Abstract:
Direct or substantive dyes are widely used in the dyeing process of cellulosic fibres because of their easy
handling. In contrast to natural dyes which must be mordanted in order to be applied on the same kind of substrates, direct dyes can be used directly. These dyes contain solubilizing sulfonic acid groups that are ionized in aqueous solution, and therefore the dye molecule is present in the dyebath as the anion. As result, an electrolyte must be added to the dye liquor because cellulose fibers have a negative surface charge in water, which would repel the dye anion. The cations of the electrolyte neutralize the negative charge and favor the adsorbtion of the dye ions on the fiber. The dye sorption can be described with the help of Nernst, Freundlich or Langmuir adsorption isotherms. The aim of the present work was to establish the influence of two important parameters (temperature and electrolyte concentration) on the dyeing of a cellulosic substrate - mercerized cotton- with one disazo direct dyes, through the study of the equilibrium isotherms. The
dyeings were carried out at three different temperatures: 60°, 86° and 96°C and at two sodium chloride concentrations: 4g/L and 10 g/L. The study of the dyeing process was performed by comparing the experimental isotherms of the studied dyes with the classical sorption isotherms: Nernst, Freundlich and Langmuir. The most appropriate sorption model was established by linear regression analysis. Better statistical results were obtained in the case of Langmuir adsorption model.For this sorption model, the saturation values, the dyes affinity, the enthalpy and the entropy of the dyeing process have been evaluated, at the three dyeing temperatures and two salt concentrations.
