Aggregation Behavior of Polyoxyethylene (20) Sorbitan Monolaurate

Publication Date (Web): July 29, 2008 ... Corresponding authors: Liqiang Zheng, tel +86 531 88366062, fax +86 531 88564750, e-mail [email protected];...
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Langmuir 2008, 24, 9314-9322

Aggregation Behavior of Polyoxyethylene (20) Sorbitan Monolaurate (Tween 20) in Imidazolium Based Ionic Liquids Jiapei Wu,† Na Li,† Liqiang Zheng,*,† Xinwei Li,† Yan’an Gao,† and Tohru Inoue*,‡ Key Laboratory of Colloid and Interface Chemistry (Shandong UniVersity), Ministry of Education, Jinan 250100, People’s Republic of China, and Department of Chemistry, Faculty of Science, Fukuoka UniVersity, Nanakuma, Jonan-ku, Fukuoka 814-0180, Japan ReceiVed April 30, 2008. ReVised Manuscript ReceiVed June 2, 2008 Surface tension measurements were carried out for the solutions of polyoxyethylene (20) sorbitan monolaurate (Tween 20) in 1-butyl-3-methylimidazolium tetrafluoroborate (bmimBF4) and hexafluorophosphate (bmimPF6) at various temperatures. Two transition points were found in the surface tension-concentration curves at each temperature. The freeze-fracture transmission electron microscopy revealed that two kinds of particles with different sizes are formed at the concentrations of each transition point. Thus, the surfactant concentrations of the two transition points are regarded as critical aggregation concentrations, CAC1 and CAC2. From the CAC values and their temperature 0 0 0 dependence, we estimated the thermodynamic parameters of the aggregate formation, ∆Gagg , ∆Hagg , and ∆Sagg . The thermodynamic parameters related to CAC1 are almost independent of temperature. On the other hand, as for 0 0 the aggregate formation at CAC2, a positive∆Sagg contributes to a negative ∆Gagg at low temperature, while a negative 0 0 ∆Hagg contributes to a negative ∆Gagg at high temperature. The behavior of the thermodynamic parameters as a function of temperature, combined with the variation of 1H NMR chemical shifts of the bmim+ protons as a function of the surfactant concentration, demonstrated that the aggregates formed at CAC1 are nanodroplets of Tween 20 segregated from the solution phase, while those formed at CAC2 are similar to the usual surfactant micelles formed in aqueous solution.

Introduction Ionic liquids (ILs) are currently attracting significant attention as a new class of solvent. They have many desirable properties, such as easy recyclability,1 nonvolatility, nonflammability, high ionic conductivity, wide electrochemical potential window, and high thermal stability.2 The ionic liquids are a series of molten salts at or near ambient temperature, whose chemical and physical properties can be easily tailored by judicious selection of cation and anion.3 These advantages make them important as novel solvents in organic synthesis,4–6 chemical separations,7,8 solar cells9 and so on. One major new direction in IL research is to explore the formation of supramolecular assemblies. Molecular self-assemblies formed in ILs are of great interest and may widen the application of ILs. Recently, ILs based on the 1-alkyl-3-methylimidazolium salts (CnmimX, where n is the carbon number of the alkyl chain and X is the anion) have been extensively studied and widely used in the field of colloid and interface science. Longchained CnmimX ionic liquids have amphiphilic character * Corresponding authors: Liqiang Zheng, tel +86 531 88366062, fax +86 531 88564750, e-mail [email protected]; Tohru Inoue, tel +81 092 871 6631, fax +81 092 865 6030, e-mail [email protected]. † Key Laboratory of Colloid and Interface Chemistry (Shandong University). ‡ Department of Chemistry, Faculty of Science, Fukuoka University. (1) Audic, N.; Clavier, H.; Mauduit, M.; Guillemin, J. C. J. Am. Chem. Soc. 2003, 125, 9248. (2) Fletcher, K. A.; Pandey, S. Langmuir 2004, 20, 33. (3) Wang, Z. N.; Liu, F.; Gao, Y. A.; Zhuang, W. C. Langmuir 2005, 21, 4931. (4) Welton, T. Chem. ReV. 1999, 35, 2071. (5) Avery, T. D.; Jenkis, N. F.; Kimber, M. C.; Lupton, D. W.; Taylor, D. K. Chem. Commun. 2002, 28. (6) Zerth, H. M.; Leonard, N. M.; Mohan, R. S. Org. Lett. 2003, 5, 55. (7) Huddieston, J. G.; Willauer, H. D.; Swauoski, R. P.; Visser, A. E.; Rogers, K. D. Chem. Commun. 1998, 1765. (8) Anderson, J. L.; Ding, J.; Welton, T.; Armstong, D. W. J. Am. Chem. Soc. 2002, 124, 14247. (9) Wang, P.; Zakeeruddion, S. M.; Comte, P.; Exnar, I.; Gratzel, M. J. Am. Chem. Soc. 2003, 125, 1166.

Chart 1. Chemical Structures of bmimBF4 (A), bmimPF6 (B), and Tween 20 (C)

like traditional cationic surfactants.10–12 CnmimX with shorter alkyl chains have been used as replacements for conventional solvents. Several recent studies have appeared concerning selfassembled aggregates of amphiphilic molecules in short-chained CnmimX ionic liquids. Li et al. studied the temperature-dependent aggregation behavior of Brij 76 in 1-butyl-3-methylimidazolium tetrafluoroborate (bmimBF4) by means of Fourier transform infrared spectroscopy, differential scanning calorimetry, NMR, and polarized optical microscopy.13 Anderson et al. reported the dry micelle formation by some traditional surfactants in two ionic liquids, 1-butyl-3-methylimidazolium chloride (bmimCl) and 1-butyl-3-methylimidazolium hexafluorophosphate (bmimPF6).14 Also, the aggregation behaviors of different kinds of (10) Inoue, T.; Dong, B.; Zheng, L. Q. J. Colloid Interface Sci. 2007, 307, 578. (11) Dong, B.; Li, N.; Zheng, L. Q.; Yu, L.; Inoue, T. Langmuir 2007, 23, 4178. (12) Zhang, G. D.; Chen, X.; Xie, Y. Z.; Zhao, Y. R. J. Colloid Interface Sci. 2007, 315, 601. (13) Tang, J.; Li, D.; Sun, C. Y.; Zheng, L. Z.; Li, J. H. Colloids Surf., A 2006, 273, 24.

