The research, led by Zhe Jiang of Liaoning Police College in Dalian, China, addresses a persistent challenge in materials science: how to unlock the potential of natural protein fibers without destroying the intricate molecular framework that makes them valuable. Wool’s outer scale layer, rich in keratin-associated proteins held together by dense disulfide bonds, acts as a barrier that restricts dye penetration, reduces hydrophilicity, and causes felting shrinkage during washing. Breaking through this barrier has traditionally required aggressive oxidation with chlorine reagents or enzymatic digestion, both of which carry significant environmental and structural costs.

Chlorination, the most widely adopted industrial approach, uses reagents such as dichloroisocyanuric acid and sodium hypochlorite to cleave the disulfide bonds that lock the scale layer in place. While effective, this method generates absorbable organic halogen (AOX) pollution in wastewater, a class of persistent contaminants that regulatory agencies worldwide have flagged as hazardous. Enzymatic methods, though gentler in principle, suffer from inconsistent performance due to the complex composition of wool and can cause severe fiber strength loss when small enzyme molecules penetrate deep into the fiber’s cortex. Ionic liquids, once heralded as green solvents for protein modification, have recently come under scrutiny because aromatic components like imidazole resist natural degradation and pose potential risks to ecosystems.

Deep eutectic solvents, first proposed by Abbott and colleagues in 2004, emerged as a promising alternative to ionic liquids because they share similar solvent properties but are far easier to prepare. Natural deep eutectic solvents take this concept further by using only natural products such as choline, organic acids, and polyols as their constituents. Jiang’s innovation lies in pairing choline chloride with ascorbic acid in a 1:1 molar ratio, dried and mixed at 80 degrees Celsius until a homogeneous liquid formed. Karl Fischer analysis confirmed the water content remained below 0.5 percent by weight, while viscosity measurements at 596.1 centipoise ensured sufficient liquidity for the treatment process.

The treatment itself is strikingly brief. Five grams of wool fiber, approximately 24 micrometers in diameter, were immersed in the NADES at a solid-to-liquid weight ratio of 1:5 for just three to five minutes at 80 degrees Celsius. After a simple water wash and room-temperature drying, the fibers emerged fundamentally changed on the surface yet structurally intact within. Scanning electron microscopy revealed that the dense scale layer of raw wool, clearly outlined before treatment, became progressively broken after three minutes and largely indistinct after five minutes, while the underlying cortex layer remained completely undamaged. Atomic force microscopy confirmed that the pronounced surface protrusions of untreated wool gave way to a relatively smooth topography.

Spectroscopic analysis provided the molecular explanation for these observations. Fourier transform infrared spectroscopy detected all four characteristic amide bands of wool’s peptide bonds—Amide A at 3270 wavenumbers, Amide I at 1652 to 1640, Amide II at 1546 to 1535, and Amide III at 1268 to 1255—in both raw and treated samples, confirming that the protein backbone survived the treatment regardless of duration. Solid-state carbon-13 nuclear magnetic resonance spectroscopy further quantified the secondary structure, showing that the alpha-helix fraction decreased only marginally from 52.7 percent in raw wool to 51.9 percent after five minutes of treatment, demonstrating that the process was confined to the fiber surface and did not disrupt the internal protein folding.

Raman spectroscopy delivered the most dramatic evidence of the solvent’s selective action. The ratio of the disulfide bond peak at 510 wavenumbers to the reference peak at 1450 dropped from 1.26 in raw wool to 1.08 after three minutes and 0.62 after five minutes, indicating that the NADES progressively cleaved the disulfide crosslinks that hold keratin-associated proteins in a rigid network. As these bonds broke, the proteins loosened and were partially removed, exposing the underlying keratin structure. X-ray photoelectron spectroscopy corroborated this mechanism, revealing decreased carbon and sulfur content alongside increased nitrogen and oxygen on the treated surface, consistent with the removal of the lipid layer and the oxidation of cleaved disulfide bonds by atmospheric oxygen.

The practical consequences of this molecular transformation were immediately apparent in performance testing. Raw wool, shielded by its lipid layer and dense scales, exhibited a water contact angle of 129.5 degrees and a wetting time exceeding 1800 seconds, marking it as strongly hydrophobic. After five minutes of NADES treatment, the contact angle fell to 87.5 degrees and the wetting time plummeted to just 28 seconds, transforming the fiber into a substantially more hydrophilic material. Dyeing performance improved even more strikingly: the Kubelka-Munk color strength value surged from approximately 5 to 16, a roughly 300 percent increase in shade depth, reflecting highly efficient dye exhaustion and deep penetration into the now-accessible fiber interior.

Critical to industrial viability, this intense coloration proved to be genuine fixation rather than superficial adsorption. After a standardized soap-washing cycle, the color strength value dropped only marginally from 16.91 to 16.13, mathematically corresponding to a 95.4 percent dye fixation rate. Washing color fastness declined by just half a grade, from Grade 4 to Grade 3–4, a physically normal consequence of dyeing to such a deep shade where the fiber surface is nearly saturated with colorant molecules. Anti-felting performance also met the machine-washing standard: the diameter of felt balls formed from treated wool surpassed the 30-millimeter threshold specified by the IWTO-20 test method, confirming that the modified fibers resist the directional frictional entanglement responsible for wool shrinkage.

Perhaps most compelling is the comparison with existing methods. The NADES approach requires lower temperatures, shorter treatment times, fewer procedural steps, and dramatically less chemical reagent than chlorination, enzymatic, or ionic liquid alternatives. Even against enzymatic methods that operate at lower temperatures, the NADES treatment holds an energy advantage due to its brief duration and the lower heat capacity of deep eutectic solvents. The work, funded by the Science and Technology Plan Joint Program of Liaoning Province, demonstrates that natural deep eutectic solvents can serve as powerful yet gentle tools for surface modification of protein materials, with potential applications extending beyond wool to feathers and other keratin-based resources. As the textile industry confronts mounting pressure to eliminate toxic effluents and reduce energy consumption, this Vitamin C-based solvent offers a glimpse of a future where the greenest chemistry is also the most effective.

Subject of Research: Green wool treatment using choline-ascorbic acid NADES

Article Title: A mild, effective, and green micro-dissolution treatment for wool fiber using choline-ascorbic acid NADES

Article References: Jiang, Z. (2026). A mild, effective, and green micro-dissolution treatment for wool fiber using choline-ascorbic acid NADES. PLOS One, 21(10), e0360539. https://doi.org/10.1371/journal.pone.0360539

Image Credits: AI Generated

DOI: 10.1371/journal.pone.0360539

Keywords: NADES, wool fiber, green chemistry, keratin, sustainable textiles, deep eutectic solvents, surface modification, anti-felting, mild, effective, green, micro-dissolution