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Interface Components: Nanoparticles, Colloids, Emulsions, Surfactants, Proteins, Polymers
Anisotropic Particles Templated by Cerberus Emulsions Lingling Ge, Jingru Cheng, Duo Wei, Yue Sun, and Rong Guo Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b00990 • Publication Date (Web): 06 Jun 2018 Downloaded from http://pubs.acs.org on June 7, 2018
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Anisotropic Particles Templated by Cerberus Emulsions
Lingling Ge*[a], Jingru Cheng[a], Duo Wei[a,b], Yue Sun[a], Rong Guo*[a]
[a] Dr. L. Ge, J Cheng, Dr. D. Wei, Y Sun, Prof. R. Guo School of Chemistry and Chemical Engineering Yangzhou University Yangzhou 225009 (China) [b] Dr. D. Wei Testing center Yangzhou University Yangzhou 225009 (China)
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Abstract: A strategy to the batch-scale fabrication of anisotropic particles with diverse morphologies and various chemical compositions is reported by applying the highly structured fluids of Cerberus emulsions as templates. The Cerberus emulsions are produced simply by traditional onestep vortex mixing the surfactant aqueous solution with three immiscible oils which are selectively photo curable or incurable. Anisotropic particles are subsequently fabricated by UV-induced polymerization. The diversity in morphology of the particles is provided by the various controllable geometries of the Cerberus droplets. Various droplet morphologies of “engulfed-linear”, “partialengulfed linear”, and “linear-singlet” are obtained by employing various oil combinations. Precise control of the volume fraction of each segment within the droplet is realized based on the three-phase diagram of the oils. The wide size range is achieved from hundreds of micrometers continuously down to nanometers, with topology remained. In addition, for a matrix droplet with a fixed morphology, the multiplicity in chemical composition and in geometry of the resultant anisotropic particles is realized by selectively polymerizing one, two, or three of the oil lobes. Morphologies of “crescent moon”, “etched Janus”, and “sandwich Janus” are obtained with homogeneous or multiple distinct chemical compositions. The reported strategy is universal and can be extended to a huge family of polymeric anisotropic particles.
Keywords: Cerberus emulsion, anisotropic particles, interface, polymerization, phase diagrams
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Introduction Colloidal polymer particles with anisotropic shapes are undergoing an unbelievable revolution in material science.1-4 These particles possess distinct advantages over the isotropic spherical counterparts as a result of their anisotropy in shape or chemical composition.5-6 Janus particles, for example, are biphasic colloids that have two sides with distinct chemistry and contrast properties.7-8 They present unique opportunity toward the building blocks, optical biosensors, functional surfactants, and electron paper9-13 due to their distinct chemical compositions and morphologies, in particular, their amphiphilic, magnetic, catalytic, optical and electrical properties.14-17 For instance, Janus particles fabricated by Janus emulsion as templates in our previous researches served as unique emulsifiers to stabilize Pickering emulsions, and more interestingly, the emulsion inversion was successfully achieved by choosing Janus particle with different morphologies.18 In the first decade of 21th century, most of reports on anisotropic particles were focused on the exploration of fabrication technique. Various methods were developed such as stamp coating, phase separation, surface modification, and microfluidic method.1,
14, 19-22
The immobilization based
methods, such as stamp coating and surface modification, rely on instruments and are consuming in time and energy. As an extension of immobilization, the Pickering emulsion route is able to achieve mass production of anisotropic beads.11, 23-24 However, the rotation of the precursor particles on the surface of emulsion droplets impedes the production of particles with sharp phase boundary. The phase separation based method provides several advantages, such as easy process and wide size range. Nevertheless, it is hard to fulfill the scale production due to either diluted solution or broad particle
