Aggregation Behavior of Pluronic Triblock Copolymer in 1-Butyl-3

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Aggregation Behavior of Pluronic Triblock Copolymer in 1-Butyl-3-methylimidazolium Type Ionic Liquids Shaohua Zhang, Na Li, Liqiang Zheng,* Xinwei Li, Yanan Gao, and Li Yu Key Laboratory of Colloid and Interface Chemistry (Shandong UniVersity), Ministry of Education, Jinan 250100, P. R. China ReceiVed: April 22, 2008; ReVised Manuscript ReceiVed: June 8, 2008

Three amphiphilic poly(oxyethylene)-poly(oxypropylene)-poly(oxyethylene) ethers triblock copolymers, denoted Pluronic L61 (PEO3PPO30PEO3), Pluronic L64 (PEO13PPO30PEO13), and Pluronic F68 (PEO79PPO30PEO79) were shown to aggregate and form micelles in ionic liquids (ILs) 1-butyl-3-methylimidazolium tetrafluoroborate (bmimBF4) and 1-butyl-3-methylimidazolium hexafluorophosphate (bmimPF6). The surface tension measurements revealed that the dissolution of the copolymers in ILs depressed the surface tension in a manner analogous to aqueous solutions. The cmcs of three triblock copolymers increase following the order of L61, L64, F68, suggesting that micellar formation was driven by solvatophobic effect. cmc and γcmc decrease with increasing temperature because hydrogen bonds between ILs and hydrophilic group of copolymers decrease and accordingly enhance the solvatophobic interaction. Micellar droplets of irregular shape with average size of 50nm were observed. The thermodynamic parameters ∆G0m, ∆H0m, ∆S0mof the micellization of block copolymers in bmimBF4 and bmimPF6 were also calculated. It was revealed that the micellization is a process of entropy driving, which was further confirmed by isothermal titration calorimetry (ITC) measurements. Introduction Ionic liquids (ILs) are a class of organic molten electrolytes at or near room temperature.1 They have special physical and chemical properties such as low volatility, wide electrochemical window, nonflammability, high thermal stability, and wide liquid range.2–6 Also, their properties can be fine-turned by varying the component ions of the ILs. So, ILs have attracted much attention as electrolytes7 and solvent media for chemical reactions and extractions,2,8,9 material preparation and gas absorbents.10,11 Moreover, recent developments have been involved in the various self-assemblies of common surfactants in these novel solvents, such as micelles, microemulsions and liquid crystals.12–27 Micelle is an aggregate of amphiphilic molecules, with the nonpolar portions in the interior and the polar portions at the exterior surface, commonly exposed to water. Aggregations of amphiphilic molecules to form micelles in ILs have been recently investigated. Evans and co-workers found that tetradecylpyridinium bromide and hexadecylpyridinium bromide formed micelles in ethylammonium nitrate, a low melting molten salt.28 The appearance of micelles in 1-butyl-3-methylimidazolium chloride (bmimCl) and 1-butyl-3-methylimidazolium hexafluorophosphate (bmimPF6) were explored using different surfactants by inverse gas chromatography.6 It was also reported that several typical nonionic surfactants, Brij-35, Brij-700, Tween20, and Triton X-100 aggregated into micelles in 1-ethyl-3methylimidazolium bis(trifluoromethylsulfonyl)imide (emimTf2N), while no aggregation was observed for cetyltrimethylammonium bromide.29 Amphiphilic block copolymers offer potential advantages over conventional low molar mass surfactants and lipids in their remarkable design flexibility for controlling nanostructure and functionality.30–36 Both their low toxicity and favorable phys* Corresponding author. Telephone: +86 531 88366062. Fax: +86 531 88564750. E-mail: [email protected].

