Emulsion Droplets Stabilized by Close-Packed Janus Regular

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Interface Components: Nanoparticles, Colloids, Emulsions, Surfactants, Proteins, Polymers

Emulsion droplets stabilized by closepacked Janus regular polygonal particles Ryotaro Koike, Yasutaka Iwashita, and Yasuyuki Kimura Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b02323 • Publication Date (Web): 19 Sep 2018 Downloaded from http://pubs.acs.org on September 20, 2018

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Emulsion droplets stabilized by close-packed Janus regular polygonal particles Ryotaro Koike, Yasutaka Iwashita,* and Yasuyuki Kimura Department of Physics, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan

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ABSTRACT In Pickering–Ramsden emulsions, the packing structure of the colloidal particles at the liquid– liquid (or liquid–gas) interface significantly affects the structure and behavior of the emulsion. Here, using a series of plate-like particles with regular polygonal shapes and Janus amphiphilicity, we created emulsion droplets stabilized by close-packed polygonal particles at the interface. The systematic variation of the particle morphology shows that the geometrical features of the regular polygons in (curved) planar packing dominate over the self-assembled structures. The structures are tessellations of triangular, square, and hexagonal particles at the surface for large droplets, and regular tetrahedral, cubic, and dodecahedral particle shells of triangular, square, and pentagonal particles for small droplets, respectively. This work creates the possibility of geometrically designing the structure and functionality of emulsions.

KEYWORDS. Self-assembly; Amphiphilic colloid; Colloidosome; Tessellation; Platonic solids


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INTRODUCTION Micro- and nanoparticles partially wetting a liquid–liquid or liquid–gas interface can form a dense monolayer at the interface and kinetically stabilize the interface against coalescence. Liquid–fluid dispersions stabilized by such particle layers (i.e., Pickering–Ramsden emulsions)1– 3 have attracted much attention because of their industrial applicability and for fundamental scientific interest. Pickering–Ramsden emulsions play important roles in the self-assembled structures in foods, cosmetics, pharmaceuticals, and living systems.4–7 The mesostructures in the emulsions are also used for nanotechnology. The colloidosomal structures of the emulsion droplets have been investigated for emulsion polymerization and as functional microcapsules for drug delivery.7–9 The high kinetic stability of the particle monolayer is because of the extremely large attachment energy of a particle. An attached particle replaces the liquid–fluid interface, and the reduction of the interfacial energy by a mesoscale particle explains the attachment energy. When a particle with a diameter of 10 nm is equally wettable by water and n-dodecane, the attachment energy to the water–dodecane interface is 103 kBT,3 where the interfacial tension is 53 mN/m,10 kB is the Boltzmann constant and T is the absolute temperature. Such large attachment energy makes the particle attachment practically irreversible and the formation of a dense monolayer energetically preferred. In recent years, this mechanism has attracted interest for two types of particle anisotropy: surface chemistry11-13 and shape.14-18 For a sphere with Janus amphiphilicity, where part of the particle surface is hydrophilic and the other part is hydrophobic (like a surfactant molecule), the amphiphilicity enhances the attachment energy by up to three times.11– 13 Such enhancement can make the emulsions not only kinetically but also thermodynamically stable.13,19 The particle shape is also directly related to the reduction of the liquid–fluid interface by particle attachment. In addition, the particle shape strongly affects the particle arrangement in the monolayer via hard-body packing or anisotropic capillary interactions owing to deformation of the interface by particle attachment.20-23 The particle arrangement at the interface is the dominant factor for the morphology of the liquid–fluid dispersed structure, rheological properties, permeability of the monolayer, and so forth.7,14–18,21-24 Furthermore, the combination of both the shape anisotropy and amphiphilicity broadens the designability of particle arrangements at a liquid–fluid interface22,25-27 and emulsion states.28-31 Although particle anisotropy plays these essential and important roles in Pickering-Ramsden emulsions, anisotropic particles used in experiments are often polydisperse.14-16,28,29 The anisotropy experimentally investigated so far in a monodisperse particle system has been limited, for example, ellipsoid, rod, and cube shapes17,18,22 and Janus spheres, dumbbells, and hexagonal rings for amphiphilicity,32-36 because it is difficult to obtain monodisperse small particles with well-defined arbitrary anisotropy. A systematic study of particle anisotropy is thus needed to elucidate its role in emulsion states. In this study, we systematically investigated how the geometric features of a series of regular polygonal plate-like particles affect the self-assembled structures in water–in–n-dodecane emulsion droplets. Regular polygons (n-gons, where n is the number of vertices) are simple anisotropic figures with high symmetry. Uniformly sized n-gons can tessellate in a plane for n = 3, 4, and 6 and form regular polyhedra (or Platonic solids) for n = 3, 4, and 5, exhibiting characteristic spatial symmetry in/of the structures. However, it is unclear how these geometric features are reflected in the emulsion structures. The dense packing of the anisotropically shaped bodies becomes easily jammed and structural development in the emulsification is usually kinetic and stochastic, preventing structural relaxation to energetically favored structures.


