Gas Bubbles Stabilized by Janus Particles with Varying Hydrophilic

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Gas bubbles stabilized by Janus particles with varying hydrophilic-hydrophobic surface characteristics Syuji Fujii, Yuichi Yokoyama, Saori Nakayama, Masanori Ito, Shin-ichi Yusa, and Yoshinobu Nakamura Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b02670 • Publication Date (Web): 05 Oct 2017 Downloaded from http://pubs.acs.org on October 9, 2017

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Gas bubbles stabilized by Janus particles with varying hydrophilic-hydrophobic surface characteristics Syuji Fujii1,2*, Yuichi Yokoyama3, Saori Nakayama3, Masanori Ito4, Shin-ichi Yusa4, Yoshinobu Nakamura1,2

1

Department of Applied Chemistry, Faculty of Engineering, Osaka Institute of Technology, 5-16-1 Omiya, Asahi-ku, Osaka 535-8585, Japan. 2

Nanomaterials Microdevices Research Center,

Osaka Institute of Technology, 5-16-1 Omiya, Asahi-ku, Osaka 535-8585, Japan. 3

Graduate School of Engineering, Osaka Institute of Technology, 5-16-1 Omiya, Asahi-ku, Osaka 535-8585, Japan. 4

Graduate School of Engineering, University of Hyogo, 2167 Shosha, Himeji, Hyogo 671-2280, Japan.

* Corresponding author ([email protected])

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Abstract Micrometer-sized polymer-grafted gold-silica (Au-SiO2) Janus particles were fabricated by vacuum evaporation followed by polymer grafting. The Janus particle diameter, diameter distribution, morphology, surface chemistry and water wettability were characterized

by

optical

microscopy,

scanning

electron

microscopy,

X-ray

photoelectron spectroscopy and contact angle measurements. The optical microscopy results showed that the polystyrene (PS)-grafted Au-SiO2 Janus particles exhibited monolayer adsorption at the air-water interface and could stabilize bubbles, preventing their coalescence for more than one month. The hydrophobic PS-grafted Au and hydrophilic SiO2 surfaces were exposed to the air and water phases, respectively. Bare Au-SiO2 and poly(2-(perfluorobutyl)ethyl methacrylate) (PPFBEM)-grafted Au-SiO2 Janus particles could also stabilize bubbles for up to two weeks. By contrast, bare silica particles did not stabilize bubbles and were dispersed in water. The bubbles formed in the PS-grafted Janus particle system were more stable than those formed in the bare Au-SiO2 Janus particle, PPFBEM-grafted Au-SiO2 Janus particle and SiO2 particle systems because of the high adsorption energy of the PS-grafted particles at the air-water interface.

Keywords: Janus particle · adsorption · bubble · interface · contact angle

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Introduction

It is well known that finely dispersed solid particles can stabilize liquid foams and bubbles by adsorbing at the gas-liquid interface1-8. Stable liquid foams/bubbles have been obtained using synthetic organic particles9-12, inorganic particles13-16 and particles in Nature17,18. Generally, particulate foam/bubble stabilizers should have more robust formulations and lower toxicity profiles than conventional molecular surfactants7. These stabilizers are likely to become more widely used in the future because of the importance of foams in various industrial applications, such as food and cosmetics. Increasing interest has been focused on colloidal particles that exhibit distinctly different chemical/physical surface properties (Janus particles) because of their anisotropic hydrophilic-hydrophobic balance and/or optical, electronic, and magnetic properties19-25. Because of their unique properties, Janus particles have been used in various applications such as self-motile colloidal materials26,27, compatibilizers for polymer blends28, imaging probes29,30, surface modifiers31 and particulate emulsifiers32-38. These colloidal particles have been synthesized by numerous techniques, including phase separation39,40, self-assembly41,42, masking43,44, microfluidics45,46, and the crushing of hollow spheres37. The adsorption of Janus particles at interfaces has attracted increasing attention47,48. Solid Janus particles have been shown to be effective emulsifiers; for example, Pickering-type emulsions obtained using Janus particles were more stable than those obtained using solid particles with homogeneous surfaces32-38,49. In contrast to particles with uniform surface wettability, Janus particles can be fabricated with two distinct surface regions that exhibit different wettabilities. Therefore, the Pickering effect and the


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amphiphilicity of a conventional molecular surfactant can be combined in Janus particles, which results in their considerably higher surface activity. Binks and Fletcher50 theoretically predicted that the adsorption of an amphiphilic Janus particle at an oil-water interface would be stronger than that of a particle with homogeneous surface wettability. It was then experimentally confirmed that Janus particles exhibit higher surface activity than particles with homogeneous surfaces36,38,51. Despite significant interest in the stabilization of Pickering-type emulsions by Janus particles, few studies of foam/bubble stabilization by Janus particles have been reported. Schröder et al.48 attempted to use surface-active Janus particles that can stabilize emulsions to stabilize bubbles in water; however, they did not obtain stable bubbles. In another study, Skelhon et al.52 indicated that Janus particles can stabilize bubbles but gave few/no experimental details or characterization results. This omission and other unsuccessful results are surprising because foams/bubbles and emulsions have similar properties, e.g., the gas can be thought of as a highly hydrophobic fluid. In this study, micrometer-sized gold-silica (Au-SiO2) Janus particles modified with polymer hairs were fabricated by Au vacuum evaporation, followed by the grafting of polymers with different hydrophobicities to the Au surface. The ability of the resulting Janus particles to stabilize bubbles in water was intensively studied. The polymers, namely, polystyrene (PS) and poly(2-(perfluorobutyl)ethyl methacrylate) (PPFBEM), were grafted via thiol-Au interactions using a grafting-to approach. Because Au can strongly scatter visible light more effectively than SiO2, the locations of the SiO2 and Au regions at the air-water interface could be easily observed in the wet state by optical microscopy (OM). The effects of the hydrophilic-hydrophobic balance of the Au side on the self-assembly of the Janus particles in aqueous media and on bubble formation and


