Effects of electrostatic repulsion on the aggregation of azo dyes in aqueous solution

Abstract

The aggregation of an azo dye containing one trifluoromethyl and two sulphonate groups (m-FTR) in aqueous solution has been investigated by means of ¹⁹F n.m.r. and electronic absorption spectroscopy. The ¹⁹F n.m.r. signals shifted to lower magnetic field, indicating that the fluorine atoms are located outside the aromatic ring of an adjacent dye molecule in the aggregate. On the other hand, in the case of an azo dye carrying one trifluoromethyl and one sulphonate group (m-FTS), the fluorine atoms were oriented above the adjacent aromatic ring. This difference gives a different aggregate structure between m-FTR and m-FTS. The absorption spectra of m-FTR aqueous solution show a single monomer/dimer equilibrium. On the basis of the results, the aggregation constant K was determined. The value of K for m-FTR was half that for m-FTS, suggesting that the latter dye aggregates more easily than the former owing to a lower electrostatic repulsion.