Langmuir 1999, 15, 2669-2673
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Order Parameter Profile of Perfluorinated Chains in a Micelle I. Furo´* and R. Sitnikov Division of Physical Chemistry, Department of Chemistry, Royal Institute of Technology, SE-10044 Stockholm, Sweden Received May 14, 1998. In Final Form: January 21, 1999 Cesium perfluorooctanoate surfactant molecules, dissolved in water and aggregated into micelles, are investigated by 19F-decoupled 13C NMR relaxation experiments at three different magnetic fields. The data estimate the order parameter profile of the perfluorooctanoate chain which differs both in magnitude and in shape from the order parameter profile of protonated octanoate.
Introduction Among the many contributions of nuclear magnetic resonance (NMR) to the field of complex fluids, establishing the decay of the interface-induced anisotropy of the local molecular motions along the hydrophobic tail of a surfactant is one of the most fundamental. The picture, obtained on a wide variety of protonated surfactants, is universal: a selected bond order parameter S, most often that of the C-H bond of the methylene groups,1-11 decays from a certain value (about 0.2 for SCH) at the interface to an almost negligible S at the terminal methyl group. This feature, often called the order parameter profile, is very illustrative; it embodies our mental image of a surfactant aggregate, with a large surface anisotropy and a liquid hydrocarbon-like interior.12,13 Moreover, experimental order parameter profiles can be compared to those obtained in various theoretical models;13-18 a close match is an important test of a realistic model. Thus, order parameter information is essential for understanding the aggregate structure and the nature of the interactions that are responsible for it. * Telephone: +46 8 7908592. Fax: +46 8 7908207. E-mail:
[email protected]. (1) Seelig, J. Q. Rev. Biophys. 1977, 10, 353. (2) Wennerstro¨m, H.; Lindman, B.; So¨derman, O.; Drakenberg, T.; Rosenholm, J. B. J. Am. Chem. Soc. 1979, 101, 6860. (3) Klason, T.; Henriksson, U. In Solution Behavior of Surfactants; Mittal, K. L., Fendler, E. J., Eds.; Plenum Press: New York, 1982; Vol. 1, p 417. (4) Canet, D.; Marchal, J. P.; Ne´ry, H.; Robin-Lherbier, B. J. Colloid Interface Sci. 1983, 93, 241. (5) Davis, J. H. Biochim. Biophys. Acta 1983, 737, 117. (6) Ahlna¨s, T.; So¨derman, O.; Walderhaug, H.; Lindman, B. In Surfactants in Solution; Mittal, K. L., Lindman, B., Eds.; Plenum Press: New York, 1984; Vol. 1, p 107. (7) Walderhaug, H.; So¨derman, O.; Stilbs, P. J. Phys. Chem. 1984, 88, 1655. (8) Ne´ry, H.; So¨derman, O.; Canet, D.; Walderhaug, H.; Lindman, B. J. Phys. Chem. 1986, 90, 5802. (9) So¨derman, O.; Walderhaug, H.; Henriksson, U.; Stilbs, P. J. Phys. Chem. 1985, 89, 3693. (10) So¨derman, O.; Henriksson, U. J. Chem. Soc., Faraday Trans. 1 1987, 83, 1515. (11) Zhao, J.; Fung, B. M. J. Phys. Chem. 1993, 97, 5185. (12) Evans, D. F.; Wennerstro¨m, H. The Colloidal Domain; VCH Publishers: New York, 1994. (13) Ben-Shaul, A.; Gelbart, W. M. In Micelles, Membranes, Microemulsions, and Monolayers; Gelbart, W. M., Ben-Shaul, A., Roux, D., Eds.; Springer: New York, 1994; p 1. (14) Gruen, D. W. R. J. Phys. Chem. 1985, 89, 153. (15) Gelbart, W. M.; Ben-Shaul, A. J. Phys. Chem. 1996, 100, 13169. (16) Watanabe, K.; Ferrario, M.; Klein, M. L. J. Phys. Chem. 1988, 92, 819. (17) Shelley, J. C.; Sprik, M.; Klein, M. L. Langmuir 1993, 9, 916. (18) MacKerell, A. D. J. Phys. Chem. 1995, 99, 1846.
