Particle Self-Assembly in Ionic Liquid-in-Water Pickering Emulsions

Dec 17, 2010 - Particle self-assembly in oil-in-ionic liquid Pickering emulsions. Elizabeth M. Walker , Denzil S. Frost , Lenore L. Dai. Journal of Co...
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Particle Self-Assembly in Ionic Liquid-in-Water Pickering Emulsions Huan Ma and Lenore L. Dai* School for Engineering of Matter, Transport, and Energy, Arizona State University, Tempe, Arizona 85287, United States Received September 24, 2010 We report the self-assembly of a single species or a binary mixture of microparticles in ionic liquid-in-water Pickering emulsions, with emphases on the interfacial self-assembled particle structure and the partitioning preference of free particles in the dispersed and continuous phases. The particles form monolayers at ionic liquid-water interfaces and are close-packed on fully covered emulsion droplets or aggregated on partially covered droplets. In contrast to those at oil-water interfaces, no long-range-ordered colloidal lattices are observed. Interestingly, other than equilibrating at the ionic liquid-water interfaces, the microparticles also exhibit a partitioning preference in the dispersed and continuous phases: the sulfate-treated polystyrene (S-PS) and aldehyde-sulfate-treated polystyrene (AS-PS) microparticles are extracted to the ionic liquid phase with a high extraction efficiency, whereas the amine-treated polystyrene (A-PS) microparticles remain in the water phase.

Ionic liquids are a unique collection of liquid materials composed solely of ions. Under ambient conditions, room-temperature ionic liquids (RTILs) stay as liquids whereas conventional salts are in the crystalline state. This is because, in RTILs, the Coulombic attractions of ion pairs are damped by the large ion size and the lattice packing is frustrated by the sterical mismatch of irregular shaped ions.1 With a combination of many unique properties such as negligible volatility, nonflammability, thermal and chemical stability, and high ionic conductivity as well as potential broad applications, increasing amounts of attention have been paid to ionic liquids, especially RTILs. For example, ionic liquids have been used as solvents in polymerizations,2,3 inorganic syntheses,4-6 enzymatic reactions,7 and extractions.8-10 