First Fluorinated Zwitterionic Micelle with Unusually Slow Exchange in

Article pubs.acs.org/Langmuir

First Fluorinated Zwitterionic Micelle with Unusually Slow Exchange in an Ionic Liquid Xiaolin Wang, Panfeng Long, Shuli Dong, and Jingcheng Hao* Key Laboratory of Colloid and Interface Chemistry and Key Laboratory of Special Aggregated Materials, Shandong University, Ministry of Education, Jinan 250100, China S Supporting Information *

ABSTRACT: The micellization of a fluorinated zwitterionic surfactant in ethylammonium nitrate (EAN) was investigated. The freeze-fracture transmission electron microscope (FF-TEM) observations confirm the formation of spherical micelles with the average diameter 25.45 ± 3.74 nm. The micellization is an entropy-driven process at low temperature but an enthalpy-driven process at high temperature. Two sets of 19F NMR signals above the critical micelle concentration (cmc) indicate that the unusually slow exchange between micelles and monomers exists in ionic liquid; meanwhile, surfactant molecules are more inclined to stay in micelle states instead of monomer states at higher concentration. Through the analysis of the half line width (Δν1/2), we can obtain the kinetic information of fluorinated zwitterionic micellization in an ionic liquid.

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INTRODUCTION Due to their attractive properties, such as negligible volatility, low melting points, outstanding chemical and thermal stabilities, high ionic conductivity, and a relatively wide electrochemical potential window,1−5 room-temperature ionic liquids (RTILs) have attracted considerable attention on chemical applications especially being treated as green solvents for self-assembly of amphiphilic molecules. The first RTIL was ethylammonium nitrate (EAN) reported by Walden in 1914.6 It is the most extensively studied protic ionic liquid because of its similarities to water,7,8 and the most water-like property is that it can also form a three-dimensional hydrogen-bonded network.9 Evans and co-workers first provided evidence for the existence of hydrogen-bonded network in EAN; therefore, amphiphilic molecules can self-assemble to different aggregates in EAN through the solvophobic interactions similar to the hydrophobic interactions in water. 7,10,11 Studies about aggregates formed in EAN were mainly concentrated on micelles7 and liquid crystals.12 Vesicles of Zn2+−fluorous surfactant or the mixture of Zn2+−fluorous surfactant/ zwitterionic surfactant in RTILs13 and DDAB in EAN were observed.14 Although a lot of work has been done to investigate the micellization of amphiphilic molecules in RTILs,15−19 the mechanism is still controversial. Rare studies have been reported concerning the micellization of fluorinated surfactants in RTILs because they are difficult to dissolve in RTILs.20−22 In the present work, we investigate the micellization of a fluorinated zwitterionic surfactant, polyfluorinated-2-dodecenyl (3-sulfate) propyl dimethyl ammonium (C9F19CFCHCH2N(CH3)2(CH2)3OSO3, PDSPDA, Figure 1), in EAN through the methods of surface tension, 19F NMR, and FF-TEM measurements. Unusually slow exchange between micelles and © 2013 American Chemical Society

Figure 1. Chemical structure of PDSPDA.

monomers was observed, showing the long lifetime of the fluorinated zwitterionic micelles in RTIL. To the best of our knowledge, it is the first report for surfactant micelles in RTIL with an unusually slow exchange, which should have a profound understanding for the surfactant aggregates in ionic liquid media.

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EXPERIMENTAL SECTION

Chemicals and Materials. Polyfluorinated-2-dodecenyl (3sulfate) propyl dimethyl ammonium (C 9 F 19 CFCHCH 2 N(CH3)2(CH2)3OSO3, PDSPDA) was a gift from Hoechst Aktiengesellschaft Werk, Gendorf (Frankfurt-am-Main, Germany). Ethylammonium nitrate (EAN) was synthesized according to the method reported by Evans et al.15 The aqueous solution of ethylamine (65−70 wt %) was cooled in an ice bath, and 3 mol/L nitric acid was added into it dropwise by stirring. The solution was stirred over two hours after finishing the addition of the nitric acid. Most of the water from the crude product was removed with a rotary evaporator at 60 °C for about three hours. To remove the remaining water, we first swept the product by blowing the nitrogen and then performed the suction filtration under the 80 °C water bath by using an oil pump. Pure EAN was obtained and stored in the dry cabinet. Its melting point is 14 °C, agreeing well with former reports,7,15 and the density is 1.2 g/cm3 (25 Received: July 30, 2013 Revised: October 31, 2013 Published: October 31, 2013 14380

