Aggregation Behavior of a Fluorinated Surfactant in 1-Butyl-3

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Aggregation Behavior of a Fluorinated Surfactant in 1-Butyl-3-methylimidazolium Bis(trifluoromethylsulfonyl)imide Ionic Liquid Na Li,† Shaohua Zhang,† Liqiang Zheng,*,† and Tohru Inoue*,‡ †

Key Laboratory of Colloid and Interface Chemistry, Shandong University, Ministry of Education, Jinan 250100, China, and ‡Department of Chemistry, Faculty of Science, Fukuoka University, Nanakuma, Jonan-ku, Fukuoka 814-0180, Japan Received April 3, 2009. Revised Manuscript Received April 28, 2009 The cationic fluorinated surfactant, FC-4, unlike other surfactants, forms micelles in the room temperature ionic liquid, 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (bmimTf2N). Surface tension, freeze-fracture transmission electron microscopy, 19F NMR, 1H NMR, and Fourier transform infrared measurements revealed that (i) the FC-4 cation forms an ion pair with the Tf2N anion, (ii) the ion pairs undergo association to form premicellar aggregates, and (iii) the premicellar aggregates transform into micelles at the critical micelle concentration (CMC). The thermodynamic parameters for micelle formation derived from the temperature dependence of the CMC demonstrated that the solvophobic interaction between the solvophobic tails of the surfactant molecules is rather weak in bmimTf2N compared with other ionic liquids, in accordance with the observation that surfactants do not readily form micelles in bmimTf2N. The fact that FC-4 forms micelles in such an inconvenient solvent is attributed to the ion-pair formation between the surfactant cation and the ionic liquid anion.

Introduction The self-assembly of surfactant molecules is of fundamental interest and is important in many applications such as nanomaterial synthesis,1-3 drug delivery,4,5 separation,6 pharmaceutical formulation, and other dispersant technologies.6,7 Recently, aggregations of amphiphilic molecules in ionic liquids (ILs) have received more and more attention because of the attractive properties of ILs. ILs have special physical and chemical properties such as low volatility, wide electrochemical window, nonflammability, high thermal stability, and wide liquid range.8-12 Compared to traditional volatile organic solvents, ILs are regarded as environmentally benign solvents, since their nonvolatility can prevent environmental pollution. ILs have been widely applied in organic synthesis,13 chemical separation,14 *Corresponding author. (L.Z.) Tel.: +86 531 88366062; fax: +86 531 88564750; e-mail: [email protected]. (T.I.) Tel: +81 092 8716631; fax: +81 092 8656030; e-mail: [email protected]. (1) Jaramillo, T. F.; Baeck, S. H.; Cuenya, B. R.; McFarland, E. W. J. Am. Chem. Soc. 2003, 125, 7148. (2) Sohn, B. H.; Choi, J. M.; Yoo, S. I.; Yun, S. H.; Zin, W. C.; Jung, J. C.; Kanehara, M.; Hirata, T.; Teranishi, T. J. Am. Chem. Soc. 2003, 125, 6368. (3) Massey, J. A.; Winnik, M. A.; Manners, I.; Chan, V. Z. H.; Ostermann, J. M.; Enchelmaier, R.; Spatz, J. P.; Moller, M. J. Am. Chem. Soc. 2001, 123, 3147. (4) Savic, R.; Luo, L. B.; Eisenberg, A.; Maysinger, D. Science 2003, 300, 615. (5) Allen, C.; Maysinger, D.; Eisenberg, A. Colloids Surf., B 1999, 16, 3. (6) Alexandridis, P., Lindman, B., Eds. Amphiphilic Block Copolymers: SelfAssembly and Applications; Elsevier: Amsterdam, 2000. (7) He, Y. Y.; Li, Z. B.; Simone, P.; Lodge, T. P. J. Am. Chem. Soc. 2001, 128, 2745. (8) Welton, T. Chem. Rev. 1999, 99, 2071. (9) Kazarian, S. G.; Briscoe, B. J.; Welton, T. Chem. Commun. (Cambridge) 2000, 2047. (10) Gao, H.; Li, J.; Han, B.; Chen, W.; Zhang, J.; Zhang, R.; Yan, D. Phys. Chem. Chem. Phys. 2004, 6, 2914. (11) Anderson, J. L.; Ding, J.; Welton, T.; Armstrong, D. W. J. Am. Chem. Soc. 2002, 124, 14247. (12) Anderson, J. L.; Pino, V.; Hagberg, E. C.; Sheares, V. V.; Armstrong, D. W. Chem. Commun. (Cambridge) 2003, 2444. (13) Mehnert, C. P.; Cook, R. A.; Dispenziere, N. C.; Afeworki, M. J. Am. Chem. Soc. 2002, 124, 12932. (14) Anderson, J. L.; Armstrong, D. W. Anal. Chem. 2005, 77, 6453. (15) Zhou, Y.; Antonietti, M. Adv. Mater. 2003, 15, 1452.

