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 Δν the half line width ( 1/2), we can obtain the kinetic information of fluorinated zwitterionic micellization in an ionic liquid. ■ 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 1−5 electrochemical potential window, room-temperature ionic Figure 1. Chemical structure of PDSPDA. liquids (RTILs) have attracted considerable attention on chemical applications especially being treated as green solvents monomers was observed, showing the long lifetime of the for self-assembly of amphiphilic molecules. The first RTIL was fl 6 uorinated zwitterionic micelles in RTIL. To the best of our ethylammonium nitrate (EAN) reported by Walden in 1914. It knowledge, it is the first report for surfactant micelles in RTIL is the most extensively studied protic ionic liquid because of its with an unusually slow exchange, which should have a profound 7,8 similarities to water, and the most water-like property is that understanding for the surfactant aggregates in ionic liquid it can also form a three-dimensional hydrogen-bonded media. network.9 Evans and co-workers first provided evidence for the existence of hydrogen-bonded network in EAN; therefore, ■ EXPERIMENTAL SECTION ff amphiphilic molecules can self-assemble to di erent aggregates Chemicals and Materials. Polyfluorinated-2-dodecenyl (3- in EAN through the solvophobic interactions similar to the sulfate) propyl dimethyl ammonium (C9F19CF CHCH2N- 7,10,11 hydrophobic interactions in water. Studies about (CH3)2(CH2)3OSO3, PDSPDA) was a gift from Hoechst Aktienge- aggregates formed in EAN were mainly concentrated on sellschaft Werk, Gendorf (Frankfurt-am-Main, Germany). Ethyl- micelles7 and liquid crystals.12 Vesicles of Zn2+−fluorous ammonium nitrate (EAN) was synthesized according to the method 15 − surfactant or the mixture of Zn2+−fluorous surfactant/ reported by Evans et al. The aqueous solution of ethylamine (65 70 zwitterionic surfactant in RTILs13 and DDAB in EAN were wt %) was cooled in an ice bath, and 3 mol/L nitric acid was added 14 into it dropwise by stirring. The solution was stirred over two hours observed. Although a lot of work has been done to investigate fi 15−19 after nishing the addition of the nitric acid. Most of the water from the micellization of amphiphilic molecules in RTILs, the the crude product was removed with a rotary evaporator at 60 °C for mechanism is still controversial. Rare studies have been about three hours. To remove the remaining water, we first swept the reported concerning the micellization of fluorinated surfactants product by blowing the nitrogen and then performed the suction − in RTILs because they are difficult to dissolve in RTILs.20 22 In filtration under the 80 °C water bath by using an oil pump. Pure EAN the present work, we investigate the micellization of a was obtained and stored in the dry cabinet. Its melting point is 14 °C, 7,15 3 fluorinated zwitterionic surfactant, polyfluorinated-2-dodecenyl agreeing well with former reports, and the density is 1.2 g/cm (25 (3-sulfate) propyl dimethyl ammonium (C9F19CF CHCH2N- (CH3)2(CH2)3OSO3, PDSPDA, Figure 1), in EAN through the Received: July 30, 2013 methods of surface tension, 19F NMR, and FF-TEM measure- Revised: October 31, 2013 ments. Unusually slow exchange between micelles and Published: October 31, 2013 © 2013 American Chemical Society 14380 dx.doi.org/10.1021/la402937w | Langmuir 2013, 29, 14380−14385 Langmuir Article solvophobic interaction of tails and the interactions of headgroups. Over the entire temperature range, the contribu- tion 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. ■ ASSOCIATED CONTENT Δν 19 Figure 7. Half line width 1/2(Hz) of F NMR signals for the *S Supporting Information − terminal CF3 group in micelle state as a function of PDSPDA Characterizations of EAN, the micelle diameter distribution, concentration in EAN. and dynamic light scattering results. This material is available free of charge via the Internet at http://pubs.acs.org. testing sample is 2.5 mmol/L, and the temperature