Thermosensitive Pickering Emulsion Stabilized by Poly(N

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Langmuir 2008, 24, 3300-3305

Thermosensitive Pickering Emulsion Stabilized by Poly(N-isopropylacrylamide)-Carrying Particles Sakiko Tsuji and Haruma Kawaguchi* Keio UniVersity, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan ReceiVed June 14, 2007. In Final Form: September 3, 2007 Poly(N-isopropylacrylamide) (PNIPAM)-carrying particles were characterized as thermosensitive Pickering emulsifiers. Emulsions were prepared from various oils, such as heptane, hexadecane, trichloroethylene, and toluene, with PNIPAM-carrying particles. PNIPAM-carrying particles preferentially formed oil-in-water (O/W)-type emulsions with a variety of oils. All the emulsions stabilized by PNIPAM-carrying particles were stable for more than 3 months as long as they were stored at room temperature. However, when the emulsions were heated from room temperature to 40 °C, at which point the PNIPAM layer caused a coil-to-globule transition, phase separation occurred. Thus, by using thermosensitive PNIPAM-carrying particles as emulsifiers, the stability of the Pickering emulsions could be controlled by a slight change in temperature.

Introduction It has long been known that small solid particles act like surfactant molecules that adsorb at fluid/fluid or gas/fluid interfaces and stabilize emulsions or foams.1,2 Solid emulsion stabilizers, called “Pickering emulsifiers,” offer a number of advantages3 over conventional surfactants. For example, (i) they can avoid or reduce the amount of low molecular surfactants and lower the toxicity; (ii) they make robust emulsions, and it is generally difficult to break Pickering emulsions by changing the chemical or physical parameters, such as the pH, salt concentration, temperature, and composition of the oil phase; and (iii) the viscosity of the emulsions can be adjusted to required practical applications by changing the solid content or the type of solid. These characteristics make Pickering emulsions attractive in the industries of pharmaceuticals, food, and cosmetics. Inorganic particles such as silica, carbon black, barium sulfate, and calcium carbonate have been widely used as Pickering emulsifiers.4-11 Even though the emulsifying properties of these solid particles have been recognized for more than a century, it is only recently that the precise role of the solid particles has begun to be elucidated in surfactant-free systems.5,12-22 The effectiveness of the particulate emulsifier depends on the particle * To whom correspondence should be addressed. Telephone: +81 45 566 1563. Fax: +81 45 564 5095. E-mail: [email protected]. (1) Pickering, S. U. J. Chem. Soc. 1907, 91, 2001. (2) Ramsden, W. Proc. R. Soc. London 1903, 72, 156-164. (3) Abend, S.; Bonnke, N.; Gutschner, U.; Lagaly, G. Colloid Polym. Sci. 1998, 276 (8), 730-737. (4) Vignati, E.; Piazza, R.; Lockhart, T. P. Langmuir 2003, 19 (17), 66506656. (5) Binks, B. P.; Lumsdon, S. O. Langmuir 2001, 17 (15), 4540-4547. (6) Saleh, N.; Sarbu, T.; Sirk, K.; Lowry, G. V.; Matyjaszewski, K.; Tilton, R. D. Langmuir 2005, 21 (22), 9873-9878. (7) Thieme, J.; Abend, S.; Lagaly, G. Colloid Polym. Sci. 1999, 277 (2-3), 257-260. (8) Binks, B. P.; Lumsdon, S. O. Langmuir 2000, 16 (6), 2539-2547. (9) Midmore, B. R. Colloids Surf., A 1998, 145 (1-3), 133-143. (10) Binks, B. P.; Lumsdon, S. O. Phys. Chem. Chem. Phys. 1999, 1 (12), 3007-3016. (11) Wang, H.; Hobbie, E. K. Langmuir 2003, 19 (8), 3091-3093. (12) Binks, B. P. Curr. Opin. Colloid Interface Sci. 2002, 7 (1-2), 2141. (13) Binks, B. P.; Lumsdon, S. O. Phys. Chem. Chem. Phys. 2000, 2 (13), 2959-2967. (14) Nonomura, Y.; Komura, S.; Tsujii, K. Langmuir 2005, 21 (21), 94099411. (15) Binks, B. P.; Lumsdon, S. O. Langmuir 2000, 16 (23), 8622-8631. (16) Levine, S.; Bowen, B. D.; Partridge, S. J. Colloids Surf. 1989, 38 (4), 325-343.