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stability were investigated. The stability, size, shape and morphology of the bubbles were rigorously characterized by OM and scanning electron microscopy. The bubble stability was evaluated based on the adsorption energy of the Janus particles at the air-water interface.


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Experiments

Materials Isopropanol (IPA; purity 99%) and tetrahydrofuran (THF; purity 99.0%) were purchased

from

Sigma-Aldrich

2-(perfluorobutyl)ethyl

methacrylate

(MO,

USA).

(PFBEM),

1-Butanol

(purity

99%),

tris(2-carboxyethyl)phosphine

hydrochloride (TCEP; purity > 93%), and 1-butylamine (purity > 98%) were purchased from Wako (Osaka, Japan) and used as received. Styrene (purity > 99%, Wako) was washed with an aqueous alkaline solution and distilled from calcium hydride under reduced pressure. The SiO2 particles (6 µm, Hipresica, Ube-Nitto Kasei Co., Tokyo, Japan) were centrifugally washed with IPA three times and dried under reduced pressure for 3 days before use. Deionized water (< 0.06 µS/cm, Advantec MFS RFD240NA: GA25A-0715, Advantec, Osaka, Japan) was used. Gold wire (0.50 mm diameter, purity 99.95%) was purchased from Nilaco Co. (Tokyo, Japan) and used as received. 2.2′-Azobis(isobutyronitrile) (AIBN; purity > 98%, Wako) was recrystallized from methanol. 1-Phenylethyl phenyldithioacetate (PEPD)53 and 4-cyanopentanoic acid dithiobenzoate (CPD)54 were prepared according to the literature methods.

Au-SiO2 Janus particle fabrication Au-SiO2 Janus particles were fabricated by a previously reported vacuum evaporation method38. Briefly, two-dimensional (2D) colloidal SiO2 crystals were prepared by dispersing the SiO2 particles (10 wt%) in 1-butanol and then forming a droplet of the dispersion at the end of a pipette. The droplet was carefully brought into contact with water on a glass substrate (Matsunami, 76 mm × 26 mm). The system was allowed to sit


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for 3 days to completely evaporate the water and 1-butanol, resulting in a 2D colloidal crystalline SiO2 monolayer on the glass substrate. A thin Au layer (100 nm thick) was vacuum-deposited on the 2D colloidal crystal using a super-mini vacuum coater (SVC-700, Sanyu Denshi Co., Ltd., Nagoya, Japan) equipped with a rate/thickness monitor (SQM-160, INFICON, Yokohama, Japan). The Janus particles were released from the glass substrate by sonication (Bransonic 221, Yamato, Tokyo, Japan) in water.

Thiol-terminated polymer synthesis Thiol-terminated polystyrene (PS) and poly(2-(perfluorobutyl)ethyl methacrylate) (PPFBEM) were synthesized by reversible addition−fragmentation chain-transfer (RAFT) using PEPD and CPD, respectively, as the chain transfer agents (CTAs) (Figures 1 and S1-S3, Supporting Information). Styrene (75.0 g, 720 mmol), PEPD (0.245 g, 0.900 mmol), and AIBN (29.6 mg, 0.180 mmol) were mixed in a flask, and the mixture was then deoxygenated by an Ar purge for 30 min. Polymerization was subsequently performed at 70°C for 6 h, and an aliquot of the sample was removed at the end of the experiment to estimate the conversion (16.5%) by 1

H NMR. The polymerization mixture was diluted with THF and then purified by

Figure 1 Thiol-terminated polymers grafted to the Au side of the Janus particles to modify their hydrophilic-hydrophobic balance.


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reprecipitation from THF in a large excess of methanol. The polymer (PS-PEPD) was dried in a vacuum oven at 30°C for 12 h (9.74 g, 12.7%). The PS-PEPD number-average molecular weight (Mn(GPC)) and molecular weight distribution (Mw/Mn) were estimated to be 13,490 and 1.27, respectively, by gel-permeation chromatography (GPC). The PS-PEPD DP and Mn(cal) values estimated from the conversion were 127 and 13,501, respectively. PS-PEPD (0.4 g, 0.0297 mmol) was dissolved in THF (4.0 mL), and 1-butylamine (110 mg, 1.50 mmol) and TCEP (3.71 mg, 0.015 mmol) were subsequently added to the solution, which was then stirred at room temperature for 1 h under Ar. The reaction mixture was poured into a large excess of methanol to precipitate thiol-terminated PS (PS-SH). The PS polymer (0.28 g, 70.0%) was dried in a vacuum oven at 30°C for 12 h (Supporting Information). PFBEM (3.00 g, 9.06 mmol), CPD (25.3 mg, 0.091 mmol), and AIBN (7.44 mg, 0.045 mmol) were dissolved in THF (10.0 mL). The resulting mixture was deoxygenated by an Ar purge for 30 min, and the polymerization reaction was subsequently performed at 60°C for 24 h. After polymerization, a sample aliquot was removed to estimate the conversion (94.0%) by

