Internal Viscosity Microviscosity of Micelles

Just as the extent of water penetration into micelles stiU remains a controversial topic, micellar internal viscosity (microviscosity) also suffers from a similar fate. As the determination of both micellar internal viscosity and the extent of water penetration in micelles involves an almost similar experimental approach, that is, the use of some kind of probe, it is not surprising to see that both topics still remain controversial in nature. 

Micellar microviscosities determined with two different probes using the same technique are often not the same. Such inconsistencies in the reported values of Pni are sometimes rationalized qualitatively in terms of various factors such as nonisotropic microviscosity of micelle, significant contribution from the rotation of the micelle to the measured steady-state anisotropy with certain fluorescent probes, different micellar locations of probe molecules, different structural features of probe molecules, and perturbation of the micellar environment in which probe molecules are embedded. 

With the intention of addressing the question, are the experimentally determined microviscosities of the micelles probe dependent, DntF ° determined microviscosities of SDS, CTABr, 2-phenyldodecanesnlfonate (2-PDS), 3-PDS, 

and 6-PDS using the time-resolved fluorescence depolarization method involving two dissimilar probes, 2,5-dimethyl-l,4-dioxo-3,6-diphenylpyr-rolo[3,4-c]pyrrole (DMDPP) and coumarin 6 (C6). The decay of anisotropy for both probes in all the six micelles has been rationalized on the basis of a two-step model consisting of fast-restricted rotation of the probe and slow lateral diffusion of the probe in the micelle that are coupled to the overall rotation of the micelle. On the basis of the assumption that the fast and slow motions are separable, the experimentally obtained slow and fast reorientation times and Xfajt) are related to the time constants for lateral diffusion (xj, wobbling motion (Xw), and rotation of the micelle as a whole (Xm) by the following relationships  

In view of the fact that the rotation of the micelle also contributes to the depolarization of the fluorescence of the probe, microviscosity should be calculated by using Xl rather than Dutt ° calculated t) , values of SDS, CTABr, 2-PDS, 3-PDS, 4-PDS, and 6-PDS micelles by using the calculated values of Xl, which show that both probes sense almost identical microviscosity for a given micelle. Both probes, DMDPP and C6, are concluded to reside in the Stem layer of all six micelles. 

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