Does the onset of water droplet formation alter the microenvironment of the hydrophobic probes solubilized in nonionic reverse micelles?

The interiors of the reverse micelles formed with the surfactant Triton X-100 (TX-100) in benzene-n-hexane mixed solvent with an increasing water content, W = [H2O]/[TX-100], have been explored by monitoring the fluorescence anisotropies of two structurally similar but chemically distinct hydrophobic probes. The objective of this work is to find out how the formation of the water pool influences the location and mobility of these probe molecules. It has been observed that the anisotropies of both the probes 2,5-dimethyl-1,4-dioxo-3,6diphenylpyrrolo[3,4-c]pyrrole (DMDPP) and 1,4-dioxo-3,6-diphenylpyrrolo[3,4-c]pyrrole (DPP) decay as a sum of two exponentials with slow and fast time constants. This experimental finding has been rationalized on the basis of a two-step model according to which the probe molecule undergoes two different kinds of motion inside the micelle. The average reorientation times of DPP are a factor of 4 longer than that of DMDPP. This observation in conjunction with the fact that the order parameters of both the probes are considerably different from one another indicates that DMDPP and DPP are located in different regions of the core of the reverse micelles. Another significant observation is that there is no variation in the average reorientation times of both DMDPP and DPP with W once the water pool formation takes place in these reverse micelles. This result implies that the respective microenvironments experienced by the probes essentially remain the same with the onset of water droplet formation.