Description of Dyeing Equilibria in the Application of Direct Dyes

Direct dyes are absorbed on the internal surfaces of cellulosic substrates, the amount of surface accessible to a dye anion depending on the form and size of the anion. Thus there are surface regions for which all the dyes in a combination compete, and others that are only accessible to only one dye in a particular combination. The internal surface of the fibre also possesses an electric charge. The extent to which this initial charge is reinforced depends on the different absorption of the dye anions by different surfaces. Besides competition for dye sites, a mutual electrostatic repulsion occurs. A system of equilibrium conditions permits the absorption of different dyes in a combination that can be calculated on the basis of the absorption characteristics of the individual dyes