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nanomaterial preparation,15-23 and polymer gel electrolytes.24 Even though their properties can be modified to satisfy the requirements by suitable selection of cation, anion, and cation substituent, molecular assemblies formed in ILs are of great interest because the aggregates are expected to solubilize many insoluble substances in ILs and widen the IL applications. In pioneering work in studying surfactants in ILs, Evans et al. reported the aggregation behavior of alkyltrimethylammonium bromides, alkylpyridinium bromides, and nonionic Triton X-100 in a molten salt, ethylammonium nitrate (EAN).25,26 The dry micelle formation of some traditional surfactants in two ILs, 1-butyl-3-methylimidazolium chloride (bmimCl) and hexafluorophosphate (bmimPF6) was reported by Anderson et al.12 Our group has intensively investigated the aggregation behavior of long-chain ILs (CnmimBr, n = 10,12,14,16) in 1-butyl-3-methylimidazolium tetrafluoroborate (bmimBF4) and of various commercial surfactants in both bmimBF4 and bmimPF6.27 Thermodynamic investigations on Tween 20 in ILs showed the formation mechanism of two types of aggregates, nanodroplets and traditional micelles, depending on the surfactant concentration.28 The fluorinated surfactant FC-4 forms traditional micelles (16) Nakashima, T.; Kimizuka, N. J. Am. Chem. Soc. 2003, 125, 6386. (17) Taubert, A. Angew. Chem., Int. Ed. 2004, 43, 5380. (18) Wang, Y.; Yang, H. J. Am. Chem. Soc. 2005, 127, 5316. (19) Aravinda, C. L.; Freyland, W. Chem. Commun. 2004, 2754. (20) Endres, F.; Bukowski, M.; Hempelmann, R.; Natter, H. Angew. Chem., Int. Ed. 2003, 42, 3428. (21) Buhler, G.; Feldmann, C. Angew. Chem., Int. Ed. 2006, 45, 4864. (22) Huang, J. F.; Sun, I. W. Chem. Mater. 2004, 16, 1829. (23) Zhou, Y.; Antonietti, M. J. Am. Chem. Soc. 2003, 125, 14960. (24) Susan, M. A.; Kaneko, T.; Noda, A.; Watanabe, M. J. Am. Chem. Soc. 2005, 127, 4976. (25) Evans, D. F.; Yamauchi, A.; Roman, R.; Casassa, E. Z. J. Colloid Interface Sci. 1982, 88, 89. (26) Evans, D. F.; Yamauchi, A.; Jason Wel, G.; Bloomfield, V. A. J. Phys. Chem. 1983, 87, 3537. (27) Li, N.; Zhang, S. H.; Zheng, L. Q.; Dong, B.; Li, X. W.; Yu, L. Phys. Chem. Chem. Phys. 2008, 10, 4375. (28) Wu, J. P.; Li, N.; Zheng, L. Q.; Li, X. W.; Gao, Y.; Inoue, T. Langmuir 2008, 24, 9314.

Published on Web 05/14/2009

DOI: 10.1021/la901170j

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Li et al. Chart 1. Chemical Structures of bmimTf2N (a) and FC-4 (b)

in bmimBF4, but nanodroplets composed of FC-4 molecules segregated from the solution phase in bmimPF6.29 Moreover, we investigated micelle formation by the Pluronic triblock copolymer in bmimBF4 and bmimPF6.30 The driving force behind micelle formation by Triton X-100 in ILs was investigated using 1 H NMR and two-dimensional rotating frame nuclear Overhauser effect experiments (2D ROESY).31 Although the aggregation behavior of surfactants in imidazolium-based ILs has been widely studied, most of the investigations have been restricted to bmimBF4 and bmimPF6. These IL species have high viscosity, and the PF6 and BF4 anions are susceptible to hydrolysis, which may release toxic hydrogen fluoride, which limits the application of these ILs.32 Bis(trifluoromethylsulfonyl) imide (Tf2N) is a robust anion that has become widely used because it combines low viscosity and high thermal and electrochemical stability. Its immiscibility with water and solvents of low polarity (ethers, haloalkanes, alkanes) opens interesting applications in organic synthesis and electrochemistry.32,33 Attempts have been made to investigate the aggregates formed in ILs with the Tf2N anion. Patrascu and his co-workers34 found that polyoxyethylene (POE)-type nonionic surfactants (CnEm) could form micelles in bmimBF4 and bmimPF6, but that micelle formation in bmimTf2N is difficult or even impossible. Fletcher and Pandey reported that several typical nonionic surfactants, Brij-35, Brij700, Tween-20, and Triton X-100, aggregated into micelles in 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide (emimTf2N), while no aggregation was observed for ionic surfactants.35 There has been no report to date regarding the aggregates formed by ionic surfactants in an IL with Tf2N anion. It would be of interest and importance from both practical and academic points of view to investigate the behavior of highly efficient ionic surfactants, such as fluorinated surfactants, in a Tf2N-based IL. Fluorinated surfactants are more effective for reducing surface tension compared with conventional hydrocarbon-based surfactants.36 They have a number of special properties such as chemical inertness, thermal stability, and oleophobicity, and stability against acidic, alkaline, oxidative, and reductive reagents.37 In the current study, we investigated the aggregation behavior of a cationic fluorinated surfactant (FC-4) in bmimTf2N using surface tension, freeze-fracture transmission electron microscopy (29) Li, N.; Zhang, S. H.; Zheng, L. Q.; Wu, J. P.; Li, X. W.; Yu, L. J. Phys. Chem. B 2008, 112, 12453. (30) Zhang, S. H.; Li, N.; Zheng, L. Q.; Li, X. W.; Gao, Y. A.; Yu, L. J. Phys. Chem. B 2008, 112, 10228. (31) Gao, Y. A.; Li, N.; Li, X. W.; Zhang, S. H.; Zheng, L. Q.; Bai, X. T.; Yu, L. J. Phys. Chem. B 2009, 113, 123. (32) Hapiot, P.; Lagrost, C. Chem. Rev. 2008, 108, 2238. (33) Bonh^ote, P.; Dias, A. P.; Papageorgiou, N.; Kalyanasundaram, K.; Graltzel, M. Inorg. Chem. 1996, 35, 1168. (34) Patrascu, C.; Gauffre, F.; Nallet, F.; Bordes, R.; Oberdisse, J.; de LauthViguerie, N.; Mingotaud, C. ChemPhysChem 2006, 7, 99. (35) Fletcher, K. A.; Pandey, S. Langmuir 2004, 20, 33. (36) Dong, S. L.; Li, X.; Xu, G. Y.; Hoffmann, H. J. Phys. Chem. B 2007, 111, 5903. (37) Shinoda, K.; Hato, M.; Hayashi, T. J. Phys. Chem. 1972, 76, 909.