They have also been employed as advanced materials including ionic lubricants11,12 and ionic gels.2,13,14 (1) Hayes, R.; Warr, G. G.; Atkin, R. At the Interface: Solvation and Designing Ionic Liquids. Phys. Chem. Chem. Phys. 2010, 12, 1709–1723. (2) Lu, J.; Yan, F.; Texter, J. Advanced Applications of Ionic Liquids in Polymer Science. Prog. Polym. Sci. 2009, 34, 431–448. (3) Kubisa, P. Ionic Liquids as Solvents for Polymerization Processes-Progress and Challenges. Prog. Polym. Sci. 2009, 34, 1333–1347. (4) Ma, Z.; Yu, J.; Dai, S. Preparation of Inorganic Materials Using Ionic Liquids. Adv. Mater. 2010, 22, 261–285. (5) Alammar, T.; Mudring, A. Facile Ultrasound-Assisted Synthesis of ZnO Nanorods in an Ionic Liquid. Mater. Lett. 2009, 63, 732–735. (6) Khare, V.; Li, Z.; Mantion, A.; Ayi, A. A.; Sonkaria, S.; Voelkl, A.; Thuenemann, A. F.; Taubert, A. Strong Anion Effects on Gold Nanoparticle Formation in Ionic Liquids. J. Mater. Chem. 2010, 20, 1332–1339. (7) Yang, Z.; Pan, W. Ionic Liquids: Green Solvents for Nonaqueous Biocatalysis. Enzyme Microb. Technol. 2005, 37, 19–28. (8) Poole, C. F.; Poole, S. K. Extraction of Organic Compounds with Room Temperature Ionic Liquids. J. Chromatogr., A 2010, 1217, 2268–2286. (9) Visser, A. E.; Swatloski, R. P.; Reichert, W. M.; Mayton, R.; Sheff, S.; Wierzbicki, A.; Davis, J. H.; Rogers, R. D. Task-Specific Ionic Liquids for the Extraction of Metal Ions from Aqueous Solutions. Chem. Commun. 2001, 135–136. (10) Huddleston, J. G.; Willauer, H. D.; Swatloski, R. P.; Visser, A. E.; Rogers, R. D. Room Temperature Ionic Liquids as Novel Media for ’Clean’ Liquid-Liquid Extraction. Chem. Commun. 1998, 1765–1766. (11) Bermudez, M.; Jimenez, A.; Sanes, J.; Carrion, F. Ionic Liquids as Advanced Lubricant Fluids. Molecules 2009, 14, 2888–2908. (12) Zhou, F.; Liang, Y.; Liu, W. Ionic Liquid Lubricants: Designed Chemistry for Engineering Applications. Chem. Soc. Rev. 2009, 38, 2590–2599. (13) Torimoto, T.; Tsuda, T.; Okazaki, K.; Kuwabata, S. New Frontiers in Materials Science Opened by Ionic Liquids. Adv. Mater. 2010, 22, 1196–1221. (14) Ueki, T.; Watanabe, M. Macromolecules in Ionic Liquids: Progress, Challenges, and Opportunities. Macromolecules 2008, 41, 3739–3749. (15) Qiu, Z.; Texter, J. Ionic Liquids in Microemulsions. Curr. Opin. Colloid Interface Sci. 2008, 13, 252–262.