The SCH order parameter profile has been measured in two popular ways. One, which is based on 13C spin relaxation, is most applicable in macroscopically isotropic systems, such as micellar solutions.2,4,6-10 The other approach is based on measuring the quadrupole splitting of 2H nuclei in perdeuterated or selectively deuterated chains;1,5 this technique is only available in anisotropic, liquid crystalline phases. (One should also note that there are more complex experiments which, through yielding the static 13C-1H dipole splitting, can provide the order parameter profile in anisotropic phases without perdeuteration.19-22) By the use of any of these methods, order parameter profiles in many different phases of many different hydrocarbon-based surfactants have been determined. The variation of the profiles is relatively small over this large selection of cases which can be well understood in terms of the large conformational flexibility of a hydrocarbon chain. In light of the small variation of the order parameter profile for protonated chains, it may seem curious why it has not been measured in surfactants with fluorocarbon chains23 that are supposed to have rather different conformational flexibility and intermolecular interactions. Fluorocarbon surfactants, even though their molecular differences could command quite different packing constraints, aggregate just as well as protonated surfactants and they form fairly similar types of aggregates. Thus, the order parameter profile of fluorinated chains could reveal interesting details about interactions that govern the formation of fluorinated aggregates. This is the motivation behind the present study performed in a micellar phase by NMR measurements of 13C spin relaxation; needless to say, deuteration is not an option in fluorinated chains. (One could note, however, that bond order parameters were measured in difluoromethylene units of selectively fluorinated lipids;24,25 the fluorination there seemed to decrease the order parameter.) Although 13C spin relaxation rates at one given NMR frequency (19) Fung, B. M.; Afzal, J. J. Am. Chem. Soc. 1986, 108, 1107. (20) Fung, B. M.; Afzal, J.; Foss, T. L.; Chau, M. H. J. Chem. Phys. 1986, 85, 4808. (21) Hong, M.; Schmidt-Rohr, K.; Pines, A. J. Am. Chem. Soc. 1995, 117, 3310. (22) Schmidt-Rohr, K.; Hong, M. J. Phys. Chem. 1996, 100, 3861. (23) Kissa, E. Fluorinated Surfactants; Marcel Dekker: New York, 1994. (24) Oldfield, E.; Lee, R. W. K.; Meadows, M.; Dowd, S. R.; Ho, C. J. Biol. Chem. 1980, 255, 11652. (25) Peng, Z. Y.; Simplaceanu, V.; Lowe, I. J.; Ho, C. Biophys. J. 1988, 54, 81.
10.1021/la980576q CCC: $18.00 © 1999 American Chemical Society Published on Web 03/25/1999
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were already measured in some fluorinated surfactants,26,27 we are aware of no previous study of the order parameter profile. Experimental Section Cesium perfluorooctanoate (CsPFO) has been synthesized as described elsewhere.28 The sample of 40 w % surfactant has been prepared by mixing CsPFO and D2O. The heavy water has been carefully degassed; the well-known ability of perfluorinated chains to collect oxygen29,30 means that spin relaxation measurements in such systems can easily be harmed by paramagnetic relaxation from dissolved molecular oxygen. All experiments have been carried out at 318 ( 1 K. At this temperature, the mixture is a micellar solution composed, most probably, of oblate-shaped aggregates.28,31-33 Although the absolute temperature is less accurate, the relative temperatures, at which the measurements were performed on the different NMR spectrometers of this study, were calibrated to be within a range of 0.1 K by using the temperature-dependent splitting of C6D6 dissolved in a thermotropic liquid crystal.34 The NMR experiments were performed on Bruker DMX500 (125 and 470 MHz for 13C and 19F, respectively), AMX300 (75 and 282 MHz), and DMX200 (50 and 188 MHz) spectrometers. Modern NMR spectrometers, such as the ones in this study, are usually equipped by broadband heteronuclear preamplifiers, which in our case were all extended to 19F frequencies. To avoid saturating the preamplifier during the decoupling, 13C relaxation studies under 19F decoupling required filters attached to the input of the heteronuclear preamplifiers (low-pass filters) and to the 19F decoupling lines (high-pass filters). 13C longitudinal relaxation rates were measured by inversion recovery with 19F decoupling at all three magnetic fields. 