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(FF-TEM), 1H NMR and 19F NMR, and Fourier transform infrared (FT-IR). The thermodynamic parameters related to the aggregation processes were estimated from the temperature dependence of the critical aggregation concentrations. The NMR measurements as a function of the surfactant concentration provided information about the details of the aggregation process. Combining the thermodynamic analysis and NMR results, we discuss the features of the aggregates formed from FC-4 molecules in bmimTf2N as well as the formation mechanism of the aggregates in bmimTf2N.

Experimental Section Materials. The IL, bmimTf2N, used here was prepared in our laboratory by the procedure reported in the literature.38 The purities of the products were checked using 1H NMR spectroscopy. FC-4 (98%) was provided by Rhodia Company. The chemical structures of FC-4 and the IL molecules are shown in Chart 1. Sample Preparation. The FC-4 in bmimTf2N samples were prepared by mixing appropriate amounts of FC-4 and bmimTf2N followed by stirring at about 80 °C until a clear solution was obtained. This procedure provided optically transparent solutions over a whole concentration range investigated with the solution viscosity slightly increasing with the FC-4 concentration. Apparatus and Procedures. Surface tension measurements were conducted on a Model JYW-200B surface tensiometer using the ring method. Temperature was controlled using a super constant temperature trough. The surface tension was determined in a single-measurement method. All measurements were repeated at least twice until the values were reproducible. FF-TEM observation was performed with a JEM-100CX II transmission electron microscope operated at 100 kV. Samples were immersed rapidly into the liquid ethane cooled by the liquid nitrogen. They were transferred into liquid nitrogen after about 5 s. The samples, after being transferred into the chamber of the freeze-etching apparatus (Balzers BAF-400D), were fractured at a temperature and pressure of -110 °C and 10-4 Pa. After being etched for 1 min, Pt-C was sprayed onto the fracture face at 45°, and then C was sprayed at 90°. FT-IR spectra were recorded in KBr pellets with a resolution of 2 cm-1 using a BIORAD FTS-165 spectrometer at 25 °C. 1H NMR measurements were carried out with a Varian ARX 400 NMR spectrometer operating at a frequency of 400.13 MHz. 19F NMR spectra were recorded on a Bruker AVANCE 400 spectrometer equipped with pulse field gradient module (Z axis) using a 5 mm BBO probe operating at 376.72 MHz. The 19F spectra are reported in the range from +30 to -170 ppm (digitized points = 32K, 90° pulse = 7.4 ms, relaxation delay = 2 s). 1H NMR and 19F NMR measurements were performed at 25 °C.

Results and Discussion Surface Tension Measurements. Surface tension measurements were performed to detect the aggregation behavior of FC-4 (38) Dupont, J.; Consorti, C. S.; Suarez, P. A. Z.; Souza, R. F. Org. Synth. 1999, 79, 236.

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Figure 1. Surface tension versus concentration plots obtained for FC-4 solutions in bmimTf2N at various temperatures. Numerical values on the vertical axis represent the surface tension at 25 °C. Surface tension curves at temperatures higher than 25 °C are drawn by shifting the surface tension values appropriately. Temperatures are indicated in the figure.

in bmimTf2N. Figure 1 shows the surface tension versus concentration plot obtained for the solutions of FC-4 in bmimTf2N at different temperatures, where the surface tension curves obtained at temperatures higher than 25 °C are drawn by shifting the scale on the vertical axis appropriately in order to avoid the overlapping of data points. At each temperature, the surface tension gradually decreases with the increase of the FC-4 concentration. The initial decrease of the surface tension with the addition of FC4 is followed by an abrupt change in the slope of the surface tension versus concentration curve. After the break point, the surface tension of the solutions no longer changes. This behavior of surface tension curves is quite similar to that observed for aqueous solutions of micelle-forming surfactants, and hence suggests that micelles are formed in bmimTf2N. FF-TEM images observed for the solution at concentrations beyond the break point in the surface tension-concentration curve are depicted in Figure 2. Particles 10-20 nm in diameter are seen, and their number increases with the increase in FC-4 concentration. Solutions with concentrations lower than the break point exhibit no such particles in their FF-TEM images (data not shown). Thus, the FF-TEM observation demonstrates that micelles are formed from FC-4 molecules in bmimTf2N in the high concentration range, and the concentration corresponding to the break point in the surface tension curve can be regarded as a critical micelle concentration (CMC). The values of CMC were determined from the intersection of two straight lines drawn in the low and high concentration regions in the surface tension curves (γ-log C curves), and are summarized in Table 1. The surface tension at the CMC, γCMC, the reduction of surface tension (or the surface pressure) at the CMC, ΠCMC (= γ0 - γCMC), and the relative maximum reduction of surface tension, ΠCMC/γ0, are also included in Table 1. It is noteworthy that FC-4 undergoes self-assembly to form micelles in bmimTf2N. This IL species is known to be a rather poor solvent for micelle formation by surfactants, at least for the hydrocarbon surfactants studied to date.34,35 Nevertheless, FC-4 forms micelles in the IL. This must be attributed to a specific interaction between the fluorocarbon chain and bmimTf2N. (39) Gao, Y. A.; Hou, W. G.; Wang, Z. N.; Li, G. Z.; Han, B. X.; Zhang, G. Y.; Lu, F. F. Chin. J. Chem. 2005, 23, 362.

