Evaluation of a dye isotherm method for characterisation of the wet-state structure and properties of lyocell fibre

An isotherm model based on classical thermodynamic principles has been explored as a route for characterisation of lyocell and comparative cellulosic fibres. The technique requires the experimental measurement of a dye isotherm, in this work Direct Blue 71, under conditions which avoid dye-site saturation and which maintain the influence of Donnan equilibrium upon the extent of uptake. The use of two salt levels was necessary for effective model fitting, which could be performed to establish values for both the fibre internal accessible volume and the free energy associated with dye–cellulose adsorption. The data could also be manipulated to determine the apparent anionic functionality of the fibre, in good agreement with published analytical results. The model has provided insights into the influence of fibre structure on dyeing and has also been valuable as a tool for interpreting the effect of fibre's physical and chemical processing on accessibility.