13C{19F} NOE factors were determined by dynamic NOE experiment35 on the DMX500 and the DMX200 spectrometers. Because (i) the scalar coupling constants between intramethylene 13C and 19F (∼250 Hz) and between 19F’s in different CF2 groups (on the order of 10 Hz) are both larger than the corresponding coupling constants in protonated systems36 and (ii) the 19F NMR spectrum spreads over a much larger frequency range than the corresponding 1H spectrum,36,37 good decoupling requires a higher decoupling power than is customary in protonated systems.38,39 Therefore, it proved essential to use WALTZ-16 decoupling40,41 with well-calibrated power levels and pulse lengths to achieve proper spectra without large heating effects; scan repetition times were also adjusted to appropriately long values. The absence of large sample heating during decoupling was verified at lower temperatures at which the system is in a nematic phase with a strongly temperature-dependent 133Cs quadrupole splitting;33 (26) Serratrice, G.; Stebe, M. J.; Delpeuch, J. J. J. Chim. Phys. Phys.Chim. Biol. 1988, 85, 875. (27) Serratrice, G.; Stebe, M. J.; Delpeuch, J. J. J. Fluorine Chem. 1984, 25, 275. (28) Jo´hannesson, H.; Furo´, I.; Halle, B. Phys. Rev. E 1996, 53, 4904. (29) Hamza, M. A.; Serratrice, G.; Stebe, M.; Delpeuch, J. J. J. Am. Chem. Soc. 1981, 103, 3733. (30) Hamza, M. A.; Serratrice, G.; Stebe, M. J.; Delpeuch, J. J. J. Magn. Reson. 1981, 42, 227. (31) Boden, N.; Corne, S. A.; Holmes, M. C.; Jackson, P. H.; Parker, D.; Jolley, K. W. J. Phys. (Paris) 1986, 47, 2135. (32) Boden, N.; Corne, S. A.; Jolley, K. W. J. Phys. Chem. 1987, 91, 4092. (33) Boden, N. In Micelles, Membranes, Microemulsions, and Monolayers; W. M. Gelbart, W. M., Ben-Shaul, A., Roux, D., Eds.; Springer: New York, 1994. (34) Farrant, R. D.; Lindon, J. C. Magn. Reson. Chem. 1994, 32, 231. (35) Freeman, R.; Hill, H. D. W.; Kaptein, R. J. Magn. Reson. 1972, 7, 327. (36) Emsley, J. W.; Feeney, J.; Sutcliffe, L. H. High-Resolution Nuclear Magnetic Resonance Spectroscopy; Pergamon Press: Oxford, 1965; Vol. 1. (37) Brey, W. S.; Brey, M. L. In Encyclopedia of Nuclear Magnetic Resonance; Grantm D. M., Harris, R. K., Eds.; Wiley: Chichester, 1996; Vol. 3; p 2063. (38) Ribeiro, A. A. J. Magn. Reson. A 1995, 117, 257. (39) Ribeiro, A. A. J. Fluorine Chem. 1997, 83, 61. (40) Waugh, J. S. J. Magn. Reson. 1982, 50, 30. (41) Shaka, A. J.; Keeler, J.; Freeman, R. J. Magn. Reson. 1983, 53, 313.
Figure 1. Decoupled and assigned 13C NMR spectrum of cesium perfluorooctanoate in aqueous solution (recorded at 125 MHz). The chemical shift scale is relative, with the C7 resonance set to -40 ppm. Through the use of long samples, which allow for better homogeneity but make the overall decoupling less effective, the C3,4 resonance can be resolved into C3 and C4 at the highest frequency (125 MHz) of this study. recording the 133Cs spectrum under 19F irradiation provides a e0.3 K limit (both temperature gradient and average) for heating effects. The 13C NMR spectrum obtained under these conditions is shown in Figure 1. The assignment of the 13C lines has been achieved by combining 2D spectra (not shown) obtained by 13C19F HETCOR and 19F-19F COSY experiments.42 The 19F spectrum has been assigned earlier;43,44 the assignment is particularly straightforward via 19F-19F COSY44 that reveals the profound F2-F4-F6-F8 and F3,5-F7 connectivities (the positions are marked by the carbon number, starting with 1 for the carboxylate carbon; one should note that four-bond F-C-C-C-F scalar couplings are usually larger than three-bond F-C-C-F scalar couplings36). On the basis of the 19F assignment, the HETCOR experiment clearly provides the C2, C4, and C6-C8 assignments and resolves, in the form of small but clearly measurable chemical shift differences, the ambiguity imposed on us by the F3-F5 overlap.44 Unfortunately, the C3 and C4 resonances overlap; the full 13C spectral assignment is given in Figure 1. This assignment is given further credibility by the same pattern (F3-F5 and C3C4 overlaps) detected in perfluoroheptanoic acid.38 To provide an independent control of the final conclusions, we have also performed some 19F line width measurements. To avoid complications from fluorine-fluorine couplings, spin-echo experiments with selective 180° pulses were performed so that the evolution caused by fluorine-fluorine scalar couplings to the selected fluorine resonances were well-refocused. Because selective pulses were easiest to produce at the highest field, these experiments were carried out only with the DMX500 spectrometer; we obtained 225, 128, and 25 s-1 for the homogeneous 19F line widths of the F2, F7, and F8 resonances, respectively (the other lines were too close to each other for being selectively excited).
