Investigation of the Polyamide 6,6 dyeing process with Acid Blue 45 dye. Part I. Thermodynamics of Acid Blue 45 adsorption

Among the many synthetic fibres, polyamides are one of the most important. Improvement in the adsorption/ absorption strength of a dye to and inside the yarns is therefore very important for their practical applications. In this study, the kinetics and thermodynamics of the sorption process of Acid Blue 45 on Polyamide 6,6 (Nylon 6,6) were investigated. The sorption was conducted at 293, 303, and 313 ± 0.1 K for 48 h from 10-5 -10-4 M solutions in the presence of 10-3 M NaCl. From the kinetic experiments it was found that this time was sufficient to attain the adsorption equilibrium. The first stage of the sorption (up to 160 min) can be well described by a first-order kinetic equation by its numerical fitting to the experimental results. It is a spontaneous process in which the rate of sorption and the sorbed amount increase with increasing temperature. However, it is a relatively slow endothermic process (positive enthalpy of sorption); thus, it must be entropy-driven to produce a negative free energy of sorption. From the sorption isotherms and calculated thermodynamic functions it can be concluded that at higher temperatures (303-313 K), more polar groups of Polyamide 6,6 are accessible for the dye molecules, which may be due to a glass transition. Data on the zeta potential changes and surface free energy components of Polyamide 6,6 will be presented in Part II of this study, which may help in a better understanding of the sorption process.