1

H NMR. The polymerization mixture was purified by

reprecipitation from THF in a large excess of methanol twice. The obtained polymer (PPFBEM-CPD, 1.92 g, 63.5%) was dried in a vacuum oven at 30°C for 12 h. The PPFBEM-CPD DP and Mn(cal) values estimated from the conversion were 94 and 31,401, respectively. The PPFBEM-CPD Mn(GPC) and Mw/Mn values determined by GPC were 13,700 and 1.20, respectively. The Mn(NMR) value estimated from the 1H NMR results was 31,779. PPFBEM-CPD (1.60 g, 0.0483 mmol) was dissolved in THF (16.0 mL), and 1-butylamine (176 mg, 2.42 mmol) and TCEP (6.92 mg, 0.024 mmol) were subsequently added to the solution, which was then stirred at room temperature for 1 h under Ar. The


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reaction mixture was poured into a large excess of methanol to precipitate thiol-terminated PPFBEM (PPFBEM-SH), which was subsequently dried in a vacuum oven at 30°C for 12 h (0.988 g, 61.8%) (Supporting Information).

Polymer grafting on Janus particles The thiol-terminated polymers were immobilized on the Au side of the Janus particles by the following procedure: dried Au-SiO2 Janus particles (10 mg) were mixed with a polymer solution in THF (1.0 wt%, 0.4 mL) by handshaking, and the dispersion was then allowed to sit at room temperature for 3 h. After the polymers adsorbed to the Au side of the Janus particles, the polymer-grafted Janus particles were sequentially washed with pure THF (5 times), IPA (5 times) and water (5 times) by gravity-assisted sedimentation-redispersion cycles to remove free polymers from the system.

Gel permeation chromatography (GPC) GPC analysis was performed at 40°C using a Shodex DS-4 pump and RI-101 refractive index detector. One Shodex KF-805L and three Shodex KF803L columns were connected in series. The THF eluent flow rate was 1.0 mL/min. The Mn and Mw/Mn values of the sample polymers were calibrated with standard PS samples.

1

H NMR

1

H NMR spectra were obtained with a Bruker DRX-500 spectrometer (MA, USA)

operating at 500 MHz.

UV-vis absorption


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UV-vis absorption spectroscopy was performed using a Jasco V-530 spectrophotometer (Tokyo, Japan) with a 1.0-cm path length quartz cell.

Characterization of the Au-SiO2 Janus particles and particle-stabilized bubbles Optical microscopy (OM) A droplet of a Janus particle aqueous dispersion was placed on a glass microscope slide and observed with an optical microscope (Shimadzu Motic BA200, Shimadzu Co., Kyoto, Japan) equipped with a digital system (Shimadzu Moticam 2000, Shimadzu Co., Kyoto, Japan). The bubbles in water were also observed in the same way. The degree of circularity was determined using the Motic Images Plus 2.2S software (Motic China Group Co., Ltd.).

Scanning electron microscopy Scanning electron microscopy (SEM; Keyence VE-8800, 1-12 kV, Osaka, Japan) studies were conducted using dried samples. The SiO2 and Janus particles were observed without Au sputter coating, and their number-average diameters (n = 100) were determined from the SEM images. The bubbles were observed in the same way after drying at 25 °C.

Contact angle measurements Water and oil droplets (10 µL) were placed on glass (Matsunami, 18 mm × 18 mm), polymer film and Au (with and without polymer grafting) substrates at 25°C, and the contact angles were measured 3 s later using an Excimer SImage02 apparatus (Kanagawa, Japan) (n=5). The polymer films were prepared by spin-coating


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thiol-terminated polymer (1.0 wt%) solutions in THF onto glass slides using a MS-A100 instrument (Mikasa Co., Ltd, Osaka, Japan) at 25 °C. The following spin-coating procedure was employed: 500 rpm for 2 s, acceleration to 2000 rpm in 5 s, 2000 rpm for 5 s, and finally deacceleration to 0 rpm in 5 s. The Au substrate consisted of an Au-coated glass slide (100 nm Au thickness) prepared by vacuum deposition. The glass substrate was cleaned by soaking it in 30 wt% aqueous NaOH for 60 min, thoroughly rinsing it with distilled water and then drying it at 25°C for 5 h. This substrate was used to model the SiO2 surface. The critical surface tensions of the bare and polymer-grafted Au surfaces were determined from the Zisman plots55.

X-ray photoelectron spectroscopy (XPS) The unmodified, Au-coated (100 nm thickness) and polymer-grafted Au-coated glass slides were analyzed by XPS. The samples were mounted on sample stubs using conductive tape. The XPS measurements were performed using an XPS spectrometer (Axis Ultra, Kratos Analytical Ltd., Manchester, UK) with a monochromatic Al Kα X-ray gun. The base pressure was < 1.0×10-8 Torr. Pass energies of 80 eV and 20 eV were employed for the survey and elemental core-line spectra, respectively. The atomic percentage compositions were determined from the high-resolution spectra using the manufacturer’s sensitivity factors.

