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

Sep 10, 2008 - surfactants, we deduced that the surface activity of the fluorinated surfactant in ILs was superior to the activity of other surfactant...
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J. Phys. Chem. B 2008, 112, 12453–12460

12453

Aggregation Behavior of a Fluorinated Surfactant in 1-Butyl-3-methylimidazolium Ionic Liquids Na Li, Shaohua Zhang, Liqiang Zheng,* Jiapei Wu, Xinwei Li, and Li Yu Key Laboratory of Colloid and Interface Chemistry (Shandong UniVersity), Ministry of Education, Jinan 250100, China ReceiVed: June 21, 2008; ReVised Manuscript ReceiVed: August 8, 2008

The aggregation behavior of a fluorinated surfactant (FC-4) was studied by surface tension measurements in 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF4]) and hexafluorophosphate ([bmim][PF6]) at various temperatures. A series of surface properties, including adsorption efficiency (pC20), effectiveness of surface tension reduction (ΠCAC), maximum surface excess concentration (Γmax) and minimum surface area/molecule (Amin) at the air-water interface were estimated. By comparing the fluorinated surfactant with traditional surfactants, we deduced that the surface activity of the fluorinated surfactant in ILs was superior to the activity of other surfactants. From the CAC values and their temperature dependence, we estimated the thermodynamic parameters of aggregate formation. The thermodynamic parameters indicate that the aggregate of FC-4 in [bmim][BF4] is a traditional micelle, while the aggregate of FC-4 in [bmim][PF6] is nanodroplets composed of FC-4 molecules segregated from the solution phase. These results were further confirmed by 1H NMR measurements. Introduction Ionic liquids (ILs) are a class of organic electrolytes with melting points below 373 K.1 They can dissolve many organic and inorganic substances. Owing to their special chemical and physical properties, such as low volatility, nonflammability, high ionic conductivity, wide electrochemical window, and thermal stability,2-5 ILs have currently attracted much interest for applications as novel solvents in many fields.6-9 The unique advantages of ILs as novel solvents compared with traditional solvents are that they can be treated as environmentally benign solvents, since their nonvolatile nature can prevent environmental pollution and their properties can be modified to satisfy the requirements by suitable selection of cation, anion, and cation substituent. Molecular self-assemblies formed in ILs are of great interest and may widen the application of ILs. Aggregations of amphiphilic molecules such as surfactants in ILs have been widely studied in the field of colloid and interface science. Lyotropic liquid crystalline phases of an amphiphilic block copolymer, P123, in 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]) have been reported.10 Wang and co-workers11 investigated hexagonal liquid crystals formed in ternary systems of Brij97/water/ILs ([bmim][BF4] and [bmim][PF6]). In addition, microemulsions including ILs have been studied by a number of groups. Han and co-workers12 first discovered that 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF4]) could act as polar nanosized droplets dispersed in cyclohexane. Subsequently, Eastoe et al.13 investigated the same system by small-angle neutron scattering (SANS). Our group investigated the formation mechanism of IL microemulsions,14 then we characterized the micropolarities and solubilization behaviors of IL/O microemulsions using UV-vis spectrophotometry.15 It has been discovered that small amounts of water have a great effect on the microstructure and stability of IL/O microemulsions.16,17 Moreover, the second virial coef* Corresponding author. Telephone: +86 531 88366062. Fax: +86 531 88564750. E-mail: [email protected].

ficient of an IL/O microemulsion was obtained using microcalorimetry.18 Apart from the studies of liquid crystals and microemulsions in ILs such as those mentioned above, micelle formation in ILs has recently attracted much attention. Anderson et al.5 reported the dry micelle formation of some traditional surfactants in two ionic liquids, 1-butyl-3-methylimidazolium chloride (bmimCl) and [bmim][PF6]. The aggregation behaviors of several polyoxyethylene (POE)-type nonionic surfactants (CnEm) in 1-butyl3-methylimidazolium type ILs have been investigated.19 It was demonstrated that changing the exact nature of the ILs can tune the size and aggregation number of the micelles. It was also 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.20 Most of the aggregates investigated in ILs to date are formed by nonionic surfactants which are not highly efficient in lowering the surface tension of pure ILs. It would be of interest and importance from both practical and academic points of view to investigate highly efficient ionic surfactants, such as fluorinated surfactants. Fluorinated surfactants are highly efficient surfactants which have a number of special properties such as chemical inertness, thermal stability, and oleophobicity that offer some advantages over hydrocarbon surfactants.21 Their outstanding chemical stability expands their applications to extreme conditions which are too severe for hydrocarbon surfactants.22 Due to the larger volume and higher electronegativity of fluorine than of hydrogen, fluorinated surfactants are more effective at reducing surface tension than conventional hydrocarbon-based surfactants.23 Although the aggregation behavior of surfactants in imidazolium based ILs have been widely studied, few reports have described the mechanism of aggregation in ILs, and even fewer reports have investigated the thermodynamics of surfactant aggregation in ILs. Regarding the thermodynamic investigations, the methodology is well established for aqueous micelles.24-29

