Particle Volume Ratio

Subscriber access provided by UNIV OF CALIFORNIA SAN DIEGO LIBRARIES

Article

Dependence of the internal structure on water/particle volume ratio in an amphiphilic Janus particle–water–oil ternary system: from micelle-like clusters to emulsions of spherical droplets Tomohiro G Noguchi, Yasutaka Iwashita, and Yasuyuki Kimura Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b03723 • Publication Date (Web): 08 Jan 2017 Downloaded from http://pubs.acs.org on January 17, 2017

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

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

Page 1 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Dependence of the internal structure on water/particle volume ratio in an amphiphilic Janus particle–water–oil ternary system: from micelle-like clusters to emulsions of spherical droplets Tomohiro G. Noguchi, Yasutaka Iwashita* and Yasuyuki Kimura Department of Physics, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan

ABSTRACT Amphiphilic Janus particles (AJP), composed of hydrophilic and hydrophobic hemispheres, are one of the simplest anisotropic colloids, and they exhibit higher surface activities than particles with homogeneous surface properties. Consequently, a ternary system of AJP, water and oil can form extremely stable Pickering emulsions, with internal structures that depend on the Janus structure of the particles and the system composition. However, the detail of these structures has not been fully explored, especially for the composition range where the amount of the minority liquid phase and AJP are comparable, where one would expect the Janus characteristics to be directly reflected. In this study, we varied the volume ratio of the particles and the minority liquid

ACS Paragon Plus Environment

1

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 26

phase, water, by two orders of magnitude around the comparable composition range, and observed the resultant structures at the resolution of the individual particle dimensions by optical microscopy. When the volume ratio of water is smaller than that of the Janus particles, capillary interactions between the hydrophilic hemispheres of the particles induce micelle-like clusters in which the hydrophilic sides of the particles face inward. With increasing water content, these clusters grow into a rod-like morphology. When the water volume exceeds that of the particles, the structure transforms into an emulsion state composed of spherical droplets, colloidosomes, because of the surface activity of particles at the liquid–liquid interface. Thus, we found that a change in volume fraction alters the mechanism of structure formation in the ternary system, and large resulting morphological changes in the self-assembled structures reflect the anisotropy of the particles. The self-assembly shows essential commonalities with that in microemulsions of surfactant molecules, however the AJP system is stabilized only kinetically. Analysis of the dependence of the emulsion droplet size on composition shows that almost all the particles are adsorbed at the water-oil interface; i.e., the particles show ideal surface activity.

Introduction Colloidal particles that show partial wetting at a liquid–liquid interface exhibit strong surface activity and can emulsify immiscible liquids.1-3 Emulsions stabilized by colloidal particles are called Pickering emulsions (PEs), discovered about a century ago.4 Because the size of colloidal particles is much larger than that of surfactant molecules, their adsorption energy at an interface is much larger than that of the latter, reaching ~102–106 𝑘B 𝑇, where 𝑘B is the Boltzmann constant and 𝑇 is the absolute temperature. Thus, the stability and rheological properties of PEs


2

Page 3 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

markedly differ from those of emulsions stabilized by surfactant molecules.5,6 PEs play important roles in self-assembled structures and material properties in various systems, including the structures found in living systems composed of bio-colloids such as proteins, and the characteristic rheological properties of foods and cosmetics.7-9 Emulsification by particles possessing homogeneous surface properties are now well understood through various experimental and theoretical studies.10-12 Recent developments in fabrication techniques for fine particles now enable the creation of various kinds of colloidal particles with anisotropic surface properties.13-15 In particular, amphiphilic Janus particles (AJP), one of the first patchy particles to be developed, comprise a hydrophobic patch on a hydrophilic surface or vice versa.16, 17 AJP have interfacial adsorption energies that can be as much as three times larger than that of homogenous particles and thus exhibit excellent surface activities.18,19 Because of the adsorption energy, it has been theoretically proven that emulsions of AJP can be thermodynamically stable, i.e., in an equilibrium state, in contrast to the case of homogenous particles.20, 21 Additionally, the contact line on an AJP at a liquid–liquid interface is held at the boundary between the hydrophilic and hydrophobic surfaces for some range of the solvents’ polarities (Figure 1a).22 Because of the high stability of such emulsions, AJP are used in emulsion polymerization or emulsion-based micro-reactors.23-26 For the use of AJP as an emulsifier, it is important to elucidate the dependence of the emulsion state on the amphiphilicity of the particles, the Janus structure, and the particle/solvent ratio. In addition to a theoretical study of these dependencies,21 the influences of amphiphilicity and Janus structure have been studied experimentally.27-30 However, the dependence on the composition of particles and solvents has not been studied experimentally in detail, and systematic studies on emulsification with AJP lag behind those for homogenous particles.


