Coloration kinetics and thermodynamics for a special red dye SCFX-AYRL on silk in supercritical carbon dioxide

Reactive disperse dyes are of great significance for the anhydrous coloration of natural fibers in supercritical carbon dioxide (SCF-CO2). In order to further disclose their coloration theory in supercritical carbon dioxide system, the coloration kinetics and thermodynamics of a special red dye SCFX-AYRL on silk fabric, mainly including coloration rate, effective adsorption isotherm model, coloration saturation value, coloration affinity and enthalpy, etc., were investigated at different system temperatures. The obtained results indicate that the coloration kinetics of the dye SCFX-AYRL on silk substrate was conformed to a pseudo-second order kinetic equation, companying with a maximum equilibrium adsorption amount (qe (exp)) at 7.95 mg g−1 and a shortest half-dyeing duration (t1/2) at 11.27 min at 120°C. The dye equilibrium adsorption amount increased with the increase of temperature, while the half-dyeing duration (t1/2) gradually decreased as the dyeing temperature was increased. The investigations on coloration thermodynamics show that the red dye SCFX-AYRL uptake behavior on silk in SCF-CO2 was well consistent with Langmuir adsorption isotherm model, by involving saturation adsorption values ([S]f) over 11.63 mol kg−1 to 19.39 mol kg−1 at different system temperatures. The affinity of the dye (-Δμ°) for silk coloration in SCF-CO2 decreased with increased temperature by accompanying with the highest value of 18.13 kJ mol at 80°C. The calculated dyeing enthalpy (ΔH°) and entropy (ΔS°) were −35.83 kJ moL−1 and −0.20 kJ K−1·moL−1, respectively, indicating that the silk coloration in SCF-CO2 was an exothermic and entropy decrease process. This basic study about the coloration kinetics and thermodynamics in this work is instrumental in disclosing and enriching the coloration theory and mechanism in SCF-CO2, which also promotes the development and application of this green and sustainable coloration method in practice to avoid heavy pollutions.