Do ionic and hydrophobic probes sense similar microenvironment in Triton X-100 nonionic reverse micelles?

Rotational diffusion of two structurally similar ionic probes, rhodamine 110 and fluorescein, has been examined in nonionic reverse micellar system of Triton X-100/benzene-n-hexane/water as a function of mole ratio of the water to surfactant, W. This study has been undertaken to find out whether ionic and hydrophobic probes experience similar microenvironment in these reverse micelles. Experimental results indicate that, from W=0 to 3, the average reorientation time, which is a measure of the microviscosity experienced by the probe molecule, increases by 90% and 40% for rhodamine 110 and fluorescein, respectively, and from W=3 to 8, it decreases by 20% for both the probes. The increase in the average reorientation time with W has been rationalized on the basis of the flexible oxyethylene chains of the TX-100 surfactant being hydrogen bonded by the water molecules, which makes the core region less fluid. However, once the hydration of the oxyethylene chains is complete, further addition of water results in formation of water droplet; which renders the micelle-water interface in the core region less compact leading to a marginal decrease in the average reorientation time of the probe molecules. These explanations are consistent with the location of the probes and the structure of the Triton X-100/benzene-hexane/water reverse micelles. To compare how the microenvironment experienced by these ionic probes is different from the hydrophobic ones, results from our earlier work [J. Phys. Chem. B 108, 7944 (2004)] have been considered. Such a comparison revealed that both ionic and hydrophobic probes experience similar microenvironment in these reverse micelles until the hydration of the oxyethylene chains is complete. In case of hydrophobic probes, however, the onset of water droplet formation does not alter their microenvironment, which is due to their location in the reverse micellar cores.