Results and Discussion The 13C longitudinal relaxation rates R1 and NOE factors η, obtained by single-exponential fits to the experimental points, are given in Table 1, along with estimates for the random experimental error. The analysis of these data is complicated by several possible sources of systematic error which deserve some discussion. First, despite our efforts to avoid it, some molecular oxygen could still be present (42) Ernst, R. R.; Bodenhausen, G.; Wokaun, A. Principles of Nuclear Magnetic Resonance in One and Two Dimensions; Clarendon Press: Oxford, 1987. (43) Muller, N.; Simsohn, H. J. Phys. Chem. 1971, 75, 942. (44) Raulet, R.; Furo´, I.; Brondeau, J.; Diter, B.; Canet, D. J. Magn. Reson. 1998, 133, 324.
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Table 1. 13C Longitudinal Relaxation Rates R1a and NOE Factors ηb Measured at Three Different 13C Frequenciesc
13C{19F}
50 MHz
carbon number
R1
η
75 MHz R1
C2 C3,4 C5 C6 C7 C8
0.597 0.561 0.539 0.500 0.417 0.346
1.01 1.10 1.07 1.02 1.01 1.50
0.479 0.445 0.413 0.386 0.299 0.310
125 MHz R1 η 0.354 0.348 0.337 0.302 0.271 0.288
1.10 1.22 1.18 1.19 1.27 1.53
a In s-1 units. The experimental uncertainty of the relaxation rates R1, yielded by repeated measurements, is about 6%, 4%, and 3% for the 50, 75, and 125 MHz data, respectively. b The same error in η is about 8% and 4% at 50 and 125 MHz, respectively.c In a 40 w % aqueous (D2O) micellar solution of cesium perfluorooctanoate.
in our fluorinated micelles and may influence the spin relaxation. This eventual artifact must be in extreme narrowing and therefore provides a frequency-independent contribution to the relaxation rates presented below. Although the internal consistency of the R1 and η data (see below) excludes a large paramagnetic effect, some small corresponding systematic error may be present in the obtained fast correlation times τf (see eq 4 and Table 2) which are the evaluation parameters most sensitive to paramagnetic impurities. Second, the relaxation of an AX2 spin system is rather complex45 and, in general, cannot be described by singleexponential decay in an inversion recovery experiment. Thorough analysis46 yields, however, that under continuous decoupling conditions (as here) the obtained R1 data are by good approximation free from cross-correlation effects and the decay is single exponential. It also looks probable46,47 that η supplied by the dynamic NOE experiment can be well-approximated by the familiar expression involving only autocorrelation spectral densities. Hence, we can express our experimental parameters as48,49
R1 )
(
)
N µ0γFγCp 2 [j(ωF - ωC) + 3j(ωC) + 6j(ωF + ωC)] 20 4πr3 FC (1)
σFC )
(
)
N µ0γFγCp 2 [6j(ωF + ωC) - j(ωF - ωC)] 20 4πr3 FC η)
γF σFC γC R1
(2)
(3)
where N is the number of fluorines bound to a carbon (3 for the terminal CF3 group) and rFC is the carbon-fluorine bond length set to 1.36 Å.50 The other parameters have their usual meaning, which provides 4.26 × 108 s-2 for the multiplicative prefactor in eqs 2 and 3 with N ) 2. The third complication appears at the level of the motional spectral densities j(ω) in eqs 2 and 3. For spherical (45) Werbelow, L. G.; Grant, D. M. In Advances in Magnetic Resonance; Waugh, J. S. Ed.; Academic Press: San Diego, 1977; Vol. 9. (46) Canet, D. Prog. Nucl. Magn. Reson. Spectrosc. 1989, 21, 237. (47) Zhu, L.; Kemple, M. D.; Landy, S. B.; Buckley, P. J. Magn. Reson. B 1995, 109, 19. (48) Kuhlmann, K. F.; Grant, D. M.; Harris, R. K. J. Chem. Phys. 1970, 52, 3439. (49) Doddrell, D.; Glushko, V.; Allerhand, A. J. Chem. Phys. 1972, 56, 3683. (50) James, A. M.; Lord, M. P. Macmillan’s Chemical and Physical Data; Macmillan Press: London, 1992.
