Article pubs.acs.org/Langmuir
Emulsions Stabilized by Silica Rods via Arrested Demixing Santosh Vasant Daware and Madivala G. Basavaraj* Polymer Engineering and Colloid Science Lab (PECS), Department of Chemical Engineering, Indian Institute of Technology Madras, Chennai, Tamil Nadu 600036, India S Supporting Information *
ABSTRACT: A binary liquid−liquid mixture with a lower critical solution temperature (LCST) when heated above a critical temperature undergoes demixing. During the initial phase of demixing process, high-energy liquid−liquid interfaces are created before both liquids eventually phase separate. By incorporating well-characterized colloidal silica rods in a homogeneous one-phase liquid−liquid mixture of lutidine/ water (L/W) before inducing phase separation, we show that colloidal rod stabilized Pickering emulsions can be obtained. We show that the droplet size of Pickering emulsions can be tuned by varying particle concentration, and the droplet size distribution follows the prediction of the limited coalescence model.
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emulsions, foams and polymer blends.18,26 In this article, we consider homogeneous dispersion of silica rods in lutidine/ water (L/W) mixture (a fluid−fluid mixture that exhibits a lower critical solution temperature at 34 °C) at critical composition and show that particle stabilized emulsions can be obtained by arresting the demixing of lutidine and water by the colloidal rods. Recently it has been shown that incorporation of spherical colloidal particles in the binary liquid mixture undergoing temperature-induced phase separation will arrest the phase separation to give stable emulsions with complex shape.27−29 However, there are very few reports30,31 on the use of shape anisotropic particles to arrest the temperature-induced phase separation. Recent simulation study shows that shape anisotropic particles can effectively stabilize temperature-induced phase separation to give emulsions.32 We show that when a binary mixture of L/W in the presence of shape anisotropic silica particles is heated, particles migrate to the interfaces created due to temperatureinduced phase separation giving rise to emulsions that are covered by the colloidal rods. Resulting silica rod stabilized emulsions are stable against further coalescence. When these emulsions are heated on a substrate, both liquids evaporate and form hollow shells of silica rod: “colloidosomes”. The size of emulsion drops can be controlled by varying particle concentration and follow predictions of the limited coalescence model.
INTRODUCTION Practically irreversible adsorption of partially wettable colloidal particles to the fluid−fluid interface provides an alternative method to obtain emulsions that are more commonly called “Pickering−Ramsden emulsion”.1,2 Such Pickering emulsion offers several advantages over traditional surfactant stabilized emulsions such as enhanced stability and provide templates for fabricating novel materials like colloidosomes. Emulsions are ubiquitous and used in many products such as food,3,4 cosmetics5 and pharmaceuticals.6 Pickering emulsions are generally created by imparting mechanical energy to a system consisting of two immiscible fluids and particles. Fluid−fluid interfaces created during emulsification process are eventually covered with particles such that the coalescence is arrested, leading to the formation of Pickering emulsions. The role of particle size,7 particle concentration,7 particle wettability,8 oil to water ratio,9 pH,10 temperature,11 solvent type,12 particle− particle interaction,13 addition of surfactant14 or salt,15 and interfacial rheology16 in relation to the formation of emulsions stabilized by spherical particles has been studied in great detail in the last few decades. Emulsion characteristics such as droplet size, emulsion type (O/W or W/O), emulsion stability, and rheological properties can be tuned by adjusting the abovementioned parameters. Recently, shape and surface anisotropic colloidal particles has been widely used to effect stabilization of emulsions and foam.17−21 When shape anisotropic particles are used, shape induced capillary attraction between the particles leads to the formation of highly viscoelastic layer of particles at the interface which plays key role in the stabilization of Pickering emulsions.17,18,22,23 Shape-induced capillary attractions are long ranged and of the order of several hundreds of kBT, which drives the particles to form aggregated network of particles at interface.24,25 Hence, particle shape anisotropy has been exploited to stabilize variety of materials such as © XXXX American Chemical Society
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MATERIALS AND METHODS
Synthesis of Silica Rods. Silica rods used in all experiments were synthesized by the method developed by Kuijk et. al33 In this
Received: February 28, 2015 Revised: June 1, 2015
A
DOI: 10.1021/acs.langmuir.5b00775 Langmuir XXXX, XXX, XXX−XXX
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Langmuir
Figure 1. (A) Scanning electron microscopy image of silica rods. (B−D) Particle size distribution (histogram with normal fit): (B) length (L = 1.99 μm and σ = 0.20 μm), (C) diameter (D = 0.35 μm and σ = 0.07 μm), and aspect ratio (L/D = 5 and σ = 0.91 μm). emulsion-based synthesis method, silica rods were obtained due to the hydrolysis of a precursor of silica, i.e., tetra-ethyl orthosilicate (TEOS) at the interface of emulsions followed by condensation of silica inside the emulsion droplets. An unidirectional supply of reagents leads to the directed growth and the formation of silica rods. All glassware were cleaned with piranha solution and distilled water. Three hundred milliliters of pentanol (Sigma-Aldrich) was taken in a clean glass Pyrex bottle and 30 g of polyvinylpyrrolidine (Mw = 40000, Sigma-Aldrich) was dissolved by sonication. Thirty milliliters of absolute ethanol (Hayman - AR), 8.4 mL of ultrapure water (Milli-Q-18.2 Ωcm), and 2 mL of 0.18 M sodium citrate (Sigma-Aldrich) were added. The bottle was manually shaken vigorously to mix the contents. Then 6.75 mL of ammonia (25% in water, HDFCL) was added. The bottle was manually shaken again followed by addition of 3 mL of TEOS (99% Merck). After thorough mixing, the reaction mixture was kept at 20 °C for 8 h. At the end of the reaction period, the particles were separated by centrifugation from the reaction media. The particles were subsequently washed with ethanol and water multiple times. To remove small particles and to obtain monodisperse particles, gravity settling was used. The scanning electron microscopy image of silica rods obtained shown in Figure 1 (A). Silica rods were of 5 ± 0.91 aspect ratio (defined as length/diameter of rods and the length of rods =1.99 ± 0.20 μm and the diameter =0.35 ± 0.07 μm). For the preparation of colloidosomes, silica rods of AR (L/D) = 15.10 ± 2.68 were used. The particles of this dimension were obtained by using the same synthesis procedure but by carrying out the reaction at 30 °C for 24 h. Characterization of Silica Rods. The rod-shaped silica particles thus synthesized were characterized for their size, density, and surface charge. The size of particles was measured from the scanning electron microscopy (SEM-Hitachi S4800, Japan) images. The average particle length and diameter calculated using ImageJ software was found as to be 1.99 ± 0.20 μm and 0.35 ± 0.07 μm, respectively, giving rise to an aspect ratio (= L/D) of 5 ± 0.91. In order to measure the density of silica rods, solution densitometry was used. The densities of all samples were measured at 20 °C with a densitometer, which works on the principle of an oscillating U-tube (DMA 4500 M, Anton Paar Austria). To calculate density of silica particles, aqueous suspensions of silica rods (L = 1.99 ± 0.20 μm, D = 0.35 ± 0.07 μm and AR = L/D = 5 ± 0.91 μm) at four different concentration were prepared, and their
density was measured. To prepare silica rod suspensions, ultrapure water (Milli-Q-18.2 Ωcm) was used. The density of the water at 20 °C was measured to be 0.99820 g/cm3. Under the assumption of ideal mixing, the particle density was calculated from eq 1, where ρS is suspension density (g/cm3), ρp is particle density, ρm medium density, and X is particle weight fraction (g of particle/g of total dispersion).
