COUNTERION SURFACE-DIFFUSION IN A LYOTROPIC MESOPHASE - A NA-23 2-DIMENSIONAL QUADRUPOLAR ECHO NMR RELAXATION STUDY

In the first systematic application of the two-dimensional quadrupolar echo (2DQE) method, we study the 23Na NMR relaxation behavior of counterions in the reversed hexagonal mesophase of the AOT/D2O/isooctane system, consisting of long cylindrical aggregates with the water and ions on the inside. Using a combination of relaxation experiments, performed on magnetically aligned samples, we determine the three spectral densities at two sample orientations. The orientational dependence of the spectral densities allows us to separate contributions from different types of molecular motion. In particular, we find a dominant contribution from surface diffusion of counterions along the periphery of the cylindrical aqueous regions (diameter ca. 50 Å). From the frequency dependence of this contribution, we determine the diffusion coefficient of sodium ions in the interfacial region, a quantity which is important for a variety of phenomena in colloid and polymer science, electrochemistry, and biophysics. We thus obtain D(Na+) = (2.8 ± 0.3) × 10-10 m2 s-1, which is a factor of 3.6 lower than in an infinitely dilute aqueous (D2O) electrolyte solution at the same temperature (20.6°C). In addition, our analysis of the 23Na relaxation rates and spectral line shapes provides information about the fast local ion and water dynamics in the interfacial region, as well as about the equilibrium structure of the liquid crystalline phase. © 1990 American Chemical Society.