Steady-state and time-resolved studies on spectral and redox properties of dye-surfactant interactions

Effect of surfactants on absorption, emission characteristics, and redox reactions of cresyl violet have been studied. Among the various surfactants investigated, anionic surfactants, particularly sodium dodecyl sulfate (SDS) and sodium dodecyl benzene sulfortate (SDDBS), showed marked effect on these properties. Changes in the absorption spectrum and decrease in the fluorescence intensities below the CMCs of anionic surfactants could be attributed to the formation of ion-pair complexes of the dye with oppositely charged surfactants. At surfactant concentrations above the CMC the dye band is restored with an increase in extinction coefficient and red shift. It is assigned to the incorporation of the dye in the surfactant micelles. The association constant, KA, for the formation of a dye-surfactant complex and the binding constant, KB, for the micellization of the dye are determined using the method of iteration and are found to increase markedly in the presence of electrolyte (NaCl). The environment in terms of dielectric constant experienced by CV inside the surfactant micelles has been estimated. Time-resolved studies using a single-mixing stopped-flow technique showed different dynamics in premicellar and micellar regions. In the premicellar region, two distinct steps with observed rates changing by orders of magnitude were observed whereas in the micellar region only a single step was noticed. A hydrated electron reaction with the dye has been found to be slowed down and the decay of the semireduced CV is considerably arrested in the micellar medium. Results of other surfactants, viz. bis(2-ethylhexyl) sulfosuccinate sodium salt (Aerosol-OT, AOT), cetyl trimethyl ammonium bromide (CTAB), and Triton-X-100 have also been presented. (C) 2001 Academic Press.