wettability,15,19 oil type,13 particle size,5,20 particle shape,4,14,23 particle concentration, and interparticle interactions.12,16 The behavior of particle-stabilized liquid/liquid interfaces has been examined as well.24-27 Recently, research in this field has focused on synthesizing designed particles,6,11,28 such as surface-modified polystyrene particles,29-33 shell cross-linked micelles,34,35 and silica-polymer composite particles,36 to use as emulsion stabilizers. Pickering emulsions are also used in miniemulsion polymerization systems37,38 and as templates for preparing advanced materials,39-41 such as Janus particles,42 microlens arrays,43 and colloid capsules.44 (17) Levine, S.; Bowen, B. D.; Partridge, S. J. Colloids Surf. 1989, 38 (4), 345-364. (18) Aveyard, R.; Clint, J. H.; Nees, D.; Paunov, V. N. Langmuir 2000, 16 (4), 1969-1979. (19) Binks, B. P.; Clint, J. H. Langmuir 2002, 18 (4), 1270-1273. (20) Golemanov, K.; Tcholakova, S.; Kralchevsky, P. A.; Ananthapadmanabhan, K. P.; Lips, A. Langmuir 2006, 22 (11), 4968-4977. (21) Danov, K. D.; Kralchevsky, P. A.; Ananthapadmanabhan, K. P.; Lips, A. Langmuir 2006, 22 (1), 106-115. (22) Binks, B. P.; Murakami, R. Nat. Mater. 2006, 5 (11), 865-869. (23) Nonomura, Y.; Komura, S.; Tsujii, K. Langmuir 2004, 20 (26), 1182111823. (24) Tarimala, S.; Dai, L. L. Langmuir 2004, 20 (9), 3492-3494. (25) Dai, L. L.; Sharma, R.; Wu, C. Y. Langmuir 2005, 21 (7), 26412643. (26) Nonomura, Y.; Fukuda, K.; Komura, S.; Tsujii, K. Langmuir 2003, 19 (24), 10152-10156. (27) Nonomura, Y.; Sugawara, T.; Kashimoto, A.; Fukuda, K.; Hotta, H.; Tsujii, K. Langmuir 2002, 18 (26), 10163-10167. (28) Alargova, R. G.; Warhadpande, D. S.; Paunov, V. N.; Velev, O. D. Langmuir 2004, 20 (24), 10371-10374. (29) Fujii, S.; Randall, D. P.; Armes, S. P. Langmuir 2004, 20 (26), 1132911335. (30) Amalvy, J. I.; Unali, G. F.; Li, Y.; Granger-Bevan, S.; Armes, S. P.; Binks, B. P.; Rodrigues, J. A.; Whitby, C. P. Langmuir 2004, 20 (11), 43454354. (31) Amalvy, J. I.; Armes, S. P.; Binks, B. P.; Rodrigues, J. A.; Unali, G. F. Chem. Commun. 2003, (15), 1826-1827. (32) Binks, B. P.; Murakami, R.; Armes, S. P.; Fujii, S. Angew. Chem., Int. Ed. 2005, 44 (30), 4795-4798. (33) Fujii, S.; Ryan, A. J.; Armes, S. P. J. Am. Chem. Soc. 2006, 128 (24), 7882-7886. (34) Fujii, S.; Cai, Y. L.; Weaver, J. V. M.; Armes, S. P. J. Am. Chem. Soc. 2005, 127 (20), 7304-7305. (35) Weaver, J. V. M.; Tang, Y. Q.; Liu, S. Y.; Iddon, P. D.; Grigg, R.; Billingham, N. C.; Armes, S. P.; Hunter, R.; Rannard, S. P. Angew. Chem., Int. Ed. 2004, 43 (11), 1389-1392. (36) Fujii, S.; Read, E. S.; Binks, B. P.; Armes, S. P. AdV. Mater. 2005, 17 (8), 1014-1018. (37) Voorn, D. J.; Ming, W.; van Herk, A. M. Macromolecules 2006, 39 (6), 2137-2143. (38) Cauvin, S.; Colver, P. J.; Bon, S. A. F. Macromolecules 2005, 38 (19), 7887-7889.