10.1021/la801358z CCC: $40.75  2008 American Chemical Society Published on Web 07/29/2008

Aggregation BehaVior

surfactants in 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (emimTf2N) were investigated on the basis of the response of solvatochromic probes.2 The micelle formed from several polyoxyethylene (POE)-type nonionic surfactants (CnEm) in bmimBF4, bmimPF6, and bmimTf2N has been studied by Patrascu et al.15 ILs have been intensively investigated as substitutes for water or oil in microemulsion systems by Han and his co-workers.16,17 Detailed information about the structure of IL microemulsion was obtained by our group.18,19 Wang et al. published a communication on lyotropic liquid crystalline phase of an amphiphilic block copolymer, P123, in bmimPF6.20 Hexagonal and cubic liquid crystalline phases formed in the ternary systems of Brij97/water/ILs and oleyl polyoxyethylene (20) ether (C18:1E20)/water/ILs have also been explored by Wang et al.3,21 As mentioned above, an increasing number of papers have been published currently concerning the aggregation behavior of surfactants in imidazolium based ILs. Regarding the surfactant micelle formation in aqueous solution, an enormous number of thermodynamic investigations have been preformed to date, and the methodology is well established. Thermodynamic parameters are quite useful to elucidate the mechanism of micelle formation and have contributed to a better understanding of the surfactant aggregation phenomena in aqueous media. However, few reports have described the mechanism of aggregation in ILs, and even fewer reports have investigated the thermodynamics of surfactant aggregation in ILs. Only some discussions on the thermodynamic parameters relating to the second virial coefficient22 and the formation mechanism18,19 of the microemulsion in ILs have been reported recently by our group. Tween 20, a common nonionic surfactant, is the trade name of polyoxyethylene (20) sorbitan monolaurate. Tween surfactants are widely used in the food and cosmetic industries due to their nontoxicity and other advantages. Various studies of Tween 20 in aqueous solution have been reported.23,24 In the present work, we investigated the aggregate formation of Tween 20 in typical ionic liquids, bmimBF4 and bmimPF6. The aggregation behavior was followed by means of surface tension, freeze-fracture transmission electron microscopy (FF-TEM), and 1H NMR spectroscopy. It was found that there appear two transition points in the surface tension vs concentration plot. FF-TEM observation demonstrated that the particles with 20-50 nm in diameter are formed at the concentration corresponding to the first transition point and those with 10 nm in diameter are formed at the concentration of the second transition point. The temperature dependence of these critical aggregation concentrations enabled us to estimate the thermodynamic parameters related to the aggregation processes. NMR measurements as a function of the (14) Anderson, J. L.; Pino, V.; Hagberg, E. C.; Sheares, V. V.; Armstrong, D. W. Chem. Commun. 2003, 2444. (15) Patrascu, C.; Gauffre, F.; Nallet, F.; Bordes, R.; Oberdisse, J.; de LauthViguerie, N.; Mingotaud, C. ChemPhysChem 2006, 7, 99. (16) Gao, H. X.; Li, J. C.; Han, B. X.; Chen, W. N.; Zhang, J. L.; Zhang, R.; Yan, D. D. Phys. Chem. Chem. Phys. 2004, 6, 2914. (17) Li, J. C.; Zhang, J. L.; Gao, H. X.; Han, B. X.; Gao, L. Colloid Polym. Sci. 2005, 283, 1371. (18) Li, N.; Cao, Y.; Gao, Y. A.; Zhang, J.; Zheng, L. Q.; Bai, X. T.; Dong, B.; Li, Z.; Zhao, M. W.; Yu, L. ChemPhysChem 2007, 8, 2211. (19) Gao, Y. A.; Li, N.; Zheng, L. Q.; Bai, X. T.; Yu, L.; Zhao, X. Y.; Zhang, J.; Zhao, M. W.; Li, Z. J. Phys. Chem. B 2007, 111, 2506. (20) Wang, L. Y.; Chen, X.; Chai, Y. C.; Hao, J. C.; Sui, Z. M.; Zhuang, W. C.; Sun, Z. W. Chem. Commun. 2004, 2840. (21) Wang, Z. N.; Zhou, W.; Li, G. Z. J. Colloid Interface Sci. 2008, 318, 405. (22) Li, N.; Zhang, S. H.; Zheng, L. Q.; Gao, Y. A.; Yu, L. Langmuir 2008, 24, 2973. (23) de Smet, Y.; Deriemaeker, L.; Parloo, E.; Finsy, R. Langmuir 1999, 15, 2327. (24) Woodward, N. C.; Wilde, P. J.; Mackie, A. R.; Gunning, A. P.; Gunning, P. A.; Morris, V. J. J. Agric. Food Chem. 2004, 52, 1287.

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Figure 1. Surface tension vs concentration plots obtained for Tween 20 solutions in bmimBF4 (a) and bmimPF6 (b) at various temperatures. Numbers on the vertical axis represent the surface tension values 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 each figure.

surfactant concentration provided information about the interactions between the surfactant and IL molecules associated with aggregate formation. Combining the thermodynamic analysis and NMR results, we discuss the features of the aggregates of Tween 20 molecules in imidazolium based ILs as well as their formation mechanisms.

