Absorption and fluorescence spectral characteristics of aromatic amines in cetyltrimethylammonium bromide

Abstract

The spectral characteristics of eight aromatic amines of different hydrophobicity have been studied in CTAB. A. red shift (8–15 nm) in the absorption spectra and a blue shift (ca. 10 nm) in the fluorescence spectra of these amines in CTAB relative to their aqueous solutions indicate that the polarity at the binding site of micelles corresponds to a relative permittivity of 24 and that water molecules are present in small amounts inside the micelles. Fluorescence lifetime data support this conclusion. Fluorescence quantum yields (ø_f) of all the amines (except 2-aminonaphthalene) increase in CTAB, whereas in CTAC, ø_f increases for all the amines relative to their aqueous solutions. The binding constant increases with increasing hydrophobicity of the amine. All the monoanions, with the exception of aniline and 9-aminophenanthrene are fluorescent, indicating that fluorescence quenching of these anions by water molecules in aqueous solution is faster than radiative decay. The pK_a values for the deprotonation reactions are lower in 0.02 mol dm^–3 CTAB compared to those in aqueous solutions. These results can be explained adequately by the pseudophase ion-exchange model.