10.1021/la701780g CCC: $40.75 © 2008 American Chemical Society Published on Web 03/07/2008

PNIPAM-Carrying Particles as Pickering Emulsifiers

Pickering emulsifiers irreversibly adsorb onto liquid/liquid surfaces to form robust emulsions that can be stored for a long time. On the other hand, the energy required to remove a particle from the interface can be much higher than the corresponding energy of removing a surfactant molecule in a conventional emulsion system. Environment-responsive emulsifiers that change the emulsion stability depending on surrounding chemical or physical parameters (e.g., the temperature, pH, or ionic strength) may solve this problem. There have been some reports of success achieved in removing solid particles from the once-formed emulsion surfaces. Fuller et al. used paramagnetite particles to prepare Pickering emulsions and detached the particles from liquid/liquid surfaces by applying a strong magnetic field.45 Various pH-responsive Pickering emulsions were prepared by Armes et al. , who synthesized pH-responsive shell cross-linked micelles and silica-polymer composite microgels and succeeded in leading Pickering emulsions prepared from these particles to phase separation into oil and water phases by lowering the pH to 2.29-31,34,36 On the other hand, there have only been a few reports on thermosensitive Pickering emulsions. In 2005, Binks et al. reported emulsions in which destabilization or inversion occurred at relatively high temperature (50-60 °C) when poly(2-(dimethylamino)ethyl methacrylate)-carrying particles were used as Pickering emulsifiers.46 As a practical thermosensitive Pickering emulsifier, a particle prepared from poly(N-isopropylacrylamide) (PNIPAM) may be a promising candidate. PNIPAM is a representative thermosensitive polymer that shows coil-to-globule transition in water when the solution is heated. The amide groups of PNIPAM make it water-soluble when the solution temperature is lower than 32 °C; however, it becomes insoluble in water above the critical temperature. The driving force of precipitation at high temperature is the entropy gain by the release of water molecules that are partially immobilized by the isopropyl groups. On the other hand, the amphiphilic nature of PNIPAM, which has both amide groups and isopropyl groups, makes it surface-active.47-49 It has also been confirmed that PNIPAM-carrying particles work as surfaceactive materials and adsorb onto an air/water interface.50,51 Behrens et al. prepared Pickering emulsions stabilized by poly(NIPAM-co-methacrylic acid) microgel particles.52,53 They reported that the emulsions were destabilized at 60 °C when the pH was adjusted. Also, Dinsmore et al. showed colloid capsules of poly(NIPAM-co-acrylic acid) microgel particles using waterin-oil (W/O) emulsions as templates and the diblock copolymers as cross-linking agents.54 The obtained colloid capsules were elastic, and reversibly changed the diameter by the temperature change. (39) Strohm, H.; Lobmann, P. J. Mater. Chem. 2004, 14 (17), 2667-2673. (40) Velev, O. D.; Furusawa, K.; Nagayama, K. Langmuir 1996, 12 (10), 2374-2384. (41) Velev, O. D.; Furusawa, K.; Nagayama, K. Langmuir 1996, 12 (10), 2385-2391. (42) Tsuji, S.; Kawaguchi, H. Langmuir 2005, 21 (18), 8439-8442. (43) Cayre, O. J.; Paunov, V. N. J. Mater. Chem. 2004, 14 (22), 3300-3302. (44) Dinsmore, A. D.; Hsu, M. F.; Nikolaides, M. G.; Marquez, M.; Bausch, A. R.; Weitz, D. A. Science 2002, 298 (5595), 1006-1009. (45) Melle, S.; Lask, M.; Fuller, G. G. Langmuir 2005, 21 (6), 2158-2162. (46) Binks, B. P.; Murakami, R.; Armes, S. P.; Fujii, S. Angew. Chem., Int. Ed. 2005, 44 (30), 4795-8. (47) Kawaguchi, M.; Saito, W.; Kato, T. Macromolecules 1994, 27 (20), 58825884. (48) Zhang, J.; Pelton, R. Langmuir 1996, 12 (10), 2611-2612. (49) Kawaguchi, M.; Hirose, Y.; Kato, T. Langmuir 1996, 12 (14), 35233526. (50) Tsuji, S.; Kawaguchi, H. Submitted for publication. (51) Chan, K.; Pelton, R.; Zhang, J. Langmuir 1999, 15 (11), 4018-4020. (52) Ngai, T.; Auweter, H.; Behrens, S. H. Macromolecules 2006, 39 (23), 8171-8177. (53) Ngai, T.; Behrens, S. H.; Auweter, H. Chem. Commun. 2005, (3), 331333.