Experimental Section Materials. The ionic liquids used here, bmimBF4 and bmimPF6, were freshly prepared in our laboratory according to the procedures reported previously.25 The purity of the obtained IL samples was checked by 1H NMR spectroscopy and mass spectrum. Chemical grade Tween 20 and formamide were purchased from Tianjin Experimental Reagent Co. and were used as received without further purification. D2O (99.96%) was provided by Sigma-Aldrich Reagent Company. The chemical structures of Tween 20 and the two IL molecules are shown in Chart 1. Apparatus and Procedures. Surface tension measurements were conducted on a model JYW-200B surface tensiometer using the ring method. The temperature was controlled by using a super-constanttemperature trough. The surface tension was determined in a singlemeasurement method. All measurements were repeated at least twice until the values were reproducible. The values of the concentrations at the two transition points in surface tension curves (see Figure 1) were determined from the intersection of the two straight lines drawn in the low and high concentration regions. Freeze-fracture transmission electron microscopic (FF-TEM) observation was performed with a JEM-100CX II transmission electron microscope operated at 100 kV by the freeze fracture technique. 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. 1H NMR measurements were carried out with a Varian ARX 400 NMR spectrometer at 40 °C. The instrument was operated at a frequency of 400.13 MHz.

Results and Discussion Surface Tension Measurements at Various Temperatures. Figure 1 shows the surface tension obtained for Tween 20 solutions in bmimBF4 (a) and bmimPF6 (b) at various temperatures as a function of the surfactant concentration. As can be clearly seen, two transition points appear in the γ-log C curve for both (25) Dupont, J.; Consorti, C. S.; Suarez, P. A. Z.; de Souza, R. F.; Fulmer, S. L.; Richardson, D. P.; Smith, T. E.; Wolff, S. Org. Synth. 2002, 79, 236.

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Figure 3. Temperature dependence of CAC1 and CAC2 for Tween 20 in bmimBF4. Circles and squares correspond to CAC1 and CAC2, respectively.

Figure 2. FF-TEM images for Tween 20 in bmimBF4. The surfactant concentrations are 0.8 mmol/L (a), 1.6 mmol/L (b), and 10.6 mmol/L (c). Table 1. Values of CAC1 and CAC2 for Tween 20 in bmimBF4 and bmimPF6 at Different Temperatures bmimBF4

bmimPF6

temp (°C)

CAC1 (mmol/L)

CAC2 (mmol/L)

CAC1 (mmol/L)

CAC2 (mmol/L)

25 30 35 40 45 55

0.615 0.611 0.708 0.885 0.871 1.11

1.60 1.45 1.15 1.30 1.33 2.29

18.5 19.4 20.0 20.3 21.6 23.7

38.8 37.4 36.9 36.6 37.2 38.4

systems at every temperature. That is, the surface tension gradually decreases with the increase in the Tween 20 concentration to a plateau region, and then it decreases again until another new plateau region is attained. As is well-known, the appearance of a transition point in the γ-log C curve indicates that certain kinds of self-assembled aggregates begin to form in the solution at the concentration of the transition point. Thus, the two transition points found in the present study suggest that two kinds of selfassembled aggregates are formed in our systems at different critical surfactant concentrations. We denote the critical aggregation concentrations as CAC1 and CAC2 corresponding to the first and second transition points, respectively. In order to confirm the possible aggregate formation at these critical concentrations, we carried out FF-TEM experiments. FF-TEM Observation. FF-TEM images obtained for Tween 20 in bmimBF4 at different concentrations are shown in Figure 2. The FF-TEM images demonstrate that spherical aggregates 20-50 nm in diameter are formed at the surfactant concentration of 0.8 mmol/L (at about CAC1, Figure 2a), while smaller aggregates 10 nm in diameter are obtained at 1.6 mmol/L (at about CAC2, Figure 2b). Further increase in the surfactant concentration provides very large aggregates with 150 nm diameter (Figure 2c), which are probably produced by fusion of small aggregates formed at CAC2. Similar results were also obtained for the Tween 20/bmimPF6 system, as is shown in Supporting Information (Figure S1). Thus, it can be concluded that the self-assembled aggregates with different sizes are formed at both transition points of the γ-C curves. The values of CAC estimated from the surface tension curves at various temperatures are summarized in Table 1.

