Since silk thread is amphoteric and has acidic (carboxylic) groups and basic (amino) groups, the pH dependence of its surface zeta potential is known to have an isoelectric point around pH 3. In the process of fiber dyeing, the process of dye adsorption to the fiber surface is said to be the key process governing. An electric double layer forms during the process; molecules forming the surface of the fibers dissociate, while dyes and other molecules in the liquid phase (dye bath phase) are adsorbed as ions. Thus, the zeta potential is believed to contribute significantly to the adsorption of dyes to fiber surfaces.
We measured the surface zeta potential of silk thread in the pH region (pH 3 or below) in which the zeta ponteial of silk thread becomes positive. We added acidic dye (azo dye (orange II)) to silk thread in concentrations ranging from 0 to 1 mM.
