Sorption isotherms of purified azo disperse dyes and their model compounds on nylon 6 from water are measured in a temperature range of 40-90°C. Most isotherms are curved, convex upward, in the range of low dye concentration in water. Analysis of the isotherms using Freundlich sorption and a dual-mode sorption model shows that the isotherms follow the latter, in which Nemst-type partitioning and simple Langmuir sorption are concurrently operative. The amount of dye (CL) on the polymer sorbed by Langmuir sorption is usually much smaller than that ( Cp) by partitioning. Dyes with the hydrogen atom capable of forming an intermolecular hydrogen bond show high substantivity for nylon 6. The hydrogen bond between nylon 6 and the hydrogen atom of the amino group or the 4-N-substituted-(2-hydroxyethyl) group in the dye seems to be involved in Langmuir sorption. For dyes with 4-amino and two 4-N-substituted-(2-hydroxyethyl) groups, the number of Langmuir sorption sites (S) decreases gradually with increasing temperature, while for dyes with only one 4-N- substituted-(2-hydroxyethyl ) group, S decreases abruptly with increasing temperature. In some cases, the Langmuir sorption mode seems to diminish at a high temperature: the sorption isotherms become almost linear.
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