10.1021/jp8054872 CCC: $40.75  2008 American Chemical Society Published on Web 09/10/2008

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CHART 1: Chemical Structures of [bmim][BF4] (a), [bmim][PF6] (b), and FC-4 (c)

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. Recently, thermodynamic investigations on the formation mechanism of two aggregates for Tween 20 in ILs were performed by our group.30 In the present work, we investigated the aggregation behavior of a cationic fluorinated surfactant (FC-4) in [bmim][BF4] and [bmim][PF6]. Freeze-fracture transmission electron microscopy (FF-TEM) revealed the formation of aggregates of FC-4 in [bmim][BF4] and [bmim][PF6]. Surface tension was measured to investigate the surface properties of the fluorinated surfactant in ILs. The temperature dependence of these critical aggregation concentrations enabled us to estimate the thermodynamic parameters related to the aggregation processes. Combining the thermodynamic analysis and 1H NMR results, we discuss the different formation mechanisms of FC-4 aggregates in [bmim][BF4] and [bmim][PF6]. Experimental Section Materials. Ionic liquids, [bmim][BF4] and [bmim][PF6], were prepared in our laboratory by the procedure reported in the literature.31 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 two IL molecules are shown in Chart 1). Apparatus and Procedures. Surface tension measurements were conducted on the apparatus of 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. 1H NMR measurements were carried out with a Varian ARX 400 NMR spectrometer at 25 °C, 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 FC-4 solutions in [bmim][BF4] (a) and [bmim][PF6] (b) at various temperatures as a function of the surfactant concentration. For each curve, the surface tension gradually decreases with the increase of the FC-4 concentration. A decrease of surface tension indicates that the surfactant is adsorbed at the air/solution interface. The initial decrease of the surface tension 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, suggesting the formation of aggregates. The critical aggregate concentrations (CAC) 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 the corresponding surface tension is defined as γCAC. The main difference between these curves and the

Figure 1. Surface tension versus concentration plots obtained for FC-4 solutions in [bmim][BF4] (a) and [bmim][PF6] (b) at various temperatures. Numerical numbers on 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.

analogous curves for aqueous solutions is that 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.5 Freeze Fracture Transmission Electron Microscopy (FFTEM) Observations. FF-TEM is a powerful method to characterize the size and shape of surfactant aggregates. To further verify the formation of FC-4 aggregates in ILs, FF-TEM observations were carried out to characterize the self-assembled aggregates in terms of size and shape. Figure 2 shows typical FF-TEM images of FC-4 in [bmim][BF4] and [bmim][PF6] above the CAC. Irregular spherical aggregates 30-50 nm in diameter are formed in [bmim][BF4], while the aggregates in [bmim][PF6] are 70-100 nm in diameter. Surface Properties of FC-4 in ILs at Various Temperatures. The values of CAC and γCAC for FC-4 in [bmim][BF4] and [bmim][PF6] at various temperatures are shown in Tables 1 and 2. It is evident that the CAC values of FC-4 increase with increasing temperature in both [bmim][BF4] and [bmim][PF6]. From the surface tension versus concentration data, we can obtain several additional parameters, i.e., the adsorption efficiency, pC20, the effectiveness of surface tension reduction, ΠCAC, the maximum surface excess concentration, Γmax, and the minimum molecular sectional area, Amin. The anterior parameter is defined as32

pC20 ) -log C20

(1)

where C is the molar concentration of surfactant and C20 stands for the concentration required to reduce the surface tension of pure solvent by 20 mN · m-1. C20 is regarded as the minimum concentration needed to saturate the surface adsorption. Thus, C20 can be a measure of the adsorption efficiency of surfactant molecules at the air-water interface. Usually, the negative logarithm of C20 is used instead of C20 itself as is shown in eq 1. The greater the pC20 value, the higher the adsorption efficiency of the surfactant is. The

Behavior of a Fluorinated Surfactant

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Figure 2. FF-TEM images for FC-4 in [bmim][BF4] (a) and [bmim][PF6] (b). The surfactant concentrations are 10 mmol/L (a), 20 mmol/L (b).

