Pluronic Micelle Shuttle between Water and an Ionic Liquid - Langmuir

Department of Chemical Engineering & Materials Science. University of Minnesota, Minneapolis, Minnesota 55455 ... an effective micelle shuttle between...
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Pluronic Micelle Shuttle between Water and an Ionic Liquid Zhifeng Bai† and Timothy P. Lodge*,†,‡ †

Department of Chemistry and ‡Department of Chemical Engineering & Materials Science, University of Minnesota, Minneapolis, Minnesota 55455 Received December 9, 2009. Revised Manuscript Received January 28, 2010

We demonstrate an effective micelle shuttle between water and a hydrophobic ionic liquid and its application in transportation in the biphasic system, using a commercially available and inexpensive Pluronic block copolymer. The poly(ethylene oxide-b-propylene oxide-b-ethylene oxide) (P123) block copolymer self-assembles into micelles in both water and the ionic liquid, 1-butyl-3-methylimidazolium hexafluorophosphate, as characterized by dynamic light scattering. The poly(ethylene oxide) blocks provide the well-solvated corona, whereas the poly(propylene oxide) blocks form the solvophobic core. The micelles can spontaneously transfer between the two phases upon a simple temperature stimulus; the transfer is reversible and repeatable, and 1H NMR analysis indicates quantitative transfer. The micelle nanocarriers are used to transport various cargoes in the biphasic system: fully reversible transport of hydrophobic small organic dyes into and out of water and facile extraction of an ionic liquid-phobic polymer from the ionic liquid. This simple round-trip delivery system may be used in delivery, separations, and extraction in synthesis and biphasic catalysis involving ionic liquids.

Introduction Among the most studied block copolymer systems are the poly(ethylene oxide-b-propylene oxide-b-ethylene oxide) (PEO-PPOPEO) triblock copolymers, commercially available over wide ranges of composition and molecular weight (Pluronics).1,2 In dilute aqueous solution, the PEO end blocks are selectively solvated by water and micelles form with hydrophobic PPO cores. The micellization has been well studied experimentally3-9 and theoretically10-13 and is of interest for many applications, including separations, detergency, dispersion, drug and gene delivery, and pharmaceutical and cosmetic formulations.1,2 Ionic liquids have been enjoying growing interest since the early 1990s.14,15 Being composed solely of ions yet with low melting points, they are bestowed with some attractive properties, including negligible volatility, widely tunable solvation properties, good chemical and thermal stability, and electrochemical transparency. Thus, they have been extensively explored as media for synthesis, *Author for correspondence: e-mail [email protected].

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catalysis, and separations.16-20 Of particular promise is biphasic catalysis, involving a biphasic system containing an ionic liquid and another immiscible solvent, whereby the advantages of homogeneous and heterogeneous catalysis can be combined.21 For example, in biphasic catalysis involving a hydrophobic ionic liquid and water, both high catalytic efficiency and facile catalyst/ product separation and catalyst recycling were achieved.22-25 Another elegant example is biphasic biocatalysis.19,26 Lye et al. reported a whole-cell biotransformation in a biphasic system containing a hydrophobic ionic liquid, 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF6]), and water,27 which may enable replacement of traditional organic solvents with ionic liquids.19,26 Related liquid-liquid extraction, e.g., of metal ions,28 nanoparticles,29 organic molecules,30 and proteins,31 using ionic liquids has also received much attention.20 Self-assembly of amphiphiles in ionic liquids can impart desirable nanostructures to ionic liquids, such as for solubilization and (16) Welton, T. Chem. Rev. 1999, 99, 2071–2083. (17) Rogers, R. D.; Seddon, K. R. Science 2003, 302, 792–793. (18) Parvulescu, V. I.; Hardacre, C. Chem. Rev. 2007, 107, 2615–2665. (19) Van Rantwijk, F.; Sheldon, R. A. Chem. Rev. 2007, 107, 2757–2785. (20) Han, X.; Armstrong, D. W. Acc. Chem. Res. 2007, 40, 1079–1086. (21) Dupont, J.; de Souza, R. F.; Suarez, P. A. Z. Chem. Rev. 2002, 102, 3667– 3692. (22) Pugin, B.; Studer, M.; Kuesters, E.; Sedelmeier, G.; Feng, X. Adv. Synth. Catal. 2004, 346, 1481–1486. (23) Zhao, D.; Fei, Z.; Geldbach, T. J.; Scopelliti, R.; Dyson, P. J. J. Am. Chem. Soc. 2004, 126, 15876–15882. (24) Brown, R. A.; Pollet, P.; McKoon, E.; Eckert, C. A.; Liotta, C. L.; Jessop, P. G. J. Am. Chem. Soc. 2001, 123, 1254–1255. (25) Wang, B.; Kang, Y.; Yang, L.; Suo, J. J. Mol. Catal. A: Chem. 2003, 203, 29–36. (26) Gangu, S. A.; Weatherley, L. R.; Scurto, A. M. Curr. Org. Chem. 2009, 13, 1242–1258. (27) Cull, S. G.; Holbrey, J. D.; Vargas-Mora, V.; Seddon, K. R.; Lye, G. J. Biotechnol. Bioeng. 2000, 69, 227–233. (28) Dai, S.; Ju, Y. H.; Barnes, C. E. J. Chem. Soc., Dalton Trans. 1999, 1201– 1202. (29) Wei, G.-Z.; Yang, Z.; Lee, C.-Y.; Yang, H.-Y.; Wang, C. R. C. J. Am. Chem. Soc. 2004, 126, 5036–5037. (30) Visser, A. E.; Swatloski, R. P.; Rogers, R. D. Green Chem. 2000, 2, 1–4. (31) Shimojo, K.; Kamiya, N.; Tani, F.; Naganawa, H.; Naruta, Y.; Goto, M. Anal. Chem. 2006, 78, 7735–7742.

