Small Angle X-ray and Neutron Scattering Study of the Micellization of (N-Alkylamino)-1-deoxylactitols in Water

In this study, we investigated the micellization in water of four surfactants from the series of (N-alkylamino)lactitols by small angle neutron and X-ray scattering. These four surfactants differed from one another by the number of carbon atoms in their alkyl chain:  8, 9, 10, or 12. The comparison of the I(q) curves of the intensity scattered from micellar solutions as a function of concentration indicated that the micelle size was weakly dispersed at small concentrations. The C<INF>8</INF> derivative presented a singular behavior which may be attributed to the formation of premicellar aggregates. The characteristic dimensions of the micelles are derived from the I(q) curve fittings. For the calculation of scattered intensity from the micelles, three models of micelles were considered:  spherical, oblate ellipsoidal, and prolate ellipsoidal. These models take into account particularities of the studied surfactants, in particular, the size of the sugar head group, which leads us to consider a nonuniform penetration of the solvent into the polar outer layer. On the other hand, the width of the polar head layer is assumed to be the same along the short and long axes of the ellipsoidal models. This means that the ellipticity of the external surface of the micelle is different from the ellipticity of the core which contains the alkyl chains. The best fit of the scattered intensity curves for the four surfactants was obtained with an oblate ellipsoidal model. The model produced a constant value of 0.55 for the ellipticity of the cores of the models. We pointed out how this particular packing is adapted to the characteristics of the (N-alkylamino)lactitol polar head, and we compared it to the other models of micelles:  spherical and prolate ellipsoidal. We also studied the evolution of several parameters (aggregation number, radius of the core, ...) as a function of the number of carbon atoms in the alkyl chain.