Rotational relaxation of hydrophobic probes in nonionic reverse micelles: Influence of water content on the location and mobility of the probe molecules

Rotational relaxation studies 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), have been carried out in Triton X-100/cyclohexane (TX-100/cyclohexane) reverse micelles with increasing water content, W = [H2O]/[TX-100]. in order to investigate the effect of added water on the location and mobility of the probe molecules. The anisotropy decays of both probes in the micelles have been adequately described by a sum of two exponentials with fast and slow reorientation times. The fast and slow components have been ascribed to the wobbling motion and the lateral diffusion of the probe in the micelle, respectively, and both of these motions are coupled to the rotation of the micelle as a whole. It has been observed that the average reorientation time <tau(r)> of DMDPP increases by a factor of 2 when W is increased from 0.0 to 4.2, and in case of DPP it is almost independent of W. These results have been rationalized on the basis of the differences in the chemical nature of the probe molecules, which enables them to interact with the micellar internal environment in a distinct manner.