508 DOI: 10.1021/la103828x

In recent years, ionic liquid-based microemulsions have been studied extensively.15,16 Such microemulsions contain two liquid phases, at least one of which is an ionic liquid, and are stabilized by small-molecule surfactants. Nonionic surfactants Triton X-100 (TX-100),17-22 Tween 20,23 and Tween 8024 have been demonstrated to effectively stabilize microemulsions of ionic liquids, such as 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF6]) and 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF4 ]), and water or other organic solvents.16 In comparison to the microemulsions, which are often considered to be thermodynamically stable, very few studies on thermodynamically unstable but kinetically stable ionic-liquidbased emulsions have been reported, especially for those stabilized by solid particles. Emulsions stabilized by solid particles, instead of surfactants, are often referred to as Pickering emulsions;25,26 they are of (16) Greaves, T. L.; Drummond, C. J. Ionic Liquids as Amphiphile SelfAssembly Media. Chem. Soc. Rev. 2008, 37, 1709–1726. (17) Behera, K.; Malek, N. I.; Pandey, S. Visual Evidence for Formation of Water-in-Ionic Liquid Microemulsions. ChemPhysChem 2009, 10, 3204–3208. (18) Cheng, S.; Fu, X.; Liu, J.; Zhang, J.; Zhang, Z.; Wei, Y.; Han, B. Study of Ethylene Glycol/TX-100/Ionic Liquid Microemulsions. Colloids Surf., A 2007, 302, 211–215. (19) Gao, Y.; Li, N.; Zheng, L.; Zhao, X.; Zhang, J.; Cao, Q.; Zhao, M.; Li, Z.; Zhang, G. The Effect of Water on the Microstructure of 1-Butyl-3-Methylimidazolium tetrafluoroborate/TX-100/benzene Ionic Liquid Microemulsions. Chem.;Eur. J. 2007, 13, 2661–2670. (20) Gao, Y.; Zhang, J.; Xu, H. Y.; Zhao, X. Y.; Zheng, L. Q.; Li, X. W.; Yu, L. Structural Studies of 1-Butyl-3-Methylimidazolium tetrafluoroborate/TX-100/ p-Xylene Ionic Liquid Microemulsions. ChemPhysChem 2006, 7, 1554–1561. (21) Gao, Y. N.; Han, S. B.; Han, B. X.; Li, G. Z.; Shen, D.; Li, Z. H.; Du, J. M.; Hou, W. G.; Zhang, G. Y. TX-100/water/1-Butyl-3-Methylimidazolium Hexafluorophosphate Microemulsions. Langmuir 2005, 21, 5681–5684. (22) Gao, H. X.; Li, J. C.; Han, B. X.; Chen, W. N.; Zhang, J. L.; Zhang, R.; Yan, D. D. Microemulsions with Ionic Liquid Polar Domains. Phys. Chem. Chem. Phys. 2004, 6, 2914–2916. (23) Gao, Y.; Li, N.; Zheng, L. Q.; Zhao, X. Y.; Zhang, S. H.; Han, B. X.; Hou, W. G.; Li, G. Z. A Cyclic Voltammetric Technique for the Detection of MicroRegions of bmimPF(6)/Tween 20/H2O Microemulsions and Their Performance Characterization by UV-Vis Spectroscopy. Green Chem. 2006, 8, 43–49. (24) Zheng, Y.; Eli, W. Study on the Polarity of bmimPF6/Tween80/Toluene Microemulsion Characterized by UV-Visible Spectroscopy. J. Dispersion Sci. Technol. 2009, 30, 698–703. (25) Binks, B. P. Particles as Surfactants;Similarities and Differences. Curr. Opin. Colloid Interface Sci. 2002, 7, 21–41. (26) Tcholakova, S.; Denkov, N. D.; Lips, A. Comparison of Solid Particles, Globular Proteins and Surfactants as Emulsifiers. Phys. Chem. Chem. Phys. 2008, 10, 1608–1627.