3

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 26

The self-assembled structure formation in an AJP - immiscible liquids ternary system possesses essential similarities to microemulsions.31-33 The PE with AJP can be the equilibrium state due to their amphiphilicity as described above. In other words, the PE can be regarded as a microemulsion stabilized with colloidal surfactants instead of surfactant molecules. Because the chemical anisotropy in AJP is analogous to that of surfactant molecules, we can expect similar self-assembled structures in the AJP system to microemulsion systems, such as (swollen) spherical, thread-like and plate-like micelles,31-33 and some of those structures have been reported in binary systems composed of AJP and solvent34, 35 and in ternary systems.30, 36 However, there are essential differences between the AJP systems and microemulsions because of the size difference of surfactants. AJP are mesoscopic particles, thus they can exhibit bulk-like properties and the interactions between AJP can easily overwhelm thermal agitation, in contrast to those between surfactant molecules. The structure formation in the AJP system can therefore be kinetic rather than thermodynamic. To put it briefly, as surfactants, AJP are expected to possess combined characteristics of homogeneous colloidal particles and molecular surfactants. In this work, we experimentally studied the effect of the amphiphilic Janus structure on the self-assembled structure formation in the AJP–water–oil ternary system, particularly focusing on the ratio of AJP and the minority liquid phase, water. We varied the volume ratio of the particles and water 𝛼 = 𝑉w ⁄𝑉p, where 𝑉w is the volume of water and 𝑉p is that of AJP, over two orders of magnitude. The resultant structures were observed at the resolution of individual particle dimensions by optical microscopy. As a result, we found that varying the particle-solvent composition changes the formation mechanism of the self-assembled structures, subsequently altering the morphology of the structure, as follows: in the case of low water content (𝛼 < 0.7), spherical or rod-shaped micelle-like structures are formed, reflecting the amphiphilic nature of the


4

Page 5 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

particles. The origin of the self-assembly is the capillary attraction of tiny water droplets bridging hydrophilic surfaces. When the water content is higher (𝛼 > 0.7), however, the particles are strongly adsorbed to the oil-water interface through their surface activity, forming spherical emulsion droplets. From a theoretical analysis accounting for the volume of partially embedded particles, we find an ideal surface-active behavior of AJP, in which almost all the particles strongly attach to and highly stabilize the oil-water interface.

Figure 1. Amphiphilic Janus particles attaching to a liquid–liquid interface. (a) A schematic of the particle. (b) An optical microscope image of AJP at a water-dodecane interface. The black and white regions of a particle are, respectively, the hydrophobic part (Au-coated surface) and hydrophilic part (silica surface). The scale bar is 5 m. The inset is a scanning electron microscope image of a spherical silica particle with a hemisphere coated with Au. The white and gray regions of the particle are, respectively, the Au-coated surface and the silica surface. The radius of the particle was 2.6 m for visual clarity.

Experimental The experimental methods used in this study are briefly described below (see Supporting Information for detail). We prepared AJP composed of hydrophilic and hydrophobic hemispheres


5

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 26

by coating a hemisphere of spherical silica particles with Au and modifying the surface of the Au layer with 1-octadecanethiol (90.0%, Wako) to make the surface hydrophobic. The radius of the silica particles was 𝑎 = 1.5 m and they were monodisperse (coefficient of variation about 2%). This particle possessed high amphiphilicity, showing clear amphiphilic Janus nature not only at pure water (18.2M, Millipore) - n-dodecane (99.0%, Wako) interface (Figure 1b) used in subsequent experiments; but also at an interface where the difference in polarity was smaller than that of water - n-dodecane (see Figure S1 in Supporting Information). We prepared a ternary system comprising AJP, pure water and n-dodecane. We used volume ratios of water to the particles, 𝛼 = 𝑉w ⁄𝑉p , over the range 0.00–8.28, where both 𝑉w and 𝑉p were precisely measured and were up to a few l. The volume of dodecane was 100–200 l. The water was thus the minority liquid phase in all samples, so that the water domain was isolated and enclosed by the particles. Each sample in a polypropylene tube was strongly agitated with an ultrasound bath (38 kHz, 100 W)(US-102, SND Inc.), then we gently stirred the mixture manually and allowed the sample to self-assemble, while avoiding sedimentation. We confirmed that the water phase was broken up into small droplets of sizes ≲ 1 m immediately after ultrasonication (see Figure S2 in Supporting Information). We then observed the sample with optical microscopy.