⎤ ⎡ 1 1 1 1 = ⎢ − ⎥X + ρs ⎣⎢ ρp ρm ⎥⎦ ρm
(1)
To obtain weight fraction of particles in the suspension, fixed amount of suspension was weighed and dried at room temperature (∼34 °C) for 3 days. From the initial weight of suspension and dry weight of particles, the particle weight fraction defined as weight of dry particles divided by weight of suspension was calculated. From the slope and intercept values (Figure 2), the density of silica rods was
Figure 2. Density of silica rods: Specific volume of suspension (cm3/ g) plotted against weight fraction of rod shaped silica particles in the dispersion (g/g). Density of rod shaped silica (ρP) was calculated from slope of the line, and eq 2 was found to be 1.99 g/cm3. B
DOI: 10.1021/acs.langmuir.5b00775 Langmuir XXXX, XXX, XXX−XXX
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Langmuir found to be 1.99 g/cm3. The zeta potential of the particles was measured with the help of a zeta sizer (SZ-100 nanopartica, Horiba Japan). For these measurements, rod-shaped silica at very dilute concentration (∼0.01 wt %) were dispersed in single fluid critical mixture of L/W. The zeta potential of 5 ± 0.91 aspect ratio particles was found to be −33.5 ± 3.08 mV. The Smoluchowski equation was used to convert the measured electrophoretic mobility value to the zeta potential. Since the zeta-potential measurements were done at high salt concentration (κb < 30, where κ is the inverse Debye−Hückel length, and b is the length of minor axis of the silica rods), the Smoluchowski equation is applicable.34,35 In order to understand the wetting properties of silica surface for water and lutidine, a 4 μL drop of each fluid was placed on a silicon wafer substrate coated with silica rods and the contact angle was measured by a Goniometer (Digidrop, GBX instruments, France) and digidrop software. The dip coating method was used to create multiple layers of silica particles on the clean silicon wafer. The wafer was moved in and out multiple times perpendicular to the surface of a concentrated suspension (∼8% by weight) of silica rods. Formation of uniform silica layer on the substrate was confirmed by scanning electron microscopy. Phase Separation Experiments. A temperature cell where a liquid can be heated at a required rate and simultaneously observed under the microscope was used for the phase separation experiments (Figure 3). The set up consist of three main components - a disc
mixture. Prior to incorporation of silica particles in the lutidine/ water mixture, control experiments were carried out by heating a single phase binary mixture of lutidine/water at a critical composition without any silica rods. When the binary mixture was heated from 10 °C (where both the liquids are miscible), above 34 °C (lower critical solution temperature for the mixture) the mixture showed complete phase separates. As soon as the temperature of the mixture approaches the lower critical solution temperature, the solution turned turbid, indicating the development of spinodal surfaces and micronsized domains.36 However, in the absence of any particles, the fluid interfaces coalesce, resulting in the formation of two distinct phases of lutidine and water. Complete phase separation analogous to that seen in pristine lutidine/water mixture was observed when silica rods (AR = 5 ± 0.91) at a particle concentration of 0.5, 1.0, and 2.0 wt % were incorporated in the phase separating mixture of lutidine/ water. In spite of the presence of silica rod, the binary mixtures completely phase separate due to insufficient number of silica rods. In the subsequent experiments, the particle concentration in the single phase mixture of lutidine water was raised to 5, 6, 7, and 8 wt % and single phase mixture of lutidine water dispersed with silica rods was heated from 10 to 43 °C, we observed the development of spinodal surfaces in the course of phase separation. However, these spinodal surfaces were not immediately arrested by the colloidal particles due to the following reasons: (1) off-neutral wetting behavior of silica rods: particles preferred to be in lutidine, hence very few particles reached the spinodal interface. The preference of silica rods for pure lutidine and pure water was investigated by wetting studies on silica particle coated surface. Wetting studies showed that a drop of pure lutidine spreads completely on the silica coated surface, whereas a drop of pure water as shown in Figure 4 forms a drop with a contact angle of 76°. Therefore,
Figure 3. Schematic representation of temperature cell used in the phase separation with lutidine −water−silica rods. shaped aluminum block, K type thermocouple and a PID controller (Fuzi PXR-5). The aluminum block has a cavity where sample (in a custom-made cylindrical glass cuvette of volume ∼0.4 mL) can be placed. The aluminum block can be heated at controlled rate with the help of band type heater that encompasses the block. The K-type thermocouple was inserted in aluminum block and attached to a PID controller (Fuzi PXR-5) for feedback and control. Typically, 0.5 g of single phase mixture of 2, 6 lutidine-water (XL= 28 wt %) was prepared for each phase separation experiment. Prior to experiments, rhodamine B dye (Sigma-Aldrich) was dissolved in the 2,6-lutidine so that the final concentration of rhodamine was approximately 6 × 10−6 M. The concentration of rod shaped particles (AR ∼ 5) was varied from 4 wt % to 8 wt %. This mixture was sonicated for 15 min prior to phase separation experiments. The temperature of water inside the sonicator (Crest Ultrasonic 132 kW) was maintained at 10 °C by immersing a cooling coil through which the water from a water bath maintained at 10 °C was circulated. 300 μL of this mixture was placed in a glass cuvette inserted into the aluminum block preheated to 43 °C. The heating rate of the sample was measured to be 28 °C/min. The microstructural changes were captured by fluorescent microscopy (Zeiss axioscope MAT 2) at different magnifications. The colloidosomes were obtained by drying emulsion on a silicon substrate at a temperature of 40 °C for 24 h.
Figure 4. Wetting studies. (A) Droplet of water after reaching equilibrium on silica surface at 43 °C. The contact angle measured by digidrop software was found to be 76°. (B) Lutidine spreads radially giving a contact angle too small to measure.
lutidine preferentially wets the silica rods more compared to water. The three phase contact angle measured for the particle−pure liquids (water and lutidine)−air should not be taken as the exact measure of equilibrium position of silica rods at the lutidine/water interface. Alternatively, we also measured the contact angle of water and lutidine drops on planar smooth silica substrates, namely, microscopy glass slide and silicon wafer. The measured contact angle on these smooth substrates (listed in Table 1 of Supporting Information) showed that the contact angle of sessile drops of lutidine is less than the contact angle of water under identical measurement conditions. These results are similar to measurements on substrates coated with silica particle multilayers and conclusively show that the silica surface is more wetted by lutidine. (2) Partial negative charge (−33.5 ± 3.08 mV) on silica rods due to silanol (SiOH−) groups and the resulting electrostatic repulsion may affect the migration and eventual adsorption of particles to the spinodal
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RESULTS AND DISCUSSION The phase behavior of a binary mixture of lutidine and water depends on the temperature and the composition of the C
DOI: 10.1021/acs.langmuir.5b00775 Langmuir XXXX, XXX, XXX−XXX
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Langmuir interfaces at initial time.37 The energy barrier for particle adsorption is due to interaction between the particle and the interface, which in the present case is a combination of van der Waals attraction, particle−interface repulsion, and particle− image charge repulsion.37 In addition, in the binary mixture close to critical temperature, Casimir forces will also be important.38 However, as the temperature is increased, thermal energy and energy of the demixing may be sufficient to overcome the energy barrier for the adsorption of particles. As a consequence, as the phase separation progresses, the spinodal surfaces containing very few particles were formed. These spinodal surfaces were eventually broken to form several spherical droplets to give rise to droplets that are partially covered with particles. When the surfaces of drops were completely covered with silica rods through the coalescence of sparely covered drops, emulsion that was stable against further coalescence was formed. The particles that eventually got adsorbed at interface were held together due to shape induced capillary attractions to form jammed layer of silica at the interface.17,39 The presence of shape induced capillary attraction leading to the formation of network of particles at interface was evident from the formation of many unbroken shells when emulsion was dried. The fluorescence microscopy images of emulsion droplets obtained at various silica rod concentration (4 to 8 wt %) are shown in Figure 5. The
Figure 6. Comparison of emulsion droplet size from experiments and limited coalescence model: The emulsion droplet size (on the y-axis) is plotted against the inverse of the number of silica rods (on the x-axis) used for emulsion stabilization. As the number of particles increases, emulsion droplet size and polydispersity (standard deviation in droplet size) decrease. The dotted line is a linear fit corresponding to the limited coalescence model.