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distribution.14 Microfluidic reactors are proved to be an outstanding route to fabricate particles with unconventional shapes and high monodispersity.2, 25-27 But the limited size range of 5-100 µm and the low production amount of g/d inhibit the commercialization. Janus emulsions prepared by one-step mixing were first reported by Friberg et al.,28-30 and they were explored as templates for the fabrication of anisotropic particles by our group.18,
31-32
However, the variousness in chemical
composition and geometry is restricted by the binary components as well as the limited topology of Janus droplet. In the second decade of this century, the research interest on anisotropic particles evolves from the fabrication technique to the exploration of their application.33-34 The structure-induced organization by employing anisotropic particles as building blocks is first worth to be mentioned. All the relevant reports verify that a modest change in either geometry or the chemical composition of the building blocks is critical in complexity of the self-assembled structures.35-40 Hence, the realization of diversity in morphology and chemical constitution as well as their precise control is the emerging requirement towards the fabrication technique. Cerberus emulsion, named after the three-head dog of Roman mythology, is a kind of complex emulsion with droplets composed by three singlets of mutually immiscible oils.41 Cerberus emulsion provides an extra free dimension to provide anisotropy in geometry compared to two-singlet Janus droplets. Seven distinct topologies of Cerberus droplets were anticipated by Fryd et al. depending on the oil combination, surfactant, and flow conditions based on micro fluidic technique. The highly controllable and reconfigurable morphologies of Cerberus emulsion was first reported by Zarzar et al.42 using stimuli-responsive and
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cleavable surfactants that underwent changes in their effectiveness as stabilizers in response to temperature, light, or pH. In addition, the topology was found to be an overwhelming extent linear singlet and controllable in volume ratio based on the oil compositions even for the emulsions prepared by traditional vortex mixing.43 All these previous successes inspire us to employ this highly structured fluid of Cerberus emulsion as template to fabricate anisotropic particles. This technique would provide distinct advantage of variety in both morphology and chemical composition in addition to the superiorities of simple, batch scale, precise topology control, and wide size range.18, 31
Experimental Section Materials Ethoxylated trimethylolpropane triacrylate (ETPTA, >99 %), 2-(perfluorooctyl) ethyl methacrylate (PFOEMA, > 98%), and Pluronic F127 (EO97PO68EO97, >99%) were obtained from Sigma-Aldrich. Tripropylene glycol diacrylate (TPGDA, >90%, Tokyo Chemical Industry), Ethylene glycol dimethacrylate (EGDMA, >99%, Sun Chemical Technology of Shanghai Co., Ltd.), methacryl oxypropyl dimethyl siloxane (DMS, >97%, Gelest, Inc.), and silicone oil (SO, 50 cst, >99%, Dowcorning) were used without further purification. Fluorocarbon oil FC-770 (>99.5%) was purchased from the 3M Company and used as received. 1-hydroxycyclohexyl phenyl ketone (HCPK, 99%) was the product of Sun Chemical Technology of Shanghai Co., Ltd.. Acetone was from the Sinopharm Chemical Reagent Co., Ltd. Oil soluble red dye was purchased from Wing fat Chemical Co., Ltd. Deionized water was used. Emulsion preparations 5 ACS Paragon Plus Environment
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The samples with a certain amount of three immiscible oils and surfactant aqueous solutions were weight into 10 mL vials. 4 wt % HCPK was solubilized into polymerizable oils as initiator. All emulsions were made at the same total oil mass fraction of 0.67 with correspondingly aqueous mass fraction of 0.33. The content of surfactant F127 in aqueous phase is 0.4 wt%. The samples were mixed by vibration with a Vortex-Genie 2 from Scientific Industries at speed of 3000 rpm for 1 min and subsequently the ultra-turrax T18 basic (IKA Co., Ltd., Germany) for 3 min. To get nanoemulsions, further emulsification was endowed by sonication with 21 kHz Ultrasonic processor (Scientz-IID, Ningbo Scientz Biotechnology Co., Ltd., China) equipped with a 6 mm diameter probe. Detailed information can be found in our previous paper.31 It should be mentioned that the weight fraction of aqueous phase for ultrasonic emulsification was 0.92. In order to identify the component of each lobe, a red dye was selectively dissolved into oils. Particle preparations A series of emulsion samples prepared as above were exposed to UV-irradiation (500 W, λ=330-380 nm) by XPA-photochemical Reactor (XPA-7, Xujiang Electromech-anical plant, China) for 3 min. The products were washed with acetone for three times, and dried at room temperature. Interfacial tension measurement Interfacial tensions were measured by SVT20 as a spinning drop tensiometer from Dataphysic Instrument Co., Ltd, Germany. The sample tube was filled with the heavier phase and a droplet of the lighter phase was pumped into the heavier phase. The sample tube was rotating by the motor and the droplet of the lighter phase was stretched at the central axis of the sample tube. Under the action of centrifugal force, gravity and interfacial tension, the droplets with smaller density gradually change from ellipsoid to cylinder (interfacial tension is less than 0.1 mN/m) or from sphere to ellipsoid (interfacial tension is greater than 0.1 mN/m), whose shape is determined by rotational speed ω and 6 ACS Paragon Plus Environment
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interfacial tension. In the process of droplet changing, the software control system will always track the shape of the droplet and fit its contour, and the software will automatically calculate the interfacial tension value. More details can be found in the reference.44 The oils were mixed to mutually saturate before the interfacial tension measurement, so as equilibrium was reached from the viewpoints of surface energies and chemical potentials.45 The interfacial tension value γ was recorded 5 min after the fresh droplet was pumped into the liquid to reach equilibrium. Each measurement was repeated more than twice, and uncertainties of these measured values were ±0.02 mN/m. Size distribution measurements The size distribution of the emulsion and particles was determined by a BT-9300H laser particle size analyzer (0.1-1000 µm, Bettersize Instruments Ltd., China) equipped with import optical fiber semiconductor laser. The Measuring principle is based on Mie scattering theory, which is developed from the theory of electromagnetic plane wave scattering by a dielectric sphere and is appropriate for measuring the size of objects whose size is similar to the wavelength of the radiation, e.g., water droplets in the atmosphere, latex particles in paint, droplets in emulsions. The emulsion was slowly added into water with gentle stirring at the speed of 320 rpm. The diluted sample was then pumped into detector at 1500 mL/min. Each measurement was lasted 1 min, and the measurements were repeated more than 3 times. The average size reported was volume-averaged diameter. For the nanosized particles, the size distribution was performed on Malvern NanoSizer (0.3 nm-5 µm, Malvern Instruments Ltd., England, ZEN3690) with He-Ne laser wavelength of 632.8 nm at fixed scattering angle of 90o. The scan time was 240-300 s. The principle is based on Rayleigh scattering which is the dominantly elastic scattering of light or other electromagnetic radiation by particles much smaller than the wavelength of the radiation. Microscopy Optical micrographs of emulsions were recorded with microscope (DMLM/P, Leica Instrument Co., Ltd., Germany). Scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) analyses 7 ACS Paragon Plus Environment
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were performed using a Hitachi S-4800 II field emission (Japan) at an accelerating voltage of 20 kV. Transmission electron microscopy (TEM, JEM 2100F Microscope, JEOL, Japan) analysis was at an accelerating voltage of 120 kV.
Results and Discussion Anisotropic particles templated by Cerberus droplets The formation of Cerberus droplets is achieved easily by dispersing three mutually immiscible oils into the aqueous solution of surfactant by one-step vortex mixing. Two of the oils are photo curable, i.e., ETPTA and DMS, and the third fluorocarbon oil (FC-770) is incurable. As shown in the scheme of Figure 1a, the engulfed-linear Cerberus droplets (O3/O2∣O1)/W are formed referring to the definition by Fryd et al.41 The smaller spherical O3 droplet is preferentially engulfed inside only in the O2 portion, and the O2 lobe is partially engulfed by O1 resulting in a well-known Janus geometry. Due to the clear imaging contrast, the larger and darker O1 lobe in the microscopy image as shown in Figure 1c is assigned to ETPTA phase since it is selectively solubilized with red dye. Upon UVinduced polymerization, solid particles with the contour profile of Janus are obtained as shown in Figure 1e, and the smaller lobe of the Janus possesses a hole. The particles obtained are termed as "etched Janus" in the present contribution. This morphology indicates that the engulfed O3 sphere and corresponding hole is incurable FC-770, which is washed away after polymerization. Hence, the rest smaller O2 lobe is attributed to DMS. Herein, the morphology of the matrix Cerberus droplet is (FC-770/DMS∣ETPTA), which means FC-770 is engulfed by DMS and the two lobes of DMS and ETPTA take a Janus topology. Typical morphology of Cerberus droplet as well as the ascription of each lobe is shown in Figure 1c. The variation of the initial emulsion composition in the following 8 ACS Paragon Plus Environment