icochemical properties gave rise to a wide industrial interest. Amphiphilic block copolymers are known to assemble in an aqueous milieu into polymeric micelles. Generally, these micelles have a fairly narrow size distribution and are characterized by their unique core-shell architecture. Compared to conventional surfactant micelles, polymeric micelles are generally more stable, with a lowered critical micellar concentration (cmc), and have shown their advantages over traditional ones as medical carriers.37 The self-assembly of four amphiphilic diblock copolymers poly((1,2-butadiene)-block-ethylene oxide) (PB-PEO) in bmimPF6 has been recently investigated by Lodge and coworkers. They found that the universal micellar structures (spherical micelle, wormlike micelle, and bilayered vesicle) were all accessed by varying the length of the corona block while holding the core block constant. Compared to aqueous solutions of the same copolymers, bmimPF6 solutions exhibit some distinct features, such as temperature-independent micellar morphologies in a large range of temperature. Interestingly, they also discovered that these micelles can thermoreversibly transfer between bmimPF6 and water without perturbing the major micellar structure.38,39 Moreover, a morphological transition from spherical to cylindrical micelles, formed by polystyrene-blockpoly(methyl methacrylate) (PS-PMMA), another block copolymer, was observed upon reduction of the PS-PMMA volume fraction.40 Also, the aggregation behaviors of a series of alkyl poly(oxyethyleneglycol) ethers diblock copolymers in 1-butyl3-methylimidazolium type of ILs have been investigated.41 It was demonstrated that size and aggregation number of the micelles can be tuned by changing the chemical structure of the ILs. Here, we describe the aggregation behaviors of a series of amphiphilic poly(oxyethylene)-poly(oxypropylene)-poly(oxyethylene) ethers triblock copolymers (Pluronics), in two common ILs, 1-butyl-3-methylimidazolium tetrafluoroborate (bmimBF4) and 1-butyl-3-methylimidazolium hexafluorophosphate (bmimPF6) (their

10.1021/jp8035132 CCC: $40.75  2008 American Chemical Society Published on Web 07/26/2008

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Figure 1. Chemical structures of ionic liquids, bmimBF4 and bmimPF6, used in this work.

molecular structures are shown in Figure 1). The choice of nonionic surfactants, denoted (PEO)n(PPO)m(PEO)n, was to avoid the exchange of counterions with ILs solvent, as counterions of surfactants have shown to have a great effect on the aggregation structure.42,43 The effect of molecular parameter by varying the length of the hydrophilic block while keeping the hydrophobic block constant on micellar aggregation behaviors was detected. Moreover, considering that these amphiphilic triblock copolymers are essentially a class of bola surfactants, therefore, it would be of interest and of importance to investigate their aggregation behaviors in ILs for the first time.

Figure 2. Surface tension of bmimBF4 solution as a function of L61 concentration at different temperatures.

Experimental Section Materials. ILs, bmimBF4 and bmimPF6, were synthesized according to the standard method by a quaternization reaction of 1-methylimidazole using 1-chlorobutane.44 The imidazolium chloride salt was crystallized in ethyl acetate at -30 °C. For hydrophilic bmimBF4, the postmetathesis product was obtained by ion exchange of 1-butyl-3-methylimidazolium chloride (bmimCl) and potassium tetrafluoroborate in distilled water and then washed with dichloromethane and dried under a high vacuum. For hydrophobic bmimPF6, the rude product was obtained by ion exchange of bmimCl and potassium hexafluorophosphate in distilled water. BmimPF6 was washed by water for several times to increase its purity and then also dried under a high vacuum. The purities of the two products were further checked by 1H NMR spectroscopy. To avoid water, the containers with the materials were sealed tightly to avoid any further contact with air before use. We measured the water concentration in ILs by Karl Fischer titration (787 KF Titrino, Deutsche Metrohm, Filderstadt, Germany). The water concentration in bmimBF4 is 0.318%, and the water concentration in bmimPF6 is 0.375%. Pluronic L61 (PEO3PPO30PEO3), Pluronic L64 (PEO13PPO30PEO13), and Pluronic F68 (PEO79PPO30PEO79) triblock copolymers used in this study were purchased from Sigma and used without further treatment. Their nominal molecular weights are 2000, 2900, and 8400 g/mol, respectively. Surface Tension Measurements. The surface tensions of the IL soluitons were measured by a surface tensiometer (Model JYW-200B, Chengde Dahua Instrument Co.), equipped with a platinum ring. Each datum is an average of five individual points, with an accuracy of (0.2 mN m-1. The samples were equilibrated in the measuring vessel for 15 min, to minimize the drift due to adsorption kinetics. The temperature was controlled by a water circulation jacket connected to a thermostat. Dynamic Light Scattering. Dynamic light scattering (DLS; Brookhaven Instrument Co., BI-200SM goniometer and BI9000AT correlator) was attempted to measure the size and size distribution of micellar solutions. FF-TEM. The fracturing and replication were carried out on Balzers BAF-400D (Germany) freeze-fracture device at the temperature and pressure of -110 °C and 10-4 Pa, respectively. The replicas were examined with a Philips Tecnai 20 and Jeal JEM-100cx electron microscope. Isotherm Titration Microcalorimetry. Isotherm Titration Microcalorimetry was performed on a TAM 2277 calorimeter

