On the basis of investigation into the dyeing equilibrium of cotton fibers with indigo dyes in decamethylcyclopentasiloxane (D5), liquid paraffin and water, the thermodynamic properties of cotton dyeing with indigo dyes in non-aqueous medium systems were studied in comparison with aqueous dyeing. The main works involved are as follows: firstly, the adsorption isotherms were created; then, the three theoretical adsorption models of Nernst, Langmuir and Freundlich were used to fit the adsorption isotherms created; finally, the thermodynamic parameters were calculated. The results showed that the adsorption isotherms were all in line with the Freundlich model. The order of dyeing affinity was in the sequence: liquid paraffin > D5 > water. The dyeing entropy in the three media showed positive values, which is mainly attributed to the adsorption of both indigo-leuco and water onto cotton fibers, thus reducing the ice-like structure formed among the water molecules in the dyeing system and the hydrophobic bonding structure formed among the non-aqueous medium molecules, then leading to an increase in the system disorder. The dyeing heat in the three media also showed positive values, due mainly to the absorption of thermal energy to “melt” the ice-like structure and to “break” the hydrophobic bonding structure. These dyeing thermodynamic properties are conducive to understanding and interpreting the dyeing performance and behavior of indigo dyes in non-aqueous dyeing systems.
