STRUCTURAL STUDIES OF A HOMOLOGOUS SERIES OF ALKYL SUCROSE ESTER MICELLE BY X-RAY-SCATTERING

The micelle structure of a homologous series of the fatty acid sucrose monoester, C(n)SE (n = 10, 12, 14, 16), was systematically studied by X-ray scattering technique. The contrast variation method was used by changing the concentration of sucrose in solutions, and the obtained scattering data were converted into the absolute intensities. The data were corrected for the intermicellar interference effect. The morphological parameters such as micelle volume nu-1, average electron density rho-BAR1, radius of gyration of micelle shape R(v), second moment of the electron density distribution a, and square average of the electron density distribution (DELTA-rho-1)2BAR were determined from the data in the range of 0 less-than-or-equal-to s less-than-or-equal-to 0.074 angstrom-1, where s = 2 sin theta/lambda, 2-theta is the scattering angle, and lambda is the wavelength of X-ray. The nu-1, R-nu, and a increase but rho-BAR1 and (DELTA-rho-1)2BAR decrease with increasing chain length of fatty acid. From relations among R-nu, maximum dimension of micelle, and first isoscattering point, it is suggested that the micelles are not spherical in shape. The other parameters such as aggregation number, hydration ratio, etc. were also obtained in this study. Logarithms of the aggregation number and the micelle volume were found to show excellently linear dependence on the chain length. The models of micelle structure were constructed on the basis of the morphological parameters. Since the two-shell structure of ellipsoid of revolution was found to be an unsuitable model for the sucrose ester micelles, the micelle was modeled by three-shell structure of polar group layer, hydrocarbon core, and the terminal methyl group region which localizes at the center of core. The best fitted model was selected by R factors calculated for both the intensity curves and distribution functions. The micelle shape was oblate for C10SE, C12SE, and C14SE but prolate for C16SE. X-ray scattering curves show a possibility that both types of ellipsoid coexist in the solution of C16SE at 20-degrees-C. Many structural parameters were found to change systematically with length of the hydrocarbon chain. However, the minor semiaxial length of hydrophobic core does not change consistently with the increase of chain length. This suggests the increase of gauche-trans-gauche (kink) sequence with increasing the chain length. It is also suggested that water molecules enter into near the boundary between hydrophobic core and hydrophilic layer but do not penetrate into the hydrophobic core. The polydispersity in micelle size was found to be small as a result of the model calculations assuming the Gaussian distribution of the micelle size. The structural change of C16SE micelle was found at 49% (w/v) sucrose concentration and interpreted as the change in the packing mode of the detergent molecule and/or the hydration.