Table 2. Variation of SCF order parameter and τf fast correlation time along the perfluorooctanoate chaina carbon number
SCF
τf (ps)
C2 C3,4 C5 C6 C7 C8
0.39 0.36 0.36 0.34 0.31 0.18
42 46 38 36 31 23
a These parameters were obtained by fitting the data of Table 1 to eqs 1-4. The global slow correlation time τsd ) 0.26 ns is also yielded by the fit.
micelles, these functions are Lorentzian and characterized by a single correlation time and order parameter. However, the CsPFO micelles are most probably oblatelike;28,31-33 at our concentration and temperature32 they have an axial ratio of ∼3, where the axial ratio is defined as the ratio of the large and small axes of the aggregate modeled as an oblate ellipsoid. Nevertheless, an exact analysis is still readily available using the treatment of spin relaxation in spheroidal micelles by Halle.51 Hence, we proceed by first customarily writing the motional spectral densities2 as
j(ω; F) ) (1 - S2CF) 2τf + S2CF js(ω, τsd; F)
(4)
In this expression, τf is the correlation time that characterizes the fast local chain motions on the , nanosecond time scale which lead to the partial averaging of the C-F dipolar coupling to a value given by the order parameter SCF. This parameter is rather independent of the particular model chosen for analyzing the spin relaxation data. The frequency dependence of spin relaxation is caused by the slower global motions of the surfactant chains. The effect of the two slower motions, the surface diffusion of the surfactant molecule and the tumbling of the micelle, are accounted for by numerically calculating51 the resulted spectral density term js(ω, τsd; F). The other three micellar parameters thus involved are the bond order parameter SCF, the correlation time τsd describing the molecular surface diffusion, and the axial ratio of the aggregate F. Of these parameters, the axial ratio has already been established to be ∼3 by X-ray scattering.32 In the next step, we fitted the expressions provided by eqs 1-4 to the experimental results; the axial ratio of the aggregates has been fixed to 3 and the small axis of the aggregate has been fixed to 1.1 nm,31,32 yielding the parameters presented in Table 2. The correlation time τsd is a property of the full molecule, and SCF and τf vary along the chain. The quality of the fit is illustrated by Figure 2. Because deviations of the fit from the experimental points were sometimes significantly bigger than indicated by the random experimental error (presented as 2σ), we conclude that our results in Table 2 may indeed include a small systematic error. To investigate this point further, we have also fitted our data with the small axis fixed to 1.2 nm and the axial ratio set to 2 and 4. Changing the small axis to 1.2 nm has practically no effect on the obtained order parameters and the slow correlation time τsd is increased by about 10%. The obtained parameters are more sensitive to the choice of the axial ratio; the order parameters are about 30% higher with F ) 4 and are about 22% lower with F ) 2. Note that the obtained relative variation of SCF along the chain (see Figure 3b) is not influenced at all by these changes in the (51) Halle, B. J. Chem. Phys. 1991, 94, 3150.
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Figure 2. The theoretical fit, via eqs 1-4, of the measured frequency-dependent longitudinal relaxation rates R1 (b) and NOE factors η (O) for the C2 carbon.
Figure 3. The variation of SCH and SCF bond order parameters in protonated2,3,6,10 (O and 3) and fluorinated (b) octanoate molecules embedded in micellar (O and b) and hexagonal (3) phases: (a) absolute order parameters, and (b) order parameter profiles normalized to the order parameters at position 2 in each particular chain; position 1 belongs to the carboxylate carbon.