Bubble preparation A Janus particle aqueous dispersion (1.00 g) was placed in a glass vessel (2 mL) with a screw cap and then shaken for 1 min at 2,500 rpm using a touch mixer (Tube Mixer Trio TM-1F, As One, Osaka, Japan). The solid contents of the bare Au-SiO2 and


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polymer-grafted Au-SiO2 particles were 2.0 wt% and 1.0 wt%, respectively. The bubbles were allowed to sit at 25 °C.


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Results and Discussion Synthesis of PS and PPFBEM The structures and properties of the obtained polymers are summarized in Figure 1 and Table 1. The Mn(cal) values were estimated from the molar ratio of the monomer to the chain transfer agent (CTA) and the conversion. The PS Mn(cal) and Mn(GPC) values were similar; however, the PPFBEM Mn(GPC) and Mn(cal) values differed. These results were probably due to the use of PS as the GPC calibration standard, i.e., the difference between the volume-to-mass ratios of the PS standard and PPFBEM. Unfortunately, the Mn(NMR) value of PS-PEPD could not be determined because the 1

H NMR signals of the CTA terminal phenyl groups completely overlapped with those

of the pendant phenyl groups (Figure S2). Therefore, the Mn(NMR) value of thiol-terminated PS-SH cannot be calculated based on the Mn(NMR) of PS-PEPD. By contrast, the Mn(NMR) value of PPFBEM-CPD was calculated from the integral intensity ratio of the 1H NMR signals at 7.4-8.0 and 2.8 ppm, which were attributed to the terminal dithioester protons and pendant methylene protons, respectively (Figure Table 1.

Molecular weights and their distribution data for thiol-terminated polymers used for modification

of hydrophilic-hydrophobic balance of Janus particles.

Mn (cal)

a)

1

Mn ( H NMR) Mw (GPC)

c)

Mw/Mn (GPC) DP e)

b)

d)

PS-SH

PPFBEM-SH

13,261

31,155

-

f)

31,500

13,400

13,000

1.23

1.26

127 g)

94 h)

a)

Theoretical molecular weight Number-average molecular weight determined using 1H NMR spectroscopy c) Weight-average molecular weight determined using GPC (UV detector) d) Polydispersity determined using GPC (UV detector) e) Degree of polymerization f) Cannot be determined by 1H NMR because the terminal protons were overlapped with those of repeating styrene units. g) Determined by GPC h) Determined by 1H NMR b)


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S3). The Mn(NMR) value of thiol-terminated PPFBEM-SH was calculated based on the

Mn(NMR) value of PPFBEM-CPD.

Au-SiO2 Janus particle fabrication and characterization The Au-SiO2 Janus particles were fabricated by vacuum evaporation (Figure S4). The number-average diameter (Dn) was measured to be 5.8±0.1 µm (Dw/Dn = 1.001, n=70) from the SEM images. This value is nearly identicaly to that of the original SiO2 particles (Dn = 5.8±0.1 µm, Dw/Dn = 1.001, n=70). It should be noted that the Au layer thickness on the SiO2 particles was 100 nm, which is approximately 60 times smaller than the Dn value of the SiO2 particles. These results are consistent with those reported in a previous study38. Several reports of the synthesis of hairy Janus particles have appeared in the literature. Although the hairy Janus particles were synthesized by a grafting-from method in some of these studies36,56, few reports described the synthesis of these Janus particles by a

grafting-to method. Recently, we fabricated Au-SiO2 Janus particles with poly(sodium 6-acrylamidohexanoate) grafted to the Au side by a grafting-to method57. In this study, hydrophobic thiol-terminated polymers, namely, PS-SH and PPFBEM-SH (Figure 1), were grafted to the Au side of the Au-SiO2 Janus particles via thiol-Au interactions, and the resulting hairy Janus particles were used to stabilize bubbles. To investigate the hydrophilic-hydrophobic properties of the SiO2 and polymer grafted-Au surfaces, static contact angle measurements were conducted (Figure S5). Glass slides with and without an Au coating (100 nm thickness) were used as model surfaces for Au and SiO2, respectively. Polymer-grafted Au surfaces were fabricated under the same conditions as those used to fabricate the Au-SiO2 Janus particles. For the glass slide and


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Au surface, the water contact angles were 5±1° and 80±3°, respectively, and the contact angles of n-dodecane droplets were 3±1° and 2±1°, respectively. These results indicated that the Au surface is more hydrophobic than the glass surface. It is known that metal surfaces with high surface energies are easily contaminated by organic compounds existing in air under ambient conditions, because contaminant adsorption decreases the surface energy58-62. It should be noted that the contact angle measurements on the Au surface were conducted 72 h after the Au coating was applied. When the Au surface was removed from vacuum, it was immediately contaminated by exposure to air. The water contact angle on the fresh Au surface (less than ten minutes after exposure to air) was 80±3°, whereas the water contact angles on the PS-grafted Au (PS-g-Au) and PPFBEM-grafted Au (PPFBEM-g-Au) surfaces were 84±2° and 99±2°, respectively. These results indicated that polymer grafting increased the hydrophobicity of the Au surface and the surface hydrophobicity decreased in the following order: PPFBEM-g-Au > PS-g-Au > bare Au. The water contact angles on the PS-SH and PPFBEM-SH films were 91±2° and 112±2°, respectively. The hydrophobicities of the polymer-grafted Au surfaces were lower than those of the original polymer surfaces, indicating that both the polymer and Au were exposed at the polymer-grafted Au surfaces. The critical surface