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EXPERIMENTAL We prepared Janus amphiphilic regular polygonal particles (JRPPs, triangles to hexagons) and disc particles with well-defined shapes as follows (Figure 1a, and see the Supporting Information for details). First, regular polygonal and disc particles made of a negative photoresist (SU-8 2002, Microchem, Westborough, USA) were produced on a glass substrate by the standard photolithography method.37,38 One of the polygonal or disc faces were then coated with gold, and the gold surfaces were modified with sodium 3-mercapto-1propanesulfonate (99%, Sigma-Aldrich (Merck), Darmstadt, Germany) by being immersed in its 2-mM solution in ethanol (99.5%, Wako, Osaka, Japan) for 20 h. The particles were then harvested with a remover (mr-Rem 660, Micro Resist Technology, Berlin, Germany), and rinsed with isopropanol (99.7%, Wako)(Figure 1b). The surface modification made the gold-coated face hydrophilic whereas the other untreated surface was left hydrophobic, as shown in Table 1. This amphiphilicity makes the attachment energy to the water–dodecane interface almost twice that of a particle with equal wettabilities to the two liquid phases. The side length of the polygons, , and diameter of the discs was 10.0 µm and the thickness 1.4 µm. We emulsified a sample as follows (see the Supporting Information for details). First, one type of particles and small amount of water, typically 20 to 80 µL, were dispersed in 1000 µL of n-dodecane (99%, Wako) in a 1.5-mL polypropylene tube by agitation in a ultrasonic bath (38 kHz, 100 W) (US-102, SND. Inc., Suwa, Japan) for 5 min (Figure 1c). The total area of the polygonal or disc faces of the particles added in a tube was ~3400 mm2. The dispersion was then vigorously shaken for 5 min, followed by gentle shaking for 10 min.

Figure 1. Sample preparation. (a) Production of Janus regular polygonal and disc particles. (b) Scanning electron microscopy image of the triangular JRPPs. The bright surfaces are coated with gold. (c)–(f) Production of emulsion droplets. All of the scale bars are 10 µm.


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Table 1. Contact angles of water in air and n-dodecane measured by the V-r method, where the angle is calculated from the volume of the water droplet (here, 2.0 µL) and the diameter of the water droplet observed from the top. The contact angle of the bare SU-8 surface in air agrees with the reported values in Refs. 39 and 40.

in air contact angle (°)

SU-8

gold

90 2

25 2

in dodecane SU-8 gold 123 3

35 2

RESULTS AND DISCUSSION After the vigorous shaking, obtained water droplets were stabilized by relatively densely packed particles (see Figure 1d for triangular JRPPs). However, there was a considerable amount of gaps between the particles and the particles were not highly ordered. By moderately agitating the droplet dispersion via the gentle shaking, we succeeded in preparing a closely packed particle shell at the droplet surface (Figure 1f). The emulsion droplets are arrested state, exhibiting various sizes and morphologies reflecting their structural development kinetics (Figure 2). Because of the dense packing, the particles show no thermal motion and the packing density is uniform over the droplet surface (see Figure S4 in the Supporting Information). No change was observed in the droplets during a few hours microscopy observation.

