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Shamrock Surfactants: Synthesis and Characterization David A. Jaeger,*,† Xiaohui Zeng,† and Robert P. Apkarian‡ Department of Chemistry, University of Wyoming, Laramie, Wyoming 82071, and Integrated Microscopy and Microanalytical Facility, Emory University, Atlanta, Georgia 30322 Received July 23, 2004 Two types of a new class of surfactants with three headgroups, which possess the general structure 1, have been prepared. Within structure 1, a central headgroup is connected to two flanking headgroups by hydrocarbon chains. The term “shamrock” is used to describe surfactants of structure 1, denoting their triple-headed character and reflecting the fact that shamrocks have leaflets in groups of three. The major lipophilic character of shamrock surfactants is provided by the two hydrocarbon chains linking the three headgroups and not by long-chain alkyl groups appended to the linking hydrocarbon chains or the headgroups. The new surfactants are 2a (2,2,15,15,28,28-hexamethyl-2,15,28-triazonianonacosane triiodide), 2b (2,2,15,15,28,28-hexamethyl-2,15,28-triazonianonacosane trichloride), 3a (O,O′-di-[10-(N,N,N-tripropylammonio)decyl]phosphorodithioate bromide), and 3b (O,O′-di-[10-(N,N,N-tributylammonio)decyl]phosphorodithioate bromide). Compound 14 (2,2,9,9,16,16-hexamethyl-2,9,16-triazoniaheptadecane triiodide) was prepared for comparison with 2a. Surfactants 2 and 3 were characterized in water by measurement of their Krafft temperatures and critical aggregation concentrations, and their aggregates were studied by 1H NMR spectroscopy, dynamic laser light scattering, and phase-contrast optical microscopy. Aqueous 2b was also studied by cryo-etch high-resolution scanning electron microscopy, which revealed irregularly shaped cells containing a complex matrix of surfactant. Coacervates were observed by optical microscopy upon the hydration of 2 and 3.
Introduction Surfactants are important in a broad spectrum of applications as diverse as oil recovery,1a drug delivery,2 and the decontamination of chemical warfare agents.3 Recently there has been increasing activity in the synthesis of novel functionalized and unfunctionalized surfactants.4 Many new surfactants have been designed for specific applications, and others have been synthesized in a search for interesting and unusual properties. The former include cleavable surfactants,5,6 and the latter, gemini surfactants.7 The synthesis of novel surfactants and the characterization of their properties, even without preconceived applications, are indeed worthwhile endeavors. The case of gemini surfactants strongly supports this contention. They have been shown to have unique physical properties, compared to conventional surfactants, which have been used to advantage in a number of important applications.8 Herein, we report two types of a new class of surfactants that possess the general structure 1. The circles represent
ionic and/or nonionic headgroups, and the wavy lines, unbranched hydrocarbon chains. Thus these surfactants contain a central headgroup connected to two flanking headgroups by hydrocarbon chains. For ease of discussion, we use the term “shamrock” to describe surfactants of structure 1, denoting their triple-headed character and reflecting the fact that shamrocks have leaflets in groups of three. Note that the major lipophilic character of shamrock surfactants is provided by the two hydrocarbon chains linking the three headgroups, and not by longchain alkyl groups appended to the linking hydrocarbon chains or the headgroups. We have synthesized and characterized shamrock surfactants 2 and 3. The former surfactants contain three quaternary ammonium headgroups, and the latter, a central dithiophosphate headgroup and two flanking quaternary ammonium headgroups.
* Author to whom correspondence should be addressed. E-mail:
[email protected]. † University of Wyoming. ‡ Emory University. (1) Myers, D. Surfactant Science and Technology, 2nd ed.; VCH Publishers: New York, 1992; (a) Chapter 1; (b) Chapter 3. (2) Liposomes: From Physical Structure to Therapeutic Applications; Knight, C. G., Ed.; Elsevier/North-Holland: New York, 1981. (3) (a) Yang, Y.-C.; Baker, J. A.; Ward, J. R. Chem. Rev. 1992, 92, 1729. (b) Yang, Y.-C. Chem. Ind. (London) 1995, 334. (c) Yang, Y.-C. Acc. Chem. Res. 1999, 32, 109. (4) Novel Surfactants, 2nd ed.; Holmberg, K., Ed.; Marcel Dekker: New York, 2003. (5) For examples, see (a) Jaeger, D. A. Supramol. Chem. 1995, 5, 27. (b) Holmberg, K. Curr. Opin. Colloid Interface Sci. 1996, 1, 572-579. (c) Jong, L. I.; Abbott, N. L. Langmuir 2000, 16, 5533. (6) Holmberg, K. In Novel Surfactants, 2nd ed.; Holmberg, K., Ed.; Marcel Dekker: New York, 2003; Chapter 11. (7) For reviews, see (a) Menger, F. M.; Keiper, J. S. Angew. Chem., Int. Ed. 2000, 39, 1906. (b) Rosen, M. J.; Tracy, D. J. J. Surfactants Deterg. 1998, 1, 547. (c) Zana, R. Curr. Opin. Colloid Interface Sci. 1996, 1, 566. (8) For a partial listing of applications, see Menger, F. M.; Mbadugha, B. J. Am. Chem. Soc. 2001, 123, 875.
