Spectroscopic investigations in molecularly organized solvent media. Part 2: Examination of the nitromethane selective quenching rule at different ‘effective’ micellar surface charge densities

Abstract

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