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It is of interest to compare the present results with those obtained for FC-4 in aqueous solution39 and in bmimBF4 solution.29 The CMC values of FC-4 have been reported to be 0.036 mmol/L39 and 2.3 mmol/L29 at 25 °C for aqueous and bmimBF4 systems, respectively. Generally, the CMCs in ILs are much higher than those in aqueous solution, reflecting a much weaker solvophobic effect in ILs than the hydrophobic effect in the aqueous system. The present bmimTf2N system exhibits an extremely high CMC value of 0.219 mol/L at 25 °C, which is 2 orders of magnitude larger than the bmimBF4 value. In other words, the solvophobicity sensed by the surfactant molecules in bmimTf2N is remarkably weak compared with other IL species. This is the reason for the difficulty of micellization in bmimTf2N for many surfactants. The significant effect of the anion in imidazolium-based ILs on the aggregation behavior of surfactants has been frequently reported.34,35 The solvophobicity is greater in bmimBF4 than in bmimPF6. When Tf2N- is included, the surfactant solvophobicity decreases in the order bmimBF4 > bmimPF6 > bmimTf2N. The relative maximum reduction of surface tension, ΠCMC/γ0, is regarded as a measure of the surface activity of the surfactant in the solvent. ΠCMC/γ0 for FC-4 in bmimTf2N is 0.31 (Table 1), whereas those for bmimBF4 and water are 0.5829 and 0.74,39 respectively. That is, the surface activity of FC-4 in the three solvents decreases in the order water > bmimBF4 > bmimTf2N. Thus, the difference in the solvophobicity of the surfactant molecule in different solvents is reflected in its surface properties as well as by micelle formation in bulk solution. The temperature dependence of CMC for FC-4 in bmimTf2N is illustrated in Figure 3. As can be seen in this figure, CMC increases with temperature showing a concave curve. Usually, a minimum CMC is observed in the CMC versus temperature plot. In the present case, the minimum is expected to appear below 25 °C. This trend is similar to the temperature dependence obtained for the CMC of FC-4 in bmimBF4.29 19 F NMR Results. (a). 19F NMR of FC-4. The 19F NMR chemical shift is known to be sensitive to the surrounding environment and is an excellent tool to study fluorinated surfactants.20-22 The physicochemical microenvironment of a surfactant molecule depends on its self-organization in solvents. It is well-known that the magnetic resonance of fluorine nuclei shows an upfield shift upon micellization.36,40 The peak of the terminal CF3 group in the fluorocarbon chain is the strongest, and its chemical shift is the most sensitive to the micellization.36,40 We performed 19F NMR measurements in order to obtain detailed information regarding the micelle formation of FC-4 in bmimTf2N. The 19F NMR spectrum obtained for 0.430 mol/L FC-4 in bmimTf2N is shown in Figure 4. The strong signal around -80.3 ppm is ascribed to the CF3 group in Tf2N-, since this signal appears in pure bmimTf2N. The signal around -81.3 ppm is ascribed to the terminal CF3 group in FC-4.36 The plot of chemical shift, δ, for terminal CF3 fluorine as a function of FC-4 concentration is shown in Figure 5a. As can be seen in this figure, δ remains constant up to about 0.1 mol/L, and then decreases monotonously with the increase in FC-4 concentration. The constant δ in the low concentration region is understandable, because FC-4 molecules exist as free monomer, and the environment around the monomeric surfactant would be the same regardless of the surfactant concentration. When the concentration of FC-4 is increased, molecular aggregates of FC-4 are formed, and the fluorocarbon chains contact each other in the aggregates. Then, the environment becomes less polar compared (40) Dong, S.; Xu, G.; Hoffmann, H. J. Phys. Chem. B 2008, 112, 9371.

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Figure 2. FF-TEM images obtained for FC-4 solution in bmimTf2N. The surfactant concentrations are 0.280 mol/L (a) and 0.344 mol/L (b). Table 1. Surface Properties and Thermodynamic Parameters of Micelle Formation for FC-4 Solution in bmimTf2N at Various Temperatures temp. (°C)

CMC (mol/L)

γCMC (mN/m)

ΠCMC (mN/m)

ΠCMC/γ0

ΔG0m (kJ/mol)

ΔH0m (kJ/mol)

ΔS0m (J/Kmol)

25 30 35 40 45 50

0.219 0.229 0.245 0.282 0.322 0.367

26.4 25.8 25.7 25.6 25.4 25.2

11.2 11.7 11.6 11.5 11.5 11.6

0.30 0.31 0.31 0.31 0.31 0.32

-6.79 -6.79 -6.73 -6.47 -6.22 -5.97

-5.54 -10.4 -15.1 -19.7 -24.0 -28.3

4.17 -11.9 -27.2 -42.1 -56.1 -69.2

Figure 3. Temperature dependence of CMC for FC-4 in bmimTf2N.

Figure 4.