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In this paper, the PNIPAM-carrying particles are evaluated as thermosensitive Pickering emulsifiers that change the emulsion stability at moderate temperature changes, 25 °C < T < 40 °C. The dependence of emulsion formation and their stability on oil types, particle structures, and temperature were examined. Experimental Section Materials. N-Isopropylacrylamide (NIPAM) was kindly given by Kojin, Co. and was purified by recrystallization from a hexanetoluene mixture. Styrene (St) and glycidyl methacrylate (GMA) were distilled under reduced pressure before use. Potassium persulfate (KPS) was purified by recrystallization from water. N,N-Methylenebisacrylamide (BIS), ammonium cerium nitrate (Ce), and divinyl benzene (DVB) were used as received. Water used in this experiment was MilliQ water. Hexadecane (HD), heptane (HP, Junsei Chemical Co. Ltd.), trichloroethylene (TCE, Junsei Chemical Co. LTd.), 1-undecanol (UD, Aldrich), and toluene (TL, Junsei Chemical Co. Ltd.) were all used as received. The reagents were purchased from Wako Pure Chemicals Co. unless otherwise noted. Characterization of Oils. The surface tension of the oils and interfacial tension of oil/water were measured with a tensiometer (ST-1S-PC1F, Shimadzu) using the Wilhelmy plate method. The sample was settled at a given temperature in a tensiometer, and the glass plate was then attached to an electrobalance. The liquid was slowly moved up using the stage until the edge of the plate touched the air/liquid or liquid/liquid interface. Preparation and Characterization of Particles. As PNIPAMcarrying particles, PNIPAM microgel particles and PNIPAM-haircarrying particles were prepared without using low-molecular weight surfactants. PNIPAM microgel particles were prepared by precipitation polymerization. The reaction vessel was loaded with 110 mL of water, 2.84 g of NIPAM, and 0.16 g of BIS. The mixture was maintained at 70 °C and purged with nitrogen for 30 min prior to polymerization. Next, 0.06 g of an initiator (KPS) was dissolved in 10 mL of water and added to the vessel to initiate polymerization. The reaction was continued for 4.5 h. The particles thus obtained were cleaned by repeating centrifugation and decantation. For the preparation of PNIPAM-carrying hairy particles, polystyrene core particles were first prepared via soap-free emulsion polymerization. Surface modification of core particles was then conducted by surface initiation graft polymerization. The core particles were synthesized by copolymerizing 1.2 g of St, 1.8 g of GMA, and 0.04 g of DVB with 0.06 g of KPS in 120 mL of water at 70 °C. Two hours after initiation, an additional 2 g of GMA was added to the vessel, and the reaction was continued for 24 h. GMA units on the particle surface were partially hydrolyzed to form glycol groups during polymerization.55 After purification of the core particles, NIPAM was graft-polymerized from core particle surfaces in water at 25 °C by using the redox reaction between Ce and the glycol groups.56 To initiate graft polymerization, 0.5 g of core particles and 0.075 g of Ce were added to 75 g of acetic buffer (pH 3.3, 15 mM). The hairy particles thus obtained were purified by repeating centrifugation and decantation.57-60 The particles were characterized by dynamic light scattering (DLS; PAR-III, Otsuka Electronic Co.) for size measurements. Preparation of Pickering Emulsions and Their Characterization. The total volume of all emulsions prepared was 7.5 mL. Each (54) Lawrence, D. B.; Cai, T.; Hu, Z.; Marquez, M.; Dinsmore, A. D. Langmuir 2007, 23 (2), 395-398. (55) Inomata, Y.; Wada, T.; Handa, H.; Fujimoto, K.; Kawaguchi, H. J. Biomater. Sci., Polym. Ed. 1994, 5 (4), 293-302. (56) Behari, K.; Agrawal, U.; Das, R. Polymer 1993, 34 (21), 4557-4561. (57) Takata, S.; Shibayama, M.; Sasabe, R.; Kawaguchi, H. Polymer 2003, 44 (2), 495-501. (58) Tsuji, S.; Kawaguchi, H. e-Polymers 2005, 076, 1-7. (59) Matsuoka, H.; Fujimoto, K.; Kawaguchi, H. Polym. J. 1999, 31 (11), 1139-1144. (60) Matsuoka, H.; Fujimoto, K.; Kawaguchi, H. Polym. Gels Networks 1998, 6 (5), 319-332.