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nanomaterial fabrication.32 Certain common surfactants have been shown to self-assemble into micelles in ionic liquids.33-39 Micellization of block copolymers in ionic liquids is also promising;40,41 good control over micelle dimensions, structure, stability, and functionality has been documented.42-54 Recently, Zheng et al. reported the micellization of Pluronic block copolymers in two imidazolium based ionic liquids, with similar surface and thermodynamic properties as those in water.49 The micelle shuttle is an interesting phenomenon whereby poly((1,2-butadiene)-b-ethylene oxide) (PB-PEO) block copolymer micelles transfer spontaneously from an aqueous phase at room temperature to a hydrophobic ionic liquid phase at elevated temperature.55 The transfer is reversible, quantitative, and with a favorable strong temperature dependence of the micelle distribution in the biphasic system.56 The micelle partitioning is driven by the lower critical solution temperature (LCST) phase behavior (solubility decreases upon heating) of the PEO corona block in water, and therefore the transfer temperature can be effectively tuned by adding additives to the aqueous phase to adjust the solubility of PEO in water.57 The kinetically trapped structures of the highly amphiphilic PB-PEO block copolymer micelles in both water58,59 and ionic liquids42,44,57 account for the stability of these nanocarriers, which exhibited intact transfer55,57 and quantitative transport of a cargo, a Rhodamine B-labeled PB homopolymer,56 between the two phases. The thermodynamics and mechanism of the transfer have also been studied in detail.56 To control loading and release in both phases in the micelle shuttle system, a reversible micellization-transfer-demicellization shuttle has been developed by using a multi-thermosensitive poly(Nisopropylacrylamide-b-ethylene oxide) (PNIPAm-PEO) block copolymer.60 The core-forming PNIPAm block has a LCST (32) Greaves, T. L.; Drummond, C. J. Chem. Soc. Rev. 2008, 37, 1709–1726. (33) Evans, D. F.; Yamauchi, A.; Roman, R.; Casassa, E. Z. J. Colloid Interface Sci. 1982, 88, 89–96. (34) Anderson, J. L.; Pino, V.; Hagberg, E. C.; Sheares, V. V.; Armstrong, D. W. Chem. Commun. 2003, 2444–2445. (35) Fletcher, K. A.; Pandey, S. Langmuir 2004, 20, 33–36. (36) Patrascu, C.; Gauffre, F.; Nallet, F.; Bordes, R.; Oberdisse, J.; de LauthViguerie, N.; Mingotaud, C. ChemPhysChem 2006, 7, 99–101. (37) Araos, M. U.; Warr, G. G. Langmuir 2008, 24, 9354–9360. (38) Gao, Y.; Li, N.; Li, X.; Zhang, S.; Zheng, L.; Bai, X.; Yu, L. J. Phys. Chem. B 2009, 113, 123–130. (39) Li, N.; Zhang, S.; Zheng, L.; Inoue, T. Langmuir 2009, 25, 10473–10482. (40) Ueki, T.; Watanabe, M. Macromolecules 2008, 41, 3739–3749. (41) Lodge, T. P. Science 2008, 321, 50–51. (42) He, Y.; Li, Z.; Simone, P. M.; Lodge, T. P. J. Am. Chem. Soc. 2006, 128, 2745–2750. (43) Simone, P. M.; Lodge, T. P. Macromolecules 2008, 41, 1753–1759. (44) Meli, L.; Lodge, T. P. Macromolecules 2009, 42, 580–583. (45) Ueki, T.; Watanabe, M.; Lodge, T. P. Macromolecules 2009, 42, 1315–1320. (46) He, Y.; Lodge, T. P. Chem. Commun. 2007, 2732–2734. (47) Noro, A.; Matsushita, Y.; Lodge, T. P. Macromolecules 2008, 41, 5839– 5844. (48) Cho, J. H.; Lee, J.; Xia, Y.; Kim, B.; He, Y.; Renn, M. J.; Lodge, T. P.; Frisbie, C. D. Nat. Mater. 2008, 7, 900–906. (49) Zhang, S.; Li, N.; Zheng, L.; Li, X.; Gao, Y.; Yu, L. J. Phys. Chem. B 2008, 112, 10228–10233. (50) Zhang, G.; Chen, X.; Zhao, Y.; Ma, F.; Jing, B.; Qiu, H. J. Phys. Chem. B 2008, 112, 6578–6584. (51) Atkin, R.; De Fina, L. -M.; Kiederling, U.; Warr, G. G. J. Phys. Chem. B 2009, 113, 12201–12213. (52) Tamura, S.; Ueki, T.; Ueno, K.; Kodama, K.; Watanabe, M. Macromolecules 2009, 42, 6239–6244. (53) Virgili, J. M.; Hexemer, A.; Pople, J. A.; Balsara, N. P.; Segalman, R. A. Macromolecules 2009, 42, 5642–5651. (54) Noro, A.; Yamagishi, H.; Matsushita, Y. Macromolecules 2009, 42, 6335– 6338. (55) He, Y.; Lodge, T. P. J. Am. Chem. Soc. 2006, 128, 12666–12667. (56) Bai, Z.; Lodge, T. P. J. Phys. Chem. B 2009, 113, 14151–14157. (57) Bai, Z.; He, Y.; Lodge, T. P. Langmuir 2008, 24, 5284–5290. (58) Won, Y. Y.; Davis, H. T.; Bates, F. S. Macromolecules 2003, 36, 953–955. (59) Jain, S.; Bates, F. S. Macromolecules 2004, 37, 1511–1523. (60) Bai, Z.; He, Y.; Young, N. P.; Lodge, T. P. Macromolecules 2008, 41, 6615– 6617.

