Insights into the sorption mechanism of anionic dye molecules on cationised cellulose fibres via zeta potential analysis

The cationisation of cellulose textiles with 3-chloro-2-hydroxypropyl- N,N,N -trimethylammonium chloride is investigated as a salt-free alternative to conventional reactive dyeing. While the ion-exchange mechanism, driven by strong electrostatic attraction between anionic dye molecules and grafted cationic groups, is well understood; uneven colouration remains a significant challenge for industrial application. Addressing this issue requires a deeper understanding of the relation between dye molecular structure and migration behaviour. This study aims to observe the structural levels at which dye sorption occurs, hypothesising that dyes initially adsorb onto the fibre surface before diffusing to cationic groups within fibre pores. To test this, the apparent zeta potential of cationised cellulose fibres (cationic group content of 48–113 mmol kg –1 ) was monitored as a function of dye concentration and time using the streaming potential technique. The zeta potential profiles show a sharp drop after first dye addition, indicating surface adsorption, followed by a gradual increase over time, reflecting intraparticle diffusion into fibre pores. The extent of diffusion is strongly influenced by the dye’s molecular structure and its affinity for the fibre. Furthermore, the zeta potential versus dye concentration profiles suggest that cationic group occupation proceeds from the pores to the surface. However, the extent of intraparticle diffusion, i.e. migration into the pores, depends on the time intervals between dye additions. The results provide valuable insights into the sorption mechanisms of anionic dyes on cationised cellulose, suggesting that the slow intra- and interparticle diffusion characteristics of certain dyes are the cause of non-uniform dyeing.