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Figure 1. Characterization of poly(N-isopropylacrylamide)-carrying hairy particles. (a) Hydrodynamic diameters of microgel particles and hairy particles depending on temperature. (b) and (c) FE-TEM observation of hairy particles. emulsion sample contained 50 mg of latex, and the latexes were dispersed in aqueous phases in the initial state. A mixture of given amounts of aqueous latex dispersions (5 mL) and oils (2.5 mL) was stirred for 5 min at normal temperature (25 °C) with a magnetic stirrer. Emulsion stabilities were assessed after standing for 24 h at a given temperature by measuring the volume fraction of the water, emulsion, and oil phases. Photographs of the emulsions were taken with a digital camera (SP-350, Olympus) in the daylight. The conductivity of the emulsions was measured immediately after emulsion preparation using a digital conductivity meter (CM115, Kyoto Electronics Manufacturing Co., Ltd.). The emulsions (O/W or W/O) were classified by their conductivities (conductivity of an emulsion lower than few µS/cm indicates a W/O emulsion formation, and a higher value indicates an O/W emulsion formation). In addition, the emulsion type was confirmed by a drop test. A drop of emulsion was added to both oil and water to check its ease of spreading. A water continuous emulsion easily disperses in water and remains as a drop in oil or vice versa. A droplet of the emulsion was mounted on a glass slide (Matsunami Glass Ind., Ltd.) and examined by using an optical microscope (BX51, Olympus) equipped with a digital camera (XD200, Flovel Co., Ltd.). The size of the emulsion was determined by calculating the average diameter of 50 droplets. Observation at the liquid/liquid interface was carried out with FLVFS-FIS Ver. 1.12 software on a computer. The contact angles of oil, water, and hairy particles at a given temperature were determined using a contact anglemeter (G-I type, ERMA Inc.) with a thermostatted cell. Oil droplets (1 µL) were put on films of hairy particles soaked in water. The films were prepared by coating hairy particles on polystyrene substrates.

Results and Discussion Formation of Pickering Emulsions. The surface activity of linear PNIPAM and PNIPAM microgel particles has been reported by several researchers.47,50,51 Thus, PNIPAM-carrying particles are expected to work as effective emulsion stabilizers. In this paper, PNIPAM microgel particles and PNIPAM-hair-carrying particles were used as thermosensitive Pickering emulsifiers. PNIPAM-carrying hairy particles were proposed by Kawaguchi et al.,57,61-63 and they are composed of polystyrene core particles with tethered PNIPAM chains on the surfaces.

