SPECTROSCOPIC INVESTIGATIONS IN MOLECULARLY ORGANIZED SOLVENT MEDIA .2. EXAMINATION OF THE NITROMETHANE SELECTIVE QUENCHING RULE AT DIFFERENT EFFECTIVE MICELLAR SURFACE-CHARGE DENSITIES

Applicability of nitromethane as a selective quenching agent for discr iminating between alternant versus nonalternant polycyclic aromatic hy drocarbons (PAHs) is examined for select solutes dissolved in micellar sodium dodecylsulfate (SDS) + 1-pentanol, sodium dodecylsulfate + 1-h exanol and cetyltrimethylammonium bromide (CTAB) + 1-pentanol solvent media. Results of measurements show that nitromethane quenched fluores cence emission of only the 6 alternant PAHs in the cationic CTAB + 1-p entanol solvent media as expected. Emission intensities of nonalternan t PAHs, except for two exceptions noted previously, were unaffected by nitromethane addition. Unexpected quenching behavior was observed, ho wever, in the case of nonalternant PAHs dissolved in micellar anionic SDS + 1-pentanol and micellar anionic SDS + 1-hexanol. Nitromethane qu enched fluorescence emission of all nonalternant PAHs studied in these two anionic surfactant systems, which is contrary to the selective qu enching rule. The reduced emission quenching that is observed at incre asing alcohol cosolvent concentration is rationalized in terms of the decreased micellar surface charge density that occurs when more alcoho l molecules are incorporated into the anionic micelle.