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tensions of the bare Au, PS-g-Au and PPFBEM-g-Au surfaces were determined to be 32.6 mN/m, 24.8 mN/m and 20.6 mN/m, respectively, from Zisman plots (Table 2). The surface chemistries of the Au, polymer-grafted Au and glass surfaces were examined by XPS. Figure S6 shows the XPS survey scans for these substrates. The F, O, C, Au, and Si surface atomic concentrations were quantified using the F 1s, O 1s, C 1s, Au 4f, and Si 2s peaks (Table 3). It was confirmed that the carbon contaminant concentration was higher on the Au surface than on the glass surface. Furthermore, oxygen contaminants were also present on the Au surface; unidentified organic compounds covered 40% of the Au surface. The adsorbed contaminants should affect the wettability of the Au surface by both water and air. The carbon and Au surface contents of PS-g-Au were determined to be 72.45% and 21.86%, respectively. This C content was higher than that of the Au substrate, whereas the Au content was lower than that of the Au substrate, indicating that PS-SH molecules were successfully grafted to the Au surface. Unfortunately, the PS surface composition could not be determined because of the lack of specific elemental markers. For PPFBEM-g-Au, F, O and C were detected on the surface in addition to Au. By comparing the percentage of F on the PPFBEM-g-Au surface and in the PPFBEM-SH homopolymer, it was determined that 56.95% of the Au surface was Table 2. Critical surface tensions of bare and polymer-grafted Au (γAAu), interfacial tensions of water-Au (γWAu) and contact angles (θ) of water droplet on the bare and polymer-grafted planar Au substrates γAAua)/mNm-1

γWAub)/mNm-1

θ c)/ °

Bare Au

32.6

20.0

80±3

PS-g-Au

24.8

17.2

84±2

PPFBEM-g-Au 20.6 32.0 99±2 Critical surface tensions determined by Zisman plot b) Interfacial tensions calculated using Young’s equation with critical surface tensions of bare and polymer-grafted Au c) Water droplets with a volume of 10 µL were used for contact angle measurement. a)


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Figure 2 OM images of the (a) Au-SiO2, (b) PS-g-Au-SiO2 and (c) PPFBEM-g-Au-SiO2 Janus particles dispersed in water. covered with PPFBEM. Unfortunately, peaks due to sulfur, which acted as the polymer grafting point, were not detected for the polymer-grafted systems because sulfur was only present in trace amounts. These observations strongly suggested that the thiol-terminated polymers were immobilized on the Au surface; therefore, it was expected that these polymers could also be immobilized on the Au side of the Janus particles.

Self-assembly behavior of the Janus particles in water Conventional molecular surfactants, such as sodium dodecyl sulfate, form nearly spherical micelles in water because of the hydrophobic interactions between Table 3. Surface atomic compositions of PPFBEM-g-Au, PPFBEM-SH homopolymer, PS-g-Au, PS-SH homopolymer, Au and pristine cover glass determined by XPS studies F/%

O/%

C/%

S/%

Au / %

Si / %

PPFBEM-g-Au

15.03

7.51

33.14

~0

44.32

~0

PPFBEM-SH

26.39

9.83

63.77

~0

~0

~0

PS-g-Au

~0

5.68

72.45

~0

21.86

~0

PS-SH

~0

5.43

94.57

~0

~0

~0

Au

~0

7.60

33.05

~0

59.35

~0

SiO2

~0

66.86

2.76

~0

0.02

30.36


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approximately 60 aggregated molecules above the critical micellar concentration63. By analogy, the amphiphilic Janus particles interacted with each other to form non-spherical, irrregular aggregates in water, as shown in Figure 238. (Note that particle-particle attraction force should be larger than surfactant-surfactant attraction force.) In particular, the hydrophobic interactions between the Au sides of the Janus particles drove aggregate formation in water. The Janus structure is crucial for aggregate formation; SiO2 particles with uniform hydrophilic surfaces were dispersed as individual particles in water. The aggregate sizes in all the Janus particle systems varied at constant particle concentrations of 1 and 2 wt%; thus, the aggregation number was not well defined, in contrast to molecular surfactants. For the Au-SiO2, PS-g-Au-SiO2 and PPFBEM-g-Au-SiO2 Janus particles, the percentages of aggregated particles were 39%, 86% and 97% (n= approximately 300), respectively, and the aggregate sizes were 12-18 µm, 12-30 µm and 12-110 µm (n= approximately 300), respectively. These percentages and sizes increased with increasing hydrophobicity of the Au side (hydrophobicity order: PPFBEM-g-Au > PS-g-Au > Au). Moreover, the percentages of single particles decreased with increasing hydrophobicity of the Au side (Au-SiO2, 61%; PS-g-Au-SiO2, 14%; PPFBEM-g-Au-SiO2, 3%: n= approximately 300), and the particle-particle hydrophobic interaction energy in water should also increase accordingly. After applying a shear stress using a magnetic stirrer (HS-6AN, As One) at 500 rpm, the dissociation degrees of and conformational changes in the aggregates in water decreased with increasing hydrophobicity of the Au side, as shown by the OM results (Figure S7).

