THE DYEING OF NATURAL COTTON WITH DIRECT DYES: A DETERMINATION OF THE HEAT OF DYEING

The equilibrium sorption of a purified direct dye on natural cotton yarn has been determined over a range of dye-bath concentrations (20 to 175 mgm. of dye per litre of solution) and temperatures (75° to 90 °C.). The liquor-to-yarn ratio in the dye bath, which contained sodium chloride (4.00 gm. per litre), was sufficiently high to permit the concentration of dye in solution to remain essentially constant throughout a dyeing experiment. The dye was removed from the skeins using a pyridine–ethanol–water solution, and the concentration of dye in the latter determined spectrophotometrically.Differential heats of dyeing may be calculated from the results obtained. For an equilibrium sorption of 475 mgm. of Calcodur Blue 4GL per 100 gm. of natural cotton yarn, the value of 15 kgm-cal. per mole was found, and it is shown that the differential heat of dyeing increases as the value of the equilibrium sorption decreases. The results are interpreted in the light of the modern view of the structure of cellulose.