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Order Parameter Profile of Perfluorinated Chains in a Lamellar Phase S. V. Dvinskikh† and I. Furo´* Department of Chemistry, Division of Physical Chemistry, Royal Institute of Technology, SE-10044 Stockholm, Sweden Received August 17, 1999. In Final Form: November 10, 1999 Cesium perfluorooctanoate molecules are investigated by NMR spectroscopy in the lamellar phase of their aqueous solution. The particular NMR method, 13C-detected, 19F-decoupled separated-local-field (SLF) spectroscopy, provides the dipolar splitting of each 13C spin to its 19F neighbors. These dipolar splittings are interpreted in terms of the molecular C-F bond order parameters of each difluoromethylene and trifluoromethyl group. The obtained variation of this order parameter along the fluoroalkyl chain strengthens the conclusion from an earlier, model-dependent study (Furo´, I.; Sitnikov, R. Langmuir 1999, 15, 2669): perfluorinated surfactant chains are significantly more rigid than their hydrogenated counterparts.
Introduction Despite recent advances, fluorosurfactants1 are less well-known than their hydrogenated cousins. In particular, information about the microstructure and dynamics of their aggregates is scarce. As one example, the NMR bond order parameter profile, which uniquely characterizes the average molecular structure of the surfactant tails within the aggregates, has been measured only recently for a fluorosurfactant,2 cesium perfluorooctanoate (CsPFO). That study has indicated that the perfluorinated chain is less flexible and less mobile than its hydrogenated counterpart in other surfactants. Since the energy barrier for segmental motion is known to be rather high for difluoromethylene groups,3 the general trend provided by this finding is along the expectations. On the other hand, the order parameter profile has only been measured in an isotropic micellar phase of CsPFO, and the obtained order parameter values, deduced from the dipolar spin relaxation rates of 13C spins, were somewhat model dependent (see Discussion). Thus, it might turn out to be useful to provide another measurement of the same quantity, though via another method, separated-local-field (SLF) NMR spectroscopy,4,5 and in another phase of the same surfactant. The order parameter of the C-F bond, SCF, measures the orientational average of the direction of that bond with respect to the normal of the aggregate surface.6 NMR is sensitive to this quantity because the direction of the C-F bond with respect to the applied magnetic field affects the magnitude of the dipolar coupling of the 13C and 19F spins. Fast (much less than nanoseconds) segmental and overall motions of the surfactant molecule average this dipolar coupling to a residual value,6-8 whose ratio to the * Author for correspondence. E-mail:
[email protected]. † On leave from the Institute of Physics, St. Petersburg State University, 198904 St. Petersburg, Russia. (1) Kissa, E. Fluorinated Surfactants; Marcel Dekker: New York, 1994. (2) Furo´, I.; Sitnikov, R. Langmuir 1999, 15, 2669. (3) Tipton, A. B.; Britt, C. O.; Boggs, J. E. J. Chem. Phys. 1967, 46, 1606. (4) Hester, R. K.; Ackerman, J. L.; Neff, B. L.; Waugh, J. S. Phys. Rev. Lett. 1976, 36, 1081. (5) Schmidt-Rohr, K.; Spiess, H. W. Multidimensional Solid-State NMR and Polymers; Academic Press: London, 1994. (6) Seelig, J. Q. Rev. Biophys. 1977, 10, 353.
unaveraged dipolar coupling provides SCF. In an isotropic micellar phase, the dipolar coupling is further averaged to zero by slower (approximately nanoseconds) motions, such as micellar tumbling and the surface diffusion of the surfactant molecules within the micelles. Thus, the NMR spectrum is a high-resolution one, with no static dipolar coupling. On the other hand, the spin relaxation of the 13C spins is dominated by this approximately nanosecond modulation of the residual dipolar coupling.9 Therefore, one can, within the frame of a suitable model, backcalculate the residual dipolar coupling from the 13C spin relaxation rates measured at different magnetic field strengths.2,9 As a result, one obtains the variation of SCF along the chain, in other words, the order parameter profile. In contrast to an isotropic micellar phase, the motion of the individual surfactant molecules in a lyotropic liquid crystalline phase is not fully isotropic. Thus, in an ideal lamellar phase, where the aggregates are modeled as rigid, coparallel bilayers, the residual dipolar coupling, left after the fast motions, is not averaged to any lower value. Hence, the NMR spectrum of a 13C spin is split by its dipolar interactions to the surrounding 19F spins. This splitting, if detected, would provide SCF directly. The first problem is that the 19F spins are dipole-coupled not only to the 13C spins but also to each other, which may complicate the appearance of the 13C spectrum.5 This can be remedied via homonuclear decoupling applied to the 19F spins. The second problem is the very dipolar splitting in the 13C spectrum that obscures the location of the different lines5 and prevents us from rendering the different dipolar splittings to different 13C spins along the perfluorinated chain. This uncertainty is removed by heteronuclear decoupling. These two decoupling schemes are combined into a two-dimensional (2D) 13C detected-19F decoupled SLF experiment that is explained in the next section. The results, obtained by this method in the lamellar phase of CsPFO, are presented thereafter. The meaning of the obtained order parameter profile is discussed at the end. (7) Dong, R. Y. Nuclear Magnetic Resonance of Liquid Crystals; Springer: New York, 1994. (8) Halle, B.; Quist, P. O.; Furo´, I. Liq. Cryst. 1993, 14, 227. (9) Wennerstro¨m, H.; Lindman, B.; So¨derman, O.; Drakenberg, T.; Rosenholm, J. B. J. Am. Chem. Soc. 1979, 101, 6860.
10.1021/la991114v CCC: $19.00 © 2000 American Chemical Society Published on Web 01/19/2000
Order Parameter Profile of Perfluorinated Chains
Figure 1. Pulse sequence used in our 13C-19F separated-localfield (SLF) NMR experiments with BLEW-12 homonuclear and COMARO-2 heteronuclear decoupling sequences. The crosspolarization (CP) period was 2 ms.
Langmuir, Vol. 16, No. 6, 2000 2963 The temperature was regulated with an accuracy of 0.1 °C by a Bruker BVT-2000 unit. The temperature shift and temperature gradient within the sample, caused by the decoupling irradiation, were calibrated by observing the change in the line splitting and line width of the 2H NMR spectrum of D2O. Decoupling power, irradiation time, and repetition rate were adjusted to limit those heating effects to
