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Langmuir 2006, 22, 42-45
Articles Aggregation Properties of Cationic Gemini Surfactants with Partially Fluorinated Spacers in Aqueous Solution Yajuan Li,† Peixun Li,‡ Chuchuan Dong,‡ Xiaoyong Wang,† Yilin Wang,*,† Haike Yan,† and Robert K. Thomas‡ Key Laboratory of Colloid and Interface Science, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, People’s Republic of China, and Physical and Theoretical Chemistry Laboratory, Oxford UniVersity, South Parks Road, Oxford OXI 3QZ, United Kingdom ReceiVed June 10, 2005. In Final Form: September 29, 2005 The aggregation properties of cationic gemini surfactants alkanediyl-R,ω-bis(dodecyldimethylammonium bromide), [C12H25(CH3)2N(CH2)m(CF2)n(CH2)mN(CH3)2C12H25]Br2 [where 2m + n ) 12 and n ) 0, 4, and 6; designated as 12-12-12, 12-12(C4F)-12, and 12-12(C6F)-12, respectively] have been studied by microcalorimetry, time-resolved fluorescence quenching, and electrical conductivity. Compared with a fully hydrocarbon spacer of 12-12-12, the fluorinated spacer with a lower ratio of CF2 to CH2 in 12-12(C4F)-12 tends to disfavor the aggregation, leading to larger critical micelle concentration (cmc), lower micelle aggregation number (N), and less negative Gibbs free energy of micellization (∆Gmic). However, the fluorinated spacer with a higher ratio of CF2 to CH2 in 12-12(C6F)-12 may prompt the aggregation, resulting in lower cmc, higher N, and more negative ∆Gmic. It is also noted that enthalpy change of micellization (∆Hmic) for 12-12(C4F)-12 is the most exothermic, but the values of ∆Hmic for 12-12-12 and 1212(C6F)-12 are almost the same. These results are rationalized in terms of competition among the enhanced hydrophobicity and the rigidity of the fluorinated spacer, and the variation of immiscibility of the fluorinated spacer with the hydrocarbon side chains.
Introduction Gemini surfactants have been attracting increased attention owing to their superior properties to those of conventional singlechain surfactants.1-3 These novel surfactants consist of two hydrophobic side chains and two polar headgroups covalently attached through a spacer group. The most widely investigated gemini surfactants are made of hydrocarbon side chains and hydrocarbon spacer. Recently, gemini surfactants with fluorinated side chains have been reported to hold many special properties.4 Oda et al.5 synthesized a series of gemini surfactants with partially fluorinated hydrophobic chains. These fluorinated gemini surfactants exhibit very low critical micelle concentration (cmc) and unusually slow exchange rates of surfactants between aggregates and solution. Our group6 has investigated the thermodynamics of partially chain-fluorinated cationic gemini surfactants. The irregularities in enthalpy change of micellization have been found to be associated with variations in the architecture of the fluorocarbon chain segments. As is well-known, many previous studies7-13 have shown that the hydrophobicity of spacer * To whom correspondence should be addressed: e-mail yilinwang@ iccas.ac.cn. † Chinese Academy of Sciences. ‡ Oxford University. (1) Menger, F. M.; Littau, C. A. J. Am. Chem. Soc. 1991, 113, 1451-1452. (2) Rosen, M. J. CHEMTECH 1993, 23, 30-33. (3) Menger, F. M.; Littau, C. A. J. Am. Chem. Soc. 1993, 115, 10083-10090. (4) Monduzzi, M. Curr. Opin. Colloid Interface Sci. 1998, 3, 467-477. (5) Oda, R.; Huc, I.; Danino, D.; Talmon, Y. Langmuir 2000, 16, 9759-9769. (6) Li, Y. J.; Li, P. X.; Wang, J. B.; Wang, Y. L.; Yan, H. K.; Dong, C. C.; Thomas, R. K. J. Colloid Interface Sci. 2005, 287, 333-337. (7) Danino, D.; Talmon, Y.; Zana, R. Langmuir 1995, 11, 1448-1456. (8) Song, L. D.; Rosen, M. J. Langmuir 1996, 12, 1149-1153. (9) De, S.; Aswal, V. K.; Goyal, P. S.; Bhattacharya, S. J. Phys. Chem. B 1998, 102, 6152-6160.
affects the aggregation of gemini surfactants significantly. Because of the larger volume and higher electronegativity of fluorine than those of hydrogen, the introduction of fluorine atoms greatly increases the amphiphilic character of surfactant molecules, resulting in enhanced surface activity and lower cmc. Therefore, it is of great interest to see how introducing fluorine to the spacer will influence the aggregation properties of gemini surfactants. In the present paper, we focus on the aggregation properties of a series of alkanediyl-R,ω-bis(dodecyldimethylammonium bromide) gemini surfactants with the structure [C12H25(CH3)2N(CH2)m(CF2)n(CH2)mN(CH3)2C12H25]Br2 [where 2m + n ) 12 and n ) 0, 4, and 6; referred to as 12-12-12, 12-12(C4F)-12, and 12-12(C6F)-12, respectively]. Isothermal titration microcalorimetry, time-resolved fluorescence quenching, and electrical conductivity are used to investigate the effect of partially fluorinated spacer on the aggregation of gemini surfactants. Materials and Methods Materials. The gemini surfactants used in this study were synthesized and purified according to the method of Menger and Littau.3 Starting materials were obtained as follows. Br(CH2)12Br was purchased from Aldrich. Br(CH2)4(CF2)4(CH2)4Br was synthesized by three steps. 1,4-Diidooctafluorobutane (Apollo Scientific Ltd.) was first reacted with excess 3-buten-1-ol (Aldrich) to produce the iodo adduct,14 which was then reduced by dehalogenation.15 The (10) Menger, F. M.; Keiper, J. S.; Azov, V. Langmuir 2000, 16, 2062-2067. (11) Wettig, S. D.; Verrall, R. E. J. Colloid Interface Sci. 2001, 235, 310-316. (12) Wettig, S. D.; Nowak, P.; Verrall, R. E. Langmuir 2002, 18, 5354-5359. (13) Wettig, S. D.; Li, X.; Verrall, R. E. Langmuir 2003, 19, 3666-3670. (14) Huang, W. H.; Wang, W.; Huang, B. N. Acta Chim. Sin. 1986, 44, 488494.
10.1021/la051544n CCC: $33.50 © 2006 American Chemical Society Published on Web 11/15/2005
Aggregation Properties of Gemini Surfactants resulting Br(CH2)4(CF2)4(CH2)4Br was synthesized by brominating the dehalogenated product using tetrabutylammonium bromide as phase transfer catalysis.16 Br(CH2)3(CF2)6(CH2)3Br was synthesized in a similar way as Br(CH2)4(CF2)4(CH2)4Br from 1,6-diiodoperfluorohexane (Apollo Scientific Ltd.) and allyl alcohol (Aldrich). Dodecyldimethylamine was prepared by direct reaction of bromododecane with dimethylamine in methanol. Then, a large excess of dodecyldimethylamine was reacted with Br(CH2)12Br, Br(CH2)4(CF2)4(CH2)4Br, or Br(CH2)3(CF2)6(CH2)3Br in warm acetone (
