Aggregation of o,o′-Dihydroxy azo Dyes III.: Effect of cationic, anionic and non-ionic surfactants on the electronic spectra of 2-hydroxy-5-nitrophenylazo-4-[3-methyl-1-(4"-sulfophenyl)-5-pyrazolone]

The effect of Septonex, TX-100 and SDS surface-active agents on the aggregation, the acid-base, and the optical properties of 2-hydroxy-5-nitrophenylazo-4-[3-methyl-1-(4"-sulfophenyl)-5-pyrazolone] was studied by the spectrophotometric method. The study was performed in submicelle and micelle surfactant concentration, and at different dye/surfactant concentrations in a buffered system. Both the aqueous and micellar dissociation constants of the dye under investigation were evaluated at 22 +/- 1 degrees C. The changes in the absorption pattern resulted from the addition of surfactants at different pHs, where different species exist, and is explained on the basis of electrostatic interactions between the different existing dye species (azo/hydrazo/dimer/monomer) and the surfactant micelles and pre-micellar aggregates. Different tautomeric forms of the dye are suggested based on the different spectral changes. A model for the interaction between surfactants and the different dye species is proposed. This model explains both the observed spectral changes in micellar solution and the obtained micellar dissociation constants of the dye. (C) 2000 Elsevier Science Ltd. All rights reserved.