Heats of Dyeing of Direct Cotton Dyes on Cellulose: A New Thermodynamic Approach to the Determination of Activity of Direct Dyes in Cellulose

Abstract

Heats of dyeing of Chrysophenine G, Chlorazol Sky Blue FF, and Congo Red on cotton, viscose, and cuprammonium rayon have been determined. It is shown that, except for the initial low concentration of adsorbed dye, the values of heat of dyeing remain more or less the same over a fairly wide range of concentration of adsorbed dye, and that beyond a critical dye concentration, the values tend to decrease continuously when the proportion of the adsorbed dye is increased. Values for the critical concentration of adsorbed dye beyond which the values of heat of dyeing tend to decrease agree closely with values for dye adsorption at which maximum volume contraction in the substrate takes place, as observed from density measurements, and also with values of dye concentrations where maximum extent of oxidation of cellulose takes place during catalytic oxidation of the fiber substance by sodium hypochlorite in presence of the dye. A new thermodynamic approach has been developed to express the activity of the dye in a cellulosic fiber substance in terms of the fraction of the total sites occupied at a particular stage of dyeing. Based on this thermodynamic approach, affinity values of the three direct dyes for the three cellulosic fibers under different experimental conditions have been determined; the validity of the treatment is discussed.