Langmuir 1991, 7, 2602-2606
2602
Synthesis and Characterization of a Homologous Series of Zwitterionic Surfactants Based on Phosphocholinet Peter M. Macdonald,' John R. Rydall, and Sigrid C. Kuebler Department of Chemistry and Erindale College, University of Toronto, 80 St. George Street, Toronto, Ontario, Canada M5S 1 A l
FranCoise M. Winnik Xerox Research Centre of Canada, 2660 S p e a k m a n Drive, Mississauga,
Ontario, Canada L5K 2L1
Received April 16, 1991. In Final Form: August 1, 1991 Three homologous alkyl trimethylammonioethyl phosphates have been synthesized starting from phosphoryl chloride, with oxazaphospholane intermediates. The critical micelle concentrations (cmc) of the surfactants, determined by using a fluorescence probe technique, were 1.3 x 10-3 mol L-1 for dodecyl (DDPC), 1.2 X 1 0 4 mol L-1 for tetradecyl (TDPC), and 3.2 X 10-6 mol L-1 for hexadecyl trimethylammonioethyl phosphate (HDPC). The individual contributions of the alkyl chain and the trimethylammonioethyl phosphate headgroup to the free energy change of micellization were determined to be comparable to the corresponding values for the micellization of other zwitterionic surfactants. The hexadecyl homologue formed a stable monolayer at an air-water interface, but DDPC and TDPC did not.
Introduction
We describe here the synthesis and characterization of a homologous series of zwitterionic surfactants in which the hydrophilic moiety consists of the trimethylammonioethyl phosphate (i.e. phosphocholine) group and the hydrophobic moiety consists of either an n-dodecyl, n-tetradecyl or n-hexadecyl alkyl chain. These phosphocholine surfactants are related structurally t o t h e naturally occurring "lecithins". Certain properties of the phosphocholine headgroup of lecithin make i t desirable to package phosphocholine in surfactant form for use in surfactant applications. These include the ability of phosphocholine to respond to and to report on surface electrostatic potentials1t2 as well as its favorable biocompatibility properties.3
The three homologous phosphocholine surfactants synthesized here were characterized by using fluorescence probe techniques to determine their critical micelle concentrations (cmc) and to estimate the fluidity and polarity of t h e micellar core. In addition, their ability to form stable monolayers at an air-water interface has been investigated using a film balance. Experimental Section Materials. Dodecanol, tetradecanol, hexadecanol, N-methylethanolamine, triethylamine, phosphoryl chloride, and methyl iodide were purchased from Aldrich ChemicalCo. (Milwaukee, WI). Spectral grade solvents were used throughout and, when necessary, were further dried by standard means. Water was deionized with a Millipore Milli-Q purification system. The long chain alcohols were distilled under vacuum prior to their use. The phosphoryl chloride fraction distilling at bp 105-107 "C was collected for use. Bis(1-pyreny1)methylether (Dipyme) was a gift of Professor M. A. Winnik, University of Toronto, and had
* To whom correspondence should be addressed.
Supported by the National Science and Engineering Research Council of Canadaand by the Ontario Centre for MaterialsResearch. (1) Macdonald, P. M.; Leisen, J.; Marassi, F. M. Biochemistry 1991, 30, 3668.
(2) Seelig,J.;Macdonald, P. M.; Scherer,P. G. Biochemistry 1987,26, 7535. (3) Hayward, J. A.; Chapman, D.Biomaterials 1984,5, 135.
been prepared and purified as described previously.' Pyrene (Py)was purchased from Aldrich Chemical Co. (Milwaukee,WI) and was purified by repeated recrystallizations from absolute ethanol and subsequent sublimation. The sulfobetaine surfactantsN-octyl-NJV-dimethylammonio-1-propanesulfonate (OPS), N-decyl-N,N-dimethylammonio-1-propanesulfonate (DPS), N-dodecyl-NJV-dimethylammonio-1-propanesulfonate (DDPS), N-tetradecyl-N~-dimethyla"onio-1-propaneaulfonate(TDPS), and N-hexadecyl-N,N-dimethylammonio-l-propanesulfonate (HDPS) were purchased from Sigma Chemical Co. (St. Louis, MO) and were used without further purification. Instrumentation. Proton NMR spectra were recorded at 200 MHz using a Varian Gemini 200 NMR spectrometer and were referenced to external tetramethylsilane. Fluorescence spectra were measured with a SPEX Fluorolog 212 spectrometer equipped with a DM3000F data system. Pressure-area isotherms were obtained using a Lauda Model 1974 monolayer trough (Sybron-Brinkmann, Toronto, Ontario, Canada) which was modified such that a Wilhelmy plate could be used for pressure transduction. Methods. Fluorescence emission spectra of pyrene were obtained using an excitation wavelengthof 336 nm, with excitation and emission slit widths set to 3.6 and 0.9 nm, respectively. The ratio [Zl/Z,lPy was calculated as the ratio of the pyrene emission intensities at 376 and 386 nm. For measurements with dipyme the excitation wavelength was 348 nm. The quantity [Z1/Zs]DP was calculated as the ratio of the dipyme emission intensities at 378 and 388 nm. The ratioZ,/Z, was calculated from the emission intensities at 495 and 399 nm. Emission spectra were not corrected. Measurements were performed at 20 OC. Solutions were not degassed. Solutions for the determination of cmc were prepared by dilution of stock solutions of surfactant in pyrenesaturated water, filtered before use. The solutions of dipyme in surfactant were prepared by adding a concentrated solution of dipyme (5pL, 1.3 x 10" mol L-l in acetone) to aqueous solutions of the surfactants at concentrations above their cmc. The solutions were sonicated for 10 min in an ultrasonic bath and were kept at room temperature in the dark until equilibrated (2-7 days). For pressure-area isotherm determinations the surfactante were first dissolved at a concentration of 1.0 mg mL-1 in 9:l (v/v) hexane/ethanol. An appropriate volume of this solution (corresponding to 6.48 x 1016 molecules) was deposited onto the aqueous subphase (0.1 M KC1 plus 0.01 M HEPES buffer, pH (4) Georgescauld, D.;Dasmawz, J. P.; Lapouyade, R.; Babeau, A.; Richard, H.; Winnik, M. A. Photochem. Photobrol. 1980,32, 639.
0 1991 American Chemical Society
Langmuir, Vol. 7, No. 11, 1991 2603
Synthesis of Zwitterionic Surfactants
II
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CHS-NH-(CH~)~-OH
-
I CH3/N\
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