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phase behavior in water,61 yet an inverse upper critical solution temperature (UCST) phase behavior in [BMIM][PF6], as first reported by Watanabe et al.;62 they also reported the interesting LCST phase behavior of certain polymers in ionic liquids.63,64 Schubert et al. recently reported an analogous micelle shuttle between water and [BMIM][PF6] using poly(2-nonyl-2-oxazolineb-2-ethyl-2-oxazoline) (PNonOx-PEtOx) block copolymers with PEtOx coronas and PNonOx cores, which indicates the generality of the micelle shuttle.65 Other related nanoparticle shuttles between aqueous and organic phases have also been documented.66-70 This simple round-trip delivery system can be used in delivery, reaction,71-73 and separations in synthesis and biphasic catalysis involving ionic liquids. Herein, we describe a micelle shuttle between water and [BMIM][PF6],74 utilizing a Pluronic P123 block copolymer, and its application in transportation in the biphasic system. The use of a commercially available and inexpensive copolymer, in place of the custom-synthesized diblocks used previously, could greatly increase the practicality of the micelle shuttle. The P123 block copolymer self-assembles into micelles in both water and [BMIM][PF6], as characterized by dynamic light scattering (DLS). The micelles can reversibly transfer many times between the two phases; the transfer is quantitative as revealed by 1H NMR analysis. The application of the micelle shuttle to transport cargo in the biphasic system is demonstrated by using the micelles as nanocarriers to reversibly transport various hydrophobic small organic dyes to and out of water and to extract a [BMIM][PF6]phobic polymer from [BMIM][PF6].