Figure 2. Emulsion droplets observed after the gentle shaking process. (a) Triangular, (b) square, (c) pentagonal, (d) hexagonal, and (e) disc particles with 82, 20, 60, 70, and 60 µL water, respectively. Multiple images are combined for (a) and (b). All of the scale bars are 200 µm.

For large emulsion droplets with a radius of ~100 µm (i.e., ~10 particles), most of the droplets exhibit a smoothly curved surface (Figure 2) and the characteristic packing structure of the particles corresponds to close packing in a flat plane for all of the particle shapes (Figure 3). The triangular, square, and hexagonal JRPPs exhibit tessellation structures, that is, there are almost no observable gaps between neighboring particles in the monolayer (Figure 3a, b, and d). Here, it should be noted that the tessellated triangles and squares can shift in lanes without any gaps and the rotational symmetry of the monolayer is not necessary three- and four-fold (Figures


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3a and b and 4a and b). The characteristics of the tessellation structures are indicated by the distributions of the interparticle distances (Figure 4). For , several droplets were analyzed for each type of particles, where the typical length of the focused region of a droplet is ~10 particles in a microscopy image. We therefore discuss the peak positions of in a shortrange, ≲ 3 , where is the position of the first peak. For tessellated regular triangles, the center-to-center distances to the first and second neighbor triangles ( and ) are /√3 and in a lane and /√3 7/12 and 3/4 to neighboring lanes (see the schematic diagram in Figure 4a). For , the positions of the first peak ( ) and second peak correspond to and in a lane, and the peak widths reflect and between neighboring lanes together with disturbance in the particle arrangement. For tessellated squares, and 2 in a lane and 5/4 and 5/4 √2 between neighboring lanes are also reflected in in Figure 4b. The peak positions in Figure 4d correspond to those in a hexagonal arrangement, which are , √3 , 2 , √7 , 3 , etc. from the first neighbor.

Figure 3. Magnified images of large emulsion droplets. (a) Triangular, (b) square, (c) pentagonal, (d) hexagonal, and (e) disc particles. In (c), a magnified part of the image and a schematic diagram of the characteristic packing structure are also shown. Some large gaps are indicated by dotted yellow circles. When particles overlap, the overlapping part appears dark. All of the scale bars are 10 µm.

For the disc and pentagonal particles, interstitial gaps are clearly observed between the particles because they cannot tessellate in a plane (Figure 3c and e). The discs are in a hexagonal arrangement (Figure 4e), as observed for droplets stabilized by spherical particles without amphiphilicity.9 The arrangement of the pentagonal particles also reflects close packing in a plane (Figure 3c).41 The positional order of the close-packed regular pentagons is similar to the hexagonal arrangement, and exhibits the corresponding peaks (Figure 4c and f). The two lattice constants shown in Figure 4c are similar ( 0.942) and the particle arrangement at ACS Paragon Plus Environment

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the droplet surface is disturbed (Figure 3c), so the characteristics unique to close-packed regular pentagons are not observed in with 1 µm bin size. In addition, the higher order peaks of the hexagonal arrangement in Figure 4f are sharper than those of the tessellated triangular and square particles in Figure 4a and b, indicating a lack of two-dimensional crystalline nature by the lane shifts for the latter.

Figure 4. Distributions of the interparticle distance , where r is the center-to-center distance between particles and the bin size is 1 µm, for (a) triangular, (b) square, (c) pentagonal, (d) hexagonal, and (e) disc particles. The frequency is normalized by the distance of the central value of r of the bin, as performed for the radial distribution function. The effect of the boundary of the focused region is not considered, however, and thus the magnitude is more underestimated for larger r. In (a) and (b), the center-to-center distances in the arrangement without the shift in lanes, i.e., honeycomb and square lattice respectively, are indicated by arrows for comparison, and the distance to the first and second neighbors in the tessellation structures are shown in the schematic drawings of the structures. (f) Replot of the distributions in (c) to (e) against the distance normalized by the position of the first peak .