The phenomenon of “limited coalescence” refers to this process of formation of kinetically stable particle stabilized emulsion drops by the coalescence of partially covered droplets.40 Similar to the formulation of limited coalescence in emulsions stabilized by spherical particles, simple geometric argument has been used to derive the limited coalescence model for the emulsions stabilized by rod-shaped particles (i.e., the relation between limiting size of the droplet and particle concentration). Following were the assumptions for developing the model: (1) All the particles in the mixture migrate to the water/lutidine interface and get adsorbed. (2) Adsorbed silica rods at the water/lutidine interface are positioned horizontallythat is the major axis of the rods is parallel to the interface. It is possible that when higher aspect ratio particles are used for emulsification, particles may “flip” such that they are oriented with major axis perpendicular to the interface.17 The presence of particles in such configuration is not considered. (3) The area occupied by the particles at the interface is rectangular, given by the product of the length and diameter of the rods that is the particles are neutrally wetting (contact angle of 90°). If ‘n’ number of particle stabilized droplets of diameter D are formed from ‘V’ volume of dispersed phase,
Figure 5. Optical micrographs of water in lutidine droplets obtained when a homogeneous mixture of lutidine/water (XL= 28 wt %) containing silica rods (AR = 5 ± 0.915) at different weight % was heated at the rate of 28°/ min from 10° to 43°. Water appears yellow in color (c) when captured with color camera for 5 wt %. (a) 4 wt %, (b) 5 wt %, (d) 6 wt %, (e) 7 wt %, and (f) 8 wt %.
⎛ D3 ⎞ n⎜π ⎟ = V ⎝ 6 ⎠
(2)
If the fraction of the drop surface covered by particles is s, assuming neutral wettability (contact angle of 90°) of silica rods (length - l and diameter - d), the relation between NP, the number of particles used for emulsification, and the number of particle stabilized droplets formed is
emulsions formed were of water in lutidine type as silica rods were wetted more by lutidine than water. The type of emulsion was confirmed by the use of suitable dyes and by the drop test. The microscopy images shown in Figure 5 were used to obtain emulsion droplet size distribution. As shown in Figure 6, droplet size and droplet size polydispersity (filled symbols and corresponding standard deviation) decreases with increasing particle concentration. At high particle concentration, as the availability of particles was more, the coalescence was arrested at an earlier time hence the resulting emulsion droplet size and polydispersity were smaller. On the contrary, at lower particle concentration, sparsely covered drops formed initially coalesce and this process was arrested at a later time when the droplets grow to sufficiently large sizes. As a result at lower particle concentration, emulsions with large droplet size having more polydispersity were formed.
πD2ns = l × d × NP
(3)
Eliminating n using eq 2 and eq 3, an expression for the diameter of the emulsion drops D can be obtained as ⎡ 6sV ⎤ 1 D=⎢ ⎣ ld ⎥⎦ NP
(4)
It must be noted that though the particle wetting is offneutral, the assumption of neutral wetting provides a simplified expression for the limited coalescence model. If accurate measurement of contact angle for silica rods at water/lutidine interface is available, area occupied by the particle must be D
DOI: 10.1021/acs.langmuir.5b00775 Langmuir XXXX, XXX, XXX−XXX
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Langmuir suitably corrected. The dotted line in Figure 6 is a linear fit to the plot of droplet size versus the inverse of the number of particles. According to eq 4, the slope of the fitted line, equal to the prefactor on the right-hand side of eq 4, was found to be 5.85 × 1012 μm. Since all other parameters in the prefactor except s are known, calculated value of s was found to be 1.9, indicating more than monolayer coverage. When silica rods of higher aspect ratio (AR = 15.10 ± 2.68) were used, as expected, the phase separation in lutidine/water mixture (at critical composition) was arrested at lower concentration. Even at a particle concentration of 2 wt %, particle stabilized lutidine drops were observed across the entire volume of the sample. When a small amount of emulsion was dried on a silicon substrate in an oven at 40 °C for 24 h, both lutidine and water evaporate, giving rise to a shell with multilayer of particles present on the surface. Structural rearrangement due to evaporation resulted in the formation of some broken shells, but many of the drops remained intact, an example of which is shown in Figure 7. The robustness of the shell can be attributed to the viscoelastic network of silica rods formed at interface due to shape-induced capillary attractions.17,18
Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.5b00775.
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Corresponding Author
*Mailing address: Polymer Engineering and Colloidal Science Lab (PECS), Department of Chemical Engineering, Indian Institute of Technology Madras, Chennai (TN), India. Fax: +91 44 2257 4152; Tel: +91 044 2257 4164; E-mail: basa@ iitm.ac.in. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We acknowledge DST- FIST for the SEM facility and the IITM-NISSAN research support grant for financial support. We thank Dilli Babu Padmanaban for assistance in contact angle measurements on planar silica substrates.
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REFERENCES
(1) Pickering, S. U. CXCVI.Emulsions. J. Chem. Soc. 1907, 91 (0), 2001−2021. (2) Ramsden, W. Separation of Solids in the Surface-Layers of Solutions and ‘Suspensions’ (Observations on Surface-Membranes, Bubbles, Emulsions, and Mechanical Coagulation). – Preliminary Account. Proc. R. Soc. London 1903, 72 (477−486), 156−164. (3) Garti, N. Progress in stabilization and transport phenomena of double emulsions in food applications. LWT–Food Sci. Technol. 1997, 30 (3), 222−235. (4) Guzey, D.; McClements, D. J. Formation, stability and properties of multilayer emulsions for application in the food industry. Adv. Colloid Interface Sci. 2006, 128, 227−248. (5) Favre, S.; Griat, J.; Lorant, R.; Terren, N. Oil-in-water emulsion, a composition comprising this emulsion and use of this emulsion in cosmetics and in hygiene. US Patent 5993832 A, 1999. (6) Davis, S. S.; Washington, C.; West, P.; Illum, L.; Liversidge, G.; Sternson, L.; Kirsh, R. Lipid emulsions as drug delivery systems. Ann. N.Y. Acad. Sci. 1987, 507 (1), 75−88. (7) Binks, B. P.; Philip, J.; Rodrigues, J. A. Inversion of silicastabilized emulsions induced by particle concentration. Langmuir 2005, 21 (8), 3296−3302. (8) Binks, B. P.; Whitby, C. P. Nanoparticle silica-stabilised oil-inwater emulsions: Improving emulsion stability. Colloids Surf., A 2005, 253 (1−3), 105−115. (9) Binks, B.; Lumsdon, S. Catastrophic phase inversion of water-inoil emulsions stabilized by hydrophobic silica. Langmuir 2000, 16 (6), 2539−2547. (10) Midmore, B. Preparation of a novel silica-stabilized oil/water emulsion. Colloids Surf., A 1998, 132 (2), 257−265. (11) Binks, B. P.; Whitby, C. P. Temperature-dependent stability of water-in-undecanol emulsions. Colloids Surf., A 2003, 224 (1), 241− 249. (12) Binks, B.; Lumsdon, S. Effects of oil type and aqueous phase composition on oil−water mixtures containing particles of intermediate hydrophobicity. Phys. Chem. Chem. Phys. 2000, 2 (13), 2959− 2967. (13) Menon, V.; Nikolov, A.; Wasan, D. Interfacial effects in solidsstabilized emulsions: Measurements of film tension and particle interaction energy. J. Colloid Interface Sci. 1988, 124 (1), 317−327. (14) Whitby, C. P.; Fornasiero, D.; Ralston, J. Effect of adding anionic surfactant on the stability of Pickering emulsions. J. Colloid Interface Sci. 2009, 329 (1), 173−181. (15) Tambe, D. E.; Sharma, M. M. Factors controlling the stability of colloid-stabilized emulsions: I. An experimental investigation. J. Colloid Interface Sci. 1993, 157 (1), 244−253.
