Table 3. Adsorption, kinetic, thermodynamic, and mechanistic evidence reported for natural-dye uptake by nylon.

Dye system/class	Nylon type	Key adsorption, kinetic, or thermodynamic evidence	Techniques used to support the proposed mechanism	Proposed mechanism	Evidence level	Main limitation	Ref.
HRP-oxidized catechol, hydroquinone, and gallic acid; chitosan-assisted phenolic pigments	Nylon 5,10	No isotherm, kinetic, or equilibrium constants reported; mild enzymatic coloration	FTIR- and MS-based characterization of oxidative products/model reactions, SEM, K/S, colorimetric and fastness evidence	Oxidative quinone/polyphenol formation, hydrogen bonding, and possible C–N coupling through amine sites	Ref. 32: level 3 for model-reaction C–N evidence, but level 2A for actual fiber attachment:33 level 2A	Model reaction supports possible coupling but does not prove universal covalent fiber bonding	32 and 33
Rice-straw phenolics/flavonoids	Nylon 6	No formal adsorption or thermodynamic analysis; plasma improved uptake and function	LC-MS/MS and UV-vis for extract characterization; FTIR, SEM, XRD, TGA/DSC and colorimetric analysis of the treated system	pH-assisted ionic attraction, hydrogen bonding, plasma-created polar sites, and surface activation	Level 2A	Surface activation was not separated from true molecular fixation	20
Annatto/norbixinate carotenoids	Polyamide-rich and PA6.6 fabrics	High exhaustion under acidic conditions; no formal thermodynamic analysis	UV-vis/exhaustion, FTIR, SEM and colorimetry; K/S, CIELAB and fastness comparisons; concentration-dependent exhaustion and K/S	Carboxylate/phenolic dye groups interacting with protonated nylon sites; protein/soy layer may entrap dye	Ref. 57: level 2A:91 level 1:121 level 2B	Biopretreatment, coating, and fiber diffusion were not separated	57, 91 and 121
Luteolin/weld flavonoids	PA56 and recycled PA6.6	Mordanting/finishing improved thermal stability and light fastness; no adsorption constants	FTIR, TGA, colorimetric, fastness and functional analyses; FTIR/chemical characterization and surface/morphological evidence associated with finishing; mainly K/S, CIELAB and process-response evidence	Hydrogen bonding before mordanting; Al/Fe-mediated dye-metal-fiber bridges after mordanting	Ref. 97: level 2A:35 level 2A:116 level 1	Metal location, leaching, and true fiber-bound fraction were not quantified	35, 97 and 116
Curcumin/turmeric	Knitted polyamide	Langmuir fit, R2 = 0.981–0.993; qe,exp = 310 mg g−1; pseudo-second-order, R2 = 0.995; exothermic ΔH = −1.376 kJ mol−1	Adsorption-isotherm fitting, kinetic modeling, thermodynamic analysis and color-strength measurements	Ionic/polar affinity, hydrogen bonding, and hydrophobic curcumin association	Level 2B	Positive ΔG values complicate spontaneity claims; poor light/wash durability remains limiting	104
Saffron/crocin-rich dye	Nylon 6.6	Freundlich-type adsorption over 0.5–12% owf; no ΔG/ΔH/ΔS reported	Dye characterization, exhaustion measurements, K/S and Freundlich-type adsorption analysis	Heterogeneous adsorption and acid-assisted dye affinity	Level 2B	Based mainly on uptake curves without direct dyed-fiber spectroscopy	119
Cochineal/carminic acid	Polyamide 66	Nernst behavior at pH 3 and 6; pseudo-second-order, average R2 ≈ 0.990; exothermic ΔH = −3771 to −4768 cal mol−1	Equilibrium uptake, nernst partition analysis, kinetic modeling and thermodynamic calculations	Carboxylated dye attraction to protonated amino sites, with non-electrostatic contributions at higher pH	Level 2B	Kinetic fit does not identify the exact bond; fiber-level XPS/metal mapping was absent	61
Madder anthraquinones	Nylon yarn/fabric	Nernst partitioning over 0.6–3.0 g L−1; equilibrium time shortened from 50–55 to 40–45 min between 40 and 80 °C; partition coefficient decreased from 1453 to 876.79	Equilibrium uptake, concentration-dependent partitioning, dyeing-rate analysis and thermodynamic calculations	Electrostatic attraction, hydrogen bonding, and hydrophobic anthraquinone partitioning	Level 2B	Individual anthraquinone contributions and aggregation state remain unresolved	81
