Photophysics of some cationic dyes in aqueous micellar dispersions of surfactants and in different solvents

The absorption and fluorescence spectra of Acridine Orange and Safranin O (both cationic dyes) have been studied in solutions of different classes of surfactants - cationic (CTAB), anionic (SDS) and non-ionic (Triton X-100). The spectral changes of the dyes were observed in varying amounts of Triton X-100, CTAB and SDS and indicated the formation of charge-transfer dye-surfactant complex between the dyes and Triton X-100 caused by incorporation of the dye molecules to micelles. It was observed that there were two types of interactions between the dyes and micelles: hydrophobic and electrostatic. While the hydrophobic interactions play a dominant role in binding of the dyes to neutral Triton X-100, the electrostatic interactions explain the binding of dyes to anionic SDS. The binding of dyes to cationic CTAB may be explained in terms of a dominant role played by hydrophobic interactions over electrostatic repulsions. The spectral studies (both absorption and fluorescence) of dyes have also been carried out in different polar and non-polar solvents, and the corresponding Stokes' shifts of fluorescence from the absorption have been calculated. The Stokes' shifts have been found to increase with increasing polarity of the solvents. An attempt has been made to ascertain the polarity of the microenvironment of the dyes in micellar media from the photophysical characteristics of the dyes in solvents of known polarity.