Table 1. Hydrodynamic Diameters of PNIPAM-Carrying Particles at Given Temperatures (Measured by DLS) at 25 °C at 40 °C

microgel particle

hairy particle

792 ( 60 nm 461 ( 30 nm

817 ( 40 nm 305 ( 20 nm

PNIPAM microgel particles and PNIPAM-carrying hairy particles were obtained by precipitation polymerization64 and by graft polymerization of NIPAM from polystyrene particles, respectively. Dynamic light-scattering analysis revealed that both particles were monodispersed (CV < 7.6%). The thermosensitivity of these particles was characterized by measuring the hydrodynamic diameters depending on the temperature (Figure 1a). The hydrodynamic diameter of the core particle was 280 nm at 25 °C, and that of the hairy particle increased to 817 nm after graft polymerization of NIPAM on the surface. The number of PNIPAM chains grafted onto the core particles was estimated as 0.0039 chain/nm2.57 Transmission electron microscopy (TEM) pictures of hairy particles are shown in the same figure. The hydrodynamic diameters of microgel particles and hairy particles in the swollen state (at 25 °C) and shrunken states (at 40 °C) are listed in Table 1. The oils used for the preparation of the Pickering emulsions and their interfacial tensions of air/oil (γao) and oil/ water (γow) are listed in Table 2. Using the PNIPAM-carrying particles as emulsifiers, a series of emulsions was prepared. Their size and type are shown in Table 3. The surface coverage of the emulsion droplets with PNIPAM-carrying particles was also measured from optical microscopy observations, and the values are listed in the table. The emulsion phases stabilized by PNIPAM-carrying particles were very stable in all cases. Their relative phase volumes did not change for more than 3 months as long as they were stored at room temperature. The energy of attachment of a single particle (61) Kawaguchi, H.; Isono, Y.; Tsuji, S. Macromol. Symp. 2002, 179, 75-87. (62) Tsuji, S.; Kawaguchi, H. Langmuir 2004, 20 (6), 2449-55. (63) Tsuji, S.; Kawaguchi, H. Macromolecules 2006, 39 (13), 4338-4344. (64) Pelton, R. H.; Chibante, P. Colloids Surf. 1986, 20 (3), 247-256.

PNIPAM-Carrying Particles as Pickering Emulsifiers

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Table 2. Characterization of the Oils Used for the Preparation of Pickering Emulsions oil

code

d (g/mL)

γao (mN/m)

γow (mN/m)

Wa (mJ/m2)

heptane hexadecane trichloroethylene toluene 1-undecanol

HP HD TCE TL UD

0.682-0.686 0.771-0.777 1.464-1.472 0.8658 0.832

19.87 ( 0.13 26.61 ( 0.34 28.74 ( 0.26 28.06 ( 0.47 28.54 ( 0.52

49.35 ( 0.44 55.15 ( 4.32 39.69 ( 0.64 35.35 ( 0.55 7.84 ( 0.16

43.27 44.21 61.80 65.46 93.45

Table 3. Characterization of Emulsions Formeda

particle microgel particle

oil

conductivity (µS/cm)

size (µm)

type

surface coverage (%)

HP 81 23 ( 13 O/W 98 HD 75 10 ( 4 O/W 100 TCE 129 46 ( 21 O/W 85 TL 78 27 ( 18 O/W 100 UD two phase, particle partially moved to oil phase

hairy particle HP HD TCE TL UD

44 25 ( 7 O/W 98 48 18 ( 12 O/W 99 96 90 ( 18 O/W 77 50 58 ( 28 O/W 100 two phase, particle moved to oil phase

a Emulsions were prepared from 5 mL of water containing 50 mg of particles with 2.5 mL of each oil.