TABLE 1: Surface Properties of FC-4 in [bmim][BF4] at Various Temperaturesa temp (°C)

CAC (mmol/L)

γCAC (mN/m)

ΠCAC (mN/m)

pC20

Γmax (µmol/m2)

Amin (Å2)

25 30 35 40 45 50

2.3 2.8 3.0 3.8 5.9 7.7

20.7 20.1 19.8 19.3 19.2 19.1

27.4 27.7 27.8 28.1 28.0 27.8

3.26 3.18 3.16 3.12 3.01 2.83

1.02 1.10 1.24 1.38 1.46 1.50

162.9 151.0 134.0 120.4 113.8 110.8

The error is (0.1 for CAC, γCAC, ΠCAC; (0.01 for pC20, Γmax; (2 for Amin. a

Figure 3. Comparison of ΠCAC for FC-4 with other conventional surfactants in ILs. Data for Brij 35, SDS, C16E8 and C16mimBr are from refs 5, 19, and 33. (The error of each ΠCAC is ( 0.1 mN/m.)

TABLE 2: Surface Properties of FC-4 in [bmim][PF6] at Various Temperaturesa temp (°C)

CAC (mmol/L)

γCAC (mN/m)

ΠCAC (mN/m)

pC20

Γmax (µmol/m2)

Amin (Å2)

25 30 35 40 45 50

9.6 10.6 12.1 12.9 13.9 15.5

21.2 21.1 20.8 20.6 20.2 20.0

27.0 26.7 26.7 26.4 26.6 26.4

2.63 2.61 2.55 2.53 2.47 2.40

0.35 0.39 0.52 0.70 0.83 1.18

474.6 425.9 319.4 237.3 200.1 140.8

The error is (0.1 for CAC; (0.1 for γCAC, ΠCAC; (0.01 for pC20, Γmax; (2 for Amin. a

second parameter, ΠCAC, is the surface pressure at the CAC, being defined by32

Πcmc ) γ0 - γCAC

(2)

where γo is the surface tension of pure solvent and γCAC is the surface tension of the solution at the CAC. This parameter indicates the maximum reduction of surface tension for pure solvent caused by the dissolution of surfactant, and hence becomes a measure for the effectiveness of the surfactant in lowering the surface tension of the solvent. The values of the two parameters at various temperatures in [bmim][BF4] and [bmim][PF6] are also shown in Tables 1 and 2. The ΠCAC for FC-4 in ILs is in the range 26-28 mN · m-1.

Figure 4. Temperature dependence of CAC for FC-4 in [bmim][BF4](a) and [bmim][PF6](b).

Figure 3 shows the comparison of ΠCAC for FC-4 with other surfactants in ILs. In this figure, the data for Brij 35, SDS, C16E8 and C16mimBr were obtained from previously published reports.5,19,33 As can be seen, the values of ΠCAC for FC-4 are greater than those for hydrocarbon surfactants in ILs. In other words, FC-4 is superior to other surfactants in terms of surface tension reduction. Though an enormous number of papers report surfactant aggregation in ILs by surface tension measurement, there is no report of the parameter pC20. Because the initial surface tensions of the neat ILs are much lower than for pure water, it is more difficult for surfactants to reduce the surface tension of the pure solvent. Common hydrocarbon surfactants cannot reduce the surface tension of pure solvent by 20 mN · m-1, so pC20 has no meaning for these surfactants in ILs.

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0 Figure 5. Plot of ∆G agg /T against 1/Tfor FC-4 in [bmim][BF4] (a) and [bmim][PF6] (b).