Results and Discussion We first qualitatively describe the change in structure of the ternary system with increasing water content, 𝛼. With increasing 𝛼, (inverse) small, micelle-like clusters appeared, followed by anisotropic growth into rod-like shapes, and finally spherical emulsion droplets formed (Figure 2b – k). At 𝛼 = 0.0, random aggregates simply formed by the van der Waals attraction between the


6

Page 7 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

particles (Figure 2a). The attraction between bare silica particles is strong enough to irreversibly aggregate in dodecane, explaining why there was no selectivity for bonding surfaces observed, because a stronger attraction was expected between Au surfaces than between Au and silica or between silica surfaces due to the large Hamaker’s constant of Au. Adding a small amount of water (𝛼 = 0.08) caused small clusters in which hydrophilic surfaces were bonded to one another, to appear (Figure 2b), with an inverse micelle-like internal structure. Larger micelle-like clusters were observed with an increase in 𝛼. Simultaneously, the number ratio of the micelle-like clusters to random aggregates increased with 𝛼, and random aggregates were not observed for 𝛼 > 0.27. When the sizes of small clusters exceeded about 15 particles, the clusters exhibited unidirectional anisotropy in shape, and clusters with clear rod-like shapes appeared at 𝛼 > 0.16 (Figure 2c–e). With further increases in 𝛼, almost all clusters became rod-like and their widths slightly increased, as shown in Figure 2c–e. For 𝛼 > 0.39, spherical emulsion droplets (colloidosomes) with a water inner phase appeared. In Figure 2J, AJP can be seen attached to the water-oil interface, reflecting their amphiphilic Janus structure and forming a monolayer. With an increase in 𝛼, the volume ratio and size of the spherical emulsion droplets increased. For 𝛼 ≥ 0.66, finally, the system formed an emulsion, in which almost all structures were spherical droplets (Figure 2g–j). A diagram of the 𝛼-ranges for each of the characteristic structures is shown in Figure 2k. At the intermediate concentration where rod-like micelles and spherical droplets coexisted, a structure was observed with a thickness that differed somewhat from other parts (Figure 2f). This might have been an intermediate structure between rod-like and spherical droplets. The reason for the increase in thickness of the rod-like clusters observed in Figure 2c–e is not only that more particles attached to the cluster to eliminate hydrophilic surfaces exposed to the external environment, which is apparent by comparing Figure 2c and d; but also that the rods slightly swelled because of the inner


7

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 26

water phase. However, the inner structure of micelle-like clusters has not been observed in detail at this stage, and the way in which thickness changed in Figure 2f and the existence of swelling remain uncertain. There is similarity between the observed structural changes induced by increasing the water content for the AJP and that known for surfactant molecules. At low water content, the formed clusters were similar to the spherical and rod-like micelles that occur in a binary system of surfactant and solvent. At 𝛼 ≳ 1, where clusters can swell, emulsions composed of spherical droplets appeared, which are well-known in (micro)emulsions of surfactant systems. However, the particles showed no thermal motion in the clusters and emulsion droplets, indicating much larger interparticle and particle-interface interaction compared with thermal agitation. Thus, the structures with AJP did not relax thermally, i.e., they are trapped deeply within metastable states, which differs from the dynamic equilibrium structures found in surfactant systems including microemulsions. Such a metastability has often been observed in mesoscopic particle systems, such as in PEs formed with homogeneous particles1. We therefore consider that the continuous change in the ratio of respective structures with the increase of 𝛼 was predominantly caused by stochastic and/or kinetic formation of metastable structures and cannot be related directly to the transition between equilibrium states. It would be noteworthy that the emulsion droplets can be regarded as swollen spherical micelles of a microemulsion when the emulsion with AJP is (almost) at equilibrium, though it would not be the case here as discussed later.


8

Page 9 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Figure 2. 𝛼-Dependence of the morphology in self-assembled structures. (a) to (i) are optical microscope images of typical structures formed at respective 𝛼. (a) A random aggregate. (b) A small micelle-like cluster. (c) – (e) Rod-shaped micelle-like clusters. (f) A structure observed at a value of 𝛼 where rod-shaped micelle-like clusters and spherical droplets coexist. (g) – (i) Spherical droplets in emulsions. (i) A hemispherical droplet attached to the bottom of the observation cell. (j) A magnified image of the framed region in (i). (k) A diagram of the 𝛼-range of the observed structures. The scale bars are 5 m in (a) and (b), 10 m in (c) to (h) and (j), 50m in (i).


9

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 26

In the following, we discuss the formation mechanism of micelle-like clusters observed at low water content. From the microscopic observation of randomly-sampled small clusters composed of two or three particles for 𝛼 < 0.27 (Figure 3a), the ratio of bonds that were not formed between hydrophilic surfaces is large at 𝛼 = 0.09, whereas most bonds were formed between hydrophilic surfaces at 𝛼 = 0.27. Figure 3b shows the orientation of hydrophobic surfaces in typical clusters over the 𝛼-range. With an increase in 𝛼, the hydrophobic surfaces of the particles tended to face outwards from the cluster, i.e., forming inverse micelle-like structures enclosing the hydrophilic surfaces, as observed in Figure 2. In inverse micelles formed with surfactant molecules in apolar solvents, the condensation force arises from the attraction between the hydrophilic parts of the molecules (or the repulsion between hydrophilic parts and apolar solvents, or hydrophobic parts). For AJP in a polar solvent, the formation of micelle-like clusters through attraction between hydrophobic surfaces has been also reported.37,

