Apparent Diffusion Coefficients for Reactive Dyes in Swollen Cellulosic Micro Capillaries
Apparent diffusion coefficients dictate internal fiber dye transport rate, governing shade levelness and fixation efficiency in swollen cellulosic processing.
Donnan Potential
Electrostatic Charge
Electrical potential differences at the boundary between a charged solid phase and an external liquid phase govern ion distribution. The Donnan potential arises when a fiber carries a fixed internal charge that prevents certain ions from moving freely between the fiber and the solution. This charge barrier affects the transport of dissolved chemical species.
Cellulose fibers in water typically carry a negative surface charge.
Dyeing Kinetic
Anionic dye molecules face strong electrostatic repulsion when approaching the negatively charged fiber surface. The Donnan potential must be overcome to allow these dyes to enter the fiber pores and react with the cellulose. If this repulsion is too strong, dye exhaustion will be low, resulting in pale shades.
Overcoming this barrier is the key to efficient dye fixation.
Electrolyte Impact
Adding neutral electrolytes like sodium chloride or sodium sulfate screens the fixed charges on the fiber. This shielding effect reduces the Donnan potential and allows the anionic dye molecules to approach the fiber surface closely. The dye molecules can then form intermolecular bonds with the cellulose.
Proper salt dosing is critical for uniform dye distribution.
Boundary Phase
Increasing the temperature of the dye bath also reduces this barrier by increasing the kinetic energy of the dye molecules. However, the effect is smaller than that achieved by salt addition. Electrolyte control remains the primary method for managing this potential.