Interactions between methylene blue and sodium dodecyl sulfate in aqueous solution studied by molecular spectroscopy

The interactions of methylene blue (MB, a cationic redox indicator and biol ogical stain) and sodium dodecyl sulfate (SDS, a micelle-forming, anionic s urfactant) in aqueous solution have been examined by using Rayleigh scatter ing, UV-visible absorption, and fluorescence spectroscopy, At SDS concentra tions significantly below the critical micelle concentration (cmc), MB form s noncovalent dimers and aggregates with SDS that scatter light but do not fluoresce, For solutions containing 1 mu M MB and < 3-5 mill SDS, shifts in the absorption spectrum characteristic of the formation of MB H-aggregates are noted. There appears to he Little effect on the fluorescence emission spectrum, indicating that these MB aggregates do not fluoresce appreciably, At and above the known SDS cmc, MB is observed to interact with the micell es. The MB excited-state fluorescence lifetime (380 ps) remains constant un til Sos micelles form, then increases to 615 ps. The MB rotational reorient ation time similarly increases from 105 to 500 ps between 6 and 8 mM SDS, T his finding suggests that the MB is encountering, on average, a microenviro nment in the SDS micelles that is 5-fold more viscous than liquid water or the molar volume of the MB/SDS species that is reorienting is 5-fold larger than MB in water.