Figure 7. Scanning electron microscopy image of hollow shells obtained by drying silica-rod stabilized emulsions: The emulsion used was obtained by phase separation in critical mixture of lutidine/water dispersed with 2 wt % rod shaped silica particles (AR ∼ 15). The dry shell in the figure indicates a multilayer of rod-shaped silica particles at the surface.
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CONCLUSION Pickering emulsion stabilized by rod-shaped silica particles were formed by arresting the temperature induced phase separation in critical mixture of L/W. Although the phase separation occurs via spinodal decomposition, probably due to low concentration of particles at the spinodal interface at initial stages of phase separation, interfaces break into spherical droplets. Emulsion droplet size was tuned by varying particle concentration, and the size of the droplets followed the prediction of the limited coalescence model. Pickering emulsions thus formed were found to be robust and resulted in hollow shells of silica rods upon evaporation. With appropriate surface modification, this methodology can be extended to fabricate bicontinuous interfacially jammed emulsion gels (‘bijels’) stabilized by anisotropic colloids, which in turn can be used as templates for novel materials.
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AUTHOR INFORMATION
ASSOCIATED CONTENT
S Supporting Information *
A short summary on the measurement of the contact angle of sessile drops of water and lutidine on planar silica substrates is available free of charge via the Internet at The Supporting E
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Langmuir (16) Tambe, D. E.; Sharma, M. M. The effect of colloidal particles on fluid−fluid interfacial properties and emulsion stability. Adv. Colloid Interface Sci. 1994, 52, 1−63. (17) Madivala, B.; Vandebril, S.; Fransaer, J.; Vermant, J. Exploiting particle shape in solid stabilized emulsions. Soft Matter 2009, 5 (8), 1717−1727. (18) Dugyala, V. R.; Daware, S. V.; Basavaraj, M. G. Shape anisotropic colloids: Synthesis, packing behavior, evaporation driven assembly, and their application in emulsion stabilization. Soft Matter 2013, 9 (29), 6711−6725. (19) Wang, H.; Hobbie, E. K. Amphiphobic Carbon Nanotubes as Macroemulsion Surfactants. Langmuir 2003, 19 (8), 3091−3093. (20) Hobbie, E. K.; Bauer, B. J.; Stephens, J.; Becker, M. L.; McGuiggan, P.; Hudson, S. D.; Wang, H. Colloidal Particles Coated and Stabilized by DNA-Wrapped Carbon Nanotubes. Langmuir 2005, 21 (23), 10284−10287. (21) Feng, T.; Hoagland, D. A.; Russell, T. P. Assembly of AcidFunctionalized Single-Walled Carbon Nanotubes at Oil/Water Interfaces. Langmuir 2014, 30 (4), 1072−1079. (22) Davies, G. B.; Krüger, T.; Coveney, P. V.; Harting, J. Detachment energies of spheroidal particles from fluid−fluid interfaces. J. Chem. Phys. 2014, 141 (15), 154902. (23) Basavaraj, M.; Fuller, G.; Fransaer, J.; Vermant, J. Packing, flipping, and buckling transitions in compressed monolayers of ellipsoidal latex particles. Langmuir 2006, 22 (15), 6605−6612. (24) Loudet, J.-C.; Pouligny, B. Self-assembled capillary arrows. Europhys. Lett. 2009, 85 (2), 28003. (25) Loudet, J. C.; Alsayed, A. M.; Zhang, J.; Yodh, A. G. Capillary Interactions Between Anisotropic Colloidal Particles. Phys. Rev. Lett. 2005, 94 (1), 018301. (26) Vandebril, S.; Vermant, J.; Moldenaers, P. Efficiently suppressing coalescence in polymer blends using nanoparticles: Role of interfacial rheology. Soft Matter 2010, 6 (14), 3353−3362. (27) Stratford, K.; Adhikari, R.; Pagonabarraga, I.; Desplat, J.-C.; Cates, M. Colloidal jamming at interfaces: A route to fluidbicontinuous gels. Science 2005, 309 (5744), 2198−2201. (28) Herzig, E.; White, K.; Schofield, A.; Poon, W.; Clegg, P. Bicontinuous emulsions stabilized solely by colloidal particles. Nat. Mater. 2007, 6 (12), 966−971. (29) Frijters, S.; Harting, J. Self-assembled porous media from particle-stabilized emulsions. arXiv.org, e-Print Arch., Condens. Matter 2014, No. arXiv:1408.2974. (30) Hijnen, N.; Clegg, P. S. Simple Synthesis of Versatile Akaganéite-Silica Core−Shell Rods. Chem. Mater. 2012, 24 (17), 3449−3457. (31) Imperiali, L.; Clasen, C.; Fransaer, J.; Macosko, C. W.; Vermant, J. A simple route towards graphene oxide frameworks. Mater. Horiz. 2014, 1 (1), 139−145. (32) Günther, F.; Janoschek, F.; Frijters, S.; Harting, J. Lattice Boltzmann simulations of anisotropic particles at liquid interfaces. Comput. Fluids 2013, 80, 184−189. (33) Kuijk, A.; van Blaaderen, A.; Imhof, A. Synthesis of monodisperse, rodlike silica colloids with tunable aspect ratio. J. Am. Chem. Soc. 2011, 133 (8), 2346−2349. (34) Ho, C. C.; Ottewill, R. H.; Yu, L. Examination of Ellipsoidal Polystyrene Particles by Electrophoresis. Langmuir 1997, 13 (7), 1925−1930. (35) O’Brien, R. W.; White, L. R. Electrophoretic Mobility of a Spherical Colloidal Particle. J. Chem. Soc. Faraday Trans. II 1978, 74, 1607−1626. (36) Chou, Y.-C.; Goldburg, W. I. Phase separation and coalescence in critically quenched isobutyric-acid−water and 2,6-lutidine−water mixtures. Phys. Rev. A 1979, 20 (5), 2105. (37) Wang, H.; Singh, V.; Behrens, S. H. Image Charge Effects on the Formation of Pickering Emulsions. J. Phys. Chem. Lett. 2012, 3 (20), 2986−2990. (38) Gambassi, A.; Maciołek, A.; Hertlein, C.; Nellen, U.; Helden, L.; Bechinger, C.; Dietrich, S. Critical Casimir effect in classical binary liquid mixtures. Phys. Rev. E 2009, 80, 061143.
(39) Lewandowski, E. P.; Cavallaro, M., Jr.; Botto, L.; Bernate, J. C.; Garbin, V.; Stebe, K. J. Orientation and self-assembly of cylindrical particles by anisotropic capillary interactions. Langmuir 2010, 26 (19), 15142−15154. (40) Arditty, S.; Whitby, C. P.; Binks, B. P.; Schmitt, V.; LealCalderon, F. Some general features of limited coalescence in solidstabilized emulsions. Eur. Phys. J. E 2003, 11 (3), 273−281.
