ζ-POTENTIAL AND SURFACE CHARGE DENSITY OF TANNIC ACID-TREATED NYLON 6 FIBERS IN ACID DYE SOLUTIONS

Abstract

In the previous paper, it was reported that the diffusion of acid dye was markedly suppressed in the tannic acid-treated nylon 6 fiber by the electrostatic repulsion between dye anion and dissociated phenolic hydroxyl group of tannic acid.In the present paper, in order to investigate in detail the effect of the electrostatic repulsion between the dye anion and the dissociated functional groups of tannic acid and nylon, ζ-potential of nylon 6 fibers containing various amounts of tannic acid in acid dye solutions at varying pH's was measured by streaming potential method. The surface charge density, σ, of the fiber was evaluated from the ζ-potential. Dyes used were Orange II (C. I. Acid Orange 7) and Diacid Supra Red 3B (C. I. Acid Red 35).As the tannic acid content of fiber increased, both ζ-potential and surface charge density changed toward the more negative value, and consequently the isoelectric point of the fiber was shifted to lower pH. These results may be attributed to an increase in the negative charges on the fiber surface resulted from the dissociation of phenolic hydroxyl group of tannic acid absorbed.A difference between the surface charge density in the solutions in the presence and absence of dye, -Δσ, decreased with the increase in tannic acid content of the fiber. This result can be explained clearly in terms of the electrostatic repulsion between dye anion and negative charge on the fiber.Although the value of -Δσ increased with the decrease in pH of the solution, the abrupt increase at pH 2 to be expected from the adsorption isotherms of the same dyes in the previous paper was not observed. This fact suggests that around pH 2 these dyes behave as dye acid and are taken up by the fiber mostly in the acid form.