STRUCTURE OF THE WATER-SURFACTANT INTERFACE IN MICELLES - AN NMR-STUDY OF SDS MICELLES LABELED WITH PARAMAGNETIC-IONS

A method is developed for determining the structure of the water/surfactant interface and the configuration of the surfactant molecules in micelles. In this method, NMR is used to measure the magnetic dipolar interactions between the nuclei of the surfactant molecules and paramagnetic counterions adsorbed on the surface of the micelles. The experiment compares the relaxation rates of nuclei belonging to different chemical groups in the surfactant molecules; in some simple cases these relaxation rates only depend on the distances between the nuclei and the ions. Then the measured relaxation rates can be related to the populations of the nuclei in layers located at various depths from the paramagnetic surface. According to the data, the structure of these successive layers in a spherical micelle can be discussed in the following terms: (i) outermost layers containing the hydrated charges (how smooth or regular are they); (ii) layers of water/hydrocarbon contact (how deep does water penetrate in the micelle? ); (iii) layers of the hydrocarbon core (how disorganized is the packing of the surfactant chains? ).