Some deviations from close-packed structures are observed in Figure 3, which are the causes of the structural disturbance. The overlap of particles is ~5% of the area, and a few large defective gaps in the hemispherical area of the droplet surface are observed in Figure 3. The small amount of these deviations from close-packed structures indicates that the moderate agitation applied after the initial emulsification process efficiently relaxed the particle arrangement, but only locally, and the system was in a stochastically developed kinetically arrested state, as are most Pickering–Ramsden emulsions. In addition, structural disturbance in the particle arrangement is inevitable when a close-packed arrangement in a flat plane forms on a curved surface, which would also contribute to the observed disturbance. For the overlap, we consider multiple causes of the attachment between polygonal or disc faces. A particle can attach to the exposed water–dodecane interface between the particles at the interface even when the gap size is smaller than a hydrophilic face, resulting in the partly overlapped arrangement. A small water droplet compared with a particle can “glue” the polygonal or disc faces together by the capillary interaction. Such a stacked structure by the interaction may be abundant shortly after the sonication,34 however it would be easily decomposed when touching a water–dodecane interface. The van der Waals attraction can naturally be a cause of an overlapped structure.


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Particles are initially aggregated by the attraction before sonication, and thus a small amount of aggregates might survive even after the emulsification process. Here, it is noteworthy that a JRPP may cause small interfacial deformation at a curved interface because of pinning of the interface to the straight hydrophilic–hydrophobic boundary on the JRPP surface. The water–dodecane interface can be pinned to the boundary at an angle between 35° and 123° measured from the hydrophilic side (Table 1). In addition, the small surface roughness of the particle (see, for example, Figure S1) may also contribute to the interfacial deformation. However, the resulting capillary interactions would not be important in the particle arrangement at a droplet surface, because the particles closely pack to minimize the liquid–liquid interface by particle attachment, where the hard-body interaction is dominant. For small emulsion droplets, the particles appear less ordered (Figure 5a). This tendency can be explained by planar close-packed structures being less preferred at a larger curvature surface. Particle number discreteness is also more pronounced for a smaller droplet, and the energetically and/or kinetically favored packing is not necessarily ordered or symmetric. Nevertheless, the triangular, square, and pentagonal JRPPs form the smallest possible closed shells, that is, regular tetrahedral, cubic, and regular dodecahedral droplets, respectively (Figure 5b–d). These polyhedral droplets are ~30% in 123 small structures whose projected area in a two-dimensional image is less than twice the area of the corresponding polyhedral droplet (Table S1). The other 70% are irregularly covered droplets and particle aggregates (Figure S5). For the other two regular polyhedra, only two icosahedral droplets were found in the several batches of the samples (Figure 5e), corresponding to ~2% when counted in the same way as for the three populated polyhedral droplets, and no droplets exhibited a distinctive regular octahedral structure. For the hexagonal and disc particles, no polyhedron-like small droplets formed (Figure S5).

Figure 5. Arrangements of the triangular, square, and pentagonal JRPPs for small droplets. (a) Droplets of various sizes with triangular JRPPs. (b) Regular tetrahedral, (c) cubic, (d) dodecahedral, and (e) icosahedral droplets together with schematic drawings of the polyhedra. All of the scale bars are 10 µm.


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We speculate that preferential formation of the tetrahedral, cubic, and dodecahedral droplets may be because they are the smallest possible closed-shell structures. Such structures are highly stable both energetically and kinetically. When the volume of a water domain is comparable with or smaller than that of one of the three polyhedral droplets, any particle shells other than these polyhedra have considerable amounts of liquid–liquid interfaces or hydrophilic faces of particles exposed to the oil phase, making such droplets grow during emulsification. In addition, the water domain of a polyhedral droplet is completely covered with highly surfaceactive particles and thus energetically very stable: when a bare water droplet is covered with the particles and a polyhedral droplet is formed, the energy reduction is about twice the interfacial energy of the bare droplet (see the Supporting Information). Realization of such a closed shell in the stochastic emulsification process would be more likely for a smaller number of particles, inducing selectivity toward the three polyhedra. This energetic and kinetic selectivity could explain formation of the three droplets. However, the (almost) lack of octahedral and icosahedral droplets requires more detailed investigation of the emulsification process. Finally, we investigated the role of the Janus amphiphilicity. Regular polygonal particles (RPPs), whose surfaces are untreated and homogeneously hydrophobic (see the Supporting Information), can stabilize droplets by forming dense particle layers on their surfaces (Figure 6a– d). However, the layers exhibit more overlap of particles and poorer ordered arrangements than the JRPP layers in Figure 3 (Table S2), that is, the particle arrangements are less monolayer-like for RPPs. In addition, regular polyhedral droplets are rarely observed for RPPs (~3% when counted in the same way as for the JRPPs). These results indicate that the Janus amphiphilicity plays a key role in forming a closely packed particle shell. The superior interface activity of JRPPs to RPPs would promote monolayer formation for the former, decreasing the overlap of particles. In addition, interfacial deformation by particle attachment should be different for JRPPs and RPPs because of their different wettability (Table 1), making the capillary interaction different. This might affect the non-closepacked structure in, for example, Figure 1d and the kinetics of particle arrangement.