38

However, because the bonds between

hydrophilic surfaces increased with 𝛼 in this study (Figure 3a), the formation of clusters cannot be explained simply by the attraction between hydrophilic surfaces in an apolar solvent but rather that water induces the selective attraction between hydrophilic surfaces. We speculate that this attractive force acts through a capillary bridge of a water droplet between hydrophilic surfaces, as shown in Figure 3c. In the mixture of homogenous particles and two immiscible liquids, it is known that the minority liquid phase that is wetted by the particles “glues” them together by the strong attraction between them.39-42 The capillary force is typically rather stronger than the other interparticle interactions such as van der Waals and electrostatic force in colloidal systems.39, 40 In our system, the capillary force between two silica surfaces in contact 𝐹c = 2𝜋𝑎𝛾 cos 𝜃 ≈ 400 nN, where the surface tension between water and n-dodecane at


10

Page 11 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

room temperature 𝛾 = 53 mN ∙ m43 and the contact angle of water in dodecane on a silica surface 𝜃 ≈ 40°.10 The typical value for the van der Waals force 𝐹vdW = 𝐴𝑎/12𝑠 2 ≈ 1 nN, where the typical interparticle distance 𝑠 = 1 nm is given by assuming the surface roughness in 1 nm and Hamaker’s constant 𝐴 ≈ 10−20 N ∙ m for silica particles in water. The force between gold gold

hemispheres 𝐹vdW = 𝐴𝑎/12𝑠 2 ≈ 50 nN, where 𝐴 ≈ 40 × 10−20 N ∙ m. Thus, the capillary force must be generally rather larger than the van der Waals force in our system, because the former does not significantly depend on the droplet volume (see Supporting Information). During the structure formation process, water was initially broken into small-diameter droplets ≲ 1 m. Water partially wets the silica surface in n-dodecane, thus the droplets would cover a relatively small area of the hydrophilic silica surface at a low water content (e.g. 𝛼 = 0.09, see the left image in Figure 3d). The number of possible capillary bridges between hydrophilic surfaces is therefore relatively small under such conditions, resulting in a low ratio of hydrophilichydrophilic bonding. With increasing water content, a larger area on the hydrophilic surface can be covered with water droplets (the right image in Figure 3d), and hydrophilic-hydrophilic bonding becomes predominant. In addition, experimental37, 38 and theoretical44 studies have reported that a rod-like structure is most stable when a hemisphere of Janus particles aggregates through a shortrange attraction. This explains the formation of the rod-shaped micelle-like clusters when hydrophilic-hydrophilic bonding becomes predominant with increasing 𝛼.


11

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 26

Figure 3. Bonding between AJP in micelle-like clusters. (a) The ratio of bonds between hydrophilic hemispheres (silica surfaces) to the total bonds in small clusters. The number of analyzed particles were 30, 28 and 19, respectively, for 𝛼 = 0.09, 0.16, and 0.27. (b) Orientation of the hydrophilic surfaces. The broken lines represent the outlines of the particles. The arrow indicates the direction of the center of a hydrophobic hemisphere in two-dimensions. (c) A schematic of Janus particles bonded by a capillary bridge between their hydrophilic surfaces. (d) Schematics of particles wetted by water droplets on their hydrophilic surfaces.

Finally, we discuss the structure of emulsion droplets formed at high water content. The size of the droplets apparently increases with water content (Figure 4a). For 𝛼 > 0.66, where almost


12

Page 13 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

all the structures in the system were spherical droplets, their standard deviation and average radius 𝑟̅ (Figure 4b), and the distribution of their radii (Figure 4c) were obtained by image analysis. The definition of r is given in the schematic shown in Figure 4b. The microscope images of the droplets shown in Figure 4a include the particles attached to the interface and thus the observed radius r* is given as r* = r + a. 𝑟̅ was almost proportional to 𝛼, as shown in Figure 4b. The size distribution was narrow at all values of 𝛼, 0.66 – 8.28, as shown in Figure 4b and c. The average of coefficient of variation over the 𝛼-range was 24%. The linear relationship between 𝑟̅ and 𝛼, i.e., 𝑟̅ ∝ 𝑉𝑊 and 𝑟̅ ∝ 𝑉𝑝 −1 , appears to have arisen from the limited coalescence upon PE formation11 as follows. It is assumed that the minority liquid phase was initially fully broken into tiny droplets, and all particles became attached irreversibly to the liquid–liquid interface during the coalescence process. The droplets then coalesced to each other until the layer of attached particles became sufficiently dense to kinetically prevent their fusion. Thus, the total surface area of droplets coincided with the surface area that particles could possibly cover, resulting in the proportionality mentioned above. The surface area density of droplets 𝑆(𝑟) followed the long-normal distribution (see the inset in Figure 4c), which also appears in the case of limited coalescence11, 21: 𝑆(𝑟) ∝

1 √2𝜋 𝜎𝑟

𝑒 −(ln 𝑟−𝑀)

2 ⁄2𝜎2

, where M and 

are the parameters whose combinations are related to the mean and variance of the distribution.


