Structure of urea clusters confined in AOT reversed micelles

The structure of sodium bis(2-ethylhexyl)sulfosuceinate (AOT) and that of urea containing AOT reversed micelles has been investigated by small-angle neutron scattering (SANS) and Fourier transform infrared (FT-IR) spectroscopy at different ACT concentrations and urea/AOT molar ratios. For the AOT/n-heptane system, SANS data analysis indicates that ACT molecules form prolate ellipsoidal aggregates, which grow asymmetrically along the major axis upon increasing the surfactant concentration. For the urea/AOT/n-heptane system, the SANS results are consistent with the hypothesis that urea is encapsulated as small-sized ellipsoidal hydrogen-bonded clusters within the hydrophilic micellar core of the ACT reversed micelles. The insertion of urea in the micellar core causes a significant increase of the aggregate size along the minor-axis direction. FT-IR data indicates that, quite independently from the urea and ACT concentrations, the encapsulation of urea clusters in the AOT micellar core involves some changes of the H-bonded structure characterizing pure solid urea. This structural change was rationalized in terms of the specific interactions between the urea NH2 and the ACT SO3- groups in small-sized urea clusters. Moreover, the CO stretching mode analysis suggests that within the cluster the urea CO groups interact with the urea NH2 groups whereas the ACT CO groups do not.