Published on Web 12/17/2010

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Letter Table 1. Properties of Microparticles

abbreviations

surface functional groups

size (μm)

S-PS sulfate 1.0 ( 0.031 AS-PS aldehyde sulfate 1.0 ( 0.028 C-PS carboxylate 1.1 ( 0.035 amine 1.0 ( 0.023 A-PSa A-PS amine 1.0 ( 0.023 a Unless noted, A-PS particles with higher surface charge were used.

increasing interest because of their tremendous applications in oil recovery, food, and pharmaceutical formulations.27-30 Most studies have focused on oil-water Pickering emulsions,31-37 and there is indeed sparse work on ionic liquid-based Pickering emulsions. In the pioneering work by Binks and co-workers,38,39 a series of stable ionic liquid emulsions stabilized solely by fumed silica nanoparticles were successfully prepared, and the emulsion stability and phase inversions were investigated. However, it remains unclear in terms of the particle self-assembled structure at ionic liquid-water interfaces. In this letter, we report the microparticle self-assembly in ionic liquid-water emulsions with emphases on the self-assembled structure at ionic liquid-water interfaces and the partitioning preferences of free microparticles in the dispersed and continuous phases. In our experiments, [BMIM][PF6]-in-water Pickering emulsions containing a single type or a binary mixture of particles were prepared using an ultrasonic processor (Sonics VibraCell, 500 W model). The solid particles are FluoSpheres fluorescent microspheres from Molecular Probes and were received as a 2% dispersion in distilled water with 2 mM sodium azide. The dispersions are free of surfactants. We selected five types of particles with different surface chemistry, charge, hydrophobicity, and fluorescent labels; their properties are summarized in Table 1. Figure 1 shows representative confocal microscope images of ionic liquid-in-water Pickering emulsion droplets containing S-PS, A-PS, AS-PS, S-PS/A-PS, and S-PS/AS-PS particles, respectively. Each bulk system was started with approximately 0.002 g of solid particles, 0.1 g of [BMIM][PF6], and 1.1 g of water. The emulsion contains droplets with a broad size distribution ranging from several (27) Dickinson, E. Food Emulsions and Foams: Stabilization by Particles. Curr. Opin. Colloid Interface Sci. 2010, 15, 40–49. (28) Frelichowska, J.; Bolzinger, M.; Pelletier, J.; Valour, J.; Chevalier, Y. Topical Delivery of Lipophilic Drugs from o/w Pickering Emulsions. Int. J. Pharm. 2009, 371, 56–63. (29) Frelichowska, J.; Bolzinger, M.; Valour, J.; Mouaziz, H.; Pelletier, J.; Chevalier, Y. Pickering w/o Emulsions: Drug Release and Topical Delivery. Int. J. Pharm. 2009, 368, 7–15. (30) Sullivan, A. P.; Kilpatrick, P. K. The Effects of Inorganic Solid Particles on Water and Crude Oil Emulsion Stability. Ind. Eng. Chem. Res. 2002, 41, 3389– 3404. (31) Tarimala, S.; Dai, L. L. Structure of Microparticles in Solid-Stabilized Emulsions. Langmuir 2004, 20, 3492–3494. (32) Tarimala, S.; Wu, C.; Dai, L. L. Dynamics and Collapse of Two-Dimensional Colloidal Lattices. Langmuir 2006, 22, 7458–7461. (33) Dai, L. L.; Tarimala, S.; Wu, C.; Guttula, S.; Wu, J. The Structure and Dynamics of Microparticles at Pickering Emulsion Interfaces. Scanning 2008, 30, 87–95. (34) Ma, H.; Dai, L. L. Structure of Multi-Component Colloidal Lattices at Oil-Water Interfaces. Langmuir 2009, 25, 11210–11215. (35) Binks, B. P.; Lumsdon, S. O. Transitional Phase Inversion of SolidStabilized Emulsions Using Particle Mixtures. Langmuir 2000, 16, 3748–3756. (36) Golemanov, K.; Tcholakova, S.; Kralchevsky, P. A.; Ananthapadmanabhan, K. P.; Lips, A. Latex-Particle-Stabilized Emulsions of Anti-Bancroft Type. Langmuir 2006, 22, 4968–4977. (37) Whitby, C. P.; Fornasiero, D.; Ralston, J. Structure of Oil-in-Water Emulsions Stabilised by Silica and Hydrophobised Titania Particles. J. Colloid Interface Sci. 2010, 342, 205–209. (38) Binks, B. P.; Dyab, A. K. F.; Fletcher, P. D. I. Novel Emulsions of Ionic Liquids Stabilised Solely by Silica Nanoparticles. Chem. Commun. 2003, 2540– 2541. (39) Binks, B. P.; Dyab, A. K. F.; Fletcher, P. D. I. Contact Angles in Relation to Emulsions Stabilised Solely by Silica Nanoparticles Including Systems Containing Room Temperature Ionic Liquids. Phys. Chem. Chem. Phys. 2007, 9, 6391– 6397.

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surface charge (C/m2)

excitation/emission wavelengths (nm)

