A kinetic and thermodynamic study of lac dye adsorption on silk yarn coated with microcrystalline chitosan

The coating of silk yarn with microcrystalline chitosan (MCCh) was carried out using the ultrasonic‐assisted method at a pulsed wave frequency of 80 kHz, which only had a slight impact on the yarn as measured by changes in Young's modulus and percentage of elongation compared with untreated silk. A significant enhancement of lac dye uptake onto MCCh‐coated silk yarn compared with the untreated silk was observed. The rate of dye uptake at different temperatures onto silk yarn coated with MCCh was investigated. It was found that the adsorption rate constant and diffusion coefficient both increased with increasing temperature, as a result of a diffusion kinetically controlled process with a diffusion activation energy of 9.40 kJ mol⁻¹. This suggests that dye adsorption on silk yarn coated with MCCh is a physisorption process. The free energy change (∆G^○), enthalpy change (∆H^○) and entropy change for dye adsorption were also determined, and the negative values of ∆G^○ and ∆H^○ obtained indicated that the lac dye adsorption process is both spontaneous and exothermic.