J . Phys. Chem. 1989, 93, 4861-4867 will likely cause microstructure to vanish; the resulting solution having no distinct oil and water domains could not be considered a topologically ordered microemulsion. Acknowledgment. We sincerely thank J. P. Canselier for assistance with the synthesis of C3OC2OC3 and J. F. Billman for assistance in obtaining the SAXS results. We acknowledge
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discussions with A. Sporer, M. Kahlweit, and R. Strey. This work was supported by the IBM Corp. Registry No. C20C20C2,629-14-1; I'-c,Oc3, 108-20-3; C40C20C4, 112-48-1;c4oc4, 142-96-1; csocs, 693-65-2; c6oc6, 112-58-3; Cl2E6, 3055-96-7; C4E2, 112-34-5;C4E3, 143-22-6; C3OC4OC3, 91 179-75-8; C30C20C3,18854-56-3; C4E,, 11 1-76-2; toluene, 108-88-3; anisole, 100-66-3;cyclohexane, 110-82-7; octane, 11 1-65-9; decane, 124-18-5.
Hydrocarbon Chain Conformation of Bipolar Surfactants in Micelles. A Magnetic Field Dependent 13C and 14N NMR Spin-Lattice Relaxation and Nuclear Overhauser Effect Study of N ,"-1 ,PO-Eicosanediyibis(triethyiammonium bromide) Tuck C. Wong,*.+vtK. Ikeda,l K. Meguro,O 0. Soderman,f U. OIsson,*and B. Lindmanf Physical Chemistry 1 , Chemical Center, University of Lund, Lund, Sweden, and Department of Applied Chemistry, Institute of Colloid and Interface Science, Science University of Tokyo, Tokyo 162, Japan (Received: July 27, 1988; In Final Form: November 23, 1988)
A field-dependent I3C and I4N spin-lattice relaxation and nuclear Overhauser effect study has been performed on an aqueous micellar system of an a,w-bifunctional surfactant, N,N'- 1,2O-eicosanediylbis(triethylammonium bromide). The I3C relaxation rates and NOESwere analyzed on the basis of the "two-step" model, and the fast correlation time and order parameter for each carbon segment on the hydrocarbon chain and an overall slow correlation time for the whole micelles were obtained. The almost flat order parameter profile for the hydrocarbon chain suggests that the surfactant chains adopt a predominantly stretched form in micelles. This is in contradiction to the conclusion drawn from the more indirect chemical relaxation results for similar systems. The slow correlation time, which primarily describes the time scale for the tumbling of the micelles, is found to be about 2-3 ns and rather concentration independent,indicating rather small micellar aggregates. Several important properties of this micellar system, such as the small micellar size (low aggregation number) and the peculiarly low capacity for solubilizing hydrophobic substances, can be explained by the conformational properties of the surfactant molecules in the micelles.
Introduction There have been a number of studies of the physical properties of a,o-bifunctional (or bolaform) surfactants in solution.'-1° The ionic a,w-type surfactants usually exhibit significantly different properties in several respects from those of "normal" surfactants with a single head group on a single hydrocarbon chain. The salient differences are as follows: First, the propensity of micelle formation of these a,w-type surfactants is generally lower;5*6the size of the micelles formed is relatively small;6s10and the cmc of these surfactants is generally higher than that of single-head surfactants of comparable chain length.4j6 Second, these a,w-type surfactancts have been shown to adopt a folded (or wicketlike) conformation at the air-water interface!,' Third, it was suggested in several studies that these surfactants may also exist predominantly in a folded conformation in However, with regard to the last point, there has been no study that provides direct evidence on the conformation of these surfactants in micellar solutions. There have been several studies of the aliphatic chain conformation of several bifunctional surfactants in liquid crystalline phases."-14 The knowledge of the principal conformation these surfactants adopt in micelles is very important because it may provide fundamental explanations for some important properties of these micelles, most notably, the small micellar size, the phase behavior, and the extremely low capacity for solubilizing hydrophobic s ~ b s t a n c e s . ~ ~ It has been demonstrated16 that, for isotropic solutions containing molecular aggregates where there is no static dipolar or quadrupolar interaction, analysis of multifield N M R relaxation rates and nuclear Overhauser effect (NOE) provides direct in-
'
On leave from the Department of Chemistry, University of Missouri, Columbia, MO 6521 1. *University of Lund. Science University of Tokyo.