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DOI: 10.1021/acs.langmuir.5b00775 Langmuir XXXX, XXX, XXX−XXX
Emulsions Stabilized by Silica Rods via Arrested Demixing Santosh Vasant Daware and Madivala G. Basavaraj* Polymer Engineering and Colloid Science Lab (PECS), Department of Chemical Engineering, Indian Institute of Technology Madras, Chennai, Tamil Nadu 600036, India S Supporting Information *
ABSTRACT: A binary liquid−liquid mixture with a lower critical solution temperature (LCST) when heated above a critical temperature undergoes demixing. During the initial phase of demixing process, high-energy liquid−liquid interfaces are created before both liquids eventually phase separate. By incorporating well-characterized colloidal silica rods in a homogeneous one-phase liquid−liquid mixture of lutidine/ water (L/W) before inducing phase separation, we show that colloidal rod stabilized Pickering emulsions can be obtained. We show that the droplet size of Pickering emulsions can be tuned by varying particle concentration, and the droplet size distribution follows the prediction of the limited coalescence model.
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emulsions, foams and polymer blends.18,26 In this article, we consider homogeneous dispersion of silica rods in lutidine/ water (L/W) mixture (a fluid−fluid mixture that exhibits a lower critical solution temperature at 34 °C) at critical composition and show that particle stabilized emulsions can be obtained by arresting the demixing of lutidine and water by the colloidal rods. Recently it has been shown that incorporation of spherical colloidal particles in the binary liquid mixture undergoing temperature-induced phase separation will arrest the phase separation to give stable emulsions with complex shape.27−29 However, there are very few reports30,31 on the use of shape anisotropic particles to arrest the temperature-induced phase separation. Recent simulation study shows that shape anisotropic particles can effectively stabilize temperature-induced phase separation to give emulsions.32 We show that when a binary mixture of L/W in the presence of shape anisotropic silica particles is heated, particles migrate to the interfaces created due to temperatureinduced phase separation giving rise to emulsions that are covered by the colloidal rods. Resulting silica rod stabilized emulsions are stable against further coalescence. When these emulsions are heated on a substrate, both liquids evaporate and form hollow shells of silica rod: “colloidosomes”. The size of emulsion drops can be controlled by varying particle concentration and follow predictions of the limited coalescence model.
INTRODUCTION Practically irreversible adsorption of partially wettable colloidal particles to the fluid−fluid interface provides an alternative method to obtain emulsions that are more commonly called “Pickering−Ramsden emulsion”.1,2 Such Pickering emulsion offers several advantages over traditional surfactant stabilized emulsions such as enhanced stability and provide templates for fabricating novel materials like colloidosomes. Emulsions are ubiquitous and used in many products such as food,3,4 cosmetics5 and pharmaceuticals.6 Pickering emulsions are generally created by imparting mechanical energy to a system consisting of two immiscible fluids and particles. Fluid−fluid interfaces created during emulsification process are eventually covered with particles such that the coalescence is arrested, leading to the formation of Pickering emulsions. The role of particle size,7 particle concentration,7 particle wettability,8 oil to water ratio,9 pH,10 temperature,11 solvent type,12 particle− particle interaction,13 addition of surfactant14 or salt,15 and interfacial rheology16 in relation to the formation of emulsions stabilized by spherical particles has been studied in great detail in the last few decades. Emulsion characteristics such as droplet size, emulsion type (O/W or W/O), emulsion stability, and rheological properties can be tuned by adjusting the abovementioned parameters. Recently, shape and surface anisotropic colloidal particles has been widely used to effect stabilization of emulsions and foam.17−21 When shape anisotropic particles are used, shape induced capillary attraction between the particles leads to the formation of highly viscoelastic layer of particles at the interface which plays key role in the stabilization of Pickering emulsions.17,18,22,23 Shape-induced capillary attractions are long ranged and of the order of several hundreds of kBT, which drives the particles to form aggregated network of particles at interface.24,25 Hence, particle shape anisotropy has been exploited to stabilize variety of materials such as © XXXX American Chemical Society
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MATERIALS AND METHODS
Synthesis of Silica Rods. Silica rods used in all experiments were synthesized by the method developed by Kuijk et. al33 In this
Received: February 28, 2015 Revised: June 1, 2015
A
DOI: 10.1021/acs.langmuir.5b00775 Langmuir XXXX, XXX, XXX−XXX
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Langmuir
Figure 1. (A) Scanning electron microscopy image of silica rods. (B−D) Particle size distribution (histogram with normal fit): (B) length (L = 1.99 μm and σ = 0.20 μm), (C) diameter (D = 0.35 μm and σ = 0.07 μm), and aspect ratio (L/D = 5 and σ = 0.91 μm). emulsion-based synthesis method, silica rods were obtained due to the hydrolysis of a precursor of silica, i.e., tetra-ethyl orthosilicate (TEOS) at the interface of emulsions followed by condensation of silica inside the emulsion droplets. An unidirectional supply of reagents leads to the directed growth and the formation of silica rods. All glassware were cleaned with piranha solution and distilled water. Three hundred milliliters of pentanol (Sigma-Aldrich) was taken in a clean glass Pyrex bottle and 30 g of polyvinylpyrrolidine (Mw = 40000, Sigma-Aldrich) was dissolved by sonication. Thirty milliliters of absolute ethanol (Hayman - AR), 8.4 mL of ultrapure water (Milli-Q-18.2 Ωcm), and 2 mL of 0.18 M sodium citrate (Sigma-Aldrich) were added. The bottle was manually shaken vigorously to mix the contents. Then 6.75 mL of ammonia (25% in water, HDFCL) was added. The bottle was manually shaken again followed by addition of 3 mL of TEOS (99% Merck). After thorough mixing, the reaction mixture was kept at 20 °C for 8 h. At the end of the reaction period, the particles were separated by centrifugation from the reaction media. The particles were subsequently washed with ethanol and water multiple times. To remove small particles and to obtain monodisperse particles, gravity settling was used. The scanning electron microscopy image of silica rods obtained shown in Figure 1 (A). Silica rods were of 5 ± 0.91 aspect ratio (defined as length/diameter of rods and the length of rods =1.99 ± 0.20 μm and the diameter =0.35 ± 0.07 μm). For the preparation of colloidosomes, silica rods of AR (L/D) = 15.10 ± 2.68 were used. The particles of this dimension were obtained by using the same synthesis procedure but by carrying out the reaction at 30 °C for 24 h. Characterization of Silica Rods. The rod-shaped silica particles thus synthesized were characterized for their size, density, and surface charge. The size of particles was measured from the scanning electron microscopy (SEM-Hitachi S4800, Japan) images. The average particle length and diameter calculated using ImageJ software was found as to be 1.99 ± 0.20 μm and 0.35 ± 0.07 μm, respectively, giving rise to an aspect ratio (= L/D) of 5 ± 0.91. In order to measure the density of silica rods, solution densitometry was used. The densities of all samples were measured at 20 °C with a densitometer, which works on the principle of an oscillating U-tube (DMA 4500 M, Anton Paar Austria). To calculate density of silica particles, aqueous suspensions of silica rods (L = 1.99 ± 0.20 μm, D = 0.35 ± 0.07 μm and AR = L/D = 5 ± 0.91 μm) at four different concentration were prepared, and their
density was measured. To prepare silica rod suspensions, ultrapure water (Milli-Q-18.2 Ωcm) was used. The density of the water at 20 °C was measured to be 0.99820 g/cm3. Under the assumption of ideal mixing, the particle density was calculated from eq 1, where ρS is suspension density (g/cm3), ρp is particle density, ρm medium density, and X is particle weight fraction (g of particle/g of total dispersion).