19

F NMR spectrum obtained for FC-4 solution in bmimTf2N. The surfactant concentration is 0.430 mol/L.

with that in the IL, where the electron density around the fluorine nuclei is enhanced. The increased electron density results in an increase in the shielding effect, and hence causes an upfield shift of δ. It is noteworthy that δ starts to decrease at a concentration 10476 DOI: 10.1021/la901170j

Figure 5. (a) Plot of the 19F NMR chemical shift for the terminal CF3 in the fluorocarbon chain of FC-4 as a function of the surfactant concentration. (b) Plot of δobs against 1/ct. Temperature is 25 °C.

much lower than the CMC. This suggests that some molecular event such as premicelle formation occurs below the CMC. Langmuir 2009, 25(18), 10473–10482

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On the basis of the NMR results, we assume that premicellar aggregates are formed at a critical concentration, Cpm, and that the monomer concentration in the region between Cpm and the CMC is approximated by Cpm. The exchange rate of the surfactant molecules between the monomeric state and the premicelles may be reasonably assumed to be rapid compared with the NMR time scale. Then, the observed chemical shift, δobs, for different concentration ranges may be expressed as follows: (i) 0 < ct < Cpm δobs ¼ δs

ð1Þ

where δs represents the chemical shift of CF3 fluorine in monomeric FC-4, and ct means the total FC-4 concentration. (ii) Cpm e ct < CMC δobs ¼ xs δs þ xpm δpm where xs and xpm are the mole fractions of the surfactant in the monomeric state and in premicellar aggregates, respectively, and δpm is the chemical shift for the premicellar aggregates. In the above equation, xs and xpm are given by xs ¼

Cpm ct

and xpm ¼

ct -Cpm ct

FC-4 concentration is quite similar to that for FC-4 fluorine. This means that Tf2N interacts with FC-4, which in turn suggests that Tf2N anion forms an ion pair with the FC-4 cation. Molecular models of the ion pair as well as the FC-4 cation and Tf2N anion are presented in Figure 7. When the ion pair is formed, the surrounding environment of Tf2N anion must change, and hence, the chemical shift of the Tf2N fluorine changes. The concentration of the ion pair must increase with FC-4 concentration. Thus, δ of Tf2N changes depending on FC-4 concentration. 1 H NMR Results. (a). 1H NMR of the bmim Protons. It is expected that some useful information can be obtained regarding molecular events occurring in FC-4 solution in bmimTf2N by following the 1H NMR chemical shifts for the surfactant protons and the bmim protons. Thus, we carried out 1 H NMR measurements on solutions of several concentrations. The 1H NMR spectrum obtained for 0.516 mol/L FC-4 solution is shown in Figure 8a. Signals observed at 2-3.5 ppm are attributed to protons in FC-4, and other signals come from bmim protons. The peak assignments of the bmim protons are shown in the figure. When FC-4 was added to bmimTf2N, the chemical shifts of the bmim protons changed with FC-4 concentration. The differences in chemical shifts of bmim protons between FC-4 solution and pure bmimTf2N, Δδ (= δsolution - δIL), are plotted in Figure 9 as a function of the surfactant concentration.

Thus, δobs in this concentration range is given by δobs ¼

Cpm ðδs -δpm Þ þ δpm ct

ð2Þ

(iii) CMC e ct δobs ¼ xs δs þ xpm δpm þ xm δm where xm is the mole fraction of the surfactant in micelles, and δm is the chemical shift for FC-4 in the micellar state. In the above equation, xm is expressed by xm ¼

ct -CMC ct

δobs ¼

Cpm ðδs -δpm Þ þ CMCðδpm -δm Þ þ δm ct

ð3Þ

Equations 1-3 predict that, when δobs is plotted against 1/ct, (i) δobs is constant above 1/Cpm, or below Cpm (eq 1), (ii) δobs changes linearly with the slope of Cpm(δs - δpm) and the intercept of δpm in the concentration range between 1/Cpm and 1/CMC (eq 2), and (iii) δobs changes linearly with the slope of Cpm(δs - δpm) + CMC (δpm - δm) and the intercept of δm. Figure 5b shows the plot of δobs against 1/ct for the data presented in Figure 5a. As expected, the plot of δ versus ct-1 can be linearly fit by three straight lines. The intersections of lines (i) and (ii), and lines (ii) and (iii), provide concentrations corresponding to Cpm and CMC, respectively. The values estimated from this plot are Cpm = 0.14 mol/L and CMC = 0.24 mol/L. The CMC is comparable to that determined by surface tension experiments. (b). 19F NMR of Tf2N Fluorine. Analysis of the 19F NMR chemical shift allows us to follow the behavior of Tf2N, the counteranion of the bmim cation. Plots of δ for Tf2N fluorine as a function of FC-4 concentration are shown in Figure 6. The trend of the variation in δ of Tf2N fluorine associated with the change in Langmuir 2009, 25(18), 10473–10482

19 F NMR chemical shift for fluorine atoms in the Tf2N anion as a function of FC-4 concentration. Temperature is 25 °C.

Figure 6. Plot of

and hence, we obtain

Figure 7. Molecular models of FC-4, bmimTf2N, and the ion pair formed from the FC-4 cation and the Tf2N anion. DOI: 10.1021/la901170j

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Figure 9. Plots of Δδ for bmim protons as a function of FC-4 concentration. Symbols for the individual protons in the bmim cation are as follows: Ha (filled circles), Hb (open circles), Hc (open squares), Hd (filled squares), He (filled triangles), and Hf and Hg (open triangles).