TABLE 3: Thermodynamic Parameters of the Aggregate Formation by FC-4 in [bmim][BF4] and [bmim][PF6] at Various Temperaturesa) aggregate formation at CAC (bmimBF4)

aggregate formation at CAC (bmimPF6)

temp (°C)

0 ∆Gagg (kJ/mol)

0 ∆Hagg (kJ/mol)

0 ∆Sagg (J/Kmol)

0 ∆Gagg (kJ/mol)

0 ∆Hagg (kJ/mol)

0 ∆Sagg (J/Kmol)

25 30 35 40 45 55

-38.3 -37.9 -38.2 -37.6 -35.8 -35.0

-18.8 -43.8 -68.0 -91.4 -114.1 -136.1

65.4 -19. 5 -96.8 -171.9 -246.2 -313.0

-30.8 -30.8 -30.7 -30.8 -30.9 -30.8

-29.9 -29.9 -29.9 -29.9 -29.9 -29.9

3.0 3.0 2.6 2.9 3.1 2.8

a

0 0 0 The errors for∆G agg ,∆H agg and ∆S agg in [bmim][BF4] are (0.2, (0.1, (1, and in [bmim][PF6] are (0.1, (0.1, (0.8, respectively.

Figure 6. Plot of thermodynamic parameters of aggregate formation at CAC against T for FC-4 in [bmim][BF4] (a) and [bmim][PF6] (b). Squares, circles and triangles correspond to ∆G0agg, ∆H0agg, and -T∆S0agg, respectively.

In the current system, we obtained the parameter pC20 for a surfactant in an IL solution for the first time. It is suggested that FC-4 has a higher adsorption efficiency than other surfactants. It can be seen that pC20 decreases as temperature increases, that is, higher temperatures lower the adsorption efficiency of the surfactant. The decrease of pC20 with temperature means that the enthalpy change associated with the surface adsorption is negative. This is common to aqueous ionic surfactant systems in which negative enthalpy changes are observed above room temperature.32 The maximum surface excess concentration, Γmax,32 and the area occupied by a single surfactant molecule at the air-water interface, Amin,32 were estimated for FC-4 in ILs by applying the Gibbs adsorption isotherm to the surface tension data. From Tables 1 and 2, it can be seen that with the increase of temperature, the values of Γmax increased both in [bmim][BF4] and [bmim][PF6] systems. As a result, Amin decreased. Γmax can reflect the surface arrangement of surfactants at the gas/liquid interface. A larger Γmax means that there are more surfactant molecules adsorbed on the surface of the solution, which also means a lower surface tension. At higher temperature, the values of Γmax increased and the number of surfactant molecules at

the air-water interface also increased, so the area occupied by a single surfactant molecule decreased. Temperature Dependence of the Critical Aggregation Concentration (CAC). The values of CAC are plotted as a function of temperature in parts a and b of Figure 4 for FC-4 in [bmim][BF4] and [bmim][PF6], respectively. It is quite interesting to find that the features of the two systems are rather different. In [bmim][BF4] systems, the increase of CAC with increasing temperature can be fit with a second-order polynomial, while in the [bmim][PF6] system, the CAC increased linearly with temperature. The CMC values of both ionic and nonionic surfactants in aqueous solution have been determined as a function of temperature by many researchers.24-29 For ionic surfactants, CMC versus temperature data fit on a U-shaped curve with a minimum around room temperature.29 The temperature dependence of CAC in [bmim][BF4] is similar to that in aqueous surfactant solutions. However, an almost linear increase with temperature rise observed for the CAC of FC-4 in [bmim][PF6], has never been reported in aqueous solutions to the best of our knowledge. Thermodynamic Analysis on the Aggregation Behavior of FC-4 in Ionic Liquids. As is well established in the thermodynamics of micelle formation, the standard Gibbs free energy of aggregate formation for an ionic surfactant is given by the following expression:34

∆G0agg ) 2RT ln CACM

(3)

where CACM is expressed in mole fraction units and it can be 0 calculated from CAC. Once∆G agg 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) ) ∆H 0agg ∂(1/T)

(4)

Finally, the standard entropy of aggregate formation ∆S 0agg is obtained from

Behavior of a Fluorinated Surfactant

∆S 0agg )

∆H 0agg - ∆G 0agg T

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(5)