⎤ ⎡ 1 1 1 1 = ⎢ − ⎥X + ρs ⎣⎢ ρp ρm ⎥⎦ ρm
(1)
To obtain weight fraction of particles in the suspension, fixed amount of suspension was weighed and dried at room temperature (∼34 °C) for 3 days. From the initial weight of suspension and dry weight of particles, the particle weight fraction defined as weight of dry particles divided by weight of suspension was calculated. From the slope and intercept values (Figure 2), the density of silica rods was
Figure 2. Density of silica rods: Specific volume of suspension (cm3/ g) plotted against weight fraction of rod shaped silica particles in the dispersion (g/g). Density of rod shaped silica (ρP) was calculated from slope of the line, and eq 2 was found to be 1.99 g/cm3. B
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Langmuir found to be 1.99 g/cm3. The zeta potential of the particles was measured with the help of a zeta sizer (SZ-100 nanopartica, Horiba Japan). For these measurements, rod-shaped silica at very dilute concentration (∼0.01 wt %) were dispersed in single fluid critical mixture of L/W. The zeta potential of 5 ± 0.91 aspect ratio particles was found to be −33.5 ± 3.08 mV. The Smoluchowski equation was used to convert the measured electrophoretic mobility value to the zeta potential. Since the zeta-potential measurements were done at high salt concentration (κb < 30, where κ is the inverse Debye−Hückel length, and b is the length of minor axis of the silica rods), the Smoluchowski equation is applicable.34,35 In order to understand the wetting properties of silica surface for water and lutidine, a 4 μL drop of each fluid was placed on a silicon wafer substrate coated with silica rods and the contact angle was measured by a Goniometer (Digidrop, GBX instruments, France) and digidrop software. The dip coating method was used to create multiple layers of silica particles on the clean silicon wafer. The wafer was moved in and out multiple times perpendicular to the surface of a concentrated suspension (∼8% by weight) of silica rods. Formation of uniform silica layer on the substrate was confirmed by scanning electron microscopy. Phase Separation Experiments. A temperature cell where a liquid can be heated at a required rate and simultaneously observed under the microscope was used for the phase separation experiments (Figure 3). The set up consist of three main components - a disc
mixture. Prior to incorporation of silica particles in the lutidine/ water mixture, control experiments were carried out by heating a single phase binary mixture of lutidine/water at a critical composition without any silica rods. When the binary mixture was heated from 10 °C (where both the liquids are miscible), above 34 °C (lower critical solution temperature for the mixture) the mixture showed complete phase separates. As soon as the temperature of the mixture approaches the lower critical solution temperature, the solution turned turbid, indicating the development of spinodal surfaces and micronsized domains.36 However, in the absence of any particles, the fluid interfaces coalesce, resulting in the formation of two distinct phases of lutidine and water. Complete phase separation analogous to that seen in pristine lutidine/water mixture was observed when silica rods (AR = 5 ± 0.91) at a particle concentration of 0.5, 1.0, and 2.0 wt % were incorporated in the phase separating mixture of lutidine/ water. In spite of the presence of silica rod, the binary mixtures completely phase separate due to insufficient number of silica rods. In the subsequent experiments, the particle concentration in the single phase mixture of lutidine water was raised to 5, 6, 7, and 8 wt % and single phase mixture of lutidine water dispersed with silica rods was heated from 10 to 43 °C, we observed the development of spinodal surfaces in the course of phase separation. However, these spinodal surfaces were not immediately arrested by the colloidal particles due to the following reasons: (1) off-neutral wetting behavior of silica rods: particles preferred to be in lutidine, hence very few particles reached the spinodal interface. The preference of silica rods for pure lutidine and pure water was investigated by wetting studies on silica particle coated surface. Wetting studies showed that a drop of pure lutidine spreads completely on the silica coated surface, whereas a drop of pure water as shown in Figure 4 forms a drop with a contact angle of 76°. Therefore,
Figure 3. Schematic representation of temperature cell used in the phase separation with lutidine −water−silica rods. shaped aluminum block, K type thermocouple and a PID controller (Fuzi PXR-5). The aluminum block has a cavity where sample (in a custom-made cylindrical glass cuvette of volume ∼0.4 mL) can be placed. The aluminum block can be heated at controlled rate with the help of band type heater that encompasses the block. The K-type thermocouple was inserted in aluminum block and attached to a PID controller (Fuzi PXR-5) for feedback and control. Typically, 0.5 g of single phase mixture of 2, 6 lutidine-water (XL= 28 wt %) was prepared for each phase separation experiment. Prior to experiments, rhodamine B dye (Sigma-Aldrich) was dissolved in the 2,6-lutidine so that the final concentration of rhodamine was approximately 6 × 10−6 M. The concentration of rod shaped particles (AR ∼ 5) was varied from 4 wt % to 8 wt %. This mixture was sonicated for 15 min prior to phase separation experiments. The temperature of water inside the sonicator (Crest Ultrasonic 132 kW) was maintained at 10 °C by immersing a cooling coil through which the water from a water bath maintained at 10 °C was circulated. 300 μL of this mixture was placed in a glass cuvette inserted into the aluminum block preheated to 43 °C. The heating rate of the sample was measured to be 28 °C/min. The microstructural changes were captured by fluorescent microscopy (Zeiss axioscope MAT 2) at different magnifications. The colloidosomes were obtained by drying emulsion on a silicon substrate at a temperature of 40 °C for 24 h.
Figure 4. Wetting studies. (A) Droplet of water after reaching equilibrium on silica surface at 43 °C. The contact angle measured by digidrop software was found to be 76°. (B) Lutidine spreads radially giving a contact angle too small to measure.
lutidine preferentially wets the silica rods more compared to water. The three phase contact angle measured for the particle−pure liquids (water and lutidine)−air should not be taken as the exact measure of equilibrium position of silica rods at the lutidine/water interface. Alternatively, we also measured the contact angle of water and lutidine drops on planar smooth silica substrates, namely, microscopy glass slide and silicon wafer. The measured contact angle on these smooth substrates (listed in Table 1 of Supporting Information) showed that the contact angle of sessile drops of lutidine is less than the contact angle of water under identical measurement conditions. These results are similar to measurements on substrates coated with silica particle multilayers and conclusively show that the silica surface is more wetted by lutidine. (2) Partial negative charge (−33.5 ± 3.08 mV) on silica rods due to silanol (SiOH−) groups and the resulting electrostatic repulsion may affect the migration and eventual adsorption of particles to the spinodal
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RESULTS AND DISCUSSION The phase behavior of a binary mixture of lutidine and water depends on the temperature and the composition of the C
DOI: 10.1021/acs.langmuir.5b00775 Langmuir XXXX, XXX, XXX−XXX
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Langmuir interfaces at initial time.37 The energy barrier for particle adsorption is due to interaction between the particle and the interface, which in the present case is a combination of van der Waals attraction, particle−interface repulsion, and particle− image charge repulsion.37 In addition, in the binary mixture close to critical temperature, Casimir forces will also be important.38 However, as the temperature is increased, thermal energy and energy of the demixing may be sufficient to overcome the energy barrier for the adsorption of particles. As a consequence, as the phase separation progresses, the spinodal surfaces containing very few particles were formed. These spinodal surfaces were eventually broken to form several spherical droplets to give rise to droplets that are partially covered with particles. When the surfaces of drops were completely covered with silica rods through the coalescence of sparely covered drops, emulsion that was stable against further coalescence was formed. The particles that eventually got adsorbed at interface were held together due to shape induced capillary attractions to form jammed layer of silica at the interface.17,39 The presence of shape induced capillary attraction leading to the formation of network of particles at interface was evident from the formation of many unbroken shells when emulsion was dried. The fluorescence microscopy images of emulsion droplets obtained at various silica rod concentration (4 to 8 wt %) are shown in Figure 5. The
Figure 6. Comparison of emulsion droplet size from experiments and limited coalescence model: The emulsion droplet size (on the y-axis) is plotted against the inverse of the number of silica rods (on the x-axis) used for emulsion stabilization. As the number of particles increases, emulsion droplet size and polydispersity (standard deviation in droplet size) decrease. The dotted line is a linear fit corresponding to the limited coalescence model.
