Effect of salts on physical interactions in wool dyeing with acid dyes

The modified Donnan model, which considers the dye distribution coefficient as a function of salt concentration and property in the dyebath, is used to describe the dyeing behavior of wool with acid dyes and different kinds of salts. This modification is based on the understanding that salt anions are either kosmotropes (water-structure maker) or chaotropes (water-structure breaker). They affect the free energy of the dyebath through their influence on water structures. As is well known, adding salt to the dyebath decreases dye sorption due to its reduction of ionic attraction between negatively charged acid dyes and positively charged wool fibers. However, adding some salts such as sulfate and dihydrogen phosphate can increase dye sorption if the salt concentration is high. This phenomenon cannot be explained by the classic theory of salt functions in dyeing. The modified Donnan model is used to explain quantitatively both the decrease and increase of acid dye sorption on wool with increasing salt concentration.