Figure 8. 1H NMR spectrum obtained for FC-4 solution in bmimTf2N. The surfactant concentration is 0.516 mol/L.

The protons bound to the imidazolium ring (Ha and Hb) exhibit a downfield shift, while the butyl and methyl protons show upfield shifts. The downfield shift of δ for Ha is much greater than for Hb, and the upfield shift of butyl protons increases from Hd to Hg (toward the terminal methyl group). The Δδ for Ha is extremely large compared with the other protons. This suggests that the Ha proton participates in hydrogen-bonding interactions. The weak upfield shift of the butyl protons is considered to be caused by nonspecific interactions or solvent effects as a result of the addition of FC-4 to the IL. That is, the polarity of environment around the IL is reduced by the addition of the fluorocarbon surfactant, FC-4, and hence δ is subjected to an upfield shift. It should be noted that a subtle change in Δδ is visible at the concentration corresponding to CMC except for the case of Ha. This discrete change may reflect the effect of micellization on the nonspecific interactions. (b). 1H NMR of FC-4 Protons. The NMR signals due to protons in the FC-4 molecule are shown in Figure 8b in an expanded scale. The signal assignment is also indicated in the figure, which was made on the basis of the relative position of the chemical shifts and peak intensities in the spectrum of FC-4 in CDCl3. The plots of δ as a function of FC-4 concentration for signals from the Ha, Hc, Hd, and He protons of FC-4 are shown in Figure 10. Although the changes in δ are relatively small, a trend is observable, as described below. The decreases in δ in the low concentration region may be attributed to the interaction between the hydrocarbon moieties of FC-4 and Tf2N due to ion-pair formation in addition to the nonspecific interactions mentioned above. It is noticeable that δ changes rather discretely at the CMC except for the case of the He proton. This means that the environment of the quaternary ammonium moiety, or in other words, the headgroup, changes abruptly when premicellar aggregates transform to micelles. This suggests that a sudden change in configuration of the headgroup takes place when micelles are formed. It is likely that the headgroup adjusts its configuration to fit the critical packing parameter required for the surfactant molecules to form micelles. For the He proton, no discrete change in δ is observed because the He proton is close to the fluorocarbon 10478 DOI: 10.1021/la901170j

chain, and the effect of headgroup configuration is small. The change in δ above the CMC may be attributed to nonspecific interactions and/or micellar growth caused by the increase in surfactant concentration. FT-IR Results. As mentioned above, the results of 1H NMR experiments suggest that the Ha proton in bmim participates in hydrogen-bonding. In order to investigate this further, FT-IR measurements were carried out. The FT-IR spectrum obtained for FC-4 solution in bmimTf2N with 0.430 mol/L is shown in Figure 11a. The peak assignment from the literature is indicated in the figure.41 The shoulder at 3107 cm-1 is of special interest, because it is assigned to νC(2)-H, i. e., the C-Ha stretching vibration of bmim. It was observed that when FC-4 is added to the IL, the absorbance around 3050 cm-1 increases with the increase in FC-4 concentration. Thus, we examined the difference spectra between FC-4 solution and pure bmimTf2N, paying attention to the region around 3050 cm-1. Figure 11b shows the difference spectra that were obtained by subtracting the spectrum for pure bmimTf2N from the spectra for FC-4 solutions after normalizing the peak intensity at 3107 cm-1. It can be seen that the broad absorption band with a maximum around 3067 cm-1 increases in its intensity with the increase in FC-4 concentration. This may be interpreted to be a result of a red shift of νC(2)-H due to the hydrogen-bonding of the proton, and is consistent with the downfield shift of Ha in the 1H NMR results. Candidates for the acceptor of the hydrogen-bond with the bmim Ha proton are considered to be the carbonyl oxygen of the amide group in FC-4, the ether oxygen in the fluorocarbon chain of FC-4, and the I- ion dissociated from FC-4. Since the IR absorption peak due to νCdO exhibits no frequency shift (data not shown), the possibility of the carbonyl oxygen is excluded. If the ether oxygen in fluorocarbon chain participates in the hydrogenbond, δ for Ha proton should exhibit a discontinuous change at the CMC because fluorocarbon chains are buried in the micellar interior. Thus, the ether oxygen is unlikely. I- ion is most probable as the hydrogen-bond acceptor, since δ for Ha changes almost linearly with the increase in FC-4 concentration. Hydrogen-bonding between the Ha proton of bmim+ and an I- ion has been reported in other systems.41 (41) Jeon, Y.; Sung, J.; Seo, C.; Lim, H.; Cheong, H.; Kang, M.; Moon, B.; Ouchi, Y.; Kim, D. J. Phys. Chem. B 2008, 112, 4735.

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Figure 10. Plots of δ for FC-4 protons as a function of FC-4 concentration.

Figure 11. (a) FT-IR spectrum obtained for FC-4 solution in bmimTf2N. FC-4 concentration is 0.430 mol/L. (b) Difference spectra between the solution and the solvent (for details, see text). The concentrations of FC-4 corresponding to each curve are indicated in the figure.