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°C). The Karl Fischer titration result shows that the water content in EAN is about 0.4 wt %. 1H NMR and 13C NMR spectra and FT-IR spectrum which are appended as Supporting Information (Figures S1 and S2) demonstrate the purity of EAN. Methods and Characterization. Surface tension measurements were performed on a Krüss K100 (Germany) surface tensiometer using plate method. The temperature was controlled with a Haake K10 (Germany) super constant temperature trough, and the error was within ±0.1 °C. All of the surface tension was measured after stirring and equilibration and was repeated at least twice until the error was negligible. 19 F NMR spectra were recorded on a Bruker AVANCE 400 spectrometer equipped with pulse field gradient module (Z axis) using a 5 mm Broadband Observe (BBO) probe operating at 376.72 MHz. The 19F spectra were reported in the range from +30 to −170 ppm (digitized points = 32 K, 90° pulse = 7.4 ms, relaxation delay = 2 s). The operating temperature was at 25.0 ± 0.1 °C. FF-TEM measurement was performed to obtain the microstructure of the micelle. A small amount of 2.5 mmol/L solution was dropped on the specimen carrier and was inserted rapidly into the liquid ethane at −175 °C cooled with liquid nitrogen. Then the sample was fractured with Leica EM BAF 060 equipment under the condition of −175 °C and 10−7 Pa. Pt/C was sprayed onto the fracture surface at 45° forming a 2.5 nm thick film, and C was sprayed at 90° forming a 18 nm thick film. The replica was observed with a JEOL JEM-1400 electron microscope with an accelerating voltage of 120 kV.

Table 1. Various Micellization Parameters of PDSPDA in EAN at Different Temperatures T (K)

cmc (mmol/L)

γcmc (mN/m)

Πcmc (mN/m)

pC20

Γmax (μmol/m2)

Amin (Å2)

298 303 308 313 318 323

0.999 1.12 1.17 1.33 1.46 1.68

25.3 25.1 24.9 24.7 24.4 24.3

24.2 24.4 24.5 24.6 24.8 24.8

3.25 3.25 3.21 3.15 3.12 3.10

1.24 1.41 1.43 1.42 1.48 1.35

114 117 116 117 112 123

where γ0 is the surface tension of the pure solvent, mN/m; γcmc is the surface tension at the cmc, mN/m. The data of the adsorption efficiency, pC20, provide direct evidence for the decrease of adsorption efficiency with the increasing temperature. It is defined as:20,23

pC20 = −log C20

(2)

where C20 refers to the required molar concentration of surfactant, mol/L, when the surface tension of the pure solvent is reduced 20 mN/m. The value of Πcmc indicates that the fluorinated surfactant is superior to the hydrocarbon surfactants in surface tension reduction, and the decreasing adsorption efficiency (pC20) with the increasing temperature implies that the enthalpy change is negative when surfactant molecules are absorbed.20 The maximum surface excess concentration (Γmax) can be derived from the Gibbs adsorption isotherm:

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RESULTS AND DISCUSSION Surface Tension Measurements. To understand the surface properties, we performed surface tension measurements for PDSPDA in EAN at different temperatures. As shown in Figure 2, at each temperature, with the increasing surfactant

Γmax = −

1 ⎛ ∂γ ⎞ ⎜ ⎟ RT ⎝ ∂ ln c ⎠T

(3)

where R is the gas constant, 8.314 J/(mol·K); T is the absolute temperature, K; γ is the surface tension of the solution, mN/m; c is the surfactant concentration, mol/L. Then the minimum area occupied per surfactant molecule at the air/solution interface (Amin) can be calculated from the following equation: 1 A min = NA Γmax (4) where NA is Avogadro’s constant, 6.022 × 1023 mol−1. Obviously the variations of Γmax and Amin with the increasing temperature are irregular. As can be seen from Table 1, compared to hydrocarbon surfactants,15 the cmc values of the fluorinated surfactant in EAN are small. Initially, the cmc value increases slowly and then sharply as a function of the temperature (Figure 3). We believe that both the solvophobic interaction of tails and the interaction of headgroups have significant effects on the variation trend of cmc. The solvophobic force has been interpreted to be the driving force for the amphiphile selfassembly in ionic liquids.20,22,24 With the increase of temperature, the solvophobic effect can be weakened, leading to the increase of cmc. Considering the structural properties, we believe that the solvation exists in EAN just like hydration in aqueous solution. As a result, the interactions between headgroups contain the solvation repulsive interaction and the electrostatic interaction. At higher temperatures, there is a stronger growth trend in cmc, for which we should take into account the influence of the changing temperature on the interactions of headgroups. Since the PDSPDA is zwitterionic, the electrostatic repulsion is weak.25 However, the increasing

