Langmuir 2006, 22, 1555-1560
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Shamrock Surfactants with Terminal Carboxylate Headgroups and a Central Phosphorodithioate or Quaternary Ammonium Headgroup David A. Jaeger,*,† Alvaro Mendoza,† 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 October 21, 2005 Surfactants 3 (tripotassium O,O′-di-[11-(carboxylato)undecyl]phosphorodithioate) and 4 (sodium 12-[dimethyl(11-carboxylatoundecyl)ammonio]dodecanoate), which are new shamrock surfactants, were prepared and characterized. Shamrock surfactants represent a novel class of surfactants that contain a central headgroup connected to two flanking headgroups by hydrocarbon chains; they do not contain long-chain alkyl groups. Surfactants 3 and 4 were characterized in water by measurement of their Krafft temperatures and critical aggregation concentrations, and their aggregates were studied by 1H and 31P NMR spectroscopy, dynamic laser light scattering, and phase-contrast optical microscopy. Aqueous 3 and 4 were also studied by cryoetch high-resolution scanning electron microscopy, which revealed fences with interposed lacelike patterns for the former and compartments formed by irregular fences for the latter. Coacervates were likely formed upon the undisturbed hydration of 3 and 4, as determined by phase-contrast optical microscopy.
Introduction
Scheme 1
surfactants,1
The synthesis and characterization of novel in the pursuit of interesting and unusual properties, are worthwhile activities, given the importance of surfactants in applications spanning drug delivery,2 oil recovery,3 and chemical agent decontamination.4 Recently, we reported5 the synthesis and characterization of 1 and 2, which are among the first examples of “shamrock” surfactants, a new class of surfactants that contain a central headgroup connected to two flanking headgroups by hydrocarbon chains. Unlike most surfactants, shamrocks do not contain long-chain alkyl groups; their major lipophilic character is provided by the two hydrocarbon chains linking the three headgroups. Herein, we report the synthesis and characterization of new shamrock surfactants 3 and 4. Like surfactants 1, the former surfactant contains a central phosphorodithioate headgroup, and like surfactants 2, the latter contains a central quaternary ammonium headgroup. Both surfactants 3 and 4 contain terminal carboxylate headgroups, as opposed to the quaternary ammonium headgroups of 1 and 2.
Results and Discussion Surfactant 3 was synthesized in three steps as illustrated in Scheme 1, starting with the conversion of commercially available * To whom correspondence should be addressed. E-mail:
[email protected]. † University of Wyoming. ‡ Emory University. (1) NoVel Surfactants, 2nd ed.; Holmberg, K., Ed.; Marcel Dekker: New York, 2003. (2) Knight, C. G. Liposomes: From Physical Structure to Therapeutic Applications; Elsevier/North-Holland: New York, 1981. (3) Myers, D. Surfactant Science and Technology; 2nd ed.; VCH Publishers: New York, 1992; Chapter 1.
Scheme 2
ω-hydroxycarboxylic acid 5 into its methyl ester 6. Then, the reaction of 6 with phosphorus pentasulfide gave compound 7, which was converted into 3 by neutralization of its phosphorodithioic acid group and saponification of its ester groups with potassium hydroxide in aqueous methanol. Surfactant 4 was prepared by the addition of 2 equiv of sodium hydroxide to an aqueous dispersion of surfactant 8 as shown in Scheme 2, and it was characterized as described below without isolation as a mixture with 1 equiv of sodium chloride. The synthesis of 8, summarized in Scheme 3, started with the esterification of ω-hydroxycarboxylic acid 5 to give its ethyl ester 9, which was oxidized to ω-oxocarboxylate ester 10. Then, the reductive alkylation of ω-aminocarboxylate ester 12, prepared from commercially available ω-aminocarboxylic acid 11, with 10 gave amino diester 13. Methylation of 13 with methyl iodide gave 14, which was converted into 15 by a metathesis reaction with silver chloride. In this reaction, the iodide ion of 14 and silver chloride combined to give chloride ion and the less soluble silver iodide. Then, the hydrolysis of the ester groups of 15 with hydrochloric acid gave surfactant 8. As noted above, surfactant 4 was obtained and characterized as a mixture with 1 equiv of sodium chloride. The characterization of surfactants 3 and 4 included the determination of their Krafft temperatures (Tk) and critical aggregation concentrations (cac). (4) (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. (5) Jaeger, D. A.; Zeng, X.; Apkarian, R. P. Langmuir 2004, 20, 10427.
10.1021/la052838b CCC: $33.50 © 2006 American Chemical Society Published on Web 01/14/2006
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Figure 2. Phase-contrast photomicrographs of the hydration at 23 °C of surfactant 3 recorded at (a) 6 min and (b) 103 min after the addition of water; scale bar ) 50 µm. Figure 1. Plots of surface tension for surfactants 3 (circles) and 4 (squares) in water. Scheme 3
Figure 3. Phase-contrast photomicrographs of the hydration at 23 °C of surfactant 4 recorded at (a) 7 min and (b) 8 min after the addition of water; scale bar ) 50 µm.
The solubility of an ionic surfactant in water generally increases with increasing temperature, but it typically increases dramatically at a point known as the Krafft temperature.6 Aggregation of an ionic surfactant into assemblies can occur only above its Tk and cac values. Aggregated surfactants 3 and 4 in water were studied by 1H NMR spectroscopy, dynamic laser light scattering (DLLS), and phase-contrast optical microscopy. Aqueous 3 and 4 were also studied by cryoetch high-resolution scanning electron microscopy (cryoetch HRSEM). The Tk values of surfactants 3 and 4 are