13

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 26

Figure 4. Dependence of emulsion droplet size on 𝛼. (a) Typical optical microscope images of spherical droplets formed at 𝛼 = 0.85, 2.99, 4.96 and 7.44. (b) Dependence of average radius of droplets 𝑟̅ on 𝛼. The error bars represent their standard deviations. The solid line is the best-fit by eq. (1). The inset is a schematic of a cross-sectional view of a spherical droplet. The radius of the droplet r is measured from its center to the liquid–liquid interface, which is at the middle of the attached particles. (c) The distribution of the radius of droplets at various 𝛼. The plots are normalized by their peak heights. The inset is the distribution plotted as a surface area ratio, i.e., surface area density, at 𝛼 = 5.80. The broken line is the best-fit to a log-normal distribution.


14

Page 15 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

We compare our experimental results with the following model, which is based on limited coalescence and takes into account the volume of the attached particles (the inset in Figure 4b). We assume that (i) droplets are monodisperse (𝑟̅ = 𝑟 in this model), (ii) the influence of curvature of the droplets can be ignored, (iii) the boundary between hydrophilic and hydrophobic surfaces of AJP lies at the water–dodecane interface, and (iv) the interfacial coverage of AJP, C, which is defined as the ratio of the area covered by particles and the lateral surface area of a droplet, 4π𝑟 2 , is constant. The resultant relation between r and 𝛼 is therefore 𝑟 = 4𝑎𝐶𝛼 + 2𝑎𝐶,

(1)

where a = 1.5 m (see Supporting Information for the derivation). The experimental result of 𝑟̅ can be fitted well by eq. (1) using Cfit = 0.60 as a fitting parameter (Figure 4b). Together with the log-normal distribution of 𝑆(𝑟), the emulsification in our experiment can be explained well by limited coalescence. A requirement for the process, strong agitation for mixing, was observed in practice, as mentioned in the experimental methods section. The value of Cfit obtained is smaller than that for the close-packing of spheres, Ccp = 0.91. The interfacial coverages obtained directly by microscopy observation of the droplets surface, Cexp, were 0.61 – 0.71 for 𝛼 = 4.14 – 7.44 (Figure 5a), respectively. The reason that Cfit and Cexp are smaller than Ccp is probably that the anisotropic interaction between particles originated from a deformation of the liquid-liquid interface. The small undulation of the boundary between hydrophilic and hydrophobic surfaces on an amphiphilic Janus particle (cf. the inset in Figure 1b) deforms the interface in the vicinity of the attached particle and induces anisotropic interaction between the particles.45 This interaction is also referred to as capillary interaction; however, the geometry of particles was different from that for capillary bridges described above. In Ref. 45 Park


15

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 26

et al. reported the formation of a loosely packed layer of AJP at a water-oil interface in a similar system. This anisotropic interaction would hinder the closest packing of AJP on the droplet surfaces. In addition, the slightly smaller values of Cfit than Cexp was caused by the size distribution of droplets (Figure 4c). Considering the size distribution, the total surface area of droplets was estimated for 𝛼 = 4.14 – 7.44 by approximating the experimentally obtained surface area density to a log-normal distribution (cf. the inset in Figure 4c). These re-calculated total surface areas for respective 𝛼are smaller by 5% on average than those estimated by assuming a constant radius, 𝑟̅, for the same total volume of droplets. The corrected values of Cfit with the re-calculated total disp

surface areas, 𝐶fit , approach Cexp and agree within statistical errors (Figure 5a). The linear relationship observed between r and 𝛼 in Figure 4b simply indicates that the total surface area of droplets was proportional to the total number of particles, and there was no direct relationship between the area and the number of the particles that actually attach to the droplet surfaces. Hence, we estimated the total volume of the attached AJP at respective 𝛼 in the experiment from the surface coverage Cexp, the distribution of droplet radius obtained by observation (Figure 4c), and the volume of the water added to the sample. These values estimated from experimental results show good agreement with those of the particles added during sample preparation (Figure 5b). This agreement indicates that almost all particles directly contributed to the stabilization of the droplets. Emulsions are formed even at fairly low values of 𝛼, especially 𝛼 < 1, where the volume of water is smaller than that of the particles, as described above. This result also indicates the high surface activity of the AJP. However, the influence of the curvature of a droplet cannot be negligible at this region of 𝛼,46 and it is necessary to explicitly consider the arrangement of the


