The effect of substituents on the aggregation and gelation of azo sulphonate dyes

The aggregation and gelation of sodium 1‐phenylazo‐2‐hydroxy‐6‐naphthalene sulphonate azo dyes containing fluoro, ethyl, n‐propyl, iso‐propyl, n‐butyl, sec‐butyl and tert‐butyl groups in the para‐position to the azo group have been compared with those containing methyl and trifluoromethyl groups. The behaviour of dyes containing a fluoro group was also studied using 19F NMR spectroscopy. Aqueous solutions of sodium 1‐(4‐sec‐butylphenylazo)‐2‐hydroxy‐6‐naphthalene sulphonate at concentrations greater than 0.01 mol dm–3 became gelatinous, whereas the other dyes containing alkyl groups did not show this effect. Although the gelation of aqueous dye solutions of fluorinated dyes has been reported previously, it had not been noticed with dyes containing hydrocarbon chains. Aggregation constants have been determined, and the thermodynamic parameters found to be influenced by chain branching and by the number of carbon atoms present. Important information has been established about the spatial arrangement of fluorine in the aromatic ring.