-0.029 -0.088 -0.325 1.760 0.152

580/605 505/515 540/560 505/515 505/515

micrometers to tens of micrometers with a varied surface coverage of particles. The surface coverage is independent of the droplet size. Partially covered droplets were observed in all systems, and fully covered droplets (Figure 1 insets) were also observed in the A-PS and S-PS/A-PS systems, respectively. In Pickering emulsions containing binary heterogeneous particles, both types of particles selfassemble at the same droplet interfaces. The particles are randomly mixed without significant phase separation. When the quantity of solid particles was increased to 0.004 g (Figure 2), the droplet size distribution remained broad but there were some noticeable differences: aggregates of fully covered droplets are frequently observed in emulsions containing S-PS/A-PS binary particles, whereas droplets in emulsions containing S-PS/AS-PS binary particles are mostly partially covered. The difference in droplet coverage is likely due to the different affinity of particles for the ionic liquid-water interfaces, and the effect of surface charge is hypothesized. Because A-PS particles are positively charged and all other particles used in this study are negatively charged, this suggests that the positively charged particles are easily adsorbed at the interfaces. It is worthwhile to compare the ionic liquid-in-water Pickering emulsions here with those in oil-in-water Pickering emulsions containing the same particles. First, aggregates of fully covered droplets are frequently observed in ionic liquid-in-water emulsions containing S-PS/A-PS binary particles (Figure 2a) whereas in poly(dimethylsiloxane) (PDMS) oil-in-water emulsions containing the same particles there were hardly any fully covered Pickering emulsions droplets. The absence of fully covered emulsion droplets was also noticed in other PDMS-water emulsion systems studied previously including S-PS, C-PS, and AS-PS particles.31-34 The contrast suggests that the S-PS/A-PS particles have a stronger affinity for the ionic liquid-water interfaces than the PDMS-water interfaces, although free particles not attached to interfaces are still present. The distinction between the ionic nature of the ionic liquid and the molecular nature of PDMS should be at least one of the important factors, if not the primary factor, in determining the affinity of particles to liquid interfaces. It is also worthwhile to note the differences in interfacial tension: the PDMS-water interfaces are 39.6 ( 0.1 mN/m whereas the ionic liquid-water interfaces are 9.9 ( 0.1 mN/m. Second, particles form aggregates instead of colloidal lattices at ionic liquid-water droplet interfaces, despite the surface packing densities being high (Figures 1 and 2) or low (Figure S1 in Supporting Information). Long-range nearly ordered colloidal lattices observed in oil-in-water Pickering emulsions, as a result of the enhanced electrostatic repulsion through the oil phase,34,40-42 (40) Aveyard, R.; Clint, J. H.; Nees, D.; Paunov, V. N. Compression and Structure of Monolayers of Charged Latex Particles at Air/Water and Octane. Langmuir 2000, 16, 1969–1979. (41) Leunissen, M. E.; Zwanikken, J.; van Roij, R.; Chaikin, P. M.; van Blaaderen, A. Ion Partitioning at the Oil-Water Interface as a Source of Tunable Electrostatic Effects in Emulsions with Colloids. Phys. Chem. Chem. Phys. 2007, 9, 6405–6414. (42) Aveyard, R.; Binks, B. P.; Clint, J. H.; Fletcher, P. D. I.; Horozov, T. S.; Neumann, B.; Paunov, V. N.; Annesley, J.; Botchway, S. W.; Nees, D.; Parker, A. W.; Ward, A. D.; Burgess, A. N. Measurement of Long-Range Repulsive Forces between Charged Particles at an Oil-Water Interface. Phys. Rev. Lett. 2002, 88, 246102.

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Figure 1. Confocal microscope images (overlays of depth-series images) of [BMIM][PF6] droplets in water in the presence of (a) S-PS particles, (b) A-PS particles, (c) AS-PS particles, (d) S-PS/A-PS binary particles, and (e) S-PS/AS-PS binary particles for 0.002 g of solid particles. Partially covered droplets were observed in all systems, and fully covered droplets (insets) were also observed in the A-PS and S-PS/ A-PS systems. The scale bars represent 10 μm.

Figure 2. Confocal microscope images (overlays of depth-series images) of [BMIM][PF6] droplets in water in the presence of (a) S-PS/A-PS binary particles and (b) S-PS/AS-PS binary particles for 0.004 g of solid particles. The scale bars represent 10 μm.

were not observed at ionic liquid-water emulsion interfaces. Obviously, the enhanced electrostatic interactions through the oil phase due to the residual charge at the particle-oil interface are no longer valid at ionic liquid-water interfaces. The ionic strength of [BMIM][PF6] is calculated to be 4.86 M. When the PDMS is substituted with [BMIM][PF6], the electrostatic repulsion through the ionic liquid phase between charged colloidal particles is screened beyond the Debye length, which is normally in the range of several nanometers43 and thus eliminates longrange lattice structure formation at the [BMIM][PF6]-water interfaces. This is consistent with the melting of the colloidal lattice with a large lattice spacing at oil-water interfaces when increasing the electrolyte concentration in the oil phase reported (43) Min, Y.; Akbulut, M.; Sangoro, J. R.; Kremer, F.; Prud’homme, R. K.; Israelachvili, J. Measurement of Forces across Room Temperature Ionic Liquids between Mica Surfaces. J. Phys. Chem. C 2009, 113, 16445–16449.