Comparison of CAC Values Obtained for bmimBF4 and bmimPF6. It is noticeable that both CAC values are much higher for bmimPF6 compared with bmimBF4. This means that the solvophilicity of Tween 20 is much higher for bmimPF6 than that in bmimBF4. The solvophilicity of POE-type nonionic surfactants originates from solvation of the POE chain. Imidazolium based ILs exhibit multiple hydrogen-bonding interaction with solute molecules; i.e., the bmim cation acts as a hydrogenbond donor through H atoms attached to the imidazolium ring, while anion species like BF4- act as hydrogen-bond acceptors.26–28 Since the oxygen atoms in the POE chains of Tween 20 must play a role as hydrogen-bond acceptors,29 a hydrogen bond may be formed between the imidazolium H atoms in the bmim cation and the O atoms in the POE chains. Hydrogen-bond formation between the O atom in methanol and the imidazolium H atoms has been suggested by a molecular dynamics simulation.30 This hydrogen bonding is responsible for the solvophilic solvation of the POE chains in imidazolium based ILs. In the imidazolium based ILs, counteranions compete with oxygen atoms in the POE chains in forming hydrogen bonds with the bmim cation, since the anions are stronger hydrogen-bond acceptors. The hydrogen bond between PF6- and the imidazolium H atom would be weaker compared with the case of BF4-, because the electron density of the bulky PF6- is lower than that of BF4-. Consequently, the hydrogen bonding between the imidazolium H and the POE oxygen would become stronger in bmimPF6 than in bmimBF4. This brings about higher solvophilicity, and hence, higher CAC values to Tween 20 in bmimPF6 than in bmimBF4. Temperature Dependence of the Critical Aggregation Concentration (CAC). The values of CAC1 and CAC2 are plotted as a function of temperature in Figure 3 for Tween 20 in bmimBF4. Similar data for the Tween 20/bmimPF6 system is given in Supporting Information (Figure S2). The common feature of both systems is that CAC1 increases monotonously with increasing temperature, while CAC2 exhibits a minimum at 35-40 °C in the CAC-temperature profile. The existence of a minimum in the critical micelle concentration (cmc) vs temperature plot is well-known for surfactants in aqueous solution;31 usually, the minimum appears around room temperature for ionic surfactants, whereas the cmc of nonionic surfactants shows a minimum at (26) Fletcher, K. A.; Pandey, S. J. Phys. Chem. B 2003, 107, 13532. (27) Reichardt, C. Green Chem. 2005, 7, 339. (28) Samanta, A. J. Phys. Chem. B 2006, 110, 13704. (29) Esaka, Y.; Tanaka, K.; Uno, B.; Goto, M. Anal. Chem. 1997, 69, 1332. (30) Lopes, J. N. C.; Gomes, M. F. C.; Padua, A. A. H. J. Phys. Chem. B 2006, 110, 16816. (31) Rosen, M. J. Surfactants and Interfacial Phenomena, 2nd ed.; John Wiley & Sons: New York, 1989; Chapter 3.

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Langmuir, Vol. 24, No. 17, 2008 9317 Table 2. Thermodynamic Parameters of the Aggregate Formation at CAC1 and CAC2 for Tween 20 in bmimBF4 (a) and bmimPF6 (b) at Various Temperatures aggregate formation at CAC1 0 ∆Gagg1

aggregate formation at CAC2

0 0 0 ∆Gagg2 ∆Hagg2 ∆Sagg2 temp (°C) (kJ/mol) (kJ/mol) (J/K mol) (kJ/mol) (kJ/mol) (J/K mol)

0 Figure 4. Plot of ∆Gagg /T against 1/T for Tween 20 in bmimBF4. Circles and squares correspond to CAC1 and CAC2, respectively. Solid lines were drawn by a least-squares curve fitting procedure using linear function (for CAC1) and quadratic function (for CAC2).

higher temperature.32 The temperature dependence of CAC2 is similar to that of the cmc of aqueous surfactant solutions. On the other hand, the almost linear increase with temperature rise as observed for CAC1 has never been reported in aqueous solutions to the best of our knowledge. Thermodynamic Consideration on the Aggregation Behavior of Tween 20 in Ionic Liquids. As is well established in the thermodynamics of micelle formation, the standard Gibbs free energy of aggregate formation is given by eq 1 for the case of nonionic surfactants31,33

∆G0agg ) RT ln xs

(1)

where xs is the mole fraction of the surfactant monomer coexisting with the aggregate. Since the monomer concentration is usually approximated by the CAC, xs is given by eq 2 taking into account the molar mass and density of the IL species. The densities are 1.15 and 1.37 g/mL for bmimBF4 and bmimPF6, respectively.25

xs )

25 30 35 40 45 55

-22.4 -22.6 -22.8 -22.6 -23.0 -23.0

25 30 35 40 45 55

-13.8 -13.9 -14.0 -14.2 -14.3 -14.5

0 ∆Hagg1

0 ∆Sagg1

(a) Tween 20 in bmimBF4 -16.3 20.5 -20.2 34.6 -16.3 20.8 -20.6 18.5 -16.3 21.1 -21.5 2.99 -16.3 20.1 -21.6 -12.0 -16.3 21.0 -21.8 -26.6 -16.3 20.6 -21.1 -54.4 (b) Tween 20 in bmimPF6 -6.47 24.5 -11.9 5.50 -6.47 24.5 -12.2 3.57 -6.47 24.6 -12.5 1.70 -6.47 24.8 -12.7 -0.11 -6.47 24.6 -12.9 -1.86 -6.47 24.5 -13.2 -5.20

184 129 79.6 30.4 -14.9 -102 58.5 52.2 46.0 40.2 34.6 24.3

which is unusual for micelle formation of surfactants in aqueous solution. On the other hand, a ∆G0agg2/T vs 1/T plot gives a concave curve with a minimum. The variation of ∆G0agg2/T as a function of 1/T was approximated by a quadratic equation, and the slopes at every temperature were evaluated, from which ∆H0agg2 values were determined. Then the values of ∆S0agg1 and ∆S0agg2 were calculated according to eq 4. The thermodynamic parameters thus obtained are listed in Table 2. Figure 5 shows the plot of ∆G0agg, ∆H0agg, and -T∆S0agg as a function of temperature for Tween 20 in bmimBF4. Similar relations between thermodynamic parameters and temperature for Tween 20/bmimPF6 system are shown in Figure S4 of Supporting Information. It can be seen for the aggregate formation at CAC2 that a factor contributing to a negative ∆G0agg is a large

CAC CAC (for bmimBF4) or (for bmimPF6) (2) 5.09 4.82

Once ∆G0agg is known as a function of temperature, the standard enthalpy of aggregate formation, ∆H0agg can be derived by applying the Gibbs-Helmholtz equation

[

]

∂(∆G0agg ⁄ T) ) ∆H0agg ∂(1 ⁄ T)

(3)