The phenomenon of “limited coalescence” refers to this process of formation of kinetically stable particle stabilized emulsion drops by the coalescence of partially covered droplets.40 Similar to the formulation of limited coalescence in emulsions stabilized by spherical particles, simple geometric argument has been used to derive the limited coalescence model for the emulsions stabilized by rod-shaped particles (i.e., the relation between limiting size of the droplet and particle concentration). Following were the assumptions for developing the model: (1) All the particles in the mixture migrate to the water/lutidine interface and get adsorbed. (2) Adsorbed silica rods at the water/lutidine interface are positioned horizontallythat is the major axis of the rods is parallel to the interface. It is possible that when higher aspect ratio particles are used for emulsification, particles may “flip” such that they are oriented with major axis perpendicular to the interface.17 The presence of particles in such configuration is not considered. (3) The area occupied by the particles at the interface is rectangular, given by the product of the length and diameter of the rods that is the particles are neutrally wetting (contact angle of 90°). If ‘n’ number of particle stabilized droplets of diameter D are formed from ‘V’ volume of dispersed phase,
Figure 5. Optical micrographs of water in lutidine droplets obtained when a homogeneous mixture of lutidine/water (XL= 28 wt %) containing silica rods (AR = 5 ± 0.915) at different weight % was heated at the rate of 28°/ min from 10° to 43°. Water appears yellow in color (c) when captured with color camera for 5 wt %. (a) 4 wt %, (b) 5 wt %, (d) 6 wt %, (e) 7 wt %, and (f) 8 wt %.
⎛ D3 ⎞ n⎜π ⎟ = V ⎝ 6 ⎠
(2)
If the fraction of the drop surface covered by particles is s, assuming neutral wettability (contact angle of 90°) of silica rods (length - l and diameter - d), the relation between NP, the number of particles used for emulsification, and the number of particle stabilized droplets formed is
emulsions formed were of water in lutidine type as silica rods were wetted more by lutidine than water. The type of emulsion was confirmed by the use of suitable dyes and by the drop test. The microscopy images shown in Figure 5 were used to obtain emulsion droplet size distribution. As shown in Figure 6, droplet size and droplet size polydispersity (filled symbols and corresponding standard deviation) decreases with increasing particle concentration. At high particle concentration, as the availability of particles was more, the coalescence was arrested at an earlier time hence the resulting emulsion droplet size and polydispersity were smaller. On the contrary, at lower particle concentration, sparsely covered drops formed initially coalesce and this process was arrested at a later time when the droplets grow to sufficiently large sizes. As a result at lower particle concentration, emulsions with large droplet size having more polydispersity were formed.
πD2ns = l × d × NP
(3)
Eliminating n using eq 2 and eq 3, an expression for the diameter of the emulsion drops D can be obtained as ⎡ 6sV ⎤ 1 D=⎢ ⎣ ld ⎥⎦ NP
(4)
It must be noted that though the particle wetting is offneutral, the assumption of neutral wetting provides a simplified expression for the limited coalescence model. If accurate measurement of contact angle for silica rods at water/lutidine interface is available, area occupied by the particle must be D
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Langmuir suitably corrected. The dotted line in Figure 6 is a linear fit to the plot of droplet size versus the inverse of the number of particles. According to eq 4, the slope of the fitted line, equal to the prefactor on the right-hand side of eq 4, was found to be 5.85 × 1012 μm. Since all other parameters in the prefactor except s are known, calculated value of s was found to be 1.9, indicating more than monolayer coverage. When silica rods of higher aspect ratio (AR = 15.10 ± 2.68) were used, as expected, the phase separation in lutidine/water mixture (at critical composition) was arrested at lower concentration. Even at a particle concentration of 2 wt %, particle stabilized lutidine drops were observed across the entire volume of the sample. When a small amount of emulsion was dried on a silicon substrate in an oven at 40 °C for 24 h, both lutidine and water evaporate, giving rise to a shell with multilayer of particles present on the surface. Structural rearrangement due to evaporation resulted in the formation of some broken shells, but many of the drops remained intact, an example of which is shown in Figure 7. The robustness of the shell can be attributed to the viscoelastic network of silica rods formed at interface due to shape-induced capillary attractions.17,18
Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.5b00775.
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Corresponding Author
*Mailing address: Polymer Engineering and Colloidal Science Lab (PECS), Department of Chemical Engineering, Indian Institute of Technology Madras, Chennai (TN), India. Fax: +91 44 2257 4152; Tel: +91 044 2257 4164; E-mail: basa@ iitm.ac.in. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We acknowledge DST- FIST for the SEM facility and the IITM-NISSAN research support grant for financial support. We thank Dilli Babu Padmanaban for assistance in contact angle measurements on planar silica substrates.
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REFERENCES
(1) Pickering, S. U. CXCVI.Emulsions. J. Chem. Soc. 1907, 91 (0), 2001−2021. (2) Ramsden, W. Separation of Solids in the Surface-Layers of Solutions and ‘Suspensions’ (Observations on Surface-Membranes, Bubbles, Emulsions, and Mechanical Coagulation). – Preliminary Account. Proc. R. Soc. London 1903, 72 (477−486), 156−164. (3) Garti, N. Progress in stabilization and transport phenomena of double emulsions in food applications. LWT–Food Sci. Technol. 1997, 30 (3), 222−235. (4) Guzey, D.; McClements, D. J. Formation, stability and properties of multilayer emulsions for application in the food industry. Adv. Colloid Interface Sci. 2006, 128, 227−248. (5) Favre, S.; Griat, J.; Lorant, R.; Terren, N. Oil-in-water emulsion, a composition comprising this emulsion and use of this emulsion in cosmetics and in hygiene. US Patent 5993832 A, 1999. (6) Davis, S. S.; Washington, C.; West, P.; Illum, L.; Liversidge, G.; Sternson, L.; Kirsh, R. Lipid emulsions as drug delivery systems. Ann. N.Y. Acad. Sci. 1987, 507 (1), 75−88. (7) Binks, B. P.; Philip, J.; Rodrigues, J. A. Inversion of silicastabilized emulsions induced by particle concentration. Langmuir 2005, 21 (8), 3296−3302. (8) Binks, B. P.; Whitby, C. P. Nanoparticle silica-stabilised oil-inwater emulsions: Improving emulsion stability. Colloids Surf., A 2005, 253 (1−3), 105−115. (9) Binks, B.; Lumsdon, S. Catastrophic phase inversion of water-inoil emulsions stabilized by hydrophobic silica. Langmuir 2000, 16 (6), 2539−2547. (10) Midmore, B. Preparation of a novel silica-stabilized oil/water emulsion. Colloids Surf., A 1998, 132 (2), 257−265. (11) Binks, B. P.; Whitby, C. P. Temperature-dependent stability of water-in-undecanol emulsions. Colloids Surf., A 2003, 224 (1), 241− 249. (12) Binks, B.; Lumsdon, S. Effects of oil type and aqueous phase composition on oil−water mixtures containing particles of intermediate hydrophobicity. Phys. Chem. Chem. Phys. 2000, 2 (13), 2959− 2967. (13) Menon, V.; Nikolov, A.; Wasan, D. Interfacial effects in solidsstabilized emulsions: Measurements of film tension and particle interaction energy. J. Colloid Interface Sci. 1988, 124 (1), 317−327. (14) Whitby, C. P.; Fornasiero, D.; Ralston, J. Effect of adding anionic surfactant on the stability of Pickering emulsions. J. Colloid Interface Sci. 2009, 329 (1), 173−181. (15) Tambe, D. E.; Sharma, M. M. Factors controlling the stability of colloid-stabilized emulsions: I. An experimental investigation. J. Colloid Interface Sci. 1993, 157 (1), 244−253.