Experimental Section Materials. All reagents were used as received, unless otherwise noted. A P123 Pluronic block copolymer (PEO20-PPO70PEO20), 1,1,4,4-tetraphenyl-1,3-butadiene (TPB, g99%), and Nile Red (NR, standard Fluka) were purchased from SigmaAldrich; 9,10-diphenylanthracene (DPA, 99%) was obtained from Acros. [BMIM][PF6] (>99%) was procured from IoLiTec and used after drying in a vacuum oven at ∼50 C for 2 days. Its temperature-dependent viscosity was previously reported.42 Rhodamine B-labeled polybutadiene (Rho-PB) was previously synthesized by coupling rhodamine B acid chloride with a hydroxyl-terminated PB (2 kDa).56 General Methods. Absorption spectra of dye-loaded micelle solutions were obtained on a Varian Cary 100Bio UV-vis spectrophotometer using a 1 cm path length cell and appropriate solvents (61) Wu, Q.; Zhou, S. Macromolecules 1995, 28, 5388–5390. (62) Ueki, T.; Watanabe, M. Chem. Lett. 2006, 35, 964–965. (63) Ueki, T.; Watanabe, M. Langmuir 2007, 23, 988–990. (64) Tsuda, R.; Kodama, K.; Ueki, T.; Kokubo, H.; Imabayashi, S.; Watanabe, M. Chem. Commun. 2008, 4939–4941. (65) Guerrero-Sanchez, C.; Gohy, J. F.; D’Haese, C.; Thijs, H.; Hoogenboom, R.; Schubert, U. S. Chem. Commun. 2008, 2753–2755. (66) Chechik, V.; Zhao, M.; Crooks, R. M. J. Am. Chem. Soc. 1999, 121, 4910– 4911. (67) Tian, H.; Chen, X.; Lin, H.; Deng, C.; Zhang, P.; Wei, Y.; Jing, X. Chem.; Eur. J. 2006, 12, 4305–4312. (68) Marcilla, R.; Curri, M. L.; Cozzoli, P. D.; Martinez, M. T.; Loinaz, I.; Grande, H.; Pomposo, J. A.; Mecerreyes, D. Small 2006, 2, 507–512. (69) Li, D.; Zhao, B. Langmuir 2007, 23, 2208–2217. (70) Desset, S. L.; Cole-Hamilton, D. J. Angew. Chem., Int. Ed. 2009, 48, 1472– 1474. (71) Vriezema, D. M.; Aragones, M. C.; Elemans, J. A. A. W.; Cornelissen, J. J. L. M.; Rowan, A. E.; Nolte, R. J. M. Chem. Rev. 2005, 105, 1445–1489. (72) Dwars, T.; Paetzold, E.; Oehme, G. Angew. Chem., Int. Ed. 2005, 44, 7174– 7199. (73) Zhao, B.; Jiang, X.; Li, D.; Jiang, X.; O’Lenick, T. G.; Li, B.; Li, C. Y. J. Polym. Sci., Part A: Polym. Chem. 2008, 46, 3438–3446. (74) [BMIM][PF6] is used since it is one of the most popular ionic liquids in synthesis, biphasic catalysis, and separations. Because of possible hydrolysis of [BMIM][PF6] in contact with water, the micelle shuttle was kept below 70 C, and no noticeable hydrolysis was observed.

