Langmuir 2001, 17, 2545-2547
Alkyltris(hydroxymethyl)phosphonium Halide Surfactants
2545 Scheme 1
David A. Jaeger* and Alexander K. Zelenin Department of Chemistry, University of Wyoming, Laramie, Wyoming 82071 Received November 6, 2000. In Final Form: January 22, 2001
Introduction There have been considerably fewer studies and applications of quaternary phosphonium surfactants than of quaternary ammonium surfactants. In part, the lesser attention to the former derives from their lesser stability under a variety of conditions1 and from their generally more complicated syntheses. Nevertheless, quaternary phosphonium surfactants are interesting and important compounds. For example, they have been successfully used as catalysts in phase transfer catalysis.2 Herein, we report the synthesis and characterization of surfactants 1a-c, which are examples of a new class of functionalized quaternary phosphonium surfactants.3 Also, 1c was converted into 2-4; 3 and 4 are precursors to long-chain trialkyl phosphines.
Results and Discussion Surfactants 1 were synthesized as outlined in Scheme 1. Diethyl alkylphosphonates 5, obtained from the corresponding 1-iodoalkanes and triethyl phosphite by the Arbuzov reaction,4 were reduced to monoalkyl phosphines 6, which were converted into 1 by reaction with formaldehyde and hydrochloric or hydrobromic acid in MeOHH2O.5 Surfactants 1, which contain hydroxymethyl groups, are less stable than unfunctionalized quaternary phosphonium surfactants.1 As pure solids, 1a-c are stable at 0 °C for several weeks, but they decompose upon melting (see Experimental Section). Also, they decompose slowly in water. Solutions of 1a-c in D2O (0.025 M) at 23 °C, prepared with sonication for 15 min at 23 °C, were monitored by 31P NMR. Each surfactant (δ ca. 29) was (1) Hudson, R. F. Structure and Mechanism in Organo-Phosphorus Chemistry; Academic Press: New York, 1965; Chapter 7. (2) Dehmlow, E. V.; Dehmlow, S. S. Phase Transfer Catalysis, 3rd ed.; VCH: New York, 1993. (3) For examples of other functionalized quaternary phosphonium surfactants, see: (a) Jaeger, D. A.; Bolikal, D. J. Org. Chem. 1985, 50, 4635. (b) Jaeger, D. A.; Bolikal, D. J. Org. Chem. 1986, 51, 1350. (c) Jaeger, D. A.; Bolikal, D. J. Org. Chem. 1986, 51, 1352. (4) Kosolapoff, G. M. J. Am. Chem. Soc. 1954, 76, 615. (5) (a) Gali, H.; Karra, S. R.; Reddy, V. S.; Katti, K. V. Angew. Chem., Int. Ed. Engl. 1999, 38, 2020. (b) Karra, S. R.; Schibli, R.; Gali, H.; Katti, K. V.; Hoffman, T. J.; Higginbotham, C.; Sieckman, G. L.; Volkert, W. A. Bioconjugate Chem. 1999, 10, 254. (c) Gali, H.; Prabhu, K. R.; Karra, S. R.; Katti, K. V. J. Org. Chem. 2000, 65, 676.
stable for g19 h at 23 °C. Thereafter, two signals began to develop at δ ca. -26 and ca. 53, which likely correspond to 7 and 8, based upon comparisons with 31P NMR chemical shifts for other alkylbis(hydroxymethyl)phosphines and their oxidation products.5 Uniformly, the characterization of surfactants 1 below was performed with fresh aqueous solutions, prior to any decomposition that was detectable by 31P NMR spectroscopy.
Surfactants 1 were characterized by Krafft temperature (Tk) and critical micelle concentration (cmc) measurements. Although the solubility of an ionic surfactant in H2O generally increases with increasing temperature, it typically increases dramatically at a point known as the Krafft temperature. Aggregation of a surfactant can occur only above its Tk value and above its critical aggregation concentration.6 Aggregated surfactants 1 were characterized by dynamic laser light scattering (DLLS) and 1H NMR spectroscopy. In H2O, the Tk values of 1a-c are