analysis. The slow correlation time unfortunately varies more, to a value 4 times bigger with F ) 2 and about 2.5 times smaller with F ) 4. In fact, some of the obtained parameters are on the unphysical limits with F * 3 (τsd with F ) 2 and SCF with F ) 4), which lends further support
Furo´ and Sitnikov
to the X-ray finding32 of the axial ratio of ∼3. Moreover, the fit is the best (lowest sum of squares of deviations of fit from experiment) with F set to 3. To summarize, the order parameter findings presented below are robust. In Figure 3, our SCF values in Table 2 are compared to the SCH profiles obtained in micellar and hexagonal phases of sodium octanoate.2,3,6,10 Obviously, the profiles are rather different for the two kinds of chains. The most profound difference is the larger overall order parameter for the fluorinated chain. Moreover, in protonated system, the order parameter decays continuously toward the end, whereas in CsPFO micelles, only the end group has a significantly smaller order parameter as illustrated by Figure 3b. The small variation of the order parameter, along the chain can be put in parallel with the almost constant value of τf (Table 2). Clearly, the τf’s are larger than the corresponding correlation times for protonated chains; in sodium octanoate micelles, τf is found to be less than 20 ps, with a continuously decreasing trend toward the end of the chain.6,52 Additional support for our order parameter findings is provided by the 19F line widths. Although chemical shift anisotropy certainly plays some role in the relaxation of the fluorine nuclei, the relative size of the line width still reflects the local order parameter (not SCF but a combination of it and SFF). Thus, the ratio of the square roots of the line widths for F2 and F7 nuclei is 1.32, which should be compared to the ratio of C2 and C7 order parameters which is 1.26 (from Table 2). The same figures for positions 2 and 8 are 3.0 and 2.17, respectively. The least accurate result of this paper is the slow correlation time τsd. Because its obtained value is very sensitive to the choice of the axial ratio, it can hardly be used to provide a good estimate of the surface diffusion coefficient51 DS ) r2/τsd. On the other hand, choosing F > 4 provides unphysically large (>0.5) order parameters, and therefore substituting the minor axis r ) 1.1 nm and τsd ) 0.6 ns (obtained at F ) 4) gives a lower boundary of DS ) 3 × 10-10 m2/s. This limiting value is reassuringly bigger than the directly measured surface diffusion coefficient of PFO chains along a defective lamellar bilayer53 (around 2 × 10-10 m2/s, extrapolated to our temperature, which is also on the same order as that obtained for protonated octanoate chains54 in a cubic phase). Conclusions As shown by Figure 3, fluorinated chains show some deviations from the characteristics of protonated chains. These might be imposed on them by their different (as compared with protonated chains) conformational rigidity and intermolecular interactions. First, the energy barrier to internal rotation is about 50% higher for tetrafluoroethylene groups than that for ethylene groups.55 Accordingly, the segmental relaxation times in poly(tetrafluoroethylene) are by more than a factor of 2 larger than those of polyethylene.56 On the intermolecular interaction side, the large electronegativity of fluorine makes that the negative charge (that is localized on the carboxylate group for protonated chains) is, to a large extent, distributed over the first few CF2 segments. In a short chain, from which results are presented here, this may (52) Mahieu, N.; Tekely, P.; Boubel, J. C.; Cases, J. M.; Canet, D. J. Phys. Chem. 1993, 97, 9513. (53) Ukleja, P.; Chidichimo, G.; Photinos, P. Liq. Cryst. 1991, 9, 359. (54) Lindblom, G.; Wennerstro¨m, H. Biophys. Chem. 1977, 6, 167. (55) Tipton, A. B.; Britt, C. O.; Boggs, J. E. J. Chem. Phys. 1967, 46, 1606. (56) Stockmayer, W. H.; Yang, H. W. H.; Matsuo, K. Macromol. Symp. 1997, 113, 107.
Order Profile of Perfluorinated Chains
imply a stronger overall interchain repulsion (at the cost of a weaker headgroup repulsion). Our present observations are that the SCF order parameter is almost constant with the exception of that for the terminal CF3 group and the fast correlation time τf, describing the chain motion on the , nanosecond time scale, is also almost constant along the chain. These findings seem to imply that the dominant modes of fast motion (setting the order parameter) are the overall “tumbling” and the axial “rotation” of an otherwise rather rigid perfluorinated chain within the micelle; fast segmental motions are more hindered than in protonated chains. Curiously, this arrangement does not seem to compromise the tight packing of the surfactant chains, as clearly indicated by the meager hydration of the F3-F8 segments revealed by 1H-19F cross-relaxation NMR experiments performed in the same system.44 One should also note that the relative order parameter profile obtained for the
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present system is rather similar to that obtained in the lamellar phase of the sodium octanoate/decanol/water mixture;3 in general, order parameter profiles of protonated chains in lamellar bilayers1 are more flat and have somewhat larger values than those in micellar and hexagonal phases. The flat order parameter profile of the fluorinated chains is a fingerprint of a chain packing that also facilitates lamellar-like aggregates, in the form of the oblate-shaped micelles of CsPFO. Acknowledgment. This work has been supported by the Swedish Natural Science Research Council (NFR). R. S. thanks the Swedish Institute for a scholarship. Tore Brinck and Mikael Bjo¨rling are thanked for useful discussions. LA980576Q