Figure 3. Surface tension of bmimPF6 solution as a function of L61 concentration at different temperatures.

(Thermometric, Sweden) with a stainless steel sample cell of 4 mL at 298.15K. The instrument had an electrical calibration with a precision better than (1% that was determined by measuring the dilution enthalpy of a concentrated sucrose solution.45 An injection schedule was automatically carried out, which was controlled by Digitam 4.1 software after setting up the number of injections, volume of each injection, and time between each injection. All experiments were performed with the ionic liquid bmimBF4 in the sample cell and the copolymer solution in the syringe. The reference cell was sealed with a fixed amount (1.2 mL) of ionic liquid bmimBF4 to obtain heat capacity balance in the system. Copolymer solution (the titrant) was injected into the sample cell containing 2.00 mL ionic liquid bmimBF4 (titrate) in 30-40 portions of 12 µL, using a 500-µL Hamilton syringe controlled by a 612 Lund Syringe Pump. The interval between two injections was 40 min, which was sufficiently long for the signal to return to the baseline. The golden turbine stirrer in the ampule was at a constant speed of 50 rpm to ensure continuous mixing efficiency. Results and Discussion Surface Tension Measurements. Surface tension measurements were performed to detect the aggregation behaviors of the block copolymers in ILs. Figure 2 and 3 represent the surface tension versus concentration plot obtained from the solutions of Pluronic L61 in bmimBF4 and bmimPF6 at different temperatures, respectively. It can be seen that, the surface tension gradually decreases with increasing the concentration of Pluronic L61. The decrease of surface tension indicated that the copolymer is adsorbed at the air/solution interface.41 The initial decrease of the surface tension is followed by an abrupt change in the slope of the surface tension versus Pluronic L61 concentration. After the breaking point, the surface tension of the solutions no longer changes obviously, suggesting the formation of micelles in the ILs where the break point

10230 J. Phys. Chem. B, Vol. 112, No. 33, 2008 corresponds to a cmc, the corresponding surface tension is defined as γcmc. The similar results were also observed for Pluronic L64 and Pluronic F68. Furthermore, it can be seen that the addition of Pluronic L61 to ILs depressed the surface tension in a manner analogous to aqueous solutions. This result indicates that there are ILs solvatophobic interactions with the hydrocarbon portion of the copolymer. The main difference between these curves and the analogous curves for aqueous solutions is the initial surface tensions of the neat ILs are lower than that for pure water, which is similar to the report by the Armstrong group.6 For the micelles of nonionic surfactants formed in aqueous solutions, the hydrophobic chains of surfactants are inclined to point toward the hydrophobic domain of micelles composed of hydrophobic chains of surfactants. The hydrophilic polyoxyethylene (PEO) groups prefer to contact with water via hydrogen bonds. The self-assembly is highly cooperative and is driven by the hydrophobic effect, because the system wants to decrease the amount of unfavorable interactions between the hydrophobic chains and water.43 As a result of high cooperation, the surfactant aggregates have different shape depending on molecular parameters of the surfactant and system variable such as concentration and temperature. The curvature of aggregate surface and the packing parameter of the surfactant molecule are two useful concepts when discussing the aggregate shape.43 ILs solvatophobic interactions with the hydrocarbon portion of nonionic surfactants Brij 35 and Brij 700 have been proposed.6 The interaction between ILs and hydrophilic PEO groups was also investigated. The electrostatic attraction between the electronegative oxygen atoms of PEO and positively charged imidazolium cation of ILs has been considered to play a crucial role in forming aggregation of nonionic surfactants in ILs media.46 The electrostatic attraction is temperature-independent, some self-assemblies therefore have shown a thermal stability in a large range of temperature change.38,47,48 In the currently investigated copolymers-ILs system, this electrostatic attraction also occurs between the PEO groups of Pluronic surfactants and positively charged imidazolium rings of either bmimBF4 or bmimPF6. Moreover, hydrogen bonds between C2-H of imidazolium cation and ethoxy/hydroxyl of PEO, between BF4and H- of the Pluronic copolymer hydroxyl terminal may also present in our studied system. A high hydrogen bond basicity was detected for the micelles of nonionic Brij 35 and Brij 700 in bmimPF6.6 The reason is ascribed to the lone pair electrons in PEO groups that are capable of accepting a C-2 proton of bmimPF6, in which C-2 proton is bonded to a carbon that is located between two positively charged nitrogen atoms, and hence C-2 proton is relatively acidic. This result further confirmed that hydrogen bond occurs between oxygen atoms of PEO and imidazolium cations of ILs. Either electrostatic interaction or hydrogen bonds between Pluronics and bmimBF4 may favor the appearance of micelles when combining with the solvophobic force. It has recently been reported that hydrophobic interaction occurs between the PPO groups of Pluronic surfactants and the n-butyl group of the imidazolium cation in aqueous solution.49 In that case, n-butyl group of ILs is relatively hydrophobic and prefers to locate in the PPO micellar cores. However, in our studied system, no water was used and thus there is no significant hydrophobic interaction between them. From these analyses, it can be seen that the PEO groups of the amphiphilic copolymers are inclined to contact with the two imidazolium type of ILs, bmimBF4 and bmimPF6, via hydrogen bonds and electrostatic attraction. That is, the PEO of the