is 25 °C. ± Obvious spherical micelles with the average diameter 25.45 ■ AUTHOR INFORMATION 3.74 nm can be observed in the FF-TEM image (Figure 8). Corresponding Author *E-mail: [email protected]. Tel.: +86-531-88366074. Fax: +86- 531-88564750. Notes The authors declare no competing financial interest. ■ ACKNOWLEDGMENTS This work was financially supported by the NSFC (Grant No. 21033005 & 21273134) and the National Basic Research Program of China (973 Program, 2009CB930103). Figure 8. A typical FF-TEM image for PDSPDA micelles in EAN. ■ REFERENCES cPDSPDA = 2.5 mmol/L. (1) Welton, T. Room-Temperature Ionic Liquids. Solvents for Synthesis and Catalysis. Chem. Rev. 1999, 99, 2071−2083. Dynamic light scattering (DLS) measurements for the sample (2) Hussey, C. L. Room Temperature Haloaluminate Ionic Liquids. solution of 2.5 mmol/L PDSPDA in EAN at 25 °C were also Novel Solvents for Transition Metal Solution Chemistry. Pure Appl. fi Chem. 1988, 60, 1763−1772. performed to further con rm the size of micelles, and the range “ ” of the hydrodynamic radius (R )is30−50 nm (see Figure S4, (3) Wasserscheid, P.; Keim, W. Ionic Liquids-New Solutions for h Transition Metal Catalysis. Angew. Chem., Int. Ed. 2000, 39, 3772− SI). Because the hydrodynamic radius is always larger than the 49,50 3789. actual size, it can be assumed that micelles with large (4) Anderson, J. L.; Ding, J.; Welton, T.; Armstrong, D. W. diameters assuredly formed in this system. The micelles formed Characterizing Ionic Liquids On the Basis of Multiple Solvation by the fluorinated zwitterionic surfactant in EAN are much Interactions. J. Am. Chem. Soc. 2002, 124, 14247−14254. larger than those of some cationic surfactants with hydrocarbon (5) Dieter, K. M.; Dymek, C. J., Jr.; Heimer, N. E.; Rovang, J. W.; chains in EAN15 and the conventional micelles formed in Wilkes, J. S. Ionic Structure and Interactions in 1-Methyl-3- aqueous solutions normally with several nanometer in size. ethylimidazolium Chloride-AlCl3 Molten Salts. J. Am. Chem. Soc. Although there is no clear explanation for the larger size of 1988, 110, 2722−2726. micelles in our observations, they confirmed that the size of (6) Walden, P. Molecular Weight and Electrical Conductivity of − − micelles in ionic liquids is apparently larger,17,20 22 and much Several Fused Salts. Bull. Acad. Imper. Sci. 1914, 1800, 405 422. (7) Evans, D. F.; Yamauchi, A.; Roman, R.; Casassa, E. Z. Micelle further characterization will be needed. For example, Anderson Formation in Ethylammonium Nitrate, a Low-melting Fused Salt. J. et al. investigated that the size of micelles in ionic liquid − 17 Colloid Interface Sci. 1982, 88,89 96. solutions is apparently larger. For their measurements of Brij (8) Greaves, T. L.; Drummond, C. J. Protic Ionic Liquids: Properties 700 in ionic liquid 1-butyl-3-methyl imidazolium hexafluor- and Applications. Chem. Rev. 2008, 108, 206−237. ophosphate (BMIM-PF6), they observed that, when the (9) Fumino, K.; Wulf, A.; Ludwig, R. Hydrogen Bonding in Protic concentrations are somewhat lower than the cmc, premicellar Ionic Liquids: Reminiscent of Water. Angew. Chem., Int. Ed. 2009, 48, aggregates were found with an aggregation number of 4 and a 3184−3186. radius of gyration of 31.4 ± 4.4 nm was calculated. (10) Evans, D. F.; Chen, S.; Schriver, G. W.; Arnett, E. M. Thermodynamics of Solution of Nonpolar Gases in a Fused Salt. ■ CONCLUSIONS “Hydrophobic Bonding” Behavior in a Nonaqueous System. J. Am. Chem. Soc. 1981, 103, 481−482. In summary, we investigated the thermodynamic and kinetic (11) Ray, A. Solvophobic Interactions and Micelle Formation in micellization mechanism of a fluorinated zwitterionic surfactant Structure Forming Nonaqueous Solvents. Nature 1971, 231, 313−315. in EAN. The cmc variation demonstrates that the influence of (12) Araos, M. U.; Warr, G. G. Self-Assembly of Nonionic temperature on the micellization is embodied in both the Surfactants into Lyotropic Liquid Crystals in Ethylammonium Nitrate, 14384 dx.doi.org/10.1021/la402937w | Langmuir 2013, 29, 14380−14385.