16

Page 17 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

individual particles and shape of the liquid–liquid interface. Elucidating the mechanism behind the formation of emulsions for a small quantity of the minority liquid phase, including the transition from rod-like clusters to spherical droplets, is left for future studies. In the above discussion, we quantitatively showed that AJP exhibit excellent surface activities to stabilize droplets where all the particles contribute to the stabilization after the limited coalescence process. Although these features are known for PEs of homogenous particles, AJP possess advantages, as described in the Introduction. The elucidation of AJP emulsification from the lower limit of the minority liquid phase ratio in this study would therefore contribute to their application. The agreement of the emulsion state with the limited coalescence model shown e.g. by the log-normal distribution in Figure 4c suggests that the droplets are kinetically stabilized in this experiment and thus they do not correspond to swollen micelles of a microemulsion. The equilibrium emulsion state is also difficult to be achieved by such a low interfacial coverage of particles at the droplet surface. In addition, because a spherical droplet (colloidosome) has interstitial space between the particles even at the closest surface packing state (Ccp = 0.91), the structure is applicable to permeable microcapsules. Our results suggest the possibility of controlling the surface coverage of the colloidosome, i.e., control of porosity using the undulation of the boundary between hydrophilic and hydrophobic surfaces of an amphiphilic Janus particle, which would also be useful for controlling permeability.


17

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 26

Figure 5. Coverage of AJP on emulsion droplets. (a) Surface coverage of AJP obtained from microscopic observation, 𝐶exp , from fitting eq. (1), 𝐶fit , and the corrected values considering disp

droplet size distribution, 𝐶fit . The numbers of analyzed droplets for the 𝐶exp data corresponding to 𝛼= 4.14, 4.96, 5.80, 6.60 and 7.44 are, respectively, 13, 14, 16, 14 and 8. (b) Comparison between the amounts of AJP attached to the droplet surface estimated from microscopic observation and those in a sample, i.e., the amounts added during sample preparation. In this range of 𝛼, Vp monotonously decreases because we increased 𝛼 by decreasing the volume of particles Vp.


18

Page 19 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Summary and Conclusions In this study, we observed the internal structures in a ternary system composed of amphiphilic Janus particles comprising hydrophilic and hydrophobic hemispheres, water and oil by varying the ratio of the minority liquid phase (water) to the particles over two orders of magnitude. With increasing water content, the self-assembled structures showed a transition from micelle-like clusters formed by anisotropic interaction between AJP to emulsions where spherical droplets were stabilized by the surface activity of AJP. At low water content, the strong attraction due to capillary interactions induced by tiny water droplets selectively appeared between the hydrophilic hemispheres, forming (inverse) micelle-like clusters. When the clusters were large, the shape of the structures became rod-like, reflecting the symmetric Janus structure of the particle. When the water content was large enough to fill the interstices between hydrophilic surfaces, i.e., the volumes of AJP and water became equivalent, droplets covered with AJP (colloidosome) were formed. This is the first experimental study that systematically exploits the remarkable change in self-assembled structures in the ternary system of AJP-water-oil through changes in composition. We have demonstrated that the chemical duality, i.e. amphiphilicity, of mesoscopic particles induces self-assembly which is qualitatively the same as that in microemulsions of surfactant molecules. On the other hand, there is an essential difference due to the surfactant’s size: The mesoscopic size of AJP makes the interparticle and particle-interface interaction much larger than thermal agitation. The self-assembly in AJP system is therefore irreversible and the structures are kinetically, not thermodynamically, stabilized, being different from dynamic equilibrium structures in microemulsions. In addition, our study clearly shows agreement between experiment


19

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 26

and a simple theoretical model for the dependence of emulsion droplet size on the composition of the system. The latter has previously been studied experimentally for homogenous particles11 but only theoretically for non-spherical AJP.21 Our experimental results clearly show that AJP have the characteristics of both surfactant molecules and homogenous colloids as emulsifiers. The formation of micelle-like clusters is useful to homogenously disperse the particles while avoiding large agglomerate formation, as a preliminary step for emulsification. The emulsification of AJP is explained by limited coalescence, as in the case for homogenous particles. This study provides fundamental understanding of the use of AJP as a superior emulsifier compared with homogeneous particles. The elucidation of the basic mechanism behind structure formation of AJP is also expected to be useful in understanding emulsification by anisotropic biomolecules such as proteins.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (Y.I.) Author Contributions T.N. performed the experiments and analyzed the results. T.N. and Y.I. jointly conceived and designed the experiments. All authors jointly wrote the paper. Notes The authors declare no competing financial interest.