Zhang et al. TABLE 1: Surface Properties and Thermodynamic Parameters of Pluronic L61 in bmimBF4 at Different Temperatures T (K)

cmc (mol/L)

298 3.73 × 10-5 308 3.35 × 10-5 318 3.03 × 10-5

γcmc Γmax ∆Hm0 -T∆Sm0 ∆Gm0 2 (mN/m) (µmol/m ) (kJ/mol) (kJ/mol) (kJ/mol) 39.9 39.4 39.1

0.63 0.73 1.08

-29.35 -30.61 -31.86

7.67 8.19 8.74

-37.02 -38.8 -40.6

copolymers behaves as an ILs-philic group in our studied systems. Therefore, it was proposed that a normal micelle formed in either bmimBF4 or bmimPF6 with PPO blocks as solvophobic cores covered by ILs-philic PEO blocks as coronas that extend into the continuous ILs phase. The micellar structure of Pluronic copolymers in ILs is similar to that in water. It is also obvious from Figure 2 and 3 that the cmc and γcmc decrease with increasing temperature for Pluronic L61 in both bmimBF4 and bmimPF6. The reason is that, hydrogen bond interactions between C2-H of imidazolium cation and ethoxy/ hydroxyl of PEO, between BF4- and H- of the Pluronic copolymer hydroxyl terminals will decrease with increasing temperature. This means that the hydrophilicity of the surfactant was decreased, which in turn enhances the solvophobic force of PPO in bmimBF4 and bmimPF6. As a result, cmc of Pluronic copolymers decreases. The same result was also obtained for nonionic surfactants in aqueous solutions.50 The enhance of solvophobic force suggests the increase of hydrophobicity of amphiphilic copolymers and thus leads to the decrease of surface tensions. The maximum surface excess concentration, Γmax, can reflect the surface arrangement of surfactants in gas/liquid interface. A lower surface tension means that there are more surfactant molecules arranged in the surface of solution. In this study, Γmax, estimated from the Gibbs adsorption isotherm, increases with increasing temperature (Table 1), also suggesting that surface tensions of the IL solutions were decreased. Moreover, we can not exclude the fact that the surface tensions of ILs itself will decrease with increasing temperature.51,52 For instance, the Γmax, of L61 in bmimPF6 at 45 °C is lower than that at 25 and 35 °C. Surface tension curves of Pluronic L61, L64, and F68 in bmimBF4 were shown in Figure 4. The cmcs of three triblock copolymers increase following the order L61, L64, and F68, which is also the order of the PEO length of the copolymers. It is known that the micelle formation is driven by solvophobic force. An increased length of the PEO group of amphiphilic molecules means that the solvophobic force decreases, so the micelle is relatively difficult to form when compared to

Figure 4. Surface tensions of bmimBF4 solutions as a function of three triblock copolymer concentration at 298K. L61 (9), L64 (b), and F68 (2).