of intermediate wettability at the oil/water interface can be very high relative to the thermal energy; as a result, particles can be irreversibly adsorbed onto the interface. The irreversible adsorption of PNIPAM-carrying particles formed robust layers at the oil/water interface, and this might achieve long storage stability of the emulsions by preventing Ostwald ripening. A similar idea is used in miniemulsion polymerization system. When a long chain alkane or alcohol is used as a cosurfactant for a miniemulsion system, it creates a barrier to droplet/droplet coalescence and makes the miniemulsion stable.65 The layer of PNIPAM-carrying particles on an emulsion surface may have a similar mechanical effect on the cosurfactants of the miniemulsion system. A typical example of emulsions stabilized by PNIPAM-carrying particles is shown in Figure 2a. In a series of experiments, O/W type emulsions were formed in most cases; however, no emulsion was generated when 1-undecanol was used for the oil phase. Remarkably, hairy particles initially dispersed in the water phase transferred into the 1-undecanol phase without forming emulsions when an external mechanical force was applied, as shown in Figure 2b and c. Representative examples of optical micrographs of the emulsions prepared from PNIPAM-carrying particles are shown in Figure 3. They are toluene (TL) in water emulsions stabilized by PNIPAM microgel particles (a-c) and TL in water emulsions stabilized by hairy particles (d-f). As can be seen from the pictures, both oil droplets were covered with coherent layers of PNIPAM-carrying particles. As listed in Table 3, the surface coverage of the droplets with particles was relatively high (>75%) in all cases. Some of the recent studies reported that emulsions can be stabilized by low coverage of the interface. For example, Midmore (65) Lovell, P. A.; El-Aasser, M. S. Emulsion polymerization and emulsion polymers; John Wiley & Sons Ltd.: Chichester, U.K., 1997; p 701. (66) Midmore, B. R. Colloids Surf., A 1998, 132 (2-3), 257-265. (67) Horozov, T. S.; Binks, B. P. Angew. Chem., Int. Ed. 2006, 45 (5), 773776. (68) Binks, B. P.; Kirkland, M. Phys. Chem. Chem. Phys. 2002, 4 (15), 37273733.

reported stable Pickering emulsions at about 29% coverage of the droplets with silica spheres,66 and Vignati et al. obtained stable emulsions even though the coverage of the emulsion was only 5%.4 In those cases, the bridging monolayer or the network of the particles on the emulsion surface is considered to be important. On the other hand, many researchers observed Pickering emulsions covered with coherent particle layers.33,39,44 Binks et al. proposed two mechanisms for stabilizing emulsions or foams by solid particles: (i) mechanical barrier formation of coherent particle monolayers around the emulsion droplets and (ii) stabilization by particle bridges when the surface coverage is low.67,68 Since the surface coverage of emulsion droplets with PNIPAM-carrying particles is relatively high in this study, the formation of robust particle layers might be the reason for the stableness of the emulsions. Effect of Oil Type on Emulsion Formation. Using PNIPAMcarrying particles as the Pickering emulsifier, O/W emulsions were generated in all the experiments (Table 3). Binks et al. studied the relationship between the polarity of oils and the preferential emulsion type.13 They evaluated the oil polarity by

Figure 2. Typical example of emulsion formation. (a) PNIPAM microgel-stabilized hexadecane (HD) emulsion in water. (b) When 1-undecanol (UD) was used as oil, hairy particles that initially dispersed in water moved into the oil phase. (c) Schematic illustration of (b).

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Figure 3. Observations of Pickering emulsions by optical microscopy. (a-c) Toluene in water emulsion stabilized by PNIPAM microgel particles. (d-f) Toluene in water emulsion stabilized by hairy particles. (c) and (f) are close up images of the emulsion surfaces.

Figure 4. Changes in volume fractions of oil (o), hairy particle-stabilized emulsion (e), and water (w) phases when the temperature changed from 25 to 40 °C.

the work of adhesion, Wa, and studied silica-stabilized emulsions with a variety of oils. The work of adhesion existing between two liquids is given by the Dupre´ equation,

