Study On The Structure And Properties Of Natural Indigo Dye

With increasing public awareness of environmental protection,the adverse effects of synthetic dyes on human health and the ecological environment during the process of production,usage,and disposal have attracted great attention.Natural dyes are favored by people because of their excellent properties,such as safety,antisepsis,anti-inflammatory,and environmental compatibility.Natural indigo is one of the most widely used natural dyes.In this work,the molecular structure,spectral properties,dyeing process,identification methods and application properties of natural indigo were systematically studied for promoting industrial-scale mass production and application of natural indigo.The molecular structure and spectral properties of indigo were studied by using both computational and experimental methods.Geometry optimized analysis showed that indigo was a planar molecule with a strong conjugation effect and belonged to the C_2h point group.The analysis of molecular electrostatic potential surface implied that two intramolecular hydrogen bonds could be formed simultaneously in a single indigo molecule.The frontier molecular orbital analysis showed that the chemical properties of indigo were relatively stable.With the computational study of the molecular structure of indigo,84 fundamental vibrational modes of indigo in the FTIR and Raman spectra were further confirmed.The luminescent mechanism of indigo in the ultraviolet-visible absorption and fluorescence spectra analyses showed that the main absorption bands of indigo located at 268 nm and 612 nm were mainly derived from???*transition,and the fluorescence peak wavelength of indigo at 486 nm also had a???*character.The effects of dyeing parameters on the K/S value of cotton fabric dyed with natural indigo were discussed by using single factor and orthogonal experiments.The optimized results showed that the optimum dyeing parameters are as follows:the dosage of natural indigo dye was 5.0%?o.w.f?,the reduction temperature was 50?,the concentrations of caustic soda,sodium dithionite and dispersant were 0.625 g/L,1.125 g/L,and 10.0 g/L,respectively.The K/S value of the dyed cotton fabric was up to 13.518 under the optimum dyeing condition.The physical properties,compositions,dyeing and application properties of natural indigo and synthetic indigo were compared.It was found that natural indigo was dim powder with the odor of Houttuynia,but synthetic indigo was granular with metallic luster.The commercially available natural indigo contained inorganic substances such as sodium chloride and sodium sulfate,which could be rapidly and accurately differentiated from synthetic indigo by using either X-ray diffraction or infrared spectroscopy.The results of the comparative dyeing properties study indicated that the diffusivity and migration property of natural indigo were inferior,because natural indigo dyes are stuck to their impurities,thereby having larger particle size.The results also showed that the initial dyeing and levelling properties of natural indigo were similar to those of synthetic indigo.The comparison in dyeing performance showed that most of the color fastnesses of natural indigo and synthetic indigo dyed fabrics were similar,both of which had met to the requirements of wearability,while the color fastness to rubbing of natural indigo dyed fabric was better.In addition,the cotton fabric dyed with natural indigo was softer and can also inhibit Escherichia coli.