Chemical modifications of embelin, a benzoquinone from Embelia schimperi Vatke and optimized application in selected textile fibers

Abstract

Globally there is resurgence in the use of natural dyes such as embelin as a suitable replacement for synthetic dyes, which have caused a negative impact to the ecosystem. However, natural dyes have poor colour quality and low solubility. Additionally, they have limited shades and inadequate fastness properties. There is, therefore, need to improve the dyeing properties of some of the natural dyes. The aim of this study was to isolate and derivatize embelin compound and assess the dyeing properties of the embelin derivatives on various textile fibers. The specific objectives were to: isolate and characterize embelin from E. schimperi berries; chemically modify and characterize embelin derivatives; evaluate and optimize their dyeing properties on cotton, silk and wool fibers. E. schimperi stem barks were collected from Kericho,Nandi and Narok Counties, however the yield was low..E. schimperi berries were collected from Kericho County, dried, ground and extracted by maceration method using acetone. Isolation of embelin (1) was done using column chromatography and chemical modification was done by condensation reactions of embelin with different aldehydes in the presence of acetic acid. Optimization of dyeing conditions was achieved using Response Surface Methodology (RSM) and mordanting was executed via simultaneous, pre-and post-mordanting techniques. The dyeing properties were evaluated using colour fastness and colour strengths. Characterization of all the compounds was done using; elemental analyses, UV/VIS spectroscopy, Fourier Transform Infra-Red spectroscopy (FTIR), Gas Chromatography-Tandem Mass Spectroscopy, Liquid Chromatography- Electron Spray Ionization - Mass Spectroscopy (LC-ESI/MS), Nuclear Magnetic Resonance (NMR), Scanning electron microscopy (SEM) and Thermal Gravimetric Analysis. Embelin of high purity isolated from berries (yield 40.0 ±0.01%) was modified to; vilangin (3); methyl vilangin (4); embelin -2, 4-dihydroxybenzaldehyde (5), and embelin ninhydrin (6). UV/Vis spectroscopy revealed λ max at 292 nm for all the compounds, due to π → π ∗ transitions. The FTIR revealed 3301.84 cm -1 (-OH), 1610.49 cm -1 (-C=O) (1), 3313.63 cm -1 (-OH), 1610.49 cm -1 (-C=O) (3), 3316.57 cm -1 (-OH), 1616.39 cm -1 (-C=O) (4), 3298 cm -1 (-OH), 1610.49 cm -1 (-C=O) (5) 3307.73 cm -1 (-OH), 1716.57 cm -1 (-C=O) (1610.49 cm -1 (-α, β- unsaturated C=O) (6). GC-MS revealed the presence of embelin,with a m/z 294 in the crude extract. LC-ESI/MS analysis showed the [M-H] - for embelin at m/z 292.9(1), m/z 599.4 for vilangin (3), m/z 613.2 (C 36 H 54 O 8 ), (4), m/z 292.9 for (C 17 H 26 O 4 ), (5) and m/z 452.9 for (C 26 H 30 O 7 ) (6). Elemental analyses confirmed theoretical C, H, and O values. NMR spectroscopy confirmed the structures of the derivatives synthesized by the disappearance of 1 H NMR peak at 5.59 ppm and 13 C NMR peak at δ 104.45 and the formation of new peaks between 172-200 ppm. Vilangin and embelin ninhydrin (ENn) were soluble in water because of added functional (-OH) groups upon modification which made them soluble at alkaline pH and were therefore used for dyeing. The ENn dyed wool fiber gave the best colour strengths of 15.8 at optimum conditions of pH 9, 90 minutes and 90 °C. The colour fastness was in the range of 4-5 in all the dyed fibers and different shades were achieved using different metallic mordants. SEM images showed the formation of aggregated dye on the surface of dyed fibers. Thermal stability of 400 - 500 °C was established in dyed fibers. The good colour strengths and dyeing properties show that embelin is a suitable precursor for the semi-synthesis of efficient and eco-friendly dyes. For maximum exploits on the derivatized dyes, this study recommends adoption of optimum dyeing conditions and use of mordants to obtain green, violet and brown shades.