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Figure 5. Surface tension as a function of concentration for triblock copolymers in bmimPF6 at 298K. L61(9), L64(b), and F68(2).

amphiphilic molecules with short PEO chain. However, γcmc of three triblock copolymers does not show a regular change with PEO length of Pluronic copolymers. It is clear from Figure 4 that Pluronic L64 displayed a most strong surface activity in comparison with Pluronic L61 and Pluronic F68. This is because Pluronic L61 has a quite short PEO chain so that it is difficult to crimp, after absorption equilibrium was reached, surface arrangement is more compact for flexible Pluronic L64. This special phenomenon is different from that of diblock amphiphilic molecules or single-chain surfactants. Moreover, the IL-philic PEO chain of Pluronic F68 is too long and solvophobic force is very poor, so Pluronic F68 did not show a strong surface activity at the ILs/gas interface. The similar result was also observed in bmimPF6 (Figure 5). The micellar aggregation behaviors of Pluronic L61, L64, and F68 in the two ILs, bmimBF4 and bmimPF6, were also compared, respectively (Figure 6). The cmcs of three amphiphilic copolymers in bmimBF4 are much lower than those in bmimPF6, indicating that solvatophobic interaction of the Pluronic copolymers in bmimBF4 is much stronger than that in bmimPF6. The result can be reflected by the fact that bmimBF4 is relatively more hydrophilic while bmimPF6 is considered to be more hydrophobic. In a more hydrophilic solvent, amphiphilic molecules are easy to aggregate and then form micelles compared to a weak hydrophilic solvent. It is well-known that most surfactant molecules can form various surfactant selfassemblies in aqueous solutions, whereas only part of surfactants can form micellar aggregation in a nonpolar solvent and in many cases, there is not even any aggregations formed. The reason is that self-assembly behavior of amphiphilic molecules is mainly driven by hydrophobic effect, when hydrophobicity between surfactants and solvent is so weak, it is difficult for surfactant molecules to form any self-assemblies. Furthermore, it seems that there is no regular for the γcmc of Pluronic L61, L64, and F68 in these two different ILs. The irregular phenomenon was also often obtained in traditional solvent systems. Freeze-Fracture Transmission Electron Microscopy (FFTEM). The morphology of micelles formed in ILs was also studied. Dynamic light scattering (DLS) was attempted to characterize the size and size distribution of micelles in both bmimBF4 and bmimPF6. No any information could be obtained maybe due to the close refraction index between ILs and copolymers, or weak scattering. Anderson et al. also discovered that DLS was infeasible method to investigate the micellar solution in ILs.6 However, freeze-fracture transmission electron microscopy (FF-TEM) can provide a direct image of the droplets, aggregates and their morphology. Figure 7 shows a typical FF-TEM image of PL64/bmimBF4 micellar solution with

Figure 6. Comparison of surface tension for L61(a), L64(b), F68(c) in bmimBF4 and bmimPF6 at 298 K: (9) bmimBF4 as the solvent for the block copolymers; (b) bmimPF6 as the solvent for the block copolymers.

Figure 7. Typical FF-TEM image of Pluronic L64 in bmimBF4 solution with Pluronic L64 concentration of 20 cmc.

L64 concentration of 20 cmc. Some irregular spherical particles with an average diameter of 50 nm were obtained. It is obvious that the micellar size is much larger than the traditional aqueous micelles that have a general size of less than 20nm. Therefore, the current micellar size exceeds the normally micellar size range. Such large micellar size was also observed by Lodge

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TABLE 2: Surface Properties and Thermodynamic Parameters of Pluronic L61 in bmimPF6 at Different Temperatures T (K)

cmc (mol/L)

γcmc (mN/m)

Γmax (µmol/m2)

∆Gm0 (kJ/mol)

∆Hm0 (kJ/mol)

-T∆Sm0 (kJ/mol)