Molecular Events Occurring in the FC-4 Solution in BmimTf2N Deduced from Spectroscopic Observation. The spectroscopic observations described above provide the following scenario for the molecular events occurring in FC-4 solution in bmimTf2N, relating to premicelle formation and micelle formation. When FC-4 is added to bmimTf2N, dissociation takes place to form a FC-4 cation and an I- anion. Then, the Tf2N anion Langmuir 2009, 25(18), 10473–10482

binds to the FC-4 cation to form an ion pair. This ion-pair formation would be facilitated by not only the electrostatic attraction between the oppositely charged species, but also the solvophobic interaction between fluorocarbon moieties included in FC-4 and Tf2N. The ion pair is in equilibrium with free ions. As for the dissociated I- ion, it binds to bmim+ through hydrogen-bonding with the Ha proton. This hydrogen-bond DOI: 10.1021/la901170j

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Figure 12. Schematic model of a part of a FC-4 micelle in bmimTf2N.

interaction facilitates the dissociation of I- from FC-4 in the medium. The ion pair takes a certain characteristic configurational structure in order to fulfill the requirements that (i) the positive charge on the quaternary nitrogen atom of FC-4 and the negative charge on the nitrogen of Tf2N are located as close as possible to each other and (ii) the fluorocarbon moieties in FC-4 and in Tf2N contact each other as much as possible. Both requirements would be satisfied if the hydrocarbon moiety of the headgroup bends at the position of the amide group as shown in Figure 7. Ion-pair formation between FC-4 cation and Tf2N anion allows self-assembly of premicellar aggregates, because electrostatic repulsion is eliminated and solvophobic interactions attract the ion pairs. The size of the premicellar aggregates is not so large as to be detected by FF-TEM, since no particles were observed in FF-TEM images taken in the premicellar region. The premicellar aggregates are in equilibrium with the ion pair. Thus, the ion pair is in equilibrium with free ions and also with premicellar aggregates. The ion pair as well as free ions may adsorb at the air/solution interface, which lowers the surface tension. When FC-4 concentration is increased, all the concentrations of free ion, ion-pair, and premicellar aggregates increase, and the surface concentration of the free FC-4 ion and the ion pair also increases. Thus, surface tension decreases continuously with the increase in FC-4 concentration. When the surface adsorption has reached saturation, the premicellar aggregates undergo transformation into micelles in order to accommodate the surfactant molecules added beyond the surface saturation concentration. This concentration corresponds to the CMC. The transformation from premicelle to micelle is accompanied by a change in configuration of the headgroup, i.e., the hydrocarbon moiety of FC-4. This configurational change must be reflected in a discrete change of 1H NMR chemical shifts of the FC-4 protons (see Figure 10). The hydrocarbon moiety extends, taking an orientation nearly vertical to the micellar surface. In this way, the area occupied by each surfactant molecule at the micellar surface decreases, and the number of surfactant molecules accommodated in a given micelle increases. A schematic model of a part of the micelle is shown in Figure 12. Thermodynamic Analysis of the Micelle Formation of FC-4 in bmimTf2N. It may be reasonably assumed that the concentration of free FC-4 cation is negligibly small compared with that of the ion pair in the high concentration range near and above the CMC, since the equilibrium constant for ion-pair formation is expected to be large due to the strong 10480 DOI: 10.1021/la901170j

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Figure 13. Plot of ΔG0m/T for FC-4 in bmimTf2N. Solid line is a curve calculated according to a second-order polynomial fit.

attractive interaction between the FC-4 cation and the Tf2N anion. Then, we can consider that the micelles are formed from the ion pairs, or in other words, the micelles are in equilibrium with the ion pairs. This situation is similar to the micelle formation of nonionic surfactants, and hence, the standard Gibbs energy change associated with the micelle formation is given by42 ΔG0m ¼ RT ln XCMC

ð4Þ

where XCMC is the CMC expressed in mole fraction units. We estimated ΔG0m using the CMC values listed in Table 1. When ΔG0m is known as a function of temperature, the standard enthalpy of micelle formation, ΔH0m, can be derived by applying the Gibbs-Helmholtz equation: "

# DðΔG0m =TÞ ¼ ΔHm0 Dð1=TÞ

ð5Þ

Following eq 5, ΔG0m/T was plotted against 1/T as is shown in Figure 13. A second-order polynomial was fitted to the data points, and the values of ΔH0m were calculated from the slopes of tangential lines at the temperatures corresponding to the experimental data points. Using these ΔG0m and ΔH0m values, the standard entropy change associated with the micelle formation, ΔS0m, was calculated following the relation 0 ΔSm ¼

ΔHm0 -ΔG0m T

ð6Þ

The thermodynamic parameters thus obtained are summarized in Table 1. Figure 14 depicts the plot of these thermodynamic parameters as a function of temperature, where, for the entropy change, -TΔS0m is plotted instead of ΔS0m in order to make clear the contribution of the entropy term to the free energy gain associated with micelle formation. It can be seen in this figure that the temperature dependence of ΔG0m is rather weak, and -TΔS0m increases with temperature, while ΔH0m decreases. This behavior of thermodynamic parameters associated with temperature rise is common to the micellization of surfactants in IL solution as well as in aqueous solution, which is caused by solvophobic (hydrophobic in aqueous systems) interaction (42) Hiemenz, P. C. Principles of Colloid and Surface Chemistry, 2nd ed.; Marcel Dekker: New York, 1986; Chapter 8.