510 DOI: 10.1021/la103828x

previously.41 This observation supports the importance and necessity of Coulomb repulsion through the oil phase in the lattice structure formation. The formation of particle aggregates at partially covered droplets might be driven by attractive interparticle forces although the origin of the attraction is still debatable. An early study on particle monolayers at air-water interfaces attributes the driving force for particle aggregation to the van der Waals force.44 Later studies on oil-water or airwater systems suggest the importance of attractive capillary forces, which could originate from the undulations of the three-phase contact line,45 the topological or chemical inhomogeneity of particle surfaces,45 the electrodipping force,46-49 or gravity.50,51 The capillary force is expected to be a significant attractive driving force for particle aggregate formation at ionic liquid-water interfaces, which is worth further study. The unique properties of ionic (44) Robinson, D. J.; Earnshaw, J. C. Experimental Study of Colloidal Aggregation in Two Dimensions. I. Structural Aspects. Phys. Rev. A 1992, 46, 2045–2054. (45) Stamou, D.; Duschl, C.; Johannsmann, D. Long-Range Attraction between Colloidal Spheres at the Air-Water Interface: The Consequence of an Irregular Meniscus. Phys. Rev. E 2000, 62, 5263–5272. (46) Danov, K. D.; Kralchevsky, P. A.; Boneva, M. P. Electrodipping Force Acting on Solid Particles at a Fluid Interface. Langmuir 2004, 20, 6139–6151. (47) Danov, K. D.; Kralchevsky, P. A. Reply to Comment on Electrodipping Force Acting on Solid Particles at a Fluid Interface. Langmuir 2006, 22, 848–849. (48) Danov, K. D.; Kralchevsky, P. A. Electric Forces Induced by a Charged Colloid Particle Attached to the Water-Nonpolar Fluid Interface. J. Colloid Interface Sci. 2006, 298, 213–231. (49) Danov, K. D.; Kralchevsky, P. A. Interaction between Like-Charged Particles at a Liquid Interface: Electrostatic Repulsion Vs. Electrocapillary Attraction. J. Colloid Interface Sci. 2010, 345, 505–514. (50) Oettel, M.; Dietrich, S. Colloidal Interactions at Fluid Interfaces. Langmuir 2008, 24, 1425–1441. (51) Boneva, M. P.; Danov, K. D.; Christov, N. C.; Kralchevsky, P. A. Attraction between Particles at a Liquid Interface due to the Interplay of Gravityand Electric-Field-Induced Interfacial Deformations. Langmuir 2009, 25, 9129– 9139.

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Figure 3. Confocal microscope images of [BMIM][PF6] droplet cross sections. (a) The droplet interface is covered with a mixture of S-PS (blue) and A-PS (green) particles, and the [BMIM][PF6] phase is rich in S-PS and the aqueous phase is rich in A-PS. (b) The droplet interface is covered with a mixture of S-PS (blue) and AS-PS (green) particles, and both types of particles are more concentrated in the [BMIM][PF6] phase than in the aqueous phase. The scale bars represent 10 μm.

