ABSTRACT
The point of zero charge is an electrokinetic characteristic related to amphoteric materials such as protein fibers, whose determination and knowledge is crucial for the dyeing and finishing processes of the same. This characteristic influences the adsorption of acid type dye, main dye class applied in dyeing of silk and wool fibers. During the dyeing the ionic charge of the protein fibers is dependent of the pH of the bath in which they are. In the pH corresponding to the point of zero charge (pHPZC), protein fiber has a neutral charge. When in the presence of a bath whose pH is lower than pHPZC, the fiber is positively charged and can thus form ionic bonds with the dye molecules that are negatively charged. In this article, the pH corresponding to the point of zero charge was determined by a calling method "experiment of 11 points". This method requires a simpler experimental apparatus and accessible than the methods used in determining the isoelectric point, another related electrokinetic characteristic of amphoteric materials. For silk fiber and wool were found pHPZC equal to 3.8 and 6.3 respectively. Dyeing with acid dye were carried out at pH values above and below the pHPZC. The samples dyed at pH lower than pHPZC showed higher color strength indicating the formation of ionic bonds between dye and fiber. The dyed samples at pH greater than pHPZC, on the contrary, resulted in lower color strength. These results demonstrate the importance of knowing the point of zero charge of protein fibers and thus determine the best dyeing pH of the same.
Point of zero charge; Silk; Wool; Dyeing