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Bai and Lodge as backgrounds. Fluorescence spectra were measured on a Varian Cary Eclipse fluorescence spectrophotometer at room temperature using corresponding absorption maxima as excitation wavelengths. Dynamic light scattering (DLS) was performed on a homemade photometer equipped with a Brookhaven BI-DS photomultiplier, a Lexel Arþ laser with a wavelength of 488 nm, and a Brookhaven BI-9000 correlator.57 The samples were prepared by filtering polymer solutions through 0.45 μm PTFE syringe filters into glass sample tubes with an inner diameter of 0.38 in. Measurements were acquired at five different scattering angles between 60 and 120. 1H NMR spectra were taken on a Varian Inova 500 MHz spectrometer. Solution Preparation. Micelle solutions without dye loading were prepared by directly dissolving the P123 block copolymer, either in water at room temperature or in [BMIM][PF6] at 50 C. “Wet” [BMIM][PF6] was the ionic liquid phase of the equilibrated biphasic water/[BMIM][PF6] system at room temperature. In the preparation of the [BMIM][PF6]-saturated water solution of the micelles, excess [BMIM][PF6] was added to the aqueous micelle solution and equilibrated at room temperature for 1 day. Dyeloaded micelles in water were prepared by a thin-film protocol. The copolymer and the corresponding dye were dissolved in dichloromethane. Most of the solvent was removed by a gentle nitrogen purge, and the residue was dried in a vacuum oven at 50 C overnight. The resulting well-mixed thin film was rehydrated by water and stirred at room temperature for 3 days. Dye-loaded micelles in [BMIM][PF6] were prepared by a cosolvent protocol. The copolymer, the corresponding dye, and [BMIM][PF6] were dissolved in dichloromethane. Most of the cosolvent was removed by a gentle nitrogen purge, and the solutions were dried in a vacuum oven at 50 C overnight. In the loading of DPA, TPB, and NR, 2 wt % copolymer and 0.2 wt % dye were used for both the water and [BMIM][PF6] solutions. In the loading of Rho-PB, 2.7 and 2 wt % copolymer and 0.0135 and 0.01 wt % Rho-PB were used for the water and [BMIM][PF6] solutions, respectively, to obtain the same volume concentrations; the density of [BMIM][PF6] is 1.35 g/mL,75 and the volumes of the two phases were the same in the biphasic system. Excess, “unloaded” dyes and Rho-PB were removed by filtering the solutions through 0.45 μm syringe filters (0.1 μm syringe filters for unloaded dyes in water). The loading of Rho-PB in [BMIM][PF6] is quantitative, which can also be achieved by directly dissolving the copolymer in a Rho-PB/[BMIM][PF6] mixture at room temperature. Corresponding solutions prepared with the same amount of dyes but without the copolymer were also made as references. Micelle Shuttle. An equal mass of the aqueous micelle solution was added to [BMIM][PF6] in a vial. The system was heated with moderate stirring from room temperature in 1 deg intervals and equilibrated at each temperature for 5 min. The color of the slightly bluish aqueous phase gradually turned intense and finally cloudy; at this temperature micelle transfer was observed, and hence it was taken as the transfer temperature (Tt) ((1 C). The micelle transfer was completed in 30 min at a temperature 5 deg higher than Tt, where the aqueous phase was clear. The reverse micelle transfer from [BMIM][PF6] to water was observed after cooling the heated system to room temperature or after adding water to a [BMIM][PF6] solution of the micelles at room temperature; this transfer was completed in 30 min with moderate stirring. In the measurement of the micelle distribution in the biphasic system, an equal mass of a D2O solution of the micelles (5 wt %) was added to [BMIM][PF6]. After equilibration at room temperature, a fraction of the solution was taken out separately from both phases for 1H NMR analysis. Another sample was heated at 65 C with moderate stirring for 30 min, and 1H NMR measurements were performed on both phases

(75) Huddleston, J.; Visser, A.; Reichert, W. M.; Willauer, H.; Brooker, G.; Rogers, R. D. Green Chem. 2001, 3, 156–164.

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Article again. Independent measurements of 5 wt % copolymer in [BMIM][PF6] and D2O were also made as references.

