Rotational Reorientation Dynamics of Xanthene Dyes within the Interior of Aerosol-OT Reversed Micelles

The rotational reorientation kinetics of several xanthene dye molecules [rhodamine 6G (R6G), rhodamine B (RB), rhodamine 101 (R101), and fluorescein (FLR)] have been investigated in sodium bis(2-ethylhexyl) sulfosuccinate (Aerosol-OT; AOT) reverse micelles formed in liquid n-heptane. We show that the water content within the micellar core and the probe structure have a significant effect on the rotational dynamics of the probe. The differential phase and polarized modulation ratio data (decays of anisotropy) are always best described by a bi-exponential decay law. We interpret these results in terms of a two-step model for lateral diffusion and a wobbling-in-a-cone model. The former model described the observed bi-exponential decay by attributing the slower of the two rotational motions to lateral diffusion along the water core/headgroup interfacial region and the faster motion to a more restricted internal motion. The results show that, for the cationic probes, lateral diffusion coefficients vary by more than an order of magnitude across the water-loading range investigated. The wobbling-in-a-cone model serves to quantify the restricted internal motion of the probe in terms of a cone angle (θ) and wobbling diffusion coefficient. The semiangle covers a wide range for the cationic probes (0 ≤ θ ≤ 35) depending on the specific probe structure and R ([water]/[AOT]) value. The anionic probe shows a larger semiangle at each R value relative to the cationic probes.