Bubble stabilization by the Janus particles


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Figure 3

OM images of air bubbles stabilized by the (a,d) Au-SiO2, (b,e)

PS-g-Au-SiO2 and (c,f) PPFBEM-g-Au-SiO2 Janus particles. Figs. (d), (e) and (f) are magnified images of air bubbles. The abilities of the bare Au-SiO2 and polymer-grafted Au-SiO2 Janus particles and the pristine SiO2 particles to stabilize bubbles in water were investigated at 25°C. The bare Au-SiO2 and PPFBEM-g-Au-SiO2 Janus particles acted as particulate bubble stabilizers by adsorbing at the air-water interface, which led to the formation of gold-colored bubbles with diameters of less than approximately 1 mm. The particle-stabilized bubbles creamed to the planar air-water interface and were stable for 10 min; however, they almost collapsed (> 90%) within 2 weeks at 25°C, which was confirmed by OM and naked eyes64. The PS-g-Au-SiO2 Janus particles could be adsorbed at the air-water interface and stabilized bubbles against defoamation for over 1 month (> 90% bubbles survived.). The diameters of the gold-colored bubbles in this system ranged from 60 µm to 700 µm. By contrast, no bubbles formed in the presence of the pristine SiO2 particles, indicating that these particles did not adsorb at the air-water interface because of their highly hydrophilic surfaces. It is worth evaluating formability and stability of bubbles stabilized by SiO2 19 ACS Paragon Plus Environment

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particles homogeneously covered with Au layer, unfortunately, we were unsuccessful in synthesizing such particles. It is expected that the homogeneously Au-coated SiO2 particles could form huge aggregates, rather than the small aggregates with a few to a few tens micrometer sizes observed in the Janus particle systems, because of hydrophobic interaction in water medium. (Note that there is no hydrophilic part on the homogeneously Au-coated SiO2 particle surface, although the Au-SiO2 Janus particles have hydrophilic part on their surfaces and can work as amphiphilic particles.) The aggregates with larger sizes could not stabilize larger air-water interfacial area, which means that the homogeneously Au-coated SiO2 particles are not an efficient bubble stabilizer, comparing with the Au-SiO2 Janus particles. Unlike molecular bubble stabilizers, Janus particles can be directly visualized at the air-water interface because of their size, enabling the effects of their presence at the interface on the bubble morphology and stability to be studied. Typical OM images of the bubbles stabilized by the Janus particles in water are shown in Figure 3. The moderately

Figure 4 OM images of air bubbles in water stabilized by the (a) Au-SiO2, (b) PS-g-Au-SiO2 and (c) PPFBEM-g-Au-SiO2 Janus particles obtained after pressurizing bubbles between a cover glass and glass slide.


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polydisperse bubbles in the Au-SiO2 and PPFBEM-g-Au-SiO2 Janus particle systems had spherical and nearly spherical morphologies. In the PS-g-Au-SiO2 Janus particle system, polydisperse non-spherical and cylindrical bubbles were observed. The circularities of the bubbles in the Au-SiO2, PS-g-Au-SiO2 and PPFBEM-g-Au-SiO2 Janus particle systems were determined to be 0.96, 0.90 and 0.94, respectively. These results indicated that the bubbles stabilized by the PS-g-Au-SiO2 Janus particles were the least spherical. To investigate the configuration of the Janus particles at the bubble surface, light pressure was manually applied to gas bubbles in water placed between a glass slide and cover glass during the OM studies (Figure 4). After the pressure was applied, the air escaped from the Janus particle-stabilized bubbles. The bare and PS-grafted Janus particles formed a monolayer at the air-water interface, with the Au side contacting both the air and water and the SiO2 side incorporated into the water (Figures 4a, b). The contact angles of the bare and PS-g-Au-SiO2 Janus particles at the air-water interface (through the water phase) were determined to be 20±5° and 84±7° (n=50), respectively (Figure 5). The contact angle of the PS-g-Au-SiO2 Janus particles was consistent with that measured for the planar PS-grafted Au substrate. The relatively large discrepancy in contact angle for the

Figure 5 Contact angle determination for the (a) Au-SiO2 and (b) PS-g-Au-SiO2 Janus particles at the air-water interface (measured through the aqueous phase).


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planar Au substrate and the Au-SiO2 particles might be due to history that the samples experienced. In the case of contact angle on a planar Au substrate, a water droplet was placed on the air-exposed substrate. In the case of the Janus particle at air-water interface, the Au surface immersed in water phase met the air phase. There is a possibility that contaminant on the Au surface can be desorbed into water media, which should cause surface contamination concentration difference (hydrophilicity of the Au surface once immersed in water media is higher than that of Au surface exposed to air.) There is another possibility that the curvature of the substrate has effect on the contact angle65. It should be noted that the orientation degree of the Janus particles at the air-water interface was lower for the bare Janus particles. This result was probably due to the smaller contact angle, i.e., the smaller contact area between the Au side and the air allowed the Janus particle orientation to vary more during adsorption. The OM studies showed that the Janus particle aggregates in the aqueous dispersion divided into smaller aggregates and single particles (Figure S7) that could be adsorbed at the air-water interface when the aqueous dispersion was mixed with air. The aggregates might have formed a monolayer at the air-water interface to decrease the interfacial energy, because the Au-Au interactions were relatively weak. By contrast, in the PPFBEM-g-Au-SiO2 system, Janus particle aggregates were observed at the air-water interface, and the contact angle of the Janus particles at the interface could not be determined (Figure 4c). Strong hydrophobic interactions between the PPFBEM-g-Au-SiO2 particles in water led to the formation of aggregates, which then adsorbed at the air-water interface, stabilizing the bubbles. The low stability of PPFBEM-g-Au-SiO2 particle-stabilized bubbles should be due to inefficient contact of the PPFBEM-g-Au surface and air-water interface. There is also a possibility that the PPFBEM-g-Au-SiO2 particles work as a bubble breaker4 because of


