Are the Experimentally Determined Microviscosities of the Micelles Probe Dependent?

Abstract

With an intent to address the question posed in the title, rotational diffusion of two dissimilar probes, 2,5-dimethyl-1,4-dioxo-3,6-diphenylpyrrolo[3,4-c]pyrrole (DMDPP) and coumarin 6 (C6), has been investigated in small aqueous micelles formed with the surfactants sodium dodecyl sulfate (SDS), cetyltrimethylammonium bromide (CTAB), 2-phenyldodecanesulfonate (2-PDS), 3-phenyldodecanesulfonate (3-PDS), 4-phenyldodecanesulfonate (4-PDS), and 6-phenyldodecanesulfonate (6-PDS) using the time-resolved fluorescence depolarization method. The decay of anisotropy for both the probes in all the micelles could be adequately described by a sum of two exponentials with slow and fast time constants. These results have 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. From the order parameter values it has been established that the probes are located in t...