Positional effects of a trifluoromethyl group on the aggregation of azo dyes in aqueous solutions

Abstract

The aggregation behavior in aqueous solutions of monoazo sulfonic dyes containing a trifluoromethyl group at various positions to the azo linkage, and the corresponding dyes containing a methyl group, was investigated by means of 19 F NMR and visible absorption spectroscopy. The visible absorption spectra changed with dye concentration, exhibiting distinct isosbestic points. From this, the aggregation number was estimated as 2 for all the dyes in the concentration region used for the visible absorption measurements and the aggregation constants were calculated. Furthermore, the thermodynamic parameters were determined from the temperature dependence of the aggregation constants