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their hydrophobic Au hemisphere with a contact angle > 90˚. The bubble stabilization mechanisms by the Au-SiO2, PS-g-Au-SiO2 and PPFBEM-g-Au-SiO2 Janus particles are illustrated in Figure 6. It was confirmed that < 8% of Au caps have been detached from the SiO2 particles (Figure 4). There was no strong interaction between Au caps and SiO2 particle surface (such as covalent bond), and it seems that small amount of Au caps was detached from the SiO2 particles during formation of bubbles. The non-spherical bubbles observed in the PS-g-Au-SiO2 Janus particle system might have formed because of the coalescence of multiple Janus particle-coated bubbles. The bubbles could not become spherical owing to the solid-like properties conferred to the interface by the presence of the Janus particles (i.e., the Janus particles were irreversibly adsorbed at the air-water interface). Because the Janus particles could not be packed any closer together at the interface, further bubble coalescence was prevented, and therefore, the bubbles remained non-spherical. These bubbles could also be formed by elongation

Figure 6 Schematic illustrations of the bubble stabilization mechanisms by the Au-SiO2, PS-g-Au-SiO2 and PPFBEM-g-Au-SiO2 Janus particles.


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due to uneven shearing during the bubble formation process; the elongated bubbles were covered with the Janus particles before they could relax to a spherical shape. Similarly, non-spherical oil droplets were formed in water in the presence of Janus particles in other studies34,38. This unique behavior of amphiphilic Janus particles is different from that of molecular surfactants, which reversibly adsorb/desorb at the air-water interface63. The adsorption energy (∆G), which is defined as the free energy of system required for a particle to desorb from the air-water interface into the bulk water phase (Ewater - Einterface), is an important parameter for evaluating the surface activity of Janus particles. Ewater and Einterface are the energies of systems (air-water interfacial energy + interfacial energy of the particle) where a Janus particle located entirely in water and at the air-water interface, respectively, and are given by Eqs. (1) and (2), respectively (Supporting Information). These equations are modified ones derived by Ondarçuhu et al.66 and Binks and Fletcher50.

‫ܧ‬௪௔௧௘௥ = 2π‫ ݎ‬ଶ ൫ߛௐ஺௨ + ߛௐௌ௜ைమ ൯ + ߛ஺ௐ ߨ‫ ݎ‬ଶ

…(1)

‫ܧ‬௜௡௧௘௥௙௔௖௘ = ߛ஺஺௨ ሼ2ߨ‫ ݎ‬ଶ ሺ1 − ܿ‫ ߠݏ݋‬ሻሽ + ߛ஺ௐ ሼߨ‫ ݎ‬ଶ ሺ1 − ‫݊݅ݏ‬ଶ ߠ ሻሽ + 2π‫ ݎ‬ଶ ൫ߛௐ஺௨ ܿ‫ ߠݏ݋‬+ …(2)

ߛௐௌ௜ைమ ൯

Then, the adsorption energy of a Janus particle at the air-water interface is given by Eq. (3).

∆G = ‫ܧ‬௪௔௧௘௥ − ‫ܧ‬௜௡௧௘௥௙௔௖௘ = π‫ ݎ‬ଶ ሼ2ߛௐ஺௨ ሺ1 − ܿ‫ߠݏ݋‬ሻ + ߛ஺ௐ ‫݊݅ݏ‬ଶ ߠ − 2ߛ஺஺௨ ሺ1 − ܿ‫ߠݏ݋‬ሻሽ …(3) 24 ACS Paragon Plus Environment

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Table 4. Adsorption energies (∆G) for Janus particle to air-water interface and contact angles (θ) of the Janus particles at air-water interface measured through water phase

Au-SiO2 Janus particle

∆G a)/ J

θ b)/ °

0.198×10-12 c)

20±5

-12 d)

PS-g-Au-SiO2 Janus particle

1.65×10

84±7

― e)

― e)

PPFBEM-g-Au-SiO2 Janus particle a)

Calculated using the equation (1) (see manuscript) Determined from optical microscopy images (n = 50) c) 4.78×107 kT d) 3.99×108 kT e) Cannot be determined because Janus particles were adsorbed to air-water interface as an aggregate. b)

Figure 7

SEM images of bubbles stabilized by the (a,d) Au-SiO2, (b,e)

PS-g-Au-SiO2 and (c,f) PPFBEM-g-Au-SiO2 Janus particles. Figs. (d), (e) and (f) are magnifications of Figs. (a), (b) and (c), respectively.

where r is the Janus particle radius; θ is the contact angle of the Janus particle at the air-water interface (measured through the water phase); and γAAu, γWAu, and γAW are the air-Au, water-Au, and air-water interfacial energies, corresponds to the equilibrium angle given

respectively.

