Salt-Free Dyeing
Salt exists in the dyebath to screen a charge. Reverse the charge instead and the salt has nothing left to do.
— mV
Surface charge after cationisation; positive attracts reactive dye
The zeta relationship is linear here and real titration curves are not — charge reversal saturates, so pushing the dose higher stops buying zeta and starts buying unfixed reagent in the effluent instead, and the coefficient must be fitted from streaming-potential measurements on the actual substrate. Fixation efficiency depends heavily on alkali ratio, temperature and time, and the unfixed fraction hydrolyses to a diol that is wasted rather than recoverable. Cationised cotton also dyes far more readily than untreated, which sounds purely good and is not: strike rate rises sharply and levelness becomes the hard problem, so recipes and dosing profiles need reworking rather than transferring.
Cationic Cotton Reagent Dosing & Zeta Potential — free, with the formula and a worked example, at Textile School.