Then the standard entropy of aggregate formation, ∆S0agg, is obtained by the use of the following relation

∆S0agg )

∆H0agg - ∆G0agg T

(4)

The values of CAC1 and CAC2 obtained above were used to estimate ∆G0agg1 and ∆G0agg2 for the aggregate formation of Tween 20 occurring at CAC1 and CAC2. Plots of ∆G0agg1/T and ∆G0agg2/T against 1/T are shown in Figure 4 for bmimBF4. The corresponding plots for the Tween 20/bmimPF6 system are shown in Supporting Information (Figure S3). ∆G0agg1/T exhibits a linear dependence on 1/T for both bmimBF4 and bmimPF6, and hence, the values of ∆H0agg1 were estimated from the slopes of the straight lines according to eq 3. The linear dependence of ∆G0agg1/T on 1/T means that the standard enthalpy change associated with the aggregate formation at CAC1 is constant regardless of temperature, (32) Chen, L. J.; Lin, S. Y.; Huang, C. C.; Chen, E. M. Colloids Surf., A 1998, 135, 175. (33) Hiemenz, P. C. Principles of Colloid and Surface Chemistry, 2nd ed.; Marcel Dekker: New York, 1986; Chapter 8.

Figure 5. Plot of thermodynamic parameters of the aggregate formation at CAC1 (a) and CAC2 (b) against temperature for Tween 20 in bmimBF4. Circles, squares, and triangles correspond to ∆G0agg, ∆H0agg, and -T∆S0agg, respectively.

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Figure 6. Schematic model of (a) the solvation of a Tween 20 molecule in bmim-type ILs and (b) the release of the solvating IL molecules around hydrocarbon chains associated with the aggregate formation of surfactant molecules.

positive entropy term (negative -T∆S0agg2) at low temperature, while the contribution from the enthalpy term becomes more and more important with the increase in temperature. This behavior of thermodynamic functions with respect to the temperature change is quite similar to the case of micelle formation of surfactants in aqueous systems.32 This implies the correspondence of the Tween 20 aggregates formed at CAC2 in the ILs to ordinary micelles formed in aqueous surfactant solutions. The micelle formation of surfactants in aqueous medium is believed to be caused by a hydrophobic effect, the origin of which is hydrophobic hydration around the hydrocarbon chains of the surfactant molecules.34 The present case, i.e., the aggregate formation of Tween 20 at CAC2 in IL solution, could be attributed to a similar mechanism to the aqueous system. A solvophobic driving force has been suggested for surfactant self-assembly in protic ionic liquids (analogous to the hydrophobic effect in aqueous systems).35 According to this mechanism, the aggregate formation of Tween 20 at CAC2 is described as follows. Aggregation Mechanism of Tween 20 in Ionic Liquids. The monomeric Tween 20 molecules must be solvated in ionic liquids, and the solvation may be classified, as a firstapproximation, into a solvophilic solvation around the POE chains and a solvophobic solvation around the hydrocarbon chain as schematically illustrated in Figure 6a. The solvophilic solvation is described in detail in the third subsection of the Results and Discussion section. Solvophobic solvation is analogous to hydrophobic hydration in aqueous surfactant systems. The hydrophobic hydration is created by a hydrogen-bond network around the surfactant hydrocarbon chain, and the water molecules form highly ordered “icelike” structures around the hydrocarbon chain.34 Similarly, it is likely that the solvophobic solvation by IL molecules is created by a network of ionic interactions instead of the hydrogen-bonding interactions, and the IL molecules in this solvation layer are highly ordered. It is reasonable to assume that the bmim cation binds to a surfactant hydrocarbon chain directing its butyl group toward the hydrocarbon chain. The molecular dynamics study30 supports this assumption, where the direct interactions between the butyl group of bmim+ and hexane were demonstrated. Taking into account the solvation of a monomeric surfactant molecule, and assuming that the solvophilic solvation around POE chain is not affected significantly by the aggregate formation (34) Tanford, C. The Hydrophobic Effect: Formation of Micelles and Biological Membranes; John Wiley & Sons: New York, 1980. (35) Greaves, T. L.; Weerawardena, A.; Fong, C.; Drummond, C. J. Langmuir 2007, 23, 402.

(see Figure 6), the energy balance in the aggregation process at CAC2 and its temperature dependence are considered as follows. The release of solvating ILs around the hydrocarbon chain, which is required for the hydrocarbon chain to contact each other to form aggregates, leads to energy loss (increase in enthalpy), while the contact of hydrocarbon chains causes energy gain (decrease in enthalpy). As a result of these two opposing effects, ∆H0agg2 becomes positive at low temperature. The release of highly ordered solvating IL molecules associated with the aggregate formation increases entropy (large negative -T∆S0agg2) at low temperature, which overcomes the enthalpy loss and results in the free energy gain (negative ∆G0agg2). When the temperature is increased, the IL molecules participating in the solvophobic solvation become less ordered due to an increase in thermal energy. This leads to less enthalpy required to release the solvating IL molecules and results in smaller ∆H0agg2. Finally, the enthalpy gain coming from the contact of the hydrocarbon chains would overcome the enthalpy loss due to the release of the solvating IL molecules, and ∆H0agg2 becomes negative at elevated temperature. Concomitantly with the reduced order of the solvating IL molecules, the entropy increase due to the release of the IL molecules becomes small. Finally, the entropy loss coming from the assembly of surfactant molecules would overcome the entropy gain due to the release of solvating IL molecules, and -T∆S0agg2 becomes positive at elevated temperature. Thus, the aggregation of Tween 20 occurring at CAC2 is interpreted similarly to micelle formation in aqueous system in terms of an analogy between solvophobic solvation and hydrophobic hydration around the surfactant hydrocarbon chain. Therefore, the aggregates formed at CAC2 may be regarded as ordinary “micelles”; the size of the aggregates, i.e., 10 nm in diameter, is reasonable for the micelles. On the other hand, the aggregation behavior at CAC1 is rather anomalous compared with micelle formation in aqueous systems. The thermodynamic parameters associated with this process are essentially independent of temperature. This suggests that the driving force for this process is not like hydrophobic interactions between surfactant molecules in aqueous system. 1H NMR Results Obtained for the Solutions of Tween 20 in Ionic Liquids. NMR is a powerful tool to obtain information about solute-solvent interactions at the molecular level. 1H NMR measurements were carried out for Tween 20 in bmimPF6 at 40 °C as a function of the surfactant concentration. The spectrum obtained for pure bmimPF6 and its peak assignment is shown in Figure 7. When Tween 20 was added to bmimPF6, the chemical