Figure 2. Surface tension vs cPDSPDA in EAN at different temperatures. The numbers on the y-axis refer to the surface tension values at 25 °C, and at other temperatures, the surface tension values are added at 5, 10, 15, 20, and 25 one after another as the temperature increases to avoid data overlap. The error of the temperature is ±0.1 °C.

concentration, the surface tension declines significantly initially, then almost keeps constant after a turning point, indicating the micelles have been formed. The critical micelle concentration (cmc) and the surface tension at the critical micelle concentration (γcmc) at each temperature were obtained from the plots. Various micellization parameters were calculated as shown in Table 1. The effectiveness of surface tension reduction (Πcmc) can be used to determine the effectiveness in reducing the solvent surface tension. As shown in Table 1, the high Πcmc value ranges from 24.2 to 24.8 mN/m. It is defined as follows:20,23 Πcmc = γ0 − γcmc

(1) 14381

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Figure 3. Dependence of cmc values on the temperature for PDSPDA in EAN.

Figure 4. Thermodynamic parameters ΔH0m (solid squares) and −TΔS0m (hollow squares) as a function of temperature for PDSPDA in EAN.

temperature will destroy the solvation of headgroups and lead to higher net charge; hence the electrostatic repulsion is enhanced, and the cmc value increases. Thermodynamic Analysis on the Micellization of PDSPDA in EAN. Generally, thermodynamic parameters are calculated to conjecture the micellization mechanism. According to the well-established thermodynamic models: the phase separation26 and the mass action model,27 the standard Gibbs free energy of micelle formation (ΔG0m) is given by the following equation:22,28 ΔGm0 = (1 + β)RT ln Xcmc

formation process is entropy-driven at low temperatures (below approximate 28 °C) but enthalpy-driven at high temperatures. The essence of entropy-driven process is the solvophobic interaction;21 however, the entropy term is dominated in the micelle formation process just in a small range of lower temperature. The release of solvent molecules around the solvophobic tails contributes to the entropy increase and is beneficial to the micelle formation, but with the temperature increasing, less solvent molecules can be released; hence the enthalpy term becomes more dominant than the entropy term. 19 F NMR Results. Many thermodynamic investigations have been reported to reveal the micellization mechanism in ionic liquid media.20−22 However, few studies have described the kinetic mechanism of micellization in RTILs. The high sensitivity to the environment of the nuclei makes NMR become a powerful tool in the kinetics study field.29,30 The 19F NMR spectra of the terminal −CF3 group of PDSPDA in EAN at various concentrations are shown in Figure 5. One can obviously see that there is only one set of signals below cmc, while there are two sets of signals above cmc, clearly indicating

(5)

where Xcmc is the mole fraction of surfactant in the solution at the cmc. The degree of counterion binding (β) can be approximately considered as 0, because the fluorinated surfactant we used is zwitterionic. The standard enthalpy of micelle formation (ΔH0m) can be obtained by applying the Gibbs−Helmholtz equation: ⎡ ∂(ΔG 0 /T ) ⎤ m ⎥ = ΔHm0 ⎢ ⎣ ∂(1/T ) ⎦

(6)

Then the standard entropy of micelle formation be easily calculated from the following equation:

(ΔS0m)

can

ΔHm0 − ΔGm0 (7) T The thermodynamic parameters of micelle formation are presented in Table 2. The contribution of the entropy term and enthalpy term to the free energy change associated with the micellization can be easily distinguished from Figure 4. The data of thermodynamic parameters correspond well to the cmc variation. In the experimental temperature range, the micelle ΔSm0 =

