Abstract
A model is proposed to explain the manner by which the uptake of direct dyes on cellulosic fibres is promoted by the addition of inorganic electrolyte (NaCl or Na2SO4) to the dyebath. According to the model, which invokes the concept of interstitial water in dyeing, the intrinsically low substantivity displayed by direct dyes towards cellulosic fibres in the absence of added electrolyte results from the dye's marked preference for the aqueous dyebath phase, which stems from the dye's high solubility. Adding NaCl or Na2SO4 to a direct dye dyebath promotes dye aggregation which reduces the solubility of the dye in the bulk dyebath phase, so that the inherent preference of the originally highly soluble dye to favour the aqueous phase shifts towards the fibre phase.
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