EFFECTS OF HYDRATING WATER-MOLECULES ON THE AGGREGATION BEHAVIOR OF AZO DYES IN AQUEOUS-SOLUTIONS

The aggregation behavior of three sulfonated monoazo dyes (C.I. Acid Red 88, C.I. Acid Red 13 and C.I. Acid Red 27) was investigated by means of visible absorption spectroscopy to elucidate the effects of the sulfonate groups. The aggregation numbers of the three dyes were estimated as two. The aggregation constant K(Agg) of the dye containing two sulfonate groups was the largest. This result is inconsistent with that expected from the electrostatic repulsion between the sulfonate groups in the dyes, suggesting that the other effects are dominant. The thermodynamic parameters determined from the temperature dependence of K(Agg) indicates that hydrophobic interaction is involved in the aggregation reaction. From this, it is concluded that the hydrating water molecules around the dyes play an important role in the aggregation process. Furthermore, the effects of urea were investigated to clarify the influence of cosolutes on the aggregation behavior.