Rotational diffusion of hydrophobic probes in Brij-35 micelles: Effect of temperature on micellar internal environment

Rotational diffusion of two structurally similar hydrophobic probes, 2,5-dimethyl-1,4-dioxo-3,6-diphenylpyrrolo-[3,4-c]pyrrole (DMDPP) and 1,4-dioxo-3,6-diphenylpyrrolo[3,4-c]pyrrole (DPP), has been studied in nonionic Brij-35 micelles as a function of temperature to explore the micellar internal environment. Analysis of the anisotropy data reveals that the probes are located in the interfacial region of the micelle and also undergo slow lateral diffusion and fast wobbling motion that are linked to the rotation of the micelle as a whole. From light scattering studies, it has been well-established that there is no variation in the size and hydration levels of Brij-35 micelles in the temperature range 283-323 K. Hence, it is logical to expect that there is no change in the site of solubilization of the probes in the micelles with temperature. An attempt has been made to determine the microviscosity of these micelles from the average reorientation times of the noninteracting probe DMDPP.