Table 2. Thermodynamic Parameters of Micellization for PDSPDA in EAN at Various Temperatures T (K)

ΔG0m (kJ/mol)

ΔH0m (kJ/mol)

−TΔS0m (kJ/mol)

ΔS0m (J/K mol)

298 303 308 313 318 323

−23.1 −23.2 −23.5 −23.5 −23.6 −23.6

−9.88 −12.8 −15.6 −18.3 −20.9 −23.5

−13.2 −10.4 −7.89 −5.22 −2.72 −0.167

44.3 34.4 25.6 16.7 8.55 0.517

Figure 5. 19F NMR spectra of the terminal −CF3 group of PDSPDA in EAN at different concentrations. T = 25.0 ± 0.1 °C. 14382

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Figure 6. 19F NMR chemical shifts of the terminal −CF3 group of PDSPDA in two states as a function of the fluorinated surfactant concentration in EAN. (a) δmon refers to the chemical shift in the monomer state; (b) δmic refers to the chemical shift in the micelle state.

(δmon) is independent of the surfactant concentration (Figure 6a) because the environment of monomers in the bulk remains constant. It should be mentioned that the chemical shift of the terminal −CF3 group in micelle state (δmic) decreases initially with the increase of surfactant concentration and then levels off when the concentration exceeds 3.0 mmol/L as shown in Figure 6b. The migration of chemical shift is due to the change of the polarizable environment. A less polarizable environment would enhance the shielding effect and lead the signal to an upfield shift.44 With the formation of micelles, the terminal −CF3 group would be transferred from the EAN environment to a CF-rich environment, that is, a less polarizable environment, and the shielding effect increased, inducing larger chemical shifts for micellar signals. It has been reported that the average aggregation number increases with the increasing surfactant concentration;42,45 consequently, more fluorinated tails enclose the terminal −CF3 group, and the environment becomes less polarizable, which leads to the declining trend. When the surfactant concentration is higher than 3.0 mmol/L, the aggregation number is less affected by the surfactant concentration, and the micelles are stable. Kinetic Analysis on the Micellization of PDSPDA in EAN. Kinetic models have been established to understand the formation and dissolution equilibrium of the micelle.31,41 Two equilibrium processes exist in micellar system: one is the fast process concerning the exchange of a single surfactant molecule between the micelle and the bulk; the other one is micellization-dissolution process which is slow and is believed to consist of many steps of the fast process.46,47 Accordingly, two kinds of relaxation time exist: the longitudinal relaxation time for the fast process and the transverse relaxation time for the slow process.41 The half line width (Δν1/2) variation versus surfactant concentration is shown in Figure 7. The kinetic information of micellization can be obtained by analyzing the half line width (Δν1/2) of micellar signals, because it is closely related to the transverse relaxation time.48 We found that the value of Δν1/2 has the same variation trend with δmic. The decrease declares that the micelles have a longer lifetime. As the aggregation number increases, the steric hindrance for the exchange of surfactant molecules between micelles and monomers becomes larger, and the intramicellar interaction also becomes more complicated. Therefore, the prolongation of the micelle lifetime is reasonable. The half line width variation is negligible when the aggregation number no longer changes. FF-TEM Observations. Finally, to determine the microstructure of micelles formed by PDSPDA in EAN, we performed FF-TEM measurements. The concentration of the

