The role of auxiliaries in the immersion dyeing of textile fibres: Part 4 theoretical model to describe the role of liquor ratio in dyeing cellulosic fibres with direct dyes in the absence and presence of inorganic electrolyte

A model is proposed to explain the manner by which the uptake of direct dyes on cellulosic fibres is promoted by a reduction in the liquor ratio used for dyeing. According to the model, which invokes the concept of interstitial water in dyeing, reducing the liquor ratio promotes dye aggregation which reduces the solubility of the dye in the bulk dyebath phase, so that the inherent preference of the dye to favour the aqueous phase shifts towards the fibre phase. It is suggested that the same mechanism accounts for the promotion of direct dye uptake imparted by the addition of inorganic electrolyte to direct dye dyebaths. As such, reduced liquor ratio and added inorganic electrolyte exert a combinatorial promotional effect on direct dye uptake.