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Figure 7. 1H NMR spectrum obtained for bmimPF6 at 40 °C.

Figure 8. The change of 1H NMR chemical shifts of various protons in bmimPF6 as a function of Tween 20 concentration, C. Temperature is 40 °C.

shift of the bmim+ protons changed with concentration. Figure 8 shows the plot of the chemical shifts, δ, for different protons in the bmim cation against the surfactant concentration. The δ of all protons exhibits a shift toward lower magnetic field with the addition of Tween 20. When we look at Figure 8 closely, it can be seen that δ increases linearly with Tween 20 concentration up to CAC1 for all protons, while the pattern of the concentration dependence of δ above CAC1 is classified into three types. (1) For Ha (the proton attached to the carbon atom between the two nitrogen atoms in the imidazolium ring), δ increases with a smaller slope in the concentration range CAC1 < C < C*, where C* represents the concentration at which the first plateau region of surface tension ends (see Figure 1). Then the slope increases steeply in the region C* < C < CAC2 where surface tension decreases again (see Figure 1), and then it increases with a slope slightly smaller than that below CAC1. (2) For Hb and Hd (the protons attached to the carbon atom of the imidazolium ring and the methyl protons attached to the imidazolium ring), δ stays at a constant value in CAC1 < C < C*, then increases steeply up to CAC2, and then it increases with a slope slightly smaller than

that below CAC1. (3) For Hc, He, and Hg (butyl protons of bmim+), δ stays at a constant value in CAC1 < C < C*, then it increases steeply up to CAC2, and after that it increases with much smaller slope than the cases of (1) and (2). These changes of δ caused by the addition of Tween 20 indicate that some interactions take place between Tween 20 and bmimPF6. It may be regarded that the change in chemical shift of Ha reflects the interactions between bmim+ and the POE chains in the surfactant, because bmim+ would bind to POE oxygen due to hydrogen-bond formation through Ha as mentioned above. On the other hand, the change in chemical shift of the butyl protons reflects the interactions between bmim+ and hydrocarbon chain in Tween 20, because the butyl group would contact the hydrocarbon chain due to solvophobic solvation. Thus, we focus our attention on the chemical shift behavior of the Ha and the butyl protons. Interpretation of the Changes in Chemical Shift Caused by the Addition of Tween 20. We consider the present situation as follows. As mentioned above, the bmim cation would bind to Tween 20 molecules in two ways; i.e., one is solvophilic

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solvation to the POE chains and the other is solvophobic solvation to the hydrocarbon chain. The bound bmim+ and free bmim+ in solution would exchange each other rapidly, i.e., the bound and free bmim+ are in exchange equilibrium. When the exchange rate is sufficiently rapid compared with the NMR time scale, the observed chemical shift, δobs, becomes weighted average of the chemical shifts of these two states, δb (bound bmim+) and δf (free bmim+). It may be reasonable to consider separately the bmim+ bound to the POE chains and that bound to the hydrocarbon chain, because the interactions between bmim+ and Tween 20 must have different character in the two cases. We denote the chemical shifts of the protons in bmim+ bound to the POE chains and to the hydrocarbon chains as δbOE and δbHC, respectively. Then, δobs would be expressed as

δobs ) xfδf + xbOEδbOE + xbHCδbHC

(5)

where x is the mole fraction of bmim+ in each state; i.e.,

xf )

nf nf + nbOE + nbHC

and so on. In the above expression, n represents the molar amount of bmim+ in each state. The total molar amount of bmimPF6 in 1 L of solution is 4.82 mol, and the molar amount of Tween 20 in 1 L of solution is C mol, where C is the concentration of Tween 20 in mol/L. Here, we denote the number of bmim+ bound to the POE chains and the hydrocarbon chain of a single Tween 20 molecule as mOE and mHC, respectively. Then, the molar amounts of bmim+ in all possible states in 1 L of solution are expressed as follows.

nbOE ) mOEC, nbHC ) mHCC, and nf ) 4.82 - (mOE + mHC)C Thus

xf )

4.82 - (mOE + mHC)C 4.82 mOEC xbOE ) 4.82

(6)

δobs ) δf +

δobs ) (δobs)CAC1 +

δobs ) (δobs)CAC1 +

In the above equations, mOE′ and mHC′ correspond to the number of bmim+ bound to a single Tween 20 molecule in this concentration range. The behavior of δobs for Ha in this concentration range is described as follows. First, it increases slightly with C until C*. This means that mOE′ is much smaller than mOE, i.e., the solvophilic solvation around the POE chains is strongly depressed when Tween 20 molecules form aggregates at CAC1. After C*, δobs increases steeply up to CAC2. This steep increase may be attributed to the increase in monomer concentration as suggested by the decrease of surface tension in this concentration range. On the other hand, δobs for butyl protons is kept constant in the concentration range CAC1 < C < C*. According to eq 11, this means mHC′ ) 0, i.e., bmim+ is completely excluded from the region around the hydrocarbon chain in the aggregates formed at CAC1. A steep increase in δobs for the butyl protons after C* may be attributed again to the increase in monomer concentration. In the concentration range above CAC2, eqs 10 and 11 should be modified again as follows. For Ha

