Fluorescence anisotropy of ionic probes in AOT reverse micelles: influence of water droplet size and electrostatic interactions on probe dynamics

Fluorescence anisotropies of two structurally similar ionic probes, rhodamine 110 and fluorescein, were measured in di(2-ethylhexyl) sodium sulfosuccinate (AOT) reverse micelles as a function of the mole ratio of water to surfactant W. This study was undertaken to explore the influence of water droplet size and electrostatic interactions on the rotational diffusion of the probe molecules. It was noticed that at W = 1 and 2, the anisotropy decays of both the probes display single-exponential behavior and for a particular value of W, the time constants sensed by rhodamine 110 and fluorescein are identical. Moreover, an increase in the reorientation time was observed from W = 1 to 2. These observations indicate that, at W = 1 and 2, it is the overall rotation of micelle which is responsible for the decay of the anisotropy and also rule out the possibility of internal rotation of the probes within the reverse micelles. However, from W = 4 to 20, the anisotropy decays of the probes could only be described by a biexponential function with two time constants. The rotational diffusion of rhodamine 110 and fluorescein in the above-mentioned range of W was rationalized using the two-step model. The average reorientation time decreases with an increase in W for both the probes, and this decrease is pronounced in the case of fluorescein compared to that in rhodamine 110. The decrease in the average reorientation time with W is due to the change in the micellar packing within the core. The significant reduction in the average reorientation time of fluorescein is a consequence of repulsive electrostatic interactions between the negatively charged probe and the anionic head groups of the surfactant AOT.