Surfactants 4 have been reported,9 which upon ionization of their carboxyl group correspond to shamrock surfactants. Other known surfactants, such as 510 and 6,11 contain three headgroups, but they are not shamrock surfactants.12 The major lipophilic character of these (9) Menger, F. M.; Mounier, C. E. J. Am. Chem. Soc. 1993, 115, 12222. (10) Yoshimura, T.; Yoshida, H.; Ohno, A.; Esumi, K. J. Colloid Interface Sci. 2003, 267, 167. (11) Masuyama, A.; Yokota, M.; Zhu, Y.-P.; Kida, T.; Nakatsuji, Y. J. Chem. Soc., Chem. Commun. 1994, 1435. (12) For other examples of triple-headed surfactants, see (a) Haldar, J.; Aswal, V. K.; Goyal, P. S.; Bhattacharya, S. Angew. Chem., Int. Ed. 2001, 40, 1228. (b) Willemen, H. M.; de Smet, L. C. P. M.; Koudijs, A.; Stuart, M. C. A.; Heikamp-de Jong, I. G. A. M.; Marcelis, A. T. M.; Sudho¨lter, E. J. R. Angew. Chem., Int. Ed. 2002, 41, 4275.
10.1021/la0481391 CCC: $27.50 © 2004 American Chemical Society Published on Web 10/29/2004
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surfactants is provided by three long-chain alkyl groups, whereas that of shamrock surfactants 2 and 3 is provided by the two hydrocarbon chains linking the three headgroups. Shamrock surfactants are structurally related to, but are more complex than, bola surfactants, which contain two headgroups connected by one or more hydrocarbon chains, as in 7.13 Also related, but structurally distinct, are hyperextended surfactants, such as 8,14 and ionene polyelectrolytes, such as 9.15
Results and Discussion Surfactant 2a was synthesized in three steps as illustrated (eq 1), starting with the conversion of com-
mercially available diamine 10 into triamine 11 with Raney Ni, followed by its reductive methylation to afford triamine 12. Then, 12 was methylated with methyl iodide to give 2a. Surfactant 2a was converted into surfactant 2b by a metathesis reaction with silver chloride (eq 2). In this
reaction, the iodide ion of 2a and silver chloride combined to give chloride ion and the less-soluble silver iodide. Compound 14, a homologue of surfactant 2a, was prepared by the permethylation of commercially available 13 with methyl iodide (eq 3). Surfactant 2a could be
(13) Menger, F. M.; Wrenn, S. J. Phys. Chem. 1974, 78, 1387. (14) Menger, F. M.; Yamasaki, Y. J. Am. Chem. Soc. 1993, 115, 3840. (15) Soldi, V.; Erismann, N. D. M.; Quina, F. H. J. Am. Chem. Soc. 1988, 110, 5137.
Jaeger et al.
obtained from triamine 11 by the same route, but it was contaminated with sodium iodide, which was difficult to remove due to the similar solubilities of 2a and sodium iodide in a variety of solvents. Surfactants 3 were synthesized in four steps as illustrated (eq 4), starting with the conversion of com-
mercially available diol 15 into bromo alcohol 16. The nucleophilic substitution reaction of tertiary amine R3N (R ) Pr, Bu) with 16 gave surfactant 17, which was converted into compound 18 by reaction with phosphorus pentasulfide. Then, a dichloromethane solution of 18 was washed with water to give 3. In this process, the dithiophosphoric acid unit of 18 ionizes, with the net loss of hydrogen bromide.16 Note that surfactants 3 contain propyl/butyl groups on their quaternary ammonium nitrogens instead of methyl groups, which are generally used as the short-chain components of quaternary ammonium surfactants. Methyl groups would be more susceptible than propyl/butyl groups to the possibility of SN2 substitution at carbon by the nucleophilic dithiophosphate headgroup.17 Average relative rates for alkyl substrates in SN2 reactions are Me (30); Et (1); Pr (0.4); Bu (0.4).18 The characterization of surfactants 2 and 3, which were obtained as oils/waxes, included the determination of their Krafft temperatures (Tk) and critical aggregation concentrations (cac). Although the solubility of an ionic surfactant in water generally increases with increasing temperature, it typically increases dramatically at a point known as the Krafft temperature (Tk).1b Aggregation of an ionic surfactant into assemblies can occur only above its Tk and cac values. Aggregated surfactants 2 and 3 in water were studied by 1H NMR spectroscopy, dynamic laser light scattering (DLLS), and phase-contrast optical microscopy. Aqueous 2b was also studied by cryo-etch high-resolution scanning electron microscopy (cryo-etch HRSEM). Since concentrations of up to 1.0 M 14 did not lower the surface tension of water at 23 °C, suggesting the lack of aggregation, its aqueous solutions were not characterized further. The Tk values of surfactants 2 and 3 are
