Molecular Mobility in Liquid and in Frozen Micellar Solutions: EPR Spectroscopy of Nitroxide Free Radicals

EPR spectra of stable nitroxide radicals in fluid and in frozen micellar solutions of cationic and anionic surfactants were recorded. Analysis of the obtained spectra gave information on the mobility of molecules in the electrical double layer, in the core of the micelle and in the bulk water phase. In fluid solution the viscosity near the surface of the micelle is higher than in the water phase. It increases with increasing chain length of the surfactant and with increasing atomic number of the counterion. The viscosity in the inner cores of the micelles exceeds that of the water phase by factors up to 30 and that near the surface by factors between 10 and 20. The mobility within the micelle decreases with increasing atomic number of the counterion. In frozen micellar solutions there is no significant mobility in the core of the micelle up to the macroscopic melting point. In the double layer, however, mobility is detected at temperatures >240 K. This can be explained by local melting point depression because of the high local ion concentration in the double layer.