According to the above formulas, the standard Gibbs free energy of aggregate formation of FC-4 in [bmim][BF4] and [bmim][PF6] at different temperatures were calculated using the CACM values. The plot of G 0agg/T against 1/T is shown in Figure 5, parts a and b, for [bmim][BF4] and [bmim][PF6]. The plot of G 0agg/T as a function of 1/T approximates a quadratic function for [bmim][BF4], and the slopes at various temperatures were evaluated, from which ∆H 0aggvalues were determined. G 0agg/T exhibits a linear dependence on1/T for [bmim][PF6], and hence, the values of ∆H 0agg were estimated from the slopes of the straight lines. The linear dependence of G 0agg/T on 1/T shows that the standard enthalpy change associated with the aggregate formation at CAC in [bmim][PF6] is independent of temperature. Then, the values of ∆S 0agg were calculated according to eq 5 for [bmim][BF4] and [bmim][PF6]. The thermodynamic parameters thus obtained are listed in Table 3. The plots of ∆G 0agg, ∆H 0agg, and -T∆S 0agg as a function of temperature for FC-4 in [bmim][BF4] and in [bmim][PF6] are shown in Figure 6, parts a and b. In [bmim][BF4], a factor contributing to a negative ∆G0agg was a large negative enthalpy term above 30 °C, while the value of enthalpy became more and more negative with the increase in temperature. Thus the aggregate formation for FC-4 in [bmim][BF4] is an enthalpy driven process in the temperature range above 30 °C. The temperature behavior of the thermodynamic functions is similar to the case in aqueous systems.34 This implies that the formation

Figure 7.

1

mechanism of FC-4 aggregates in [bmim][BF4] and of traditional micelles in aqueous solutions may be the same. Micelle formation in aqueous media is caused by a hydrophobic effect, the origin of which is hydrophobic hydration around the hydrocarbon chains of the surfactant molecules.35 The formation of FC-4 aggregates in [bmim][BF4] could be attributed to an analogous mechanism. A solvophobic driving force has been suggested for FC-4 self-assembly in [bmim][BF4].36 The formation mechanism for FC-4 assemblies in [bmim][PF6] is quite different from that in [bmim][BF4]. ∆G0agg ,∆H0agg, and -T∆S0agg for the aggregate formation in [bmim][PF6] remain constant as the temperature increases, which is not seen for micelle formation by surfactants in aqueous solution. This phenomenon is similar to what we observed in a previous report30 where two critical aggregation concentrations, CAC1 and CAC2, were found in the surface tension-concentration curves for Tween 20 in ILs. The thermodynamic parameters associated with CAC1 are almost independent of temperature. On the other hand, for the aggregate formed at CAC2, at low temperature the process is driven by entropy, and at high temperature it is driven by enthalpy. We 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. We propose that FC-4 forms segregated nanodroplets in [bmim][PF6]. The aggregation mechanisms for FC-4 in [bmim][BF4] and [bmim][PF6] are explained below. When FC-4 is dissolved in [bmim][BF4], which is hydrophilic, it has strong solvophobic

H NMR spectra obtained for [bmim][BF4] (a) and [bmim][PF6] (b) at 298 K.

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Figure 8.

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Figure 9.

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H NMR chemical shifts of various protons in [bmim][BF4] as a function of FC-4 concentration at 298 K.

H NMR chemical shifts of various protons in [bmim][PF6] as a function of FC-4 concentration at 298 K.

solvation around the hydrophobic part of FC-4 molecule. The solvophobic solvation is similar to the hydrophobic effect in aqueous solution, where a hydrogen-bond network around the surfactant hydrocarbon chain causes the hydrophobic effect.34 The solvophobic solvation by IL molecules is created by a network of ionic interactions instead of the hydrogen-bonding interactions, and the bmim+ cation binds to a surfactant hydrophobic chain directing its butyl group toward the hydrophobic chain.37 Thus, the aggregates of FC-4 in [bmim][BF4] are regarded as traditional “micelles”, and the driving force of the aggregate formation is solvophobic solvation between FC-4 and [bmim][BF4]. On the other hand, the aggregation behavior of FC-4 in [bmim][PF6] is rather different from that in [bmim][BF4], and anomalous compared with micelle formation in aqueous systems. The thermodynamic parameters related to CAC are almost independent of temperature. [bmim][PF6] is hydrophobic, so the solvophobic effect between FC-4 and [bmim][PF6] is much less than that between FC-4 and

