Electrokinetic study of water hardness during acid dyeing with silk

In this paper, the effect of water hardness, expressed in CaCO3, on the dyeing of silk with acid dye under acid alkaline and isoelectric point dyeing conditions was studied using the zeta potential method. Under acidic conditions and in the presence of calcium ion, the positive zeta potential of silk was found to decrease with a reduction in dye adsorption. Such a phenomenon might be due to the presence of cation which increased the dyeing potential barrier at the interface between fibre and dye solution. This would result in a higher resistance of dye anions passing through the interface. Under alkaline conditions the zeta potential of silk was negative and resulted in a strong potential barrier against the dye anions. The presence of calcium ions would result in a decrease in the absolute value of the zeta potential of silk fibre, with an overall increase in dye absorption. At the isoelectric point, the zeta potential of the silk fibre was found to be near zero, and the dye adsorption was not influenced by the cations. These results showed that calcium ions could have a strong electrolytic effect on dyeing, even at very low concentrations.