COUNTERION SPIN RELAXATION IN MICROEMULSION DROPLETS

Counterion Na-23 spin relaxation data are reported from the microemulsion phase in the AOT/water/isooctane system as a function of the water/AOT ratio, which determines the size of the aqueous droplets. The Na-23 NMR measurements comprise three independent relaxation rates, allowing the individual spectral density values to be determined, as well as the second-order quadrupolar dynamic shift. The effect on the relaxation observables of counterion diffusion within the aqueous droplet core is calculated by solving the diffusion equation in the presence of the electrostatic mean field. This allows the Na-23 NMR data to be interpreted in terms of the Na+ lateral diffusion coefficient D(s) in the surface region, the residual quadrupole coupling constant chi(s), and a spectral density contribution J(f) due to fast local motions. These quantities have also been determined in a previous Na-23 NMR study of the reversed hexagonal phase of the same system, with similar results. Although the present data are basically consistent with the expected microstructure of closed droplets with highly mobile counterions, a model of monodisperse spherical droplets cannot account quantitatively for all the data. This discrepancy is tentatively ascribed to the existence of small reversed micelles coexisting with the classical microemulsion droplets.