the existence of the unusually slow exchange process between micelles and monomers in RTIL. The cmc is within 1.3−1.5 mmol/L, and it is in agreement with the result obtained by the method of surface tension measurement (0.999 mmol/L at 25 °C). The subtle difference is ascribed to the measuring error during surface tension measurement caused by the relatively higher viscosity of EAN. Thus, we believe that NMR is superior to the surface tension in determining the cmc values of surfactants in RTILs. Generally, the exchange of surfactant molecules between micelles and monomers is fast on the NMR time scale; therefore, the NMR spectra only show one set of averaged signals for surfactant molecules in their different states.31,32 The slow exchange processes for surfactants in aqueous solution have been reported,33−37 including some novel structure surfactants such as gemini surfactants, hybrid surfactants, and fluorocarbon surfactants, and some amphiphilic calixarenes also show similar phenomena.38−40 Our report first provides evidence for the unusually slow exchange of surfactant molecules between micelles and monomers in ionic liquid media. To the best of our knowledge, this is the first report on the unusually slow exchange of surfactant molecules between the micelles and monomers in ionic liquid media. The set of signals at the down magnetic field is supposed to be the monomers in the bulk; another set of signals appearing at the upper magnetic field corresponds to the micelle states. With the increase of the concentration, above 3.0 mmol/L, surprisingly, only the micellar signals could be detected. The results can be interpreted that surfactant molecules are more inclined to stay in micelle states instead of monomer states at higher concentration; therefore, the weak monomer signals are difficult to be detected on the NMR time scale. We give explanations of the unusually slow exchange between micelles and monomers in two aspects. On the one hand, because of the solvation around headgroups by EAN, the steric hindrance and electrostatic interaction restrict the exchanging process for surfactant molecules between micelles and the bulk. On the other hand, the exchange of molecules between micelles and monomers is believed to be controlled by diffusion.32,41,42 The viscosity of EAN is much higher than water,8 and the fluorinated chain is stiffer than the hydrocarbon chain;43 therefore, the motion of fluorinated tails is not flexible, and the diffusion is restricted, which leads to the slow exchange. 19 F NMR Chemical Shifts. 19F NMR chemical shifts of the terminal −CF3 group of PDSPDA in the monomer state and the micelle state against the total surfactant concentration are plotted in Figure 6. The chemical shift in the monomer state 14383

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solvophobic interaction of tails and the interactions of headgroups. Over the entire temperature range, the contribution of the enthalpy term to the micellization is more dominant than the entropy term. Both the solvation of headgroups and properties of the RTIL and the fluorinated surfactant contribute to the unusually slow exchange of surfactant molecules between micelles and monomers. With the increase of surfactant concentration, the micelles have a longer lifetime because of the increase of aggregation number. Since studies for the micellization of fluorinated surfactants in RTILs are rare, our investigation could provide a deeper understanding for the micellization mechanism in RTIL media.

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ASSOCIATED CONTENT

* Supporting Information

Figure 7. Half line width Δν1/2(Hz) of 19F NMR signals for the terminal −CF3 group in micelle state as a function of PDSPDA concentration in EAN.

S

Characterizations of EAN, the micelle diameter distribution, and dynamic light scattering results. This material is available free of charge via the Internet at http://pubs.acs.org.

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testing sample is 2.5 mmol/L, and the temperature is 25 °C. Obvious spherical micelles with the average diameter 25.45 ± 3.74 nm can be observed in the FF-TEM image (Figure 8).

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +86-531-88366074. Fax: +86531-88564750. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the NSFC (Grant No. 21033005 & 21273134) and the National Basic Research Program of China (973 Program, 2009CB930103).

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Figure 8. A typical FF-TEM image for PDSPDA micelles in EAN. cPDSPDA = 2.5 mmol/L.

REFERENCES

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Dynamic light scattering (DLS) measurements for the sample solution of 2.5 mmol/L PDSPDA in EAN at 25 °C were also performed to further confirm the size of micelles, and the range of the hydrodynamic radius (Rh) is 30−50 nm (see Figure S4, SI). Because the hydrodynamic radius is always larger than the actual size,49,50 it can be assumed that micelles with large diameters assuredly formed in this system. The micelles formed by the fluorinated zwitterionic surfactant in EAN are much larger than those of some cationic surfactants with hydrocarbon chains in EAN15 and the conventional micelles formed in aqueous solutions normally with several nanometer in size. Although there is no clear explanation for the larger size of micelles in our observations, they confirmed that the size of micelles in ionic liquids is apparently larger,17,20−22 and much further characterization will be needed. For example, Anderson et al. investigated that the size of micelles in ionic liquid solutions is apparently larger.17 For their measurements of Brij 700 in ionic liquid 1-butyl-3-methyl imidazolium hexafluorophosphate (BMIM-PF6), they observed that, when the concentrations are somewhat lower than the cmc, premicellar aggregates were found with an aggregation number of 4 and a radius of gyration of 31.4 ± 4.4 nm was calculated.

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CONCLUSIONS In summary, we investigated the thermodynamic and kinetic micellization mechanism of a fluorinated zwitterionic surfactant in EAN. The cmc variation demonstrates that the influence of temperature on the micellization is embodied in both the 14384

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dx.doi.org/10.1021/la402937w | Langmuir 2013, 29, 14380−14385