liquids, such as high ionic strength and structural directionality,52 might lead to new origins for attractive forces and call for theoretical study. Interestingly, other than equilibrating at the ionic liquid-water interfaces, the microparticles also exhibit phase preference in the dispersed and continuous phases, although all particles were initially dispersed in the aqueous phase. Figure 3a shows a representative cross-sectional image of an ionic liquid droplet in an emulsion containing binary particle mixtures of S-PS/A-PS. The droplet interface is covered with a mixture of S-PS and A-PS particles, and the [BMIM][PF6] phase is rich in S-PS and the continuous (aqueous) phase is rich in A-PS. In contrast, for the S-PS/AS-PS binary system, both types of particles are more concentrated in the [BMIM][PF6] phase than in the aqueous phase (Figure 3b). In the emulsions containing single species of particles, a similar preference was observed: the S-PS and AS-PS particles prefer the [BMIM][PF6] phase, and the A-PS particles prefer the aqueous phase. The concentration of different species of particles in each aqueous phase was characterized by quantifying the fluorescence intensity over the emission wavelength range of the fluorescent particles. Figure 4 compares the fluorescence intensity spectra of the emulsion aqueous phase before (solid lines) and after (dashed lines) emulsification. For the S-PS/A-PS system (Figure 4a), the fluorescence intensity spectrum shows the initial presence of both types of particles in the aqueous phase before emulsification. After the formation of ionic liquid-in-water Pickering emulsions, almost all of the S-PS particles migrate out of the aqueous phase, as indicated by the observation that the fluorescence intensity in its emission wavelength range (645-744 nm) falls nearly to the background level. However, the peak fluorescence intensity of A-PS decreases from 53.6 to 37.2, suggesting that a significant number of A-PS particles remain in the aqueous phase after ultrasonic emulsification. In contrast, Figure 4b shows that for the S-PS/AS-PS system both types of particles are scarcely present in the aqueous phase after emulsification. The spectra of emulsion systems containing a single species of particles show results that are consistent with the binary particle systems. The fluorescence intensity spectra support the visual observation from confocal images. The preferred equilibrium positions of the microparticles in ionic liquid-in-water Pickering emulsions are probably determined by the particle properties, including the surface hydrophobicity and the sign and magnitude of the surface charge as well

as their interactions with the ionic liquid and water. First, the hydrophobicity of the particles is examined as a possible influencing factor. The particles in our study are surface treated to have various surface dissociable groups and thus differ in hydrophobicity. The result shows that hydrophobic S-PS and AS-PS particles prefer the hydrophobic ionic liquid phase whereas the hydrophilic A-PS particles prefer the water phase. However, in PDMS-in-water Pickering emulsions, other than being adsorbed at droplet interfaces, the S-PS and AS-PS particles prefer the water phase. This contrast indicates that other factors should be considered in addition to the hydrophobicity. Second, the sign of the particle surface charge was initially hypothesized as an influencing factor on the phase preference, considering the repulsions between the positively charged A-PS particles with the large cations in the ionic liquid. However, additional experiments were performed, and it was found that the negatively charged C-PS particles (which are also relatively hydrophilic) can partition in both the ionic liquid and aqueous phases without a significant preference. Furthermore, the potential influences of the surface charge density were studied, and we noticed that the charge density of A-PS particles (1.760 C/m2), which prefer the water phase, is at least 20 times higher than those of S-PS and AS-PS particles (-0.029 and -0.088 C/m2, respectively). Additional experiments were performed by employing A-PS particles with a lower surface charge density (0.152 C/m2); the results show that the particles also prefer to stay in the aqueous phase and suggest minor influences of the surface charge density. The high transfer efficiency of the S-PS and AS-PS particles to the ionic liquid phases is noteworthy. The applications of roomtemperature ionic liquids in the extraction of organic compounds have been studied extensively;8 however, investigations of the phase transfer of solid particles, especially in the micrometer size range, are very limited. Gold nanoparticles have been reported to transfer from an aqueous phase to a [BMIM][PF6] phase more efficiently than to an alcohol phase of the same polarity; increasing ionic strength in the alcohol improves the transfer efficiency.53

(52) Dupont, J.; Scholten, J. D. On the Structural and Surface Properties of Transition-Metal Nanoparticles in Ionic Liquids. Chem. Soc. Rev. 2010, 39, 1780– 1804.

(53) Wei, G. T.; Yang, Z. S.; Lee, C. Y.; Yang, H. Y.; Wang, C. R. C. Aqueous-Organic Phase Transfer of Gold Nanoparticles and Gold Nanorods Using an Ionic Liquid. J. Am. Chem. Soc. 2004, 126, 5036–5037.