[bmim][BF4]. All the above evidence indicates that the driving force for the [bmim][PF6] system is not like the hydrophobic effects in an aqueous system. The aggregate may be nanodroplets composed of FC-4 molecules segregated from the solution phase. 1H NMR Analysis of the Aggregation Mechanisms of FC-4 in Ionic Liquids. 1H NMR spectroscopic analysis can give more detailed information about solute-solvent interactions and provide insight into interactions at the molecular level. In our previous report, 1H NMR was used to investigate the microstructure of the IL microemulsions.14 In the current study, 1H NMR measurements were carried out for FC-4 in [bmim][BF4] and [bmim][PF6] as a function of the surfactant concentration. Figure 7 shows the spectrum obtained for pure [bmim][BF4] (a) and [bmim][PF6] (b), as well as their peak assignments. Plots of the chemical shifts, δ, for different protons in the bmim+ cation against the FC-4 concentration in [bmim][BF4] and [bmim][PF6] are shown in Figures 8 and

Behavior of a Fluorinated Surfactant 9. As can be seen, the chemical shift of the bmim+ protons changes with the increase of the FC-4 concentration. The δ of all protons exhibits a shift toward lower magnetic field with the addition of FC-4 for the [bmim][BF4] system. In [bmim][PF6], the shift for δ of all protons exhibits the same trend. Below CAC, the δ shifts toward lower magnetic field with the increase of FC-4 concentration, which is same as in [bmim][BF4]. To our surprise, when the aggregates formed, the δ abruptly shift toward high magnetic field, then remain nearly constant with further increase of FC-4 concentration. These changes of δ caused by the addition of FC-4 indicate that some interactions take place between FC-4 and ILs. When FC-4 is added to [bmim][BF4], there is a solvophobic effect between the hydrophobic part of the FC-4 molecule and the [bmim][BF4] molecule, so the δ of all protons exhibit a shift toward lower magnetic field. The slope increases steeply below the CAC, but after the formation of the aggregates, the slope is slightly smaller than that below the CAC. The propylene group and ethyl groups attached to nitrogen of the FC-4 molecules in the aggregate was solvated by bmim+ more or less similarly to the FC-4 monomers, while the hydrophobic part in the aggregates had much less solvating bmim+, because the solvophobic chains were buried in the interior of the micelles. So the δ continued to shift toward lower magnetic field with a slightly smaller slope. According to the analysis of the thermodynamic parameters and the NMR results, the FC-4 aggregates in [bmim][BF4] can be considered traditional micelles. As for the aggregates formed in [bmim][PF6], when FC-4 was added to [bmim][PF6], because of the weak hydrophilicity of [bmim][PF6], there is a weak solvophobic effect between the hydrophobic parts of FC-4 and [bmim][PF6], similar to the FC-4 monomers in [bmim][BF4]. So in the lower concentration range, the δ of all protons exhibits a shift toward lower magnetic field with the addition of FC-4. At the CAC, the δ abruptly moved toward high magnetic field. This fact suggests that the aggregates are nanodroplets composed of FC-4 molecules segregated from the solution phase. The interior of the droplets must be similar to bulk FC-4, in which no solvation takes place. The solvation of the hydrophilic part of FC-4 is quite weak and can be neglected, so upon further addition of FC-4, the δ values remain steady. The thermodynamic parameters derived from the temperature dependence of FC-4 in [bmim][PF6] become thermodynamic functions of dissolution when we invert their signs. For example, the enthalpy change can be regarded as the heat of dissolution. Typically, dissolution heat is positive, and insensitive to temperature. This accords with the rather anomalous result for FC-4 in [bmim][PF6]. The 1H NMR result together with the thermodynamic analysis all show that the aggregates of FC-4 in [bmim][PF6] are not traditional micelles, but rather nanodroplets composed of FC-4 molecules segregated from the solution phase. Conclusions In summary, the aggregation behavior of a cationic fluorinated surfactant, FC-4, in [bmim][BF4] and [bmim][PF6] has been investigated. FF-TEM revealed the formation of FC-4 aggregates in ILs. A series of useful surface properties were measured to assess the surface activity of the surfactant in ILs. The surface activity of the fluorinated surfactant in ILs was superior to traditional surfactants in ILs. Thermodynamic analysis of surface tension data as a function of temperature revealed that the aggregate formation of FC-4

