Determination of Acid‐dye Standard Affinities for Nylon Fibres from Sorption Isotherms

The dyeing equilibria of acid dyes on nylon 6.6 are studied using a model in which the substrate is divided into solid polymer and an internal solution. A Nernst distribution is assumed for the distribution of dye between these two phases, while a Donnan equilibrium is presupposed for that between the internal and external solutions. It is shown experimentally that the Donnan coefficient of the nylon increases exponentially with the amount of fixed dye provided that the substrate is initially present in the isoelectric state. Based on this finding, the standard affinity of an acid dye for nylon 6.6 can be derived from the sorption isotherm, without the necessity of knowing the acidity and basicity of the carboxyl and amino groups respectively. The effect of introducing an alkyl group on the standard affinity is demonstrated with four anthraquinone dyes.