δobs ) (δobs)CAC2 +

)

mHC mOE + mHC mOE δ + δ δf C (7) 4.82 bOE 4.82 bHC 4.82

It may be reasonable to assume that δbHC ≈ δf for Ha, because the magnetic environment for Ha in bmim+ bound to the hydrocarbon chain must be similar to that in free bmim+ due to the separation of Ha from the interaction site of the bmim+ with the hydrocarbon chain. For the butyl protons, we can assume δbOE ≈ δf based on a similar reason to the above. Then, δobs for Ha and the butyl protons are simplified as follows. For Ha

For butyl protons

mOE (δ - δf)C 4.82 bOE

mHC ′ (δ - δf)(C - CAC1) (11) 4.82 bHC

mOE ″ (δ - δf)(C - CAC2) (12) 4.82 bOE

For butyl protons

Substituting eq 6 to eq 5 and rearranging appropriately, one can obtain the following relation expressing the observed chemical shift.

δobs ) δf +

mOE ′ (δ - δf)(C - CAC1) (10) 4.82 bOE

For butyl protons

δobs ) (δobs)CAC2 +

mHCC xbHC ) 4.82

(

(9)

Equations 8 and 9 predict that δobs increases linearly with C for both Ha and butyl protons and are in agreement with the observed behavior in the concentration range up to CAC1. For the concentration range CAC1 < C < CAC2, eqs 8 and 9 are modified as eqs 10 and 11, respectively. For Ha

and

δobs ) δf +

mHC (δ - δf)C 4.82 bHC

(8)

mHC ′ (δ - δf)(C - CAC2) (13) 4.82 bHC

where mOE′′ and mHC′′ represent the number of bmim+ bound to a single Tween 20 molecule in the aggregates formed at CAC2. As for Ha, δobs increases linearly with C and the slope is only slightly smaller than that observed below CAC1. This means that the number of bmim+ bound to the POE chains of Tween 20 in the aggregates formed at CAC2 is only slightly less than the number bound to Tween 20 monomer in the solution. However, the increase in δobs for butyl protons shows quite a small slope above CAC2. This indicates that the solvation of hydrocarbon chains of Tween 20 in the aggregates formed at CAC2 is considerably restricted. Characterization of Tween 20 Aggregates in Imidazolium Ionic Liquids Based on Thermodynamic Analysis and NMR Results. Thermodynamic considerations of the surface tension results and NMR observations provide a rather consistent picture for the aggregates formed at CAC2. Thermodynamic analysis suggests that the driving force for the aggregate formation is the solvophobic effect of the hydrocarbon chain in the Tween 20 molecule at room temperature; that is, a large entropy increase due to the release of solvating bmim+ around the hydrocarbon chains is a dominant factor promoting the aggregate formation.

Aggregation BehaVior

In this sense, the aggregates are analogous to surfactant micelles in aqueous solution. Then, it would be expected that the POE chains of Tween 20 in the aggregates must be solvated by bmim+ more or less similarly to the Tween 20 monomers, while the hydrocarbon chains in the aggregates have much less solvating bmim+, because the hydrocarbon chains are buried in the interior of micelles. This picture is in accordance with the behavior of the chemical shifts for the Ha and butyl protons in the concentration range above CAC2. Also, the size of the aggregates formed at CAC2, approximately 10 nm in diameter, fits this picture. Thus, we can conclude that this aggregate is just like usual micelles formed in aqueous solution, and hence, CAC2 corresponds to the cmc. The reported value of cmc of Tween 20 in aqueous solution is 7.1 × 10-5 mol/L.23 The values of CAC2 (i.e., cmc) of Tween 20 in bmimBF4 and bmimPF6 are 2 to 3 orders of magnitude higher than the cmc in aqueous solution (see Table 1). This is in good agreement with the results obtained for several POEtype nonionic surfactants in imidazolium based ILs.15 As for the aggregates formed at CAC1, the NMR results demonstrate the fact that the hydrocarbon chains of Tween 20 in the aggregates are not solvated by bmim+ and even the POE chains are only weakly solvated. The weak solvation of the POE chains is shown by the small slope of the δobs vs C plot for Ha in the region CAC1 < C < C*. This fact suggests that the aggregates are nanodroplets composed of Tween 20 molecules segregated from the solution phase. The surfaces of the spherical droplets must be covered with the POE chains, and hence, bmim+ can bind to the surface. On the other hand, the interior of the droplets must be similar to bulk Tween 20, in which no solvation takes place. The proposed nanodroplets can explain the binding situation, i.e., much lesser extent of binding to the POE chains and lack of binding to the hydrocarbon chain. A rather broad size distribution of the aggregates, 20-50 nm, also coincides with the droplet picture. According to this model, CAC1 can be regarded as a solubility of Tween 20 in the ILs, and the thermodynamic parameters derived from the temperature dependence of CAC1 become thermodynamic functions of dissolution when we invert their signs. For example, the enthalpy change (this time, they become positive) can be regarded as heat of dissolution. It is usual that heat of dissolution is positive and insensitive to temperature. This is in accordance with the rather anomalous result for CAC1, i.e., ∆H0agg1 is independent of temperature. When the polarity of solvent is increased, the solubility of Tween 20 would increase because of enhanced solvophilicity of the POE chains. Thus, the effect of solvent polarity on CAC1 was examined. We used formamide to modify the polarity of the ionic liquids; the dielectric constant, , of formamide is 109.5,36 whereas  ) 11.7 and 11.4 for bmimBF4 and bmimPF6,37 respectively. The experiments were designed as follows. At first, a certain proportion of formamide was added to the ILs, and then Tween 20 was dissolved to measure the surface tension. Figure 9 shows the effect of formamide addition on γ-C curves obtained for Tween 20 in bmimPF6. As can be seen in this figure, CAC1 shifts toward higher concentration with the increase in the amount of formamide and finally disappears. This behavior can be interpreted as that the solubility of Tween 20 in bmimPF6 was increased due to an enhanced polarity of the solvent caused by the addition of formamide. Here, a question arises; why do the droplets once formed by segregation change to micelles as the Tween 20 concentration increases, instead of separating into a macroscopic phase? The (36) Singh, H. N.; Saleem, S. M.; Singh, R. P.; Birdi, K. S. J. Phys. Chem. 1980, 84, 2191. (37) Wakai, C.; Oleinikova, A.; Ott, M.; Hermann, W. J. Phys. Chem. B 2005, 109, 17028.