Results and Discussion Reversible Transport of Small Hydrophobes into and out of Water. The Pluronic micelle shuttle was used to transport three hydrophobic small organic dyes into water and then reversibly out of water (Figure 1). All three dyes, 9,10-diphenylanthracene (DPA), 1,1,4,4-tetraphenyl-1,3-butadiene (TPB), and Nile Red (NR), are insoluble in water but can be solubilized in water by loading into the P123 micelles; they are poorly soluble in [BMIM][PF6]. The dye-loaded micelles can reversibly transfer between water and [BMIM][PF6] upon a simple temperature stimulus; at room temperature, they prefer residing in the aqueous phase, and a spontaneous transfer to the aqueous phase was observed if they are originally in the ionic liquid phase, while upon heating to 60 C, they transfer to the ionic liquid phase. The clear contrast between the presence of the dyes in the aqueous phase at room temperature but absence at high temperature in Figure 1 indicates a fully reversible transport. This can also be directly visualized by absorption and/or fluorescence, as illustrated by the experimental images in Figure 2. The transport can be successfully repeated many times, and its applicability to the three distinct dyes indicates its generality. No leakage of the loaded dyes to the [BMIM][PF6] phase was observed in the transport when the dyes were initially loaded by the micelles in [BMIM][PF6], since the [BMIM][PF6] phase has been saturated with free dyes. If the aqueous solution of the dye-loaded micelles was mixed with an equal mass of pure [BMIM][PF6] and equilibrated at room temperature for 1 day, various amounts of loaded dyes, 37% of TPB, 65% of DPA, and 89% of NR, were released to the [BMIM][PF6] phase as determined by UV-vis spectroscopic analysis. This variation presumably depends on the relative affinity of the dyes for the micelle core and [BMIM][PF6]. Extraction of Ionic Liquid-phobic Substance from Ionic Liquid. Synthesis in ionic liquids can yield ionic liquid-phobic products16,18,21 that are usually nonpolar.76 Their recovery is typically addressed by extraction with organic solvents, which may induce safety and environmental issues arising from volatility, flammability, and/or toxicity of organic solvents. The micelle shuttle involving extraction with water washing could be an appealing alternative. As shown in Figure 3, while the nonpolar Rhodamine B-labeled polybutadiene (Rho-PB) is insoluble in both water and [BMIM][PF6], it can be solubilized by loading into the P123 micelles. It should be noted that it is not difficult to dissolve the P123 block copolymer and pick up the cargo in the relatively viscous [BMIM][PF6], which is consistent with the relatively high critical micelle concentrations (cmc) of Pluronic block copolymers in [BMIM][PF6].49 The micelles loaded with Rho-PB in [BMIM][PF6] can be extracted to an aqueous phase by simply washing the [BMIM][PF6] solution with water at room temperature (Figure 3), whereby the aqueous phase could then be decanted for further purification and the ionic liquid phase remain for reuse. Removal of Rho-PB from a mixture of RhoPB/[BMIM][PF6] can also be achieved by washing the mixture with a water solution of the P123 micelles at room temperature (Figure 3), although more time and more extensive mixing are required. Transfer Temperature and Micelle Distribution. The transfer temperature and the micelle distribution are two important parameters of the micelle shuttle. The P123 micelles begin to (76) Reichardt, C. Green Chem. 2005, 7, 339–351.

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Figure 1. Absorption spectra of DPA, TPB, and NR in various solutions (the black dotted line, green short dashed line, and purple dashed line overlap in each plot). The black dotted lines indicate the dyes are insoluble in water. The blue lines show the dyes are solubilized by loading into the P123 micelles in water. The red lines indicate the dyes are present in the aqueous phase of the micelle shuttle system at room temperature loaded by the P123 micelles, and the red dotted lines show their fluorescence. The green short dashed lines indicate that the dyes are absent in the aqueous phase of the micelle shuttle system at 60 C; they are transferred to the ionic liquid phase. The purple dashed lines are references indicating that the dyes are absent in the aqueous phase of the biphasic water/[BMIM][PF6] system without the P123 copolymer. In the micelle shuttle, the dyes were initially loaded by the micelles in [BMIM][PF6], then transferred to an aqueous phase at room temperature, and subsequently transferred back to the [BMIM][PF6] phase at 60 C.

Figure 2. Images of the dye-loaded Pluronic micelle shuttles between water (the upper phase) and [BMIM][PF6] (the lower phase). The systems were equilibrated at 22 C (left) and 60 C (right) and irradiated separately by a UV lamp with a wavelength of 365 nm (TPB) and daylight lamps (NR). The dyes were initially loaded by the micelles in [BMIM][PF6].

transfer from the aqueous phase to the ionic liquid phase at 50 C (transfer temperature (Tt), see the Experimental Section). It could be desirable to tune the Tt to control the micelle partitioning in the biphasic system. The micelle partitioning depends on the solvent affinity of the PEO corona, and the LCST phase behavior of PEO in water drives the micelle transfer.57 It has been previously shown that both an ionic salt, sodium chloride, and a nonionic sugar, isomaltotriose, can depress the Tt of the PB-PEO micelle shuttle, in accordance with the depression in the LCST of PEO in water by these additives.57 In the current system, adding 0.2 M sodium 8890 DOI: 10.1021/la9046462