20

Page 21 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

ACKNOWLEDGEMENTS The authors gratefully thank the Center of Advanced Instrumental Analysis, Kyushu University for scanning electron microscope observations. T.N. acknowledges financial support from QREC Academic Challenge 2013 and Y.I. to JSPS KAKENHI under grant no. 23740319 and Kurata Grants. ABBREVIATIONS PE Pickering emulsion; AJP amphiphilic Janus particles. REFERENCES (1) Binks, B. P. Particles as surfactants-similarities and differences. Curr. Opin. Colloid In. 2002, 7, 21-41. (2) Chevalier, Y.; Bolzinger, M. A. Emulsions stabilized with solid nanoparticles: Pickering emulsions. Colloids and Surfaces A: Physicochem. Eng. Aspects 2013, 439, 23-34. (3) Zeng, C; Bissig, H.; Dinsmore, A. D. Particles on droplets: From fundamental physics to novel materials. Solid State Commun. 2006, 139, 547-556. (4) Pickering, S. U. CXCVI.-Emulsions. J. Chem. Soc., Trans. 1907, 91, 2001-2021. (5) Aveyard, R.; Clint, J. H.; Horozov, T. S. Aspects of the stabilisation of emulsions by solid particles: Effects of line tension and monolayer curvature energy. Phys. Chem. Chem. Phys. 2003, 5, 2398-2409. (6) Binks, B. P.; Lumsdon, S. O. Influence of Particle Wettability on the Type and Stability of Surfactant-Free Emulsions. Langmuir 2000, 16, 8622-8631.


21

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 26

(7) Dickinson, E. Food emulsions and foams: Stabilization by particles. Curr. Opin. Colloid In. 2010, 15, 40–49. (8) Tcholakova, S.; Denkov, N. D.; Lips, A. Comparison of solid particles, globular proteins and surfactants as emulsifiers. Phys. Chem. Chem. Phys. 2008, 10, 1608–1627. (9) Murray, B. S.; Ettelaie, R. Foam stability: proteins and nanoparticles. Curr. Opin. Colloid In. 2004, 9, 314–320. (10) Aveyard, R.; Binks, B. P.; Clint, J. H. Emulsions stabilised solely by colloidal particles. Adv. Colloid Interfac. 2003, 100, 503-546. (11) Arditty, S.; Whitby, C. P.; Binks, B. P.; Schmitt, V.; Leal-Calderon, F. Some general features of limited coalescence in solid-stabilized emulsions. Eur. Phys. J. E 2003, 11, 273-281. (12) Abismaïl, B.; Canselier, J. P.; Wilhelm, A. M.; Delmas, H.; Gourdon, C. Emulsification by ultrasound: drop size distribution and stability. Ultrasonics Sonochemistry 1999, 6, 75-83. (13) Perro, A.; Reculusa, S.; Ravaine, S.; Bourgeat-Lami, E.; Duguet, E. Design and synthesis of Janus micro-and nanoparticles. J. Mater. Chem. 2005, 15, 3745-3760. (14) Walther, A.; Müller, A. H. E. Janus particles. Soft Matter 2008, 4, 663-668. (15) Lattuada, M.; Hatton, T. A. Synthesis, properties and applications of Janus nanoparticles. Nano Today 2011, 6, 286-308. (16) Kumar, A.; Park, B. J.; Tu, F.; Lee, D. Amphiphilic Janus particles at fluid interfaces. Soft Matter 2013, 9, 6604–6617.


22

Page 23 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

(17) Casagrande, C.; Veyssié, M. "Janus Beads": realization and first observation of interfacial properties. C. R. Acad. Sci. Paris II 1988, 306, 1423-1425. (18) Binks, B. P.; Fletcher, P. D. I. Particles Adsorbed at the Oil-Water Interface: A Theoretical Comparison between Spheres of Uniform Wettability and “Janus” Particles. Langmuir 2001, 17, 4708-4710. (19) Glaser, N.; Adams, D. J.; Böker, A.; Krausch, G. Janus Particles at Liquid-Liquid Interfaces. Langmuir 2006, 22, 5227-5229. (20) Aveyard, R. Can Janus particles give thermodynamically stable Pickering emulsions? Soft Matter 2012, 8, 5233–5240. (21) Tu, F.; Park, B. J.; Lee, D. Thermodynamically Stable Emulsions Using Janus Dumbbells as Colloid Surfactants. Langmuir 2013, 29, 12679-12687. (22) Casagrande, C.; Fabre, P.; Raphaël, E.; Veyssié, M. "Janus Beads": Realization and Behaviour at Water/Oil Interfaces. Europhys. Lett. 1989, 9, 251-255. (23) Walther, A.; Hoffmann, M.; Müller, A. H. E. Emulsion Polymerization Using Janus Particles as Stabilizers. Angew. Chem. 2008, 120, 723-726. (24) Kim, J. W.; Cho, J.; Cho, J.; Park, B. J.; Kim, Y. J.; Choi, K. H.; Kim, J. W. Synthesis of Monodisperse Bi-Compartmentalized Amphiphilic Janus Microparticles for Tailored Assembly at the Oil–Water Interface. Angew. Chem. Int. Ed. 2016, 55, 4509-4513.