formation on the conformation of the surfactant molecules via the determination of the order parameters of the various segments of the molecules. We have, therefore, undertaken a multifield I3C spin-lattice relaxation rate and N O E study of the aqueous micellar solutions formed by a cationic a,w-type surfactant, N,N'- 1,20-eicosanediylbis(triethylammonium bromide) (Cz0(NEt3)*Br2,hereafter referred to as C&t6), the cmc, self-diffusion, and the phase diagram of which have recently been investigated in this 1aborat0ry.I~I4N spin-lattice and spin-spin relaxation rates (1) Elworthy, P. H. J . Pharm. Pharmacol. 1959, 11, 557. (2) Elworthy, P. H. J . Pharm. Pharmacol. 1959,11, 624. (3) Ueno, M.; Hikota, T.; Mitama, T.; Meguro, K.J. Am. Oil Chem. Soc. 1972,49,250. Ueno, M.; Yamamoto, S.; Meguro, K.J. Am. Oil Chem. Soc. 1974, 51, 373. (4) Menger, F. M.; Wrenn, S. J. Phys. Chem. 1974, 78, 1387. ( 5 ) Johnson, J. R.; Fleming, R. J . Phys. Chem. 1975, 79, 2327. (6) (a) Yiv, S.; Kale, K.M.; Lang, J.; Zana, R. J . Phys. Chem. 1976, 80, 2651. (b) Yiv, S.; Zana, R. J. Colloid Interface Sci. 1980, 77,449. (c) Zana, R.; Yiv, S.; Kale, K. M. J . Colloid Interface Sci. 1980, 77, 456. (7) Meguro, K.; Ikeda, K.;Otsuji, A.; Taya, M.; Yasuda, M.; Esumi, K. J . Colloid Interface Sci. 1987, 118, 372. (8) Zana, R.; Muto, Y.; Esumi, K.;Meguro, K.J . Colloid Interface Sci.
1988, 123, 502. (9) Cipiciani, A,; Fracassini, M. C.; Germani, R.; Savelli, G.;Bunton, C. A. J . Chem. SOC.,Perkin Trans. 2 1987, 547. (10) McKenzie, D. C.; Bunton, C. A,; Nicoli, D. F.; Savelli, G. J . Phys. Chem. 1987, 91, 5709. (11) Gallot, B. R. Mol. Cryst. Liq. Cryst. 1971, 13, 323. (12) Seelig, J.; Limacher, H.; Bader, P. J. Am. Chem. SOC.1972, 94,6364. (13) Forrest, B. J.; de Carvalho, L. H.; Reeves, L. W.; Rodger, C. J. Am. Chem. SOC.1981, 103, 245. (14) Gutman, H.; Luz, Z.; Charvolin, J.; Loewenstein, A. Liq. 1987, - Cryst. . 2, 739. (15) Ikeda, K.; Khan, A,; Meguro, K.; Lindman, B., to be submitted for
publication.
(16) See, for example: Lindman, B.; Saerman, 0.;Wennerstrom, H. In Surfactant Solutions, New Methods of Itwestigation; Zana, R., Ed.; Dekker: New York. 1987.
0022-365418912093-4861$01.50/0 0 1989 American Chemical Societv
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The Journal of Physical Chemistry, Vol. 93, No. 12. 19‘89
of the NEt, head group have also been measured to complement the I3C study.
Theory The I3C spin-lattice relaxation rate (R,) and the nuclear Overhauser enhancement, 7 for each carbon segment, under a proton decoupling condition, are given by1’
where the dipolar interaction between the carbon nucleus and the directly bonded protons is considered to be the only significant relaxation mechanism. For I4N, assuming that the fluctuating quadrupolar interaction is the dominating relaxation mechanism, the spin-lattice and spinspin relaxation rates, R I and R2, are given byI8
+
Rl = ( 3 ~ ~ / 4 0 ) ~ * ( 2 j ( ~Sj(2wN)) ,)
(3)
+
R2 = ( 3 ~ ~ / 4 0 ) ~ ~ ( 3 + 3 (50J )( ~ p 4 ) 2 1 ( 2 ~ ~ ) ) (4) In eq 1-4, x is the quadrupolar coupling constant, go is the permeability of the vacuum, yH and yc represent the magnetogyric ratios of proton and carbon, respectively, and r is the C-H bond length and is set to 1.09 A throughout the calculations. J(w) are the various reduced spectral density functions, wc, wH, and wN are the Larmor frequencies of the respective nuclei at the actual magnetic field of measurement, N is the number of protons directly bonded to the carbon nucleus, and h is the reduced Planck’s constant. In eq 3 and 4, it is assumed that the asymmetry parameter of the electric field gradient (EFG) tensor is equal to zero. In isotropic micellar solutions, it is common to observe significant magnetic field dependence in the 13C R, and 7 and substantial reduction of 7 from the extreme narrowing value of 1.99. These observations have been interpreted by the “two-step” modellg for many such systems. In this model, the molecular dynamics of the system is described by a fast and slightly anisotropic internal motion of each segment of the molecule, superimposed on a slow isotropic motion for the whole molecule. If the slow motion is described by a single exponential function, and the fast motion is in the extreme narrowing limit (Le., rcfw
