Aggregation of Anionic Dyes in Aqueous Solutions

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Abstract
The aggregation at 30 and 40d̀C of the sodium salts of l‐amino‐4‐alkylaminoanthraquinone‐2‐sulphonic acid has been studied polarographically. Aggregation proceeded up to the dimerisation stage only. The critical concentrations for formation of dye aggregates were considerably lower in the presence of KCl than in the presence of NaCl of equivalent molar concentrations. The calculated values of heats of dimerisation were exothermic and small, of the same order of magnitude as the corresponding free‐energy changes. The values of the unitary entropy changes ΔSU were generally positive and increased with increasing nonpolar character of the parent dye, indicating that hydrophobic interactions play an important role in the aggregation of these dyes. The good agreement between theoretical values of the thermodynamic parameters for pairwise hydrophobic‐bond formation between alkyl‐alkyl side‐chains and the experimental values emphasises the role of hydrophobic bonding in aggregation. Furthermore, addition of urea disaggregated the dye by reducing hydrophobic interactions.