J. Phys. Chem. B, Vol. 112, No. 39, 2008 12459 in [bmim][BF4] was enthalpy-driven above 30 °C, analogous to surfactant micelle formation in aqueous solution. In addition, the results of thermodynamic parameters and 1H NMR show that the FC-4 micelle formation in [bmim][BF4] was caused by solvophobic interactions between the solvophobic chains of the FC-4 molecules just like hydrophobic interactions in aqueous solution. This demonstrates that the aggregates formed in [bmim][BF4] are traditional micelles. On the other hand, the results of thermodynamic analysis along with the chemical shift behavior of the bmim+ protons suggest strongly that the aggregates of FC-4 formed in [bmim][PF6] are nanodroplets composed of FC-4 molecules segregated from the solution phase. The present work provides insight into aggregates of highly efficient surfactants in ILs, which may widen the application of ILs as novel solvents. Understanding the different formation mechanisms of FC-4 self-assemblies in ionic liquids (with different hydrophilicity), may contribute a better understanding of the self-assembly of surfactants in different ionic liquids. 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, Z2007B03). This work was partially supported by the Laboratory 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. References and Notes (1) Rogers, R. D.; Seddon, K. R. Science 2003, 302, 792. (2) Welton, T. Chem. ReV. 1999, 99, 2071. (3) Kazarian, S. G.; Briscoe, B. J.; Welton, T. Chem. Commun. (Cambridge) 2000, 2047. (4) Anderson, J. L.; Ding, J.; Welton, T.; Armstrong, D. W. J. Am. Chem. Soc. 2002, 124, 14247. (5) Anderson, J. L.; Pino, V.; Hagberg, E. C.; Sheares, V. V.; Armstrong, D. W. Chem. Commun. (Cambridge) 2003, 2444. (6) Avery, T. D.; Jenkis, N. F.; Kimber, M. C.; Lupton, D. W.; Taylor, D. K. Chem. Commun. 2002, 1, 28. (7) Zerth, H. M.; Leonard, N. M.; Mohan, R. S. Org. Lett. 2003, 5, 55. (8) Huddieston, J. G.; Willauer, H. D.; Swauoski, R. P.; Visser, A. E.; Rogers, K. D. Chem. Commun. 1998, 16, 1765. (9) Wang, P.; Zakeeruddion, S. M.; Comte, P.; Exnar, I.; Gratzel, M. J. Am. Chem. Soc. 2003, 125, 1166. (10) Wang, L. Y.; Chen, X.; Chai, Y. C.; Hao, J. C.; Sui, Z. M.; Zhuang, W. C.; Sun, Z. W. Chem. Commun. 2004, 24, 2840. (11) Wang, Z. N.; Liu, F.; Gao, Y. A.; Zhuang, W. C.; Xu, L. M.; Han, B. X.; Li, G. Z.; Zhang, G. Y. Langmuir 2005, 21, 4931. (12) Gao, H.; Li, J.; Han, B.; Chen, W.; Zhang, J.; Zhang, R.; Yan, D. Phys. Chem. Chem. Phys. 2004, 6, 2914. (13) Eastoe, J.; Gold, S.; Rogers, S. E.; Paul, A.; Welton, T.; Heenan, R. K.; Grillo, I. J. Am. Chem. Soc. 2005, 127, 7302. (14) Gao, Y. A.; Zhang, J.; Xu, H. Y.; Zhao, X. Y.; Zheng, L. Q.; Li, X. W.; Yu, L. ChemPhysChem 2006, 7, 1554. (15) Li, N.; Gao, Y. A.; Zheng, L. Q.; Zhang, J.; Yu, L.; Li, X. W. Langmuir 2007, 23, 1091. (16) Gao, Y. A.; Li, N.; Zheng, L. Q.; Zhao, X. Y.; Zhang, J.; Cao, Q.; Zhao, M. W.; Li, Z.; Zhang, G. Y Chem. Eur. J. 2007, 13, 2661. (17) Li, N.; Cao, Q.; Gao, Y.; Zhang, J.; Zheng, L. Q.; Bai, X. T.; Dong, B.; Li, Z.; Zhao, M. W.; Yu, L Chem Phys Chem 2007, 8, 2211. (18) Li, N.; Zhang, S.; Zheng, L. Q.; Gao, Y. A.; Yu, L. Langmuir 2008, 24, 2973. (19) Patrascu, C.; Gauffre, F.; Nallet, F.; Bordes, R.; Oberdisse, J.; de Lauth-Viguerie, N.; Mingotaud, C. ChemPhysChem 2006, 7, 99. (20) Fletcher, K. A.; Pandey, S. Langmuir 2004, 20, 33. (21) Abe, M. Curr. Opin. Colloid Interface Sci. 1999, 4, 354. (22) Dong, S. L.; Li, X.; Xu, G. Y.; Hoffmann, H. J. Phys. Chem. B 2007, 111, 590.

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