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Figure 6. Arrangements of the RPPs for large droplets. (a) Triangular, (b) square, (c) pentagonal, and (d) hexagonal particles. All of the scale bars are 10 µm.

SUMMARY AND CONCLUSIONS We have shown that Pickering–Ramsden emulsions stabilized by a close-packed regular polygonal particle monolayer can be produced by a simple emulsification method by giving Janus amphiphilicity to the particles. In the emulsions, the geometric features of the regular polygons govern the structure. For a large droplet whose radius is ≳10 particles, the arrangement is planar packing of polygons, which are tessellations for triangular, square, and hexagonal particles. In small droplets, regular tetrahedral, cubic, and regular dodecahedral structures are preferred for triangular, square, and pentagonal particles, respectively, that is, the droplet morphology is determined by the particle shape. The liquid–liquid interface is almost minimized for tessellation and the polyhedral structures. Our conceptual study demonstrates the important role of the particle shape in designing self-assembled structures and the resulting functionalities of Pickering–Ramsden emulsions. There are various other possibilities to design emulsions via the particle shape. The interface morphology in the emulsion is coupled with the particle arrangement. The tessellation structures of triangles and squares can bend without producing gaps in the perpendicular direction to the lanes, whereas bending accompanies gaps in the parallel direction to the lanes (Figure S6), possibly making the bending energy of the interface anisotropic. In multiparticle systems, Archimedean and quasicrystalline tiling at a small curvature interface, semiregular polyhedral droplets, and so forth are expected. However, no coupling between the interface morphology and the particle arrangement was identified in this study, and preliminary experiments with JRPP mixtures exhibited poorly ordered particle arrangements at the droplet surfaces (Figure S7). This suggests that energetic optimization of the emulsion states in the current preparation process


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(Figure 1c and d) is relatively local, possibly in the range of neighboring particles, and finer tuning of the preparation conditions, such as the particle properties and emulsification method, are required to more globally optimize the emulsion states. Three-dimensionally designing the particle shape and surface chemistry are future research directions, whose role in determining self-assembly has been proven for millimeter-sized particles at a flat interface.25,26 Downsizing the emulsion structures by using designed nanoparticles is also an exciting challenge.42 An interesting question arises: How much can the polyhedral droplets in Figure 5 be downsized? These questions are relevant to viral capsids, which are one of the smallest functional nanocapsules composed of small colloidal particles with sophisticated chemical and morphological anisotropy (i.e., proteins).43

ASSOCIATED CONTENT Supporting Information. Experimental procedures; amphiphilicity and surface activity of the particles; additional results (PDF). AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (Y.I.) Author Contributions R.K. performed the experiments. R.K. and Y.I. jointly conceived and designed the experiments and analyzed the results. All of the authors jointly wrote the paper. Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS The authors gratefully thank Prof. Clemens Bechinger and Dr. Felix Kümmel for the particle preparation method, and Assoc. Prof. Yusuke Maeda for the photolithography setup and fruitful discussions. Y.I. gratefully acknowledges support from JSPS KAKENHI (Grant no. 16K14463). We thank Edanz Group (www.edanzediting.com/ac) for editing a draft of this manuscript. ABBREVIATIONS JRPP Janus regular polygonal particle; RPP Regular polygonal particle.


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Emulsion Droplets Stabilized by Close-Packed Janus Regular

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