Time-resolved recording of ionic dyestuff aggregation by static light scattering

Mixtures of anionic direct dyes are used to achieve distinct color shades during dyeing of cellulose. Occasionally, the presence of bivalent alkaline-earth ions interferes with this dyeing process. A possible reason for this impairment is dyestuff aggregation. To investigate the aggregation process together with the resulting structure of the aggregates, time-resolved static light scattering was carried out with dilute solutions of a dyestuff mixture which contains 50 wt % of each component and which has an orange color shade. All scattering data of the aggregates could be interpreted consistently by means of the wormlike chain model. Comparatively short aggregates with a contour length of 250-300 nm were observed in the presence of Na2SO4 as an added salt. If the Na+ ions were exchanged by Mg2+ ions, a gradual increase of the aggregates was observed, extending over a time regime of several minutes and leading to contour lengths which were up to 10 times longer than those found in the presence of Na+ ions.