a light/fluorescence microscope in ambient environment. Under static conditions, two stable configurations of the g-C3N4 particles with respect to the emulsion droplets were revealed: (i) g-C3N4 particles located at interfaces of adjoining oil droplets and (ii) g-C3N4 particles fully covering oil droplets. Further, we carried out real-time monitoring of the g-C3N4 particles and the associated emulsion droplets, recording their movements and adsorption kinetics toward each other. These results directly demonstrated the dynamic stabilization of Pickering emulsions and may explain their superior stability compared to normal emulsions. Moreover, the oil droplets were stable enough to be physically manipulated and probed, permitting a study of their response to external forces. Emulsion droplets readily deformed upon stress but did not break apart. Finally, the possible effect of the
Corresponding Author
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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.langmuir.8b02347.
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LM and FM pictures, homemade setup, contact angle measurement, and TEM images (PDF)
AUTHOR INFORMATION
*E-mail:
[email protected]. ORCID
Qianling Cui: 0000-0002-4983-6445 Hengquan Yang: 0000-0001-7955-0512 Jingsan Xu: 0000-0003-1172-3864 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS J.X. is grateful to the Discovery Early Career Researcher Award ( D EC R A ) b y t h e A u s t r a l i a n R e s e a r c h C o u n ci l (DE160101488). Science and Engineering Faculty and Central Analytical Research Facility (CARF) at QUT are greatly acknowledged for technical access.
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REFERENCES
(1) Ramsden, W. Proc. Royal Soc. A 1903, 72, 156. (2) Pickering, S. U. J. Chem. Soc., Trans. 1907, 91, 2001.
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DOI: 10.1021/acs.langmuir.8b02347 Langmuir XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.langmuir.8b02347 Langmuir XXXX, XXX, XXX−XXX