23

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 26

(25) Fujii, S.; Yokoyama, Y.; Miyanari, Y.; Shiono, T.; Ito, M.; Yusa, S.; Nakamura, Y. Micrometer-Sized Gold−Silica Janus Particles as Particulate Emulsifiers. Langmuir 2013, 29, 5457-5465. (26) Faria, J.; Ruiz, M. P.; Resasco, D. E. Phase-Selective Catalysis in Emulsions Stabilized by Janus Silica-Nanoparticles. Adv. Synth. Catal. 2010, 352, 2359-2364. (27) Tu, F.; Lee, D. Shape-Changing and Amphiphilicity-Reversing Janus Particles with pHResponsive Surfactant Properties. J. Am. Chem. Soc. 2014, 136, 9999-10006. (28) Passas-Lagos, E.; Schüth, F. Amphiphilic Pickering Emulsifiers Based on Mushroom-Type Janus Particles. Langmuir 2015, 31, 7749-7757. (29) Jiang, S.; Chen, Q.; Tripathy, M.; Luijten, E.; Schweizer, K. S.; Granick, S. Janus Particle Synthesis and Assembly. Adv. Mater. 2010, 22, 1060–1071. (30) Wu, D.; Chew, J. W.; Honciuc, A. Polarity Reversal in Homologous Series of SurfactantFree Janus Nanoparticles: Toward the Next Generation of Amphiphiles. Langmuir. 2016, 32, 6376–6386. (31) Israelachvili, J. N. Intermolecular and surface forces, 2nd. Ed.; Academic press Inc.: USA, 1991, 381. (32) Gompper, G.; Schick, M. Self-Assembling Amphiphilic Systems, Vol. 16; Domb, C., Lebowitz, J. L., Eds.; Phase Transitions and Critical Phenomena; Academic press: USA, 1994. (33) Komura, S. Mesoscale structures in microemulsions. J. Phys.: Condens. Matter 2007, 19, 463101.


24

Page 25 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

(34) He, J.; Hourwitz, M. J.; Liu, Y.; Perez, M. T.; Nie, Z. One-pot facile synthesis of Janus particles with tailored shape and functionality. Chem. Commun. 2011, 47, 12450–12452. (35) Iwashita, Y.; Kimura, Y. Stable cluster phase of Janus particles in two dimensions. Soft Matter 2013, 9, 10694–10698. (36) Evers, C. H. J.; Luiken, J. A.; Bolhuis, P. G.; Kegel, W. K. Self-assembly of microcapsules via colloidal bond hybridization and anisotropy. Nature 2016, 534, 364–368. (37) Chen, Q.; Whitmer, J. K.; Jiang, S.; Bae, S. C.; Luijten, E.; Granick, S. Supracolloidal Reaction Kinetics of Janus Spheres. Science 2011, 331, 199-202. (38) Hong, L.; Cacciuto, A.; Luijten, E.; Granick, S. Clusters of Amphiphilic Colloidal Spheres. Langmuir 2008, 24, 621-625. (39) Seville, J. P. K.; Willett, C. D.; Knight, P. C. Interparticle forces in fluidisation: a review. Powder Technol. 2000, 113, 261-268. (40) Koos, E.; Willenbacher, N. Capillary Forces in Suspension Rheology. Science 2011, 331, 897-900. (41) Herminghaus, S. Dynamics of wet granular matter. Adv. Phys. 2005, 54, 221-267. (42) Gögelein, C.; Brinkmann, M.; Schröter, M.; Herminghaus, S. Controlling the Formation of Capillary Bridges in Binary Liquid Mixtures. Langmuir 2010, 26, 17184-17189. (43) Zeppieri, S.; Rodríguez, J.; Ramos, A. L. L. Interfacial Tension of Alkane + Water Systems. J. Chem. Eng. Data 2001, 46, 1086-1088.


25

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 26

(44) Miller, W. L.; Cacciuto, A. Hierarchical self-assembly of asymmetric amphiphatic spherical colloidal particles. Phys. Rev. E 2009, 80, 021404. (45) Park, B. J.; Brugarolas, T.; Lee, D. Janus particles at an oil–water interface. Soft Matter 2011, 7, 6413–6417. (46) Hirose, Y.; Komura, S.; Nonomura, Y. Adsorption of Janus particles to curved interfaces. J. Chem. Phys. 2007, 127, 054707.

Table of Contents Graphic

Volume of water Micelle-like cluster Amphiphilic Janus particle

small

unidirectional growth

Emulsion droplet (colloidosome)

hydrophobic hydrophilic

Capillary bridge of water

water


26

Particle Volume Ratio

Recommend Documents