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Figure 4. Fluorescence intensity spectra of the emulsion aqueous phase before (-) and after (---) emulsification for (a) the S-PS/ A-PS system and (b) the S-PS/AS-PS system.

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For some organic compounds8 and nanocrystals,54 phase transfer usually occurs upon stirring and the mechanism is proposed to be an ion-exchange mechanism under the principle of maintaining the electroneutrality of the ionic liquid phase. For example, the transfer of cationic CdTe nanocrystals to the [BMIM][NTf2] phase is accompanied by the compensatory transfer of the [BMIM] cation to the aqueous phase.54 For microparticles, it is questionable if the ion-exchange mechanism is still applicable because of geometric differences between the charged microparticles and ions. Another possible contributing factor can be the hydrophobic or hydrophilic interactions between the microparticles and the ionic liquid and water phases. In this letter, we report the self-assembly of a single species or a binary mixture of microparticles in ionic liquid-in-water Pickering emulsions, with emphasis on the interfacial selfassembled particle structure and the partitioning preference of free particles in the dispersed and continuous phases. The ionic liquid droplets are either fully or partially covered with microparticles. The particles form monolayers at ionic liquid-water interfaces and are close-packed on fully covered emulsion droplets or aggregated on partially covered droplets; in contrast to those at oil-water interfaces, no longrange nearly ordered colloidal lattices were observed. Interestingly, other than equilibrating at the ionic liquid-water interfaces, the microparticles also exhibit phase preference in the dispersed and continuous phases: the sulfate-treated polystyrene (S-PS) and aldehyde-sulfate-treated polystyrene (AS-PS) microparticles are extracted into the ionic liquid phase with a high extraction efficiency whereas the aminetreated polystyrene (A-PS) microparticles remain in the water phase. (54) Nakashima, T.; Nonoguchi, Y.; Kawai, T. Ionic Liquid-Based Luminescent Composite Materials. Polym. Adv. Technol. 2008, 19, 1401–1405.

512 DOI: 10.1021/la103828x

Ma and Dai

Experimental Section The basic ionic liquid-in-water emulsion recipe contains 1 g of water (HPLC grade, Acros Organics), 0.1 g of [BMIM][PF6] (g97.0%, Aldrich), and 0.1 g of particle dispersions at a solid concentration of 2 wt % (single particle species or a binary species of 1:1 S-PS/AS-PS or 1:1 S-PS/A-PS). For samples with a higher particle concentration, 0.9 g of water, 0.1 g of [BMIM][PF6], and 0.2 g of particle dispersions were used. The emulsions were prepared using an ultrasonic processor (Sonics VibraCell, 500 W model). Typically, a sample was prepared by adding water, particles, and ionic liquid sequentially to a sample vial and processing ultrasonically in an ice-water bath three times at 21% amplitude for a duration of 1 s each time. The images of the samples were obtained using a Leica SP5 confocal laser scanning microscope under ambient conditions (with a typical temperature of 21-22 °C). When preparing emulsions with particle mixtures, we also tried adding water, one type of particle, and the ionic liquid first, followed by ultrasonic processing twice, and then adding the second type of particle and ultrasonic processing twice. Confocal microscope observations show that the different preparation procedures do not have a significant effect on the self-assembly of colloidal lattices. The fluorescence intensity spectra were obtained via a wavelength scan over the range of 500-599 nm for A-PS particles and AS-PS particles under excitation at 488 nm and over the range of 645-744 nm for S-PS particles under excitation at 633 nm. The spectra for the aqueous phases before and after emulsification were collected under the same laser intensity and at approximately the same depth within the samples.

Acknowledgment. We thank the National Science Foundation for financial support (CBET-0922277). Supporting Information Available: Representative confocal microscope image and the corresponding transmitted light image of a [BMIM][PF6] droplet in water with a low coverage of A-PS particles. This material is available free of charge via the Internet at http://pubs.acs.org.

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