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The molecular organization in the hexagonal and lamellar phases of the ternary systems lecithin--sodium cholate--water has been investigated by using a variety of nuclear magnetic resonance techniques. The main findings and conclusions are the following: (i) When calculated on a mole fraction basis, the phase equilibria are insensitive to changes in the alkyl chains of the lecithin. (ii) When incorporated into a lecithin bilayer, cholate exerts a strong perturbation on the lecithin alkyl chain order, giving a large decrease of the order parameters. (iii) This decrease of the order occurs since the average cross-sectional area per alkyl chain increases probably as a result of cholate placing itself flat on the bilayer surface. (iv) The diffusion of lecithin molecules is approximately equally rapid in the lamellar and hexagonal phases. (v) The hexagonal phase is formed by rodlike aggregates with the polar groups at the surface of the rods and with a continuous hydrocarbon core. The rods are not formed by stacking disklike mixed micelles. (vi) With the interpretations of the molecular packing and the phase structures, the observed phase equilibria are in good agreement with current theories of the factors that govern phase behavior in amphiphile--water systems.

Decylammonium chloride dissolved in water at concentrations of 0.5 and 2.0 M has been studied by multifield carbon-13 and deuterium nuclear magnetic relaxation. Carbon-13 longitudinal relaxation times , and nuclear Overhauser effects have been obtained for all carbons in the aliphatic chain, at frequencies ranging from 15 to 90 MHz. Decylammonium chloride has been selectively deuterated at the a position with respect to the polar head. 2H longitudinal relaxation times T[ and, in some instances, transverse relaxation times T2 have been measured from 2 to 55 MHz. At 0.5 M decylammonium chloride forms spherical micelles. The relevant data are consistently analyzed according to the "two-step model" (Halle, B.; Wennerstrom, H. J. Chem. Phys. 1981, 75, 1928, which implies correlation times associated with the local fast motions, ß (/ = 1, 10), a correlation time associated with the slow overall motion of surfactant molecules, rs, and local order parameters S¡. The 2.0 M solution data required a more elaborate analysis (three-step model), which involves two correlation times (corresponding to rotation about the different axes of an ellipsoid) for describing the slow motions. This is because, at this concentration, micelles have nonspherical shape. In both cases, slow correlation times could be interpreted in terms of micelle tumbling and lateral diffusion of surfactant molecules around the curved surface of the aggregate. Quantitative parameters could be deduced. The radius of spherical micelles is identical with the surfactant extended chain length. At 2 M concentration, micelles are rod-shaped. When approximated by an ellipsoid, it is shown that the minor axis is identical with the radius of spherical micelles whereas the major axis is 9 times as large as the latter. The lateral diffusion coefficient is found to be the same in the two types of micelles D = (1.5 ± 0.2) X 10"10 m2 s"1. The fast correlation times confirm the liquidlike character of the micelle interior and the order parameters exhibit the usual profile starting from 0.2 at the a position to very small values at the terminal methyl.

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The molecular self-diffusion coefficients of water and surfactant were measured in the isotropic liquid L3 and bicontinuous cubic phases of the ternary sodium bis(2-ethylhexyl) sulfosuccinate (A0T)-water-NaC1 system. It is demonstrated that the selfdiffusion data from the L3 phase is consistent with a multiply connected bilayer microstructure, of low average coordination number, but inconsistent with a discrete particle, i.e., micellar, structure. The self-diffusion constants of water and surfactant decrease smoothly with increasing surfactant concentration, even as one passes from the L3 to the cubic phase. This observation lends strong support to the view that the microstructures of the L3 and bicontinuous cubic phases are topologically related and that the liquid L3 phase, which lacks long-range order, may be pictured as a disordered, or melted, cubic phase.

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