Effect of Dye Aggregation on the Sorption Behavior of Anionic Dyes onto Cationized Cellulose Fibers

The aim of this work was to investigate the sorption mechanism of anionic dyes (acid, reactive) at the liquid-solid interface between the dye solution and cationized cellulose. Viscose fibers were cationized with 3-chloro-2-hydroxypropyl-N,N,N-trimethylammonium chloride to a cationic group content of 25 to 113 mmol kg^-1, and their sorption propensities were first examined in the absence of NaCl. With C.I. Acid Blue 25 and C.I. Reactive Red 120, the dye uptake followed a Langmuir isotherm at low dye concentrations, but at higher concentrations, an increase in dye uptake attributed to a nonionic sorption mechanism was observed. In the case of C.I. Reactive Blue 19, no nonionic sorption process occurred; rather, a decrease in dye uptake was observed. The latter is thought to occur due to the competitive sorption of anionic additives from the dye formulation on the cationic centers of the fiber. To gain a more comprehensive understanding of the processes involved, sorption equilibria were recorded in the presence of NaCl, together with assessments of the dye aggregation propensity in solution with ultraviolet-visible (UV/vis) spectrophotometry. A sorption model was developed using an adapted Langmuir-Sips isotherm that describes the sorption isotherm and accounts for dye aggregation in solution. Further, reactive dye fixation was investigated on a cationized cotton jersey to examine the transition from the sorption to the fixation stage in the dyeing process. The results of the work give insights into the adaptations required in dyeing recipes in the salt-free reactive dyeing of cellulosic materials.