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results will also verify the ascription. We have to emphasize that the morphology and composition of the droplets are highly uniform since it is thermodynamically determined by interfacial free energy;29, 42
although size of these droplets are polydisperse due to the shearing effect of vortex mixing.18
The equilibrium topology of droplets in a multiple emulsion depends on two independent factors: the interfacial tension equilibrium at the contact line between three liquids,43,
46-48
and the relative
volumes of each oil. The first aspect depends only on the compounds per se, while the second element is determined by the amount of equilibrium liquids.28 In the present system of ETPTA/FC770/DMS/F127(aq), six different interfacial tension values from the random combination of four immiscible liquids provide the insight of the exclusively preferred topology (Table 1). The lower interfacial tension between ETPTA and aqueous phase (γETPTA/aq=3.99 mN/m) as compared to that between DMS and aqueous phase (γDMS/aq=6.72 mN/m) generates greater affinity of ETPTA towards aqueous phase. Thus, ETPTA partially engulf DMS resulting in Janus topology. The asymmetry of ETPTA and DMS is the same as our previous reported system of ETPTA/HF 7200/ F127(aq).31 As compared to the relatively lower interfacial tension values of DMS/aq, the distinct higher FC-770/aq interfacial tension value of 10.50 mN/m renders the complete engulfment of FC-770 by DMS, resulting in the engulfed-linear geometry of (FC-770/DMS∣ETPTA) observed in Figure 1c. To confirm the engulfed-linear geometry, the corresponding microscopy images (Figure 1d) without cover glass to ensure free rotation and also avoid sheared and squeezed by the cover glass,47, 49 where shows the complete engulfment of DMS on FC-770 sphere. In addition to the interfacial tensions towards aqueous phase, the interfacial tensions of oil combinations per se also shed light on the
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preferred geometry. The interfacial tension between ETPTA and FC-770 is extremely high (8.22 mN/m) compared to oil combinations of ETPTA/DMS (5.41 mN/m) and DMS/FC-770 (4.52 mN/m). Thus, the thermodynamically preferred low interfacial energy drives FC-770 phase keep separating with ETPTA by selectively dispersing in the lobe of DMS. The case is the same with the three immiscible poly(siloxane) oils reported by Fryd et al.41
Figure 1. a- A schematic representation of the fabrication of anisotropic particles with Cerberus emulsion droplets as template; b - Size distributions of Cerberus emulsions (filled symbols) and the resultant particles (hollow symbols); c and d - Microscopy images of Cerberus emulsions with and without cover glass, respectively; e-SEM images of the resultant polymer particles. The mass ratio of ETPTA/FC-770/DMS is 0.40/0.13/0.13, the weight fraction of 0.4 wt% F127 aqueous solutions is 0.33.
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Table 1. Interfacial Tensions (γ, mN/m) between Pre-equilibrium Oils and an Aqueous Solution of 0.5 wt% F127. Combination
Oil phase/aqueous phase ETPTA/a DMS/a FC-770/aq q q
Oil phase/oil phase DMS/ETPT ETPTA/FC-770 DMS/FC-770 A
ETPTA/FC-770/DMS/aq
ETPTA/FC-770/SO/aq
ETPTA/PFOEMA/DMS/a q
FC-770/TPGDA/DMS/aq
ETPTA/PFOEMA/SO/aq
3.99
10.5
6.72
8.22
5.41
4.52
ETPTA/a q
FC-770/aq
SO/aq
ETPTA/FC-770
SO/ETPTA
SO/FC-770
4.58
9.94
8.28
7.69
7.81
5.03
ETPTA/a q
PFOEMA/a q
DMS/a q
ETPTA/PFMA
DMS/ETPT A
DMS/PFOEM A
3.94
5.50
7.80
1.97
5.44
1.88
FC770/aq
TPGDA/aq
DMS/a q
FC-770/TPGDA
FC770/DMS
TPGDA/DMS
9.18
4.88
7.48
5.67
4.38
2.36
ETPTA/a q
PFOEMA/a q
SO/aq
ETPTA/PFOEM A
SO/ETPTA
SO/PFOEMA
4.26
6.00
7.20
1.71
5.91
2.30
As indicated by the theory of Nisisako et al.5 that the DMS monomer will spread entirely across the FC-770 oil and form a core-shell geometry, only if S=γFC-770/aq-(γDMS/aq+γDMS/FC)>0. However, the values of the three interfacial tensions determined independently (γFC
-770/aq
=10.50, γDMS/aq =6.72
mN/m, γDMS/FC =4.52 mN/m,) show that S