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Figure 14. Plot of thermodynamic parameters of micelle formation at the CMC against temperature for FC-4 in bmimTf2N. Circles, squares, and triangles correspond to ΔG0m, ΔH0m, and -TΔS0m, respectively.

between the solvophobic tails in the surfactant molecules. However, a distinctive feature of the present system (FC-4 in bmimTf2N) is that the main contribution to a negative ΔG0m is the enthalpy term instead of the entropy term, even at room temperature. Usually, a large negative -TΔS0m overcomes a positive ΔH0m to produce a negative ΔG0m at low temperature, as is well-known for aqueous micellar systems and also as observed for Tween 20 solution in bmimBF4 and bmimPF6.28 The importance of the entropic contribution is a characteristic of solvophobic interactions and is interpreted as follows. When surfactant molecules are dissolved in water or ILs in monomeric form, a highly ordered arrangement of solvent molecules is created around the solvophobic tails of the surfactants due to hydrogen-bonding (in water) or ionic interactions (in ILs). This causes free energy loss coming from the entropic term. Thus, surfactant molecules assemble to form micelles and release the solvating molecules around the solvophobic tails in order to avoid this entropic loss. The contribution from the entropy term is reduced more and more with the increase in temperature, while the enthalpic contribution is enhanced instead. The small entropic contribution even at room temperature observed in the present system means that the solvophobic effect is rather weak for FC-4 micelles formed in bmimTf2N, and this is a reason for the extremely high CMC of this system. Enthalpy-Entropy Compensation for Micellization of FC-4 in ILs. A linear relationship exists between the enthalpy change and entropy change for various processes in aqueous solution such as oxidation-reduction, hydrolysis, protein unfolding.43,44 This phenomenon is known as enthalpy-entropy compensation. The micelle formation process in surfactant solution also exhibits such a compensation phenomenon.45-48 It is of interest to examine the relation between enthalpy and entropy for micelle formation of FC-4 in ILs. Figure 15 shows a plot of ΔH0m against ΔS0m obtained for FC-4 solutions in bmimTf2N and in bmimBF4.29 A good linear relationship is seen, i.e., the enthalpyentropy compensation relation holds well for the micelle formation in these ILs. (43) Hammett, L. P. Physical Organic Chemistry: Reaction Rates, Equilibrium, and Mechanism, 2nd ed.; MacGraw-Hill: New York, 1970. (44) Lumry, R. In Methods in Enzymology; Ackers, G. K., Johnson, M. L., Eds.; Academic Press: New York, 1995; Vol. 259, pp 628-720. (45) Bedo, Z.; Berecz, E.; Lakatos, I. Colloid Polym. Sci. 1992, 270, 799. (46) Singh, H. N.; Saleem, S. M.; Singh, R. P.; Birdi, K. S. J. Phys. Chem. 1980, 84, 2191. (47) Goto, A.; Takemoto, M.; Endo, F. Bull. Chem. Soc. Jpn. 1985, 58, 247. (48) Lee, D. J. Colloid Polym. Sci. 1995, 273, 539.

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Figure 15. Enthalpy-entropy compensation relation for micelle formation of FC-4 in bmimTf2N (circles, this work) and in bmimBF4 (squares, ref 29).

The compensation phenomenon is expressed in the form of 0 ΔHm0 ¼ ΔHm/ þ Tc ΔSm

The slope of the straight line in the ΔH0m versus ΔS0m plot has a dimension of temperature, and is called the compensation temperature. The same Tc value of 310 K was obtained for both IL species. It has been reported that Tc values are in a range from 304 to 314 K for micelle formation of ionic surfactants in aqueous solution.49 The Tc of 310 K obtained for micellization of cationic FC-4 in both ILs is in this range, and implies that the compensation temperature is rather independent of the solvent species. The intercept, ΔH*m, stands for the enthalpy effect under the condition ΔS0m = 0, and becomes a measure of the stability of the micelles. A considerable difference is seen in the intercept between bmimTf2N and bmimBF4. The greater ΔH*m value (smaller in absolute value) in bmimTf2N compared with in bmimBF4 demonstrates that the FC-4 micelles formed in bmimTf2N are less stable than those formed in bmimBF4.

Conclusions In the present work, we investigated the aggregation behavior of the cationic fluorinated surfactant, FC-4, in an IL, bmimTf2N. FC-4 cations form ion pairs with Tf2N anions, the counteranions of the IL. The ion pairs undergo association to form premicellar aggregates at a critical concentration. The premicellar aggregates transform into micelles at the CMC. Micelles are difficult to form in bmimTf2N. Nevertheless, FC-4 can assemble to form micelles, although the CMC is much higher than in water or other ILs (bmimBF4 and bmimPF6). This unique property of FC-4 may be attributed to the ion-pair formation that is facilitated by solvophobic interactions between the fluorocarbon moieties in FC-4 and Tf2N, in addition to electrostatic interactions between the opposite charges. Thermodynamic analysis of micellization revealed that the contribution of the entropy term to the free energy of micelle formation is rather weak compared with other IL systems. This means that the solvophobicity sensed by surfactant molecules in bmimTf2N is weaker than that in other ILs, which is reflected in the difficulty of micelle formation by surfactant molecules in bmimTf2N. The present work contributes to a better understanding of the self-assembly of amphiphilic molecules in ILs. (49) Chen, L. J.; Lin, S. Y.; Huang, C. C. J. Phys. Chem. B 1998, 102, 4350.

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Acknowledgment. The authors are grateful to the National Natural Science Foundation of China (No. 20773081), the National Basic Research Program (2007CB808004), and the Natural Scientific Foundation of Shandong Province of China (Z2007B06). This work was partially supported by the Laboratory

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of Organic Optoelectronic Functional Materials and Molecular Engineering, TIPC, CAS. And we gratefully acknowledge the help of Shufeng Sun of the Institute of Biophysics of Chinese Academy of Sciences for taking the FF-TEM pictures. We also thank Dr. Pamela Holt for editing the manuscript.

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