chloride to the aqueous phase depresses the Tt to 43 C, which agrees with the reported salt-induced depression of the cloud points of Pluronic micelles in water77 and indicates a way to tune the Tt. The micelle distribution in the biphasic system was quantified by 1H NMR spectroscopy (Figure 4). The micelle transfer is essentially quantitative; at 22 C, greater than 99.5% of the micelles are in the aqueous phase, while at 65 C, more than 99.5% transfer to the ionic liquid phase. This is consistent with the previous quantitative shuttle of a PEO homopolymer57 and quantitative transport of a cargo by the PB-PEO micelle shuttle56 between the two phases. Furthermore, the amount of [BMIM][PF6] in the aqueous phase of the biphasic systems with and without the P123 micelles at room temperature is essentially the same, 1.6 wt %78 and 1.8 wt %,57,79 respectively, indicating no significant partitioning of [BMIM][PF6] into the core of the micelles in water. Micelle Size. The size of the P123 micelles was characterized by DLS. In water, the micelles have an apparent hydrodynamic (77) Desai, P. R.; Jain, N. J.; Sharma, R. K.; Bahadur, P. Colloids Surf., A 2001, 178, 57–69. (78) The value is calculated using 5 wt % P123 as reference. (79) Freire, M. G.; Neves, C. M. S. S.; Carvalho, P. J.; Gardas, R. L.; Fernandes, A. M.; Marrucho, I. M.; Santos, L. M. N. B. F.; Coutinho, J. A. P. J. Phys. Chem. B 2007, 111, 13082–13089.

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Figure 3. Absorption spectra of Rho-PB measured in (a) [BMIM][PF6] and (b) water/aqueous phase. The solid lines indicate the insolubility of Rho-PB in [BMIM][PF6] and water, while the dotted lines indicate the solubilization of Rho-PB in [BMIM][PF6] and water by loading into the P123 micelles. The dashed line indicates the extraction of Rho-PB from [BMIM][PF6] to an aqueous phase upon washing the ionic liquid containing Rho-PB-loaded P123 micelles with water at room temperature. The dashed and dotted line shows the spectrum of the aqueous phase after vigorously agitating a Rho-PB/[BMIM][PF6] mixture with a water solution of the P123 micelles at room temperature for 1 day.

Figure 4. 1H NMR spectra (500 MHz) of the aqueous phase and the ionic liquid phase in the biphasic D2O/[BMIM][PF6] system containing 5 wt % P123 at 22 and 65 C. References are 5 wt % P123 in D2O and [BMIM][PF6] separately.

radius (ÆRhæ) of 9.3 nm and a narrow size distribution at 25 C, as indicated by the reduced second cumulant80 (μ2/Γ2) (Table 1) and the ÆRhæ distribution,81 which is consistent with previous reports.9 Similarly, the micelles in [BMIM][PF6] have an ÆRhæ of 8.8 nm and a narrow size distribution at 25 C. It was reported that Pluronic block copolymers have similar aggregation behavior in [BMIM][PF6] as that in water.49 Hydrogen-bond interactions between the ether oxygen of PPO and water1 as well as the acidic protons on the imidazolium ring of [BMIM][PF6]64 play a significant role in the micellization of the copolymers in both solvents. They lead to an unfavorable positive enthalpy of micellization arising from breaking of the hydrogen bonds, an favorable positive entropy of micellization arising from release of solvent molecules structured by PPO, and thus entropy-driven micellization.3,49 The P123 micelles in [BMIM][PF6]-saturated water and wet [BMIM][PF6] were also characterized (Table 1). The presence of small amounts of the other selective solvent (i.e., water in ionic liquid and vice versa) leads to a modest decrease in the ÆRhæ of the micelles in both solvents. The size decrease suggests (80) A measure of the width of micelle size distribution. Koppel, D. E. J. Chem. Phys. 1972, 57, 4814–4820. (81) See the Supporting Information, obtained using the Laplace inversion routine REPES. Jakes, J. Collect. Czech. Chem. Commun. 1995, 60, 1781–1797.

Langmuir 2010, 26(11), 8887–8892

Table 1. Mean Hydrodynamic Radii and Size Polydispersity of the P123 Micelles in Water, [BMIM][PF6], [BMIM][PF6]-Saturated Water, and Wet [BMIM][PF6] at 25 C solvents

water

[BMIM][PF6]

[BMIM][PF6]saturated watera

wet [BMIM][PF6]a

ÆRhæ 9.3 8.8 8.0 7.2 0.03 0.07 0.05 0.18 μ2/Γ2 a [BMIM][PF6] content in [BMIM][PF6]-saturated water: 1.9 wt %. Water content in wet [BMIM][PF6]: 2.1 wt %. See ref 79.

a decrease in the core-corona interfacial tension, i.e., a slight increase in solvent quality toward the core-forming block.82,83 Small amounts of the other solvent bring more possible hydrogenbond interactions and may therefore result in a change of solvent quality. Further work would be needed to elucidate the detailed mechanism; however, the effect of various additives on Pluronic micelles in water has been reported.1 Comparison of the Various Micelle Shuttles. It is of interest to compare the attributes of the several micelle shuttles that have been described. In the PB-PEO micelle shuttle,55-57 the (82) Shen, H.; Eisenberg, A. J. Phys. Chem. B 1999, 103, 9473–9487. (83) Bang, J.; Jain, S.; Li, Z.; Lodge, T. P.; Pedersen, J. S.; Kesselman, E.; Talmon, Y. Macromolecules 2006, 39, 1199–1208.