298 308 318

0.0181 0.0122 0.0084

39.5 39.2 38.1

1.87 1.89 1.27

-13.84 -15.31 -16.80

28.34 30.27 32.27

-42.17 -45.58 -49.06

group when they studied the micellar aggregation behavior of amphiphilic poly((1,2-butadiene)-block-ethylene oxide) (PBPEO) diblock copolymers in bmimPF6.38 Moreover, it also can be seen that some micellar droplets are inclined to assembly together and form a warm-like structure. These warm-like aggregations have an average length of 200 nm and a width of 50 nm, which therefore may be regarded as the fusion of micelles. Thermodynamic Functions of Micellization. A standard free energy ∆G0m of micellization for nonionic surfactant is given as the following expression:

∆Gm0 ) RT ln cmc

(1)

and here the cmc is expressed in mole fraction units. Application of the Gibbs-Helmholtz equation to eq 1 yields

( )

0 ∆Hm0 ∂ ∆Gm ∂(ln[cmc]) )R )- 2 ∂T T ∂T P T

(

)

(2)

Hence, the standard enthalpy of micellization per mole of monomer ∆Hm0 is

( ∂(ln[cmc]) ) ∂T

∆Hm0 ) -RT2

P

(3)

Finally the standard entropy of micellization per mole of monomer ∆Sm0 obtained from

( ∂(ln[cmc]) ) -R ln cmc ∂T

∆Sm0 ) -RT

P

9. The dilution curve in pure ionic liquid is sigmoidal in shape and can be subdivided into two concentration regions separated by a transition region associated with micelle formation, corresponding to cmc. When the final dilution concentration is below cmc, the enthalpy change results from the breakup of the added micelles and from the further dilution of the monomer solution. When the final dilution concentration is above cmc, only the micelle solution is diluted. The cmc value was determined from the first-order differential curve of the observed dilution enthalpic curve as shown in the inset of Figure 9, while the enthalpy of micellization (∆Hm) was determined from the difference in the enthalpy between the two horizontal parts of the S-shaped curve as marked. There was an excellent agreement between cmc values obtained from microcalorimetry and calculated by determining cmcs at different temperatures by surface tension measurement, as can be seen from the results listed in Table 3. Moreover, the enthalpy of micellization (∆Hm) is also positive value, further suggesting that the micellization process of the copolymers L64, L61 is endothermic. This result in turn confirmed that the micellization of copolymers in ILs was controlled by entropy driving. Conclusions In summary, the aggregation behaviors of three kinds of nonionic surfactants poly(oxyethylene)-poly(oxypropylene)-

(4)

According to the above formulas, the standard thermodynamic functions of the micellization of the block copolymers in bmimBF4 and bmimPF6 at different temperatures were calculated and the results were together listed in Tables 1 and 2, respectively. All of the values of ∆Gm0 are negative, suggesting that the micellization of the block copolymer in ILs is spontaneous. From Tables 1 and 2, we can see that the values of ∆Hm0 are positive, while the values of -T∆Sm0 are negative, revealing that the micellization of the copolymers is an entropy driven process. This behavior of thermodynamic functions with respect to the micellization of the block copolymers in ILs is similar to the case of their micellar formation in aqueous system. Alexandridis and his co-workers found that the micellization process of Pluronic triblock copolymers in aqueous solutions has a negative free energy and a positive micellization enthalpy, so the micellization process is entropy-driven.53–55 These results also confirmed that the micellar structure of Pluronic copolymers in ILs have a common feature with corresponding aqueous systems from another point of view. Isothermal Titration Calorimetry (ITC). To confirm the above conclusion, isothermal titration calorimetry (ITC) experiments were further carried out. The typical experimental isothermal calorimetric titration curve for the concentrated solution of copolymer L64 dropped into pure bmimBF4 at 298.15 K is presented in Figure 8, which shows the thermal power P as a function of time t. Integration of the area under the raw signal curve at each injection, after subtraction of the baseline, and normalized to the amount of injected surfactant gives the observed dilution enthalpic curve, as shown in Figure

Figure 8. Isothermal calorimetric titration curve of 2.30 mM PL64 dilution into bmimBF4 at 298.15 K.

