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Pyrophosphate-Based Gemini Surfactants David A. Jaeger,* Yapin Wang, and Richard L. Pennington Department of Chemistry, University of Wyoming, Laramie, Wyoming 82071-3838 Received July 1, 2002. In Final Form: August 26, 2002 Two series of pyrophosphate-based gemini surfactants [diimidazolium (1) and disodium P,P′-dialkyl pyrophosphate (2) (a, R ) 1-octyl; b, R ) 1-dodecyl; c, R ) 2-dodecyl)] were synthesized, along with a series of imidazolium monoalkyl phosphate surfactants (3a-c) for comparison. The surfactants were characterized by measurement of their Krafft temperatures (Tk) and critical aggregation concentrations (cac’s). The Tk and cac values of the pyrophosphate surfactants are lower than those of their conventional phosphate surfactant counterparts. Aggregated surfactants 1 and 3 in D2O were characterized by 1H and 31P NMR spectroscopy, and solutions of 1c and 3c in H2O were characterized by phase-contrast optical microscopy and cryogenic high-resolution scanning electron microscopy (cryo-HRSEM). By optical microscopy, fiberlike aggregates were observed for gemini surfactant 1c, and both fibers and rod/tubule-like aggregates were observed for its single-chain counterpart 3c. By cryo-HRSEM, lamellar ice was observed for both 1c and 3c. Also, a single-crystal X-ray diffraction study of 1b was performed, and its cleavable nature was demonstrated.
Introduction Gemini surfactants, which nominally contain, in order, a long hydrocarbon chain, an ionic or neutral headgroup, a nonpolar or polar spacer, an ionic or neutral headgroup, and a long hydrocarbon chain, have been the subject of numerous reports in recent years.1 They generally display unique properties2 that can result in improved performance in a variety of applications,3 compared to their conventional surfactant counterparts, which contain one headgroup and one hydrocarbon chain. Herein we report the synthesis and characterization of two new series of new gemini surfactants, 1 and 2, which are based on the pyrophosphate (diphosphate) group. Note that the spacer group of geminis 1 and 2 is an oxygen atom. Thus these gemini surfactants contain the shortest possible spacer, namely, a single atom. Single-chain surfactants 3 were prepared for comparison. Pyrophosphate-based surfactants 1 and 2 also represent a new class of cleavable surfactants.4 In particular, the cleavable nature of 1b was demonstrated. A few examples of related pyrophosphatebased compounds have been reported previously.5 They were prepared by routes other than those employed below, and their surfactant properties were not evaluated.
first, the reaction of 2 equiv of 4 with 1 equiv of N,N,N′,N′tetramethylchloroformamidinium chloride (6) gave the symmetrical dialkyl pyrophosphoric acid 7, and in the second step, the neutralization of 7 with NaOH yielded 2. Surfactants 3 were obtained from the reaction of acids 4 with imidazole. Note that two diastereomers are possible for each of 1c and 2c. Although only one of the two (meso or d,l) was obtained for each purified surfactant, as determined by 31P NMR, evidence for the formation of a second diastereomer was obtained in the preparation of 1c (see the Experimental Section).
Surfactant Characterization. Surfactants 1-3 were characterized by measurement of their Krafft temperatures and critical aggregation concentrations (cac’s) in H2O. Aggregated surfactants 1 and 3 were characterized
Results and Discussion Syntheses. Gemini surfactants 1 were prepared in one step from the reaction of 2 equiv of monoalkyl phosphoric acid 4 with 1 equiv of 1,1′-carbonyldiimidazole (5) (eq 1). Surfactants 2 were synthesized in two steps (eq 2). In the
(1) 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. (2) Rosen, M. J. CHEMTECH 1993, 23, 30. (3) For a partial listing of applications, see: Menger, F. M.; Mbadugha, B. J. Am. Chem. Soc. 2001, 123, 875. (4) For examples, see: (a) Jaeger, D. A. Supramol. Chem. 1995, 5, 27. (b) Holmberg, K. Curr. Opin. Colloid Interface Sci. 1996, 1, 572. (5) For examples, see: (a) Sprecher, M.; Breslow, R.; PhilosofOppenheimer, R.; Chavet, E. Tetrahedron 1999, 55, 5465. (b) Ramirez, F.; Marecek, J. F.; Chaw, Y.; McCaffrey, T. Synthesis 1978, 519. (c) Cremlyn, R. J. W.; Olsson, N. A. J. Chem. Soc. C 1970, 1889. (d) Cocker, J. D.; Elks, J.; May, P. J.; Nice, F. A.; Phillipps, G. H.; Wall, W. F. J. Med. Chem. 1965, 8, 417.
10.1021/la020601z CCC: $22.00 © 2002 American Chemical Society Published on Web 10/30/2002
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Jaeger et al.
Table 1. Values of cac, γcac, C20, and Tk for Surfactants in H2Oa surfactant
103 cac (M)
γcac (mN/m)
103 C20 (M)
Tk (°C)
1a 1b 1c 2a 2b 2c 3a 3b 3c 8
22 1.4 1.4 28
30 31 32 32
9.6 0.57 0.35 9.0
80 5.8b 9.8 140c,d
30