Wa ) γoa + γaw - γow

(1)

where γaw represents the surface tension of water. Using partially hydrophobized silica particles, oils having low Wa values preferentially formed O/W emulsions even at low water content, and oils of higher Wa values only formed W/O emulsions. When the Wa of oils ranged from 43 to 65 mJ/m2 (Table 2), O/W emulsions were preferentially formed (Table 3). Interestingly, when 1-undecanol was used as the oil phase, PNIPAMcarrying hairy particles moved to the oil phase from initially dispersed aqueous media and did not form emulsions (Figure 2b and c). This indicated that the wettability of hairy particles is higher for 1-undecanol than for water. A similar observation was reported by Behrens et al.,53 who showed that poly(NIPAM-

co-methacrylic acid) microgel particles initially dispersed in water were driven into the bulk octanol phase when carboxyl groups of methacrylic acid units were protonated. Thus, PNIPAMcarrying particles may preferentially exist in alcohols than in the aqueous phase. From these observations, it can be concluded that PNIPAMcarrying particles preferentially form O/W type emulsions with a variety of oils. Effect of Temperature. When emulsions prepared from hairy particles at room temperature were heated to 40 °C, the volume fractions of the water, emulsion, and oil phases changed (Figure 4). Emulsions were remarkably stable at 25 °C; however, the size distribution of the emulsions became broad at elevated temperatures, and the volumes of the oil phases increased when they were heated. These changes indicated that the emulsions were destabilized at 40 °C and part of them coalesced. Notable changes happened in emulsions prepared from hairy particles

PNIPAM-Carrying Particles as Pickering Emulsifiers

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Equation 2 indicates that the particle adsorption energy onto the interface is reduced as the particle shrinks at elevated temperature. Since little change was observed for the cos θ term in eq 2 (see the Supporting Information), size change might be a factor in the destabilization of the emulsions at 40 °C. The hairy particles dispersed in phase-separated emulsions were reusable. After the emulsions collapsed at 40 °C, stable emulsions were regenerated by agitation at 25 °C. Figure 5. Photographs of the phase separation of trichloroethylene (TCE) emulsions stabilized by hairy particles.

with TCE/water and TL/water. They exhibited macroscopic separation into oil and aqueous phases at elevated temperature, as shown in Figure 5. The TCE phase, which is denser than water (dTCE ) 1.46-1.47 g/mL), came to the bottom phase, and the water phase, which contained hairy particles, came to the upper phase. When PNIPAM microgel particles were used as Pickering emulsifiers, such macroscopic phase separation did not occur, although partial coalescence of droplets was observed. Temperature-dependent destabilization was remarkable when hairy particles were used, since the diameter of hairy particles changes more drastically than that of microgel particles when heated (Table 1). Thus, temperature-dependent emulsion stability might be explained with the surface coverage of emulsions by particles. Upon heating, the PNIPAM layer immersed in the water phase shrinks and reduces the coverage of the surface of the oil droplet. The oil droplet then starts to ripen and coalesce to compensate for the insufficient coverage by the reduction of the total interfacial area. In addition, the energy, E, required to remove a particle from the interface changes depending on the particle size. E is given by

E ) πr2γow(1 - cos θ)2

(2)

Conclusion Two types of particles, PNIPAM microgel particles and PNIPAM chain grafted hairy particles, were evaluated as Pickering emulsifiers. Both particles worked as Pickering emulsifiers to generate stable emulsions, and they preferentially formed O/W emulsions with a variety of oils. The surfaces of the emulsions were covered with coherent layers of particles, and the surface coverage was relatively high (75-100%). Pickering emulsions prepared from both types of particles were very stable as long as they were stored at room temperature, but they were destabilized when heated to 40 °C. O/W emulsions prepared from hairy particles, water, and oils of middle polarity (trichloroethylene and toluene) exhibited macroscopic phase separation into oil and water phases at elevated temperature (40 °C). Acknowledgment. We gratefully acknowledge valuable discussions with Prof. Shuichi Osanai, Prof. Kimihisa Yamamoto, and Associate Prof. Keiji Fujimoto. S.T. is grateful for a JSPS research fellowship for young scientists. Supporting Information Available: Contact angles and calculated energies of particle attachment onto oil/water interfaces. This material is available free of charge via the Internet at http://pubs.acs.org. LA701780G