Figure 9. Observed dilution enthalpic curve of 2.30 mM L64 dilution into bmimBF4 at 298.15K. The inset is the first-order differential curve of the observed dilution enthalpic curve. The determination of the critical micelle concentration (cmc) and the ∆Hm of L64 micellization are marked.

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TABLE 3: Obtained Critical Micellar Concentration (cmc), and ∆Hm for the Copolymers L64 and L61 at 298.15 K using Surface Tension and Calorimetric Measurements surface tension cmc (mol/L) L64 L61

microcalorimetry

a

∆Hm (KJ/mol)

cmc (mol/L)

∆Hm(KJ/mol)

21.33 7.67

2.14 × 3.12 × 10-5

31.1 275.9

2.21 × 3.73 × 10-5 10-4

10-4

a Calculated by determining cmcs at different temperatures by surface tension measurement.

poly(oxyethylene) triblock copolymers, named Pluronic L61, L64, and F68 in two common ionic liquids (ILs), bmimBF4 and bmimPF6, were investigated by means of surface tension measurements. It was revealed that triblock copolymers can form micellar aggregation in these two ILs and a similar surface tension curve as aqueous surfactant solutions was obtained. The micellar formation in ILs was found to be driven by solvatophobic effect. cmc and γcmc decrease with increasing temperature because hydrogen bonds between ILs and hydrophilic group of copolymers decreases and accordingly enhances the solvatophobic interaction. The micelles formed in ILs have an irregular droplet shape with average size of 50nm. On the basis of surface tension curves, the thermodynamic parameters ∆Gm0 , ∆Hm0 , and ∆Sm0 of the micellization of block copolymers in bmimBF4 and bmimPF6 were further calculated. It was revealed that the micellization is a process of entropy driving, which was followed to be confirmed by isothermal titration calorimetry (ITC) measurements. Acknowledgment. The authors are grateful to the National Natural Science Foundation of China (No.20773081), National Basic Research Program (2007CB808004), the Natural Scientific Foundation of Shandong Province of China (Z2007B06).The authors thank Dezhi Sun, Qian Zhang of Liaocheng University for operating isothermal titration calorimetry (ITC) experiments. We also thank Mr. Shufeng Sun of the Institute of Biophysics of Chinese Academy of Sciences for taking the FF-TEM pictures. And this work was partially supported by Laboratory of Organic Optoelectronic Functional Materials and Molecular Engineering, TIPC, CAS. References and Notes (1) Li, R. X. Green SolVent: synthesis and application of ionic liquids; Chemistry Technology Press: Beijing, 2004. (2) Welton, T. Chem. ReV. 1999, 99, 2071. (3) Kazarian, S. G.; Briscoe, B. J.; Welton, T. Chem. Commun. (Cambridge) 2000, 2047. (4) Gao, H.; Li, J.; Han, B.; Chen, W.; Zhang, J.; Zhang, R.; Yan, D. Phys. Chem. Chem. Phys. 2004, 6, 2914. (5) Anderson, J. L.; Ding, J.; Welton, T.; Armstrong, D. W. J. Am. Chem. Soc. 2002, 124, 14247. (6) Anderson, J. L.; Pino, V.; Hagberg, E. C.; Sheares, V. V.; Armstrong, D. W. Chem. Commun. (Cambridge) 2003, 2444. (7) McEwen, A. B.; Ngo, H. L.; Lecompte, K.; Goldman, J. L. J. Electrochem. Soc. 1999, 146, 1687. (8) Blanchard, L. A.; Hancu, A.; Bechman, E. J.; Brennecke, J. F. Nature 1999, 399, 28. (9) Anthony, J. L.; Maginn, E. J.; Brennecke, J. F. J. Phys. Chem. B 2002, 106, 7315. (10) Nakashima, T.; Kimizuka, N. J. Am. Chem. Soc. 2003, 125, 6386. (11) Camper, D.; Scovazzo, P.; Koval, C.; Noble, R. Ind. Eng. Chem. Res. 2004, 43, 3049. (12) Eastoe, J.; Gold, S.; Rogers, S. E.; Paul, A.; Welton, T.; Heenan, R. K. J. Am. Chem. Soc. 2005, 127, 7302. (13) Merrigan, T. L.; Bates, E. D.; Dorman, S. C.; Davis, J. H. Chem. Commun. 2000, 2051.

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