FLUORESCENCE DECAY OF THE ACRIDINE ORANGE‐SODIUM DODECYL SULFATE SYSTEM: FORMATION OF DYE‐RICH INDUCED MICELLES IN THE PREMICELLAR REGION *

The fluorescence decay behavior was studied for the acridine orange (AO)‐sodium dodecyl sulfate (SDS) system in order to investigate the nature of the dye‐detergent interaction, especially in the premicellar region, i.e. below the critical micelle concentration (cmc). The fluorescence spectra and decay behavior were found to depend on [AO] and [SDS]. An exponential fluorescence decay with a lifetime of 3‐4 ns was found for large [SDS]s together with a fluorescence band at 545 nm and the monomer‐type absorption spectrum. This can be attributed to the monomer dye associated with micelles. For smaller [SDS]s where the fluorescence band at 640 nm appeared together with a dimer band in absorption spectra, slower fluorescence decays were observed when we probed the long‐wavelength region (λobs > 620 nm). These decays can be attributed to the dimer of the dye associated with micelles. The presence of this slower decay below the cmc directly proves the presence of dye‐rich induced micelles. The distribution of dye molecules among the dye‐rich induced micelles is discussed assuming that the same fluorescence behavior is the consequence of the same distribution of dye molecules among micelles (above the cmc) or among dye‐rich induced micelles (below the cmc).