The

θ

value

by Young’s equation. Using Eq. (3), the

∆G value of an Au-SiO2 Janus particle at the air-water interface was calculated to be


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0.198×10-12 J (Table 4), which is one order of magnitude lower than that of a Au-SiO2 Janus particle adsorbed at the n-dodecane-water interface38. For this calculation, γWAu, γAW, and γAAu values of 20.0 mN/m, 72.8 mN/m and 32.6 mN/m, respectively, were employed (Table 2). For the PS-g-Au-SiO2 Janus particles, γWAu and γAAu were determined to be 17.2 mN/m and 24.8 mN/m, respectively, and ∆G was calculated to be 1.65×10-12 J (Table 4), which is one order of magnitude higher than that calculated for the Au-SiO2 Janus particles and high enough to stabilize stable bubbles67,68. These results indicate that the PS-g-Au-SiO2 Janus particles adsorbed at the air-water interface more strongly than the Au-SiO2 particles, resulting in higher bubble stability. From the contact angle point of view, stable bubbles can be prepared using solid particles which show the contact angle at air-water interface (measured through water phase) between 43˚ and 90˚69. Based on these results, the Au-SiO2 and the PS-g-Au-SiO2 Janus particles, which show the contact angles of 20˚ and 84˚, can be categorized as a poor and an effective bubble stabilizer, respectively. Unfortunately, the ∆G value of the PPFBEM-g-Au-SiO2 particles could not be calculated because they formed aggregates at the air-water interface and the contact angle could not be accurately determined.


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Figure 8

Cross-sectional SEM images of bubbles stabilized by the PS-g-Au-SiO2

Janus particles. Fig. b is a magnification of Fig. a. Figure 7 shows SEM images of Janus particle-stabilized bubbles after drying. In all the systems, the bubbles retained their three-dimensional structures, even in the high vacuum SEM chamber, owing to interparticle forces63. Interestingly, the bubble surfaces were relatively smooth, and Janus particle arrays were observed in the Au-SiO2 and PS-g-Au-SiO2 Janus particle systems. By contrast, the bubbles in the PPFBEM-g-SiO2 particle system had rough surfaces. These results further demonstrated that the Au-SiO2 and PS-g-Au-SiO2 Janus particles formed a monolayer at the air-water interface, whereas the PPFBEM-g-SiO2 particles adsorbed as aggregates at the interface. It should be noted that the percentage of Janus particles that adsorbed with a specific orientation (Au and SiO2 sides toward the air and water phases, respectively) was higher for the PS-g-Au-SiO2 particles (91%) than for the Au-SiO2 particles (76%) (n=200). A cross-sectional SEM image of a bubble stabilized by the PS-g-Au-SiO2 particles clearly demonstrated that the Janus particles adsorbed as a monolayer at the air-water interface with the Au and SiO2 sides oriented toward the air and water phases, respectively (Figure 8). The higher adsorption energy and lower degree of orientational freedom of the PS-g-Au-SiO2 Janus


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particles at the air-water interface should lead to a higher percentage of oriented Janus particles in this system than in the bare Janus particle system.

Conclusions Monodisperse, micrometer-sized, polymer-grafted Au-SiO2 Janus particles were successfully fabricated by a grafting-to method that exploited thiol-Au interactions. The particle size, size distribution and self-assembly behavior in water were well characterized. Owing to differences in the hydrophobicity of the polymer-grafted Au surfaces and unidentified organic compounds adsorbed on the particle surfaces and to the hydrophilicity of the SiO2 surface, the Janus particles formed aggregates of various sizes in water. The ability of the Janus particles to act as a particulate bubble stabilizer was compared to that of SiO2 particles. The Au-SiO2 and PS-g-Au-SiO2 Janus particles exhibited monolayer adsorption at the air-water interface with the Au and SiO2 sides facing the air and water phases, respectively, and could effectively stabilize the bubbles. By contrast, the PPFBEM-g-Au-SiO2 Janus particles adsorbed at the air-water interface as aggregates; however, they could also stabilize bubbles. The PS-g-Au-SiO2 Janus particles stabilized bubbles for more than 1 month due to a sufficiently high adsorption energy at the air-water interface, which was estimated from the Gibbs free energy. By contrast, bubbles stabilized by the Au-SiO2 and PPFBEM-g-Au-SiO2 Janus particles collapsed within 2 weeks because of inefficient Janus particle and aggregate adsorption at the interface. However, bare SiO2 particles could not adsorb at the air-water interface because of their highly hydrophilic surface; therefore, no bubbles were formed in this system. The use of Janus particles to encapsulate air bubbles in water has potential for


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application in food manufacturing, cosmetic formulations and personal care products.

Acknowledgments

This work was supported by Grants-in-Aid for Scientific Research (B) (JSPS KAKENHI Grant No. JP16H04207) and for Scientific Research on Innovative Areas “Engineering Neo-Biomimetics (No. 4402)” (JSPS KAKENHI Grant Nos. JP15H01602 and JP25120511), “New Polymeric Materials Based on Element-Blocks (No. 2401)” (JSPS KAKENHI Grant Nos. JP15H00767 and JP25102542) and “Molecular Soft Interface Science (No. 2005)” (JSPS KAKENHI Grant No. 23106720). Reviewers are also thanked for their fruitful comments.

Supporting Information Available Details on the characterization of thiol-terminated PS and PPFBEM, surface analysis of polymer-grafted Au surfaces, morphologies of Janus particles and characterization of Janus particle aggregates in water. This information is available free of charge via the Internet at http://pubs.acs.org/.


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Graphic for TOC


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Gas Bubbles Stabilized by Janus Particles with Varying Hydrophilic

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