Photophysics of Rhodamine B in the nanosized water droplets: A concentration dependence study

Abstract

The photophysics of Rhodamine B (RB) dye at both low and high concentration was studied in confined water nanodroplets within water/AOT/n-hexane microemulsions (MEs) at constant water content as a function of mass fraction of droplet (mfd) using techniques such as Uv–Vis and fluorescence spectroscopy, as well as dynamic light scattering (DLS). The study of the diffusion coefficient of nanometer-sized water droplets by the DLS technique showed that with increasing concentration of Rhodamine B (RB) dye, the interactions between nano-droplets were altered from attractive to repulsive as a function of mass fraction of droplets increased. Uv–Vis spectroscopy showed that at high concentrations of Rhodamine B (RB) dye molecules, the absorption spectra of samples deviated from Beer's law, and is broader at larger mfd due to the interactions of RB dye molecules. The comparison of fluorescence spectra of water droplets containing RB dye at low(0.0000625) and high concentrations (0.001) showed that the fluorescence emission intensity of AOT MEs at high concentration of RB molecules quenched due to aggregation of dyesmolecules, and the red shift in λmax of fluorescencewas observed by increasing the dye's concentration. It was also observed that changes in intensity and λmax of fluorescence were most significant in a region where the mfd of the water/AOT/n-hexane MEs was relatively small, and approached an extreme at larger mfd. The stokes shift and dipole moments' ratio of ground to excited state (μg/μe) of water/AOT/n-hexane microemulsion containing Rhodamine B at a high concentration of 0.001 increased as a function of mfd, whereas at a 0.0000625 concentration, it included two slopes of mfd = 0.04, but in both concentration, they remained almost constant at larger mfds. ©