Figure 7. Scanning electron microscopy image of hollow shells obtained by drying silica-rod stabilized emulsions: The emulsion used was obtained by phase separation in critical mixture of lutidine/water dispersed with 2 wt % rod shaped silica particles (AR ∼ 15). The dry shell in the figure indicates a multilayer of rod-shaped silica particles at the surface.
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CONCLUSION Pickering emulsion stabilized by rod-shaped silica particles were formed by arresting the temperature induced phase separation in critical mixture of L/W. Although the phase separation occurs via spinodal decomposition, probably due to low concentration of particles at the spinodal interface at initial stages of phase separation, interfaces break into spherical droplets. Emulsion droplet size was tuned by varying particle concentration, and the size of the droplets followed the prediction of the limited coalescence model. Pickering emulsions thus formed were found to be robust and resulted in hollow shells of silica rods upon evaporation. With appropriate surface modification, this methodology can be extended to fabricate bicontinuous interfacially jammed emulsion gels (‘bijels’) stabilized by anisotropic colloids, which in turn can be used as templates for novel materials.
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AUTHOR INFORMATION
ASSOCIATED CONTENT
S Supporting Information *
A short summary on the measurement of the contact angle of sessile drops of water and lutidine on planar silica substrates is available free of charge via the Internet at The Supporting E
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Langmuir (16) Tambe, D. E.; Sharma, M. M. The effect of colloidal particles on fluid−fluid interfacial properties and emulsion stability. Adv. Colloid Interface Sci. 1994, 52, 1−63. (17) Madivala, B.; Vandebril, S.; Fransaer, J.; Vermant, J. Exploiting particle shape in solid stabilized emulsions. Soft Matter 2009, 5 (8), 1717−1727. (18) Dugyala, V. R.; Daware, S. V.; Basavaraj, M. G. Shape anisotropic colloids: Synthesis, packing behavior, evaporation driven assembly, and their application in emulsion stabilization. Soft Matter 2013, 9 (29), 6711−6725. (19) Wang, H.; Hobbie, E. K. Amphiphobic Carbon Nanotubes as Macroemulsion Surfactants. Langmuir 2003, 19 (8), 3091−3093. (20) Hobbie, E. K.; Bauer, B. J.; Stephens, J.; Becker, M. L.; McGuiggan, P.; Hudson, S. D.; Wang, H. Colloidal Particles Coated and Stabilized by DNA-Wrapped Carbon Nanotubes. Langmuir 2005, 21 (23), 10284−10287. (21) Feng, T.; Hoagland, D. A.; Russell, T. P. Assembly of AcidFunctionalized Single-Walled Carbon Nanotubes at Oil/Water Interfaces. Langmuir 2014, 30 (4), 1072−1079. (22) Davies, G. B.; Krüger, T.; Coveney, P. V.; Harting, J. Detachment energies of spheroidal particles from fluid−fluid interfaces. J. Chem. Phys. 2014, 141 (15), 154902. (23) Basavaraj, M.; Fuller, G.; Fransaer, J.; Vermant, J. Packing, flipping, and buckling transitions in compressed monolayers of ellipsoidal latex particles. Langmuir 2006, 22 (15), 6605−6612. (24) Loudet, J.-C.; Pouligny, B. Self-assembled capillary arrows. Europhys. Lett. 2009, 85 (2), 28003. (25) Loudet, J. C.; Alsayed, A. M.; Zhang, J.; Yodh, A. G. Capillary Interactions Between Anisotropic Colloidal Particles. Phys. Rev. Lett. 2005, 94 (1), 018301. (26) Vandebril, S.; Vermant, J.; Moldenaers, P. Efficiently suppressing coalescence in polymer blends using nanoparticles: Role of interfacial rheology. Soft Matter 2010, 6 (14), 3353−3362. (27) Stratford, K.; Adhikari, R.; Pagonabarraga, I.; Desplat, J.-C.; Cates, M. Colloidal jamming at interfaces: A route to fluidbicontinuous gels. Science 2005, 309 (5744), 2198−2201. (28) Herzig, E.; White, K.; Schofield, A.; Poon, W.; Clegg, P. Bicontinuous emulsions stabilized solely by colloidal particles. Nat. Mater. 2007, 6 (12), 966−971. (29) Frijters, S.; Harting, J. Self-assembled porous media from particle-stabilized emulsions. arXiv.org, e-Print Arch., Condens. Matter 2014, No. arXiv:1408.2974. (30) Hijnen, N.; Clegg, P. S. Simple Synthesis of Versatile Akaganéite-Silica Core−Shell Rods. Chem. Mater. 2012, 24 (17), 3449−3457. (31) Imperiali, L.; Clasen, C.; Fransaer, J.; Macosko, C. W.; Vermant, J. A simple route towards graphene oxide frameworks. Mater. Horiz. 2014, 1 (1), 139−145. (32) Günther, F.; Janoschek, F.; Frijters, S.; Harting, J. Lattice Boltzmann simulations of anisotropic particles at liquid interfaces. Comput. Fluids 2013, 80, 184−189. (33) Kuijk, A.; van Blaaderen, A.; Imhof, A. Synthesis of monodisperse, rodlike silica colloids with tunable aspect ratio. J. Am. Chem. Soc. 2011, 133 (8), 2346−2349. (34) Ho, C. C.; Ottewill, R. H.; Yu, L. Examination of Ellipsoidal Polystyrene Particles by Electrophoresis. Langmuir 1997, 13 (7), 1925−1930. (35) O’Brien, R. W.; White, L. R. Electrophoretic Mobility of a Spherical Colloidal Particle. J. Chem. Soc. Faraday Trans. II 1978, 74, 1607−1626. (36) Chou, Y.-C.; Goldburg, W. I. Phase separation and coalescence in critically quenched isobutyric-acid−water and 2,6-lutidine−water mixtures. Phys. Rev. A 1979, 20 (5), 2105. (37) Wang, H.; Singh, V.; Behrens, S. H. Image Charge Effects on the Formation of Pickering Emulsions. J. Phys. Chem. Lett. 2012, 3 (20), 2986−2990. (38) Gambassi, A.; Maciołek, A.; Hertlein, C.; Nellen, U.; Helden, L.; Bechinger, C.; Dietrich, S. Critical Casimir effect in classical binary liquid mixtures. Phys. Rev. E 2009, 80, 061143.
(39) Lewandowski, E. P.; Cavallaro, M., Jr.; Botto, L.; Bernate, J. C.; Garbin, V.; Stebe, K. J. Orientation and self-assembly of cylindrical particles by anisotropic capillary interactions. Langmuir 2010, 26 (19), 15142−15154. (40) Arditty, S.; Whitby, C. P.; Binks, B. P.; Schmitt, V.; LealCalderon, F. Some general features of limited coalescence in solidstabilized emulsions. Eur. Phys. J. E 2003, 11 (3), 273−281.
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