Magnetic resonance studies of a redox probe in a reverse sodium bis(2-ethylhexyl)sulfosuccinate/octane/water microemulsion

The location and dynamics of the [Ru(bpy)3]2+ complex inside sodium bis(2-ethylhexyl)sulfosuccinate (AOT)/octane/water microemulsions were studied, over a range of droplet sizes, using magnetic resonance spectroscopy, dynamic light scattering, and molecular modeling. The T1 magnetic resonance relaxation times of water inside the AOT reverse micelles (RMs) were measured in both the presence and the absence of the [Ru(bpy)3]2+ complex. Large size droplet RMs (ω0 > 20) were found to be sensitive to the presence of the [Ru(bpy)3]2+ complex, which was detected through a decrease in the T1 relaxation time of the water inside the RM core, as compared to RMs containing no [Ru(bpy)3]2+. However, no difference in T1 relaxation time was observed for water in small RMs (ω0 < 20). Two-dimensional 1H−1H NOESY spectroscopy was performed to probe the location of the [Ru(bpy)3]2+ complex in both small (ω0 = 9.2) and large droplets (ω0 = 34.9). Cross-peaks between protons in the AOT tail groups and bipyridyl ligands were observed, showing that the [Ru(bpy)3]2+ complex resided in the RM interface. Finally, molecular modeling simulations were performed to probe the location of the [Ru(bpy)3]2+ complex and the structure of the RM. Molecular dynamics simulations confirmed the location of the [Ru(bpy)3]2+ complex in the RM interface and detected differences in the surfactant layer and the amount of water penetration into this layer with changing droplet size.