Development of magnetic ionic liquid-based ultrasound-assisted method for green walnut husk naphthoquinone pigments extraction and application of the pigment to black hair dye

The work established a new method combining MILs with ultrasound to effectively extract naphthoquinone pigments from green walnut husk. With the yield of juglone as the index, some parameters were optimized by single factor experiment with UV. Optimal extraction conditions were determined to be as follows: extraction solvent of [C4MIM] [FeCl4], ratio of liquid to material of 10 mL/g, extraction temperature of 60 °C and extraction time of 60 min. Based on this method, the extraction yield of green walnut husk pigment was as high as 85.52% (g/g), which was a lot higher than that of heat-reflux extractions with ethanol and NaOH. Subsequently, the extracted pigment was used as a colorant in the hair dyeing combined with eugenol and tea polyphenol as natural accelerating agent and mordant. The process was optimized by orthogonal experiment, and the hair treated with the extracts got by MB-UA-E was the darkest when pH was kept at 5.0, the pigment concentration was 60%, the dyeing temperature was kept at 70 °C, and the dyeing time was 40 min. Compared to two products obtained by reflux extraction, the extracts got by MB-UA-E showcased superior black dyeing capacity, higher tensile strength, less damage to hair scales or α-keratin and greater UV protection property. It caused less damage to structural of hair fiber than commercial oxidative dye despite participation of conditioner in the oxidative dye. Our study could open new possibilities for using recycled walnut waste in the field of hair dyeing with an eco-friendly and efficient method. [Display omitted] •A new extraction method combining magnetic ionic liquid with ultrasound was established.•Magnetic ionic liquid was easy to recycle in response to external magnetic fields.•The extraction yield of pigment was as high as 85.52% (g/g).•The pigment had excellent black dyeing capacity.