Langmuir, Vol. 24, No. 17, 2008 9321

Figure 9. Surface tension vs concentration plots obtained for Tween 20 solutions in bmimPF6 in the absence and presence of added formamide at 25 °C. Numerical numbers on vertical axis represent the surface tension values for nonadditive system. Surface tension curves in the presence of formamide are drawn by shifting the surface tension values appropriately. The concentrations of formamide in volume percent are indicated in the figure.

reason might be attributed to a rather broad distribution of the POE chain length in the Tween 20 sample. It is likely that the sample contains Tween species with shorter/longer POE chain than 20 OE units. The surfactants with longer POE chains must be more solvophilic, and they remain dissolved in the IL even after other surfactants with shorter chains have segregated to form droplets due to their low solvophilicity. When the surfactant concentration is increased, these solvophilic species increase, although the increment may not be large enough to be detected by surface tension measurements. At a certain concentration, these species start to form micelles. Once micelles are formed, they solubilize the surfactant molecules in the droplets, and finally, all the Tween 20 molecules are incorporated into the micelles. In conformity with this interpretation, the double transition points in surface tension vs concentration plot similar to the present case have been reported for Brij 35 in bmimPF6,14 for Brij 35 also has a wide distribution of the POE chain length. In contrast, only a single transition point has been observed for CnEm which usually have narrow distribution of the POE chain length.15 The concentration at which the surfactants with longer POE chains start to form micelles may correspond to C*. Thus, the concentration range C* < C < CAC2 would be regarded as a transient region where micelles are formed more and more with the consumption of nanodroplets. The surface tension at CAC2 is lower than that at CAC1, so the concentration of monomers coexisting with micelles at CAC2 is higher than that coexisting with the droplets at CAC1. The monomer would be supplied from the droplets in addition to the external addition of surfactant, because the disappearing droplets can turn into either free monomer or micelles. The supply of the monomer from the nanodroplets may cause a rapid increase of monomer concentration, which brings about a rather steep increase in δobs as well as a decrease in surface tension in this concentration range. This kind of anomalous micellization behavior caused by the polydispersity of chain length distribution has also been observed for the micelle formation of amphiphilic block-copolymers in aqueous solution.38,39 (38) Desai, P. R.; Jain, N. J.; Bahadur, P. Colloids Surf., A 2002, 197, 19. (39) Inoue, T.; Yamashita, K. J. Colloid Interface Sci. 2007, 308, 525.

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Summary The conclusion derived in the present work is summarized as follows. Tween 20 forms two types of self-assembled aggregates in imidazolium based ionic liquids, bmimBF4 and bmimPF6, at different critical concentrations, CAC1 (lower concentration) and CAC2 (higher concentration). Thermodynamic analysis of surface tension data as a function of temperature revealed that the aggregate formation at CAC2 changes from entropy-driven at low temperature to enthalpy-driven at high temperature analogously to the surfactant micelle formation in aqueous solution. This demonstrates that the aggregate formed at CAC2 is micelle, and hence, CAC2 corresponds to the cmc in the ionic liquids. In addition, the temperature dependence of the thermodynamic parameters proves that the micelle formation of the surfactant in ionic liquids is caused by solvophobic interactions between the hydrocarbon chains of the surfactant molecules just like hydrophobic interactions in aqueous medium. The micellar characteristics coincide with the picture obtained from the NMR chemical shift analysis of bmim+ protons as a function of Tween 20 concentration. The results of thermodynamic analysis and chemical shift behavior of bmim+ protons suggest strongly that the aggregates formed at CAC1 are nanodroplets composed of

Wu et al.

Tween 20 molecules segregated from the solution phase. This anomalous behavior of the appearance of droplets in a low concentration region is attributed to the broad distribution of the POE chain length of Tween 20 inherent in the commercial products. The present work provides insight into the aggregation mechanism of surfactants in ionic liquids and contributes to a better understanding of the self-assembly of amphiphilic compounds in ionic liquids. Acknowledgment. The authors are grateful to the Natural Science Foundation of China (Grant No. 20773081), the National Basic Research Program (2007CB808004), and the Natural Science Foundation of Shandong Province (Z2007B06). This work was partially supported by the Laboratory of Organic Optoelectronic Functional Materials and Molecular Engineering, TIPC, CAS. The authors also thank Dr. Pamela Holt for editing the manuscript. Supporting Information Available: FF-TEM images of Tween 0 20 in bmimPF6 and plots of CACs vs temperature, ∆Gagg /T vs 1/T, and thermodynamic parameters vs temperature. This material is available free of charge via the Internet at http://pubs.acs.org. LA801358Z