Surfactant dynamics in spherical and nonspherical micelles. A nuclear magnetic resonance study

Abstract

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.

Effect of nanocavity confinement on the rotational relaxation dynamics: 3-acetyl-4-oxo-6,7-dihydro-12H indolo-[2,3-a] quinolizine in micelles

“…NMR relaxation data suggest a higher degree of order near the surface than in the interior of the micelle. 42,43 So the high values of the order parameters are also a supportive indication of the fact that the probe molecules are located in the micelle-water interfacial region rather than the interior of the micelles.…”

“…Based on the NMR relaxation data, the lateral diffusion of the surfactant molecules under study in micelles are reported to be in the range of ͑0.2-1.5͒ ϫ 10 −6 cm 2 s −1 . 42,43 Thus it is important to note that the lateral diffusion of the AODIQ is faster than that of the surfactant molecules in these micelles. The order parameter of the probe in water is zero because the dye molecule tumbles freely and the distribution is completely random.…”

Exploring membrane organization and dynamics by the wavelength-selective fluorescence approach

“…The organization and dynamics of micellar environments, namely, the core, the interface, and the immediate layers of water on the interface, have been investigated using experimental (Shinitzky et al, 1971;Kalyanasundaram and Thomas, 1977;Mukerjee and Cardinal, 1978;Leung and Shah, 1986;Nery et al, 1986;Maiti et al, 1995;Saroja and Samanta, 1995) and theoretical (Gruen, 1985) approaches. It is fairly well established that practically all types of molecules have a surface-seeking tendency in micelles (due to very large surface area to volume ratio) and that the interfacial region is the preferred site for solubilization, even for hydrophobic molecules (Mukerjee and Cardinal, 1978;Ganesh et al, 1982;Shobha and Balasubramanian, 1986;Shobha et al, 1989).…”

Photoinduced Electron Transfer on the Surfaces of Micelles

“…Lateral diffusion of the donor and acceptor molecules over the micelle surface is included and expected to be significant. 1,7,[14][15][16] Since each micelle will have a different configuration of acceptor/donors distributed over the surface, calculation of the observed electron-transfer dynamics requires performing the ensemble average with the lateral diffusion of the molecules appropriately included. Details of the theory were given in ref 1.…”