DOI: 10.1021/la9046462

8891

Article

PB-PEO micelles adopt kinetically trapped structures, originating from the high amphiphilicity of the PB and PEO blocks and the very low solvent compatibility of the PB core block. These stable nanocarriers feature intact and quantitative transport, and thus this micelle shuttle may be used in transport, e.g., of valuable catalysts or environmentally sensitive cargoes. Three ionic liquids, [BMIM][PF6], 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][TFSI]), and 1-butyl-3methylimidazolium bis(trifluoromethylsulfonyl)imide ([BMIM][TFSI]), have been successfully used to perform the PB-PEO micelle shuttle, indicating the possibility of tuning the properties of the ionic liquid phase to fit the needs of different applications. The Pluronic micelle shuttle is advantageous in the commercial availability and low cost of Pluronic block copolymers. Moreover, their tunable critical micellization temperatures (cmts) around room temperature, e.g., 14 C for 2 wt % P123 in water,3 could enable straightforward release of cargoes by cooling, and their moderate cmcs in water3 and some ionic liquids49 could allow facile and rapid assembly and loading of the nanocarriers. In the micellization-transfer-demicellization shuttle,60 the PNIPAm-PEO block copolymer is multiply thermosensitive. Thus, the LCST phase behavior of the PEO corona block in water drives the micelle transfer (“delivery”), and the double and inverse thermosensibility of the PNIPAm core block in water (LCST) and [BMIM][PF6] (UCST) triggers the assembly (“loading”) and disassembly (“release”) of the nanocarriers. This could enable full thermocontrol of the loading, delivery, and release in the biphasic system. In the PNonOx-PEtOx micelle shuttle between water and [BMIM][PF6],65 the block copolymers are based on a different polymer family, poly(2-oxazoline)s, which themselves exhibit interesting and widely tunable properties.84 In particular, the transfer is also driven by the LCST phase behavior of the PEtOx coronas in water, indicating the potential of using other polymers beyond PEO as the micelle coronas for the micelle shuttle. Control of the micelle transfer in this biphasic system by (84) Hoogenboom, R. Angew. Chem., Int. Ed. 2009, 48, 7978–7994.

8892 DOI: 10.1021/la9046462

Bai and Lodge

thermal trigger was also demonstrated. This micelle shuttle is perhaps most comparable to the PB-PEO system, based on the stability and characteristics of the PNonOx-PEtOx micelles in [BMIM][PF6]65 and water.85

Conclusion We have demonstrated a Pluronic micelle shuttle between water and [BMIM][PF6] and its application in molecular transport in the biphasic system. The P123 block copolymers form micelles in both water and [BMIM][PF6], as characterized by DLS. The micelles can reversibly transfer many times between the two fluids; 1H NMR analysis indicates quantitative transfer. The transfer temperature that determines the micelle partitioning in the biphasic system is tunable by adding salt to the aqueous phase. The micelle shuttle is used to transport various cargoes in the biphasic system using the micelles as nanocarriers: fully reversible transport of three distinct hydrophobic small organic dyes to and out of water and facile extraction of a [BMIM][PF6]-phobic polymer from [BMIM][PF6]. This simple round-trip delivery system may be used in delivery, reaction, and separations in synthesis and biphasic catalysis involving ionic liquids. The commercial availability and low cost of Pluronic block copolymers enhance the practicality of this approach. Acknowledgment. This work was supported by the National Science Foundation through Award DMR-0804197 and by the Frieda Martha Kunze Fellowship (Z.B.). We acknowledge Prof. Valerie C. Pierre for providing access to the UV-vis and fluorescence spectrophotometers. Supporting Information Available: Normalized squared correlation function and size distribution of the P123 micelles measured by DLS. This material is available free of charge via the Internet at http://pubs.acs.org. (85) Lambermont-Thijs, H. M. L.; Hoogenboom, R.; Fustin, C.-A.; BomalD’Haese, C.; Gohy, J.-F.; Schubert, U. S. J. Polym. Sci., Part A 2009, 47, 515–522.

Langmuir 2010, 26(11), 8887–8892