Abstract
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 <i>n</i>-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 ration 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 <i>R</i> ([water]/[AOT]) value. The anionic probe shows a larger semiangle at each <i>R</i> value relative to the cationic probes.
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