Lac dye/laccaic acids	Nylon	Langmuir Q = 26.60 mg g−1 and b = 20.19 mL mg−1; pseudo-second-order, R2 = 0.9987–0.9998; t1/2 decreased from 3.313 to 1.522 min between 70 and 90 °C; Ea = 45.31 kJ mol−1	Adsorption-isotherm analysis, kinetic modeling, equilibrium uptake and activation-energy calculation	Ion–ion interaction between dye carboxyl groups and protonated nylon sites	Level 2B	Chemisorption was inferred mainly from model fitting; equilibrium uptake decreased at higher temperature	62
Turmeric yellow	Nylon	Redlich–Peterson best fit; β increased from 0.72 to 0.88 between 80 °C and 90 °C; endothermic ΔH = 71.34 kJ mol−1; ΔS = 274 J mol−1 K−1	Adsorption-model comparison and thermodynamic analysis	Mixed finite-site and heterogeneous adsorption with diffusion contribution	Level 2B	Model selection depends on fit statistics; fiber-level proof is limited	109
Red sandalwood/santalin	Nylon fibers	Nernst slope decreased from 1127.883 to 589.951 between 70 °C and 90 °C; etters diffusion increased from 5.412 to 8.123; exothermic ΔH = −24.689 kJ mol−1	Nernst partition analysis, diffusion measurements and thermodynamic calculations	Hydrophobic partitioning and internal diffusion	Level 2B	Higher temperature accelerated diffusion but lowered equilibrium loading	69
Purpurin and nordamncanthal anthraquinones	Nylon	Nernst/linear partitioning; purpurin t1/2 decreased from 60.1 to 9.0 min; nordamncanthal D ≈ 1.4–1.6 × 10−8 cm2 s−1; ΔH = 5.19–13.36 kJ mol−1	Uptake/partition and diffusion analysis; for chemically isolated anthraquinones, molecular identification was supported by chromatographic/spectroscopic methods together with equilibrium analysis	Disperse-like hydrophobic partitioning into nylon	Level 2B	Thermal stability limits processing; high uptake does not necessarily indicate durable fastness	112 and 113
Fungal anthraquinones: emodin and dermocybin	Polyamide knitted fabric	Uptake measured but not modeled; acidic baths gave 97–100% emodin uptake	Pigment purity/identity analysis, residual-bath uptake measurements, CIELAB and standardized fastness testing	Hydrogen bonding, π-related aggregation, and possible metal-anthraquinone coordination with amide sites	Level 2B	High uptake may include aggregation or surface adsorption; depth and fastness data are needed	79
Aspergillus turcosus yellow pigment	PA6	No formal adsorption, kinetic, or thermodynamic constants reported	Pigment FTIR, NMR and UV-vis; dyed-fiber FTIR/XRD, K/S, CIELAB and functional testing	Acidic ionic attraction, hydrogen bonding, and hydrophobic/aromatic interactions	Level 2A	Alum reduced K/S; adsorption constants and penetration depth were not reported; proposed covalent-type assignment remains unconfirmed	44
Monascus pigments	Silk/polyamide two-component fabric	No formal adsorption, kinetic, or thermodynamic constants reported; pigment distribution between fiber components was experimentally examined	Optical/cross-sectional microscopy, SEM, separated-fiber analysis, UV-vis and XPS	Hydrophobic interaction, hydrogen bonding and van der Waals association at the polyamide/pigment interface	Level 3 for the investigated fiber/interface evidence	The individual contributions of the proposed interactions were not quantitatively separated	163
Alizarin-ZnO hybrid colorant	PA4,10	No nylon adsorption isotherm or kinetic analysis; alizarin–ZnO binding was characterized before incorporation into the polyamide melt	UV-vis, FTIR, 1H NMR, 13C CP-MAS NMR, XPS, TGA, TEM and DFT	Bidentate catechol-type binding of alizarin to ZnO, followed by physical incorporation of the stabilized colorant into polyamide	Level 3 for the alizarin-ZnO interface	Strong molecular validation of alizarin–ZnO binding does not prove a covalent alizarin-polyamide bond	71
Natural-dye diffusion comparison among polyamides	PA56, PA6 and PA66	Comparative dye uptake associated with differences in crystallinity and fractional free volume; PA56 showed easier dye diffusion than PA6/PA66	Crystallinity/free-volume characterization, comparative dye uptake and molecular simulation	Free-volume-controlled diffusion into amorphous polyamide regions	Level 2B	Strong evidence for transport/diffusion, but not direct identification of a specific dye-polyamide bond	70